A. Durán-Herrera

Thermal properties of high-volume fly ash mortars and concretes:

As sustainability moves to the forefront of construction, the utilization of high-volume fly ash concrete mixtures to reduce CO2 emissions and cement consumption per unit volume of concrete placed is receiving renewed interest. Concrete mixtures in which the fly ash replaces 50% or more of the Portland cement are both economically and technically viable. This article focuses on a characterization of the thermal properties, namely, specific heat capacity and thermal conductivity, of such mixtures. Both the raw materials and the finished products (mortars and concretes) are evaluated using a transient plane source method. Because the specimens being examined are well hydrated, estimates of the specific heat capacity based on a law of mixtures, with a ‘bound water’ specific heat capacity value being employed for the water in the mixture, provide reasonable predictions of the measured performance. As with most materials, thermal conductivity is found to be a function of density, while also being dependent on whether the aggregate source is siliceous or limestone. The measured values should provide a useful database for evaluating the thermal performance of high-volume fly ash concrete structures.

Comparison of ASTM C311 Strength Activity Index Testing versus Testing Based on Constant Volumetric Proportions

Currently, the (pozzolanic) strength activity indices of fly ashes and natural pozzolans are typically evaluated using the procedures outlined in ASTM C311 [“Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use in Portland-Cement Concrete,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2007]. In this test, the 7 and 28 d compressive strengths of mortar cubes with a 20 % mass replacement of cement by pozzolan are compared to those of a control without pozzolan, at constant flow conditions. In its current form, this procedure confounds two other properties of the pozzolan with its strength activity, namely its density and its water-reducing/increasing capabilities. In this study, the current C311 testing procedure is evaluated against an alternative in which the 20 % fly ash replacement for cement is performed on a volumetric basis and the volume fractions of water and sand are held constant, which should provide a true evaluation of the strength activity index of the pozzolan, free of these confounding influences. Class C and Class F fly ashes, a natural pozzolan, and a sugar cane ash are evaluated using both approaches, with some significant differences being noted. For a subset of the materials, the strength measurements are complemented by measurements of isothermal calorimetry on the mortars to an age of 7 d. For the constant volumetric proportions approach, a good correlation is exhibited between the cumulative heat release of the mortar at 7 d and the measured 7 d strength, suggesting the potential to evaluate 7 d pozzolanic activity via calorimetric measurements on much smaller specimens.

Effect of a micro-copolymer addition on the thermal conductivity of fly ash mortars

In this study, a copolymer composed of hollow spherical particles with an average particle size of 90 µm was evaluated as a lightweight aggregate in Portland cement–fly ash mortars to improve the thermal conductivity (k) of the composite. Mortars were produced for three different water/binder ratios by mass (w/b), 0.4, 0.5, and 0.6. Optimized proportions were obtained for a minimum target compressive strength of 35 kgf/cm2 (3.4 MPa) according to the requirements of Mexican standards for nonstructural masonry units. Thermal conductivity was determined for dry and saturated samples through the transient plane technique with average results of 0.16 and 0.31 W/(m K), respectively. These values represent an increment of 23% and a reduction of 33% in comparison to an efficient Portland cement–based commercially available thermal insulator.

Autogenous Shrinkage in High Performance Concrete with Chemical Admixtures

Durability in High Performance Concrete have resulted in the development of admixtures to mitigate several concrete deterioration mechanisms. Due to the low water/binder, HPC enables autogenous shrinkage that could lead to cracks at early ages. Superabsorbent polymers, shrinkage reducing admixtures and corrosion inhibitor based on calcium nitrite were used in HPC with silica fume to evaluate autogenous shrinkage. SAP was added with an amount of intenal curing water determined by the modification proposed by Jensen to the Power’s Model. Results showed that admixtures could improve or aggravate autogenous shrinkage.