A finite element model is used to examine how the properties of cementitious mortar are related to the stress development in the dual ring test. The results of this investigation are used to explain the thermal cracking behavior of mixtures containing prewetted lightweight aggregates (LWA) by quantifying the contribution of several material properties individually. In addition to the beneficial effects of using the LWA as an internal curing agent to reduce the autogenous shrinkage of concrete, the LWA also helps to reduce the potential for thermal cracking due to a lower elastic modulus and increased stress relaxation. The rate of stress development, age of cracking, and magnitude of the temperature drop necessary to induce cracking in a dual ring specimen are dependent on a variety of factors, including the coefficient of thermal expansion of both the cementitious mortar and the restraining rings, elastic modulus of the mortar, creep effect of the mortar, and rate of thermal loading. Depending on the rate of cooling, cracking may or may not occur. The slowest rate of cooling (C/h) minimizes the effects of creep while cooling rates faster than C/h can produce a thermal gradient through the mortar cross-section that needs to be considered.

these materials to be used and specified more widely in Indiana. It is recommended that a training video be developed that highlights the benefits of this material, describes its use, and discusses important features associated with placement and testing

A new test is being developed to evaluate the performance of concretes undergoing both expansion and shrinkage during hydration and/or temperature changes under mechanically restrained conditions. The standard restrained ring test (ASTM C1581-09) is a simple, economical method to evaluate a concrete mixture’s susceptibility to develop shrinkage cracking when it is restrained. The standard test only provides restraint against samples that shrink at isothermal temperature and cannot be used to characterize materials undergoing expansion. Further, the conventional restrained ring test is difficult to apply in cases in which the concrete undergoes large variations in temperature since the restraint changes dimension under heating and cooling. This paper describes a test that was designed to overcome both of these limitations. The new dual concentric ring test provides restraint for both shrinkage and expansion and has incorporated the ability to study thermal stresses by using a restraining ring having a very low thermal expansion coefficient. This paper will discuss the design and construction of the test device and will present preliminary data that characterizes its ability to quantify and evaluate restrained expansive, shrinkage, and thermal stresses.

mixture (paste, mortar, or concrete) around a steel ring. Residual tensile stress develops in the sample as it attempts to shrink but is restrained by the ring. A crack results if the stress that develops due to restraint (called "residual stress" by some) exceeds the developing tensile strength. The standard 18 suggests that the relative cracking potential of mixtures can be quantified by comparing the amount of time required to crack the samples. Shorter measured cracking times indicate a relatively higher cracking potential, whereas longer cracking times indicate a lower cracking potential. While ASTM C1581-04 18 describes a material behavior in terms of the age of cracking, the authors believe that describing the stress development, stress-to-strength ratio, or probability of cracking may be better indicators of cracking susceptibility. 19 While the standard restrained ring test is a useful standardized test, 20 it is limited in at least three ways. First, it should be operated at a constant temperature because changing the temperature of a standard ring would significantly expand or shrink the restraining ring, thereby altering the degree of restraint and stress. This effect would make it difficult to isolate and study the behavior of the concrete under the changing temperature conditions that are commonly encountered in a field structure. The second limitation is that the standard restrained ring test is a passive test. 21,22 As a result, the test waits for the shrinkage of the specimen to generate enough stress to induce cracking. This provides useful timeto-cracking comparisons between multiple mixtures and can permit stress development to be calculated, but determining the probability of cracking 16 is more difficult. 23,24 Further, the passive behavior of the test may require considerable time to induce cracking. Third, the restrained ring test can only provide restraint against samples that shrink. Samples that exhibit net expansion typically come out of contact with the restraining ring and expand freely. This limitation becomes significant when studying expansive cements, 25 shrinkagereducing admixtures, 26-28 and internal curing, 7,8,10,12,29 as these technologies often produce early-age expansion. A dual-ring testing device 30 can be used to extend the applicability of the restrained ring test. The dual-ring device, as its name implies, incorporates a second restraining ring that is located on the outer face of the sample. This additional ring provides restraint against expansion and captures the restrained behavior of expansive samples. The dual ring used in this study can also be used to study the effects of Concrete can develop tensile stress when it is restrained from shrinking freely. Standard tests, such as the restrained ring test (ASTM C1581), can be used to quantify how likely it is that a mixture will crack due to the stresses developed under constant temperature conditions. The standardized restrained ring test is a passive test where the residual stress that develops due to restraint ca...

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