Dale P. Bentz

Protected paste volume in concrete: Extension to internal curing using saturated lightweight fine aggregate

Abstract One difficulty in the field use of high-performance concrete is the extensive self-desiccation and autogenous shrinkage that may occur due to its low water/cement ratio and the addition of silica fume to the mixture proportions. Several researchers have proposed the use of saturated lightweight aggregates to provide “internal” curing for the concrete. In this communication, simple equations are developed to estimate the replacement level needed to ensure adequate water for complete curing of the concrete. Additionally, a three-dimensional concrete microstructural model is applied to determine the fraction of the cement paste within a given distance from the lightweight aggregate surfaces. The simulation results are compared with analytical approximations developed previously. This new concept for curing is similar to the protected paste volume concept conventionally applied to characterizing air void systems in air-entrained concrete.

Water permeability and chloride ion diffusion in portland cement mortars: Relationship to sand content and critical pore diameter

The pore structure of hydrated cement in mortar and concrete is quite different from that of neat cement paste. The porous transition zones formed at the aggregate-paste interfaces affect the pore size distribution. The effect of the sand content on the development of pore structure, the permeability to water, and the diffusivity of chloride ions was studied on portland cement mortars. Mortars of two water-to-cement ratios and three sand volume fractions were cast together with pastes and tested at degrees of hydration ranging from 45 to 70%. An electrically-accelerated concentration cell test was used to determine the coefficient of chloride ion diffusion while a high pressure permeability cell was employed to assess liquid permeability. The coefficient of chloride ion diffusion varied linearly with the critical pore radius as determined by mercury intrusion porosimetry while permeability was found to follow a power-law relationship vs. this critical radius. The data set provides an opportunity to directly examine the application of the Katz-Thompson relationship to cement-based materials.

Percolation of phases in a three-dimensional cement paste microstructural model

Abstract A three-dimensional digital-image-based simulation model of cement hydration is used to study the percolation or connectivity of phases as a function of hydration. Results from an investigation of the effects of water-to-cement ratio, degree of hydration, and the substitution of inert and pozzolanic mineral admixtures for cement, on the connectivity of the capillary porosity are presented. For all scenarios studied, plotting pore connectivity vs. total porosity results in a single universal curve. Based on this curve, the degree of hydration required to achieve pore discontinuity as a function of water-to-cement ratio and pozzolanic mineral admixture concentration has been determined. Similar universal curves have been obtained for the connectivity of the calcium silicate hydrate and calcium hydroxide phases in hydrated neat cement when plotted against the appropriate phase fraction. Simulation results are analyzed using percolation theory, and are applied to interpreting observed experimental results concerning cement properties as a function of hydration.

Computer simulation of the diffusivity of cement-based materials

A digital image-based model of the microstructure of cement paste, coupled with exact transport algorithms, is used to study the diffusivity of Portland cement paste. The principal variables considered are water∶cement ratio, degree of cement hydration and capillary porosity. Computational methods are described and diffusivity results are presented, which are found to agree with the available experimental measurements within experimental error. Model cement pastes prepared with different water∶cement ratios, and having different degrees of hydration, are found to have diffusivities that lie on a single master curve when plotted as a function of capillary porosity. Concepts from percolation theory are used to explain quantitatively the dependence of diffusivity on capillary porosity. The effect of silica fume addition on diffusivity is also examined.

Percolation and pore structure in mortars and concrete

Abstract The cement paste in concrete and mortar has been shown to have a pore size distribution different than that of plain paste hydrated without aggregate. For mortar and concrete, additional porosity occurs in pore sizes larger than the plain pastes threshold diameter as measured by mercury intrusion. Based on the assumption that these larger pores are essentially present only in the interfacial zones surrounding each aggregate, an experimental program was designed in which the volume fraction of sand in a mortar was varied in a systematic fashion and the resultant pore system probed using mercury intrusion porosimetry. The intrusion characteristics were observed to change drastically at a critical sand content. Similar results are observed for a series of mortar specimens in which the cement paste contains 10% silica fume. To better interpret the experimental results, a hard core/soft shell computer model has been developed to examine the percolation characteristics of these interfacial zone pores. Using the model, interfacial zone percolation in concretes is also examined. Finally, the implications of interfacial zone percolation for transport properties and durability of mortar and concrete are discussed.

Effects of cement particle size distribution on performance properties of Portland cement-based materials

The original size, spatial distribution, and composition of Portland cement particles have a large influence on hydration kinetics, microstructure development, and ultimate properties of cement-based materials. In this paper, the effects of cement particle size distribution on a variety of performance properties are explored via computer simulation and a few experimental studies. Properties examined include setting time, heat release, capillary porosity percolation, diffusivity, chemical shrinkage, autogenous shrinkage, internal relative humidity evolution, and interfacial transition zone microstructure. The effects of flocculation and dispersion of the cement particles in the starting microstructures on resultant properties are also briefly evaluated. The computer simulations are conducted using two cement particle size distributions that bound those commonly in use today and three different water-to-cement ratios: 0.5, 0.3, and 0.246. For lower water-to-cement ratio systems, the use of coarser cements may offer equivalent or superior performance, as well as reducing production costs for the manufacturer.

Shrinkage-Reducing Admixtures and Early-Age Desiccation in Cement Pastes and Mortars

Fundamental studies of the early-age desiccation of cement-based materials with and without a shrinkage-reducing admixture (SRA) have been performed. Studies have been conducted under both sealed and drying conditions. Physical measurements include mass loss, surface tension, X-ray absorption to map the drying profile, internal relative humidity (RH), and autogenous deformation. Interestingly, although the SRA accelerates the drying of bulk solutions, in cement paste with a water-to-cement (w/c) ratio of 0.35, it actually reduces the measured drying rate. Based on the accompanying X-ray absorption measurements and a simple three-dimensional microstructure model, an explanation for this observation is proposed. In sealed systems, at equivalent hydration times, the SRA maintains a greater internal RH and reduces the induced autogenous deformation. Thus, these admixtures should be beneficial to low w/c ratio concretes undergoing self-desiccation, in addition to their normal usage to reduce drying shrinkage.

Analytical Formulas for Interfacial Transition Zone Properties.

Abstract Two analytical results are presented that are of use to concrete material technologists. Using a model of concrete in which the aggregates are spherical, but with an arbitrary size distribution, a result from statistical geometry can be used to accurately give the total interfacial transition zone (ITZ) volume for any width ITZ and any volume fraction of aggregates. In reality, the ITZ contains a gradient of porosity and therefore a gradient of properties. When only a small volume fraction of aggregates is present (called the dilute limit), it is possible to analytically solve for the effect of the ITZ on the overall concrete properties. This calculation can be carried out for the effective linear elastic moduli, linear electrical conductivity/ionic diffusivity, and linear thermal/moisture shrinkage/expansion. The details of the calculation are summarized and applications described.

Mitigation Strategies for Autogenous Shrinkage Cracking

Abstract As the use of high-performance concrete has increased, problems with early-age cracking have become prominent. The reduction in water-to-cement ratio, the incorporation of silica fume, and the increase in binder content of high-performance concretes all contribute to this problem. In this paper, the fundamental parameters contributing to the autogenous shrinkage and resultant early-age cracking of concrete are presented. Basic characteristics of the cement paste that contribute to or control the autogenous shrinkage response include the surface tension of the pore solution, the geometry of the pore network, the visco-elastic response of the developing solid framework, and the kinetics of the cementitious reactions. While the complexity of this phenomenon may hinder a quantitative interpretation of a specific cement-based system, it also offers a wide variety of possible solutions to the problem of early-age cracking due to autogenous shrinkage. Mitigation strategies discussed in this paper include: the addition of shrinkage-reducing admixtures more commonly used to control drying shrinkage, control of the cement particle size distribution, modification of the mineralogical composition of the cement, the addition of saturated lightweight fine aggregates, the use of controlled permeability formwork, and the new concept of “water-entrained” concrete. As with any remedy, new problems may be created by the application of each of these strategies. But, with careful attention to detail in the field, it should be possible to minimize cracking due to autogenous shrinkage via some combination of the presented approaches.

Influence of silica fume on diffusivity in cement-based materials: I. Experimental and computer modeling studies on cement pastes

Experimental and computer modeling studies are applied in determining the influence of silica fume on the microstructure and diffusivity of cement paste. It is suggested that silica fume modifies the inherent nanostructure of the calcium silicate hydrate (C-S-H) gel, reducing its porosity and thus increasing its resistance to diffusion of both tritiated water and chloride ions. Because the pores in the C-S-H are extremely fine, the relative reduction in diffusion depends on the specific diffusing species. Based on the NIST cement hydration and microstructural model, for tritiated water diffusion, the reduction in the diffusivity of the gel caused by silica fume is about a factor of five. For chloride ions, when a diffusivity value 25 times lower than that used for conventional high Ca/Si ratio C-S-H is assigned to the pozzolanic lower Ca/Si ratio C-S-H, excellent agreement is obtained between experimental chloride ion diffusivity data and results generated based on the NIST model, for silica fume additions ranging from 0% to 10%. For higher addition rates, the experimentally observed reduction in diffusivity is significantly greater than that predicted from the computer models, suggesting that at these very high dosages, the nanostructure of the pozzolanic C-S-H may be even further modified. Based on the hydration model, a percolation-based explanation of the influence of silica fume on diffusivity is proposed and a set of equations relating diffusivity to capillary porosity and silica fume addition rate is developed. A 10% addition of silica fume may result in a factor of 15 or more reduction in chloride ion diffusion and could potentially lead to a substantial increase in the service life of steel-reinforced concrete exposed to a severe environment.