Timing of sawcutting is very critical. The follow- ing items must be considered:
1. Sawing needs to commence as soon as the concrete has hardened sufficiently to permit cutting of concrete without exces- sive chipping, spalling or raveling (see Figure 3.3).
2. Factors that influence the rate of harden- ing of concrete are:
a. Cement content of mixture,
b. Air and concrete temperatures during placement, and
c. Mixture characteristics.
3. The contractor must be prepared to saw as soon as the concrete is ready for saw- ing, regardless of the time of day or night.
4. During warm weather, concrete will usu- ally be ready for sawing 4 to 12 hours after placement. In cold weather, or when mixture water is below 50°F, saw- ing could be delayed as long as 24 hours.
5. Generally, concrete mixtures with soft coarse aggregate (e.g., limestone) do not require as much strength development prior to sawing as mixtures with hard coarse aggregates.
6. If sawing is delayed, random cracking may occur.
7. Several factors can reduce the length of the joint sawing window (Figure 9.5). If the window becomes to short, cracking may occur.
8. The concrete must be capable of support- ing the weight of the sawing equipment and the personnel involved in the opera- tion.
9. During sawing, if spalling occurs along the sawcut, or if the sawcut tears the ag- gregate from the surface rather than go through the coarse aggregate, it is an in- dication that the concrete has not hard- ened sufficiently.
PAVING ALERT
Factors that Shorten the Joint Sawing Window:
• Sudden temperature drop.
• High wind, low humidity.
• High friction bases.
• Bonding between base and slab.
• Porous base.
• Retarded set.
• Paving fill in lanes.
• Delay in curing application.
Joint Sawing Window Factors
1. The earliest time to cut joints is usually determined based on the sawing equip- ment operator’s scratch test or observa- tion of the raveling or spalling at joints while making the initial cut.
2. A rule of thumb for the last limit of sawing opportunity is to sawcut before the sur- face concrete temperature decreases sig- nificantly.
a. Under most paving conditions, the top surface temperature eventually will start to decrease (Figure 9.6), while sub-surface concrete tempera- tures continue to increase.
b. Once the concrete surface tempera- ture decreases, creating a thermal gradient, thermal curling restraint stresses start to develop. Concrete cracking will result if the restraint stresses exceed the concrete tensile strength (see Figure 9.5).
c. If joints are sawed prior to significant surface cooling, curling restraint stresses remain low and cracking de- velops only at planned joint locations.
d. Monitoring of the concrete surface temperature can be done using sur- face thermometers or infrared guns.
e. On larger projects, slab surface tem- perature decreases can be moni- tored to establish guidelines for al- lowable surface temperature de- creases.
i. For example, assuming relatively constant paving conditions, if no slab cracking results in sections with a 5-degree drop in surface temperature, the last limit guide- line would be established at a temperature drop of 5 degrees.
ii. This guideline would be followed until weather condition changes or other data warrant establish- ment of new maximum allowable temperature decreases.
iii. The factor of safety is reduced as the maximum allowable tem- perature decrease increases.
Time Too Early:
Raveling Sawing
Window Too Late:
Cracking
Internal Stress Equals Concrete Strength Minimum Strength to Avert
Excessive Saw Cut Raveling Concrete Strength Internal Stress
Figure 9.5. Concrete strength and internal stress de- velopment and how these two variables define the sawing window.
Figure 9.6. Early-age slab surface temperature.
Chapter 9 – Joint Design, Layout, Construction, Sawing and Sealing
3. An improved method to establish the start of the sawing window is to use concrete maturity meters. The maturity method ac- counts for the combined effects of tem- perature and time on strength develop- ment of the concrete.
a. Concrete maturity meters (Figure 9.7) use thermocouples installed in plastic concrete to automatically record tem- peratures at given time intervals.
b. By accounting for both curing temper- ature and time, it is assumed that a given concrete mix will have the same strength at equal maturities independ- ent of curing time and temperature histories.
c. Thermocouples are typically inserted approximately 2 in. deep as soon as possible after finishing operations.
Maturity meters then need to be set to acquire temperatures at approxi- mately 15 to 30 minute intervals. The meters automatically calculate matu- rity. Early-age strength development is a function of ambient conditions, initial concrete temperatures, cement type, cement quantity, coarse aggre- gate type, and water-cementitious materials (w/cm) ratio. Maturity val- ues can also be used to establish ear- liest sawcutting times correlated with acceptable amounts of raveling or vi- sual ratings
Figure 9.7. Maturity meter testing.
QUALITY ALERT
Special Attention to Sawcut Timing:
Concrete pavement placed on a stabilized base is sensitive to sawcut timing. High slab/base interface friction can develop if ad- equate precautions are not taken, possibly resulting in uncontrolled cracking.
A rapid overnight temperature drop will cause shrinkage stresses in the concrete that can exceed the tensile strength of the concrete and lead to uncontrolled cracking. When ad- verse conditions are expected, sawing needs to take place as soon as possible and con- tinue until complete. This is especially impor- tant for concrete placed over a stabilized base.
The surface of the subbase can become hot during summer conditions. This increases the temperature gradient through the slab.
Sawing time will be decreased dramatically when these conditions occur. For asphalt- treated bases, the surface of the material can be white washed to increase reflectivity.