Kenneth C. Hover

Mercury porosimetry of hardened cement pastes

Abstract Mercury porosimetry was performed on 92 hardened cement paste specimens of water/cement (w/c) ratios 0.3, 0.4, 0.5, 0.6, and 0.7 and curing times of 1, 3, 7, 14, 28, and 56 days. This paper presents the experimental techniques, results, and their possible implications with respect to pore connectivity. As expected, longer curing times and lower w/c ratios resulted in smaller indicated total porosities and smaller threshold pore widths. Longer curing times and higher w/c ratios resulted in greater degrees of hydration. In most of the mercury intrusion results, two peaks could be observed in the differential curves that were identified as the “initial” and “rounded” peaks. The initial peak may correspond to the intrusion of mercury through a connected capillary network, while the rounded peak may correspond to the crushing of interposed hydration products.

APPLICATION OF MATURITY APPROACH TO SETTING TIMES

The influence of temperature on setting times was studied using the maturity approach and the Freiesleben-Hansen and Pederson maturity function. This function requires a value for the apparent activation energy (Ea) of cement hydration. A method to estimate Ea at the early stages of hydration was developed, therefore, based on setting times measured by the penetration resistance method (American Society for Testing and Materials C 403). An experimental verification of the proposed method was carried out. A mortar mixture was cast and cured at various temperatures. It was concluded that Ea for the mixture studied at the early stages of cement hydration could be successfully estimated by setting times and that the maturity approach could be used to predict variations in setting times caused by different curing temperatures.

MERCURY POROSIMETRY OF CEMENT-BASED MATERIALS AND ASSOICATED CORRECTION FACTORS

The paper gives a brief review of the technique of mercury porosimetry, and presents assumptions and corrections relevant to the application of mercury porosimetry to cement-based materials. The paper examines the impact of these corrections and assumptions on calculations based upon porositmetry data. The paper shows that assumptions and corrections employed in processing porosimetry data have a significant effect on porosimetry results, affecting total porosity values, threshold pore size values, and curve shape. Different assumptions and corrections can, therefore, lead to significantly different predictions of strength, permeability, and durability.

Insitu identification of ASR products in concrete

Abstract A new technique to identify ASR products insitu is presented. It is proposed that the ASR products found in concrete be visualised as gelatinous silica, and its crystalline modifications, with adsorbed sodium, potassium and calcium ions. Based on this view a brief survey of the adsorption and cation exchange properties of silica gel is presented. It is shown that a wide variety of cations can replace previously adsorbed Na + , K + or Ca ++ ions on silica gel. The uranyl ion (UO 2 ++ ) is one such species, and is chosen for the proposed technique for gel identification. Under ultraviolet light, the uranyl ion flouresces and identifies the areas where gel is present. The technique is applied to reactive aggregates, reactive aggregates treated with NaOH and mortar bars made with reactive aggregates. The limitations of the technique and its future refinements are discussed.

Mercury porosimetry of cement-based materials and associated correction factors

Abstract This paper reviews briefly the technique of mercury porosimetry, presents assumptions and corrections relevant to the application of mercury porosimetry to cement-based materials, and examines the impact of these corrections and assumptions on calculations based upon porosimetry data. A generalized form of the Washburn equation is given; corrections are given for machine expansion, differential mercury compression, sample compression and hydrostatic pressure; and values appropriate for cement-based materials are given for contact angle, surface tension and pore shape. Numerical relationships that predict strength, permeability and durability on the basis of pore structure information are then given, and these equations are used to assess quantitatively the impact of different assumptions and corrections. It is shown that assumptions and corrections employed in processing porosimetry data have a significant effect on porosimetry results, affecting total porosity values, threshold pore size values and curve shape. Different assumptions and corrections can, therefore, lead to significantly different predictions of strength, permeability and durability. Thus, researchers should include details on the assumptions and corrections used when presenting porosimetry results to enable comparisons with other work.

Rapid evaporation from freshly cast concrete and the Gulf environment

Abstract In hot dry climates, such as exist in the Arabian Gulf region, plastic shrinkage cracking can develop when the rate of evaporation exceeds the rate at which bleed water rises to the concrete surface. Furthermore, with or without plastic shrinkage cracking, surface drying can still lead to a reduction in abrasion resistance and an increase in porosity and permeability at the critical near surface, or ‘cure affected zone’. This paper discusses evaporation and surface drying in hot, dry environments, as in the Arabian Gulf region, based on experimental measurements of the rates of both evaporation and bleeding. Measured rates of evaporation are compared to predicted values obtained from the Menzel equation, the most commonly used formula for this purpose. The paper concludes by pointing out the precautions to be taken in order to prevent exposing recently placed concrete to excessive drying.

CONCRETE MIXTURE PROPORTIONING WITH WATER-REDUCING ADMIXTURES TO ENHANCE DURABILITY: A QUANTITATIVE MODEL

Abstract Durable concrete is characterized as concrete with a low porosity, in which the individual grains of cement are tightly packed before initiation of hydration. Such concrete is typically characterized by a low water cement ratio, which is described herein on a volumetric, rather than a mass basis. Low water cement ratio is responsible for improved mechanical properties and for enhanced durability. It is shown, however, that use of a low water cement ratio necessitates either a sacrifice in workability, or the use of high cement content, neither a desirable consequence. The use of water-reducing admixtures is discussed as an alternative. The use of normal-, mid-, and high-range waterreducing admixtures is described in the context of a series of graphs based on the water demand relationships implicit in the ACI 211.1 procedure for proportioning mixtures. By introducing the concept of water-reducer ‘effectiveness’, these graphs can be used to describe the differences among normal, mid-range, and high-range water reducers, and can be used to illustrate the use of water reducers to reduce water content, reduce water cement ratio, and to increase workability.

Experiments on the contact angle between mercury and hardened cement paste

Abstract Experiments on the contact angle of mercury with hardened cement paste (HCP) were performed. Circular holes of 150 to 1,050 micrometers in diameter were formed by casting HCP around small diameter wires. The pressure required to intrude the unevacuated, optically measured holes was used to determine a range of upper-bound values. For HCP, this was found to be from 139° to 150°. Furthermore, the contact angle was not significantly affected by the cleanliness of the mercury, the water-cement ratio of the paste, nor vacuum drying of the HCP.

Evaporation of Water from Concrete Surfaces

This article explores the background and application of the evaporation rate nomograph, to better understand its use in predicting the rate of evaporation on concrete. The origins of the nomograph, designed more than 200 years ago, are explored. Also explored is the current form of the nomograph, which is based on observable evaporation rates at Lake Hefner in Oklahoma between 1950 and 1951.

Effect of Silica Fume and Superplasticizer Addition on Setting Behavior of High-Strength Mixtures

An investigation was performed to assess the influence of silica fume and superplasticizer on setting behavior of high-strength concrete mixtures. Both admixtures were added in various proportions to a mortar mixture, and setting times were determined by means of penetration resistance of freshly cast mortar. The influence of temperature was also investigated by exposing mortar specimens to different storage temperatures. Silica fume was found to shorten initial set times, whereas superplasticizer was found to delay initial set. Statistical analysis identified an interaction effect between these admixtures: the influence of one admixture on initial set depended on the amount of the other admixture. Mortar temperature not only affected initial and final set times, but also influenced the time interval between initial and final set.