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High Strength Concrete

 High Strength Concrete




PREFACE:

Perhaps an appropriate way to begin this book is not to discuss what highstrength concrete is, but rather, what it is not. Having the word “strength” in its name undeniably suggests a bias towards one property only; however, high-strength concrete can be an advantageous material with respect to other properties, both mechanical and durability related. Nevertheless, it is crucially important to recognize that the achievement of high strength alone should neversummarily serve as a surrogate to satisfying other important concrete properties. It would seem logical that strong concrete would be more durable, and in many respects, the lower permeability that comes along with higher strength often does improve concrete’s resistance to certain durabilityrelated distress, but unlike strength, the prerequisites for durability are not easily defined. In fact, depending on the manner in which higher-strength is achieved, the durability of high-strength concrete could actually diminish.

 For example, if cementing materials are not carefully chosen, higher-strength mixes could conceivably contain an objectionably high quantity of soluble alkalis that could promote cracking if aggregates that are potentially susceptible to alkali reactivity are used.

 Throughout this book, the reader will frequently encounter references stressing the importance of identifying all relevant properties when developing high-strength concrete. However, equally important is identifying properties that are notrelevant that could impede the ability to achieve the truly important properties. 

There are extraordinary differences when comparing the properties of a very high-strength concrete having a compressive strength of 140 MPa (20,000 psi) to that of a conventional-strength structural concrete with a compressive strength of 30 MPa (4000 psi). When considering the adjustments to the principles of mix proportioning necessary in order to satisfy mixture performance requirements, it is interesting to note that no abrupt change in material technology occurs at any one particular level of strength, or at a particular water–binder (W/B) ratio. Rather, the changes that occur when progressing up the strength ladder are quite subtle with each advancing step. As the W/B ratio changes, so do the principles governing mix proportioning, which in turn establishes strength and other mechanical properties.

 In order to develop an intuitive understanding of how it is possible to  produce concretes four to five times stronger than conventional concrete, any beliefs that the principles governing concrete proportioning change little should be abandoned from this point on. It is only natural that hydraulic cement concrete would be viewed as a single material, but in reality, concrete is much better understood when viewed as a composite material comprised of two fundamentally different materials—filler (i.e. aggregate) and binder (i.e. paste). Material properties, principally those mechanical in nature are fundamentally derived from the relative similarities (or differences) in the properties of the aggregate and paste. 

For this reason, the laws governing the selection of materials and proportions of concrete are by no means static. The most influential factor affecting the strength and largely influencing the durability of concrete is the water-binder (water-cement) ratio. Hydraulic cement concrete is a two-component composite material fundamentally consisting of aggregates and paste. The principles applicable to proportioning structural concrete are primarily driven by the relative mechanical properties of paste and aggregate. For this reason, proportioning guidelines that might be viewed as “best practice” for one strength level might be quite inappropriate for concrete of a different strength class.

 The requisite properties of constituents and material proportions will subtly vary from one W/B ratio to another. This fundamental principle applies to the entire spectrum of strength achievable with hydraulic cement concrete when using mainstream, non-exotic constituent materials. This book primarily addresses normal-weight high-strength concrete using constituents and construction practices appropriate for producing compressive strengths with an upper limit of approximately 140 to 150 MPa (20,000 to 22,000 psi) using mainstream materials and testing standards. This book does not address high-strength concrete produced with exotic materials or uncommon manufacturing or evaluation methods.



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