Strength can be defined as ability to resist change. One of the most valuable properties of the concrete is its strength. Strength is most important parameter that gives the picture of overall quality of concrete. Strength of concrete usually directly related to cement paste. Many factors influence the rate at which the strength of concrete increases after mixing. Before coming toward the factors that influence the strength gain of concrete, it is important to have concept of these terminologies:
Hardening is the process of growth of strength. This is often confused with 'setting' but setting and hardening are not the same.
Setting is the stiffening of the concrete after it has been placed. Hardening may continue for weeks or months after the concrete has been mixed and placed.
Factors affecting strength gain & rate of strength gain of concrete
Voids in concrete can be filled with air or with water. Broadly speaking, the more porous the concrete, the weaker it will be. Probably the most important source of porosity in concrete is the ratio of water to cement in the mix, known as the 'water to cement' ratio.
This is defined as the mass of water divided by the mass of cement in a mix. The water/cement ratio may be abbreviated to 'w/c ratio' or just 'w/c'. In mixes where the w/c is greater than approximately 0.4, all the cement can, react with water to form cement hydration products. At higher w/c ratios it follows that the space occupied by the additional water above w/c = 0.4 will remain as pore space filled with water, or with air if the concrete dries out.
Consequently, as the w/c ratio increases, the porosity of the cement paste in the concrete also increases. As the porosity increases, the compressive strength of the concrete will decrease.
Soundness of aggregate
If the aggregate in concrete is weak, the concrete will also be weak. Rocks with low strength, such as chalk, are clearly unsuitable for use as aggregate.
Aggregate paste bond
The compactness of the bond between the paste and the aggregate is critical. If there is no bond, the aggregate effectively represents a void & voids are a source of weakness in concrete.
Many parameters relating to the composition of the cement constituents and their proportions in the cement can affect the rate of strength gain and the final strength achieved. These include:
- Alite content (Tri-Calcium silicates) & Belite contents (Di-calcium silicates)
- Alite & belite reactivity
- Sulfate contents
Alite is the most reactive cement mineral that contributes significantly to concrete strength. More Alite should give better early strengths ('early' means up to about 7 days).
Sulfate in cement, both the clinker sulfate and added gypsum, retards the hydration phase. If there is insufficient sulfate, a flash set (rapid hardening of freshly mixed cement paste with noticeable heat evolution) may occur. on the other hand too much sulfate contents can cause false-setting(rapid hardening of freshly mixed cement paste with minimum heat evolution)
Some physical parameters of cement also play role in strength gain of concrete like Cement surface area and particle size distribution.
Fineness is often expressed in terms of total particle surface area. More fine is cement; greater will be its hydration rate. Particle size distribution is also very important prospect in strength gain of concrete. Cement with very finely-ground gypsum and clinker particles results in slower hydration.
Tests To Determine The Strength Gain & Rate of Strength Gain Of Concrete
In concrete practice the strength of concrete is characterized by the 28 day value and some other properties are also related to the 28 day strength. After 28 days, different tests are usually performed to determine the strength gain of the concrete. These are as under:
For Strength Gain:
Compressive Strength Test
- Cylinder test
- Cube test
Tensile Strength Test
Split cylinder test
Flexural Strength Test
- Two point loading test
- Three point loading test
Rate of strength gain of concrete:
To determine the rate of gain of strength of concrete, there is a need to select period shorter than 28 day, as 28 day is considered to be the reference time. In concrete practice, it is accepted that after 28 days concrete usually gains most of its strength. Strength determined at an early stage say after 7th day of placing of concrete can be compared to strength determined after 28 days, which is considered to be the reference time. In this way, rate of gain of strength of concrete can be determined.