A View of Concrete Technologies and Required Related Research for Materials of Construction and Their Testing Methods

Dr. Y. P. Gupta, Consultant, Allahabad Bypass Project; BCEOM-LASA JV Chairman, ICI UP Centre Professor (RTD.) Civil Engineering, MNNIT, Allahabad

Introduction

In the past 60 years, significant changes have taken place in the type, properties of concrete and its constituent materials. During the 1940’s to 2000’s substantial basic research was conducted in the United States and many other Countries which produced a thorough understanding of the properties of concrete materials, such as cement and aggregates, and the effect of these materials on the green and hardened properties of concrete. Material standards and specifications, concrete mix design and ingredient proportions, test procedures, and construction techniques were developed extensively on the basis of this knowledge.

In recent years, Construction Industry has been placing strong emphasis on high-strength and high-performance concrete and on shorter construction times. In response to this challenge, research has been focused on producing changes in the properties of the basic ingredients of concrete, such as cement, and on developing new ingredients to achieve better-quality, higher-strength, and more-durable concrete.

Admixtures: Needs and Challenges in Concrete Technologies

For several generations, concrete admixtures have been developed with the aim of altering a wide range of green and hardened concrete properties to achieve high-early-strength and high-performance concrete. Use of admixtures has allowed a dramatic reduction in the water-cementitious materials ratio (w/cm) in the concrete mix, which in turn has resulted in higher-strength and more-durable concrete. Significant research has also been done on the development and use of cementitious and pozzolanic materials, such as fly ash, silica fume and slag to replace or supplement the cement content in the concrete mixture. These materials have significantly improved the durability of concrete by reducing its permeability.

Today, it is quite common for admixtures and cementitious / pozzolanic materials to be included in concrete in addition to the standard concrete ingredients. Such complex concrete mixtures are significantly different from the simple Concrete mixes produced in the 1960’s to 2000’s in India. Yet many specifications and construction practices developed in accordance with basic research of the 1950s are still being applied to today’s concrete materials and construction industry, especially on small scale projects.

In addition, there are still unresolved problems and many unanswered questions associated with today’s concrete. For example, excessive shrinkage and shrinkage cracking are being observed in many of the high-performance and high-strength concrete. These unintended consequences impact the durability of the concrete and thus tend to defeat the purpose of using such concrete mixes. Another important set of issues with today’s concrete relates to the timing, duration, and type of curing, and the balance between curing time and speed of construction.

Still another issue is the knowledge gap among many practitioners with regard to the properties of individual concrete ingredients, how the various ingredients interact in the concrete mix, and how to arrive at the optimum mixture for the type of application and level of exposure to adverse environments. An effective technology transfer plan is needed to convey to practitioner’s state-of-the art information and the latest research findings on materials and concrete properties should be informed to field Engineers & implemented.

Testing Methods

Current testing methods for concrete and its ingredients are another challenging issue. Some of these methods are simple but time-consuming and tend to slow the pace of construction. New or improved tests for determining the properties of concrete and its materials need to be developed like Concrete strength is known after 28 days which is too long. These test methods should combine speed, accuracy, and precision. Technologies from other fields, such as medicine or the military action that can be non-intrusive should be considered in determining the concrete strength.

Research for New Concepts

The challenge to the research community in this millennium is to promote and develop a thorough and comprehensive understanding of the properties of concrete and of its multiple ingredients. This challenge can be met through a well-planned basic research program. This program should include the development of new and improved methods for testing concrete and its materials. Another program should focus on the best and most effective means of transferring the knowledge and methods thus developed to concrete practitioners for implementation. The following are some specific directions which these programs might take.

a) Cement
Many changes have occurred in the sources and production of cement, including the raw materials, fuel used and the grinding of the clinker. Today’s cements are much finer than those of the 1950’s and 1980’s. Research in the basic properties of cement is needed to evaluate the effect of such properties as fineness, chemical composition on the heat of hydration, and on shrinkage characteristics.

b) Admixtures and Pozzolanic / Cementitious Materials
There is a need to evaluate all the properties of the various admixtures and cementitious or pozzolanic materials. Issues associated with the use of these materials in concrete, including setting time, plastic and hardened concrete shrinkage, and the need for extensive curing should be investigated. The research should produce ready to use tables of the types and dosage or proportion of these materials in concrete, and the specific level of performance and strength achieved with each. The research should also focus on developing a new family of admixtures that would improve the tensile strength of concrete and facilitate the fast construction of concrete structures. For example, new admixtures now being produced help in the self-compaction of concrete in structures. This must be simplified.

c) Curing Materials
The industry has moved away from moist curing toward the use of curing compounds which are more convenient to use. However, the use of high cement content, higher fineness of cement, silica fume, and low basic research and emerging Technologies related to concrete w/cm ratio has made the concrete more prone to shrinkage and thermal cracking. Curing compounds are not effective in preventing shrinkage or cracking. New curing compounds are needed not only to prevent evaporation, but also to replenish lost concrete mix water. For example, the curing compound might include chemicals that could condense ambient moisture on the concrete surface to provide much needed moisture. Further, Concrete ingredients like aggregates, Admixtures are to be developed which can help in self- curing of Concrete without the use of water or curing compound and not giving rise to shrinkage cracking.

d) Fibers for use in Concrete
Apart from cement, water, aggregate and admixture; different types of fibers are also developed. Fiber Reinforced Concrete has a very high resistance to abrasion and impact loading that means it has good ductility similar to mild steel. It also has a higher tensile strength compared to normal concrete and better abrasion resistance. Apart from high strength, it also has high performance & fracture energy. Following are some of the various types of fibers, which may be used in FRC.
  • Carbon Fibers
  • Steel Fibers
  • Glass Fibers
  • Polypropylene Fibers etc.
Polypropylene Fibers have become very popular these days. Thus concrete mix in green stage should be such that fibers are not collected in pockets and they are well dispersed in the entire concrete matrix. Studies for use of Fiber reinforcement in concrete must be done extensively. This should also be done for Composite concrete Construction using normal concrete and fiber reinforced concrete.

e) Tests For Concrete

1. Tests for Green Concrete
Tests for green concrete properties such as slump, air content, and unit weight have been useful in controlling the quality and consistency of concrete mixtures. However, it can be expected that more emphasis will be placed on shorter construction duration on the roads, bridges, and airports. The present tests for plastic concrete tend to cause delays in construction. New technology is needed to enable testing of the workability, air content, and unit weight of mixtures in a non-intrusive manner. For example, a non-intrusive device similar to a radar gun could be developed for measuring the concrete workability from the concrete chute itself during its discharge.

2. Tests for Hardened Concrete
A better means of predicting the strength and durability of concrete is needed. Tests based on the hydration process, rate of strength development, and other physical and chemical indicators should be developed for predicting the ultimate strength and permeability / durability of concrete. The availability of such tests would allow better optimization of the concrete mixture with respect to the types and proportions of its ingredients. In addition, the concept of 28-day strength may become obsolete as an acceptance requirement. Concrete mixtures of the future may reach their ultimate strength in less than 7 days. This accelerated development of strength may alter the microstructure of the concrete. Research is needed to better understand the physical and chemical properties of hydration process and its related compounds, as well as the extent of micro-cracking and volume change in the mortar matrix.

3. Tests for Permeability
Advances have been made in measuring the permeability of concrete to better predict its durability. Nonetheless, existing devices either are too slow or provide an indirect measure of concrete permeability. Thus a fast, accurate, and repeatable device for determining the permeability of concrete is needed. A procedure should also be developed for predicting the durability of concrete from the automatic analysis of permeability data.

Technology Transfer To Field

Good-quality research in concrete technology and its constituent materials is being conducted at many places. This research is generating new information and technologies. However, effective means of transferring the research findings and products from the research phase to application in field are needed. Many practitioners do not attend conferences, workshops, or meetings. These practitioners often do not receive full information on the properties of new materials and how these materials, individually or collectively, affect the strength, durability, and volume change of the concrete. A detailed plan for transferring the knowledge and new products resulting from completed research in concrete technology and its materials should be developed and implemented. The Concrete making materials & Design handbooks and Internet information should be the centerpiece of such plan.

Summary

This millennium brings challenges and opportunities for research on the basic properties of concrete and its materials. New non-intrusive devices and other test methods should be devised to allow faster, more accurate testing of concrete materials and construction procedures. Performance based specifications should be developed for concrete materials and construction aspects in field. Appropriate tests should be designed to assess compliance with the requirements. An effective technology transfer plan should be developed to translate research results and new products for implementation by practitioners or Field Engineers.
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