Fundamentals of Jointing for Volume Volume Changes in Concrete Concrete is at its greatest volume when it is freshly placed. As the concrete cures and hardens, water is consumed in the chemical reaction that occurs between the water and portland cement. Excess water is also evaporated from the concrete as heat is liberated in this chemical reaction. This chemical hardening and water loss causes a relatively large initial reduction in volume of the concrete mass, known as shrinkage. 2 Winter Quarter 2015 1
Volume Changes in Concrete One-third of the average shrinkage occurs during the first month and most of the remaining shrinkage occurs by the end of one year. 3 Volume Changes in Concrete Concrete, like most other building materials, will change in volume a definable amount due to moisture and temperature changes. Concrete will expand with a rise in temperature or a gain in moisture and will contract with a fall in temperature or a loss of moisture. This means that whether concrete will expand or contract depends upon the temperature and moisture condition at the time. Opposite conditions tend to counteract each other and similar conditions compound each other. 4 Winter Quarter 2015 2
Volume Changes in Concrete If concrete was unrestrained -meaning that it was free to move in any direction without resistance to changes in volume- there would be no stress in the concrete. However, concrete in use is rarely in a total unrestrained condition. Therefore, once the volume change stresses become greater than the tensile strength of the concrete, the concrete cracks. 5 Jointing in Concrete Most cracking in concrete can be controlled by joints that reduce stress to manageable limits. The three types of joints are: Contraction (or control) joints Used to subdivide large sections into smaller units. Isolation (sometimes called expansion) joints Used to isolate sections of unusual shape or to separate slabs from fixed objects. Construction joints Used to provide transition between one day s work and the next 6 Winter Quarter 2015 3
Contraction Joints Contraction joints are necessary to control natural cracking from stresses caused by concrete shrinkage, thermal contraction, and moisture or thermal gradients within the concrete. Typically, transverse contraction joints are cut at a right angle to the pavement centerline and edges. 7 Both longitudinal and transverse contraction joints are required in most situations. The theory behind proper jointing is to provide a weakened section that forces the concrete to crack at predetermined locations. 8 Winter Quarter 2015 4
The depth of the joint must be at least onefourth the slab thickness in order to provide a sufficiently weakened section to control cracking. 9 A general rule-of-thumb is that the spacing in feet should not exceed two to three times the slab thickness in inches. For example, a 5-inch slab will be jointed at 10 to 15 feet intervals in both directions. Plain pavements with short joint spacings designed to control cracking are usually limited to spacings of 15 to 20 feet. 10 Winter Quarter 2015 5
Pavement lane widths must be limited to 12 to 13 feet in order to control longitudinal cracking. Longitudinal joints are usually tied together with deformed tie bars to keep the lanes from separating at these joints. 11 Pavements with heavy truck traffic will also require load transfer dowels to prevent differential movements of adjacent panels. 12 Winter Quarter 2015 6
Highway pavements with welded wire fabric placed two-inches from the slab surface have joints spaced up to 40 feet apart. Joints in floors of buildings to control early shrinkage stresses vary from about ten feet to thirty feet depending on floor thickness, size of aggregate and slump of the concrete. 13 Contraction Joints in Buildings Joints should be no more than twenty feet apart in exterior walls with frequent openings. In walls without openings, the joint spacing should never exceed twenty-five feet to be most effective. Joints should be placed within ten to fifteen feet of a corner, if possible. Where small openings are more than twenty feet apart at the first-story level, there should be a joint in line with each jamb below the openings. Above the first-story, a single joint at the centerline of each opening will usually be adequate. 14 Winter Quarter 2015 7
Isolation (or Expansion) Joints They permit both horizontal and vertical differential movements at a joining parts of the structure, such as: Around the perimeter of a floor on ground Around columns Around machine foundations The objective is to separate the slab from the more rigid parts of the structure. Care must be taken during the construction of isolation joints to assure that the joint passes completely through the structure in order to provide complete isolation. A joint that provides only partial isolation will not function properly. 15 Isolation Joint The material could be as thin as ¼ inch or less, but ½ inch materials are commonly used. 16 Winter Quarter 2015 8
Construction Joint A true construction joint should bond new concrete to existing concrete and prevent movement. Deformed tie-bars are often used in construction joints to restrict movements. Since extra care is needed to make a true construction joint, they are usually designed and built to function as contraction joints, and therefore may purposely be made unbonded. 17 Construction Joint Construction joints join concrete that is paved at different times. Transverse construction joints are necessary at the end of a paving segment. Longitudinal construction joints join lanes that are paved at different times, or join through-lanes to curb and gutter. 18 Winter Quarter 2015 9