Cohesiveness in concrete
Cohesiveness is how well concrete HOLDS TOGETHER when plastic.
Making a more workable mix
To make a more workable mix:

  • Add more CEMENT PASTE.
  • Use WELL GRADED aggregates.
  • Use an ADMIXTURE.

  • Never try to make a mixture more workable by just adding more water because this lowers the strength.and durability of concrete.
Strength & Durability in Concrete
  • Well made concrete is a naturally strong and durable material.
  • It is DENSE, reasonably WATERTIGHT, able to resist changes in TEMPERATURE, as well as wear and tear from WEATHERING.
  • Strength and Durability are affected by the density of the concrete. Denser concrete is more watertight (or less permeable).

  • Concrete durability INCREASES with strength.
  • Well made concrete is very important to protect the steel in reinforced concrete
Measurement of compressive strength in concrete
Strength of concrete in the hardened state is usually measured by the COMPRESSIVE STRENGTH using the Compression Test .
Factors affecting strength & durability of concrete
Strength and Durability are affected by:

COMPACTION : Compaction is removing the air from concrete. Proper compaction results in concrete with an increased density which is stronger and more durable.
Curing is keeping concrete damp for a period, to allow it to reach maximum strength. Longer curing will give more durable concrete.
Necessity of curing
Concrete that is cured is:

  • More DURABLE.

  • Cured concrete has a surface that wears better, lasts longer and better protects the steel reinforcement.
Process of curing
Concrete is cured by:
  • APPLYING EXTRA WATER to the surface of the concrete, orSTOPPING water loss from the concrete.

Methods : The most common methods of curing are explained below.
  • The simplest method of APPLYING WATER is to put acontinuous fine, misty spray of water over the concrete.
  • The spray must be a very fine mist or else it will damage the surface of the concrete.
  • Concrete will dry out more quickly in hot weather.
  • Keep the concrete continuously moist.
  • The most important thing in curing is to keep the concrete moist at all times. Hosing in the morning and again at night and letting the concrete dry out in between is no good.
  • Another way to cure concrete is to cover with PLASTIC SHEETS to slow down water loss. This method is easy and cheap. The only problem is that the sheets may cause concrete to become darker in places. To avoid this keep concrete EVENLY moist. The sheets must be held down to stop them blowing away and the concrete surface drying out. The sheets can be overlapped and stuck together and/or held down with sand, timber or bricks.Always check under the plastic from time to time to make sure the concrete is EVENLY moist. If it feels dry, sprinkle with water and put back the plastic sheets carefully. Condensation on the underside of the plastic is a good sign.

Concrete may also be cured by applying a CURING COMPOUND which slows water loss.

This should be applied soon after finishing. Always follow the manufacturer’s instructions carefully. Curing compounds may be sprayed or brushed on.
  • In rapid drying conditions (i.e. high wind, dry air and/or hot air) the use of an EVAPORATION RETARDANT minimises the rapid loss of surface moisture and as such reduces the incidence of early age plastic cracking
Duration of curing
Concrete keeps getting HARDER AND STRONGER over TIME.
  • The LONGER concrete is cured, the closer it will be to its best possible strength and durability.
  • A minimum of 14 day curing is recommended for best results.
Effect of weather on strength of concrete
Warmer weather will cause concrete to have a higher early strength.
Water Cement ratio
Too much water and not enough cement means concrete will be weaker and less durable.

  • The water to cement ratio (W/C) is the weight of the water divided by the weight of cement.
  • Water 20 litres, Cement 40 kg means W/C = 0.5.
  • The lower the ratio, the stronger the concrete.
Two main tests for concrete done at site
There are two main tests to be done on concrete:

  • The SLUMP test.
  • The SLUMP test shows the WORKABILITY of concrete.
  • Workability measures how easy the concrete is to place, handle and compact.
  • The COMPRESSION test.

  • The COMPRESSION test shows the best possible strength concrete can reach in perfect conditions.
  • The compression test measures concrete strength in the hardened state.
  • Testing should always be done carefully. Wrong test results can be costly.
Concrete Mix Design
There are two main tests to be done on concrete:

  • A CONCRETE MIX is designed to produce concrete that can be easily placed at the lowest cost.
  • The concrete must be workable and cohesive when plastic, then set and harden to give strong and durable concrete.
  • The mix design must consider the environment that the concrete will be in; ie exposure to sea water, trucks, cars, forklifts, foot traffic or extremes of hot and cold.
  • PROPORTIONING Concrete is a mixture of Cement, Water, Coarse and Fine Aggregates and Admixtures.
  • The proportions of each material in the mixture affects the properties of the final hardened concrete. These proportions are best measured by weight. Measurement by volume is not as accurate, but is suitable for minor projects.

  • CEMENT CONTENT As the cement content increases, so does strength and durability. Therefore to increase the strength, increase the cement content of a mix.
  • WATER CONTENT Adding MORE WATER to a mix gives a WEAKER hardened concrete.
  • Always use as little water as possible, only enough to make the mix workable.
  • WATER TO CEMENT RATIO As the Water to Cement ratio INCREASES, the strength and durability of hardened concrete DECREASES.
  • To increase the strength and durability of concrete, decrease the Water-Cement ratio.
Avoiding segregation in concrete
There are two main tests to be done on concrete:

  • Segregation is when the coarse and fine aggregate, and cement paste, become separated. Segregation may happen when the concrete is mixed, transported, placed or compacted.
  • Segregation makes the concrete:
  • To avoid segregation:
    Ensure that the concrete is not 'too wet' or 'too dry' – it is properly mixed. It is important that the concrete is mixed at the correct speed in a transit mixer for at least two minutes immediately prior to discharge. The concrete should be placed as soon as possible. When transporting the mix, load carefully. If placing concrete straight from a truck, pour vertically and never let the concrete fall more than one-and-a-half metres. Always pour new concrete into the face of concrete already in place. When compacting with a poker vibrator, be sure to use it carefully.




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