Control Joints for Concrete Slabs
Concrete, like other construction materials, expands and contracts with moisture and temperature changes. These volume changes may produce cracks in hardened concrete unless they are properly controlled. Provision for volume changes at predetermined locations prevents a concentration of crack producing stress forces. Such provisions are termed control relief joints. Adequately designed and constructed, these joints serve to eliminate unsightly random surface cracks (which detract esthetically from any type of member) by gathering, distributing and dissipating stress forces resulting from temperature and moisture variations.
Lack of, or inadequate, control joints can produce unsightly and damaging cracking. If control joints are to be effective, and perform their intended function, they must be located and installed correctly.
Plastic concrete in place, after being initially proportioned and mixed to homogenity, occupies greatest volume. When curing is discontinued, the concrete dries out, loosing by capillary action free, uncombined mixing water to the subgrade, or by evaporation to the atmosphere. This loss of moisture causes the concrete to contract and decrease in length. On changing from a saturated to a dry condition, average concrete will contract approximately two-thirds of an inch per 100 linear feet. This change is also approximately of the same magnitude that a concrete undergoes with a 100°F temperature decrease. Unless these volume changes are controlled through use of relief joints, cracking may occur.
Concrete restrained, and not free to contract as a monolithic unit, may crack on drying as the restraint produces a concentration of tensile stress forces. It is the restraint of the shrinkage, not the shrinkage itself, which causes cracking. Concrete that is not restrained, will not crack. Restraint may, or may not, be intentional.
It is virtually impossible to support a concrete member without some restraint. Differences in elevations of subgrade, type of subgrade upon which the concrete is placed, bond to existing wall or footing members, structural connections, etc., are typical examples of unintentional restraint. Any restraint which will concentrate drying stress forces is capable of producing cracks in the restrained member, unless control joints are utilized. Adequately designed control joints properly placed will relieve this restraint.
Many factors contribute to the drying shrinkage coefficient of concrete. Drying shrinkage values can be controlled and minimized through use of factors conducive to quality, serviceable and durable concrete. These include:
- Use of as low a water-cement ratio as possible to obtain adequate placement and consolidation requirements.
- Use of a minimum of fine size aggregate in relation and deference to coarse size aggregate. Fine aggregate quantity should be maintained at a minimum which will just produce adequate workability and finishing characteristics.
- Proper selection of clean, well graded specification quality aggregates.
- Use of functional water reducing agents that also reduce drying shrinkage, with advantage taken of the water reduction to lower the water-cement ratio.
- Use of low slump to place concrete.
- Proper consolidation of concrete.
- Adequate and continuous curing effected as soon as concrete is finished greatly enhances strength properties and minimizes development of shrinkage distress cracks.
Construction joints, unlike expansion and contraction joints, are no intended to allow for movement of concrete members, but generally are effected at the end of a lift, at the end of a day’s concrete placement, etc. This type of joint is a plane surface between two sections of concrete-concrete placed against concrete already in place which has hardened to the extent that consolidation cannot be effected by vibration or re-vibration. Construction joints may be horizontal as in a structure or column, or vertical as in a slab, or both as in a wall. This type of joint is commonly termed as a “cold joint.”
Quality of a construction joint is directly related to quality and placement of the concrete. Maximum bond and watertightness are obtained with quality concrete of the lowest slump which will just permit adequate placement and consolidation. Bleeding and segregation tendencies of high slump concrete promote laitance and weak surfaces of low bonding character. A clean, structurally sound surface is desirable. Coarse aggregate pieces protruding from the plane, as well as slight indentations, are not beneficial or recommended. Surface retardants are often used to achieve a proper surface.
Reinforcing steel, if used, normally extends across a construction joint. The steel should be placed in such a manner that only half the steel spans the joint. This may necessitate cutting every other piece of reinforcing rod extending across the joint. Unless some reduction of steel restraint is established, a plane of weakness is not in effect.
Some slab designs use load transfer-slip dowel devices. These are usually installed only across transverse joints. One end of the dowel must be free to permit the concrete to contract and move unrestrained from that end of the dowel. Usually this is accomplished by use of a metal, paper, rubber sleeve in which the free end of the dowel device rests. The other end is embedded in concrete of the adjoining slab.
Another method of load transfer is shear keys, or keyways, which are often used to accomplish the same purpose. Keyways are formed with templates of metal or wood. Standard steel forms are often constructed with a keyway in place. Any wood insert in concrete, such as a keyway, should be well soaked in water prior to concrete placement, or be resin sealed. Unless pre-soaked, the wood will draw water from the concrete, swell and promote stress cracking in the concrete. Keyways should never be so thin as to be possibly sheared off with movement. Beveled strips can be used to form keyways, being removed, after the concrete has hardened. Spalling of edges is avoided by careful handling during removal of the strips.
Expansion joints permit volume change movement of a concrete structure or member. These are usually constructed by installing pre-formed, or pre-molded elastic/resilient material of approximately ¼″ to ½″ thickness as wide as the concrete is thick, before the concrete is placed. Expansion joints should never be less than ¼″ wide. Pre-molded expansion joints for installation in residential, commercial, or industrial slabs may be of fiber, sponge rubber, plastic, or cork composition. Such materials must be highly resilient, and non-extruding in hot weather, or brittle in cold weather.
An expansion joint should always be utilized where a concrete member will join or abut an existing structure of any type. This would include a junction of sidewalks, sidewalk with a driveway, building, curb, or other similar members, as well as where a floor slab joins a column, staircase, etc. The square formed by the intersection of two sidewalks should have pre-molded expansion material enclosing the perimeter. Normally, expansion joints are not provided in sidewalks other than where the walk abuts an existing structure.
Expansion joints should also be provided in a building floor slab where the slab abuts walls or footings. Sealing of expansion joints is desirable in many outdoor or industrial/commercial applications.
Large flat areas, or long lengths of concrete placed monolithically, require contraction joints. These are essentially weakened planes constructed in a concrete member to provide a reduction in member thickness for the purpose of controlling shrinkage stresses to that specific area. These are commonly referred to as “dummy joints,” which are place at predetermined points of possible stress concentration. Thin joints spaced at frequent intervals are more effective than thicker joints spaced less frequently.
For sidewalks, transverse dummy joints are usually spaced at 5 to 6 foot intervals. Driveway, patio, and floor slab dummy joints should be spaced 15 to 20 feet apart. A longitudinal joint constructed down the center of a double width driveway, dividing the driveway into two equal width sections, is equally as beneficial as the transverse joints. When widths exceed 20 feet, joints should be used to break up those expanses into architectural widths not to exceed 20 feet. Reinforced concrete pavement joints are usually spaced at greater intervals of 40 to 80 feet. When concrete is reinforced, the steel should be placed in such a manner that only one half the reinforcing bars will span the joint. This establishes the plane of weakness at the joint area.
Tooled joints, if improperly constructed, can detract from appearance of the concrete. Joints must be perpendicular to the edge and straight. Use of a straight edge, such as a one inch thick board, to serve as a guide is recommended in obtaining a straight-lint joint. The joint may be filled with a flexible joint sealing compound to prevent water penetration, if desired. This is not necessary for sidewalks or patios.
Restraint corners usually are highly stressed areas, and are starting points for cracks unless rounded or jointed.
Many methods are used to construct joints. One of the most commonly used methods for sidewalks, slabs, driveways and similar members is by grooving the plastic concrete with a grooving or jointing tool. The cutting edge of a grooving tool is V shaped, to cut a V joint partly into the plastic concrete of the member. This creates a reduction in member thickness which localizes cracking to that weakened plane area. When the concrete dries out and contracts, the joint opens up further to accommodate that volume change. Installation of a dummy joint is effected after the concrete has been edged, and prior to float-finishing of the surface. Forming strips of wood (pre-soaked or pre-sealed) or metal may be embedded in the plastic concrete and carefully removed after the concrete has hardened. This leaves a joint of predetermined width and depth in the concrete. Premolded tongue and grooved joints are often used to form contraction joints in industrial floors.
A later, more recent innovation to construct contraction joints which is gaining rapid industry acceptance utilizes electric or gasoline powered saws equipped with shatterproof abrasive or diamond rimmed blades. The blade cuts a joint into the hardened concrete as soon as the surface will not be torn, abraided or damaged by the cutting action.
Concrete jointing or grooving tools are metal, about 6 inches long, 3 to 4 inches wide with different length bits ranging from 3⁄16 of an inch to 1 inch in depth. The bit is V shaped to eliminate spalling from pinching at the rim area. The V groove approximates 3⁄8 of an inch in width at the top and ¼ of an inch in width at the bottom.
To be functional, it is recommended contraction joints be at least ¾, and preferably 1″ in depth. A practical rule of thumb guide for depth of dummy joints is-a depth equal to at least one-fourth the thickness of the member. Joints which are too shallow serve only a decorative purpose, and are not functional in respect to controlling and localizing stress cracking to that area. Frequently, shrinkage cracks are just ahead of, or slightly behind dummy joints which are too shallow, existing in a random, haphazard pattern.
Electric or gasoline powered circular saws fitted with either reinforced abrasive blades or metal bonded diamond blades are used to saw contraction joints in concrete. Sawed joints are uniform and straight with sharp edges. Water is generally required as a coolant for the blades to dissipate frictional heat. When used, a constant flow of approximately 2 ½ gallons per minute is sufficient.
In areas where extremely soft aggregates prevail, sawing can be effected “dry” if performed at an early stage. In this case, moisture of the concrete acts as a coolant. When diamond blades are used, water is an absolute necessity. The water also serves to flush fine particles of concrete away from the blade. Blades, classified as soft, medium and hard, are available for different concretes depending upon hardness of the aggregate, strength of the concrete when sawed, and speed of sawing.
Thanks to Kaiser Cement who supplied information for this document.