The development of maturity concept started when some papers dealing with accelerated curing method that carried out in England were published in the late 1940s and early 1950s[101]. In 1949, McIntosh[102] reported the procedures of his proposed method to estimate the strength development of concrete, during electrical curing. He compared the strengths of concrete cured under normal condition to that of concrete cured at elevated temperatures under electrical curing. McIntosh was probably the first to develop a parameter in 1949, which he called ‘basic age’, to combine the influence of temperature and time. He concluded that the combination of time and concrete temperature above a datum temperature of -1.10C could be used to measure the effects of curing temperature history.
A few months after, Nurse[103] reported on the effects of steam curing on concrete strength gain. He agreed with McIntosh that the combinations of time and temperature could be used to quantify the effect of different steam-curing cycles on the strength gain. However, he did not use a datum temperature in calculating his predicted strength, as McIntosh did. Nurse also did not take into account the actual concrete temperatures, but used chamber-curing temperatures instead. In order to compare the combination influence of the time and temperature on the compressive strength of different concrete mixtures investigated, Nurse expressed the strength as a percentage of strength after the age of 3-days of concrete cured at normal condition (moist air - 180C). The resulting percentages were then plotted
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Figure 2.2: Strength development vs. the product of time – temperature (non-
reactive aggregate)[103]
Figure 2.3:Strength development vs. the product of time – temperature (reactive aggregate)[103]
against the product of time and temperature as are presented in Figure 2.2 and Figure 2.3 for the non-reactive and reactive aggregates respectively, which produced a single non-linear curve. He then suggested using the curve to estimate
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the strength of concrete cured at other temperatures. This was the first evidence that showed that strength development of concrete could be approximated from the two factors of time and temperature.
In 1951, Saul[104] summarized the conclusion that was obtained from experimental work carried out both at the Cement and Concrete Association Research Station and others regarding the principles of underlying steam curing at atmospheric pressure. He introduced the term of ‘maturity’ for the first time and linked it as an indicator of strength gain, dependent on the product of concrete time and temperature. Furthermore, he suggested that the maturity should be determined
with respect to a ‘datum temperature’, which is below the temperature, the
hydration process then will cease and no strength be obtained. He recognized that once concrete has set, the strength of concrete would be continuously developed even with temperatures lower than the freezing point, 00C (320F). Thus, he suggested taking a value of the datum temperature of -10.50C (130F) to be used in determining the maturity of concrete[101]. In 1956, McIntosh[105] confirmed in his paper that the value of a datum temperature of -100C could be better than that of the value he previously proposed.
Saul (1951) then proposed the ‘maturity rule’ as the following[106]:
"Concrete of the same mix at the same maturity (reckoned in temperature-time) has approximately the same strength whatever combination of temperature and time go to make up that maturity."
The Nurse-Saul maturity function is defined as follows[6, 101]:
Equation 2.2
where:
M = maturity or the temperature-time factor at age t (0C.days or 0C.hours) t = elapsed time or concrete age, days or hours
t = a time interval, days or hours
T = average temperature of the concrete during time interval, t, 0C T0 = datum temperature
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Saul found that the maturity method function would give an accurate result when it was used to estimate the strength of the concrete, which had a temperature that did not reach 500C in the first two hours or about 1000C within the first 6 hours after mixing[101].
The principle of the maturity method can be used to determine the strength development of a given concrete mixture cured at different temperatures. Therefore, when the concrete is cured in either cold or hot conditions, the maturity should be the same and the strength of the concrete can be predicted accurately. Figure 2.4 illustrates the maturity concept for concrete that is cured at different temperatures such as at lower and higher temperatures. When the concrete is cured at lower temperature, it will take a longer time than that of cured at higher temperature to reach the same level of maturity.
Figure 2.4:Saul’s maturity rule using temperature-time factor[107]
Kehl et al.[107] illustrated the maturity rule through the figure above. They found that when the same concrete mixture was cured in both cold and hot conditions, it would reach the same maturity when the temperature-time areas were equal, as shown in the figure M1 = M2 = M. It is clear that the concrete will reach a certain maturity much quicker in hot curing conditions than that of in cold curing
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conditions. Therefore, the time needed to reach the maturity of the two curing conditions will be different as well.