Although the water / cementious material ratio is an important factor affecting the strength of concrete, the aggregate properties can not be ignored. The aggregate strength is usually not a factor except in lightweight and high strength concrete. However, aggregate characteristics other than strength, such as the size, shape, surface texture, grading and mineralogy are known to affect concrete strength in varying degrees.
The maximum size of a well-graded coarse aggregate of a given mineralogy can have two opposing effects on the strength of normal concrete. With the same cement content and consistency, concrete mixtures containing larger aggregate particles require less mixing water than those containing smaller aggregate. On the contrary, larger aggregates tend to form weaker transition zones containing more micro-cracks. Therefore, the result of these two opposing effects when large aggregates are used is only slight.
To obtain high strength concrete, coarse aggregate size is usually held to a maximum size of 19 mm, but additional cement is required for the additional surface area. The fine aggregate can generally contain less material passing 300 um and 150 um sieve because of the higher cement content. Proportionally, the amount of fine aggregate should also be somewhat less than that used for normal strength concrete.
Shape refers to geometrical characteristics such as round, angular, elongated, flaky, etc. Aggregate particles that are cubicle or spherical in shape and correct mineral composition are ideal for maximizing concrete strength. The use of flat and elongated aggregate particles should be avoided or at least limited to a minimum of 15 percent. These shapes are further described on Table (1) below.
Table (1), Classification of Aggregate Shape, Mindess
Flaky and elongated
|Fully water –worn or completely
Naturally irregular, or partly shaped by
attrition and having rounded edges
Possessing well-defined edges formed at
the intersection of roughly planar faces
Material of which the thickness is small
relative to the other two dimensions
Material, usually angular, in which the
length is considerably larger than the
other two dimensions
Material having the length considerably
larger than the width, and the width
considerably larger than the thickness
|River or seashore gravel;
desert, seashore, and
Other gravels; sand or dug
Crushed rocks of all types;
talus; crushed slag
Concrete mixtures containing rough textured or crushed aggregate would show somewhat higher strength at early ages than corresponding concrete containing smooth or naturally weathered aggregate of similar mineralogy. A stronger physical bond between the aggregate and the hydrated cement paste is assumed to be responsible for this. At later ages the influence of the surface texture of aggregate on strength may be reduced. Also, with given cement content, more mixing water is usually needed to obtain the desired workability in a concrete mixture containing rough textured aggregates. Different surface texture characteristics and examples are given in Table (2).
Table (2) Classification of Aggregate Texture
Water-worn, or smooth due to
fracture of laminated or fine-
Fracture showing more-or-less
uniform rounded grains
Rough fracture of fine- or medium –
grained rock containing no easily
visible crystalline constituents
Containing easily visible crystalline
With visible pores and cavities
|Black flint, obsidian,
Gravels, chert, slate,
Brick, pumice, foamed slag,
clinker, expanded clay
The grading of an aggregate is determined by a sieve analysis, which is the distribution of particles of granular materials among various sizes, usually expressed in terms of cumulative percentages larger or smaller than a series of sizes of sieve openings (or the percentages between certain ranges of sieve openings). Results from a sieve analysis are used in three ways: (1) to determine if the material meets specifications; (2) to select the most suitable material; and (3) to detect variations in grading that are sufficient to warrant blending select sizes or an adjustment of concrete mix proportions.
The ASTM C 33, Standard Specification for Concrete Aggregates, grading requirements for coarse and fine aggregates are shown on Tables (3) and (4), respectively.
There are several reasons for specifying grading limits and maximum aggregate size, most importantly, workability and cost. For example, very coarse sands produce harsh and unworkable concrete mixtures, and very fine sands increase water and cement requirements, and are uneconomical. Aggregates that do not have a large deficiency or excess of any particular size produce the most workable and economical concrete mixtures.
Table (3), Grading Requirements for Coarse Aggregates
Annual Book of ASTM Standards, Section 4, Vol.
O4.02 Copyright, ASTM, 1916 Race Street, Phila-
Delphia, PA 19103.
Differences in mineralogical composition of aggregates are known to affect concrete strength. The substitution of calcareous (limestone) for a siliceous (sandstone) aggregate, under identical conditions, resulted in substantial improvement in concrete strength. Not only a decrease in the maximum size of coarse aggregate as shown in figure (1a), but a substitution of limestone for sandstone as shown in figure (1b), improved the strength of concrete significantly after 56 days.
compressive strength, Mehta and Monteiro