Slump loss effect

Despite wide publication of the adverse effects that retempering causes, job site addition of extra water to compensate for slump loss is still a common practice, particularly in hot-weather conditions. Under these conditions, the use of both conventional and superplasticizing admixtures helps to minimize the amount of water required for re-tempering so that the loss of strength is minimized [46, 47], This is shown in Table 7.14. 

Fig. 7.33 Effect of temperature increase on slump loss (Mailvaganam [90]).

F/g. 7.35 Effect of type D admixture on slump loss initial slump 95-115 mm, temperature 30°C (Soroka and Ravina [94]). 

The replacement of Portland cement by fly ash class F (ASTM C 618) has been found to reduce the rate of slump loss in a prolonged mixed concrete, and the extent of the reduction is greater with increased cement replacement (Fig. 7.37). Fly ash also was found to be beneficial in reducing slump loss in concretes with conventional water-reducing and retarding admixtures [95], The effect of fly ash on reducing slump loss can be attributed to chemical and physical factors. It was found that the surface of fly ash particles may be partly covered with a vapor-deposited alkali sulfate that is readily soluble [103, 104], Thus the early hydration process of Portland cement is effected because sulfate ions have a retarding effect on the formation of the aluminates. Indeed, fly ash was found to be a more effective retarder than an... 

With some cement-superplasticizer combinations, various problems have been reported such as low fluidification effect, rapid slump loss, severe segregation, extended set retardation and loss of entrained air. These are briefly reviewed below. 

The workability of any fresh concrete decreases with time after mixing, but this effect, which is called slump loss, is more marked if a water reducer is used. The slump nevertheless remains higher than if the latter was absent. Slump loss is caused by the slow commencement of the hydration reactions, and its increased magnitude in concrete containing water reducers is probably due to the gradual absorption of the admixture by the hydration products. Delay in adding the admixture until a few minutes after mixing minimizes it. 

Most of the charaeteristics of conventional water reducers are shown also by superplasticizers, at least qualitatively thus, they are more effective if added a few minutes after mixing, retard setting and increase slump loss. Retardation of set is slight with SMF but more marked with SNF. Slump... 

The water requirement of alkali-activated slag cements is relatively low, owing to the plasticizing effect of the alkali compounds that are present, resulting in a lower total porosity of the hardened material, as compared with Portland cement mixes. The produced fresh AAS cement based concrete mixes exhibit a distinct thixotropy and require continuous mixing, to prevent a quick slump loss and setting. Owing to their thixotropic properties even stiff mixes may be compacted if vibration is applied. 

Figure 18.1 Effect of microsilica (MS) addition on the slump loss of Portland cement based fresh concrete mixes [W/(C+MS)=0.25].

Workability Loss Most WRA also have a retardation effect. Therefore, they reduce the workability loss, which is usually described in terms of slump (measure of concrete consistency) loss. For normal WRA, a distinction should be made between concrete mixes with a specified waterrcement ratio and a specified slump. At a given waterxement ratio the slump of concrete with WRA increases considerably. Although the rate of slump loss increases upon normal WRA use, the higher slump value at the beginning causes later slump values to be still higher than those of the concrete without WRA. On the other hand, for the same initial slump, the workability loss is more rapid in normal WRA incorporated concretes than that of control concretes. A similar but more pronounced effect of loss of workability is observed in high range WRA incorporated mixes. 

Gluconate was found to be a good retarder. Its action is possibly related to its poisoning action on the hydration products of cement. It has also been advocated for controlling slump loss in concrete. Conduction calorimetric curves of cement with gluconate show large hump effects for the hydration of the silicate phase (Fig. 18a). At a dosage of 0.15%, the... 

Figure 21. Effect of gluconate and its mixtures with SMF on slump loss.

Ramachandran, V. S., Effect of Retarders/ Water Reducers on Slump Loss in Superplasticized Concrete, Developments in the Use of Superplasticizers, Am. Conor. Inst SpecialPubl, 68 393-407 (1981)... 

In order to study the effect of SO3 on slump loss in superplasticized cement, Khalil and Ward l adopted the conduction calorimetric technique. The heat effects were used to explain the workability loss. It was concluded that for cements cured at 25°C, the optimum SO3 content was higher in the presence of the superplasticizer. 

Khalil, S. M., and Ward, M. A., Effect of Sulfate Content of Cement Upon Heat Evolution and Slump Loss of Concrete Containing High Range Water Reducers, Mag. Concr. Res., 32 28-38 (1980)... 

Fig. 1.30 The effect of different dosage levels on loss of slump (Ravina).

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