International Concrete Abstracts Portal

Self-compacting concretes (SCC) are special cement mixes which can be placed without any vibration at all. The fresh mixes are characterized by very high fluidity and cohesion. These properties can be more easily obtained by decreasing the maximum size of the coarse aggregate and increasing the cement content. This characteristic is just the opposite of that required for concrete mixes for massive structures. These mixtures need coarse aggregate with high maximum size and low cement content in order to reduce the risk of thermal cracking related to the high heat development due to cement hydration. Special SCC have been studied. They contain a gravel with a maximum size of 18 mm, a very low cement content (- 150 kg/m3) a large amount of limestone filler (250-380 kg/m3) and fly ash in the range of 50-150 kg/m3 so that the amount of fine materials is approximately 500 kg/m3. Due to the combined use of an acrylic superplasticizer and a viscosity agent based on a colloidal byopolimeric emulsion, unsegregable SCC with a slump flow of about 700 mm were manufactured. The compressive strength was about 20 MPa at 3 days and 30-40 MPa at 28 days.

Acrylic based superplasticizers are characterised by high water reducing ability and high retention of the workability in comparison to the traditional superplasticizers based on naphthalene sulfonate formaldehyde condensate (NSFC) and melamine sulfonate formaldehyde condensate (MSFC). On the other hand, polycarboxylate polymers may lead to a retardation of the cement hydration. This disadvantage can limit the use of these superplasticizers in cold climates, particularly in the cases where early strength development is required. In the present work, a new polycarboxylate superplasticizer based on high molecular weight polyoxyethylene side chains is presented. This new polymer allows the production of cement mixtures characterised by low water-cement ratio and high early age mechanical strength development, even at low temperature of curing. These results can greatly improve the productivity of precast concrete industry by reducing or even eliminating the steam curing cycle and the time for the moulds reuse. The polymer has been studied in comparison to a traditional acrylic based superplasticizer in terms of superplasticizing effect, development of mechanical strength, adsorption rate, heat development and morphological analysis of the cement hydration products by means of ESEM-FEG (Environmental Scanning Electron MicroscopeField Emision Gun).

Concrete that is to be placed under cold weather conditions must be protected from freezing and may be required to have adequate setting behavior and strength development. There are a number of different approaches that can be used to ensure protection from freezing and the development of mechanical properties of such concrete. One approach is the addition of chemical admixtures that accelerate the cement hydration in the concrete. This paper presents the results of studies that examined the performance of a new cold weather admixture (CWA). The performance of the CWA was evaluated in concrete with an initial concrete temperature of 11-13°C that was cured at an air temperature of either -1 °C, -7°C, or -11 °C. The addition of the CWA allowed the concrete to set and gain strength under the cold weather conditions. Lowering the w/c of the concrete with a high-range water reducing admixture coupled with the addition of the CWA further improved the setting and strength development of the concrete at the lowest curing temperature tested. These results suggest that the use of the CWA will allow current cold weather concreting guidelines to be revised to allow for lower initial concrete temperatures.

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.