Science and Technology of Concrete Admixtures
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Science and Technology of Concrete Admixtures

Science and Technology of Concrete Admixtures Edited by Pierre Claude Aitcin and Robert J Flatt | PDF Free Download.

Preface to Science and Technology of Concrete Admixtures

During the last 40 years, concrete technology has made considerable progress—not due to some spectacular improvement in the properties of modern cements, but rather to the utilization of very efficient admixtures.

For example, in the 1970s in the United States and Canada, concrete structures were typically built with concretes having a maximum compressive strength of 30 MPa and a slump of 100 mm.

Today, 80–100 MPa concretes having a slump of 200 mm are used to build the lower portions of the columns of high-rise buildings (Aïtcin and Wilson, 2015). These concretes are pumped from the first floor to the very top (Aldred, 2010, Kwon et al., 2013a,b).

Moreover, 40 MPa self-compacting concretes are being used for the prestressed floors in these high-rise buildings (Clark, 2014).

Currently, 200 MPa ultra-high strength concretes are being used. Such achievements are the result of a massive research effort that has created a true science of concrete and a true science of admixtures.

It is the prime objective of this book to present the current state of the art of the science and technology of concrete admixtures.

It is now possible to explain not only the fundamental mechanisms of the actions of the most important admixtures, but also to design specific new admixtures to improve particular properties of both fresh and hardened concretes.

The time is long past when different industrial byproducts were selected by trial and error as concrete admixtures. Today, most concrete admixtures are synthetic chemicals designed to act specifically on some particular property of the fresh or hardened concrete.

At the end of the Second World War, the price of Portland cement was quite low because oil was not expensive. Thus, it was cheaper to increase concrete compressive strength by adding more cement to the mix rather than using concrete admixtures.

This explains, at least partially, why the admixture industry was forced to use cheap industrial byproducts to produce and sell their admixtures. Today, oil is no longer cheap and the price of Portland cement has increased dramatically.

Thus, it is now possible for the admixture industry to base their admixture formulations on more sophisticated molecules synthesised specifically for the concrete industry.

As a result, in some sophisticated concrete formulations, it now happens that the cost of the admixtures is greater than the cost of the cement—a situation unbelievable just a few years ago.

The development of a new science of admixtures has also resulted in a questioning of current acceptance standards for cement.

For example, a given superplasticizer may perform differently from a rheological point of view with different Portland cements, although these cements comply with the same acceptance standards.

Expressions such as “cement/superplasticizer compatibility” or “robustness of cement/superplasticizer combinations” are often used to qualify these strange behaviors.

It is now evident that the current acceptance standards for cement, which were developed for concretes of low strength having high water–cement ratios (w/c), are totally inadequate to optimize the characteristics of a cement that is to be used for the production of high-performance concrete having low w/c or water–binder ratios.

It is a matter of sense to revise these acceptance standards because, in too many cases, they represent a serious obstacle to the progress of concrete technology.

Moreover, we are now more and more concerned by the environmental impact of civil engineering structures, which favors the use of low w/c concretes that require the use of superplasticizers.

It is easy to show that a judicious use of concrete admixtures can result in a significant reduction of the carbon footprint of concrete structures.

In some cases, this reduction may be greater than that resulting from the substitution of a certain percentage of Portland cement clinker by some supplementary cementitious material or filler.

To illustrate this point very simply, let us suppose that to support a given load L we decide to build two unreinforced concrete columns—one with a 25 MPa concrete and the other with a 75 MPa concrete, as shown in Figure 1. As seen in Figure 1, the cross-sectional area of the 25 MPa column is three times as large as that of the 75 MPa column.

Therefore, to support the same load L, it will be necessary to place three times more concrete and to use approximately three times more sand, three times as much coarse aggregate, and three times much water.

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