Civil Engineering Horizon - application of retarding admixture in hot weather by S K Ahmad

The Performance requirements of hardened concrete are defined with respect to shape, finish, strength, durability, shrinkage and creep. To achieve these objectives economically, the fresh concrete should have a suitable composition in terms of quality and quantity of cement, aggregate, water and admixtures [10]. It should also satisfy a number of requirements from the mixing stage till it is transported, placed in a formwork and compacted.

The requirements to achieve good quality of concrete may be summarised as follows:

(i) The mix should be able to produce homogeneous fresh concrete from the constituent material of the batch under the action of the mixing forces.

(ii) The mix should be stable, in that it should not segregate during transportation and placing when it is subjected to forces during handling operation of limited nature.

(iii) The mix should be cohesive and sufficiently mobile to be placed in the form around the reinforcement. It should also be able to be cast into the required shape without losing continuity or its homogeneous state under the available techniques of placing the concrete at a particular job.

(iv) The mix should be amenable to proper and thorough compaction to make a dense and compact concrete with minimum voids under the existing facilities of compaction at the site.

(v) It should be possible to attain a satisfactory surface finish without honeycombing or blowing holes from the formwork and free surface by trowelling and other processes.

Investigations have shown that very fine cracks at the interface between coarse aggregate and cement paste exist even prior to application of load on the concrete. They are probably due to the inevitable differences in mechanical properties between the coarse aggregate and the hydrated cement paste, coupled with shrinkage or thermal movement.

The observation that micro-cracking is initiated at the interface between coarse aggregate and the surrounding mortar and that at failure, the crack pattern includes the interface, points to the importance of this part of the concrete. It is therefore necessary to understand the properties and behaviour of the interface zone sometimes called the transition zone. During mixing dry bulk of the cement particles are unable to become closely packed against the relatively large particles of the aggregate. This situation is similar to the wall‑effect at the surface of cast concrete surfaces although on a much smaller scale. There is thus less cement present to hydrate and fill the original voids. It?s a consequence the interface zone has much higher porosity than the hydrated cement paste further away from the coarse aggregate.

The weather conditions while casting and curing concrete may not always be ideal, but concreting is often necessary due to time constraints, environmental factors, etc. Casting and curing concrete in a hot environment requires special precautions to reduce the effects hot weather may have on the concrete. In some cases, it has been reported that both the initial and final setting times are halved when the temperature of cement mortar with water/cement (w/c) ratio of 0.6 is increased from 27^oC to 45.5 ^oC and other difficulties like decrease of slump, plastic shrinkage cracking etc. may also arise [9]. The chemical hydration reaction between the Portland cement and water is the major contributing factor that makes it difficult to cast and cure concrete in hot environments.

Retarding admixtures or retarders are highly recommended for use in all concrete where a delay in rate of hardening is necessary [1]. There are also problems related to early volume changes and cracking. More specifically, fresh concrete that is allowed to prematurely dry experiences plastic shrinkage, which is essentially contraction of the concrete. Due to this effect, cracks may develop in the surface of concrete after the first few hours of placement. Retarders do not alter the composition or identity of products of hydration [7]. Retarders tend to increase plastic shrinkage because the duration of the plastic stage is extended but drying shrinkage is not affected [4]. Air temperature also affects the performance of admixture [8]. The use of retarding admixtures is important where thin concrete sections are present. The retarding admixtures provide more uniform setting characteristics. Since concrete setting times are dramatically influenced by ambient temperature, the uses of retarding admixtures are strongly recommended during hot weather. These admixtures allow for normal setting times under these conditions. The use of retarding admixtures usually leads to higher ultimate strength in concrete.

Cement: Ordinary Portland cement manufactured by an Omani company.

Admixture: Pozzolith

Pozzolith is an admixture, which is used to enhance the properties of concrete. It is a versatile product, which can be utilized to maintain workability and effect water reduction throughout a range of concrete mix designs. In this study different percentages of Pozzolith are added to the concrete mix and the variations in properties are reported. The physical properties of Pozzolith are given in Table[1].

Colour:	Dark brown/black liquid
Specific gravity:	1.194 at 25?C
Air entrainment:	1-2% dependent on dosage.
Chloride content:	Nil to BS 5075
Nitrate content	Nil
Freezing point:	2?C. Can be reconstituted if stirred after leaving to thaw.
Flashpoint:	None

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Syed Khaleeq Ahmad

Cement: Ordinary Portland cement manufactured by an Omani company.

Admixture: Pozzolith