University of Sydney – Structures CONCRETE

Peter Smith 1998

Concrete Making and Testing

Unique material

Made specially for each job

Handling on job affects quality

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Concrete Materials Cement

Portland, rapid-hardening, white, coloured, ... reacts with water to form a gel - curing process important

Coarse aggregate provides bulk grading and shape essential must be at least as strong as final concrete

Fine aggregate fills spaces between

Water cheapest but most critical element

Additives plasticizers, accelerators & retarders, waterproofing, ...

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Proportions

Cement paste coats all surfaces of aggregate

Fine fills the spaces between coarse aggregate

Coarse aggregate of uniform size doesn’t pack as well as a graded aggregate

Detailed grading by supplier of readymixed concrete ratio cement to total aggregate 1:4 to 1:7 by weight

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Weighing and Mixing

Weigh-batching necessary for accuracy

Mixed in factory and transported in agitator truck

Site mixing uneconomical, not accurate enough

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Handling on Site Chute and/or barrow Concrete pump

most common

Vibrators to eliminate air pocketsensure compaction around reinforcement

& into corners immersion vibrators most common

Trowellingshovel, trowelling machine, wood float or steel

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Stages of Setting Initial set

stiffening of the concretebegins within about 2 hours of adding water

Hardeningmain gaining of strengthmain gain in first weekmost of strength after 28 days - but continues

Ageingslowly for years

6/19

University of Sydney – Structures CONCRETE

Peter Smith 1998

Curing Procedure for retaining moisture in

concrete for several days

Prolongs the chemical reaction of hydration

Will improve compressive strength

Reduces drying shrinkage & cracking

Improves protection of reinforcement

7/19

University of Sydney – Structures CONCRETE

Peter Smith 1998

Curing (cont.1) Keep moisture available throughout the

setting process - for at least 3 daysmust be continuous

Must start no longer than 3 hours after placing

Fast and slow setting cementsretardants if want special finishes

Temperature affects rateheat speeds up process

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Curing (cont.2)

extremely important partof concrete process

will get cracking otherwise

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Curing Methods Curing compounds

applied by spray or roller

Plastic sheeting to prevent evaporation

Formwork for edge beams and face panels

left in place

Ponding of water where practicable

intermittent wetting down morning and nightIS NOT CURING

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Strength

Water/cement ratio affects strength

More water = less strength

Try to limit water content

But it must be workable

11/19

University of Sydney – Structures CONCRETE

Peter Smith 1998

Workability Concrete must be fully compacted to

remove air bubbles Water assists compaction Try to have it wet enough

dry mix too difficult to compactwet mix too sloppy - weak

Ideal combination requires experience Use vibration to improve workability Use additives to improve workability

12/19

University of Sydney – Structures CONCRETE

Peter Smith 1998

Measuring Workability The slump test Not perfect, but simple

and quick Good guide to uniformity

between batches

100

300

200

50-100 mm(normal)

13/19

University of Sydney – Structures CONCRETE

Peter Smith 1998

Economy

Cement is dearer than aggregate

Try to limit cement contentcement responsible for strength

but also for most of cost

and the shrinkage

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Shrinkage

Concrete shrinks on setting loss of volume when water and cement react

chemically loss of excess water

Starts off rapidly - continues for years Cement paste shrinks, aggregate doesn’t Try to limit water content Good curing delays shrinkage Reinforcement helps limit cracks

15/19

University of Sydney – Structures CONCRETE

Peter Smith 1998

Conflicting Requirements

The various requirements above are

in conflict

good compromise solutions

are possible

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Specifying and Measuring Strength

The cylinder crushing strength (in MPa)

Usually measured at 28 days

Test cylinders cast on site

Cured in lab before testing

17/19

University of Sydney – Structures CONCRETE

Peter Smith 1998

Testing Cylinders

Cylinders crushed in lab at 28 days

Some early tests at 7 days

Drastic consequences if under-strength after 28 days

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University of Sydney – Structures CONCRETE

Peter Smith 1998

Testing Other Materials

Quality control of other materials is usually done by manufacturers

Homogeneous materials like steel are made to close tolerances of strength

Natural materials like timber vary greatly, and are classified into several grades

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