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In order to withstand service and structural loads, a hardened concrete should be strong and durable enough and should also withstand the exposure of the environment to which it’s designed for. The most vital properties of a hardened concrete should be; durability, strength, shrinkage, creep, water tightness and bulk modulus of elasticity. Aggregates porosity and absorption will hugely affect most of this properties of concrete.
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The absorption, permeability, and aggregate porosity, influences the aggregate’s ability to bond with hydrated cement paste, concretes resistance thawing and freezing and its chemical stability, and abrasion resistance. Porosity also affects the overall yield of concrete per given aggregate mass; this is because aggregates specific gravity depends on porosity. Pores with less than 4 µm size affects the aggregate durability as it is subjected to thawing and freezing (Neville, 1995).
The volume of concrete is represented by three-quarters of aggregate hence, the porosity of concrete is largely represented by porosity of aggregate. The surface pores affects bonding between aggregate and cement paste. Since aggregate is more porous, water absorption becomes high. As a result more bonding occurs and this results to increase in the strength of concrete. When aggregates absorb water, concrete workability reduces with time, but after 15 minutes the loss minimizes. The more the void are occupied by water the less the concretes workability of hardened concrete (Neville, 1995).
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Strength of concrete also depends highly in the type of aggregate used. A fine aggregate will give a hard low-porosity aggregate with less compaction, as compared to a coarse aggregate that will give a soft high-porosity aggregate. A well compacted aggregate will contribute to the strength of concrete. Compressive strength of concrete depends also largely on the water – cement ratio. The higher the water – cement ratio the lower the compressive strength of concrete and the vise vasa is true. Coarse aggregates results in good bonding between the aggregate and cement. On the other hand, a rounded smooth surfaced aggregate would reduce absorbance of water and hence give weaker bonds. This would result in low compressive strength of the hardened concrete. When aggregate is compacted the pore spaces in the particles are reduced, this results to reduced volume of water occupying the pore spaces during mixing and hence low water – cement ratio (Domone & Illston, 2010).
Under high porosity, a concrete structure can undergo deformation. When pressure and stress are piled up for a long time, the concrete change in shape hence deformation.
Aggregate greatly affect the water tightness of concrete. Since aggregate constitutes to the larger percentage of concrete volume, the resulting hardened concrete assumes high permeability. When more water is absorbed into the hardened concrete, the concrete becomes more susceptible to cracking. This overall affects the durability of concrete (Khan, 2017).
Porosity and absorption properties of an aggregate widely affects the performance of the resulting concrete. A good aggregate mix should aim at increasing the durability of the resulting concerete and at the same time the compressive strength of concrete.
- Domone, P. & Illston, J., 2010. porosity and absorption. In: Construction Material: Their Nature and Behaviour. 4th ed. london ; New york: Spon Press, p. 126.
- Khan, I., 2017. Properties of hardened concrete. [Online]
Neville, A. M., 1995. Porosity and absorption of aggregate. In: Properties of concrete. 4th ed. s.l.:Pearson Education, pp. 462-464.