Design properties of materials

Abstract

Introduction Selection of materials and a decision on the associated properties constitute an essential first step in any structural design. In this chapter, the relevant information and data required for the design of reinforced and prestressed concrete structures are covered in some detail. Concrete Characteristic strengths The characteristic compressive strength at 28 days is defined as the strength value of a given concrete that is exceeded by 95% of concrete cylinders tested (made from the same concrete). The test method for determining is described in AS 1012.9–2014. The value of is used in all Standard-based design calculations. Thus, in mix design, the strength to aim for (or the 28-day mean in situ compressive strength) is where s is the standard deviation of the test data and k varies from 1.25 to 2.5 depending on the grade of concrete and the manufacturing process (see AS 1379–2007 for all aspects of the testing and assessment of concrete or AS 1012.9–2014 for determining the compressive strength of concrete specimens). As per Clause 3.1.1.2 of AS 3600–2009 (the Standard), f cm at a given age may be taken as 90% of the mean cylinder strength of the same age or f cmi . Refer to Clause 3.1.1.1 for more recommendations. Various design procedures, such as those described by Ryan and Samarin (1992) and by Murdock and Brook (1982), may be used to determine the proportions of the constituent materials (water, cement, fine and course aggregates and admixtures as appropriate). As a function of, the characteristic flexural tensile strength as per Clause 3.1.1.3 of the Standard, may be taken as or it may be determined for a given concrete by the laboratory test described in AS 1012.11–2000 (R2014). This value is used mainly in cracking moment and deflection calculations (see Clauses 8.1.6.1 and 8.5.3.1, respectively, in the Standard), and in determining the minimum slab and pad-footing reinforcement (Clause 9.1.1). A second tensile strength, the characteristic principal (or splitting) tensile strength, is required mainly in the design for longitudinal shear (Clause 8.4.3 in the Standard).