freezing and thawing, wetting and drying, leaching or erosion caused by any 

 concentrated or excessive influx of water at joints. Foreign solid matter, 

 including ice, must be prevented from collecting in open joints; otherwise 

 the joints cannot close freely later. Should this happen, high stresses 

 may be generated and damage to the concrete may occur. 



In industrial floors the concrete at the edges of joints often needs 

 the protections of a filler or sealant (possibly between armored faces) 

 capable of preventing damage from impact of concentrated loads such as 

 steel-wheeled traffic. The specific function of sealants is to prevent 

 the instrusion of liquids (sometimes under pressure), solids or gases, and 

 to protect the concrete against damage. In certain applications secondary 

 functions are to improve thermal and acoustical insulation, damp down 

 vibrations, or prevent unwanted matter collecting in crevices. They must 

 often perform their prime function while subject to repeated contractions 

 and expansions as the joint opens and closes and while exposed to heat, 

 cold moisture, sunlight, and sometimes, aggresive chemicals. 



In most concrete structures all concrete-to-concrete joints (contrac- 

 tion, expansion, and construction), and the periphery of openings left for 

 other purposes, require sealing. One exception is contraction joints (and 

 cracks) that have very narrow openings, e.g., those in certain short plain 

 slab or reinforced pavement designs. Other exceptions are certain construc- 

 tion joints, e.g., monolithic joints, not subject to fluid pressure or 

 joints, between precast units used either internally or externally with 

 intentional open draining joints. 



d. Types of Joints and Their Function . 



(1) Contraction (Control) Joints . These are purposely made 

 planes of weakness designed to regulate cracking that might otherwise occur 

 due to the unavoidable, often unpredictable, contraction of concrete 

 structural units. They are appropriate only where the net result of the 

 contraction and any subsequent expansion during service is such that the 

 units abutting are always shorter than at the time the concrete was placed. 

 They are frequently used to divide large, relatively thin structural 

 units, e.g., pavements, floors, canal linings, retaining and other walls 

 into smaller panels. Contraction joints in structures are often called 

 control joints because they are intended to control crack location. 



Contraction joints may form a complete break, dividing the original 

 concrete unit into two or more units. Where the joint is not wide, some 

 continuity may be maintained by aggregate interlock. Where greater con- 

 tinuity is required without restricting freedom to open and close, dowels 

 and in certain cases steps or keyways, may be used. Where restriction of 

 the joint opening is required for structural stability, appropriate tie 

 bars or continuation of the reinforcing steel across the joint may be 

 provided. The necessary plane of weakness may be formed either by partly 

 or fully reducing the concrete cross section. This may be done by in- 

 stalling thin metallic, plastic or wooden strips when the concrete is 

 placed or by sawing the concrete soon after it has hardened. 



(2) Expansion (Isolation) Joints . These are designed to prevent 

 the crushing and distortion (including displacement, buckling, and warping) 



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