High Performance Self-Consolidating Cementitious Composites

Ganesh Babu Kodeboyina

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781138063044
• eBook ISBN: 9781351664974
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This book attempts to bring together some of the basic intricacies in the production of the complete range of self-consolidating cementitious composites, with a proper understanding of the contributions of different materials and their combinations, including performance and limitations.

Presents a comprehensive perspective of the state of the art in self-compacting concretes while explaining the basic background and principles, includes possible alternatives of making SCC with different powder extenders and pozzolanic materials

Explores concepts through theoretical and graphical representations

1
Introduction

1.1Concept

Concrete has been the preferred material for construction because it can be molded into any shape dictated by the structural configuration requirements while being the most economical. Cementitious composites have undergone several changes over the years, due to the advancements in cement production processes resulting in higher-grade cements and the advent of modern pozzolanic and chemical admixtures, leading to strengths reaching the levels of steel in compression, while having significantly improved performance characteristics even in the most aggressive of environments. However, being a highly complex composite with so many materials of varying sizes, shapes, and textures, the prediction of the strength and performance of the resultant material has always been a matter of serious concern, without stringent control on the constitutes. The large and ever expanding database of research findings with several different local constituents, be it aggregates or the cements produced or the admixtures utilized in these concrete composites, also causes a certain amount of confusion in understanding the material.

In the context of the normally vibrated concretes, be it the general-purpose concretes, high-strength concretes, no slump concretes, or even lightweight concretes, recommendations are available for arriving at a concrete of a specific strength and consistency required for a particular application in the construction activity. In brief, the methodology adopted was to define the water–cement ratio in terms of the strength required for the structural member. Depending on parameters such as the construction practice, member size, and dimensions, the consistency or the workability of concrete was fixed. The workability ranges from no slump to a collapse slump based on the compaction procedures adopted. Depending on the constituent materials, particularly the maximum size of the aggregate, the water content is defined for that workability. After this the coarse aggregate and fine aggregate proportions are fixed or a continuously graded aggregate is recommended for each maximum size of the aggregate. All these were possible through an understanding of the gradation of aggregates and the amount of paste and mortar contents including water, which were all chosen from a very large database of the experimental evaluations and field experiences that are available on these types of normally vibrated concrete systems. One important factor that was also learned during these investigations was that an inadequate compaction of the resulting mass can significantly affect the strength of the composite ultimately. Several compaction procedures such as vibro-compaction, high-frequency vibration, vacuum dewatering, spinning, and pressure application have all been used effectively, depending upon the member details and construction practices available. Naturally, the emphasis is more on appropriate concrete compaction methodologies to alleviate the problems associated with the lowering of strengths during these processes. Though the most desired parameter in defining the characteristics of concrete is strength, which is related to the water to cement ratio, the fact that it can be produced with various workabilities at different water contents was also well established. Slump is a measure of the workability, and structural concretes with no slump to collapse slump characteristics can be produced in practice with water contents ranging from 150 to 215 kg/m³, as recommended in the ACI guidelines (ACI 211.1, 1997).

In this complex scenario wherein the concrete has undergone several significant modifications to cater to the varying needs of the construction industry, the modification that has attracted the attention of the industry in recent times is the concept of self-compacting or self-consolidating concrete (SCC), a concrete that consolidates under its own weight. This concept of SCC, originally articulated by Okamura (1995), came into existence in an effort to design a concrete mix that could consolidate and fill without any difficulty the massive end anchorages of the Akashi-Kaikyo bridge with its highly impenetrable surface reinforcement cage and anchorage details. These SCCs have also been referred to in the literature as SCCs, self-placing concretes, and self-levelling concretes by a few, depending upon the requirements and perception. Semioli (2001) opines that a less dramatic terminology such as self-accommodating concrete might be more appropriate to describe the self-levelling and self-compacting properties that permit easier installation and working at sites for Lafarge’s “Agilia,” which was termed a self-placing concrete by them. He also discussed its use for both horizontal and vertical applications as well as in the repair and rehabilitation of an old courthouse structure. Szecsy (2002) presented a broad discussion on the nomenclature as well as the technology. He opines that each of these different names defines a specific aspect of the type of concrete, namely, SCC—a concrete that consolidates through gravity to achieve maximum density without the need for vibration, self-levelling concrete (SLC)—a concrete that can seek a level grade within the formwork, and self-placing concrete (SPC)—a concrete that has ease of placement and has the ability to be both self-compacting and self-levelling. The various admixtures that help in achieving self-compactability (mostly polycarboxylate-based superplasticizers in recent years) and a few tests that establish the different workability characteristics have also been presented. Also, others followed a similar approach in defining what is expected of SCC over the years. To be more specific the name carries the entire meaning of the expectation from the cementitious composites developed.

1.2Historical Development

One of the significant developments in the area of concrete technology could be traced to the advent of superplasticizers, though certain classes of plasticizers have always been available from times immemorial, even from the early lime concrete technology periods. This change is essentially because concretes with even a limited slump of just 25–50 mm could be readily reduced to a fluid consistency with collapse slump, if only it is ensured that there is enough sand and fines (even cement as in the case of high-strength concretes of lower water–cement ratio) in the mix. The simple yet often forgotten rule in the development of these concretes was to increase the sand content by about 10% and also to have a good gradation in the coarse aggregate fractions. These are then termed as superplasticized concretes; while the same superplasticizers could be utilized in concretes where the slump was nearly retained at the original level of 25–50 mm by appropriately reducing the water content (almost by about 20% if possible) and be termed as high-range water reducing admixtures, resulting in significantly higher strengths in these concretes. This in a way promoted the initial development of high-strength concretes, which were presumed, even if wrongly, to be an approach for realizing high-performance concretes due to their lower water–cement ratio and the resultant discontinuous capillary pore structure that could inhibit the permeation of moisture and environment.

The second significant development in concrete technology is the introduction of silica fume (an industrial waste from the ferrosilicon industry) as a pozzolan and the consequent development of very high-strength high-performance concretes. Simultaneously a lot of interest was generated in the use of secondary cementitious materials or mineral admixtures, which not only ensure a saving of cement and economy but also impart a higher performance. The need for an effective utilization of the abundantly available industrial wastes like fly ash from thermal power stations as an industrial waste and slag from steel plants (in the form of ground granulated blast furnace slag, GGBS) also contributed to the formulation of binary cement composites. These pozzolans in conjunction with the available superplasticizers paved the way for an entirely new regime of concrete and concrete composites that proved to be of a significantly higher performance with the possibility of also achieving very high strengths. Many other materials such as metakaolin, zeolite, and rice husk ash (RHA) have also come to be used as pozzolanic materials.

SCCs have essentially evolved in the background of these significant changes that took place in the past few decades. Okamura published a few studies prior to his proposed mix design methodology for SCCs leading to its use in the end anchorages of the Akashi-Kaikyo bridge (Okamura, 1994, 1995, 2003). The method, simply stated, attempted to achieve a highly workable and thixotropic mix through augmented fines and superplasticizers while reducing the coarse aggregate content. After this initial semi-empirical approach of modifying the conventional concrete design, researchers appear to have looked for avenues to achieve self-compactability though several different approaches, which are la...

Table of contents

• Cover
• Halftitle Page
• Title Page
• Copyright Page
• Contents
• Preface
• Acknowledgments
• Author
• 1. Introduction
• 2. Constituent Materials
• 3. Insights into Standards and Specifications
• 4. Methodologies for the Proportioning of SCC Mixtures
• 5. Concepts and Criteria for High-Performance Self-Compacting Concretes
• 6. SCCs Based on Powder Extenders and Low-End Pozzolans
• 7. SCCs Based on High Efficiency and Nano Pozzolans
• 8. Fresh Concrete Characteristics of SCCs
• 9. Mechanical Characteristics of SCCs
• 10. Performance and Service Life of Self-Compacting Concrete
• 11. Frontiers and Research Needs
• Index