Civil Engineering Reference
In-Depth Information
7
When clinker (the kiln product that is ground to make
portland cement) is examined under a microscope, most of
the individual cement compounds can be identified and
their amounts determined. However, the smallest grains
elude visual detection. The average diameter of a typical
cement particle is approximately 15 micrometers. If all
cement particles were average, portland cement would
contain about 300 billion particles per kilogram, but in fact
there are some 16,000 billion particles per kilogram because
of the broad range of particle sizes. The particles in a kilo-
gram of portland cement have a surface area of approxi-
mately 400 square meters.
The two calcium silicates, which constitute about 75%
of the weight of portland cement, react with water to form
two new compounds: calcium hydroxide and
calcium sili-
cate hydrate
. The latter is by far the most important cement-
ing component in concrete. The engineering properties of
concrete—setting and hardening, strength, and dimen-
sional stability—depend primarily on calcium silicate
hydrate. It is the heart of concrete.
The chemical composition of calcium silicate hydrate is
somewhat variable, but it contains lime (CaO) and silicate
(SiO
2
) in a ratio on the order of 3 to 2. The surface area of
calcium silicate hydrate is some 300 square meters per
gram. In hardened cement paste, the calcium silicate
hydrate forms dense, bonded aggregations between the
other crystalline phases and the remaining unhydrated
cement grains; they also adhere to grains of sand and to
pieces of coarse aggregate, cementing everything together
(
Copeland and Schulz 1962
)
.
As concrete hardens, its gross volume remains almost
unchanged, but hardened concrete contains pores filled
with water and air that have no strength. The strength is in
the solid part of the paste, mostly in the calcium silicate
hydrate and crystalline compounds.
The less porous the cement paste, the stronger the
concrete. When mixing concrete, therefore, no more water
than is absolutely necessary to make the concrete plastic
and workable should be used. Even then, the water used is
usually more than is required for complete hydration of the
cement. About 0.4 grams of water per gram of cement are
needed to completely hydrate cement
(
Powers 1948
and
1949
)
. However, complete hydration is rare in field
concrete due to a lack of moisture and the long period of
time (decades) required to achieve complete hydration.
Knowledge of the amount of heat released as cement
hydrates can be useful in planning construction. In winter,
the heat of hydration will help protect the concrete against
damage from freezing temperatures. The heat may be harm-
ful, however, in massive structures such as dams because it
may produce undesirable temperature differentials.
Knowledge of the rate of reaction between cement and
water is important because it determines the rate of hard-
ening. The initial reaction must be slow enough to allow
time for the concrete to be transported and placed. Once
the concrete has been placed and finished, however, rapid
Cured at 32
°
C (90
°
F)
ASTM C 403
(AASHTO T 22)
6
40
23
°
C (73
°
F)
5
30
Final Set
4
10
°
C (50
°
F)
3
20
2
10
1
Initial Set
0
0
0
2
4
6
8
10
12
14
Time, hr
Fig. 1-11. Initial and final set times for a concrete mixture at
different temperatures
(
Burg 1996
)
.
hardening is desirable. Gypsum, added at the cement mill
when clinker is ground, acts as a regulator of the initial rate
of setting of portland cement. Other factors that influence
the rate of hydration include cement fineness, admixtures,
amount of water added, and temperature of the materials
at the time of mixing. Fig. 1-11 illustrates the setting prop-
erties of a concrete mixture at different temperatures.
HARDENED CONCRETE
Curing
Increase in strength with age continues provided (1) un-
hydrated cement is still present, (2) the concrete remains
moist or has a relative humidity above approximately 80%
(
Powers 1948
)
, (3) the concrete temperature remains favor-
able, and (4) sufficient space is available for hydration prod-
ucts to form. When the relative humidity within the concrete
drops to about 80%, or the temperature of the concrete drops
below freezing, hydration and strength gain virtually stop.
Fig. 1-12 illustrates the relationship between strength gain
and moist curing, while Fig. 1-13 illustrates the relationship
between strength gain and curing temperature.
If concrete is resaturated after a drying period, hydra-
tion is resumed and strength will again increase. However,
it is best to moist-cure concrete continuously from the time
it is placed until it has attained the desired quality; once
concrete has dried out it is difficult to resaturate. Fig. 1-14
illustrates the long-term strength gain of concrete in an
outdoor exposure. Outdoor exposures often continue to
provide moisture through ground contact and rainfall.
Indoor concretes often dry out after curing and do not
continue to gain strength (Fig. 1-12).
