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 final finish was born out of necessity. A California concrete contractor became tired of having slabs crack randomly either before conventional wet sawcutting could begin or while in the midst of making a cut.
PURPOSE OF JOINTS
The basic purpose of sawcut joints is to encourage shrinkage restraint cracking at predetermined locations. The sawcut creates a weakened plane (thinnest point of 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, sawcutting can provide crisp 90º joint nosing and a reservoir that allows a semirigid joint filler to be installed (full depth of sawcut) 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 sawcutting 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. Sawcuts 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 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 sawcut 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 sawcutting 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.
