Section Lightweight Concrete

11.1 General

11.3 Materials

SECTION 11 LIGHTWEIGHT CONCRETE C11.1 General

A favourable circumstance for the preparation of Chapter 11 was that during the last years substantial work was conducted in updating the state-on-the-art on lightweight aggregate concrete to the most actual level. In this respect the joint CEB/FIP (now fib) Task Group 8.1 published in 1999 fib Bulletin 4 "Lightweight Aggregate Concrete: Codes and Standards", a State-of-the-Art Report giving a good overview of common practice with regard to design practice in various countries [1]. Another report "Structural Lightweight Aggregate Concrete: Recommended Extensions to Model Code 90", by the same Task Group, is now ready to be published [2]. Other valuable work was done within the scope of the Brite-Euram Project "EuroLightcon". Important reports produced by the partners in this project were [3] and [4].

Chapter 10 in the new draft for EC-2 of 1/1/2000 applies to all concretes with a closed structure made with natural or artificial mineral lightweight aggregates, unless reliable experience indicates that provisions different from those given can be adopted safely.

"Lightweight aggregate concrete" is defined as a concrete having a closed structure and an oven-dry density of not more than 2200 kg/m3 consisting of or containing a proportion of artificial or natural lightweight aggregates having a particle density of less than 2000 kg/m3.

C11.3 Material properties

The material properties of lightweight aggregate concrete are related to the corresponding properties of normal aggregate concrete as defined in section 3.1. The following conversion factors have been introduced in order to derive the properties of lightweight concrete from those of normal weight concrete:

^e conversion factor for the calculation of the modulus of elasticity coefficient for the determination of the tensile strength ^2 coefficient for the determination of the creep coefficient ^3 coefficient for the determination of the drying shrinkage p oven-dry density of lightweight aggregate concrete in kg/m3

Concrete strength and stress-strain relations

The strength classes for lightweight aggregate concrete range from LC 12/15 to LC 80/95, where the first figure stands for the characteristic cylinder strength and the second for the characteristic cube strength.

Altogether 3 stress strain relations have been defined. The first one describes the average behaviour as realistically as possible and is meant for calculating the distribution of forces and moments in a structure. The second and the third are both given, as alternatives, for the design of cross sections. The second and the third differ only in the ascending branch, the second being parabolic and the third being linear, Fig. 11.1.

Lightweight concrete is more brittle than normal density concrete of the same strength class. This is reflected in the formulation of the ultimate strain, where the factor has been added.

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