Question:
How do we measure the cracking tendency of concrete?
Answer:
The AASHTO PP 34—Standard Practice for Estimating the Cracking Tendency of Concrete covers the determination of the cracking tendency of restrained concrete ring specimens. The procedure is mainly comparative and does not predict the cracking of concrete cast in a specific structure. It does, however, facilitate selecting concrete mixes that are less likely to crack.
The major advantage of the ring test, originally conceived in about 1940, is that all the material factors influencing cracking are present in one straightforward test procedure that simultaneously includes stress development, shrinkage, and creep at early ages, including those during the setting period.
The test method measures the strain in a steel ring as a surrounding concrete ring shrinks. Tensile stresses develop in the concrete and compressive stresses in the steel ring. The standard steel ring has a wall thickness of 0.5 in. (12.5 mm), an outside diameter of 12 in. (305 mm), and a height of 6 in. (152 mm). Strain gages are attached at four equidistant mid-height locations on the interior of the steel ring. A data acquisition unit automatically records each strain gage independently.
Typically, the samples are wet cured for 24 hours then the exterior side surfaces only are exposed to a standard drying environment with a constant air temperature of 73.4ºF ±3ºF (21ºC ±1.7ºC) and a relative humidity of 50 ±4 percent. Other curing conditions, however, can be used to simulate site conditions. Time-to-cracking is the age when strains measured by one or more of the strain gages mounted on the steel ring suddenly decrease. A strain decrease of more than 30 microstrain usually indicates cracking.
The measured stresses in the steel rings behave in one of three ways: (a) the stress in the specimen increases uniformly until cracking of the concrete occurs, (b) the stress increases initially to a given value, and then remains near a constant high value near the cracking strength, and (c) the stress increases initially, then decreases and remains nearly constant at a value well below the cracking strength. In case (c), large creep relaxation dissipates the tensile stresses. The goal is to formulate concrete mixtures that will not crack and will allow dissipation of accumulated stresses, as in case (c), since temperature and loading stresses will be superimposed during service.
More Information
For additional information about the cracking tendency of concretes, see NCHRP Report No. 380, Transverse Cracking in Newly Constructed Bridge Decks.
Answer contributed by Paul Krauss, Wiss Janney Elstner Associates, Inc. He may be contacted at [email protected] or 847-272-7400 for further information.