For the production of good quality concrete, it is necessary to understand the impacts of various factors which affect the layers of concrete during finishing operations, or when it brings with it, fine cement particles result in the properties of concrete in the plastic and hardened state. These properties have been sanctioned earlier that can be easily measured and which is also indirectly related qualitatively to other desirable properties.
Strength variation of concrete:
Experiment results indicate the principal sources of strength variation as arising out of the difference in strength producing properties of the concrete mixture.
- Water Cement Ratio
- Moisture Content
- Quality of cement
- Storage of cement
- Degree of Compaction
- Curing of Concrete
- Moulding Temperature
- Curing Temperature
- The moisture content at the time of testing
- Rate of application of load
- Duration of Load
- Degree of lateral restraint
- Compression machine and operation factors
Water Cement Ratio:
- For a given cement and approved aggregate, the strength of all fully compacted concrete depends principally on Water Cement Ratio irrespective of mass or the cement content.
- Water Cement Ratio is the main parameter affecting the compressive strength of concrete at all ages. However, Water Cement Ratio strength relationship is different for different types of cement.
- Generally, a change of 0.01 in Water Cement Ratio affects the strength by 8-12 kg/cm2 at 28 days.
- Control of Water Cement Ratio ratio involves adjustments in cement and water content.
- Errors relating to Water Cement Ratio ratio will be reflected in the strength and non-uniformity of the mix.
Effect of variation in moisture content are as follows:
- Poor control of water affects Water Cement Ratio of the concrete mix directly.
- For a given cement content any variation in the quantity of water added to a batch resulting out of improper or lack of adjustment for moisture content/absorption of aggregate and sand or in measuring would affect Water Cement Ratio and consequent strength.
- Every extra litre of water added over the desired quantity in a 50 kg of cement will alter Water Cement Ratio by 0.02 and would affect the strength of concrete by 15-25 kg/cm2
Quality of cement:
Water Cement Ratio, strength and age of concrete relation for a fully compacted and cured concrete is different for different types of cement as well as for the particular type of cement from the same factory over a period of time, and these relationships should be established for any job periodically.
Storage of Cement:
Cement stored in godowns which are gradually deteriorating due to hydration. The loss in strength for different periods of storage is 15% at three months and 30% and 50% at 6 and 12 months respectively.
Degree of Compaction:
Good, workable concrete can be fully compacted with medium effort, but if improperly compacted, every 1.0 % of the void will reduce strength by about 5%.
Curing of Concrete:
Usually, the strength specified is at 28 days on concrete kept continuously wet. Twenty-eight days strength expressing as 100%, seven and fourteen days strengths are 85% and 93% whereas concrete cured only for the first 24 hours and on the 28 days the strength is only 59%. These values are for a particular location and should not be generalized.
Generally, the indicated strength of the concrete cube moulded at a higher temperature which is above 20-25°C-will show increased strength, but long-range strengths are found to be superior when concrete is moulded at below 20°C but not below 5°C.
At very low temperatures, concrete gets frozen and may not hydrate. Curing at elevated temperatures accelerates strength development. The strength gain is related to “Maturity’ which is a function of the product of cutting time and temperature.
The moisture content at the time of testing:
The indicated strength of a concrete cube depends upon its moisture content. The concrete cubes should not be completely dry at the time of the experiment. They should be tested by drying just the surface. Drier the cube, higher the indicated strength. Oven dry cube can give about 80% higher strength.
The direction of loading:
Cubes are generally tested at right angles to the direction in which they are filled. There can be a 10% decrease in the strength if tested other day.
Rate of application of load:
The rate of loading is usually 14N/mm2/min. Cubes tested at very high speed may show increased strengths of up to 80%.
Duration of Load:
Constant or continuous loading of a structure will cause creep strains and reduce the strength of concrete by 50% of its ultimate strength. Under a sustained load, failure may occur at 90% strength for one-hour loading, 80% for 2-month loading and 70% for 30-year loading.
Degree of lateral restraint:
If lateral support is provided to the cubes, increased strengths are likely to be obtained triaxial compression. Tests have revealed that a 28 N/mm2 with a lateral restraint of 175N/mm2.
Compression machine and operation factors:
Due to continuous operation, the machine needs recalibration as the pressure gauges may falter. Errors may be introduced even while cantering the cubes or due to wear of the platens. More friction of the spherical seat can affect the strength by nearly 20%. 0.75mm misalignment in the machine may reduce the strength of concrete by 9%.