CONCRETE DESIGN

The design and use of appropriate concrete technology to suit a particular circumstance is not an exact science. It requires sound engineering judgment and relies heavily on individuals’ experience. In this topic, an attempt is made to highlight, through practical examples, some of the important issues concerning the use of concrete technology. It is emphasized that concrete technology means more than just the production of concrete of adequate strength and a wide range of issues have to be considered, e.g. the durability of the concrete element, its practicality and economy. On basis of the discussions, criteria are proposed to assess appropriateness in concrete design.

There are two aspects to the design of concrete: materials and techniques. The purpose of design is to ensure that, under given circumstances, the most appropriate materials are selected and that compatible techniques of concreting are deployed so that the project specification is achieved. There are broadly two types of specifications, either based on materials or on performance. With the former type of specification, materials requirements have already been researched and are laid down by the specifier, leaving little scope for modifications by the contractor. The “design” thus becomes the selection of the right techniques to go with the chosen materials. With the latter type of specification, the performance of the end product is specified and the contractor has the freedom in his design to suit the particular circumstance. This type of specification encourages innovative and economical design and is gaining popularity. Whichever type of specification is used, however, someone somewhere has to translate the project specifications into specific requirements for concrete technology. Therefore, this paper takes a broad approach to concrete design and aims to highlight some important, and yet often neglected, considerations in the design process. The design of concrete, unlike structural design, is not a well developed process. In the past, it was often taken as a means of achieving an adequate strength. Over recent decades, especially with the wide-spread use of reinforced concrete, it has gradually been accepted that concrete needs durability as much as strength, if not more so. This transition is justified by the fact that many structures have failed, not due to inadequate strength of their members, but their inability to stand up to deterioration caused by the environment. In today’s world, concrete design takes on new meaning and puts emphasis on the practicality, economy and environmental aspects of the design. As a volume construction material, concrete can be found in every developed and developing country and, with thoughtful design, concrete should and can meet the varying and sometimes very stringent requirements. This paper aims to categorize and discuss some important issues concerning concrete design. It does not intend to become a design manual, but to provoke further thought by people involved in concrete design. On the basis of the discussions, criteria are proposed for the assessment of the appropriateness of a design. CONCRETE TECHNOLOGY: DESIGN CONSIDERATIONS It is considered that there are four major areas which concrete design has to take into account and they are strength, durability, practicality and economy. There are also some other factors, such as the environment, and they are discussed briefly under a separate heading. While it is possible to design concrete with a range of properties, flexibility in design is often restricted by codes of practice and Building Regulations. It has been suggested that codifying aspects such as durability is not possible (1) . However, amendments in codes and regulations are certainly required to incorporate ideas of modifying materials and techniques in order to achieve the flexibility in concrete design. Strength Concrete is usually classified by its strength, typically at an age of 28 days. Whilst this is convenient and does provide a base for comparison, it may not be the most appropriate under certain situations. For the design of strength one needs to know which of the following criteria to satisfy: • strength at stripping shutters • strength at different times of construction • strength at application of full service load The problem of accepting different strengths at different times is one of current code requirements. While early strength gain is often checked by testing cubes or cylinders at 1 and 3 days to determine whether formwork can be stripped or removed (as in slip forming), the full service load in many situations is only applied a substantial time after construction commences, hence a design strength at 90 or 180 days or sometimes even longer may be appropriate. The significance of this change in specification is that it would increase the choice of cementitious materials and allows a more effective use of blended cements. It is known that, at the same water-cement ratio, the strength development of blended cement is slower than its ordinary Portland cement (OPC) counterpart, but its ultimate strength may be considerably higher Reference: Ravindra_Dhir,_Michael_McCarthy]_Concrete_in_the_