1. Ground-Supported Slabs • Exterior Flatwork
Chapter 4
Flatwork
Ground-Supported Slabs • Exterior Flatwork

2. Rigid insulation extends 24 horizontally or vertically from the perimeter of a slab‑on‑grade foundation. The top surface of the slab is a minimum of 8 above the outside grade level.
When a slab-on-grade floor is placed after the foundation has been constructed, rigid insulation is recommended around the perimeter of the slab to reduce heat loss. The insulation should be at least 1 thick and extend 24 vertically below grade level or 24 horizontally under the concrete slab. See Figure 4-1.

3. The building site for a slab‑on‑grade floor must be excavated to undisturbed soil. Compacted fill, a gravel base course, and a vapor barrier are placed in the excavation.
Site preparation, including all groundwork, must be completed before the concrete is placed for a slab-on-grade floor. Site preparation may only require removing the topsoil to reach undisturbed soil, or it may require excavating deep enough to place a layer of compacted fill and a gravel base course. See Figure 4-2.

4. Bulkheads are used to form construction joints when a concrete slab is placed in sections. Premolded key strips or metal dowels are permanently embedded in the slab.
A construction joint is a joint used where two successive placements of concrete meet, across which a bond is maintained between the placements. A construction joint is formed where a fresh concrete section butts up against the edge of a concrete section that has already set. A construction joint is formed by staking down a 2 thick bulkhead at the outer edges of the concrete placement area. The top of the bulkhead is positioned at the height of the floor surface and a beveled key strip is fastened to the bulkhead. Metal, wood, and premolded key strips are commonly used to form a keyway for the floor. The keyway secures the edge of the next section in position. Keyed joints are not recommended for industrial floors. Metal dowels should be used in slabs that carry heavy loads. See Figure 4-3.

5. Concrete is placed in sections for large industrial or commercial concrete slabs.
Concrete for large floor areas in commercial buildings such as warehouses, factories, and stores is placed in sections. Therefore, provision must be made for construction joints when placing large floor slabs. See Figure 4-4.

6. A screed system is used to strike off concrete placed for a concrete floor slab. The screed is supported by wood stakes or adjustable metal supports.
Placing concrete for large floor sections requires the use of a screed system to maintain proper floor elevations in the interior areas of the slab. The screed is positioned with its bottom edge at the finish elevation of the floor surface. Wood stakes or screed supports hold the screed off the ground and allow rebar to be positioned. Lines are stretched from the top of the outside walls or form boards to adjust the screeds to their proper height. A strike board acting as a straightedge to level the concrete is placed between the screed boards. A strike board is a wood or metal straightedge used for screeding concrete. Strike boards are held at the same level as the screeds by cleats nailed at opposite ends. See Figure 4-5. As the concrete is being placed and consolidated, cement masons strike off the concrete by moving the strike board along the screeds with a saw-like motion. The screeds and their supports are then removed from the concrete.

7. Metal pipe screeds are used to support a strike board.
Screeds can also be placed with the top edge flush with the finish surface of the concrete. Two screeding methods may be used with this system. In one method, a section of the floor slab is placed and struck off to the screeds. The screeds are then removed and the concrete is placed for the next floor section. In the second method, the screeds remain in place until the entire slab has been placed. The screeds and their supports are removed and the cavities are filled with concrete. Metal pipe screeds supported by wooden stakes or adjustable chairs are often used with this method. See Figure 4-6. Mechanical equipment is also available for screeding operations and is often used when placing larger slabs.

8. Control joints in a concrete slab confine cracking resulting from expansion and contraction of the slab. Control joints are hand tooled or cut into the slab.
Control joints (also called relief or contraction joints) confine and control cracking in concrete slabs caused by expansion and contraction. A control joint is a groove made in a horizontal or vertical concrete surface to create a weakened plane and control the location of cracking. A control joint is one-fourth the slab thickness. Cracks occurring in the future will be confined to the area beneath the control joints. Control joints may be formed with a special grooving tool when the concrete is being finished. They may also be cut into the slab after the concrete has set using a power saw equipped with an abrasive blade. Recommended spacing for control joints is 15′ to 20′. See Figure 4-7.

9. An expansion joint is used when a great amount of expansion and contraction is anticipated.
An expansion joint (isolation joint) is a joint that separates adjoining sections of concrete to allow for movement caused by expansion and contraction of the slabs. Expansion joints are used in slabs that cover large areas of commercial buildings, and where a great amount of expansion and contraction is anticipated. Expansion joints run through the complete thickness of the slab. One common method is to place a piece of preformed asphalt-impregnated material in the joint. The fiber material is tacked to the form board before the concrete is placed and remains in place when the form board is removed. See Figure 4-8.

10. An expansion joint contains caulking or a preformed asphalt‑ impregnated strip.
When using an asphalt-impregnated strip, a 1/2 thick premolded strip is placed against the foundation wall. After the slab is placed, the asphalt-impregnated strip remains in the concrete. See Figure 4-9.

11. Welded wire reinforcement is used to reinforce concrete floor slabs.
The WWR is laid in position before the concrete is placed. Information on the size and type of welded wire reinforcement to be placed in the slab is shown on the foundation section view of the prints. Welded wire reinforcement is represented by a long dashed line in a section drawing. Spacing of the wire, type of wire, and size of wire required are identified. For example, a drawing calling for 6 × 6 −W2.0 × W2.0 indicates that the spacing between wires is 6 longitudinally and 6 transversally. The W indicates a smooth wire, and the 2.0 × 2.0 indicates wire size, or cross-sectional area of the wire. See Figure 4-10.

12. To ensure maximum strength of concrete welded wire reinforcement must be set no more than 2 from the top surface of the concrete, or in the middle of the slab.
Welded wire reinforcement should be set no more than 2 from the top surface of concrete. In a 4 slab, welded wire reinforcement is set in the middle of the slab. Setting welded wire reinforcement near the bottom of the slab adds no additional bending strength to concrete and allows surface cracks to open. During placement, welded wire reinforcement must be protected from being pushed toward the bottom of the slab. See Figure 4-11.

13. Rebar is held in place using chairs and is tied together using wire ties. Protective coatings are used to decrease corrosion.
Rebar is held securely in place during concrete placement using continuous chairs, which hold rebar off the ground and away from forms. Rebar is set on the chairs perpendicular to the direction the chairs are set. Rebar is tied together using wire ties at alternating intersections. See Figure The ties hold rebar together and in place during concrete placement. Rebar is spliced when the area of concrete to be placed is larger than the length of the rebar.

14. Fibers in concrete reduce cracking caused by drying shrinkage and thermal expansion.
Fiber-reinforced concrete (FRC) is a concrete mixture that uses glass, metal, or plastic fibers mixed with concrete to provide extra strength. See Figure Fibers can be used as the sole reinforcement or they can be used in combination with rebar and WWR.

15. The section view of a print provides information regarding the location, thickness, and reinforcement of the basement floor slab.
Basement floor information is included in section view drawings of the foundation plans. These drawings show slab thickness, type of reinforcement used, and the distance between the surface of the slab and the ceiling joists above. See Figure 4-14.

16. Screeds are set up close to the foundation walls when concrete is placed for a basement floor slab.
Before placing the concrete slab, floor elevations must be established by measuring down from the bottom of the ceiling joists. Screeds are set up close to the foundation walls and at intervals throughout the slab area. See Figure 4-15.

17. Garage floors are sloped 1/8 to 1/4 per foot toward the front of the garage. The garage floor is formed on three sides by the foundation walls and on one side by a form board.
Once points for the floor elevation at the rear of the garage are established, the slope from back to front is calculated by multiplying the length of the floor by the slope per foot. This amount is subtracted from the elevation at the rear of the garage and marked at the front wall. For example, a garage floor measuring 16 from front to back with a 1/8 per foot slope has a total floor slope of 2 (16 × 1/8 = 2). After garage floor elevations are established, chalk lines are snapped along the back and side walls. A line is then stretched across the garage opening and a form is constructed to this line. No other forms are required because the slab is formed on three sides by the foundation walls. See Figure 4-16.

18. Exterior flatwork includes driveways, walkways, and patios
Exterior flatwork includes driveways, walkways, and patios. Expansion and control joints are used to control cracking.
Rebar or welded wire reinforcement is used where the slabs are subjected to great pressures such as driveways or other areas supporting moving vehicles. Proper sloping of exterior flatwork is very important for water drainage. Expansion and control joints are necessary to control cracking. See Figure Exterior flatwork is usually the last concrete work performed on a construction project.

19. The finished surface of the garage or carport end of a driveway should be 1/2 below the surface of the garage floor. The driveway is sloped away from the garage to facilitate drainage.
Driveway slabs for passenger cars are usually 4 thick. Driveways that support truck movement, such as in commercial and industrial structures, should be 6 thick. Reinforcement consisting of welded wire reinforcement or rebar is normally required for both passenger cars and truck traffic. The finished surface of the garage (or carport) end of the driveway should be 1/2 below the surface of the garage floor for proper water drainage. See Figure 4-18.

20. A concrete trough with a removable grate diverts water away from a garage that has a driveway sloping toward the garage.
Driveways that slope down to the garage area should have a drain directly in front of the garage entrance. A recommended drain system involves installing a removable grate over a concrete trough that runs the full width of the garage. See Figure Water collected in the trough is discharged by plastic pipe or drain tile into the surrounding soil. The water can also be discharged through a culvert or storm sewer, if allowed by local code.

21. Two edge forms are required to form a driveway
Two edge forms are required to form a driveway. The garage floor and sidewalk are used to form the concrete on both ends.
Driveway forms usually consist of staked 2 × 4s or 2 × 6s placed on edge running the length of the driveway. Form boards are not required at the garage end or at the opposite end if the garage and front sidewalk have been placed. However, provision should be made for expansion joints where the driveway butts up against the garage floor or sidewalk. See Figure 4-20.

22. 2 x 4 or 2 x 6 form boards are used to form the sides of walks
2 x 4 or 2 x 6 form boards are used to form the sides of walks. A public sidewalk and building stoop are used to form the concrete on both ends.
Walkway forms are usually constructed of 2 × 4s or 2 × 6s held in place by wood or metal stakes. See Figure Reusable metal forms are also available. Control joints are tooled or cut into the surface of the walk to control cracking of the concrete.

23. Curved forms may be required to form curves for sidewalks or driveways
Curved forms may be required to form curves for sidewalks or driveways. The example is a 5‑0 wide sidewalk with a 15‑0 radius.
A curve is laid out using a line that is equal in length to the curve’s radius. A round stake is driven at the center point of the curve, and one end of the line is tied loosely around the stake. The other end of the line is tied to an individual flat stake that is driven along the edge of the curve. The line is then released from the flat stake and the procedure is repeated for the remaining stakes in the curve. See Figure 4-22.

24. Plywood is used to form curves
Plywood is used to form curves. Saw kerfs are made in 3/4 plywood to provide greater flexibility.
The radius of the curve determines the material used to form the curve. A 3/4 piece of plywood can bend sufficiently for long-radius curves. Hardboard or 1/4 plywood is used for short-radius curves. Saw-kerfing 3/4 material is another method used to form short-radius curves. See Figure 4-23.

25. Patios serve as outdoor recreational areas in residential construction
Patios serve as outdoor recreational areas in residential construction. Wood dividers add to the attractiveness of the patio and serve as screeds to strike off the patio.
A concrete patio is an exterior slab constructed next to a building. It is primarily used for recreational purposes and can be designed in a variety of shapes. See Figure Ground preparation is basically the same as for other slab work.

26. Job‑built or prefabricated curb and gutter forms are used to form curbs and gutters. The top of the curb is usually flush with the sidewalk surface and the top of the gutter is flush with surface of the roadway.
Prefabricated metal curb and gutter forms are commonly used to form curbs and gutters for large construction projects. See Figure The edge forms are staked to the ground with metal stakes. The division plates are positioned and the curb face form is suspended from the division plates. After the concrete has set, the form and plates are removed and the curb and gutter are finished.

27. Tendons are placed in the slab area before the concrete is placed
Tendons are placed in the slab area before the concrete is placed. After the concrete has hardened, the tendons are stressed with hydraulic jacks and held in place with anchoring devices.
The concrete is placed after the tendons have been positioned. When the concrete has hardened to the required strength, the tendons are stressed with hydraulic jacks to an effective force of 25,000 lb or more. Afterward, anchoring devices placed at the ends of the tendons transfer the stressing force to the concrete slab. See Figure 4-26.