AVOID CRACKS IN CONCRETE SLABS-ON-GRADE
By Harvey Haynes
Consulting Concrete Engineer
Part 2 of 2 :: Recommendations to Avoid Cracks
The critical factor in preventing expansive clay soils from causing problems with heave is to be certain that the clay soil is damp (above its optimum moisture content) before concrete is cast. Watering the clay may take days to get it damp. It is advisable to have a soils engineer provide a letter stating that the clay is at proper moisture content.
Slabs, such as garage floors or walkways next to a house, should be free to float on top of the soil as opposed to being tied into the house footing. Place isolation joint material between the slab and building.
Use reinforcing bars to distribute and limit the width of cracks and prevent vertical offsets. As a guide, use #3 bars at 18-inch spacing each way in 4-inch thick slabs, and #4 bars at 20-inch spacing in 5-inch slabs. Do not use welded wire mesh of size 6x6-W1.4xW1.4 (also known by its old designation of 6x6-10x10). Mesh of size 6x6-W2.9xW2.9 is marginal, but acceptable.
Compacted base rock four inches deep will provide a uniform, firm support for the slab. If base rock is not used, be sure that the subgrade (native soil) can provide good support even when the subgrade is wet. Use a 5-inch thick slab, which is 50% stronger than a 4-inch thick slab in flexural strength.
THERMAL BEHAVIOR OF CONCRETE
Seasonal Temperature Change
To minimize cracks, different types of joints are installed in slabs.
Contraction or Control Joints: These joints are installed by placing plastic inserts or grooves into the fresh concrete to a depth of one-fourth the thickness of the slab. Saw cuts are also used, but should be made the same day that the concrete is cast. Spacing and layout are discussed later in the drying shrinkage section.
Expansion or Isolation Joints: These joints are of a compressible material, such as fiberboard, installed the full depth of the slab. For expansion joints, spacing is between 50 to 100 feet.
Construction Joints: These joints mark where new concrete abuts existing concrete. The joint can function as a contraction joint when detailed using bond breaker on one concrete face, or when using keyways or smooth dowels greased on one side. Constructions joints will not function as contraction joints when design details call for roughening one concrete face or having reinforcing bars tie together the new and existing concrete.
Daily Temperature Change and Heat of Hydration
To minimize cracks that appear the morning after concrete is cast, install saw cut contraction joints on the same day that concrete is cast. Grooves and inserts also are good joints because they are installed while the concrete is fresh.
SHRINKAGE BEHAVIOR OF CONCRETE
These cracks are prevented by proper finishing methods; use light tamping, do not overwork the surface, and do not add dry cement to the surface to absorb bleed water. If necessary, bleed water can be vacuumed or dragged off.
Plastic Shrinkage Cracks
Windy or hot weather is a sign of danger. Keep the temperature of the concrete as low as possible by having transit mix trucks stand-by in the shade, or cool the mixing drum by spraying water on the outside surface. Have a sufficient crew size available for rapid placement and finishing. In extreme hot weather, avoid exposing young concrete to the hot part of the day by starting to cast concrete in the late afternoon or early evening.
Delay evaporation of bleed water by spraying fog mist across the work area; or better, use an evaporation control coating sprayed on the fresh concrete after bullfloating. Store this material on the job so it is available on windy or hot days.
Drying Shrinkage Cracks
Mix Design: The more water used in making concrete, the greater the amount of shrinkage. Hence, do not add water beyond the amount necessary for proper slump.
Use as large an aggregate size as possible. Concrete with 1-1/2 inch maximum size aggregate will shrink less than concrete with 3/4-inch maximum size, and both of these will shrink considerably less than pea gravel concrete, which uses a 3/8-inch maximum size aggregate.
Vapor Retarders: Vapor retarders are used primarily to stop ground moisture from moving up through the concrete slab. Acceptable vapor retarders are:
- Clean 3/4-inch drain rock, 4 to 6 inches deep
- Plastic sheeting covered by 1 to 2 inches of clean sand
- Plastic sheeting of minimum 10-mil thickness
The preferred system is (1) and (2) together. System (3) can result in excessive cracks in the slab if a high slump concrete is used. Therefore, use system (3) only with a low slump concrete (3-inch maximum).
Contraction Joints: Use saw cuts, inserts, or grooves to install contraction joints. Make certain that saw cuts are installed on the same day the concrete is cast.
Spacing of contraction joints is given in the following table:
Any slab cast in an open environment is an exterior slab, which means that tilt-up slabs are exterior slabs. Patios and walkways are usually 3.5 inches thick and they should have a contraction joint spacing of around 6 feet. If pea gravel concrete is used, then reduce the recommended spacing in the table by 3 feet. The simplest and surest method to minimize most cracks in slabs is to follow this guidance on contraction joint spacing.
The layout of joints is best when square sections are made. When rectangular sections are made, the length should not exceed 1.5 times the width. Re-entry corners must have contraction joints.
Reinforcement: Welded wire mesh of size 6x6-W1.4xW1.4 (or old designation of 6x6-10x10) is the most common type of reinforcement in slabs-on-grade, and it is essentially useless. If welded wire mesh is used, then use 6x6-W2.9xW2.9; however, this is still a small amount of steel and will do little in crack control.
Slabs-on-grade do not have a code requirement for being reinforced; hence, they can be of unreinforced concrete with cracks controlled by a close spacing of contraction joints.
Cracks can occur even if reinforcing steel is used. The reinforcement is to control crack width and spacing. A recommended amount of steel (which only provides partial effectiveness in controlling crack widths) is the amount given for shrinkage and temperature reinforcement for structural concrete in the American Concrete Institute Building Code Requirements for Structural Concrete, ACI 318-95, Section 7.12. This amount is equivalent to #3 bars at 18-inch spacing in 4-inch thick slabs and #4 bars at 20-inch spacing in 5-inch thick slabs. Contraction joints are still required, and the spacing can be larger than that for unreinforced slabs. To allow these joints to properly function, it is advisable to cut every other bar crossing the joints.
To obtain maximum effect in controlling cracks by steel reinforcement, an amount of steel three times that given above is required. These slabs are considered as continuously reinforced concrete, and contraction joints can be omitted.
Reinforcing bars are good because they can be chaired. Place the bars in the top half of the slab. For slabs of 5 inches or thicker, locate the bars 2 inches below the top of the slab. Don't allow the chairs, or dobbies, to settle in the sand cushion layer. The reinforcing bars need to be near the top of the slab to minimize the width of the cracks. Cracks are widest at the top because the greatest shrinkage occurs at the top.
Steel fiber reinforcement works well to control cracks. The fibers are added to the transit mix truck in quantities of at least 30 to 50 pounds per cubic yard of concrete. Normal placing and finishing procedures are used. Contraction joints are required, but at a larger spacing. Steel fibers are highly recommended.
Polypropylene and nylon fibers do not provide any benefit in controlling drying shrinkage cracks, but they do have value in controlling plastic shrinkage cracks. These fibers are added to the transit mix truck in quantities of about 1 to 1.5 pounds per cubic yard of concrete.
Curing: The objective of curing is to allow concrete to gain strength. To do this, the temperature must be above 40° F and water must be present within the concrete for the cement to hydrate. The best curing method is to flood concrete, but the most common method is to spray curing compound on concrete. This method attempts to contain the mix water inside concrete, and it is moderately effective. In windy or hot weather, the coverage rate specified by the manufacturer should be increased by 1.5 times. A rough surface also requires more compound than a smooth surface.
As long as water is held inside the concrete, drying shrinkage does not occur. Eventually the concrete decreases in moisture content, and then shrinkage begins. Curing allows concrete to gain strength; stronger concrete will crack less than poorly cured or weaker concrete.
SUMMARY OF RECOMMENDATIONS
- Expansive clay soil must be damp before concrete is placed.
- The subgrade must provide fairly uniform and firm support for the slab, even when the subgrade is wet. If not, use base rock.
Thermal Behavior of Concrete
- To accommodate seasonal temperature changes, install expansion joints to prevent slab from buckling, and install contraction joints (also called control joints) to minimize random cracks due to contraction.
- To avoid cracks that appear the day after casting slabs, install contraction joints on the same day the concrete is cast.
Shrinkage Behavior of Concrete
- To avoid crazing cracks, prevent excessive paste on the top surface of fresh concrete by light tamping, do not overwork the surface, and do not apply dry cement to the top surface.
- To avoid plastic shrinkage cracks, use an evaporation-control coating on fresh concrete during windy or hot weather. Also, use polypropylene or nylon fiber reinforcement to minimize plastic shrinkage cracks.
- Do not add mix water to concrete beyond the amount for proper slump.
- Use concrete with 3/4-inch maximum size aggregate or larger. For pea gravel concrete, use extra-close spacing of contraction joints.
- During hot weather, keep the temperature of the fresh concrete cool while in the transit mix truck by having truck stand-by in the shade and spraying the outside of the drum with water.
- Space contraction joints properly (see table). This is the simplest and surest method to minimize most cracks.
- Layout contraction joints for square sections. For rectangular sections, make the length less than 1.5 times the width.
- Put contraction joints at re-entry corners.
- Unreinforced concrete slabs are acceptable, but be certain that contraction joints are properly spaced.
- Don't use welded wire mesh of size 6x6-W1.4xW1.4. In its place, use mesh of size 6x6-W2.9xW2.9 or larger.
- For reinforcement to provide a fair benefit in crack control, use #3 bars at 18-inch spacing in slabs 4-inches thick, and #4 bars at 20-inch spacing in slabs 5-inches thick.
- Chair reinforcing bars and mesh so they are located in the top half of slabs.
- Use steel reinforcement to control shrinkage cracks; steel fibers are more effective than bars.
- Spray curing compound on fresh concrete immediately after the final finishing operation. Check to be sure the coverage rate is equal or greater than the manufacturer's recommendation and 1.5 times the recommendation in windy or hot weather.
Sincere thanks to the following individuals for reviewing the tape series: Professor Kumar Mehta of the University of California at Berkeley, Professor Paulo Monteiro of the University of California at Berkeley, Professor Lawrence Kahn of the Georgia Institute of Technology, Mr. Wayne Ferree of TerraTech Inc., Mr. Ashok Kakade of Concrete Science, Mr. Donald Pearman of Pearman Construction Inc., and Mr. Andrew Bardakos of R.H. Wehner Construction Co.
REFERENCES FOR FURTHER STUDY
Concrete: Structure, Properties and Materials, by P. Kumar Mehta and Paulo J. M. Monteiro, Prentice Hall, New Jersey, 2nd Edition, 1993, pp. 548.
Properties of Concrete, by A.M. Neville, John Wiley & Sons Inc., New York, 7th Edition, 1996, pp. 842.
ACI Committee 224, Control of Cracking in Concrete Structures, Manual of Concrete Practice, Part 3, American Concrete Institute, Detroit, Michigan, published annually, pp. 42.
ACI Committee 302, Guide for Concrete Floor and Slab Construction, Manual of Concrete Practice, Part 2, American Concrete Institute, Detroit, Michigan, published annually, pp. 46.
Concrete Floors on Ground, Portland Cement Association, Skokie, Illinois, 1983, pp. 36.
Designing Floor Slabs On Grade, by Boyd C. Ringo and Robert B. Anderson, The Aberdeen Group, Addision, Illinois, 1992, pp.199.