Concrete slump

When added at dosage of 2% by weight of cement to a concrete mixture with 460 kg m of cement without adjustment for the volume of the water introduced by the admixture, the concrete s slump and porosity are increased. However, when substituted for an equal volume of water, the SRA has little or no effect on concrete slump. It does have a slight retarding effect on the rate of hydration and may extend the setting time up to about an hour. The admixture also affects the air content of fresh concrete and therefore when used in air-entrained concrete, the air-entraining admixture dosage must be increased to achieve a specified air content. 

The soluble sulfate versus time curve for the cement containing the natural anhydrite is radically changed when CLS is present [130, 131]. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a sulfate-starved system in the concrete, often producing rapid set and an increase in rate of concrete slump loss (Fig. 7.42). Apparently the adsorption of the lignosulfonate by the natural anhydride plus the rapid reaction between the soluble SO3 and the... 

Concrete slump without admixture end with admixture... 

Test Fresh concrete Slump flow Air contents Unit volume weight ... 

South African National Standards. SANS 5862-1 2006, Concrete tests - consistency of freshly mixed concrete - slump test. South African National Standard. Standards South Africa. Pretoria, Gauteng, Republic of South Africa (2006). 

In a t5 ical concrete mixing process, a large amoimt of water is added to increase flowability of the concrete (23). However, a water overdose lowers concrete compression strength and adversely affects other properties. In contrast, a lack of water causes concrete slump and deterioration, which is harmful to a construction process. Many chemical additives have been described to improve the concrete flowability without the need for increasing the amount of water. 

Traditional concrete water reducers are formed by mixing lignin as main component with naphthalene sulfonic acid sodium salt. Although the cost of such kind of additives is relatively low, they cannot provide desirable concrete water reduction when the effective content in the concrete is low. For example, when a type F water reducer, which consists mainly of naphthalene-based compoimds, is used, rapid concrete slump will result. 

These additives, even at a relatively low additive level, can provide an improved water reduction, increase concrete flowability, reduce concrete slump and enhance compression strength (23). 

In concrete, slump can be increased substantially by the addition of superplasticizers. For example, a slump of 260 mm from an initial slump value of 50 mm is obtained by an addition of 0.6% SMF the same value is realized with an addition of 0.4% SNF. The time when the superplasticizer is added to concrete affects the slump value. By adding the admixture with the mixing water, the slump is increased, but even higher values result if the admixture is added a few minutes after the concrete is mixed with water. 

Unconfined compressive strength of cohesive soil Unconfined compressive strength index of chemical-grouted soils Slump of portland cement concrete... 

The water-reducing admixtures are the group of products which possess as their primary function the ability to produce concrete of a given workability, as measured by slump or compacting factor, at a lower water-cement ratio than that of a control concrete containing no admixture. 

Fig. 1.32 Changes in slump and VeBe values for concrete containing straight addition of a hydroxycarboxylic-acid-based water-reducing agent.

The initial plastic state of the fresh concrete when properties such as the workability as measured by slump or flow table test or air content can be determined by the relevant standard method. 

Fig. 2.17 Slump tests after various intervals of time for concrete incorporating 3% SMF by weight of cement (Mailvaganam). 

Fig. 2.18 Slump loss at 21°C of superplasticized concretes with OPC and CAE or SNF ploymer-based admixtures. The figures on the slump-loss curves indicate the percentage of the superplasticizer active ingredient by mass of cement.

Fig. 2.19 Strength development of high-strength flowing concrete containing melamine-based superplasticizer compared to concrete made with 400 kg of normal Portland cement per m in the stiff to low workability ranges (25-100 mm slump). 

If the addition of the air-entraining agent is maintained at a constant level, a more workable mix will entrain more air than a less workable one. However, for very workable concrete of slump greater than 180 mm, the air will be more rapidly lost before placing. 

The temperature of the concrete has a significant influence on the amount of air entrained in concrete by a standard addition level of admixture the higher the temperature, the lower the air content. A typical set of results is shown in Fig. 3.19 [23]. The effect is more marked at higher slump values. 

Air-entraining admixture Concrete proportions Air (%) Slump (cm) Drying shrinkage x lO -6 ... 

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