operations end for the day or where one structural element is cast against previously placed concrete. These joints allow some load to be transferred from one structural element to another through the use of keys or (for some slabs and pavement) dowels. Note that the construction joint extends entirely through the concrete element.

SAWING CONCRETE

LEARNING OBJECTIVE: Upon completing this section, you should be able to determine proper occasions for using the concrete saw.

THE CONCRETE SAW

The concrete saw is used to cut longitudinal and transverse joints in finished concrete pavements. The saw is small and can be operated by one person (figure 7-40). Once the cut has been started, the machine provides its own tractive power. A water spray is used to flush the saw cuttings from the cutting area and to cool the cutting blade.

Several types of blades are available. The most common blades have either diamond or Carborundum cutting surfaces. The diamond blade is used for cutting hard or old concrete; the Carborundum blade is used for cutting green concrete (under 30 hours old). Let’s take a closer look at these two blades.

DIAMOND BLADES

Diamond blades have segments made from a sintered mixture of industrial diamonds and metal powders, which are brazed to a steel disk. They are generally used to cut old concrete, asphalt, and green concrete containing the harder aggregates. Diamond blades must always be used wet. Many grades of diamond blades are available to suit the conditions of the job.

Twelve-inch-diameter diamond blades are the most popular size. This size makes a cut about 3 1/4- inches deep. Larger-size blades are used for deeper cuts.

CARBORUNDUM BLADES

Low-cost, abrasive blades are now widely used to cut green concrete made with soft aggregates, such as limestone, dolomite, coral, or slag. These blades are made from a mixture of silicon carbide grains and a resin bond. This mixture is pressed and baked. In

Figure 7-40.-Concrete saw. and baked.

In many cases, some of the medium-hard aggregates can be cut if the step-cutting method is used. This method uses two or more saws to cut the same joint, each cutting only a part of the total depth. This principle is also used on the longitudinal saw, which has two individually adjustable cutting heads. When a total depth of 2 1/2 inches is to be cut, the leading blade cuts the first inch and the trailing blade, which is slightly narrower, cuts the remaining depth.

Abrasive blades come in 14- and 18-inch diameters. They are made in various thicknesses to cut joints from 1/4-inch to 1/2-inch wide.

When to Use

When is the best time to saw green concrete? In the case of abrasive blades, there is only one answer—as soon as the concrete can support the equipment and the joint can be cut with a minimum of chipping. In the case of diamond blades, two factors must be considered. In the interest of blade life, sawing should be delayed, but control of random cracking requires sawing at the transverse joints as early as possible. Where transverse joints are closely spaced, every second or third joint can be cut initially and the rest cut later. Sawing longitudinal joints can be delayed for 7 days or longer.

For proper operation and maintenance of the concrete saw, follow the manufacturer’s manual.