Concrete is produced in three basic forms:

Soon after the aggregates, water, and the cement are combined, the mixture starts to harden. All portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction -called hydration – a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates.

The building up process results in progressive stiffening, hardening, and strength development. Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes too stiff. This hardening process continues for years meaning that concrete gets stronger as it gets older.

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.

The key to achieving a strong, durable concrete rests in the careful proportioning and mixing of the ingredients. A concrete mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough, honeycombed surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete is likely to shrink more and be uneconomical.

Cement and water form a paste that coats each particle of stone and sand. Through a chemical reaction called hydration, the cement paste hardens and gains strength. The character of the concrete is determined mainly by quality of the paste. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete.

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.

Almost any natural water that is drinkable and has no pronounced taste or odour may be used as mixing water for concrete. However, some waters that are not fit for drinking may be suitable for concrete.

Excessive impurities in mixing water not only may affect setting time and concrete strength, but also may cause efflorescence, staining, corrosion of reinforcement, volume instability, and reduced durability. Specifications usually set limits on chlorides, sulphates, alkalis, and solids in mixing water unless tests can be performed to determine the effect the impurity has on various properties.

It is not advisable to cast concrete when temperatures reach 5°C and decreasing, or when the ambient temperature is above 32°C. However, when temperatures are higher than 32°C, aggregates and water can be cooled or chilled, in order to continue concreting. Taking this into account the time of placement should be determined.

Example:
In winter, concrete can be placed at sunrise so that it can make use of the energy/heat of the sun to increase the chemical reaction / setting and strength gain before the cold comes at night.

In summer, early morning or late afternoon is preferable, because as little help as possible is required from the sun.

Concrete is vibrated to remove entrapped air in the concrete, which causes a negative impact on compressive strengths in concrete. Here are some advantages of compacting/ vibrating concrete.

•   Have a higher compressive strength.
•   Will increase the bonding capacity between concrete and rebar.
•   Provides a better sealed concrete surface, reducing its permeability.
•   Reduces honeycombing
•   If builder knows how to correctly vibrate concrete, he can order drier mixtures that require less cement
•   Offers greater durability
•   Increases bonding strength between layers of concrete
•   Horizontally spread layers of 20cm thick, will provide with best results in 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.

The most crucial time for strength gain of concrete is immediately following placement. In field conditions, heat and wind can “suck out” the moisture from the placed mixture. Concrete that is allowed to dry in air will gain less strength compared continuously moist-cured concrete.
Rapid evaporation also causes the concrete to shrink, which leads to tensile stresses within the concrete. As a result, surface cracking may occur, especially if the stresses develop before the concrete attains adequate tensile strength.
Selecting an appropriate curing process also helps in temperature control in the concrete and to prevent moisture loss to the external environment.

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.

Concrete surfaces can flake or spall for one or more of the following reasons:

– In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface.

– The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface.

– The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable.

Workability is one of the physical parameters of concrete which affects the strength and durability, as well as the cost of labour and appearance of the finished product. Concrete is said to be workable when it is easily placed and compacted homogeneously i.e. with minimum bleeding and no segregation. Unworkable concrete needs more work or effort to be compacted in place, also honeycombs and/or pockets may also be visible in finished concrete.

Factors affecting workability:

It is not advised to add water to fresh concrete without the permission of the mix designer. The more you add water the more the workability of concrete, but it decreases the strength of the mix. Since, by simply adding water, the inter-particle lubrication is increased, high water content results in a higher fluidity and greater workability, but this can have a negative impact on your concrete strengths. Increased water content also results in bleeding and segregation. Another effect of increased water content can be that cement paste will escape through joints of formwork.

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 5°C during the curing period.

Temperature is a very key part of the curing process. On hot days the heat causes excessive moisture to evaporate from the concrete. It can also increase the chemical reaction in concrete and thus increase the heat of hydration in concrete. The increased heat of hydration can cause thermal cracks in the concrete.

However, if the temperature drops too close to freezing, the hydration reaction will slow and even stop. If the concrete is cured below freezing, the water inside the concrete will freeze, expand and completely ruin the internal structure of the concrete.

For concrete to gain strength it is essential for the temperature to be moderate. It is extremely important that the temperature of new concrete should not be permitted to fall below 5-10°C during the curing period.

The three that are most commonly used are:

• slump
• air content
• compressive strengths

A slump test is essentially a measure of the workability of fresh concrete. Concrete, placed in an inverted cone, is allowed to settle as the cone is removed. The amount of settlement is measured to determine the “slump” of concrete. Too low a slump (stiff mix) may indicate workability problems with the mixes. These could include segregation problems, reduced strength, shrinkage problems, finishing difficulties, and increased scaling problems.

The air content test measures the percentage of entrained air in the concrete. This test is important, when using a new admixture, as some admixtures or admixture combinations can entrain air in concrete. Normal concrete and admixture combinations entrain 1-2% air into a concrete mix. In some cases, like freezer floors (5-8%) you need to entrain more air and this needs to be tested by the air meter to make sure of mix stability and air entrainment.

The compressive test is the most widely used test to determine potential concrete strength. Compressive tests can be conducted on sampled concrete or from specimens taken out of the concrete member/structure. Depending from where samples emanate, the analyzing of results will be different and are indicated in national standards.

To ensure it is representative of the whole load a standard sample consists of scoopfuls taken from four different parts of the load and collected in buckets. Scoopfuls must be taken through the moving stream, as the load is discharged, sampling the whole width and depth – not just the top part. The size of sample taken should be 1,5 times the estimated volume required for testing.

Sample sizes required:

Discharge the first 15% of concrete from the ready-mix truck. Take scoopfuls from the moving stream of concrete, until the required sample size is reached.

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.

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.