Site Practice

Site Practice

Concrete Construction

Alistair Russell

• Site practice affects finished product

• Handling & placing• Compaction & finishing• Curing• Cracking & expectations• Joints & tolerances

26 March 2015 Concrete Construction - Site Practice 2

Site Practice

Context: Concrete slabs

• We have 4 courses on offer including a 2-day course devoted to just this topic – we can’t do that in 1 hour!

• Guide to Concrete Construction book –free

• Best practice video on YouTube – free


CCANZ Courses

• Starts as soon as concrete discharged from truck – 1st step in concrete’s life

• Move as quickly & efficiently as possible• Workable, without segregation – balance


Handling & Placing

• Get this right and the life of the structure will go well

• Get this wrong, and your life will not go well –compaction etc &remediation, cost…


Handling & Placing


Primary Considerations

• Workability basically means how easy concrete is to use – and place

• More water = more workable

• Generally “runnier” concrete is easy to useTOO RUNNY = SEGREGATION

• Long term performance

• Workability related to nature of job– Thin forms – more

workable concrete– Mass sections – less

workability required– Concrete pumps –

require a plastic and cohesive mix


Workability (slump)

• Avoiding Segregation• Aiding Compaction

• Correct term is “placing” not “pouring”– Implies some control over process


Placing


Avoiding Segregation

Flatwork

General Principles• Whatever method,

place without delay• Deposit in face

already poured• Slopes-start at

bottom

• Chutes• Barrows• Crane & skip or Bucket/skip• Pumps and Pipelines

• Concrete must be uniform and delivered at appropriate rate – x m3/hour


Distribution Methods


Pumps and Pipelines

• Specialised pumping operations• Notify concrete supplier of method of

placement – NZS 3109, 6.2• Pump mixes

– High slump– High(er) shrinkage– Not great off-form finish


Pumps and Pipelines

• Often compaction and finishing are part of the same operation– Often for flat work – flat horizontal surfaces

• Remember a smooth surface is not necessarily evidence of good compaction


Compaction & Finishing


Compaction

• Compaction enables concrete to reach:– Design strength– Density– Low permeability– Specified surface finish

• Compaction sometimes merges into finishing


Compaction

• Expel entrapped air• Pack aggregate particles• Increase

– Reinforcement bond– Ultimate strength– Abrasion resistance– Durability

• Minimise shrinkage and cracking


Purpose of Compaction

Increases density

• 2 stage process– Liquefaction occurs quickly– Expelling entrapped air more slow

• Compaction is complete when no more air bubbles rise to surface


Process of Compaction

PROCESS: Liquefaction of the concrete which allows it to slump and fill the form

TIME: 3 to 5 seconds

PROCESS: The expulsion of air

TIME: 7 to 15 seconds

. Total time: 10-20 seconds


Effect on Hard Concrete


Finishing

• While concrete is still plastic:– Levelling– Floating– Trowelling– Other treatments

• Purpose of finishing is to achieve:– Level or specified profile– Flatness– Surface density and texture


Flatwork Finishing

1. Screeding – hand/timber/mechanical screed

2. Floating– To level surface– Compact surface layers

3. Trowelling– To produce smooth, hard and durable

surface & densify surface


Flatwork Finishing


Screeding

• Even but open surface texture– Embed large particles– Remove imperfections– Compact and consolidate fines at surface– Close minor cracks

• When bleed water stops• Must be able to stand on concrete – minor

indentations


Floating

• Bull floating• Hand floating• Machine floating


Floating

• Final surface treatment– Working the surface– Compact top surface

• Delay between floating and trowelling– Stiffening of concrete

• Smoothness, density & wear resistance


Trowelling

• Flat blade – don’t create ripples• Subsequent trowels tilt the blade

– Once concrete is firm enough

• Blisters mean blade is too pitched• Blisters reduce durability + ugly


Trowelling


Trowelling

Hand trowellingShould give a ringing sound

Machine trowellingRegular pattern

Hand trowel in corners


Trowelling


Trowelling

• Footpaths, patios, paths & steps• Not in industrial floors – need crisp edge


Edging


Curing

• Cement hydration – chemical reaction– Requires water– Takes days, weeks– Starts fast, gets slower, will continue (forever)

• Curing – process of controlling loss of moisture– Allows time for hydration to occur


Curing

• Prevent loss of moisture– Leave formwork in place– Cover with impermeable membrane

• Continuously wetting surface– Ponding– Spraying


Curing

• Impermeable Membrane– Formwork– Plastic sheeting

• Water– Ponding– Sprinkling*

– Wet coverings• Accelerated curing


Methods of Curing

*Water blaster spraying upwards into wind

• Comprehensive physical barriers not always practical/economical

• Curing Compounds achieve same resultsSika Antisol®

• Apply after bleed water stops

• Be aware of coatings– Wax based – no painting– Resin based – ok to paint (acrylic)


Curing Compounds

• For appearance or function• Surface treatments depends on:

– Traffic & frequency– Chemicals– Impact loads

• Warehouse floors with steel wheel loads (forklifts) – high strength concrete– Specified by designer– Proper compaction, finishing and curing


Surface Treatments

• Densifiers – for polishing floors


Polished Floors

• Slip – for pedestrians• Skid – for wheels

• NZS 3114 defines surface finishes• F – Formed finish (from formwork)• U – Unformed finish (exposed to air)


Slip & Skid


• NZS 3114 defines surface finishes


Slip & Skid

F1 F6


• NZS 3114 defines surface finishes


Slip & Skid

U2 U5

• Examples U3 Trowelled Finish– Gradual variations limited to a maximum of 5

mm.– Abrupt variations limited to a maximum of 3

mm.


Floor Tolerances

Formed Finish Classification Hot Conditions

> 20°

AverageConditions≤ 20° > 12°

Cold Conditions≤ 12° > 5°

Beam and slab soffits

Forms 4 days 6 days 8 days

Supporting members (shores or backprops)

12 days 18 days 24 days

Vertical faces Finishes F6, F5, F4

1 day 2 days 3 days

Finishes F3, F2, F1

9 hours 12 hours 18 hours

A minimum of 2 days applied to the stripping of vertical faces where frost damage is likely.


Formwork Removal

Table 5.3 NZS 3109 – minimum formwork stripping times for in situ concrete


Other Finishes

Exposed Aggregate

Brushed

Sand Blasted

Polished

Exposed Aggregate

• Natural rounded coarse aggregate– Not quarried rock (too

jagged)• Chemical retarder on

surface– Apply after bleed stops– Before setting

• 20MPa (not 17.5MPa)– Reduces stone loss


Exposed Aggregate

• Surface retarder – e.g. Sika Rugasol®

• Delays hydration of cement for 3-6mm depth

• Garden sprayer• Uniform coverage is

essential• Retardation period ≤ 24h

– Wash off prior to set– Wash low pressure


Cracks in Concrete• Concrete has low tensile strength

0 – 24 hours - plastic shrinkage cracksprevent rapid moisture loss- plastic settlement cracksrevibrate concrete- thermal movement cracksuse low heat mix (eg flyash)

7 days – 2 years - restrained shrinkage crackingadequate reinforcingdetailing of movement joints


• Fill – well compacted and even• Control joints

– Saw cuts initiate cracking– 12–18 hours after finishing– Insert crack inducer

• Joint layout – 5m between (6m in NZS3604)

– Align with walls where possible– Square bays are best


Crack Avoidance


• Contraction Joints

• Expansion Joints


Joints

Concrete shrinks 6 – 10mm per 10m length


Contraction Joints

• Oxides – most coloured concrete– Mixing is important

• Spray on coloured compounds– Can fix as you go– Less durable

• Coloured concrete usually doesn’t affect the strength


Coloured Concrete


Colouring Oxides

• Powdered or liquids– Penetrate all through the concrete– Add to mixing concrete

• Peter Fell• Permacolour NZ• Rockwood – Elements and Atmospheric range

• Part of the specification• Seek expert advice• Beware multi-batch variations


Colouring Oxides

[email protected]

• IB 18: Architectural Surface Finishes• IB 33: Specification and Production of Concrete

Surface Finishes• IB 86: Self-Compacting Concrete

• CCANZ Concrete Construction Course 8-9 September, Auckland

www.ccanz.org.nz


Further Reading

mailto:[email protected]

http://www.ccanz.org.nz/

Site Practice

Concrete Construction

Alistair Russell

Baldev Kesha

Concrete Mix designs and Applications

Concrete Standards

• NZS 3101 Concrete Structures- A Standard for designers

• NZS 3109 Concrete Construction- Should be on every job site outlines good construction practice.

• NZS 3104 Specification for Concrete Production-Outlines the quality controls rules for producing concrete.

• NZS 3121, NZS 3122, AS1478, Quality of materials which are use to make concrete

• NZS3111, NZS3112 Method of conducting tests to prove properties

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Basic concrete materials

• Portland – cement reacts with water to provide strength

• Sand (fine agg) – provides cohesion & workability

• Stone (coarse agg) – provides stiffness and limits movement

• Mixing water – provides lubrication and hydrates cement

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Chemical Admixtures

• Water reducers – improve dispersion and cut water content

• Air-entrainers – create micro-bubbles that improve

workability

• Accelerators – increase chemical reaction & reduce setting

time

• Retarders – decease chemical reaction & increase setting

time

• Pump aids – stabilisers that increase viscosity of concrete

• Shrinkage – reduce drying shrinkage of concrete

• Others – corrosion inhibitors, expansive agents, foamers59

Supplementary Cementitious Materials (SCM’s)

• Flyash, fine ash from burning coal, supplied by GBC from Huntly.

• GGBS, Ground Granulated Blast Furnace Slag. Waste product from iron manufacture. Sold as Duracem as a Slag/GP blend 65/35.

• Microsilica- very fine natural silica sold by GBC.

All improve the durability of concrete but have differing characteristics.

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Other Materials

• Recycled slurry water: – Increasing plants are reusing washing water in concrete rather than disposing

• Steel and synthetic fibres:– Increasing demand for fibre reinforcedconcrete in floor slabs, tunneling & mining

• Surface treatments, pigments – Aesthetics of concrete is critical for somearchitectural projects

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Components of Concrete

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Concrete performance

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Fresh propertiesFirst 6-10 hours

Hardened propertiesNext 50-100 years

WorkabilityBleedingPumpabilityFinishabilitySetting time

StrengthDurabilityStabilityAesthetics

Performance

Components of Concrete

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Concrete performance

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Fresh propertiesFirst 6-10 hours

Hardened propertiesNext 50-100 years

WorkabilityBleedingPumpabilityFinishabilitySetting time

StrengthDurabilityStabilityAesthetics

Performance

Displays

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Making Samples

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Kerikeri Stone Store – Historicalsite

Fontera – ColaberationInternal and External finishesClient, Architect, Interior Designer, Landscape Architect,Main contractor, Placer

We will endeavour to find a suitable site to view

Muriwai Atmospheric Black

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Silverdale Shopping Centre –Concrete tells a History story

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Waikato University

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Speights Ale House - Nelson

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ASB Black Polished Floor

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Tait - Christchurch

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Thank you

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Site Practice

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