International Concrete Abstracts Portal

One of the most challenging tasks in cement chemistry is to understand exactly how superplasticizers work and why they occasionally fail. The higher fluidity of superplasticized concrete is considered to be caused by an increased dispersive force. This dispersive force can be increased in two ways, either by electrostatic repulsion or by steric repulsion. The way in which dispersive forces are increased depends on the kind of superplasticizer used. It is' commonly known that sulfonate-based admixtures increase electrostatic repulsion while polycarboxylate-based admixtures increase steric repulsion. This paper analyses the influence of various superplasticizers, mixing energy and adding time of superplasticizer on the electrostatic potential of different cement pastes. The electrostatic potential has been observed by measuring the zeta-potential of cement pastes using the electro kinetic sonic amplitude method (ESA). Additionally, suspensions of cement and silica have been used to study the dispersing mechanisms of different superplasticizers. The results lead to a better understanding of the way sulfonateand polycarboxylate-based admixtures work.

Concrete is one of the most durable construction materials. However, cracking adversely affects its durability, appearance, and functionality. A major cause of the cracking is related to shrinkage-induced strains creating stresses when concrete is restrained. Shrinkage reducing admixtures (SRA's) have been used for several years to reduce drying and autogenous shrinkage. However, their uses in temperate environments were limited due to difficulties in producing good quality air void systems needed for freezing and thawing resistance. In this paper we show that good air void systems and freezing and thawing performance are obtainable with a new formulation of a glycol ether SRA. This formulation is very compatible with polycarboxylate type superplasticizers and calcium nitrite corrosion inhibitor. Furthermore, it is demonstrated that in addition to improving shrinkage and cracking performance, permeability and corrosion performance are improved.

The effects of calcium nitrite based corrosion inhibitor (CNI) and fly ash on the corrosion of high performance concrete subjected to a simulated marine environment were investigated. Small-scale concrete slabs containing steel reinforcement were cast with a cover depth of 20 mm and with a construction joint intersecting the steel reinforcing at right angles. The slabs were subjected to a simulated marine environment with two cycles of wetting and drying per day. Corrosion activity of the reinforcing bars was evaluated using the linear polarization resistance technique and free water-soluble chloride content at the rebar level was determined at the end of the testing period. Testing in areas away from the construction joint and away from corrosion induced cracking revealed that the water to cementitious materials ratio (w/cm) has a significant effect on the ingress of chlorides. Thus, chloride content obtained for 0.29 and 0.37 w/c were less than one third that of 0.45w/c concrete. Furthermore, a reduction in the amount of chlorides at the rebar level was clearly demonstrated when fly ash was used in 0.45w/c concrete.

The paper summarizes the results on flowing and cohesive superplasticized mixtures studied and placed in the 1970's and 1980's with properties very close to those of Self-Compacting Concretes (SCCs) presently considered to be the most advanced cementitious material. Case histories (from Hong Kong, New York, and Trieste, Italy) concerning placing of superplasticized self-levelling concrete without any vibration at all, published in the 1980's, are re-examined to compare them with the present SCCs. In particular, the paper deals with the ingredients of these mixtures (superplasticizer, cement, fly ash, ground limestone, silica fume, etc.) by examining their specific role in determining the main properties of these concretes, such as fluidity, on the one hand, and resistance to segregation, on the other. Some interesting new materials, such as ground fly ash or powder from recycled aggregates, appear to be very promising for manufacturing SCC in agreement with the requirements needed for sustainable progress.

A detailed investigation on the concrete specimens made with different chemical admixtures was carried out after 10 years of exposure in the marine splash environment. Chemical admixtures include air-entraining admixture (vinsol), water-reducing admixture (lingosulfonate group), various high-range water-reducing and air-entraining admixtures (naphthalene, melamine, polycarboxyl and amino-sulfonate group), and drying shrinkage reducing admixture (glycol ether plus amino alcohol derivatives). The specimens were tested for carbonation depths, chloride ingress, oxygen permeability, electrochemical and physical evaluations of corrosion of steel bars in concrete, porosity and mineralogy of the mortar portion, and SEM (Scanning Electron Microscopy) investigation of steel-concrete interface. Naphthalene group of high-range water-reducing and air-entraining chemical admixture shows relatively better performance with respect to the strength development and chloride ion ingress in concrete. The use of shrinkage reducing admixture shows no harmful effect after 10 years of exposure. The specially adopted method of casting concrete used in this study causes a formation of good steel-concrete interface that prevents the initiation of corrosion even for water soluble chloride concentration around 1.5% of cement mass.