Basic HTML Version
74
IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 1
The influence of low temperature on the evolution of concrete strength
day is increasing, that is, the higher the cure temperature, the
higher the compression resistance at this age. This trend is
again observed at 7 days, but more smoothly. At 14 days the
equation is almost linear, thus independent of the curing tem-
perature on the compression resistance is similar. While at 28
to 91 days the behavior of the results is decreasing, that is, the
concrete compression resistance is affected by the increase of
the curing temperature.
For all ages there were determined behavioral equations, shown
in Table 8. The described behavior can be analyzed through equa-
tions, for the first number, which multiplies the ‘t’ (cure tempera-
ture) is positive for increasing results at 1 and 7 days, close to zero
at 14 days, showing the linearity and negative when the values
decrease to 28 and 91 days. Yet, the isolated portion of the equa-
tion represents the value that the line intersects the axis ‘y’, that is,
the compressive strength when cured at 0 °C. This value is higher
for curing temperatures that provided the greatest resistance to
compression and lower for those who reached the lowest values, it
means, higher for higher temperatures.
4.2 Trial to determine the resistance to traction
by diametrical compression of the cylindrical
specimens
The presentation of the resistance to traction results at 28 days of
curing are shown in Table 9. Figure 5 illustrates the behavior of the
equation, relating the resistance to traction, in the ‘y’ axis, with the
curing temperature in °C, in the ‘x’ axis.
It is observed that, again, the results at 28 days were higher than
the cured concretes at lower temperatures, confirming the behav-
ior seen for the resistance to compression. The resistance varia-
tion for age was 9.0 MPa to 25 °C, 13.2 MPa for a temperature of 0
ºC, considering a 31% reduction in the range of 25 °C, approaching
the results presented in referenced bibliography. And the correla-
tion coefficient was 0.85, considered to be acceptable for this type
of trial.
5. Conclusions
After carrying out this study, it can be observed in the early
ages, between 1 and 7 days, the resistance to compression
of cured concrete at higher temperatures was higher com-
pared to the resistance to lower curing temperatures, as was
expected. At these ages the increase in resistance was due to
the high degree of hydration of these parts, explained by the
high value of the activation energy. It is noteworthy the curing
temperatures of 20 °C and 25 °C for 1 day, which showed the
highest values. However, on day 7, when the activation energy
does not exercise much influence in the parts, the resistance
had increased closeness
From the day 14 there was an inversion in the samples of greatest
resistance, that is, those who were initially cured at low temperatures
achieved the best results, but with lower differences between them.
These differences increased at 28 days, with the best performance
to the parts initially cured at 0 °C. Finally, it was proven that the more
slowly it is the hydration of Portland cement, the better the formation
of its crystalline structure, thus justifying superior performance for
the parts cured in its early ages at low temperatures, that is, tem-
peratures which slow down and / or slow the hydration process.
6. References
[01] HELENE, P. R. L.; LEVY S. M. “Estado da arte” do
concreto como material de construção. São Paulo:
Exacta. 2003. 8p.
Table 8 – Behavioral equations relating the
compressive strength and curing temperature
Property
Equation behavior
1 day
fc = 0,4303*t + 1,6048
7 days
fc = 1,1851*t + 18,986
14 days
fc = -0,0503*t + 27,129
28 days
fc = -0,4709*t + 36,819
91 days
fc = -0,3857*t + 37,071
Table 9 – Tensile strength at 28 days
Traction resistance
Temperature (°C)
28 days (MPa)
25
9,0
20
10,5
15
10,0
10
10,8
5
11,3
0
13,2
Figure 5 – Traction resistance at 28 days