Civil Engineering Reference
In-Depth Information
100
Compressive Strength
1990s cement
90
Compressive strength as specified by ASTM cement stan-
dards is that obtained from tests of 50-mm (2-in.) mortar
cubes tested in accordance with ASTM C 109 (AASHTO T
106) (Fig. 2-41). These cubes are made and cured in a
prescribed manner using a standard sand.
Compressive strength is influenced by the cement
type, or more precisely, the compound composition and
fineness of the cement. ASTM C 1157 has both minimum
and maximum strength requirements while ASTM C 150
and C 595 (AASHTO M 85 and M 240) set only a minimum
strength requirement. Minimum strength requirements in
cement specifications are exceeded comfortably by most
manufacturers. Therefore, it should not be assumed that
two types of cement meeting the same minimum require-
ments will produce the same strength of mortar or concrete
without modification of mix proportions.
In general, cement strengths (based on mortar-cube
tests) cannot be used to predict concrete strengths with a
great degree of accuracy because of the many variables in
aggregate characteristics, concrete mixtures, construction
procedures, and environmental conditions in the field
(
Weaver, Isabelle and Williamson 1970
and
DeHayes 1990
).
Figs. 2-42 and 2-43 illustrate the strength development for
standard mortars made with various types of portland
cement.
Wood (1992)
provides long-term strength proper-
ties of mortars and concretes made with portland and
blended cements. The strength uniformity of a cement from
a single source may be determined by following the proce-
dures outlined in ASTM C 917.
80
70
Cement
Type I
Type II
Type III
Type IV
Type V
60
50
40
30
20
ASTM C 109 mortar
w/c = 0.485
10
0
0
5
10
15
20
25
30
Time, days
Fig. 2-42. Relative strength development of portland cement
mortar cubes as a percentage of 28-day strength. Mean
values adapted from
Gebhardt 1995
.
chiefly upon the chemical composition of the cement, with
C
3
A and C
3
S being the compounds primarily responsible
for high heat evolution. The water-cement ratio, fineness of
the cement, and temperature of curing also are factors. An
increase in the fineness, cement content, and curing
temperature increases the heat of hydration. Although
portland cement can evolve heat for many years, the rate of
heat generation is greatest at early ages. A large amount of
heat evolves within the first three days with the greatest
rate of heat liberation usually occurring within the first 24
hours (
Copeland and others 1960
). The heat of hydration is
tested in accordance with ASTM C 186 or by conduction
calorimetry (Figs. 2-44).
For most concrete elements, such as slabs, heat gener-
ation is not a concern because the heat is quickly dissipated
into the environment. However, in structures of consider-
able mass, greater than a meter (yard) thick, the rate and
amount of heat generated are important. If this heat is not
rapidly dissipated, a significant rise in concrete tempera-
ture can occur. This may be undesirable since, after hard-
ening at an elevated temperature, nonuniform cooling of
the concrete mass to ambient temperature may create
undesirable tensile stresses. On the other hand, a rise in
concrete temperature caused by heat of hydration is often
beneficial in cold weather since it helps maintain favorable
curing temperatures.
Table 2-8 provides heat of hydration values for a vari-
ety of portland cements. This limited data show that Type
III cement has a higher heat of hydration than other types
while Type IV has the lowest. Also note the difference in
heat generation between regular Type II cements and
moderate heat Type II cements.
Heat of Hydration
Heat of hydration is the heat generated when cement and
water react. The amount of heat generated is dependent
Fig. 2-41. 50-mm (2-in.) mortar cubes are cast (left) and
crushed (right) to determine strength characteristics of ce-
ment. (69128, 69124)
