A model that simulates the two-dimensional (2-D) coupled heat and mass transfer phenomena in heated concrete is proposed. A fully implicit finite difference (FD) method was used in the discretization of the partial differential equations in both domain and time. The control volume approach was employed in the formulation of the FD equations, ensuring both local and global conservation properties are satisfied by the numerical solution. The solid, liquid, and gaseous (both air and vapor) phases are considered, including evaporation, condensation, and dehydration. The discretized equations of all species along with the temporal discretization of an interior node, surface node, and corner node are presented. Numerical case studies based on an object-oriented code for extremely rapid heating of concrete and nonsymmetric boundary conditions are validated against experimental results. Temperature, pressure, and moisture contours indicate the model’s ability to capture the complex 2-D behaviors of fire-exposed concrete over the entire flow domain.

This paper presents a model developed to predict the internal relative humidity (RH) of concrete during drying. The model makes use of simplified inputs, including the concrete mixture design and the cement Bogue composition, thus making it accessible to engineers and practitioners. These inputs are used to separately determine the permeability of both liquid and vapor phases, hence solving for moisture transport through an empirical derivation of the (de) sorption isotherm, total porosity, and pore tortuosity. The model is validated using previously published literature data as well as experiments designed specifically for model validation. The model was found successful in predicting RH profiles for the validation data with the simple inputs required. However, it was found that in cases where the standardized ASTM F2170 method is used to measure RH, the agreement between the model and experimental data decreases. This was found to be related to errors associated with performing humidity measurements within cavities drilled in concrete. Such errors are discussed, and room for improvement in in-place humidity measurements is proposed. Finally, the model is used to validate the use of RH measurements at a specific concrete depth to evaluate the susceptibility of moisture-sensitive flooring to failures.

The experimental program reported in this paper sought to evaluate the efficiency of a range of curing methods in view of minimizing the evaporation rate at the surface of freshly placed shotcrete and preventing the detrimental consequences of early-age shrinkage. CSA A23.1-14 states that severe drying conditions should be considered to exist when the surface moisture evaporation rate exceeds 0.50 kg/m2/h (0.1 lb/ft2/h). In fact, the environmental conditions that lead to such evaporation rates are regularly experienced on construction sites, requiring that adequate protection of the concrete surface be carried out in a timely manner after placement. This research effort is aimed at quantifying the influence of selected curing methods upon the early-age moisture loss and the resulting shrinkage. The results show that early-age volume change of freshly sprayed shotcrete can be significantly reduced by adequate surface protection. Among the investigated methods, moist curing is found to be the most effective.

In this study, different types of surface coatings were applied to concrete to assess their suitability for resisting physical salt attack (PSA). Concretes with different water-binder ratios (w/b) were tested and severe PSA conditions were implemented, using sodium sulfate, to obtain conclusive trends on the performance of coatings. Visual assessment and mass loss of concrete specimens were used as physical indicators to quantify the damage, while mineralogical and microstructural studies were conducted to elucidate the damage mechanisms. Epoxy, ethyl silicate, and acrylic emulsion coatings were found successful at protecting concrete from PSA regardless of the quality of the substrate concrete, while other coatings tested were highly dependent on the concrete quality. Coatings that permit a high rate of absorption and/or desorption (evaporation) led to more severe PSA damage compared with coatings with low absorption/desorption.

Use of high-strength concrete (HSC) in built infrastructure allows efficient structural systems with higher strength and durability. However, HSC being sensitive to high temperatures results in reduced residual strength after fire exposure, which impairs postfire structural serviceability. This lower performance of HSC results from its characteristic dense microstructure that prohibits dissipation of pore pressure at high temperatures. Under fire conditions, similar to sublimation (melting) of polypropylene fibers in HSC, air entrainment can reduce microstructural damage by allowing dissipation of vapor pressure. In this study, residual mechanical and physical properties of air-entrained HSC (AEH) were investigated after exposure to high temperatures up to 800°C (1472°F). Residual mechanical properties comprised of compressive and splitting tensile strength, stress-strain response, elastic modulus, and changes in physical properties consisted of mass loss, cracking behavior, and microstructural changes. Results show that AEH retains better residual mechanical properties after exposure to high temperatures; however, a higher air content proved less beneficial.