The software's backbone
Concrete Works models the three possible ways heat may be transferred—conduction, convection, and radiation. The conduction portion uses nonlinear material thermal properties that are dependant on the materials selected, time, and temperature. The convection portion accounts for the roughness of different surfaces (concrete, wood, steel, blankets) and the effect of wind speed. The radiation portion models the solar radiation, surface shading (for example, the bottom of a bent cap would be shaded), atmospheric radiation, ground-emitted radiation, and irradiation from the concrete member's surface.
Together, these modules allow the user to quickly and easily model different construction sequences and materials to ultimately build better, longer lasting, and hopefully more cost-effective concrete members. Mixture-specific heat of hydration values are used to accurately model the effect of various cementitious materials on the in-place concrete temperature distribution. The model has been calibrated with over 33,000 hours of temperature data collected from 12 concrete members instrumented in Texas.
The user selects the mixture proportions on this screen.
Some of the unique tests performed as part of this project are semi-adiabatic calorimetry (250 tests performed), isothermal calorimetry (over 1000 tests performed), cracking frames (75 tests performed), rapid chloride permeability (ASTM C 1202, 300 tests performed), compressive strength (ASTM C 39), splitting tensile strength (ASTM C 496), modulus of elasticity (ASTM C 469), and time of set (ASTM C 403).
Estimating the cracking risk of concrete
Estimating the risk of thermal cracking is not as simple as breaking a few cylinders, or performing a few calculations on the back of an envelope. Extensive testing and computational models are needed to assess this complex phenomenon.
In mass concrete, the tensile strength increases as the hydration of the cementitious system progresses and is strongly affected by the type of cementitious material, the water-to-cementitious materials ratio, the aggregate type, and the maturity level of the hardening concrete. The restraint that leads to cracking is primarily due to the thermal gradient that exists between the core and the surface of the structure. This thermal gradient causes differential thermal stress that may exceed the tensile strength capacity of the concrete.
Next, the user inputs various construction details.
The development of in-place stresses are influenced by the effective modulus of elasticity (a time-dependent measure of stiffness development), the coefficient of thermal expansion, setting characteristics, restraint conditions, and temperature history of the hardening concrete. Trying to quantify these variables is complicated at early ages, and many have complex interactions.
For the TxDOT research project, an early-age cracking frame test is being used to evaluate the 35° F maximum temperature differential criteria. The susceptibility of hardening concrete to cracking can be assessed by this cracking frame that restrains a concrete specimen against movement. These frames are designed to allow fresh concrete to be cast into molds within each frame, which enable the study of very early-age behavior of concrete mixtures. Shrinkage effects can be assessed in actual concrete specimens, and the information will be used in ConcreteWorks to quantify the cracking sensitivity of various mixtures used in mass concrete applications.