Page 77 - Handbook of Thermal Analysis of Construction Materials

60 Chapter 2 - Introduction to Portland Cement Concrete

11.0 CONCRETE PROPERTIES

The role of pore structure and cement paste has already been
described. Aggregates, occupying 60–80% of the volume of concrete
influence its unit weight, elastic modulus, dimensional stability, and
durability. Generally, aggregates are stronger than the matrix. Coarse
aggregates are larger than 4.75 mm and fine aggregates are smaller than 4.75
mm. Typically, fine aggregates comprise particles of size in the range of 75
mm to 4.75 mm, whereas coarse aggregates are of sizes from 4.75 mm to 50
mm. Mass concrete, the size of coarse aggregates reaching 150 mm, is
generally composed of sand, gravel, and crushed rock. The synthetic
aggregates, i.e., thermally processed materials such as expanded clay/shale,
slag, and fly ash, are also used. Natural silica is used extensively as a fine
aggregate. ASTM-294 provides a descriptive nomenclature of the com-
monly occurring minerals in rocks.
Some minor constituents of fine or coarse aggregates such as clay
lumps, friable particles, coal, lignite, chert, etc. may adversely affect the
workability, setting, handling, and durability characteristics of concrete. A
list of harmful substances and permissible limits is given in ASTM C-33.

11.1 Workability

The quality of fresh concrete is determined by the ease and
homogeneity with which it can be mixed, transported, compacted, and
finished. It has also been defined as the amount of internal work necessary
to produce full compaction. [28] The rheological behavior of concrete is
related to the rheological terms such as plasticity and visco-elasticity of
cement paste. As the workability depends on the conditions of placement,
the intended use will determine whether the concrete has the required
workability. A good workable concrete should not exhibit excessive
bleeding or segregation. Thus, workability includes properties such as
flowability, moldability, cohesiveness, and compactibility. One of the main
factors affecting workability is the water content in the concrete mixture.
Workability may also be improved by the addition of plasticizers and air-
entraining admixtures. The factors that affect workability include quanti-
ties of paste and aggregates, plasticity of the cement paste, maximum size
and grading of the aggregates, and shape and surface characteristics of the
aggregate.