Fresh concrete

G. H. TattersaH and P. F. G. BanfiU, The Bheology of Fresh Concrete, Pitman Books, London, 1983. 

T. C. Powers, The Properties of Fresh Concrete,]ohxs Wiley Sons, Inc., New York, 1968. 

In order to understand more fully the effect that water-reducing admixtures have on the plastic properties of fresh concrete, and to gain an insight into the mechanism of action of this category of materials, it is useful to study the effect on the water-cement system. The topic can be considered from the... 

It is known that some of the properties of fresh concrete can be considered in terms of the rheological properties of the cement paste contained in the concrete. Thus a high water-cement ratio concrete will contain a paste content which is more fluid than that of a low water-cement ratio concrete. 

The initial plastic state of the fresh concrete subsequent to the mixing process, where properties such as the air content, density and workability are normally measured by relevant standard tests, and utilized as a means of control of production. The magnitude of these properties is affected by the addition of water-reducing admixtures, either intentionally or as a side effect, which could result not only in a change in the characteristics in the plastic state, but could also be reflected in changed properties in the hardened state. 

Setting characteristics of fresh concrete containing water-reducing... 

The stability of the concrete mix can be considered in terms of its cohesion , which is a subjective term used to describe its ability to maintain a homogeneous appearance when subjected to applied stress. Lack of cohesion leads to segregation of the mix components into layers relevant to their densities. A further term associated with mix stability is that of bleeding , which is the movement of water to the surface of the fresh concrete. This phenomenon can occur either in isolation or as a manifestation of segregation. Bleeding in excess is normally considered to be undesirable because of the dangers of water runs at the shutter/concrete interface and cracking due to plastic settlements, and there is also the possibility of adverse effect on the concrete-reinforcement bond due to the collection of water beneath the steel. 

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. 

Different types of superplasticizers alter the air content of fresh concrete in varying degrees and the effect is also dependent on the way the superplasticizer is used. In general, the following observations are noted ... 

Mix Type of Mixture proportions and Series concrete properties of fresh concrete... 

Mix no. Properties of fresh concrete Air-Void determinations on hardened concrete ... 

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. 

Fig. 7.44 Effect of mixing time after addition of air-entraining admixture (mix 3.1) in combination with superplasticizer (mix 3.2) on the total air content in (a) fresh concrete and (b) hardened concrete (Mark and Gjoerv [131]).

The replacement of cement with silica fume results in reduced air contents in the fresh concrete compared to control concrete containing no silica fume. Achieving the required air content necessitates an increase in the AKA dosage, but the AEA demand decreases in the presence of a WRA or SP. There is also an evident increase in SP and WRA amounts required to produce desired modifications in the presence of silica fiune. The mechanisms responsible for such effects are described as follows. 

Fresh concrete and cement mortars—which are identical from a chemical point of view—are relatively strongly alkaline (pH approximately 12.5). It later falls, however, due to the binding of carbon dioxide from the air. Depending on the special chemistry of the cement mortar, this process proceeds very slowly in the depth of the material. According to the composition of the cement mortar, this may last from a few months to many decades, until the pH value of such a mortar or concrete becomes neutral, even in the deepest layers.396"398 This chemical behavior explains the entire secret of the stability of reinforced concrete, which prevents the embedded steel from rusting further in the environment within the concrete, which remains alkaline for lengthy periods of time.399... 

Hygro-themo-chemo-mechanical behaviour of concrete is of great practical importance in many fields of civil engineering. Modelling these phenomena, especially in fresh concrete structures or concrete elements exposed to fire, is a complex problem. Several non-linear phenomena, like heat and mass sources associated with hydration or dehydration processes, phase changes, hysteresis of sorption isotherms, material properties dependent on moisture content, tem-... 

In order to understand more fully the effect that water-reducing admixtures have on the plastic properties of fresh concrete, and to gain an insight into the... 

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