Civil Engineering Reference
In-Depth Information
CHAPTER 1
Fundamentals of Concrete
Concrete is basically a mixture of two components: aggre-
gates and paste. The paste, comprised of portland cement
and water, binds the aggregates (usually sand and gravel
or crushed stone) into a rocklike mass as the paste hardens
because of the chemical reaction of the cement and water
(Fig. 1-1). Supplementary cementitious materials and
chemical admixtures may also be included in the paste.*
Aggregates are generally divided into two groups: fine
and coarse. Fine aggregates consist of natural or manufac-
tured sand with particle sizes ranging up to 9.5 mm (
3
⁄
8
in.);
coarse aggregates are particles retained on the 1.18 mm
(No. 16) sieve and ranging up to 150 mm (6 in.) in size. The
maximum size of coarse aggregate is typically 19 mm or
25 mm (
3
⁄
4
in. or 1 in.). An intermediate-sized aggregate,
around 9.5 mm (
3
⁄
8
in.), is sometimes added to improve the
overall aggregate gradation.
Cement
15%
Water
18%
Air
8%
Fine agg.
28%
Coarse agg.
31%
Mix 1
Air-
entrained
concrete
7%
14% 4%
24%
51%
Mix 2
15%
21%
3%
30%
31%
Mix 3
Non-air-
entrained
concrete
7%
16% 1%
25%
51%
Mix 4
Fig. 1-2. Range in proportions of materials used in concrete,
by absolute volume. Bars 1 and 3 represent rich mixes with
small size aggregates. Bars 2 and 4 represent lean mixes
with large size aggregates.
The paste is composed of cementitious materials,
water, and entrapped air or purposely entrained air. The
paste constitutes about 25% to 40% of the total volume of
concrete. Fig. 1-2 shows that the absolute volume of cement
is usually between 7% and 15% and the water between 14%
and 21%. Air content in air-entrained concrete ranges from
about 4% to 8% of the volume.
Since aggregates make up about 60% to 75% of the
total volume of concrete, their selection is important.
Aggregates should consist of particles with adequate
strength and resistance to exposure conditions and should
not contain materials that will cause deterioration of the
concrete. A continuous gradation of aggregate particle
sizes is desirable for efficient use of the paste. Throughout
this text, it will be assumed that suitable aggregates are
being used, except where otherwise noted.
The quality of the concrete depends upon the quality
of the paste and aggregate, and the bond between the two.
In properly made concrete, each and every particle of
aggregate is completely coated with paste and all of the
spaces between aggregate particles are completely filled
with paste, as illustrated in Fig. 1-3.
Fig. 1-1. Concrete components: cement, water, fine aggre-
gate and coarse aggregate, are combined to form concrete.
(55361)
* This text addresses the utilization of portland cement in the production of
concrete. The term “portland cement” pertains to a calcium silicate hydraulic
cement produced by heating materials containing calcium, silicon,
aluminum, and iron. The term “cement” used throughout the text pertains to
portland cement or blended hydraulic cement unless otherwise stated. The
term “cementitious materials” means portland or blended cement, used with
or without supplementary cementitious materials.
