International Concrete Abstracts Portal

The aim of this paper is to characterize different organic and mineral admixtures in order to evaluate their rheological behavior in cement pastes and their potential for incorporation into self compacting concrete (SCC). The admixtures tested are five commercial superplasticizers of various generations, acting by electrostatic, steric hindrance or combined effects; and four mineral admixtures (silica fume, metakaolin, fly ash and limestone filler) commonly used as cementitious materials and selected to achieve a large range of particle size, shape and chemical compositions. Rheological tests, using a modified Couette rheometer were performed on cement pastes to characterize the flow behavior of the mixtures and their rheological properties (yield stress and apparent viscosity). The results allow to propose some justifications concerning the suitability of these materials for their use in self compacting concrete. One example is the case of silica fume, which confers specific rheological characteristics to cement pastes that could be incompatible with the properties expected for SCC.

A new type of high-performance concrete was developed, based on calcium sulfoaluminate cement. It was designed to reach the following requirements: - high workability: slump higher than 200 mm for at least 50 minutes, - high-early strength: 40 MPa, 6 hours after its preparation, and higher than 55 MPa, after 24 hours. Different polycarboxylate-based superplasticizers and accelerators (lithium carbonate, calcium hydroxide, and normal portland cement) were studied. When the mixture does not contain any accelerator, no strength is recorded after 6 hours. The best accelerating mixture is that containing both lithium carbonate and normal portland cement. The cementitious material is composed of 80% calcium sulfoaluminate cement and 20% normal portland cement. The dosage of lithium carbonate is 0.005% by mass of cementitious material. For a cementitious material content of 600 kg/m3 and W/CM = 0.37, the following performances were obtained: - working time: 65 minutes, - compressive strength: 48 MPa after 6 hours, 57 MPa after 24 hours, and 81 MPa after 60 days.

Flash setting accelerators are commonly used for shotcrete which is basically a sprayed concrete for underground constructions. Such admixtures cause a very rapid hardening of cementitious systems thus allowing overhead and vertical applications. Two main classes of products are currently available in the market: alkali-free (non alkaline aluminium salts) and alkali rich (sodium or potassium silicates or aluminates) accelerators. The first ones were commercially available in the last few years but only recently they have started to be appreciated by the market. The wide literature on the subject shows that the use of alkali-free accelerators gives many advantages: reduced chemical reactivity of the admixture toward human skin; less rebound, high water impermeability, increased long term compressive strength of sprayed concrete. The action mechanism of alkali-free flash setting admixtures is still not clear; therefore, a chemical morphological analysis was carried out in order to study the effects of this new class of accelerators on the physical/chemical properties of cement pastes. XRD and temperature profile studies were performed on cement systems added with different amounts of accelerator. An ESEM study was also carried out in a time interval from a few minutes to 8 hours. Specific surface area was determined according to BET method on mortar samples mixed with the accelerator (2% and 6% by cement mass) after 24 hours and 28 days of curing. Furthermore soluble ions concentration (Al3+, S042-) was also determined by ICP spectroscopy. Setting time and compressive strength were measured to compare the effects of the admixture on the mechanical properties of modified mortars with respect to its influence on the chemistry and morphology of hardening cement.

The development of concrete, which need not be vibrated, and which consolidates under its own weight, is a challenge to the building industry. In order to achieve this behavior, fresh concrete must show high fluidity and good cohesiveness. For the purpose of evaluating these properties, some concretes were prepared with a water to cement ratio of 0.45 by alternatively adding two different kinds of acrylic-based superplasticizer to the mixture at a dosage of about 1 % by weight of cement. Either fly ash or limestone powder or rubble powder (that is a powder obtainded from the recycling process of rubble from building demolition) were used as mineral admixture, in order to ensure adequate theological properties to the self-compacting concretes in the absence of viscosity modifying agents. Preliminary rheological tests were carried out on pastes in which cement was partially replaced by these fine materials. The fresh concrete properties were evaluated through both the slump flow and the L-box tests. The segregation resistance was also determined. Compressive strength was also measured on hardened concretes after 1, 3, 7 and 28 days of wet curing.

Shrinkage is an inherent characteristic of cementitious materials and its adverse effects can compromise the mechanical integrity and durability of concrete. The use of construction joints, restraining reinforcement and special cements is not entirely effective in preventing the development of damaging cracks. Current methods of offsetting shrinkage involve the use of shrinkage compensating cements (SCC) and expansion producing calcium sulfoaluminate (CSA) based admixtures. The most important limitations to the use of these materials are rapid set, loss of workability and the amount and timing of the expansion produced. A recently developed new type of CSA admixturebased on calcium aluminate hydrates-has proved to be effective in compensating the shrinkage produced in medium strength (30-35 Mpa) concrete. This paper reviews the basic characteristics of currently available shrinkage compensating materials. Details are given on composition, mode of action, use and performance of the different admixtures.