Risk & Opportunities Associated with Lightweight Concrete Structures

7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures

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RISK & OPPORTUNITIES ASSOCIATED WITH LIGHTWEIGHT CONCRETE

STRUCTURES

A Paper presented

By

Uttam Nangre-Patil & Mahadeo Nalawade

INTRODUCTION

Concrete design has evolved rapidly in the last 30 years. Construction

technology has seen the introduction of a variety of concrete products

to the market as well as an increased use of supplementary

cementitious materials and recently blended cements. Emphasis has

been placed on creating more durable concrete through changes to the

mix constituents and proportions, including the aggregates, admixtures

and the water-cement ratio. This evolution, along with improved

reinforcing steel strength and the use of lightweight fiber reinforcement

steel has lead to modifications in design philosophy - most notably the

use of thinner structural members.

Lightweight concrete can be defined as a type of concrete which

includes an expanding agent in that it increases the volume of the

mixture while giving additional qualities such as nailibility and lessened

the dead weight. It is lighter than the conventional concrete with a dry

density of 300 kg/m3 up to 1840 kg/m3; 87 to 23% lighter. It was first

introduced by the Romans in the second century where The Pantheon

has been constructed using pumice, the most common type of

aggregate used in that particular year. From there on, the use of

lightweight concrete has been widely spread across other countries

such as USA, United Kingdom and Sweden. The main specialties of

lightweight concrete are its low density and thermal conductivity. Its

advantages are that there is a reduction of dead load, faster building

rates in construction and lower haulage and handling costs. The

building of The Pantheon of lightweight concrete material is stil l


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standing eminently in Rome until now. It shows that the lighter

materials can be used in concrete construction and has an economical

advantage.

But despite all these advantages there are research which shows

the risk associated with use of lightweight concrete structures. To

highlight the issue we would like to discuss bellow the results of

the tests done by researcher on lightweight concrete.

RISKS..

Strength & Density Comparison: The purpose of this test is to identify

the performance of aerated lightweight concrete in term of density and

compressive strength. The result is presented in Table 1. Based on, it

can

Density(Kg/M3)

CompressiveStrength

(Kn/M2)1470 2.52

1720 5.5

1770 10.34

1780 9.19

1810 13.12

1820 11.87

1840 13.21

1840 16.78

1920 16.73

1990 16.58

2040 17.27

2040 12.18

2050 9.35

2060 22.99

Table 1 : Density of Hardened Concrete and Compressive Strength at 28 days

be seen that compressive strength for aerated lightweight concrete are

low for lower density mixture. The increment of voids throughout the

sample caused by the foam in the mixture will lower the density. As a

0

500

1000

1500

2000

2500

1 2 3 4 5 6 7 8 9 10 11 12 13 14

DensityKg/M3

Compressive Strength Kn/M2


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result, compressive strength will also decrease with the increment of

those voids. The required compressive strength of lightweight concrete

is 3.45 MPa at 28 days as a non load bearing wall. The compressive

strengths obtained from these mixtures carried out are higher than 3.45

MPa and therefore it is acceptable to be produced as non-load bearing

structure. However, the compressive strength for the mixture with

density of 2050 kg/m3 is slightly low compared with density of 2040

kg/m3. This is due to the compaction problem during mixing process.

The final mixture is quite dry and since compaction is not perfectly

done, samples are not well compacted. This has resulted the

compressive strength to be lower than the mixture with lower density.

As been discussed before, tr ial and error method was used in

determining the most suitable mixture in preparing research samples.

Fourteen trial mixes have been prepared during the research and from

the results, the mixture with the highest compressive strength with low

density will be used for further investigation. Compressive strength of

aerated lightweight concrete is determined on the 7, 14, 21 and 28

days for each sample. There were three samples for each test and the

results would be taken as the average of these three. Fewer variables

had been set for different mixture, this variable would be changed

accordingly while the others were fixed to forecast their effect on the

mixture. Percentage of foam, foam agent and water, cement and sand

ratio were the variables made during the mixing process. For example,

three mixtures were prepared to determine the effect of different foam

agent and water, cement and sand ratio. The percentage of foamapplied is fixed for three mixtures and the difference in the results

would occur because of the foam agent and water ratio. All the results

were based on the 75% foam injected in the mixture. Figure 2 shows

the compressive strength of aerated lightweight concrete according to

the percentage of foam in each mixture. It can be seen that the mixture

with 25% of foam is higher than the compressive strength of 100%

foam. This is because, with higher percentage of foam, voids

throughout the sample will be increased, and as has been discussed


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ealier, this would result in the decrease of the compressive strength.

Compressive strength of mixture with 50% foam is slightly higher than

mixture with 75% foam. The density of 25%, 50%, 75%, and 100% of

foam is 2040 kg/m3, 1820 kg/m3, 1810 kg/m3, and 1470 kg/m3

respectively. The density of 50% and 75% of foam mixture is the same

as been showed in Figure 2, compressive strength for this two mixture

did not differ much. But it can be seen that there is a difference

between 25% of foam mixture and 100% of foam mixture. The density

of 25% of foam mixture is 27% higher as compared to 100% of foam

mixture and seen in Table 2, the compressive strength is 85.4% higher

at 28 days. For a 25% mixture the compressive strength is 17.27 MPa

and for 100% mixture is 2.52 MPa.

DaysCompressive Strength ( Kn/m2)

At 25% Foam At 50% Foam At 75% Foam At 100% Foam

7 32.20 9.45 8.12 1.43

14 14.68 8.88 11.02 2.44

21 16.41 14.42 11.96 2.23

28 17.27 11.87 13.12 2.52


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As per above test i t is seems that the reduction in density or the

addition of voids in concrete would effect on the strength of the

concrete. Secondly there are some other research shows that the major

disadvantages of lightweight concrete have been the inability to provide

consistent compressive strengths and density throughout the entire

area. Lightweight concrete derives its compressive strength from the

molecules in the foam additive that when properly mixed form

around the cement and serve as aggregate does in standard concrete

mixes. If the foam additive is not properly mixed, there is a probability

of foam collapse, which weakens the products compressive strength.

Secondly if the lightweight concrete produced with using the lightweight

aggregates then uniform workability of the mix is more difficult to

maintain with due to their general high absorption and the wide

variation in rate of absorption from particle to particle. And also

because the coarse aggregates are lighter than the concrete mass,

they tend to float to the surface when improperly placed. There is also

more possibility of lightweight mixes tend to entrap air and honeycomb

more than normal weight concretes. There are researches going on to

minimise these risks to increase the use of lightweight concrete in

structures. For highrise structures, it has been suggested that the use

of lightweight concrete shall be restricted for slabs and horizontal

components and regular concrete for vertical components like columns

and shear walls to mitigate the risk associated with lightweight

concretes.

There are so many ways developed to minimise the risk andimprove the performance of the lightweight concrete and it has

been successfully used for construction of landmark structures

across the globe. But in the Indian scenario as our construction

industry is facing various challenges including the quality of

labour force, use of technology, machineries and equipments, we

need to be more cautious while using innovative materials and

technologies. Now let us know some of great opportunities, we

Indians can explore in which these above mentioned risks are


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minimised with using controlled process environment. Concrete

Cloth and pre-cast concrete components with lightweight concrete

with glass fibre are two best options available for us which change

paradigm of Indian Infrastructure scenario.

OPPORTUNITIES..

Basically one of the biggest disadvantages of normal conventional

concrete is its self weight of about 2200 to 2600 Kg/m3 which is so

high and attempt have been made in past to reduce the self weight of

the concrete and to increase the efficiency of the concrete as a

structural material. Therefore day by day the util ization of normalconcrete in building across the globe is going down due to its

inflexibility, material cost and the associated cost of labour for handling

the materials. The weight of building on foundations is important factor

considered while designing the structures particularly in case of weak

soil and highrise structures. We know that a solid ordinary concrete

made of only fly ash, Portland cement and aggregates can gives the

strength of 55to62 N/mm2. This strength is much more than the

required strength for most of the structural applications. So the need

for going for developing alternative ways to lighten the strength of

concrete as well as make it l ightweight with keeping desired properties

required for most of the structural applications. But developing a viable

lightweight structural concrete with least amount-of materials and

manufacturing cost is a complex science as its not that easy to fulfi l l

all the desired parameters. With using innovations various products

have been used across the globe which are not only enhancing the

quality of structures but reducing the time and cost to execute the

projects. Let us discuss the two wonderful application of technologies,

Concrete Cloth and Pre-cast Fiber Reinforced Concrete structures.


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AMAZING INOVATION- CONCRETE CLOTH

Concrete is a freshly mixed material, which can be moulded into

required shape. There are many advantages of concrete, but there is

one drawback is that, it is not flexible, when it is hardened. To

overcome through this drawback of concrete, a new construction

material was developed by British Engineering Company called

Concrete Canvas. Concrete cloth (CC) is a unique proprietary material.

It has a very wide range of applications throughout the building & civil

engineering industry. Concrete cloth is a flexible; cement impregnated

fabric that hardens when hydrated to form a thin, durable, water & fire

proof concrete layer. CC allows concrete construction without the need

for plant or mixing equipment. Simply position the canvas & just add

water. CC has a design life of above 10 years and is significantly

quicker and less expensive to install compared to conventional

concrete. CC consists of a 3- dimensional fiber matrix containing a

specially formulated dry Concrete mix. A PVC backing on one surface

of the cloth ensures the material is completely waterproof, while

hydrophilic fibers (Polyethylene and Polypropylene yarns) on the


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opposite surface aid hydration by drawing water into the mixture. The

material can be hydrated either by spraying or by being fully immersed

in water. It can be easily nailed, stapled through or coated with an

adhesive for easy attachment to other surfaces. Once set, the fibers

reinforce the concrete, preventing crack propagation & providing a safe

plastic failure mode. CC is available in 5, 8 & 13 mm thicknesses

Fig. Concrete Cloth Section

Specifications For CC

CC TypeThickness

in mm

Roll

Width

in mm

Dry

Weight

(Kg/Sqm)

Batched

Roll

Coverage

(Sqm)

Batched

Roll

Length(

m)

Bulk Roll

Coverages

(Sqm)

Bulk Roll

Length (m)

CC5 5 1000 7.00 10.00 10.00 200.00 200.00

CC8 8 1100 12.00 5.00 4.50 125.00 113.60CC13 13 1100 19.00 N/A N?A 80.00 72.70

CC Material Properties

Strength: Very high early strength is a fundamental characteristic of

CC. Typical strengths and physical characteristics are as follows:

Compressive Strength

- 10 day compressive failure stress (MPa) 40


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- 10 day compressive Youngs modulus (MPa) 1500

Bending tests

- 10 day bending failure stress (MPa) 3.4

- 10 day bending Youngs modulus (MPa) 180

Abrasion Resistance

- CC lost 60% less weight than marble over 1000 cycles.

Tensile Test

- Similar to twice that of OPC Max 0.10 gm/cm 2

CBR Puncture Resistance

- Min. Push-through force 2.69 kN

- Max. Deflection at Peak 38mm

Resistance to Imposed Loads on Vehicle Traffic Areas - Gross weight of 2 axle vehicle 30 to 160 kN- Uniformly distributed load not exceeding 5kN/sqm

Method of Hydration : CC can be hydrated using saline or non saline

water. The minimum ratio of water to CC is 1:2 by weight. CC cannot

be over hydrated so an excess is recommended. The recommende d

methods are: In a hot/arid environment, re-wet the material 2 - 4 hours

after the initial hydration.

F i g . S p r a y T h e F i b e r S u r f a c e W i t h W a t e r U n t i l I t F e e l s W e t T o T o u c h F o r S e v e r a l M i n u t e s A f t e r S p r a y i n g .


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Cutting of CC : A disposable blade is the most suitable tool for cutt ing

CC before it is hydrated or set. When cutting dry CC, a 20mm

allowance should be left from the cut edge due to lost fi l l. This can be

avoided by wetting the CC prior to cutting.CC can also be cut using

handheld self sharpening powered disc cutters.

Angle Gi rder Di sc cut ter

Applications of CC : .Some of the applications of the CC are as follows

:

Fig. Slope Protection

Application Products

Dust Suppression CC5

Foundation Binding CC5

Weather proofing/ slope

stabilization CC5

Ditch Lining CC5,CC8, CC13

Bund Lining CC5,CC8, CC13

Sandbag/Gobian

ReinforcementCC8 & CC13

Trackway/Flooring CC8 & CC13

Pipe Protection CC5,CC8, CC13

Cable Covering CC13


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Fig.: Ground Resurfacing Fig.: Mining Applications

Fig: Bund Lining Fig.: Sandbag Reinforcement

GLASS FIBERE-REINFORCED PRE-CAST CONCRETE COMPONANTS (GRFC)

Glass fiberreinforced concrete, commonly known as GFRC, is a composite

concrete product fabricated by many precast concrete manufacturers in many

developed countries. It consists of a portland-cement-based composite that is

reinforced with an absolute, minimum of 4% by weight of alkali-resistant glass fibers

to total mix, which are randomly dispersed through the material. The fibers serve as

reinforcement to enhance the concretes flexural, tensile, and impact strength. The

low weight of GFRC panels decreases superimposed loads on the buildings

structural framing and foundation, providing potential savings in multistory

construction and in areas with poor supporting soil. Its light weight also makes it

ideal for use on low-rise frame buildings where heavier cladding systems would

increase the size of framing members required.

A variety of precast concrete components can be used in creating a completepassive-design system for a building. Foremost among these are:


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Hollow-core slabs, which serve as combined floor/ceiling systems and can also be

used as wall panels in either vertical or horizontal configurations.

Wall panels, which offer high fire ratings and work with other components to create

a noncombustible envelope. Insulated sandwich wall panels can also be used.

Double tees, which can be used similar to hollow-core planks for roofs, ceilings,

floors, or wall panels.

Columns and beams, which create a framework that will resist intense heat and will

not add fuel to a fire.

A total-precast concrete system provides an effective design for minimizing fire

damage and containing the effects within the smallest space possible for the longest

time.

The Use of all Pre-Cast Structure including tees, beams, columns, slab & walls

Precast concrete components provide a variety of savings to a project in ways that

are not always considered when looking at upfront costing versus other materials,

including masonry and curtain wall. These savings include:

Speed. Precast concrete components provide a variety of ways to speed the

construction process, from design through fabrication and erection. These

efficiencies can shave as much as one-third of the time needed for construction,

meeting tight deadlines and generating revenues quicker. Time can be saved

through:


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The fabrication process. Precast concrete components can be fabricated while

permitting and foundation work progress, so they are ready to begin erection as soon

as foundations are complete. As a single-source supplier for a large portion of the

structural system, precasters help maintain the critical path scheduling.

The erection process. Foundations can be placed one day and precast concrete

loadbearing or non-loadbearing panels can be erected as soon as the foundations

have cured sufficiently. Wall panels, double tees, and hollowcore planking also erect

quickly, often cutting weeks or months from the schedule. This speed allows

construction to get into the dry quicker. The fast enclosure also lessens concern for

weather or material damage during erection, reducing the contractors risks and

costs.

The finishing process. Precast concrete insulated sandwich panels create a

finished interior wall that avoids the time and cost of furring and drywalling.

Architectural panels can have a variety of colors and textures cast into them,

including several in one panel, eliminating the need to field-set trim pieces or paint

the faade after the structure is built.Design Economy. The custom, sculptured designs that are possible with precast

concrete may be achieved within a limited budget by selecting economical

aggregates and textures combined with repetitive units and effective production and

erection details. By reusing the same dimensions for components, the same molds

can be used, minimizing the total number needed and the changes between casting.

Efficiency is created by making it possible for similar, if not identical, shapes to be

produced from the same basic (master) mold and by minimizing the time required to

disassemble a mold and reassemble it for the next piece.

Hybrid post-tensioned precast frame: This method has the precast concrete

beams connected to multistory columns by unbonded, post-tensioned strands that

run through a duct in the center of the beam and through the columns. Mild steel

reinforcement is placed in ducts at the top and bottom of the beam, which is sleeved

through the column and grouted. The reinforcement yields alternately in tension and

compression and provides energy dissipation, while the post-tensioning strands

essentially act as rubber bands that help right the structure after the seismic event


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ends. There are no column corbels, with the vertical shear resistance provided by

the post-tensioning strand. The post-tensioning steel balances the mild steel

reinforcement so the frame re-centers after flexing during a seismic event.

Pre-Cast Concrete Components Manufacturing Assembly Line

A Pre-tensioned precast frame, which is applied at locations where the most

economical connection method features one-story columns with multispan beams.

The multispan beams are cast with partially deboned pre-tensioning strand set on

the columns. The columns reinforcing steel extends through the sleeves inside the

beams. Reinforcing-bar splices ensure continuity above the beam. As the frame

displaces laterally, the de-bonded strand remains elastic. While the system


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dissipates relatively less energy than other systems, it re-centres the structure after a

major seismic event.

Day 1 -Foundation Ready Day 7 Taking Shape

Day 15- Interior Shear Walls Day 25- Shear Structure

Day 44- I am ready

Finally we would like to close this article with one wonderful note

in which we can see that how human imagination is working for

innovations to create wonderful structures not only on earth but on

Moon.


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Lunar Concrete: Mr. Larry Beyer of University of Pittsburg

conceived idea of lightweight concrete formed from lunar regolith

which is called as Lunacrete or Mooncrete. It can be produced on

moon with using cementitious material on the moon and regolith as

aggregates with water and sulfur. Water can be produced there

with mix of oxygen with hydrogen produced from lunar soil. This

concrete will be useful for future permanent construction on the

moon saving lot of cost for transportation from earth.

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Risk & Opportunities Associated with Lightweight Concrete Structures

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