Two modifications have been proposed for the Nurse-Saul maturity function to get better estimates of compressive strength of concrete cured at different temperatures. The modifications account for the effect of w/c ratio on the temperature dependence of strength development and the effect of curing temperature on the long-term strength. The effect of the proposed modifications on the estimation of concrete strength using the Nurse Saul maturity function have been compared with the estimation using unmodified Nurse-Saul equation with two different datum temperatures (i.e., [T.sub.0] = -10[degrees]C and [T.sub.0] = 0 [degrees]C). The results show that applying the proposed modifications improves the accuracy of estimated concrete strength at different curing temperatures, especially at later ages.
Keywords: Maturity; Nurse-Saul; Curing temperature; Compressive strength; Concrete; Mortar; Strength development
During the last decades, the maturity method has been developed and used as one of the most favorable methods for estimating in-place concrete strength. The in-place strength is estimated based on the in-place maturity index and a previously established relationship between maturity index and strength [1-5].
In 1987, ASTM adopted a standard practice on the use of the maturity method to estimate in-place strength (ASTM C 1074). This standard permits the user to express the maturity index either in terms of the temperature-time factor using the Nurse Saul equation or in terms of the equivalent age at a specified temperature using the Arrhenius equation.
Using the Nurse-Saul equation, the temperature-time factor is computed as follows:
(1) M(t) = [summation]([T.sub.a] - [T.sub.0][DELTA]t,
where M(t) is the temperature-time factor at age t; degree-days or degree-hours; [DELTA]t is a time interval, days or hours; [T.sub.a] the average concrete temperature during time interval, [DELTA]t, [degrees]C, and [T.sub.0] is the datum temperature, [degrees]C.
According to ASTM C1074-98 , it is recommended that the datum temperature be determined experimentally or may be taken as 0[degrees]C if ASTM Type I cement is used without admixtures and the expected curing temperature is within 0 and 40[degrees]C. However, the [T.sub.0] value used for decades and still used by most maturity instruments is -10[degrees]C, which is approximately the temperature at which the hydration of cement ceases.
Using the Arrhenius equation, the equivalent age at a specified temperature is computed as follows:
(2) [t.sub.e] = [t.summation over 0] exp[-E/R(1/[T.sub.a] - 1/[T.sub.s])] x [DELTA]t,
where [t.sub.e] is the equivalent age at a specified temperature [T.sub.s], days or hours; E is the activation energy of cement hydration, (kJ/mol); R is the gas constant (8.31 J/ mol K); [T.sub.a] is the average concrete temperature during time interval, [DELTA]t, K; [T.sub.s] is the specified temperature, K, and [DELTA]t is the time interval, days or hours.
According to ASTM C1074-98, for concrete made with type I cement without admixtures, an activation energy of 41.5 kJ/mol is recommended. For other conditions or when maximum accuracy is desired, the activation energy should be determined experimentally.
The ASTM standard provides procedures for developing the strength-maturity relationship and for estimating the in-place strength. In addition, a procedure is provided for obtaining the datum temperature or...