1 Concrete is produced by mixing together cement, coarse and fine aggregates (gravel and sand respectively) and water. Normally, 1 m3 of concrete contains 300350 kg cement, approx. 2000 kg aggregates and 130200 l water. Depending on the intended use of the concrete, additives and/or admixtures can be mixed in (admixture: approx. 0.5-10 kg/m3; additive: approx. 5-50 kg/m3). After mixing, the concrete must be placed and compacted as soon as possible.
After mixing, the concrete must be placed and compacted as soon as possible.
2 Together, the cement and the water form the paste which sets to form hydrated cement and binds together the aggregates. The cement is supplied as a powder and is therefore added to the fine/coarse aggregate blend based on weight.
Stored in the dry, cement can be kept for months. But as soon as it becomes moist, it forms lumps and is then unusable.
3 Aggregates must be washed clean. Contaminated, greasy and incrusted aggregates are unsuitable for use in concrete. Slate-like and marlaceous constituents or mica also impair the quality of concrete.
The aggregates must exhibit an appropriate grading that is as consistent as possible. The maximum grain size is usually 32 mm.
4 The water content has a crucial influence on the quality of the concrete: less water means fewer pores and hence a concrete with improved strength, density and durability.
The water content is specified by the water/cement ratio (w/c ratio). This ratio is calculated by dividing the weight of water (moisture in aggregate plus mixing water) by the weight of cement.
Good concrete requires a w/c ratio between 0.45 and 0.55; w/c ratios > 0.60 should be avoided. A concrete with a high sand content requires more water than one with coarser-grained aggregate. Good-quality concrete therefore contains more coarse than fine aggregate.
5 Admixtures and/or additives can be mixed into the concrete in order to modify certain properties of the wet and/or hardened concrete. The most important of these are:
- Plasticisers: to improve the workability of the concrete or enable the water content to be reduced and hence achieve a better quality concrete.
- Accelerators and retarders: to influence the onset and duration of the curing process.
- Air entrainers: to improve the frost resistance - essential when the concrete will be exposed to de-icing salts, but micro hollow beads are often more advantageous for very stiff wet concrete.
- Additives: fillers and fly ash can replace ultra-fine particles - but not the cement - and improve the workability;
hydraulic lime is also used as an additive; pigments can be added to produce coloured concrete.
6 The formwork should be thoroughly cleaned out prior to concreting. Water in the formwork, excessive release agent, sawdust and any form of soiling can impair the appearance of the concrete. The f ormwork should be leakproof. The distance between r einforcement and formwork must be correct and the reinforcement must be secured to prevent displacement.
7 Proper mixing of the concrete is vital for its quality and workability. The optimum mixing time is > 1 min. Prolonging the mixing time improves the workability and has a favourable effect on exposed surfaces. Insufficient mixing is not beneficial to the properties of the wet or hardened concrete.
8 When using ready-mixed concrete it must be ensured that the loss of water during transport is kept to a minimum. Concrete transported in open vehicles must be covered. During periods of hot weather the available working time on the building site can be severely shortened due to the effects of the heat during transport. Adding water on site to "dilute" the concrete impairs the quality of the concrete.
Ready-mixed concrete must be ordered in good time and specified in full.
9 Concrete should be placeo in even, horizontal layers. The concrete should not be tipped in piles and then spread with a poker vibrator because this can result in segregation (honeycombing).
Every layer must be compacted immediately after being placed until all the air has escaped. The distance between successive immersion points for the poker vibrator is 25-70 cm depending on the diameter of the vibrator. Excessive vibration causes segregation of the concrete because the large constituents sink to the bottom and the cement slurry and water rise to the top. On exposed concrete surfaces such segregation causes permanent blemishes. A stiff mix lowers the risk of segregation. 10 Curing is an essential part of concreting because it prevents premature drying-out of the concrete. Exposed concrete surfaces should be covered or continuously sprinkled with water for at least four days after being placed, especially if exposed to draughts or direct sunlight.
During cold weather, freshly placed concrete must be protected against freezing by covering it and, if necessary, by heating.
Source: Cementbulletin, April 1987
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