How to Ascertain the Drying Shrinkage of Hardened Concrete

Objective of Shrinkage of Hardened Concrete

Drying Shrinkage of Hardened Concrete method of testing describes the process of ascertaining the drying shrinkage or modification in length of concrete’s specimen because of alteration in moisture content.

Apparatus

• An apparatus consisting of dial gauge with length measuring capacity equipped to measure accurate length for upto 0.005 mm.
• A drying oven which can be used to maintain a temperature of 50±10

Preparation of Test Sample

• The test sample should consist of specimens the length of which can range between 15 cm to 30 cm, breadth 7.5 cm and width 7.5 cm.
• Make sure to cast the specimens of concrete according to suggested dimension and keep it stored in moist air for 3 to 7 days.
• Place a 6.5 mm diameter steel ball at each end’s center of the specimen by drilling a shallow depression.
• Once the steel balls have been fixed, clean them and make sure to remove any cement and then apply lubricating grease in order to avoid

corrosion.

• Allow the cement to get hard by keeping the specimen in moist condition for a day.
• Following 24 hours, get the test specimen immersed into water of temperature 24 to 300 C for atleast 28 days.

Procedure of Testing

After 28 days, remove the specimen from the water. Now clean the balls of grease and measure the length of the specimen quickly to approximate 0.005mm using measuring apparatus.

Put the specimen in oven for approx 44 hours and after that remove the same from oven and make it cool for about 4 hours in a desicator. Once cooled, using a length comparator measure the length of the specimen. This reading has to be recorded as the ‘original wet measurement’.

Keep repeating this process, until you get a constant length, i.e. when the difference between the two successive length readings of the measurement is less than 0.01 mm for a 15 cm specimenand similarly greater for the larger specimen. Record the last reading as ‘dry measurement’.

Measure the length of the specimen that is aside the balls to the nearest 0.55 mm after dry measurement has been done. You can record this reading as the “dry length”.

Calculation

Th primary drying shrinkage is summed up as the deviation between the real wet measurement and the measurement which is dry wet, which is termed as a proportion of dry length.

Initial drying shrinkage = [(original wet measurement – dry measurement)/dry length] *100