Why concrete cracks
Concrete is weak in tension and, therefore, if it’s natural tendency to shrink is restrained, tensile stresses that exceed its tensile strength can develop, resulting in cracking.
At early ages, before the concrete dries out, most cracking is caused by temperature changes or by the slight contraction that takes place as concrete sets and hardens. Later, as the concrete dries, it will shrink further and either an additional crack will form or existing cracks will become wider.
Joints provide relief from the tensile stresses, are easy to maintain and are less objectionable than uncontrolled or irregular cracks.
Types of Concrete Joints
Isolation or Expansion Joints should be used wherever complete freedom of vertical and horizontal movement is required between the floor and adjoining building elements. Isolation joints should be used at junctions with walls, (not requiring lateral restraints from the slab), columns, drains, manholes and stairways. Isolation joints are formed by the insertion of preformed joint filler between the floor and the adjacent element. The joint material should extend the full depth of the slab and not protrude above it. Isolation joint are usually active joints and are rarely without movement as they permit independent vertical and horizontal movement between adjoining parts of the structure.
Construction joints are placed in a slab to define the extent of individual placements, generally in conformity with a predetermined joint layout. They are typically place at the end of a day’s work but may be required when concrete placement is stopped for longer than the setting time of concrete. Construction joints may be doweled, keyed or butted depending upon the intended usage of the slab. Regardless of the usage construction joints are generally active and should be treated as such.
Contraction or Control (Sawcuts) control joints are used to limit random, out of floor joint, floor slab cracking. These joints are usually on column lines, with intermediate joints located at equal spaces between column lines. These joints must be carefully designed and properly constructed if uncontrolled cracking of concrete slabs are to be avoided. Depending upon the size of the large aggregate, the maximum spacing of contraction joints should be 24 to 36 times the thickness of the slab. For example, in a 4-inch [100mm] thick slab the joint spacing should be about 10 feet [3 m]. It is further recommended that joint spacing be limited to a maximum of 15 feet [4.5 m]. The depth of the sawcut should be a minimum of 1/4 the depth of the thickness of the slab and/or a minimum of 1 inch [25mm]. All construction joints should be square or nearly so and L-shaped panels should be avoided. Sawcut joints are generally cut 4 – 12 hours after the concrete has been finished. Contraction/control joints can be either active or dormant. Active contraction joints are caused by the slab not being dry or stabilized by temperature and humidity.
Concrete movement is caused by temperature and moisture and/or humidity. The movement from thermal coefficient is about twice that of moisture. This combined movement is a very small amount roughly 1/8-inch per 100 l/f per 5 degrees Fahrenheit. It is enough to affect the joints. For instance, say the joints are 10 feet apart this would represent about 1/64-inch of movement at each joint. Now imagine you have filled the joint with high compressive strength Portland-based patching compound restricting the slab movement. The movement is going to occur, so something has to give. Is it going to be additional cracking, more telegraphing of existing cracks or a pushing up of the patching compound?
The moisture side of movement is even more complex. Moisture in concrete causes expansion of the Portland cement, the only part of concrete that moves. If a slab is not entirely dry, water as a liquid will go to the bottom of the slab, causing it to expand at the bottom. The surface of the slab is exposed open elements which, normally is a drying condition, as the slab dries the surface of the slab shrinks. This effect is known as slab curl. Slab curl is variable and will change as the slab continues to dry. Once the slab is dry the slab will go into equilibrium, this means the moisture content is equal from top to bottom. That is when the slab movement becomes dormant.
Flooring contractors will send their installation crew out and start the floor preparation with no idea of wither there is any potential for movement at the concrete joints. First of all, a calcium chloride test, done to ASTM F-1869 standards, will give you no indication of the amount of moisture down in the slab they measure moisture movement at the surface of the slab down as far as about 3/4”. The hygrometer probe test, (in-situ), done to ASTM F-2170 standards, will give you a more realistic look at the internal moisture content. If you were to do both tests and the calcium chloride was indicating a slab dry enough to install over and the hygrometer probe test was high you could expect the slab to be curled and the joints to be active. Any high compressive Portland-based filler, used to fill any active joints, will be pushed up showing through the finished floor, only to be found on the deficiency (punch) list.