Many factors affect the strength of concrete after mixing. Some of these are discussed below. Firstly, some definitions may be helpful:
The process of ‘setting’ and ‘hardening’ are often confused:
The setting is a rigid concrete after it has been placed. A concrete can be ‘set’ in that it is no longer fluid, but may still be very weak; You may not be able to walk on it, for example. This arrangement is due to the formation of ettringite and calcium silicate hydrate early stage. The term ‘initial set’ and ‘final set’ is commonly used. For all your Concrete Pumping requirements, visit a site like https://www.monstermixconcrete.co.uk/concrete-pumping/
Hardening is when the concrete is growing in strength and this may be ongoing for weeks or months after the concrete has been mixed and placed. Hardening occurs due in large part to the formation of calcium silicate hydrate that continues to hydrate.
The rate at which the concrete sets is independent of the rate at which hardens. Rapid-hardening cement may have a similar arrangement to ordinary Portland cement.
Measurement of the strength of concrete
Traditionally, this is done by setting up a concrete cube or prism, then curing them for a predetermined time. General cure times are 2, 7, 28 and 90 days. Curing temperature is usually 20 degrees Celsius. The SI unit of measurement of the strength of concrete is Mega Pascal, although the ‘Newton per square millimeter’ is still widely used as a more comfortable number.
Factors affecting the strength of concrete
There are many relevant factors; some of the more important ones include:
Concrete porosity: voids in the concrete can be filled with air or with water. Airspaces are real examples and easily-seen from the pores in the concrete. Broadly speaking, the more porous concrete, the weaker it will be. Perhaps the most important source of porosity of concrete is cement water ratio in the mixture, known as ‘water on cement’ ratio.
Water to cement ratio: this is explained as dividing the mass of water by the mass of cement in the mixture. For example, the concrete mixture containing cement 400 kg and 240 liters (= 240 kg) of water would have a ratio of water / cement 240/400 = 0.6.
Consequently, as the water-cement ratio is increased, the porosity of the cement paste in the concrete is also increased. As the porosity increases, the compressive strength of concrete decreases.
Health aggregate: it will be clear that if the weak aggregate in concrete, the concrete will be weak. Inherently weak rock, such as limestone, which is obviously not suitable for use as aggregate.
Aggregate/paste bond: the integrity of the bond between the aggregate and the paste is vital. When there is no bond, the aggregate effectively becomes a void; as discussed above, voids are a source of weakness in concrete.