8 Temperature effects
8.4 Mechanical properties
The effect of the temperature at the time of testing on the mechanical properties of high strength concrete has not been investigated so far. From the investigations on normal strength concrete without exchange of moisture it is known that for concrete with a higher content of cement paste, which is often the case for high strength concrete, a more pronounced decrease of the strength and stiffness can be observed (Mechtcherine [2000]).
In the cases where a moisture exchange takes place, the effect of temperature on the mechanical properties depends on the size and shape of the member. According to CEB-FIP MC 1990, as a first approximation for the compressive strength the effect of the temperature
can be neglected since the reduction in strength due to a temperature increase is offset by an increase in strength due to drying. The effect of drying might however be less pronounced in the case of high strength concrete, which has only a small content of capillary water. So the reduction of the compressive strength with increasing temperature should be higher.
According to CEB-FIP MC 1990 the uniaxial tensile strength and the tensile splitting strength are not significantly affected by the temperature at the time of testing. However, newer research results by Mechtcherine et al. (1995) and Slowik (1995) show, that for normal strength concrete a considerable decrease of the uniaxial tensile strength can be observed with increasing temperature. Further, the moisture gradients due to drying cause a significant decrease of the tensile strength, in particular of the uniaxial tensile strength and the flexural strength. The temperature gradients may influence the obtained values of the tensile strength as well.
According to the formula by CEB-FIP MC 1990 the fracture energy of concrete decreases significantly with increasing temperature. Though, Mechtcherine et al. (1995) and Slowik (1995) found no effect of the temperature on this material parameter.
In the following sections mainly the data concerning the effect of curing temperature on the mechanical properties will be presented, i.e. the maturity of concrete is the main parameter.
8.4.1 Compressive strength
Bergner (1997) investigated the temperature effect by curing concrete specimens (w/c = 0.33) for 7 days in a climatic chamber at temperatures of 4 °C, 7 °C, 23 °C, 36 °C, 42 °C and 60 °C. Specimens cured at higher temperatures possessed, after 2 days and 7 days, already 80 % and 95 % of the 28 days strength, respectively. Those cured at low temperatures exhibited after 7 days just 70 % of the 28 days strength, cf. Figure 8-2.
Marzouk and Hussein (1990) studied the behaviour of a high strength concrete containing 12 percent of fly ash and 8 percent of silica fume by mass of Portland cement. Cast specimens were cured for 24 hours at a temperature of 20 °C in a climatic chamber and then exposed to ocean water in pre-prepared tanks at 20 °C, 10 °C, 0 °C, -5 °C and -10 °C, respectively, for a time period of 1 to 91 days (the study was carried out with regard to the utilization of the high strength concrete for offshore structures in ocean cold regions).
Figure 8-3 shows a considerably slower strength development at lower temperatures of the applied ocean water compared with the corresponding values for the concrete kept in ocean water at a temperature of 20 °C. With decreasing temperature this tendency becomes more and more pronounced. This effect might be traced back to both, a slower cement hydration and to a low reactivity of the calcium hydroxide, a byproduct of the hydration process. Its reaction with silica fume and fly ash, which is referred to as the secondary hydration process forming a tobermorite gel, can only slowly proceed at low temperatures.
Marzouk and Hussein (1990) found that a logarithmic function according to Eq. (8-5) is in good agreement with their experimental results.
fcc = a + b · log t (8-5)
where: fcc compressive strength in MPa t time in days
a, b constants.
Fig. 8-2: Development of the compressive strength – effect of temperature, acc. to Bergner (1997)
Fig. 8-3: Development of the compressive strength – effect of storage in ocean water at different temperatures, acc. to Marzouk and Hussein (1990)
30 40 50 60 70 80 90 100
1 10 100 1000
exposure time ∆t [d]
compressive strength fcc(t) [MPa]
+ 20 °C + 10 °C 0 °C - 5 °C - 10 °C water temperature age of concrete = 1 day
SF = 8 %, FA = 12 % w/(c+fa+sf) = 0.27 0
10 20 30 40 50 60 70 80 90 100
0 1 10 100
exposure time ∆t [d]
4 °C 7 °C 20 °C 36 °C 42 °C 60 °C
compressive strength fcc(t) [MPa]
age of concrete = 1 day CEM I 42,5 R
w/c = 0.33
In a further study Marzouk and Hussein (1995) found out for the same composition of concrete that a prolongation of curing at room temperature of 20 °C from 1 day to 14 or 28 days before placing the specimens into the tanks with cold ocean water very positively affects the strength development, see Figure 8-4.
Fig. 8-4: Development of the compressive strength – effect of curing period prior to storage in ocean water, acc. to Marzouk and Hussein (1995)
Wild et al. (1995) investigated the effect of the temperature, storage in water at 20 °C and 50 °C, respectively, on the strength development of a high strength concrete, which contained different amounts of silica fume (SF), between 0 and 24 % by cement weight. The results are presented in Figure 8-5.
At higher concrete ages an apparent increase of the compressive strength of concrete with increasing amounts of silica fume was observed for both series, whereas the effect of silica fume on the strength of concrete stored at the temperature of 50 °C was more pronounced.
However, the development of the compressive strength with time was strongly affected by the storage temperature.
0 10 20 30 40 50 60 70 80 90 100
0 1 10
exposure time ∆t [d]
1 day, 20 °C 1 day, 0 °C 1 day, -10 °C
14 days, 20 °C 14 days, 0 °C 14 days, -10 °C
28 days, 20 °C 28 days, 0 °C 28 days, -10 °C compressive strength fc(t) [MPa]
Curing time prior to storage in ocean water, temperature of the ocean water SF = 8 %, FA = 12 % w/(c+fa+sf) = 0.27
100
Fig. 8-5: Development of the compressive strength – effect of the content of silica fume and temperature;
storage in water, acc. to Wild et al. (1995)
For the storage at the temperature of 20 °C the addition of silica fume causes a decrease of the compressive strength at early concrete age. The explanation might be an increase of the formation of Portlandite due to the presence of silica. Pozzolanic reactions are at this temperature rather slow. With increasing age of concrete the contribution of the reactions between silica fume and Portlandite crystals and as a result the formation of additional C-S-H gel at the interfacial zone between the cement paste and aggregate becomes more and more pronounced. For concretes containing a higher amount of silica fume a higher increase of strength in time could be observed.
At the temperature of 50 °C not only the hydration process of alite and other cement components are faster, but also the pozzolanic reactions proceed at a much higher rate. This results in very high values of the compressive strength for concrete containing a high amount of silica fume, already at an age of 3 days. On the other hand the further increase of the strength is much less pronounced in comparison with concretes stored at the room temperature. This is especially true for concretes with a very high content of silica fume.
8.4.2 Tensile strength and modulus of elasticity
Marzouk and Hussein (1990, 1995) studied the effect of curing in ocean water at different temperatures on the tensile splitting strength and the modulus of elasticity of high strength concrete containing 12 percent of fly ash and 8 percent of silica fume by mass of Portland cement. The results are presented in Figure 8-6 and Figure 8-7, respectively. The same tendencies as for the corresponding compression tests could be observed (see section 8.4.1).
30 40 50 60 70 80 90 100
1 10 100 1000
exposure time ∆t [d]
20 °C, 0 % SF 20 °C, 12 % SF 20 °C, 16 % SF 20 °C, 20 % SF 20 °C, 24 % SF
50 °C, 0 % SF 50 °C, 12 % SF 50 °C, 16 % SF 50 °C, 20 % SF 50 °C, 24 % SF compressive strength fc(t) [MPa]
Curing time prior to storage in water = 1 day
splitting tensile strength fct,sp [MPa]
0 1 2 3 4 5 6 7
0 1 10 100
exposure time ∆t [d]
1 day, 20 °C 1 day, 0 °C
1 day, -10 °C
14 days, 20 °C 14 days, 0 °C 14 days, -10 °C
28 days, 20 °C 28 days, 0 °C 28 days, -10 °C SF = 8 %, FA = 12 %
w/(c+fa+sf) = 0.27
Fig. 8-6: Development of the splitting tensile strength – effect of curing time prior to storage in ocean water, acc. to Marzouk and Hussein (1995)
20 25 30 35 40
1 10 100 1000
exposure time ∆t [d]
modulus of elasticity E0(t) [GPa]
+ 20 °C + 10 °C 0 °C -5 °C - 10 °C water temperature age of concrete = 1 day
SF = 8 %, FA = 12 % w/(c+fa+sf) = 0.27
Fig. 8-7: Development of the modulus of elasticity – effect of storage in ocean water at different temperatures, acc. to Marzouk and Hussein (1990)