Shape and texture classification
The external characteristics of the aggregate, in particular the particle shape and surface texture, are of importance with regard to the properties of fresh and hardened concrete. The shape of three-dimensional bodies is difficult to describe, and it is convenient to define certain geometrical characteristics of such bodies.
Roundness measures the relative sharpness or angularity of the edges and corners of a particle. The actual roundness is the consequence of the strength and abrasion resistance of the parent rock and of the amount of wear to which the particle has been subjected. In the case of crushed aggre-gate, the shape depends on the nature of the parent material and on the type of crusher and its reduction ratio, i.e. the ratio of initial size to that of the crushed product. A convenient broad classification of particle shape is given in Table
3.2.-Table 3.2: Particle shape classification of aggregates with examples Classification Description Examples Rounded Fully water-worn or completely
shaped by attrition River or seashore gravel; desert, seashore and wind-blown sand Irregular Naturally irregular, or partly shaped by
attrition and having rounded edges Other gravels; land or dug flint Flaky Material of which the thickness is small
relative to the other two dimensions Laminated rock Angular Possessing well-defined edges formed
at the intersection of roughly planar faces
Crushed rocks of all types; talus;
crushed slag Elongated Material, usually angular, in which
the length is considerably larger than the other two dimensions Flaky and
Elongated Material having the length considerably larger than the width, and the width considerably larger than the thickness
Although there is no ASTM standard, a classification sometimes used in the US is as follows:
Well rounded - no original faces left Rounded - faces almost gone
Subrounded - considerable wear, faces reduced in area Subangular - some wear but faces untouched
Angular - little evidence of wear.
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Since the degree of packing of particles all of one size depends on their shape, the angularity of aggregate can be estimated from the proportion of voids among particles compacted in a prescribed manner. Originally, BS 812: Part 1: 1975 quantified the effect by the angularity number, i.e.
67 minus the percentage of solid volume in a vessel filled with aggregate in a standard manner. The size of particles used in the test must be con-trolled within narrow limits, and should preferably lie within any of the following four ranges: 20.0 and 14.0 mm (f and \ in.); 14.0 and 10.0 mm (\ and | in.); 10.0 and 6.3 mm (f and \ in.); 6.3 and 5.0 mm (\ and in.).
The number 67 in the expression for the angularity number represents the solid volume of the most rounded gravel, so that the angularity number measures the percentage of voids in excess of that in the rounded gravel (i.e. 33). The higher the number, the more angular the aggregate, the range for practical aggregates being between 0 and 11.
Another aspect of the shape of coarse aggregate is its sphericity, defined as a function of the ratio of the surface area of the particle to its volume (specific surface). Sphericity is related to the bedding and cleavage of the parent rock, and is also influenced by the type of crushing equipment when the size of particles has been artificially reduced. Particles with a high ratio of surface area to volume are of particular interest as they lower the work-ability of the mix (see page 79). Elongated and flaky particles are of this type. The latter can also adversely affect the durability of concrete as they tend to be oriented in one plane, with water and air voids forming under-neath. The presence of elongated or flaky particles in excess of 10 to 15 per cent of the mass of coarse aggregate is generally considered undesirable, although no recognized limits are laid down.
The classification of such particles is made by means of simple gauges described in BS 812-105.1 and 2. The method is based on the assumption that a particle is flaky if its thickness (least dimension) is less than 0.6 times the mean sieve size of the size fraction to which the particle belongs.
Similarly, a particle whose length (largest dimension) is more than 1.8 times the mean sieve size of the size fraction is said to be elongated. The mean size is defined as the arithmetic mean of the sieve size on which the par-ticle is just retained and the sieve size through which the parpar-ticle just passes.
As closer size control is necessary, the sieves considered are not those of the standard concrete aggregate series but 75.0, 63.0 50.0 37.5, 28.0, 20.0, 14.0, 10.0, 6.30 and 5.00 mm (or about 3, 2\, 2, \\, 1, f, \, f, \ and ^ in.) sieves. BS EN 1933-4: 2000 describes a shape test that is similiar to the elongation test but, although useful, none of those tests adequately describes the particle shape.
The mass of flaky particles, expressed as a percentage of the mass of the sample, is called the flakiness index. Elongation index and shape index are similarly defined. Some particles are both flaky and elongated, and are therefore counted in both categories.
While BS EN 12620: 2002 limits the flakiness index of coarse aggregate to 50, BS 882: 1992 specifies the same limit for natural gravel, but 40 for crushed or partially crushed aggregate.
Sea aggregates may contain shells whose content needs to be controlled because they are brittle and they also reduce the workability of the mix.
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SHAPE AND TEXTURE CLASSIFICATION
The shell content is determined by weighing hand-picked shells and shell fragments from a sample of aggregate greater than 5 mm (-^ in.); the details of the test are prescribed by BS 812-106: 1985 and BS EN 933-7: 1998.
According to BS EN 12620: 2002, when required, the shell content of coarse aggregate should be allocated into two categories: greater or less than
10 per cent. British Standard BS 882: 1992 limits the shell of coarse aggre-gate content to 20 per cent when the maximum size is 10 mm (3/8 in.) and to 8 per cent when it is larger. The limits apply to single size, graded and all-in aggregate. There are no limits on the shell content of fine aggregate.
The classification of the surface texture is based on the degree to which the particle surfaces are polished or dull, smooth or rough; the type of roughness has also to be described. Surface texture depends on the hard-ness, grain size and pore characteristics of the parent material (hard, dense and fine-grained rocks generally having smooth fracture surfaces) as well as on the degree to which forces acting on the particle surface have smoothed or roughened it. Visual estimate of roughness is quite reliable, but in order to reduce misunderstanding the classification of Table 3.3 could be followed.
Table 3.3: Surface texture classification of aggregates with examples Group Surface Texture Characteristics Examples
1 Glassy Conchoidal fracture Black flint, vitreous slag
2 Smooth Water-worn, or smooth due to fracture of laminated or fine-grained rock
Gravels, chert, slate, marble, some rhyolites 3 Granular Fracture showing more or
less uniform rounded
grains Sandstone, oolite 4 Rough Rough fracture of fine- or
medium-grained rock containing no easily visible crystalline constituents
Basalt, felsite, porphyry, limestone 5 Crystalline Containing easily visible
crystalline constituents Granite, gabbro, gneiss
6 Honeycombed With visible pores and
cavities Brick, pumice, foamed slag, clinker, expanded clay
The shape and surface texture of aggregate, especially of fine aggregate, have a strong influence on the water requirement of the mix (see page 79).
In practical terms, more water is required when there is a greater void con-tent of the loosely-packed aggregate. Generally, flakiness and shape of the coarse aggregate have an appreciable effect on the workability of concrete, the workability decreasing with an increase in the angularity number.