CONCRETE COMPRESSIVE STRENGTH VARIATION WITH TIME
Effect of Age on Concrete Strength
The compressive strength of concrete is affected with its age. The concrete member capacity is usually a function of concrete cross-section dimensions, the steel bar area, the concrete compressive strength, and the steel reinforcement yield strength. After some years, the concrete element will deteriorate due to corrosion of the steel bars, which will reduce the steel cross-section dimension. The yield strength will be the same with time, but the concrete cross-section dimension will be less effective due to concrete cracks and high reduction on the section in cases of falling concrete cover.
There is something to gain: The concrete strength. Much research has discussed this in detail based on environmental conditions, which need to be addressed in detail in order to have a sense about reinforced concrete structure’s reliability. In practice, one can see a deteriorated structure with corrosion on the steel bars without complete failure. With time, the concrete strength is increased; this increase in strength compensates for some of the member strength due to reduction in the steel cross-section area. One should not be satisfied with making a concrete design because of knowledge that the steel bars are carrying more load; any reduction on it will be very risky, so one should not depend on gaining strength on concrete with time. However, it does need to be taken into consideration.
Much research has been done to predict concrete strength after 28 days. In the majority of cases, the tests are conducted at concrete age of 28 days, when its strength is considerably lower than its long-term strength will be. Different methods have been suggested to predict concrete strength with age and different codes have different recommendations for predicting this strength. For example, Baykof and Syglof (1976) compared the gain in strength of concrete specimens stored in wet and dry conditions. They found that, in dry conditions, after 1 year there is no increase in concrete strength, as shown in figure 1. On the other hand, the strength of specimens stored in a wet environment (at 15°C) is considerably increased (fig. 1).
Figure 1: variation of concrete strength with time
In Madison, Wisconsin, Washa and Wendt (1975) tested concrete specimens stored in special environmental conditions to predict the concrete strength with age and found a valuable result. The specimens were moist cured for 28 days before placement outdoors on leveled ground in an uncovered, open location. Thermocouple data indicate that the outdoor compressive cylinders were subjected to about 25 cycles of freezing and thawing each winter. The relative humidity normally varied from 65–100%, with an average of 75%. The annual precipitation, including snowfall, was about 32 in. Air temperatures usually ranged between 25 and 90°F (32 and 35°C). The average compressive strength with time is shown in figure 2.
Figure 2: Concrete compressive strength variation with time
Washa and Wendt (1989) concluded that the compressive strength of concrete cylinders made with cement that had a relatively low C2S content, had a high surface area, and was stored outdoors for 50 years generally increased as a logarithm of the age for about 10 years. After 10 years, the compressive strength decreased or remained essentially the same. MacGregor (1983) used the same study and formulated an equation of the relation between compressive strength and age; in the case of 28-day-specified compressive strength of concrete, this is equal to 281.5 kg/cm2 (27.6 Mpa). The corresponding mean of compressive strength at 28 days is 292.7 kg/cm2 (28.7 Mpa):
f(t) = 158.5+40.4ln(t) kg/cm2 for t<10 years
f(t) = 489 kg/cm2 t>10 years.
where fc(t) is the concrete compressive strength with time and t is the time in days.