Formwork Digest

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8 V1 N1 May 2013

Formwork - Future Approach in India

It is not the strongest of the species that survives, or the

most intelligent. It is the one that is the most adaptable tochange that does it. - Charles Darwin.

Winds of change are blowing across every sphere of

construction in India. Same is the case with the formwork

and scaffolding systems in India. An approx. data on

formwork derived from the cement consumption in India

reveals that in India, formwork executed is around 750

million Sqm. out of which formwork executed using system

or engineered formwork hardly constitutes around 10%. It

is a known fact that formwork constitutes around 6%-8% of

the cost of concrete and 60% of the time of the structure. So

it is the right time an emphasis is laid on the right approach

on formwork for the future of the Indian construction.

With the increased growth in high-rise construction,

demanding infrastructure projects shaping up the metros

and tier 2 cities in India, the questions that arise now

are - Are the formwork systems available in India today

sufficient enough for executing such demanding projects?

Are the major formwork suppliers across the world that

have entered the Indian market able to give end to end

solutions to the Indian construction industry? Though the

utilisation of formwork has gone up by leaps and bounds

over the years our approach is still old fashioned. Have we

modernized our approach is still a question to be answeredby all the stake-holders.

This paper deals with focal points which will shape up the

Future Approach of Formwork for the Indian construction

industry. They can be broadly defined as Value Chain Linkage,

Safety Integration in Formwork, Comprehensiveness in

Quality, Standardisation, Green concept and sustainability

and finally the Costing of Formwork.

Value Chain Linkage

Formwork is one of the vital links in the total Value Chain,

the other two links being reinforcement and concrete. At

present, the Indian construction industrys major concernis the stringent timelines (duration) in the projects. With the

clients demands increasing day by day, the construction

companies focus is mainly on the floor to floor cycle time

to meet the timelines of the projects. But to achieve this, a

good engineered formwork system is alone not a solution.

A good formwork system by itself might not give all the

desired results; it only enables to reduce the timelines in

one of the vital links of the value chain. There should be

a wholistic approach considering the other two links of

rebar and concrete. Also there should be emphasis on the

development of the skill levels of the supervisors, labour

and the approach of the engineers rather than just on the

A.L.Sekar, B.Murugesan and C.N.V.S. Rao

Larsen & Toubro Ltd

Focus Formwork Systems


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selection of the right system. To put it in a simpler way,

our future approach while selecting a formwork system

should be such that it should integrate the necessary

features to support the other two links of the value chain

i.e. rebar and concrete which will enable us to carry out

these two activities also in a fast track manner. Only sucha comprehensive approach would yield the desired results

and help us to meet the demands of the customers.

Safety

Safety in formwork is another major concern today

especially in high-rise construction and large infrastructure

projects like metros, flyovers, airports etc. It is a known

fact that in India, Safety levels are yet to catch up to the

International Standards. There is a lot of pressure on

the Indian construction companies today to improve the

same by the Govt. of India, Foreign Investors and also the

increased number of PMCs (which are basically reputedMNCs). Safety cannot be treated as a separate entity,

rather it should be an integral part of the formwork system.

Formwork & scaffolding being the major contributors to the

safety in construction sites as they are also used for the

rebar and concreting works, it is time we pay proper heed

to how these have to be integrated with safety so as to

ensure the overall safety at sites. The various areas of safety

that we need to focus and integrate with formwork are:

- Access (both Vertical & Horizontal)

- Working platforms

- Lifelines and Safety Catch Nets- Erection & Dismantling of Formwork

- Storage & Maintenance of Formwork

- Simple Tools & Tackles

- Design and Engineering

So our future approach when choosing a formwork system

should address the above aspects and how they are

integrated into the formwork system. Only then, in our way

forward, we will be able to live upto the expectations of

the customers and also reach to the level of International

Standards.

Quality

Quality of the finished product is another aspect resulting

from a good and efficient formwork system. For achieving a

good concrete surface, the right kind of sheathing member

should be used in any formwork system, depending on thetype of finish demanded by the client. Invariably, plywood

has been the most commonly used sheathing member

world-wide and has yielded the best results till date with

regards to form finish. Nevertheless considering other

factors in choosing the right kind of formwork system

for the right job, today aluminium formwork has started

penetrating and off late captured the market rapidly with a

share of about 15% of the overall formwork value in India.

Due to its easier handling, good quality surface finish,

repeatability and durability, and best suited for high-rise

residential buildings which are the trend today, aluminium

in future might be a strong contender as far as sheathingis concerned in formwork. Apart from this, to achieve a

good quality product, the formwork system should deal

with critical issues such as Grout tightness, Deformation,

Facilitating Concrete Compaction, Provision of Clean-out

doors and Box-outs etc. Only when all these issues are

addressed along with the selection of the right sheathing

member, a good quality product can be delivered. Looking

into the future, our approach in selection of the system

should keep all these aspects in view to deliver quality

products.

Standardisation

Standardisation of the various formwork systems is also an

aspect to introspect because we cannot afford to have too

many systems at sites which leads to lot of complications

in terms of usage as well as accountability. The formwork

systems should be standardized such that a single system

is adaptable to various structural elements and also across

various projects. Though it has its own limitations, still

standardization can be done to an extent which reduces

the number of components involved in a system, increase

efficiency of the components involved and the flexibility

in usage of these (in terms of sizing and detailing).

This automatically reduces the pain for the engineers /supervisors and also the labour who are the end-users

of the system and gives better results as they can easily

account for the materials and use them efficiently. In this

particular aspect, our future approach should be Using

less for more output through Innovative Solutions.

Green Formwork

Rapid industrialization, growth in population and urbanization

in the two previous millennia and in the current century

have not only taken a heavy toll on non-renewable natural

resources of the planet but also caused unprecedented



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rise in global warming. Most leading business houses

and industries across the world have adopted Corporate

Social Responsibility (CSR) as the roadmap of their current

and future business ethics and principles. Whether this

principle is adhered to while manufacturing of formwork

systems? A confident YES may not be forthcoming.Currently no importance is being given to this aspect of

Green Concept and Sustainability. Stepping into the future,

our approach should be Greener Formwork Systems to

do our part for the betterment of environment. The focus

here can be on some of the important parameters like

Energy Consumption, Wastage, Recycling and Depletion

of Natural Resources. If these aspects are dealt with in the

sourcing of raw materials, manufacturing of the products

involved in the formwork systems as well as utilisation

of the system as a whole, it helps in delivering Greener

Formwork Systems.

(Mobilisation delay, work front delay, delay due to shortage

of other resources & demobilization)

Also the associated costs like the upkeep and maintenance

can be dealt with a central approach by building it up in the

investment cost or with a localized approach to create asense of ownership for the sites using the formwork systems.

The above example clearly indicates that the life of

formwork plays a major role in the Costing of Formwork.

Formwork cannot be a scapegoat for inefficiency within

and across sites which revolve around these time-bound

methods of costing. However if the realistic costing is done

as per the cost incurred per use, it can help construction

companies in India to take a positive call on purchase

or hire of modern formwork systems and change their

approach in future.

Conclusion

Finally to conclude, Formwork Systems cannot be decided

just by suppliers alone as they might not think of all the

related elements in the value chain, instead it has to be

decided by the end-users and engineers who are the future

change-managers. And the guiding principle should be -

Formwork must be approached not in isolation, but in a

comprehensive manner to include the entire Value Chain,

Safety, Quality and Sustainability. Also the thrust should be

on realistic costing of formwork to enable viable usage of

Modern Formwork Systems.

Description Hire charges or WDV method(5% per month)

Cost peruse method

Investment Cost

Apportioned

9960 4800

Fixing and Removing

Cost

3000 3000

Upkeep and

Maintenance Cost

1000 1000

Total Cost 13960 8000

Costing of formwork

With the rapid growth in the construction industry,

introduction of modern formwork systems is essential to

meet the delivery requirements of the customers and at the

same time be competitive. However the modern formwork

systems come with a high-price tag. Hence costing of

formwork for a particular project is very critical for the

engineers. However different costing methods are used

by different contractors. Considering the factors like the

efficiency of formwork being linked to the succeeding &

preceding activities, idling at sites and poor planning; the

time-bound costing method (Written Down Value or Hire-

charges) ends up with higher formwork costs especiallyon materials for no fault of formwork. A small example

below gives a clear picture of how the time-bound costing

methods can be compared:

Sample Calculations of Formwork Costing for Aluminium

Formwork

- Cost of formwork - Rs. 16000 / Sqm. (say)

- Duration of the project - 10 Months (Only for Structure)

- No. of possible re-uses - 100 (say)

- No. of re-uses expected / month - 3

- Actual duration considering all delays - 20 Months (say)



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Certain Safety Considerationsfor Formwork

F

ormwork, the temporary mould and support for fresh

concrete until the concrete is strong enough to support

its own weight and other construction loads, itself requiresa support called falsework. In many codes, formwork and

falsework together are called formwork structure or just

formwork - which last will be the terminology used in this paper.

The problem with formwork is that it is temporary. In many

under-developed and even some developing countries, the word

temporary is automatically associated with lack of need for

planning, design and care, and with neglect of appearance,

strength, and safety. As the owner pays only for the finished

permanent structure and not the temporary structure, least cost

(including cheapest labour and materials, and in the worst

case scenario, low compensation for accident and fatality

claims) are often the easiest way to cut costs on this non-

essential item.

In advanced countries however, it is recognised that most

accidents and in fact most fatalities and property damage

occur during the brief construction stage and not during the

long usage phase of a structure. The business case for safety

in these countries also has amply demonstrated the wisdom

of preventing or mitigating the effects of accidents as against

paying for large compensation and work disruption costs due

to accidents. This is exactly why hazards present in formwork

must be identified, and the risks arising from them must be

assessed and controlled.

In this paper, not being sufficiently familiar with Indian practices

in regard to formwork safety - except as a lay observer during

his visits to India - author will focus on his experience with

Singapore practices, in the hope that Indian professionals may

make their own comparisons and draw their own lessons for

local application.

Basic Safety Requirements

The basic safety requirement is set in the Singapore Workplace

Safety and Health Act of 2006 as the responsibility of every

employer, as far as is reasonably practicable, to protect every

employee from injury and ill-health at the workplace.

This aim of providing a safe place to work is achieved by

adopting guidelines provided by the Ministry of Manpower

and Workplace Safety and Health Council, including the

following:

- Risk assessment and control, before work starts [Ref.1].

- Safe Work Procedure for every activity at the workplace

which may involve risk.

- Permit to Work for all hazardous activities such as work at

height.

- Construction Reg. 2007, Sec. 22(2) reads: In a worksite,

every open side or opening into or through which a person

is liable to fall more than 2m, shall be covered or guarded

by effective guard-rails, barriers or other equally effective

means to prevent fall.

- Construction Reg. 2007, Sec. 63(2) reads: Any formwork

structure that (a) exceeds 9m in height; (b) consists of any

formwork which is supported by shores constructed in 2

or more tiers; or (c) consists of any formwork where the

thickness of the slab or beam to be cast in the formwork

exceeds 300mm, shall be designed by a P.E.

Figure 1 depicts formwork for a condominium block in

Singapore.

Hazards in Formwork

Hazards are potential dangers. Hazardous activities in

formwork design, erection, use and dismantling are as follows:

- Incorrect or incomplete formwork design

- Erecting frames and bracing

- Erecting bearers and joists

- Placing deck and beam formwork

- Moving around on formwork during rebar placement,

concreting, and curing

- Dismantling formwork

N. KrishnamurthySafety and Structures Consultant,

Singapore

Formwork Safety


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In erection, use and dismantling phases, most activitiesinvolve following common hazards:

- Climbing up to or down from formwork, usually by ladders

- Working at height with unprotected edges on platforms

- Tripping and falling at level

- Falling through gaps and holes in formwork

- Falling from incomplete or badly designed formwork

- Hit by formwork components

- Carrying heavy loads

- Struggling with awkward shapes

- Fitting damaged connections and components

- Handling sharp objects and corrosive materials

- Working in harsh (sunny, cold, wet, windy, dusty, noisy

etc.) environments

- Uneven, sloping and cramped work surfaces

- Overloading of formwork

In addition to these, dangers may also arise from inadequate

supervision, material flaws etc. To cover all these in a paper

would be an onerous task. The author will therefore focus only

on the following factors in this paper:

1. Some design considerations,

2. Working safely at height, and,

3. Manual handling of heavy loads.

Some Design Considerations

Factor of safety

India has its own design norms, and they are likely to be world

class. Problems may arise during implementation, and in the

safety culture that may be prevalent in various enterprises.

Author has seen some excellent formwork in big projects in

cities. (Fig. 2.)

But more commonly, especially with formwork for residential

and office building floors, a common sight that greets one is a

forest of supposedly vertical and straight but actually twisted,

bent, de-barked tree branches leaning at all angles some as

much as 20 degrees to the vertical, supporting the beam and

slab formwork. (Fig. 3.)

Other Asian countries also use natural timber for falsework.

In the Far East, bamboo is common, with the advantage that

bamboo is straight and nearly uniform in size along its length,

In India we use all kinds of timber which are twisted, bent, and

non-uniform along their length.

Having just finished an assignment on the formwork code

committee in Singapore, author is very conversant with the

need for strict and conservative design for formwork and other

temporary structures, as already mentioned in the Introduction.

Fig. 1. Author with site engineers in front of extensive formwork for acondominium construction in Singapore.

Fig. 2. A recent picture taken by author in India.

In the past, load factors of 1.5 were commonly used for

falsework design. Often wear and tear in use, and poor field

conditions of connections and erection encroached into this

factor, and in certain cases resulted in accidents involving

injuries including fatalities and property damage.

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Current Singapore Formwork Code [Ref. 2] stipulates a

minimum Load safety factor of 2.0 to be applied to all

designs by whatever method, and for all testing, so that the

designed or tested capacity is at least twice the maximum

requirement under the worst combination of loadings.

What is the corresponding design requirement for Indian

construction with such timbers?

Inclined shores

There is another aspect of such ad-hoc arrangement of shoresthat raises the question: If formwork has to be approved to

satisfy design criteria, how are sloping shores handled?

An inclined member AB at an angle to the vertical subjected

to a vertical compressive force V will develop a horizontal

component H, which would be 18% and 36% of V for angles

of 10 and 20. This horizontal component will tend to increase

the angle . (Fig. 4.)

Then, how come we have not had all inclined members slide

and fall down? That is because the horizontal components

have been successfully resisted, as at top they may be nailed

to some boards, and at bottom the friction and random

projections will usually prevent sliding.

If in a particular case everything is fine when erected, but

when wet the friction coefficient vanishes, and/or when the

load increases the slide resistance is inadequate, disaster

may strike.

Smart people may think that they can cancel out the slope

effects by arranging adjacent members AB and AB sloping

in opposite directions. But there will still be the same horizontal

separating force H at the top and bottom, and if the resistance

to opening up at top and bottom is not enough, woe be unto

the formwork! (Fig. 5.) Of course, someone who knows what

is happening can easily take care of this problem by two

simple ties at or near AA and BB - but this is not much in

evidence.

Sloping shores may be the fast and cost-effective way to use

available poles without cutting them down to required size. It

may also be true that they have worked well for decades, and

the permanent structures that emerge from these temporary

structures of whatever shape, have been finished beautifully.

The point author is making here is that any structural resistance

to failure is not by design, but by chance. Contractors have

just been lucky, and professionals have not even considered,let alone provided for the horizontal component. That they

survive is because of modifications by trial and error. Potential

for failure continues to exist.

Author shows special concern about this sloping shore,

because in a court case in which he was involved, he

demonstrated that it was exactly such an undesigned inclined

strut - although it happened to be a straight steel rod - that

might have contributed to the formwork failure.

In this day and age, when India is contributing globally to the cyber

era and space effort, engineers should be a little more scientific,

Fig. 3. A common sight?

Fig. 4. Forces on an inclined prop. Fig.5. Props sloping opposite ways.

Formwork Safety


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contractors a little more professional, and the owners who

pay for all this a little more considerate of essential expenses

in what they do, at least in the interests of ultimate structural

safety, if not for the sake of appearance.

Working Safely at HeightWorking at height has been the most hazardous activity all

over the world from time immemorial, and continues to attract the

maximum number of accidents and the maximum number of

fatalities. There are many ways in which safety may be ensured

while working at height [ Ref. 3], as follows:

A. Guardrail and toeboard (Fig.6A)

B. Work restraint, attachment to lifeline (Fig. 6B)

C. Retractable lifeline (Fig. 6C)

D. Auxiliary scaffolding (Fig. 6D)

E. Safety net below (Fig. 6E)F. Safety harness (Fig. 6F)

In providing risk control against falling from height, collective

control for all workers (A, D, or E) is better than individual

control (B, C, or F); fall prevention (A, B, C, or D) is better

than fall arrest (meaning termination of a fall before hitting

the base) to reduce the effects of fall impact after one has

fallen (E or F).

In terms of hierarchy of safety then, A or D is the best, and F is

the worst. The full-body harness (E) also comes with a number

of other auxiliary requirements for effective deployment,

including proper fit, sufficient fall distance, strong anchorage,

and prompt rescue. [Ref. 4]

All these requirements are mandatory according to the

Singapore Code of Practice for Working Safely at Height. [Ref. 5]

Manual Handling of Heavy Loads

In formwork - in common with most construction and factory

activities - regularly carrying loads larger than about 25kg is

an insidious risk, not sudden and dramatic like falling from

height, but slowly causing musculoskeletal disorder (MSD)

and escalating to permanent damage of the spine over a period

of about an year.

Musculoskeletal disorders (MSD) are among the most commonworker complaints in the West. In Asia and other under-

developed countries however, it is not reported as much or taken

as seriously, possibly because natives of these countries

are more pain tolerant than citizens of the more developed

countries, or because management will not do anything about

it, or both. It may also be that both management and workforce

do not realise that what starts as a little persistent discomfort

can escalate into a permanent painful problem. In any case,

Fig. 6. Safeguards for working at height.

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most do not recognise it as a problem, and even workers who

experience it resign themselves to it as their lot in life, enduring

lifelong discomfort if not suffering as a consequence.

So workers regularly carry heavy loads over long distances

or keep doing repetitive physical activity; supervisors and

bosses let them, expect it from them, and even order them to

do so. The reason is simple: Labourers (by very name) have

always been doing it. If they dont, who will? They are paid for

it, arent they? We are not forcing them against their will!

This topic comes under Ergonomics the science of work

posture. Authors recent paper [Ref.6] covers many aspects

of construction ergonomics.

Why is this important? What do we do about it? The answers

are not simple. It becomes a matter of safety culture in a society,

the concern of the more powerful groups of people for the

weaker and less fortunate sections of society. Author hopesthat once he explains his stand, professionals will rethink about

how we are using or abusing our fellow human beings.

Many do not know that each kilogram of weight we bend and

pick up and carry in front of our body develops a force of

about 12kg on our low back muscle and bone. (Fig. 7.)

So a 50kg cement bag will put a load of 600kg on the back

of a worker. An average Asians back is designed by nature

to carry a maximum force of about half that (after allowing for

the force imposed by our own torso weight), which means

that nobody should be carrying more than 25kg on a regular

basis.

Australia, where the average person would be larger in size

and stronger than Asians, legislated a few years ago that no

worker should carry more than 20kg routinely. UK had done

likewise a few years earlier when their workers complained

about 40kg hollow concrete blocks.

Singapore recommends a limit of 25kg for worker loads.

Author is not sure about any limitations mandated in India, but

purely on humanitarian grounds he appeals to employers not

to burden their workers with more than 25kg in their normal

work.

If any activity requires lifting and movement of larger loads,

mechanical aids like trolleys may be provided for moving

the heavier weights around; two or more workers may be

deployed to lift them on to trolleys, or carry them for short

distances. Even the simple expedient of rotating the task

between different workers would reduce exposure to risk tomore tolerable levels. Proper procedure to lift heavy loads by

squatting and getting up with the load is also easily learnt.

Needless to say, this analysis and recommendations for

this particular hazard, apply to white collar non-construction

workers too, such as office and lab assistants.

Conclusion

Author has highlighted a few of the hazards in formwork

design, erection, use and dismantling with which he has

personal experience in Singapore. Not all the hazards may be

perceived as equally critical in India. But in a nation committedto democracy and concern for all citizens, the risks described

and the solutions proffered by the author may serve to trigger

improvement of overall safety culture.

References

1. Krishnamurthy, N., Introduction to Risk Management,(Self-Published), Singapore, May 2007, 86p, ISBN: 978-981-05-7924-1.

2. SS580:2012 (ICS 91.080.99), Code of Practice for Formwork(Formerly CP23), SPRING, Singapore, Nov. 2012, 40p.

3. Figures 5A to 5E sourced from Falls from height during the

floor slab formwork of buildings: Current situation in Spain, byJose M. Adam, Francisco J. Pallars, and Pedro A. Caldern,Copyright 2009 National Safety Council and Elsevier Ltd.

4. Krishnamurthy, N., Full Body Harness - Blessing or Bane?, The

Singapore Engineer, Magazine of the Institution of Engineers,Health and Safety Engineering issue, August 2012, p. 18-22.

5. WSH Council, Code of Practice for Working Safely at Height,Workplace Safety and health Council, Singapore, October 2009,50p.

6. Krishnamurthy, N., Ergonomics at the Construction Sites, TheSingapore Engineer, Magazine of the Institution of Engineers,Health and Safety Engineering issue, February 2013, p. 20-27.Fig. 7. Forces on vertebrae

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Formwork, Insulation, Wall thicknessand Fly Ash: Do They Affect

Concrete Maturity?

Concrete is the most widely used construction material

and its formability is one very important property.

Several different types of formwork are available in

the market. One way of classifying them is based on whether

they are stripped or not: (1) conventional formwork and (2)

stay-in-place (SIP) formwork made using different materials

such as steel, PVC (poly vinyl chloride), FRP (fiber-reinforced

polymers), EPS (expanded polystyrene), etc. Typically, wood

formwork is used to form concrete. However, the amount of

wood that can be harvested has been reduced, increasing

the cost and reducing the availability of wood. In addition, it

is environmentally advantageous to decrease the amount of

wood needed in construction.

New concrete forming technologies designed to reduce

wood consumption include reusable and SIP formwork.

After being used, reusable metal and wood forms must

be removed, cleaned, transported and then stored. These

systems limit design versatility since they generally come

in large, flat panels. Unlike traditional formwork that are

stripped after concrete has gained enough maturity, the SIP

forms remain an integral part of the structure; some even

provide structural strength and ductility (Kuder, Gupta et al.

2009), some provide higher R value and some just provide

a finished surface. Some forming systems such as Insulated

Concrete Forms (ICF) increase the insulative properties and

R value of the concrete walls and some SIP systems also

integrate insulation in the forming system. However, the effect

of such highly insulated walls on concrete hydration at early-

ages is not fully understood. One such category of SIP forms

are the plastic forming systems that are also more versatile

than wood and metal because various shapes can be easilymanufactured given its flexibility.

Since the SIP forms are not stripped, hence never exposing

the surface of concrete, it is very important to ascertain

that concrete in the forms has met or exceeded the project

specifications. One such type of forming system is a SIP

system that utilizes PVC panels and connectors as formwork

(Octaform Systems Inc, 2009). This forming system can be

used with and without insulation and its effect on the maturity

of concrete is not fully understood.

On the material side, fly ash is a commonly used Supplementary

Rishi Gupta1and Katie Kuder21Faculty & Program Coordinator, Department of Civil Engineering,

British Columbia Institute of Technology2Assistant Professor, Dept. of Civil and Environmental Engineering,

Seattle University

Use of different forming material, insulation, and stripping time can significantly affect the maturity and hence the strength gain ofconcrete within such forming systems. This information can be vital in determining the stripping time of scaffolding and formwork.

In this project, maturity and compression tests were performed on specimens (simulating scaled-down walls) formed using a PVCstay-in-place (SIP) forming system with and without insulation. These findings were then compared to data obtained from wallsformed by wood formwork, which is the material typically used in the field. The various parameters studied in this project werewall thickness, type of forming material, insulation, and addition of fly ash. Results indicate that with an increase in wall thickness,the peak temperature and the temperature development index (TDI) increase proportionally. TDI is defined as the area under thetemperature versus time curve measured from the dormant temperature to the peak temperature. The data show that the proposedTDI is a good indicator of the extent of the hydration reaction, and with further research the relationship between temperaturedevelopment and strength gain of concrete could be clearly identified. Both wood forming when compared to the SIP system, andinsulated systems when compared to un-insulated systems, increase the peak temperature and TDI. Use of fly ash in concreteresults in a lower temperature peak and TDI and a delay in reaching peak temperature. However, use of concrete containing flyash in insulated SIP systems has a higher TDI than a conventional concrete mix formed in wood forms, indicating better concrete

maturity at the same age.

FormworkResearch


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Cementing Material (SCM) which enhances the fresh

properties of concrete including increased workability

(Mindess, Young et al. 2003; Malhotra 2006; Mehta 2009).

High volume fly ash contents are now replacing cement

because (1) this results in lower consumption of cement,

hence reducing the energy required to produce cement andalso reducing the associated green house gas emissions,

(2) production of many self consolidating mixes require high

contents of fly ash, and (3) this results in cost-savings and is

a more sustainable process since an industrial by-product (fly

ash) is now being utilized which otherwise would end-up in a

landfill. However, addition of fly ash can decrease the rate of

the hydration reaction, negatively impacting the construction

process as the stripping of forms may be delayed. The effect

of using fly ash in concrete on the maturity of concrete was

studied in this project. During the hydration reaction, heat is

generated and released to the surroundings; the rate of the

reaction is proportional to the heat generated. The dissipationof this heat of hydration to the environment will depend on

the type of forming material used, thickness of the concrete

mass, and use of insulation (Khan, Cook et al. 1998; Wang,

Zhi et al. 2006). The effect of using insulation, wood or a PVC

SIP system on the maturity of concrete was studied in this

project. The maturity of concrete was evaluated by calculating

a Temperature Development Index (TDI), which is described

later.

The TDI is a close function of the hydration process and hence

it is important to note the different stages of the hydration

process. The first stage is the rapid heat evolution, which

occurs very quickly, the concrete then moves into the dormant

stage where the concrete is workable. The dormant stage

ends with the initial set and moves into the acceleration stage

as the reaction begins to accelerate. The concrete remains

workable until the final set where the greatest temperature is

achieved. During the deceleration stage, the reaction slows

down and temperature is reduced, bringing the concrete to

a steady state. A practical and effective way to evaluate this

hydration process is to monitor the temperature released by

the hydration reaction over time. The temperature data also

serves as an indicator of the rate of reaction, as temperature

increase is proportional to the heat generated.

Materials and Forming Systems

Concrete was prepared in a rotary drum mixer using Type I

cement (manufacturer- Lafarge), river sand, coarse aggregate

with maximum size 10 mm, Class F fly ash (Plant- Centralia),

and admixtures including superplasticizer (product- Glenium

3000 NS) and air entrainer (product- MB VR Standard). For

constructing the wood forms, lumber meeting the following

specifications was used: 23/32 inch DF-DF plywood, 48/24

span rated. The forms were oiled using a release agent (WD-

40) before pouring concrete.

A concrete mix design typical of what is used in field

construction with the PVC SIP system was used. The control

concrete mix had a water-cement ratio of 0.49 with 350 kg/

m3 of cement, 1160 kg/m3 coarse aggregate, 700 kg/m3 of

sand, dosage of 600 ml/m3 of superplasticizer and 200 ml/

m3 air entrainer. Another mix was prepared by replacing 40%of the cement with fly ash by weight.

PVC SIP Forming System

The SIP forming system used in this study is briefly described

below. This forming system is composed of PVC panels,

connectors, and braces that form cells. The panels are

typically 150 mm wide and come in variable heights. The

connectors, which are available in various widths, are placed

perpendicular to the panels and have openings that are

provided for placement of rebar and to allow the concrete

to flow through the wall. Figure 1 (a) shows one such cell

of the forming system braced with standard connectors, T-connectors, and the 45 braces to the panels. Insulation is

also available (Figure 1 (b)) for an increased thermal mass,

leading to higher energy efficiency.

The panels that make up the interior and exterior of the

formwork and can be curved to conform to the shape needed

for the specific application, such as an aquaculture tank

shown in Figure 2 (a). Once the vertical formwork has been

assembled and raised, wood bracing is used, as shown in

Figure 2 (b). This bracing is similar to the bracing required

when the wood formwork is used and is removed once

concrete within the forms has gained sufficient strength.

Figure 1. Components of SIP formwork cell (a) Top vies of cell containing allcomponents, (b) Schematic of cell with insulation (Octaform 2009)

Specimen Preparation

To compare the influence of PVC SIP formwork on the heat of

hydration (maturity/temperature release), the results from PVC

SIP system were compared to wood formwork. The SIP test

wall configuration, shown in Figures 3 and 4, was designed to

have three rectangular cells. This configuration was chosen

so that the two extra cells on either side of the middle cell

would eliminate the temperature effects of closeness to the

end of the wall (boundary effects). The sides of the end cells

were filled with wood pieces to prevent concrete from flowing

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Temperature measurements

Before the concrete was mixed and poured into the formwork,

the data acquisition system was started to obtain the initial

ambient temperature. The thermocouple wires were then

placed in the concrete as described earlier. Temperature

readings were collected at a rate of 3 readings per minute,

each reading being an average of 100 scans. Once the test

had run for an amount of time determined by previously

conducted preliminary tests indicating complete hydration,

the data acquisition system was stopped and the data was

saved for analysis.

Compression testing

Cast cylinders

To determine the compressive strength, cylinders (100 x 200

mm) were cast according to ASTM C 31. Cylinders were de-molded after 24 hours and moist cured for 56 days. Testing

was done using a Riehle hydraulic testing machine with a 300

kip load cell. Specimens were loaded by displacement-control

at a rate of 0.085 mm/min. The data acquisition system was

set up to measure the applied load at a rate of 25 readings

per second, each reading being an average of 1000 scans.

Four cylinders were tested for each mix type (NC and FA).

Neoprene caps were used in lieu of capping or grinding of

cylinders.

Cores

To study the effect of the PVC SIP system and insulationon the concrete compressive strength, drilled cores were

extracted after monitoring the temperature for 36 hours.

This was also done to determine if there was any correlation

between temperature and strength development. Cores

were taken from various 200 mm walls and subjected to

compressive testing equipment as described above. The

various configurations from which were extracted are shown

in Table 2.

Results and discussion

Data averaging

The temperature data were recorded from all six

thermocouples over time. After analyzing the data, it was

noted that the temperature readings from the five embedded

thermocouples did not vary significantly with the location of the

thermocouples; therefore the average curves were deemed

suitable for analysis. This finding is illustrated in Figure 7. Note

that the bottom and the top thermocouples reach the second

lowest and the highest temperatures, respectively, even

though these were located approximately the same distancefrom the center of the wall.

Formwork type Concrete type Insulation

SIPNormal Present

Fly Ash Absent

WoodNormal Present

Fly Ash Absent

Table 2: Various specimen types used for extracting core samples

Each wall was cored with a concrete coring machine as

shown in Figure 6, with a 100 mm diameter drill. Three cores

were taken from each wall: one from the middle cell, and

one from each of the two side cells. Each 100 mm diameter

core was then cut down to a height of 200 mm and tested for

compressive strength.

Figure 6. Coring 100 mm diameter sections for compression testing from SIPformed specimens

Figure 7. Typical plot: temperature versus time as a function of thermocouplelocation for a 200 mm thick wall formed with wood

Temperature Development Index (TDI)

From the averaged data, the peak temperature (Tp) was

determined along with the time at which the peak (tp) occurred.

Figure 8 presents a typical plot of average temperature versus

time, indicating the Tp and tp. Throughout the testing program,

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the ambient temperature in the lab fluctuated, resulting in

varying initial temperatures from specimen to specimen. This

situation would also be typical of a construction site where the

ambient conditions will be different from day to day. Figure 8

presents the average temperature versus time for two tests

performed during primary testing. The ambient temperaturerecorded during each test is not presented in the plot for clarity.

The variation in ambient temperature was less than 1C in

each test and hence the effect was considered negligible.

Although it was expected that the 100 mm FA specimen

would achieve a lower peak temperature than the 200 mm FA

specimen because it contained a smaller volume of concrete,

the results show just the opposite. As research has shown,

ambient temperatures affect the rate of the hydration process

(Wang, Zhi et al. 2006); warmer temperatures speed up the

hydration process and contribute to higher peak temperatures,

while colder temperatures slow down the hydration process

and contribute to lower peak temperatures. Because of theeffect of ambient temperatures on the hydration process,

and that ambient temperatures were not controlled during

testing, there was no linear relationship between hydration

and ambient temperature. Therefore this experimental project

cannot directly account for this effect.

the 100 mm specimen with fly ash (Table 3). Refer to Figure 8

for identification of critical points for the temperature analysis

and an illustration of the areas calculated. After analysis it

was seen that the 100 mm FA specimen (shown in Figure 8)

achieved a smaller A1 calculation in comparison to the larger,

200 mm FA specimen. The area calculation was conductedfor each test and the areas were compared. These values

were used as an indication of temperature development

during hydration. The A1 value representing temperature rise

and maturity immediately after the dormant stage was found

to be more relevant than that of A2 and is used extensively

throughout this report. The authors have called this value the

Temperature Development Index (TDI or simply A1). The

results from these comparisons are discussed later.

Figure 8. Average temperature versus time curve for 100 and 200 mm thickwalls

To minimize the effect of the ambient temperature during the

analysis, it was proposed that the area under the hydration

curve be calculated and analyzed. This area was split intotwo smaller areas: A1 being the region bound by the initial

minimum temperature (indicating the dormant period) and

the peak temperature and A2 being the region bound by

the peak temperature and the final temperature at 30 hours.

Preliminary tests (not reported here for maintaining brevity)

had indicated that the internal temperature in specimens

more or less dropped to ambient temperature after 30 hours

(Lowrie, Sommer et al. 2007). In certain specimens the peak

temperature was very similar to the ambient temperature and

sometimes lower than that recorded at 30 hours. In such

cases, the value of A2 would be negative as in the case of

Figure 9. Average temperature versus time for 100, 200 and 300 mm thick

walls formed with SIP formwork with normal concrete (NC) and 40% fly ashreplacement (FA)

Wall TypePeakTemp

(C-hr)

Time atPeak(hrs)

A1 (TDI)(C-hr)

A2(C-hr)

Normal

Concrete

100 mm

200 mm

300 mm

24.08

27.79

29.69

11.9

13.3

15.0

13.91

25.93

40.75

22.36

30.38

60.80

Fly Ash

100 mm

200 mm

300 mm

22.07

25.81

27.39

13.0

15.5

16.4

5.72

24.04

30.08

-0.19

28.95

36.79

Table 3: Temperature at peak and A1 (Temperature Development Index, TDI),

A2 for the SIP system

Wall TypePeakTemp

(C-hr)

Time atPeak(hrs)

A1 (TDI)(C-hr)

A2(C-hr)

Normal

Concrete

100 mm

200 mm

300 mm

25.31

29.12

29.85

15.0

13.5

15.7

24.12

38.71

53.71

34.07

60.93

76.99

Fly Ash

100 mm

200 mm

300 mm

24.09

24.02

26.11

12.1

16.7

18.1

8.44

22.65

32.47

3.99

29.67

37.85

Table 4: Temperature at peak and A1(Temperature Development Index, TDI),

A2 for the Wood formwork

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used. In general, the

addition of fly ash appears to slow the hydration process,

lowering the total amount of temperature developed, and

cause the specimen to reach a lower peak temperature at a

later time for both the wood and SIP walls. In the case of the

200 mm SIP fly ash specimen, a slightly higher temperature

development was achieved in comparison to the wood

formed specimen. When varying the composition, there is a

greater difference in the TDI for the wood walls, an average

of a 49% difference, than for the SIP walls, with an average

of a 31% difference. This may indicate that the SIP system

may contain more moisture and develop more cumulative

temperature relative to wood formwork during the hydration

process when fly ash is used.

In general, the inclusion of insulation for both wood and SIP

systems increases the peak temperature during hydration, and

contributes to greater temperature development. In generalthe peak temperatures for specimens containing insulation

occur later in comparison to specimens without insulation.

For the specimens tested, the inclusion of insulation with

the SIP system appears to have a more significant effect on

temperature development and peak temperatures achieved

in comparison to the wood system. This finding may be a

result of the wood system itself providing insulation and the

additional insulation having little effect. It is interesting to note

that the SIP system used with the fly ash mix and insulation

achieved a greater temperature development than the wood

forming system used with the normal concrete mix and

no insulation. These results are an indication that the SIPsystem used in combination with fly ash and insulation more

positively contributes to the hydration process in comparison

to standard wood forms used with the normal concrete mix.

The compression test results for cored specimens along with

the analyzed temperature data is presented in Table 5. In

comparing the wood and SIP formed walls, the wood formed

walls generally achieved a higher temperature development

than the SIP formed walls for all size walls. To understand

this correlation, the R-Value of each formwork material was

determined. The R-Value for the PVC SIP is reported as 0.60

(Octaform Systems Inc., 2009) while that for 20 mm ()

plywood is reported as 0.90 (TECO, 2010). The lower R-Value

of the SIP indicates that it is less resistant to thermal change

than wood, and hence may explain why it achieved lower

temperature development overall in non-insulated systems.

Compression Testing

Cast cylinders: When 40% cement was replaced with fly ash,

Sample

Compressive

Strength (fc) (MPa)

Standard

Deviation (MPa) TDI / A1 (Chr)

Avg. Peak Temp

(Tp) (oC)

Avg.Time at

Peak (tp) (hrs)

Normal ConcreteSIP 17.86 4.83 42.24 29.45 14.34

Wood 20.13 7.38 43.50 31.00 11.84

Fly AshSIP 12.07 2.69 24.31 25.42 17.87

Wood 12.41 1.72 34.53 26.57 17.99

NC InsulatedSIP 15.05 0.92 64.01 32.54 16.00

Wood 12.51 7.52 44.52 28.80 17.33

FA InsulatedSIP 20.40 0.35 51.68 35.50 12.80

Wood 5.85 0.24 30.25 28.89 14.87

Table 5: Compressive strength data from cored samples (36 hours after casting) and the corresponding temperature data

Figure 12. Average thermocouple temperature versus time for walls formedwith SIP formwork with NC and FA with and without insulation (Insul)

Figure 13. Average thermocouple temperature versus time for walls formedwith wood formwork with NC and FA with and without insulation (Insul)

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the compressive strength decreased from an average of 32 +

3 MPa to 29 + 6 MPa at 56 days.

Cores: Cores were taken from four 200 mm walls and four

250 mm walls with 50 mm insulation (and 200 mm concrete):

two each from SIP NC, SIP FA, Wood NC, and Wood FA.

These cores were tested and averaged for each wall. The

compression testing data is summarized in Table 5. It should

be noted that the compressive strengths reported in Table 5

are at an age of 36 hours and hence significantly lower than

that measured for cast specimens tested after 56 days. One

of the other methods of comparison for these walls was the

TDI (A1 in Table 5), which has been explained already.

Treating the compressive strength for FA insulated wood

specimen as an anomaly, a reasonable correlation between

fc measured for cored samples and TDI was observed.

This correlation existed only when the same concrete type

and formwork configurations were considered. The generaltrend shows that when the TDI increases there is an increase

in compressive strength. However, when comparing the

insulated walls to the non-insulated walls, similar compressive

strengths were measured for dissimilar TDIs. This may be

attributed to limited number of cores and the high standard

deviation observed in the compressive test results; as high

as 60% for the NC insulated wood specimen). Establishing

a straightforward correlation between fc and TDI was difficult

also because TDI corresponded to thermal activity up to

peak (time to peak ranged between 11 and 18 hrs), whereas

all coring occurred at 36 hours, hence making a direct

comparison more difficult. Further research is necessary toclearly establish this correlation.

Conclusions

1. The proposed TDI was an effective method of analyzing

the temperature data. TDI could be effectively used to

minimize the effect of different ambient conditions and to

capture the hydration that occurs immediately after the

dormant hydration stage.

2. The wood forming system contributes to higher peak

temperatures, which occur later when compared to the

PVC SIP forming system. The extent of hydration processdoes appear to be greater in the wood system for a normal

concrete mix. This corroborates well with the R value for

both forming systems.

3. The SIP system used in combination with a high volume

fly ash mix and insulation achieved greater temperature

development in comparison to the non-insulated wood

forming system used with normal concrete. These results

indicate that the insulated SIP system used with fly ash

more positively contributes to the hydration process in

comparison to the noninsulated wood formed system

used with the normal concrete mix.

4. Special attention is required to ensure strength gain before

stripping forms especially for thin wall cast using concrete

containing high volume fly ash.

Further Research

In addition to wood and one type of SIP formwork, furtherresearch should be done to compare the effect of other

forming systems used in the industry on maturity of concrete.

In particular, larger wall sizes should be tested to better

simulate the conditions experienced in the field. Stripping

time variability should be incorporated into this testing. Validity

of TDI should be examined by conducting tests at extreme

ambient conditions to simulate colder winter climates and

warmer summer climates. Further research is suggested

to clearly establish the relationship between concrete

compressive strength and TDI by having a larger sample size

of cored specimens.

Acknowledgements

The authors would like to thank Octaform Systems Inc. for

sponsoring this project and for providing the materials and

technical expertise for this project. The authors would also

like to acknowledge the contributions of the Seattle University

senior design team that was comprised of Kristian Lowrie,

David Sommer, and Nikki Wheeler.

References

- Khan, A. A., W. D. Cook, et al. (1998). Thermal Properties and

Transient Thermal Analysis of Structural Members During Hydration.ACI Materials Journal 95(3), 293-303.

- Kuder, K. G., R. Gupta, et al. (2009). Effect of PVC Stay-in-PlaceFormwork on Mechanical Performance of Concrete. Journal ofMaterials in Civil Engineering 21(7), 309-315.

- Lowrie, K., D. Sommer, et al. (2007). Effect of PVC Stay-In-PlaceFormwork on the Hydration of Concrete, Seattle University: 40.

- Malhotra, M. (2006). Reducing CO2 Emissions: The Role of FlyAsh and Other Supplementary Cementitious Materials. ConcreteInternational, 42-45.

- Mehta, P. K. (2009). Global Concrete Industry Sustainability: Tools forMoving Forward to Cut Carbon Emissions. Concrete International,45-48.

- Mindess, S., F. J. Young, et al. (2003). Concrete. Upper Saddle River,

Prentice Hall.- Octaform Systems Inc., Technical Guide (accessed October, 2009),

- TECO, Panel R Values (accessed June 2010),

- Wang, K., G. Zhi, et al. (2006). Developing a Simple and Rapid Testfor Monitoring the Heat Evolution of Concrete Mixtures for Both

Laboratory and Field Applications. N. C. P. T. Center.

Publishers Note:This paper was presented at the Proceedings of the One DaySeminar on Modern Formwork Systems for Building Construction Held in IITMadras, Chennai. The Masterbuilder was the official Media Partner for the aboveevent.

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Effect of Concrete Temperature andFormwork Width on Variation PressureFormwork of Self-compacting Concrete

1Amir Hosein Bakhtiarain,2Morteza Askari

1The Bsc. Student of Islamic Azad University, Iran2The Faculty Member of Islamic Azad University, Iran

The design of formwork systems for vertically cast elements

is controlled by the lateral pressure developed by the

flesh concrete. It is well established that concrete

consistency, method of placement, consolidation, type

of cement, temperature of concrete, maximum aggregatesize, head of concrete, pore water pressure, rate of

placement, and size and shape of the formwork have all

marked effect on the development of lateral pressure

[3-6-9].

Maxton (from Rodin [9]) studied the coupled effect of the

casting rate and concrete temperature on the lateral

pressure envelope for conventional concrete. Different

series of low-slump concrete mixtures placed at casting

rates varying between 0.6 and 2 m/h were investigated.

The concrete temperature varied from 4.5 to 27C.

Maximum lateral pressure was found to increase with the

increase in the casting rate and/or decrease in concrete

temperature. Irrespective of the tested parameters, the

pressure envelope was reported to be hydrostatic from

the free surface to a certain maximum value, and then

remained constant until the bottom of the formwork.

For formwork design purposes, ACI Committee 622 [2]

proposed the following design equations for column and

wall elements, both of which take into account the rate of

casting and concrete temperature: For columns:

In this article two complete programs about effect of concrete temperature, formwork width, on lateral pressure formwork of Self-Compacting Concrete are discussed. For considering effect of concrete temperature concrete mixtures which are built under 10-30c , are used and the result show that concrete temperature hasn't considerable effect on initial pressure (after casting finishing).But in time passing, pressure reduction is significant for surveying in formwork width effect, two columns with 200 and 920mmdiameter, are applied

For walls:

Where Pmax

: maximum lateral pressure, KPa

R: rate of casting, m/h

T: concrete temperature, C

H: head of concrete, m

Effect of concrete temperature on formwork pressure

For investigation of effect of concrete temperature onlateral formwork pressure, experimental research of Assad

[7] and his colleagues was used and described thosebelow:

Materials

The ternary cement contained 6% silica fume, 22% fly

ash, and 72% CSA Type 10 cement. The Type 30 cement,Type 10 cement, and fly ash had blaine specific surface

values of 600, 325, and 410m2/kg, respectively. The silicafume had a B.E.T specific surface of 20,250m2/kg.

Continuously graded crushed limestone aggregate withnominal size of 10mm and well-graded siliceous sand

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31V1 N1May 2013

were employed. The coarse aggregate and sand hadfineness module of 6.4 and 2.5, bulk specific gravities of

2.71 and 2.69, and absorption values of 0.4% and 1.2%,respectively. Polycarboxylate-based high-range water-

reducing admixture (HRWRA) of 1.1 specific gravity and27% solid content was used. A high molecular weight

cellulosic-based material was employed for the VEA toenhance stability of mixtures proportioned with 0.40 w/cm.

Mixture proportion

As summarized in Table 1, the investigated mixtures were

prepared with 450 kg/m3of binder content and w/cm of

0.40.The effect of concrete temperature on lateral pressure

variations was evaluated by testing mixtures prepared at

10, 20, and 30 2C for the TER-10, TER-20, and TER-30mixtures, respectively. Ambient temperatures during the

sampling and testing were 14, 20, and 27C, respectively,to minimize heat loss of the tested concrete. The effect of

using Type 30 cement and set accelerating admixture onthe variations in lateral pressure was investigated, as they

have marked effect on the rate of cement hydration. The

dosage of the set accelerator was set at 1000 mL/100 kg

of binder. The T30-20 and TER-20-ACC mixtures prepared

with Type30 cement and set accelerating admixture,

respectively, were proportioned at 20 2C and testedat 20C ambient temperature. The VEA dosage was fixedat 260 mL/100 kg of binder, and the sand-to-total

aggregate ratio remained constant at 0.46 for all tested

mixtures. The HRWRA and AEA concentrations were

adjusted to secure initial slump flow of 650 15mm and

air content of 6 2%.

Instrumented column systems

Two experimental columns were used to determine the

lateral pressure exerted by plastic concrete. The firstcolumn measures 2800mm in height and 200mm in

diameter, and was used to evaluate pressure variations ofthe plastic concrete. The lateral pressure was determined

using five pressure sensors mounted at 50, 250, 450, 850,and 1550mm from the base. In order to enable the

evaluation of pressure variation up to the hardening ofthe concrete, a shorter column measuring 1100mm in

height and 200mm in diameter was used. Three pressure

sensors similar to those employed in the former columnwere mounted at 50, 250, and 450mm from the base.

Both experimental columns were made of PVC with asmooth inner face to minimize friction with the concrete.

Fabrication and testing program

The slump flow, concrete temperature, unit weight, airvolume, L-box flow characteristics, surface settlement,and setting time were determined, and the results aresummarized in Table 2.

Table 2. Properties of evaluated SCC mixtures

Fresh concrete properties

All SCC mixtures had L-box blocking ratios (h2/h1) greater

than 0.80 indicating adequate passing ability, andrelatively low surface settlement (


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32 V1 N1May 2013

for the TER-30 mixture is given in Fig.1. The slump flowvalues noted at various times are also indicated. Right

after casting, the concrete is shown to develop lateralpressure close to the theoretical hydrostatic pressure. The

hydrostatic pressure (Phyd

) is calculated as: Phyd

= g

H; where , g, and H refer to the concrete unit weight,gravity constant, and head of concrete in the formwork,

respectively. The relative pressures compared to Phyd

atthe base of the column determined at end of casting and

then after 1, 2, and 3 hours were 91%, 77%, 68%, and 61%respectively.

Fig. 1: Variations of lateral pressure envelope with time for the TER-30 mixture

Fig. 2: Effect of concrete temperature, cement Type 30, and use of set-

accelerating admixture on pressure variations determined at the bottom of the

2800-mm high column

Effect of concrete temperature on variations in lateral

pressure

Variations of the P(maximum)/P(hydrostatic) values

measured along the 2800-mm column of the five SCCmixtures placed at 10 m/h are plotted in Fig.2. Slump

values determined at the end of pressure monitoring arenoted. Mixtures prepared with ternary cement at initial

temperatures of 10, 22, and 30C develop similar relativepressures of 91% at the end of casting. This indicates

that concrete temperature has no significant effect on the

development of initial pressure. The maximum initialpressure is rather affected by the degree of internal frictionthat depends on the coarse aggregate volume andmixture consistency. On the other hand, the rate of

pressure drop with time is significantly affected byconcrete temperature. For example, the time to reduce

the relative pressure by 25% decreased from 400 to 250and 160 minutes for the TER-10, TER-20, and TER-30

mixtures, respectively.

Alexandridis and Gardner [1] reported that concrete cast

at higher initial temperature can exhibit higher cohesionthrough the formation of a gel structure. This can enablethe plastic concrete to develop higher shear strength

capable of carrying a greater fraction of the vertical load,thus resulting in increased rate of pressure drop with time.

It is important to note that higher initial temperature canresult in greater rate of loss in slump flow consistency,

thus reducing the degree of lateral pressure. For example,slump values of 170 and 180mm were measured 5 and

3.5 hours after casting for the TER-10 and TER-30mixtures, respectively.

The T30-20 and TER-20-ACC mixtures developed thelowest initial relative pressures of 78% and 83%,respectively, compared to 91% for those cast at 10 to30C initial temperatures and placed at similar castingrates of 10 m/h (Fig. 2). The incorporation of set-accelerating admixture in the TER-20-ACC mixtureresulted in the highest rate of pressure drop with time;the elapsed period required to reduce the relativepressure by 25% was 88 minutes. The increased rate ofcement hydration due to the incorporation of set-accelerating admixture can lead to greater cohesiveness,and hence sharper rate of drop in lateral pressure.[4]

Effect of section width on formwork pressure

For investigation of effect of concrete temperature onlateral formwork pressure, experimental research of

Khayat[8] and his colleagues was used and describedthose below:

Materials

A ternary cement made with approximately 6% sil icafume, 22% Class F fly ash, and 72% Type 10 cement wasused. A continuously graded crushed limestoneaggregate with nominal size of 10 mm and well-gradedsiliceous sand were employed. The sand had a finenessmodulus of 2.5. The bulk specific gravities of the aggregateand sand were 2.72 and 2.69, and their absorptions were0.4% and 1.2%, respectively. A naphthalene-based high-

range water reducer (HRWR) with solid content of 41%

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33V1 N1May 2013

and specific gravity of 1.21 was used. A liquid-basedpolysaccharide was used for the viscosity-modifying

admixture (VMA) to enhance stability of the plasticconcrete. A synthetic detergent-based air-entraining

admixture (AEA) and a carboxylic acid-based water-

reducing admixture were incorporated.

Mixture proportion

For the SCC mixture used in this study, a proven mixtureprepared using 490 kg/m3of binder, 0.38 w/cm, and 0.44

sand to-coarse aggregate ratio was used. The VMA wasincorporated at a dosage of 1325 mL/100 kg of water,

and the HRWR dosage was adjusted at 6 L/m3 to secureinitial slump flow of 650 mm. A dosage of 150 mL/100 kg

of cementitious materials of the AEA was used. The unitweight and air content were 2280 kg/m 3 and 6.1%,

respectively.

Instrumented formworks

As already mentioned, two experimental formworks wereused. The first measured 2100 mm in height and 200 mmin diameter. The PVC tube had a wall thickness of 10 mmand a smooth inner face to minimize friction during andafter concrete placement. The stress in the diaphragmcaused by concrete lateral pressure was determinedusing five pressure sensors mounted at 850, 1250, 1650,1850, and 2050 nun from the top. The monitoring ofpressure distribution was stopped once the concrete hadan approximate slump consistency of 100 mm. Thesecond column consisted of a sonotube of 3600 mm inheight and 920 mm in diameter. The column wasadequately braced and reinforced. The lateral pressurewas determined using two pressure sensors located at2050 and 2880 mm from the top.

The monitoring of pressure distribution was stopped oncethe concrete had an approximate slump consistency of100 mm. The second column consisted of a sonotube of3600 mm in height and 920 mm in diameter. The columnwas adequately braced and reinforced. The lateralpressure was determined using two pressure sensorslocated at 2050 and 2880 mm from the top. In this case,the lateral pressure was monitored until the hardening of

the concrete.

Fabrication and testing program

Ready-mixed concrete was delivered to the experimentalsite. The ambient and concrete temperatures were 16

and 19C respectively. The slump flow, air content, JRingand Lbox flow characteristics, and surface settlement weredetermined for the SCC. The measurement correspondsto the mean diameter of the spread concrete at the end offlow. The JRing spread values was 600 mm and for Lboxtest the measure was 0.81 and maximum surfacesettlement was 0.34%.

The concrete was directly discharged from the mixing

truck into the formwork from the top at the desired pouringrate without stoppage or vibration. In the case of the 3600-

ram high column, the concrete was placed at a rate ofrise of 10m/hr. For the 2100-ram high column, the formwork

pressure was evaluated twice; once using a rate of

placement of 10m/hr and then at 25 m/hr for a secondcolumn. The slump flow values determined upon the

arrival on site of the concrete and after 1 and 2 hours were650, 635, and 450 mm, respectively. After 3 and 3.5 hours,

slump consistencies of 180 and 65 mm were measured,respectively.

The initial and final setting times were determined in the

laboratory at 20C in compliance with ASTM C403 andare given in Fig.3. The adiabatic temperature was alsoevaluated in an adiabatic calorimeter on mortar obtained

by sieving fresh concrete through a 4.75-mm sieve. Theheat evolved was determined by deriving the temperaturerise as a function of time. The time between the initial

contact of cement with water and that corresponding tothe beginning of the acceleration of temperature rise was

6 hours, as also shown in Fig.3.

Fig 3: Variations of hydration and stiffening kinetics with time

Lateral pressure variations

The variations of the lateral pressure envelope determined

on the 2100-ram high column along with the consistencyare plotted in Fig.4. Immediately after filling the formwork,

the concrete is shown to act as a fluid exerting almosthydrostatic head. However, a gradual decrease in lateral

pressure takes place with time. The relative pressures atthe base of the column determined initially and after 1, 2

and 3 hours were 98%, 89%, 83% and 76% of hydrostaticpressure respectively.

Results of the section width Influence on formwork

The effect of column diameter (200 vs. 920 mm) onchanges in lateral pressure is illustrated in Fig.5 by plotting

the variations of the P(measured)/P(hydrostatic) valuescalculated at 2050 mm from the top of the formworks as a

function of time. It is important to mention that bothcolumns were cast on the job site at the same casting

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rate of 10 m/hr. Initially, the mixture placed in the largercolumn exhibited slightly greater pressure of 99% of

hydrostatic pressure compared to 96% for the 200-mmdiameter column. However, the rate of drop in pressure

was significantly different. In the case of the former

concrete placed in the 920-mm diameter column, thetime required to reduce lateral pressure by 5% of the

hydrostatic value was 20 minutes, resulting in a slope of5.3 kPa/hr. Conversely, for the 200-mm diameter column,

this period was 38 minutes resulting in a slope of 3.3kPa/hr. In general, the rate of drop in lateral pressure of plastic

concrete depends on the degree of thixotropy or shearrecovery [9]. This phenomenon causes a build-up of the

structure and an increase in cohesiveness soon after the

Fig 4: Variations of hydration and stiffening kinetics with time

material is left standing at rest without any shearing action.In the case of the 200-mm diameter column, the arching

effect can be relatively more pronounced than thatresulting from the 920-ram diameter column.

Conclusions

- Variations in fresh concrete temperature have limitedeffect on the maximum lateral pressure developed by

SCC at the time of casting. However, the rate ofpressure drop with time increases with the concrete

temperature that promotes faster development ofcohesion.

- The use of Type 30 cement or set-acceleratingadmixture can lead to 10% reduction in the initialpressure and accelerate the rate of pressure drop by

two folds compared to similar concrete prepared witha ternary cement.

- The scale effect had an influence on the rate of drop inlateral pressure of SCC with time; however, no

appreciable difference in the maximum initial pressurewas observed.

- Immediately after casting, the SCC placed in the 200-

ram diameter column was found to exert slightly lesspressure than that cast in the 920-ram column. This

can be due to an arching effect in the relativelyrestricted section.

References

[1] ACI Committee 347 (2001) "Guide to formwork for concrete",

Farmington Hills, 32.

[2] ACI Committee 622 (1958)" Pressures on formwork", ACI Journal,

Proceedings, and 55(2):173-190.

[3] ACI Committee 622, "Pressures on formwork", ACI

Journal,Proceedings, 55 (2) (1958) 173-190.

[4] Assaad J, Khayat KH, Mesbah H (2003) "Variation of formwork

pressure with thixotropy of self-consolidating concrete." ACI

Materials Journal, 100(1):29-37.

[5] Bartos, P.J.M., "An appraisal of the orimet test as a method for

on-site assessment of fresh SCC concrete", Int. Workshop on

Self-Compacting Concrete, Japan, (1998) 121-135.

[6] Gardner, N.J. and Ho, P.T.-J., "Lateral pressure of fresh concrete",ACI Journal, Technical Paper, Title No. 76-35 (1979) 809-820.

[7] Joseph J. Assaad Kamal H. Khayat "Effect of casting rate and

concrete temperature on formwork pressure of self-consolidating

concrete",Rilem Materials and Structures (2006) 39:333-341

[8] K. Khayat, J. Assaad, H. Mesbah, and M. Lessard "Effect of

section width and casting rate on variations of formwork pressure

of self-consolidating concrete ", Rilem Materials and Structures

38 (January-February 2005) 73-78

[9] Rodin, S., "Pressure of concrete on formwork", Proceedings

Institution of Civil Engineers (London) 1 Part 1 (6) (1952) 709-

746.Fig. 5: Effect of the section width on lateral pressure

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Formwork Failure: Cases & Causes

Special Correspondent

Collapse of concrete structures during construction

has been happening since concrete has been

placed in formwork. Cases and causes of these

type of failures have been documented and recorded in

many texts, articles and journals. This article will try and

focus on a few of them from the available reports, starting

with The New York Coliseum on May 9, 1955, 2000

Commonwealth Ave. on January 5,1971, Skyline Plaza in

Bailey's Crossroads on March 2, 1973, The Harbour Cay

Condominium in Cocoa Beach, Florida in March 1981 and

ending with The Tropicana in Atlantic City on October 30,

2003.The focus will be on what has been learned over time

from these failures and what has been done to keep these

type of tradgedies from occurring in the future.

Although there were many cases of concrete failures during

construction prior to the New York Coliseum collapse as

illustrated in (McKaig 13-27, 1962), only a few will be

looked at after this point because of the changes and

progressions being made in the construction industry at

this time in history.

(A) New York Coliseum on May 9, 1955

Pic source: http://www.ppconstructionsafety.com

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The construction method was a flat plate waffle slab with

solid slabs at the column caps. It was one of the first times

the use of motorized buggies had been used in the pouring

of this type of structure. The floor that collapsed was the

first floor above grade supported on two tiers of shores at

a total of 22' high. It can be seen from Figure 1 how collapse

happened. The buggies weighed about 3000 lb loaded,

ran at about 12 mph, and there were eight of them at the

time of the failure with about 500 cubic yards of concrete

already placed. The investigation that followed put the

blame solely on inadequate provisions in the formwork to

resist lateral forces, it even went on to say that "if there had

been sufficient diagonal, horizontal, and end bacing ofthe temporary supporting structure, the collapse could

have been prevented entirely,...", (McKaig 15-16, 1962).

After the collapse the district attorney called attention to

the lack of inspections and made recommendations to

revising the building code with respect to formwork

because of the new advances.

(B) 2000 Commonwealth Avenue: January 5, 1971

This was a progressive collapse of a cast-in-place

reinforced concrete flat-slab structure. Punching shear was

determined to have been the triggering mechanism but

the real problem was in the numerous errors and omissionsby every party involved in the project (Delatte 133-143).

The investigating committee determined that if the

construction had had a proper building permit and had

followed codes, then the failure could have been avoided

(Delatte 142) (See Figure 2 and 3 how failure occurred).

Some of the problems leading to the collapse are

- Not following the structural engineers specifications

for shoring and formwork

- Lack of concrete design strength

- Lack of shoring or removed too soon

- Improper placement of reinforcement

- Little construction control on site

- Owner changed hands many times

- Almost all jobs were sub contracted

- No architectural opr engineering inspection done

- Inadequate inspection by the city of Boston

- The general contractors representative was not a

licensed builder

- Construction was based on arrangements done by the

subcontractors

- No direct supervision of subcontractors

Figure 2: Typical flatplate with uniform distributed loading

Figure 3: Punching shear failure diagram

Figure 4: Skyline Plaza at Bailey's Crossroads, National Archives

Figure 1: N.Y. Coliseum Collapse, National Archives

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(C) Skyline Plaza: March 2,1973

Skyline Plaza (See Figure 4) in Bailey's Crossroads is an

example of a catastrophic collapse of a 30 story cast-in-

place reinforced concrete structure. This was also a flat-

plate design structure that failed due to punching shearon the 23rd floor and resulted in a progressive collapse.

Some of the reasons for this failure again were 1) premature

removal of shores and reshores, 2) insufficient concrete

stength, 3) no preconstruction plans of concrete casting,

formwork plans, removal of formwork schedules, or

reshoring program (Kaminetzky 66-67).

(D) Harbour Cay Condominium: March 1981

Built just 10 years after 2000 Commonwealth Ave. and 8

years after Skyline Plaza, was another cast-in-place

reinforced concrete structure that collapsed during

construction. It was determined that the most importantfactor towards its failure was a design error coupled with a

construction error of the wrong size rebar and chair height.

The designer never performed any calculations to check

for punching shear, the most common form of failure in

these type of structures (Feld & Carper 18).

Figure 5: Tropicana Casino; Parking Garage Picture taken from

www.CTLGroup.com

(E) The Tropicana Casino parking garage in Atlantic

City, N.J.: October 30,2003

The structure collapsed during construction killing another

four construction workers and and leaving more than 30

others injured. Larry Bendesky, Mongeluzzi's partner of the

Philadelphia law firm Saltz, Moongeluzzi, Barrett &

Bendesky, P.C, the lead counsel for the litigation with Paul

D'Amato of the D'Amato Law Office and a member of the

trial team, said that "the simple explanation of the cause of

the collapse is that the floors were not connected to the

walls with the required reinforcing steel. Built without the

necessary steel, it is no wonder it collapsed like a house of

cards." (pr newswire) The vertical columns left standing

and the fact that the floors were not connected implies

that this was another punching. Refer Figure 5 for the

collapse picture.

Codes & Regulations

Codes in Place

ACI, The American Concrete Institute's origins started in

1905 with its first building code published in 1910 and

changing its name to the current designation in 1913. ACI's

first design handbook came out in1939 and the first

building code titled ACI 318 came out in 1941. The

beginning volumes of ACI were less tha fifty pages with

the current code specification being nearly 470 pages of

design specifications and commentaries (ACI 318). Thisclearly shows the history of ACI is closely tied to the ever

changing demands of concrete construction and

technology. The ACI sees itelf as an expanding, alert,and

informed organization prepared to stimulate imaginative

applications of concrete and better knowledge of its

properties and uses, and will take an increasingly active

part in solving problems affecting the public welfare

(History of ACI).

Lessons Learned

(A) New York Coliseum on May 9, 1955

From this failure the construction industry learned that

shoring systems should be well braced to resist lateral

loads and to consider the effect of power or motorized

buggies/carts on the formwork (Auburn University).

(B) 2000 Commonwealth Avenue: January 5, 1971

From 2000 Commonwealth Ave. the industry learned that

this type of failure is a critical failure mechanism for flat-

plate-slab concrete construction. Structural safety

depends on adequate slab thickness, proper placement

of reinforcement, and adequate concrete strength (Delatte144).

(C) Skyline Plaza: March 2,1973

Six lessons learned from the colloapse of Skyline Plaza at

Bailey's Crossroads are listed in (Kaminetzky 67)

- the contractor should prepare formwork drawings

showing details of the formwork, shores, and reshores.

- The contractor should prepare a detailed concrete

testing program, to include cylinder testing, before

stripping forms.

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- The engineer of record should ascertain that the

contractor has all the pertinent design data (such as

live loads, superimposed dead loads, and any other

information which is unique to the project).

- Inspectors and other quality control agencies should

verify that items 1 and 2 above are being adhered to.

- Uncontrolled acceleration of formwork removal may

lead to serious consquences. 6) Top and bottom rebars

running continuously within the column periphery must

be incorporated in the design.

(D) Harbour Cay Condominium: March 1981

The Harbour Cay Condominiums presented the industry

with six more lessons learned in this type of construction

also listed in (Kaminetzky 77-78). This tradgedy happened

only eight years after the Skyline Plaza tradgedy and yet

some of the same lessons are listed again, they are

- A punching shear strength check s critical to the

success of a flat-slab, since punching shear is the most

common failure mode of concrete slabs.

- Minimum depth of a flat-slab must be checked to

assure proper strength and acceptable deflections.

- Reinforcing bars, both at the top and at the bottom of

the slab, should be placed directly within the column

periphery to avoid progressive collapse. This can easily

be accomplished routinely in all flat-slab jobs at no

additional cost at all.

- Proper construction control must be provided in thefield, including design of formwork by professionals.

This must include shoring and reshoring plans,

procedures, and schedules, with data on minimum

allowable stripping strength of the concrete.

- When there are failure warning signs of any type on a

construction site, work must stop. All aspects of the

project must be carefully evaluated by experienced

Formwork Digest

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