Design Of A Testing System For Concrete Tensile Tests Under Controlled Temperature And Humidity Conditions And Verification Of Primary Data

The cracking of concrete is usually governed by concrete’s tensile stress. Tensilecreep and tensile elastic deformation are the principle mechanisms to relieve thetensile (cracking) stress. The tensile creep test and the tensile E-modulus test—withthe applied loads in the elastic range—deal with very small load-induceddeformations, which are difficult to detect. Also, the chemical and physical actionswithin the concrete (especially in the early age), coupled with the environmentalinfluence from without, always result in complex volume change (such as autogenousshrinkage and drying shrinkage). The volume instability poses further complicationfor measurement. Yet these tests are crucial to the understanding of the tensile elasticdeformation and tensile creep. Accurate prediction of cracking and deformation restson correct assessment of tensile elastic deformation and tensile creep.For this purpose, a testing system was contrived to perform the tensile strengthtest, the E-modulus test, and the tensile creep test on early age concrete specimensunder controllable temperature and relative humidity conditions. The system used aclosed-loop servo-electric loading system, which could control the loading rate andthe strain rate. The loading system could operate continuously for many weeks, thusoffering a stable testing platform for early age concrete tensile creep tests. The testingsystem also employed a set of LVDT displacement probes, and a removableenvironment chamber, which is capable of controlling the moisture and temperatureindependently. With such a system, the elastic tensile deformation and tensile creepcan be measured; and the influence of temperature and humidity on creep may also beisolated and observed.In this study, different concrete mixtures were tested for tensile strength,E-modulus, and basic tensile creep. The primary data were used to verify thereliability of the system. The tensile strength data were compared with the split tensilestrength (calculated from compressive strength), and (for some specimens) with themeasured flexure tensile strength too. The tensile E-modulus was confirmed with calculated compressive E-modulus obtained from code equations. The instantaneouselastic E-modulus was derived from the initial elastic deformation of the creep tests.They were compared with the directly measured tensile E-modulus data, and thevalues showed resemblance. Moreover, the recorded basic creep data werecurve-fitted with modified B3model. A rheological-based model (i.e. Kelvin modelsin series) was also used to approximate the measured creep data. The retardationspectrum was also plotted. It graphically presented the E-moduli and the viscosities ofthe elements in the Kelvin chain, and the parameters of these (spring and dashpot)elements were of reasonable values. Thus the modeling results indicated the creepdata from this testing system were reliable and the system was capable of repeatableperformance.Some tentative attempts were made to test the tensile creep under elevatedtemperature. The trail tests, although merely explorative, were still valuable; they ledto the improvement of the hardware. In general, this system involved manycomponents and the operation was not flawless. It was of considerable effort to tuneand to enhance the performance of the system. After all, improving the testing systemwas also part of the work.The significance of this study is that it produced a versatile and stable platformto test the tensile properties of concrete, and it generated primary data for the tensilestrength, E-modulus properties, and basic creep of various concrete mixtures. Thesedata provided valuable indication for future creep tests to be conducted under avariety of temperature and humidity combinations.