Civil Engineering Reference
In-Depth Information
tial between the surface and center is too great. The width
and depth of cracks depends upon the temperature dif-
ferential, physical properties of the concrete, and the rein-
forcing steel.
A definite member size beyond which a concrete struc-
ture should be classified as mass concrete is not readily
available. Many large structural elements may be massive
enough that heat generation should be considered; this is
particularly critical when the minimum cross-sectional
dimensions of a solid concrete member approach or exceed
1 meter (3 feet) or when cement contents exceed 355 kg/m
3
(600 lb per cubic yard). Temperature rise in mass concrete
is related to the initial concrete temperature (Fig. 18-9),
ambient temperature, size of the concrete element (volume
to surface ratio and minimum dimension), and type and
quantity of cementitious materials. Smaller concrete mem-
bers less than 0.3 meters (1 ft) thick with moderate
amounts of cementitious materials are typically of little
concern as the generated heat is rapidly dissipated.
To avoid cracking, the internal concrete temperature
for dams and other nonreinforced mass concrete struc-
tures of relatively low compressive strength should not be
allowed to rise more than 11°C to 14°C (20°F to 25°F)
above the mean annual ambient temperature (
ACI 308
).
Internal concrete temperature gain can be controlled a
number of ways: (1) a low cement content—120 to 270
kg/m
3
(200 to 450 lb per cubic yard); (2) large aggregate
size—75 to 150 mm (3 to 6 in.); (3) high coarse aggregate
content—up to 80% of total aggregate; (4) low-heat-of-
hydration cement; (5) pozzolans—where heat of hydra-
tion of a pozzolan can be 25% to 75% that of cement;
(6) reductions in the initial concrete temperature to by
cooling the concrete ingredients; (7) cooling the concrete
through the use of embedded cooling pipes; (8) steel
forms for rapid heat dissipation; (9) water curing; and
(10) low lifts—1.5 m (5 ft) or less during placement. In
massive structures of high volume-to-surface ratio, an
estimate of the adiabatic temperature rise can be made
using equations in a PCA publication (
PCA 1987
).
Massive structural reinforced concrete members with
high cement contents (300 to 600 kg per cubic meter or 500
to 1000 lb per cu yard) cannot use many of the placing tech-
niques and controlling factors mentioned above to maintain
low temperatures to control cracking. For these concretes
(often used in bridges, foundations, and power plants), a
good technique is to (1) avoid external restraint from adja-
cent concrete elements, (2) reduce the size of the member by
placing the concrete in multiple smaller pours, or (3) control
internal differential thermal strains by preventing the con-
crete from experiencing an excessive temperature differen-
tial between the surface and the center. The latter is done by
properly designing the concrete and either keeping the con-
crete surface warm through use of insulation or reducing
the internal concrete temperature by precooling of the con-
crete or postcooling with internal cooling pipes.
Studies and experience have shown that by limiting
the maximum temperature differential between the inte-
rior and exterior surface of the concrete to less than about
20°C (36°F), surface cracking can be minimized or avoided
(
FitzGibbon 1977
and
Fintel and Ghosh 1978
). Some
sources indicate that the maximum temperature differen-
tial (MTD) for concrete containing granite or limestone
(low-thermal-coefficient aggregates) should be 25°C and
31°C (45°F and 56°F), respectively (
Bamforth 1981
). The
actual MTD for a particular mass concrete placement and
concrete mix design can be determined using equations in
ACI 207 (1995)
.
In general, an MTD of 20°C (36°F) should be assumed
unless a demonstration or calculations based on physical
properties of the actual concrete mix the geometry of the con-
crete member show that higher MTD values are allowable.
By limiting the temperature differential to 20°C (36°F)
or less, the concrete will cool slowly to ambient tempera-
ture with little or no surface cracking; however, this is true
only if the member is not restrained by continuous rein-
forcement crossing the interface of adjacent or opposite
sections of hardened concrete. Restrained concrete will
tend to crack due to eventual thermal contraction after the
cool down. Unrestrained concrete should not crack if
proper procedures are followed and the temperature dif-
ferential is monitored and controlled. If there is any con-
cern over excess temperature differentials in a concrete
member, the element should be considered as mass con-
crete and appropriate precautions taken.
Fig. 18-10 illustrates the relationship between temper-
ature rise, cooling, and temperature differentials for a sec-
tion of mass concrete. As can be observed, if the forms
(which are providing adequate insulation in this case) are
removed too early, cracking will occur once the difference
between interior and surface concrete temperatures
exceeds the critical temperature differential of 20°C (36°F).
If higher temperature differentials are permissible, the
forms can be removed sooner. For large concrete place-
ments, surface insulation may be need for an extended
period of time of up to several weeks or longer.
40
per
ature
70
35
60
30
50
25
40
20
30
15
20
10
Type I cement
10
5
0
0
0
1
2
3
4
7
14
28
Time, days
Fig. 18-9. The effect of concrete-placing temperature on
temperature rise in mass concrete with 223 kg/m
3
(376
lb/yd
3
) of cement. Higher placing temperatures accelerate
temperature rise (
ACI 207.2R
).
