A saw cut joint show-through in a VCT tile along with movement of the VCT tile, as illustrated by the opening of the tile joint.

One of the most perplexing segments of the commercial flooring installation is how to handle joints in a concrete slab. Flooring manufacturers insist that flooring contractors “honor” all joints that are active (moving) and to cover all dormant (non-moving) joints. The general contractor, architects, designers or owners do not want to place a joint cover down the middle of the room just to accommodate the flooring manufacturer, nor do they want to deal with the aesthetics left by the joint cover. With fast-track construction, lack of environmental controls, concrete joints are going to keep moving long after the structure is occupied. Just how do you know whether a concrete joint is active or dormant? You cannot look at a concrete joint and make that determination. Just how long will the concrete movement be a problem? We know that 80 percent of the concrete movement peaks once the slab is stabilized, normally in 6-9 months, but has been known to move for as long as 3 years after the temperature and humidity have been stabilized. In this article we are going to look at this dilemma and what needs to be done with joints. Cracks in concrete cannot be prevented entirely, but they can be controlled and minimized by properly designed joints.

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. Concrete cracks that are over 0.035” will loose their aggregate interlock and allow ledging.

Types of Concrete Joints

 Isolation or Expansion 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
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 placed 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.

Construction Joints
Sawcut Contraction joints (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.

Slab curling can make this situation worse. If the slab curls after the joint is cut and then relaxes after the floor covering is placed, that movement can also cause the joint filler to bulge upward. Most floor covering installers choose to repair this problem by removing the row (or strip) of covering directly above the joint bulge and using a razor blade to trim the joint filler that has bulged. If this solution is implemented before the slab moisture is in equilibrium, additional moisture movement may cause the joint to bulge further and require a second repair.

The following is a series of events that happen to the slab starting with the pour and ending with a bulge occurring in the finished floor.

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 ¾”.  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.