High-Strength Concrete

A Practical Guide

Michael A. Caldarone

• Publisher: CRC Press
• Year: 2014
• ISBN:
  9781498718288
• Edition: 1
• Topic: Architecture
• Subtopic: Architecture Methods & Materials

This practical book from a highly experienced author presents clearly the means and methods for designing, producing and using high-strength concrete.

High-strength concrete offers many benefits. Higher compressive strengths allow for a reduction in the cross-sectional dimensions of columns and walls in buildings. Its greater stiffness allows for increasing building heights while controlling sway and occupant comfort. Civil structures such as bridges have benefited from greater span lengths, shallower beam sections, wider girder spacing, and extended service life.

Illustrated with real life examples, through documented case histories, High-Strength Concrete will be a valuable resource for contractors, producers, inspection agencies, as well as engineers and researchers.

1 Introduction

Perhaps an appropriate way to begin this book is not to discuss what high-strength 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 never summarily 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 durability-related 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 not relevant 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.

Unit conversions

Both SI and inch-pound units are expressed in this book, with SI being the primary unit of measurement. In most of the information presented, the values stated in each system will be rounded to only reasonable approximations, but more precise conversion values will be made when warranted. For example, when addressing a general principle, 60 MPa would be rounded to 9000 psi, yet when discussing a particular project where 60 MPa was specified, 8700 psi will be expressed.

Terminology

A section addressing terminology has been placed at the beginning of this book in the hope that the reader will be able to navigate through the coming pages with a minimal amount of needless, terminology-induced stress. The meanings of most of the terms used in this book are generally accepted among the major standards writing organizations and institutes worldwide: however, in some circumstances, terms used in one part of the world can have a different meaning in other parts. For example, in the US, the term “admixture” refers to a material other than water, aggregates, hydraulic cementitious material, and fiber reinforcement that is used as an ingredient of a cementitious mixture to modify its freshly mixed, setting, or hardened properties and that is added to the batch before or during its mixing. In the UK, “admixture” is used to mean a material added during the mixing process of concrete in small quantities related to the mass of cement to modify the properties of fresh or hardened concrete. When a powdered admixture is added to factory-made cement during its production, it is called an “additive” and not an admixture. Most would probably agree that the implications of misapplying the term “admixture” would be relatively innocuous; however, with other terms, the consequences can be more serious. For example, in the US, “slag cement” is one of several terms used for the material most accurately described as “ground granulated blast-furnace slag.” However, in other parts of the world, “slag cement” can refer to blended hydraulic cement containing ground granulated blast-furnace slag as a major constituent (adding to the confusion in the US, until recently, “slag cement” also referred to blended hydraulic cement containing ground granulated blast-furnace slag!).

Please note that the terms discussed in this section and defined in the Glossary are strictly for the purpose of this book and are based largely on the author’s personal preferences.

Water-binder ratio (W/B)

When first presented by Duff Abrams in 1918, the meaning behind the term “water–cement ratio” was indisputable. At the time, Portland cement was essentially the only binder used for making hydraulic cement concrete. In the early twentieth century, fly ash was still drifting up power plant chimneys, and other materials, such as silica fume, did not yet exist. Ground granulated blast-furnace slag and natural pozzolans, although in use, were not yet “mainstream” to the industry. In later years, with the increased use of supplementary binders, terms such as water-cement plus pozzolan ratio (W/(C+P)) and water-cementitious materials ratio (W/CM) came into use. When the chemical and physical properties and relative proportions of cementing materials vary (including Portland cements), the relationship between strength and water content, or pore space and water content changes. However, for reasons that will be provided in a more detailed discussion of this important subject in Chapter 3, the author has chosen to adopt the single term water-binder ratio (W/B) for expressing the mass ratio of mix water to the combined mass sum of all the binding materials used.

Supplementary cementitious materials

Pozzolanic materials and hydraulic materials other than Portland cement have traditionally been referred to as mineral admixtures. Recently, there has been a shift in terminology to refer to materials such as fly ash, silica fume, ground granulated blast-furnace slag, and natural pozzolans as “supplementary cementitious” or “supplementary cementing” materials. The origin of the term mineral admixture probably traces back to the days when most concretes essentially were comprised of aggregates, Portland cement, and water. Any other material introduced into the mix was considered an “additive” or “admixture.” The term mineral admixture has been extremely useful for classification purposes, since it differentiates admixtures that are mineral in nature from those that are chemical in nature. Unlike chemical admixtures, which alter the minerals present in a binding system via chemical interaction, mineral admixtures contribute additional mineral oxides to the paste.

For the purpose of this book, the terms supplementary cementitious materials, supplementary cementing materials, and mineral admixtures will be used interchangeably.

Strength

This following discussion is presented principally as a premise to providing definitions for the three strength-related terms that will be used most frequently in this book. They are:

• target strength;

• specified strength; and

• required average strength.

In the broadest of terms, strength refers to the maximum amount of stress that a material is capable of resisting until some predefined mode of failure occurs. In engineering, stress flow can be resolved into five fundamental categories—uniaxial compression, uniaxial tension, flexure, shear, and torsion. In the case of hydraulic cement concrete, stresses are most efficiently resisted under uniaxial compression; therefore, attention is almost invariably given to characterizing the mechanical properties of concrete in terms of compressive strength. Being an inherently brittle material, failure in compression is reasonably straightforward to define. A consequence of the internal fracturing that occurs when a brittle material is loaded in compression is that failure usually occurs suddenly. Being less brittle, conventional-strength concrete is capable of more inelastic strain than higher-strength concrete. As the strength of concrete increases, the static modulus of elasticity generally increases proportionally with compressive strength.

Hydraulic cement concrete is considered to have “failed” in compression when it is no longer capable of resisting stress due to the internal fracturing that has occurred.

Strength is a relative, not absolute material property. The strength of a material depends on more than just the manner in which stresses are distributed. The measured strength of concrete depends on numerous factors, several of which are age at time of testing, curing history, specimen size, shape, and loading rate. To state that the design compressive strength of a concrete is 60 MPa (9000 psi) has no substantive meaning whatsoever. For example, the measured compressive strength of cylindrically shaped specimens having a 2:1 length-to-diameter ratio (l/d)¹ will usually result in a different (usually lower) value of compressive strength compared to the measured strength of cubically shaped specimens having the same cross-sectional area cured and tested under identical conditions.

Target strength

Target strength simply refers to a desired level of measured strength at a given age, usually when evaluated under a standardized method of testing. It is important to recognize that target strength and design strength are unrelated terms. If a concrete mix was only proportioned to achieve a median average level of strength at which the structure has been designed, the statistical probability that the results of a compression test would be below design strength would be 50 percent. It is important for users of concrete, particularly specifying authorities, to understand that even under the most stringent production and testing processes, there will always exist a statistical pro...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• List of illustrations
• Preface
• Acknowledgments
• List of abbreviations
• 1 Introduction
• 2 Constituent materials
• 3 Mixture proportioning and evaluation
• 4 Properties
• 5 Specifications
• 6 Production and delivery
• 7 Placement, consolidation, and finishing
• 8 Curing
• 9 Quality control and testing
• 10 Problem solving
• 11 Summary
• Glossary
• Institutes and standard writing organizations
• Index