As one thinks back over the past 30 years, a number of innovations have significantly changed and improved the way we do things. For example, the word processor has made the typewriter and correction fluid collectors’ items. Microwave cooking has shortened the time it takes to prepare meals. In concrete construction, the laser screed has, on many projects, replaced conventional hand screeding, improved floor tolerances, and allowed for much larger placements to take place at one time.
Another innovation that has significantly changed and improved the way concrete floors are constructed is the “early-entry” saw. The idea of creating a saw that would allow contraction/control joints to be cut soon after the final finish was born out of necessity. A California concrete contractor became tired of having slabs crack randomly either before conventional wet saw cutting could begin or while in the midst of making a cut.
PURPOSE OF JOINTS
The basic purpose of saw cut joints is to encourage shrinkage restraint cracking at predetermined locations.
The saw cut creates a weakened plane (thinnest point of the slab) where shrinkage restraint cracking, called joint activation, is intended. While tooling notches in the surface of plastic concrete slabs may still work on small projects, saw cutting can provide crisp 90º joint nosing and a reservoir that allows a semi-rigid joint filler to be installed (full depth of saw cut) that helps protect the joint from heavy, hard-wheeled traffic. These joints are often called control joints as they are intended to control random cracking. Studies have investigated the “window of opportunity” for saw cutting and the factors involved in proper timing. The “near” window is determined by the degree of raveling that occurs and depends on the rate of strength development in the concrete, the condition of the sawing equipment, operator experience, and the aggregate type. The “far” window is determined by the development of shrinkage restraint stresses that result in random cracking.
The total amount of shrinkage of a slab depends on the temperature drop (cooling) and the loss of moisture (drying). At early ages, the slab is heated by the chemical reaction between cement and water (cement hydration), which is exothermic. However, the reaction results in a concrete temperature peak, which often corresponds to a drop in ambient temperature at night. As the slab cools, concrete contracts. Saw cuts are also referred to as contraction joints as they are intended to relieve the stresses that develop due to this early-age thermal contraction and subsequent drying shrinkage.
Drying shrinkage also occurs as moisture is lost from the concrete slab. Concrete mixtures typically contain more water than is required for the hydration of cement to enable proper placement, consolidation, and finishing. While some of this “water of convenience” is lost during bleeding (up to 10% depending on the setting behavior of the concrete), a portion remains in the slab after hardening occurs. The evaporation of this free water within the slab causes drying shrinkage. The rate of moisture loss is dependent on the porosity of the slab, ambient conditions, curing method, and the presence of a sealer/hardener/densifier.
Tensile stress develops in concrete slabs on the ground due to the restraint of temperature contraction and drying shrinkage. Slab movement is restrained by friction from the base, structural elements, and steel reinforcement. Cracking occurs if the tensile stress caused by shrinkage restraint exceeds the developed tensile strength of the concrete. As shrinkage begins to occur before the development of significant strength, it is critical to install saw cut contraction/control joints early to avoid random cracking.
Historically, conventional wet-cut saws were used to install joints the day after concrete placement. By the time the slab was ready for a conventional wet cut, shrinkage stresses had already begun to develop. In some cases, a random crack may already have formed off a re-entrant corner or other forms of restraint, or a random crack would have developed immediately ahead of the saw cutting operation as it progressed across the slab. Experience and research have proven that sawing should be completed before the concrete reaches its peak heat of hydration, typically around 8 to 12 hours after finishing the slab. The specific window of opportunity is project specific and depends on the concrete mix, slab thickness, and ambient conditions.
Early-entry saws, sometimes called green concrete saws, were first introduced to the market in 1988. The biggest challenge to saw cutting at an early age was the raveling that would take place at the top edges of the joint if cutting was started too soon. A number of approaches were tried. The first successful early entry saw was a small, 25-pound electric unit that made a 3/4-inch-deep cut. This unit could be used to make a saw cut control/contraction joint immediately after a burnished slab finish was accomplished. While a 3/4-inch-deep cut was not often the one-fourth of slab thickness recommended for saw-cut joints, it did seem to work very well for most slabs up to 6 inches thick. Placing the plane of relief into the slab before the heat of hydration subsided and drying shrinkage began, a 3/4-inch-deep saw cut would work most of the time. This theory proved to be true except with concrete mixtures where coarse aggregates larger than 3/4 inch were used.
Since then, early-entry saw cutting of joints progressed far beyond the small electric unit to larger gas powered saws, some capable of cutting up to 4 inches deep. Significantly lighter than conventional wet-cut sawing equipment, these saws enable sawing to be initiated much earlier, generally as soon as the concrete can be walked on. The saws operate at very high RPMs and typically have a blade thinner than conventional diamond saw blades. Also, most early-entry blades do not require water for cooling so they are a dry-cutting operation that results in less mess and required cleanup.
One study using conventional wet-cut saws found that raveling was within acceptable limits when the concrete compressive strengths were between 300 and 1015 psi depending upon the ambient conditions and the paste content, water-cementitious material ratio, and type of cementitious material and aggregate used in the mixture. However, with early-entry saws, joint installation can begin once the concrete has achieved a compressive strength of 150 to 500 psi or even 25 to 50 psi. These saws typically allow joint installation to begin as soon as finishing is completed or within 1 to 4 hours after final finish. A special cardboard or plastic insert can be placed into the saw cut at each intersection to keep the corners from spalling as the crosscut is made.
Proprietary systems, sometimes called ultra early- entry saws incorporate patented skid plates that effectively provide support to the joint nosing as the blade cuts. In addition, the blade rotates in an up-cutting direction, resulting in cleaner joints. These systems allow saw cutting to begin within 1 to 2 hours after completion of the final finish. Earlier cutting provides several advantages to the contractor and benefits the resulting concrete slab.
While greatly improving random crack control, early-entry saw cutting has other advantages over wet-cut sawing. It is important to begin curing a new concrete slab as soon as possible after the final finish. On many projects, wet or dry cover curing methods are used to allow for the subsequent application of a liquid surface hardener/densifier or to hasten the drying time of slabs scheduled to receive a moisture-sensitive flooring material or coating. When a wet-cutting process is used, the start of curing either must be delayed until cutting can begin or the cover removed and replaced after cutting is completed. Because early-entry saw cutting can begin immediately after the slab receives a hard steel-troweled finish, curing can begin far sooner than when wet-cutting is used; the need to remove and replace the curing cover may be eliminated.
Another potential advantage to early-entry saw cutting is that the joint depth may be reduced and still result in joint activation. For over 50 years, the recommended joint depth has been between one-fourth and one-third the slab thickness. While experience has shown that early-entry sawing can be performed at shallower depths, research has supported the theory. The researchers used fracture mechanics to show that the needed depth of saw cut depends on the fracture strength of the concrete and that saw cut timing is much more important than saw cut depth in controlling random cracking. They found that the early saw cuts, while shallower, promote joint activation because the tensile stress development is due to moisture and temperature reductions that occur at the surface of the slab at an early age. This top-down tensile stress development is also responsible for changes in surface flatness due to thermal (curling) and moisture (warping) gradients. The upper region of the slab shortens relative to the bottom. Therefore, it was found that shallow joints (as little as 1 inch for a 13-inch-thick slab) activate, and the crack extends downward as the depth of shrinkage progresses. Of course, the ability of the joint to promote cracking is dependent on the variability of slab thickness.
While the laser screed and modern pavers enable very strict elevation control for interior concrete slabs and pavements, respectively, if the base elevation isn’t controlled, the slab thickness can vary significantly. Sometimes, thickness deviations result in areas of weakness beyond that created by the saw cut, and cracking occurs randomly. Therefore, while early-entry saw cutting has been demonstrated to be effective at shallower depths, industry standards still recommend saw cutting to a minimum depth of one-fourth the slab thickness to reduce the risk associated with some potential thickness variation. Minimizing the shrinkage potential of the concrete mixture also decreases the likelihood of random cracking.
Early-entry saw cutting provides many benefits to the construction and long-term performance of concrete slabs. Because the cutting is done when the concrete has achieved little strength, the duration of saw cutting operations is typically less than if sawing is performed at higher concrete strength. If the slab thickness is controlled, reduced joint depth also shortens the installation period. Not only does this save the contractor time and money, but curing can be started sooner. In addition, shallower joints also result in the potential for increased load transfer as there is a greater aggregate interlock area below the saw cut. However, while shallower joints may also result in less wear on the sawing equipment, the high tolerance of the blade and skid plate assembly requires regular replacement to maintain minimal joint raveling. In addition, the saw should not be allowed to ride up over coarse aggregate particles as a consistent saw cut depth is a factor in proper joint activation.