Building Foundations

1. Building Foundations Concrete Flatwork

2. Concrete Flatwork • Flat, horizontal areas of concrete, usually 5” or less in thickness • Placed directly on ground or over compacted gravel or sand • Examples: foundation slabs, driveways, & sidewalks

3. Foundation Slabs Advantages of slab foundation: • Excavation costs reduced – little earth to move • No extensive or complex formwork required] • Eliminates need for separate subfloor • Construction costs are lower • Provides solid base for concrete block walls – often used in warm climates as exterior walls Disadvantage: Utilities must be planned & roughed-in carefully in advance – Changes difficult once slab is in place

4. Types of Foundation Slabs • Two types of slabs: • Monolithic – Footing and floor slab poured in one continuous pour. Footing must be min. of 12” below grade. Best for warm climates. Preferred where termites are common. • Independent – slab poured separate from footing & foundation walls. Used in colder climates where ground freezes fairly deep.

5. Foundation Slab Basics • Lumber or plywood forms used to contain wet concrete. • Outer edges braced to prevent spreading • Forms removed when concrete is partially cured

6. Drainage • Top of slab minimum of 8” above grade. • Perimeter grain placed around outside edge of exterior wall footings, • Keeps ground moisture from wicking into slab

7. Reinforcement • Metal reinforcement often placed in slab to increase tensile strength and reduce cracking • Usually rebar or welded-wire mesh fabric • Reinforcement placed before concrete is poured • Wire fabric placed near center of slab and supported on chairs • Chairs – small device that supports wire fabric at consistent height when pouring concrete. Left in place after pour.

8. Insulation • May use rigid, extruded or expanded polystyrene sheets • Can be placed around outside of monolithic slab. • Can be placed between foundation wall and edge of floor slab in independent • May also be placed below slab

9. Termite Protection • May chemically treat soil before placing slab • Install metal termite shields. Located between slab or foundation and wall plate • In known termite areas insulation should not be used on foundation. Termites tunnel through to reach wood framing.

10. Subgrade (earth below slab) must be well and uniformly compacted to prevent uneven settling and cracking. All organic matter (sod & roots) removed Power tamper or plate tamper often used to compact fill Finished subgrade carefully checked for height and levelness Soil should not be too wet or too dry under slab. Soil should cling to itself when squeezed by hand (not too muddy or plastic). If too dry should be sprinkled with water before compacting. Preparing for a Concrete Slab

11. Preparing the Subbase • Coarse fill placed over compacted subgrade. Coarse slag, gravel, or crushed stone no more than 2” in dia. • Subbase helps to drain water that might collect under slab. • When slab is below grade, subbase must be minimum 4” thick • Subbase should be compacted when brought to desired level

12. Installing Vapor Barriers • Polyethylene vapor barrier with joints lapped min. 6” placed between slab and subbase. • Wide sheets of 6-mil polyethylene may be used. But Cross-laminated polyethylene is more durable. • Must not be punctured before or during pour.

13. Other Types of Flatwork • Basement floors – Min. 3 ½” thick, should slope toward floor drain. Leveled with a screed. (long, straight length of metal or wood used to “strike off” (level) concrete. • Driveways – Concrete should be given a broom finish to prevent cars & people from slipping. About 5” thick, no vapor barrier, wire fabric added to reduce cracking. Expansion joints (asphalt saturated felt strips) added where driveway joins other cured concrete. Should be air-entrained in cold climates.

14. Other Types of Flatwork continued • Sidewalks – Minimum 4” thick, control joints (minimize random cracks) placed every 4’ on center

15. Check Your Knowledge • What is concrete flatwork? • Why is it important to compact the subgrade? • What is the purpose of the subbase?

16. Steps in Finishing Flatwork • Screeding – 1st step, concrete struck off after it’s placed. Screed rides on edges of forms, 2 people move along slab in sawing motion. May also be done with power screed. Leaves a coarse finish. • Bullfloating - Makes surface more even with no high or low spots. Bull float is a wide, flat metal or wood pad with a long handle that is pushed back and forth over concrete. Must be no water visible on concrete before bullfloating.

17. Steps in Finishing continued • Edging – Done when concrete has started to stiffen. Produces a rounded edge on slab to prevent chipping or damage. • Jointing – Large slabs jointed or grooved after edging. Reduces thickness of slab to control where cracks may occur. Jointing tool should be guided with a straight edge (1x8 or 1x10) • Floating – Hand floating further evens surface of concrete & compacts surface in prep for next finishing step. Produces very even surface with light texture. Removes ridges left by jointing tool.

18. Steps in Finishing continued • Troweling – Makes a dense, smooth, finish. Done with a steel trowel. Power trowel used for large areas. Should leave surface smooth, even, and free of marks and ripples. Kneeboards - (12” x24” boards) may be used when hand troweling. Help prevent leaving marks in concrete and spread out weight of worker.

19. Special Finishes • Color, pattern, or texture may be added. • Done after concrete is places but before it cures. • Patterns – metal stamps pressed into wet concrete. • Colors – various types of masonry dyes • Texture – small, smooth pebbles scattered into fresh concrete after bullfloating & pressed into place during later finishing operations.

20. Curing Flatwork • Should be kept moist for at least 2 days. • Burlap or canvas or a waterproof concrete curing paper may be used to cover slab during this period.

21. Temperature Extremes • Placing concrete in unusually hot weather can reduce strength. • Water & aggregates should be kept as cool as possible before being mixed with cement. • Forms, rebar & subgrade should be cooled by sprinkling with water. • Advised to place concrete early in morning or at night to avoid hot temps. • Moist curing should be started ASAP.

22. Temperature Extremes • Concrete placed in temps below freezing can lose strength if not protected. May lose up to 50% if frozen shortly after placed • Can be protected by placing in insulated forms or by covering with temporary insulation. • High-early strength, air-entrained, and low-slump concrete may be used to counteract such conditions.

23. Check Your Knowledge • What is the purpose of a bull float? • When is a concrete slab ready for jointing and edging? • What are kneeboards used for?