This test method describes the procedure for determining the drying shrinkage or change in length of concrete specimen due to changes in moisture content.
A length measuring apparatus incorporating a dial gauge capable of measuring the length accurately to 0.005 mm. (see fig shown below)
Drying oven capable of maintaining a temperature of 50±10
Test Sample Preparation
A test sample consists of specimens which cast conforming to the following dimension.
Can vary from 15 cm to 30 cm
Cast concrete specimens as per the required dimension and store it in moist air for 3 to 7 days.
Drill a shallow depression at the center of each end of the specimen so that it can accommodate a 6.5 mm diameter steel ball. These steel balls are fixed in to this depression by cementing the balls with neat rapid hardening Portland cement.
After fixing the steel balls, clean the balls to remove any cement adhering to it and then apply lubricating grease to prevent corrosion.
Then the test specimens are kept in moist condition for at least 24 hours in order to allow the cement to harden. After 24 hours, immerse the test specimens in water maintained at a temperature of 24 to 300 C until 28 days. (Remember the no o f days are counted starting from the day when the specimens are casted)
After removing the specimens from water, clean the grease from the balls and immediately measure the length of the specimen to an accuracy of 0.005mm by the length measuring apparatus.
Then dry the specimens in oven for at least 44 hours. After 44 hours of drying, remove it from oven and cool it for at least 4 hours in a desicator. After cooling, again measure the length of the specimen using the length comparator. Record this reading as the ‘original wet measurement’.
Repeat the process of oven drying, cooling and measuring until constant length is attained, that is , when the difference between two consecutive length measurement readings is less than 0.01 mm for a 15 cm specimen and proportionately greater for larger specimen. Record the final reading as ‘dry measurement’.
After the dry measurement has been taken, measure the length of the specimen adjacent to the balls to the nearest 0.5 mm and record this as the ‘dry length’.
The initial drying shrinkage or drying shrinkage is calculated as the difference between the original wet measurement and the dry measurement expressed as a percentage of dry length.