University of Sydney – Structures CONCRETE Peter Smith 1998 Concrete Making and Testing l Unique material çMade specially for each job çHandling on job affects quality 1/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Concrete Materials l Cement çPortland, rapid-hardening, white, coloured,... çreacts with water to form a gel - curing process important l Coarse aggregate çprovides bulk çgrading and shape essential çmust be at least as strong as final concrete l Fine aggregate çfills spaces between l Water çcheapest but most critical element l Additives çplasticizers, accelerators & retarders, waterproofing,... 2/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Proportions l Cement paste coats all surfaces of aggregate l Fine fills the spaces between coarse aggregate Coarse aggregate of uniform size doesn’t pack as well as a graded aggregate l Detailed grading by supplier of readymixed concrete çratio cement to total aggregate 1:4 to 1:7 by weight 3/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Weighing and Mixing l Weigh-batching necessary for accuracy l Mixed in factory and transported in agitator truck l Site mixing uneconomical, not accurate enough 4/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Handling on Site l Chute and/or barrow l Concrete pump çmost common l Vibrators çto eliminate air pockets çensure compaction around reinforcement & into corners çimmersion vibrators most common l Trowelling çshovel, trowelling machine, wood float or steel 5/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Stages of Setting l Initial set çstiffening of the concrete çbegins within about 2 hours of adding water l Hardening çmain gaining of strength çmain gain in first week çmost of strength after 28 days - but continues l Ageing çslowly for years 6/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Curing l Procedure for retaining moisture in concrete for several days l Prolongs the chemical reaction of hydration l Will improve compressive strength l Reduces drying shrinkage & cracking l Improves protection of reinforcement 7/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Curing (cont.1) l Keep moisture available throughout the setting process - for at least 3 days çmust be continuous l Must start no longer than 3 hours after placing l Fast and slow setting cements çretardants if want special finishes l Temperature affects rate çheat speeds up process 8/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Curing (cont.2) extremely important part of concrete process will get cracking otherwise 9/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Curing Methods l Curing compounds çapplied by spray or roller l Plastic sheeting to prevent evaporation l Formwork for edge beams and face panels left in place l Ponding of water where practicable intermittent wetting down morning and night IS NOT CURING 10/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Strength l Water/cement ratio affects strength l More water = less strength l Try to limit water content l But it must be workable 11/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Workability l Concrete must be fully compacted to remove air bubbles l Water assists compaction l Try to have it wet enough çdry mix too difficult to compact çwet mix too sloppy - weak l Ideal combination requires experience l Use vibration to improve workability l Use additives to improve workability 12/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Measuring Workability l The slump test l Not perfect, but simple and quick l Good guide to uniformity between batches mm (normal) 13/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Economy l Cement is dearer than aggregate l Try to limit cement content çcement responsible for strength but also for most of cost and the shrinkage 14/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Shrinkage l Concrete shrinks on setting çloss of volume when water and cement react chemically çloss of excess water l Starts off rapidly - continues for years l Cement paste shrinks, aggregate doesn’t l Try to limit water content l Good curing delays shrinkage l Reinforcement helps limit cracks 15/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Conflicting Requirements l The various requirements above are in conflict good compromise solutions are possible 16/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Specifying and Measuring Strength l The cylinder crushing strength (in MPa) l Usually measured at 28 days l Test cylinders cast on site l Cured in lab before testing 17/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Testing Cylinders l Cylinders crushed in lab at 28 days l Some early tests at 7 days l Drastic consequences if under-strength after 28 days 18/19

University of Sydney – Structures CONCRETE Peter Smith 1998 Testing Other Materials l Quality control of other materials is usually done by manufacturers l Homogeneous materials like steel are made to close tolerances of strength l Natural materials like timber vary greatly, and are classified into several grades 19/19