International Concrete Abstracts Portal

This paper reports on part of a substantial research programme on properties of condensed silica fume (CSF) concretes cured in temperate and climates, carried out in the Department of Civil Engineering at Loughborough. The hot The research approach was to investigate mixtures proportioned to have equal workability and 28 day strength (when water cured at 20°C). This paper examines the effect of superplastizer, curing method (water and polythene) and curing environment (temperate and hot) on the compressive strength, permeability and pore structure of 40 MPa concretes. More specifically, the paper contrasts the performance of two 15% CSF mixtures (replacement by weight of cement) where workabilities were controlled by the addition of extra water or superplasticizer. The development of the concretes’ strength and subsurface permeability index (air and water) with age (from 7 to 180 days) is described, together with the intrinsic permeability (air and water) and pore structure of their equivalent mortar fraction. The use of superplasticizer to control workability increased the compressive strength of CSF concrete mixtures by around 18% and 10% in the temperate and hot environments respectively. The super-plasticized concrete had lower air and water permeabilities which is attributed to an improved pore structure as confirmed by mercury intrusion porosimetry date. The improvements were more marked in the CSF concretes cured in a hot environment.

Various types of superplasticizer that maintain concrete slump for longer periods have been extensively investigated. A new type of superplasticizer with high-range water-reducing slump-maintaining capacities, composed of sulfonic acid polymer with methacrylic acid derivatives, has recently been developed. In this study, influence of cement type, concrete temperature, and pozzolans on properties of fresh and hardened concrete with this type of superplasticizer was investigated. Two reference superplasticizers were widely used naphthalene-based and amino sulfonic acid-based. A significant increase in water-reducing capacity to obtain the same consistency was observed at a much lower dosage. Absolute value of zeta potential of cement particles with the superplasticizer increased with elapsed time until 90 min after mixing, which explains the high-slump-retention capacity of the concrete. Plasticizing effects of superplasticizers were more pronounced for concretes with fly ash or blast furnace slag as blending agents. Concrete bleeding decreased slightly. Properties of hardened concrete, such as compressive strength and drying shrinkage, were at nearly the same level as those of concrete with naphthalene-based superplasticizer.

Among the requirements for Type C and E accelerators in the ASTM C 494 Specification on Admixtures, limits for set and strength performance are stipulated for concrete mixed and cured at 23 C (73 F). However, set-accelerating admixtures are predominantly used for cold weather concreting operations, where they can significantly increase the rate of early strength development at low temperatures, thereby reducing the curing and protection periods necessary to achieve specified strength. Paper discusses a laboratory program designed to evaluate the effectiveness of various set-accelerating admixture formulations. The scope of the program includes measuring set, strength, and air-entraining performance over a wide range of mix and curing conditions, and selection of cementitious materials. Furthermore, considering that a wide range of soluble inorganic salts, used over a relatively large dosage range, has been shown to accelerate the setting early hardening of portland cement, protocol for evaluating the corrosive potential of accelerator formulations containing these salts is discussed.

Gypsumless portland cements (GPC) are inorganic binders that may be described as systems of ground portland clinker with a specific surface of 400 to 500 kg/m 2 (Blaine), a superplasticizer with hydroxyl groups, and a hydrolyzable alkali metal salt. The major feature of GPC is the use of a low water-cement ratio (under 0.30), based on the strong liquefying action of the superplasticizer and alkali salt in suspension of ground clinker in the absence of gypsum. The effect of the dosage of sodium lignosulfonate in combination with sodium carbonate on compressive strength, setting time, and consistency of cement paste was studied. It was found that paste properties strongly depend on dosage and mutual ratio of the admixtures. The optimum composition of the setting and liquefying regulator causes high compressive strengths 3 to 7 hr after mixing of binders (15 to 45 MPa).

Inadequate performance of concrete structures is often caused by deficient construction practices and lack of appropriate specifications for controlling concrete properties that are related to adequate performance during the expected service life of a structure. Work carried out for many years at the Civil Engineering Materials Unit (CEMU) of the University of Leeds has shown that the durability of concrete can be assessed effectively by measuring its permeability to gases, liquids, and ions. Paper presents the findings of a laboratory study of the properties of normal portland cement and fly ash-normal portland cement mortar and concrete mixtures that influence their oxygen and chloride-ion permeability. The study involves 28 mixtures incorporating the use of five chemically different superplasticizers and three water-cementitious materials ratios. Statistical models that relate compressive strength, porosity, pore-size distribution, and water-cementitious materials ratio to oxygen and chloride-ion permeability are presented.