Influence of Variations in Aggregate Gradations on Selected Properties of Cement Concrete
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Date
2021-07
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Addis Ababa University
Abstract
Most of the time, natural sands in Ethiopia are failing to meet the standard requirements set in
ERA (Ethiopian Roads Authority) standard technical Specification 2013, for gradation for use in
cement concrete. Besides, the grading envelop for course aggregate is also very narrow and it is
sometimes very difficult to find an aggregate falling within the specification limit. The effect of
using aggregates that fall out of specification on properties of concrete and the consequent effect
on short and long term performance of concrete is poorly understood. Besides, a rational basis to
use or reject concretes made with such aggregates which are out of the gradation requirement is
not available.
The goal of this project is to provide guidance and recommendation to the extent that the
specification should be revised/relaxed so that concretes made with aggregates which are out of
the grading limits set in ERA could be utilized. The main objective of this research paper is to
study the influence of variations in aggregate gradations within and out of ERA’s Specification on
workability, compressive strength, and water absorption of concrete. The methodology used in this
research is, deliberately separating the as obtained aggregates into different sieve sizes and
recombining them in a range of predetermined proportions to obtain aggregate gradations that fall
on the middle, upper limit and lower limit of the specification for both the coarse and fine
aggregates, which are called the control mixtures, and some other pre specified mixtures which
fall out of the specification beyond the coarser and the finer limits.
Statistical analysis made on the means of the test results shows that, to 95% significant level (alpha
value of 0.05) varying fine aggregate gradation from fineness modulus of 1.8 to 3.74 had no impact
on the compressive strength of concrete, while the workability is significantly increased as the fine
aggregate becomes coarser. To 95% significant level, deviation in aggregate gradation of coarse
aggregate up to 10% coarser beyond the coarser limit had no significant impact on compressive
strength and workability of concrete, whereas mixtures prepared using 10% more finer and 5%
more finer coarse aggregates resulted in significant increase in compressive strength and
significant decrease in workability. Water absorption of concrete is not affected by varying both
the fine aggregate and the coarse aggregate beyond either the finer or the coarser limit. However,
within the specification limit, as the coarseness of either the coarse aggregate or the fine aggregate
is increased, the water absorption decreased.
In general, workability of cement concrete is more significantly influenced by deviations in
aggregate gradation from the specification limit, while the compressive strength and water
absorption are little affected.
In conclusion, it is recommended to relax the upper limit set in the specification for the fine
aggregate up to fineness modulus of 2.4, because using more finer fine aggregates with fineness
modulus of 1.8, 2.0 and 2.2 resulted in a mixtures with very low workability and high segregation.
It is also recommended to relax the lower limit of coarse aggregate up to 10% more coarser because
there is no significant variation in concrete properties in between mixtures made with the coarser
limit, 5% more coarser, and 10% more coarser coarse aggregates.
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Keywords
fine aggregate, coarse aggregate, gradation, aggregate, cement concrete