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In-situ concrete which is manufactured either by the contractor or a ready-mix company on the site of the project (also referred to as site-mix). The operation is completely under the contractor’s control, and a high degree of flexibility in site management is possible, e.g. small quantities can be made at short notice.
Ready-mixed concrete, which in South Africa accounts for almost 50% of all concrete, is batched at local plants by specialist manufacturers for delivery in the familiar trucks with revolving drums. This allows more space to be made available on site (important in many urban projects); the supplier takes responsibility for quality control of the concrete and also has the resources and technical expertise to provide a wide range of mixes.
Precast concrete products are cast in a factory setting. These products benefit from tight quality control achievable at a production plant. Precast products range from concrete bricks and paving stones to bridge girders, structural components, and panels for cladding.
What is Hydration?
How do you control the strength of concrete?
The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely affected by adding more water.
Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used.
Generally, using less water produces a higher quality concrete provided the concrete is properly placed, consolidated, and cured. The following chart provides a range of trial mixes for a given strength of concrete at 28 days.
Cements with higher extender contents (e.g. CEMII/B or CEM III) may develop strength more slowly and will require particular care with curing.
Can it be too hot or too cold to place new concrete?
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete, unless sufficient measures are in place to prevent this. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 5oC during the curing period.
Can I use any water for mixing concrete?
Why is it so important to "cure" concrete?
Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period - from five to seven days after placement for conventional concrete - the concrete surface needs to be kept moist to permit the hydration process. New concrete can be wetted with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.
Air-entrained concrete contains billions of microscopic air cells per cubic metre. These air pockets relieve internal pressure on the concrete by providing tiny chambers for into which water can expand when it freezes. Air-entrained concrete is produced through the use of air-entraining portland cement, or by the introduction of air-entraining agents, under careful engineering supervision as the concrete is mixed on the job. The amount of entrained air is usually between 4 % and 7% of the volume of the concrete, but may be varied as required by special conditions.
Air-entraining agents are used to produce a number of effects in the concrete mix:
· To improve cohesion and reduce bleeding
· To improve compaction of low workability concrete
· To provide stability to extruded concrete
· To give improved handling properties, stability and cohesion to bedding mortar
· To improve freeze/thaw resistance of hardened concrete (not a major problem in South Africa)
When designing air-entrained concrete it should be remembered that the compressive strength is reduced, compared to non-air-entrained concrete.
Why does concrete crack only a short time after placing?
This is referred to as plastic cracking and can take two forms – shrinkage and settlement. The principal cause of plastic shrinkage cracking is the rapid removal of water from the concrete. Water loss is mainly from the exposed surface of the concrete (e.g. concrete slabs). When the evaporation rate exceeds he rate of bleeding, the surface concrete loses water and decreases in volume. Tensile stresses are induced in the because of restraint by the non-shrinking inner concrete. Plastic cracking can be minimised or avoided through proper mix design and effective early curing
Plastic settlement cracking occurs after the concrete has been compacted. After compaction there is a tendency for solid particles to settle and displace some mixing water which rises to the surface. This settlement will continue until the concrete stiffens. In a section where there is no restraint (e.g. top reinforcement, changes in section, etc.), such settlement rarely causes any problems.
What are the most common tests for fresh concrete?
Slump, air content, unit weight and compressive strength tests are the most common tests.
How do I perform a slump test?
The slump test is a relatively simple test to perform, whereby a slump cone mould is placed on a flat plate and filled with fresh concrete in three approximately equals layer. Each layer is subjected to 25 ‘blows’ from a tamping rod, the mould being firmly held down by standing on the foot pieces. The blows are evenly distributed over the whole area of the layer; for the second and third layers, the rod should just penetrate the previous layer.
The surface is struck off by rolling the tampering rod across the top edge of the mould. After careful removal of the mould, the slump of the concrete present is measured to the nearest 5 mm. The slump as measured is the distance between the top of the inverted mould and the highest point of the concrete.
Why do concrete surfaces flake and spall?
Concrete surfaces can flake or spall for one or more of the following reasons: