Contents preface (VII) introduction 1—37

16.9. STRUCTURAL JOINTS
Joints in concrete dams are essentially designed cracks which are suitably located and treated to minimise undesirable effects. These joints are of three types.
Contraction joints prevent tensile cracks on account of volumetric shrinkage due to drop in temperature. A concrete dam is usually constructed in blocks separated by the transverse contraction joints which are normal to the axis of the dam and are continuous from the upstream face to the downstream face. Longitudinal contraction joints in the blocks formed by the transverse contraction joints are also sometimes considered necessary. The contraction joints are vertical and normally extend from the foundation to the top of the dam. Reinforcement should not extend across a contraction joint.
Expansion joints accommodate volumetric increase due to rise in temperature besides preventing transfer of stress between different units of the structure. Contraction and expansion joints are constructed in such a manner that there is no bond between the adjacent units of the structure.
Construction joint is the surface of the previously placed concrete upon or against which new concrete is to be placed. Besides permitting subsequent placing of concrete, these joints facilitate construction, reduce shrinkage stresses, and permit installation and embedded metal work. Suitable measures are adopted to ensure proper bond between the previously placed concrete and new concrete. These measures include cleaning by high-velocity water jets and roughening the surface of the previously placed concrete. A thin mortar layer is sometimes placed on the surface before placing new concrete. Construction joints do not require water seals and keys.