Shrinkage resulting from moisture loss is a common property of most materials. Drying shrinkage of concrete is a well known occurrence that engineers account for in their design. Creep is the additional amount, beyond drying shrinkage, which concrete shrinks under load over an extended period of time. Creep testing measures how much additional length change occurs for specific concrete mixtures when exposed to long-term stress.
Shrinkage happens because the moisture within concrete dissipates over time, a process that can continue for years. Creep also gradually changes the physical dimensions of finished concrete and in certain applications, engineers must account for creep in their structural designs.
Creep is a critical concern in construction projects where concrete will change dimension such as height of a tall tower on a cable stay bridge. The engineer needs to know the final height of the tower after the loading, shrinkage and creep.
Successful creep tests will provide a piece of information to predict the final geometry of the concrete structure after the loading, shrinkage and creep have stabilized.
Concrete Creep Testing: Getting it Right
These five factors will play a role in concrete creep testing:
1. Lab setting. The laboratory must carefully control the temperature, and relative humidity in the testing area. CTL Engineering, for instance, conducts creep tests at 23 degrees C and 50 percent humidity in accordance with ASTM C512. The temperature range is tightly controlled at plus or minus 1 degree. The humidity is plus or minus 4%. Other environmental conditions are available by special arrangement.
2. Length of testing time. Test durations can last up to two years, depending on the application. The creep model used by the engineer can be accommodated by establishing the parameters for a specific application. The testing must have the proper duration their samples require.
3. Size and Number of the concrete samples. The sample size specified by ASTM C512 is a 6-inch diameter cylinder sample. It is recommended to test three samples for each condition and average the results.
4. Accuracy of testing instruments. Creep happens in very small increments that must be measured in the lab. CTL’s lab uses a Whitmore strain gauge that records to one-ten-thousandth in 10 inches. That’s just one example of the tight tolerances creep testing can require.
5. Need for compressive strength tests. Creep is linear proportional to stress up to 40% of the concrete compressive strength. Creep tests require companion cylinders for shrinkage and compressive strength.
Why CTL is the Right Choice for Creep Tests
CTL started performing creep tests over 30 year ago. CTL has the experienced lab personnel, the high-precision equipment, capacity, and flexibility to handle a range of creep-testing applications that are pivotal to the design of concrete structures.