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Grading, Surface texture, Shape, Strength and Stiffness of aggregate particles

Grading

Strength of concrete is basically a function of water-cement ratio provided that concrete is workable. For producing a workable concrete the important factor is a good “gradation” of aggregates. However it should be understood that there is no “ideal aggregate grading” because satisfactory concrete can be made with various aggregate gradings within certain limits.

However, if a sample of aggregate is well graded, it would contain minimum voids; requires minimum cement paste to fill up the voids in the aggregates. Minimum paste will mean less quantity of cement and less quantity of water, which will further mean increased economy, higher strength and greater durability.

Surface Texture & Shape

Generally angular aggregate particles have rough texture and round aggregate particles are smooth textured. From the bonding point of view it seems that smooth textured rounded particles form a poor bond with cement paste. But the smooth looking surface of rounded particles is also rough enough at the microscopic level and the cement-gel that forms a bond with aggregate surfaces also has particle sizes in level of microns. Both, surface and the cement gel reacts at the sub-microscopic level; hence the theory that smooth surfaced particles yields poor concrete is not fully justified.

Angular aggregates have higher specific surface area than smooth rounded aggregate. With a greater specific surface area the angular aggregate may show higher bond strength than rounded aggregates. Also, angular aggregates exhibit a better interlocking effect in concrete that contributes in strength of concrete.

Higher specific surface area of angular aggregates with rough texture demands more water for a given workability than rounded aggregates. From this point of view for a given water-cement ratio, rounded aggregates give higher strength. Superimposing all these facts, it can be concluded that:

For water-cement ratio below 0.4 the use of crushed aggregate has resulted in strength up to 38 percent higher than the rounded aggregate. With an increase in water-cement ratio the influence of roughness of surface of the aggregate gets reduced and at a water-cement ratio of 0.65, no difference in strength of concrete made with angular aggregate or rounded aggregate has been observed.

Strength & Stiffness of Aggregate

Usually aggregates used in the concrete have higher value of “Modulus of Elasticity” than the bonding material cement-paste and concrete itself. Also, strength of aggregate particles ranges from 80 MPa to 200 MPa; which is much higher than that of low strength concrete.

(i) Low strength concrete

Aggregates of low strength and low modulus of elasticity can be valuable for producing the low strength concrete. As the strength and stiffness of corresponding matrix phase is always lower than aggregate particles, in low strength concrete, use of aggregate particles of low strength and stiffness preserves the integrity of concrete. It is to be remembered that strength of “low strength concrete” is governed by the cracking of “transition zone” or by the properties of cement-paste itself. In low strength concrete, as volume changes occurs during loading, compressible aggregates preserves the integrity of concrete while rigid aggregates might lead to cracking of the surrounding cement-paste.

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