International Concrete Abstracts Portal

Welan gum is a viscosity-enhancing admixture used to retain some of the free water in cement-based materials and modify flow properties. The incorporation of welan gum and super-plasticizer can enable the production of highly flowable, yet stable systems with relatively low yield value and moderate viscosity. Such flow characteristics are useful for proportioning underwater concrete, post-tensioning grout, and self-consolidating concrete. Despite recent advances in such technological areas, limited data are available on the effect of welan gum-superplasticizer combinations on physico-chemical characteristics and kinetics of cement hydration. The study reported here was undertaken to evaluate the influence of welan gum and a naphthalene-based superplasticizer on fresh and hardened properties of cement paste with 0.40 W/C. The performance of reference grouts made without any admixture and those made with super-plasticizer were compared to the performance of mixtures made with 0.025, 0.055 and 0.075% welan gum by mass of cement. The effect of the admixtures on consistency, rheological properties, and stability of cement paste as well as setting time and changes in heat flux are discussed in this paper. Test results on hardened cement paste involving mercury porosimetry, scanning electron microscopy, and compressive strength development are also given.

Self-leveling concrete presents a remarkable fluidity: the static flow measured by means of a truncated cone (0inf = 225 mm; O sup = 170 mm ; h = 12Omm) reaches 600 mm within 10 seconds. Such concrete is particularlysuitable for casting horizontal structural elements and allows a considerable workmanship reduction. Sometimes, this concrete has to be transported and the flow must be maintained for one hour or more. Therefore, a study was conducted to investigate the influence of the superplasticizer on the loss of workability of self-leveling concrete. The viscosity agent used in this study was a solution of modified starch (10 % solids); its content was kept constant over all the study. Two types of inorganic materials were tested; fly ash and powdered limestone. The total cement + mineral admixture was kept constant at 400 kg/m3, and the amount of water was adjusted for each composition. Five types of commercially available superplasticizers were analyzed: a melamine sulfonate, a naphthalene sulfonate, a polyacryalate, and two polycarboxylates. Their dosage was adjusted to a get an initial flow of 600 mm within 10 seconds. The following tests were carried out: loss of workability within one hour, segregation, bleeding, plastic shrinkage, strength at 18 hours, and 28 days. The main finding of this study is that only one type of polycarboxylate associated with modified starch allows the concrete to keep its initial flow for more than one hour.

A newly developed ‘polyether-based super-plasticizer’ was found to be much more suitable for self-compacting concrete than the conventional ones because of several advantages, such as high dispersability with minimal setting retardation while maintaining segregation resistance. Polyether-based superplasticizers’ features long ethylene oxide (EO) graft chains which give high dispersability by their steric repulsion, and short main chain with smaller amount of carboxyl group which leads to high early-strength. Furthermore, segregation resistance is improved by using this new superplasticizer with higher content of EO. This new superplasticizer has made possible the self-compacting concrete applications resulting in manpower savings, and improvement of quality and productivity.

The objective of this study is to establish a method that evaluates the stabilization effect of a viscosity enhancing agent. These agents are commonly used to stabilize fluctuations in the quality of fresh concrete during manufacturing and placement. It is assumed that the stability of self compacting concrete can be achieved over a wide range of mixture proportions. Thus, the dosage range of superplasticizer and allowable variation of water-powder ratio were adopted as an index of the stabilization effect. The feature of the proposed method is to provide an estimate of the stability of self-compacting concrete from mortar fluidity. Mortar and concrete tests were carried out to verify the proposed evaluation method. As a result, it was experimentally verified that several types of viscosity enhancing agents have the ability to stabilize the fluidity of fresh concrete.

In order to obtain specific concrete properties for distinct applications on site, the use of blends of different admixtures should be considered. During the construction of a roadway bridge, with long and large piles in an aggressive chemical environment, a comparative study was made to analyse the effect of blending of a melamine-based superplasticizer with triethanolamine-based admixtures, and polycarboxylate with lignosulphonate-based admixtures by the other side, on the slump loss of a flowable concrete. The performance of a mixture of plasticizer/superplasticizer was compared with that obtained by the single use of the individual admixtures. A naphthalene sulphonate-based admixture was also investigated. The results show that blending of modified lignosulphonate with polycarboxylate based admixtures in lower dosages, produces a significant synergistic action that provides improvements on slump-loss, with only a small delay in the initial setting time. With polycarboxylate based admixtures it is possible to get significant slump retention without compromising the early strengths