Sliding joints may be required where one unit of a structure must move 

 in a plane at right angles to the plane of another unit; e.g., in certain 

 reservoirs where the walls are permitted to move independently of the floor 

 or roof slab. These joints- are usually made with a bond-breaking material 

 such as a bituminous compound, paper or felt that also facilitates sliding. 



(5) Cracks . Although joints are placed in concrete so that 

 cracks do not occur elsewhere, it is seemingly impossible to prevent 

 occasional cracks between joints for a variety of reasons. As far as the 

 problem of sealing is concerned, cracks may be regarded as contraction 

 joints of irregular line and form. 



e. Joint Configurations . Two basic configurations of the schematic 

 joint details for various types of concrete structures occur from the 

 standpoint of the functioning of the sealant. These are known as butt 

 joints and lap joints. 



In butt joints, the structural units being joined abut each other and 

 any movement is largely at right angles to the plane of the joint. In lap 

 joints, the units being joined override each other and any relative move- 

 ment is one of sliding. Butt joints, and these include most stepped 

 joints, are by far the most common. Lap joints may occur in certain 

 sliding joints, between precast units or panels in curtain walls, and at 

 the junctions of these and of cladding and glazing with their concrete or 

 other framing. The difference in the mode of the relative movement between 

 structural units at butt joints and lap joints, in part, controls the 

 functioning of the sealant. In many of the applications of concern pure 

 lap joints do not occur, and the functioning of the lap joint is in practice 

 a combination of butt and lap joint action. 



From the viewpoint of the sealant, two sealing systems should be 

 recognized. First, there are open surface joints, as in pavements and 

 buildings in which the joint sealant is exposed to outside conditions on at 

 least one face. Secondly, there are joints such as in containers, dams, 

 and pipelines, in which the primary line of defense against the passage of 

 water is a sealant such as a waterstop or gasket buried deeper in the 

 joint. The functioning and type of sealant material that is suitable and 

 the method of installation are affected by these considerations. 



In conclusion, two terms should be mentioned that are in wide, though 

 imprecise use. Irrespective of their type or configuration, joints are 

 often called "working joints" where significant movement occurs and "non- 

 working joints" where movement does not occur or is negligible. 



f. Sealant Materials. 

 



(1) Performance in Open Joints . For satisfactory performance a 

 sealant in open surface joints must: 



(a) Be an impermeable material; 



(b) deform to accommodate the movement and rate of movement 

 occurring at the joint; 



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