INTRODUCTION
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Before The Architect recognizes no more abused, less well understood subjects in house foundation concrete design, concrete details, and concrete construction than slab-on-grade design, slab-on-grade detail, and slab-on-grade construction.
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The following article is intended to address the basics of house foundation slab-on-grade design, detail, and construction matters generally. The central aspect of this address is the preparation of slab-on-grade substrates in regard to both materials and methods.
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A companion piece addresses four related matters of slab-on-grade – joints, pitches, interior and exterior corner reinforcement, and gas curbs at http://searchwarp.com/swa211243.htm -
Before The Architect offers best wishes such that when your time comes to get specific about a house foundation concrete slab-on-grade, get real pros, preferably engineering real pros, who can reckon with local conditions and your house design and this design and construction guidance. Before The Architect has known a few. 4 whom we can recall.
NOTE: This website also has an article with guidance on stabilizing a reinforced concrete slab-on-grade within its horizontal plane at http://searchwarp.com/swa210615.htm.
HOUSE FOUNDATION DESIGN DETAILS, SLAB-ON-GRADE - THE BASICS
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A slab-on-grade, grade beam, footing, raised curb, column, brick or masonry ledge, pilaster, and any other concrete foundation element
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Shall be applied in the field in a single, or monolithic, placement if at all possible and
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If not then, for foundations separating exterior and interior space
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There shall be a continuous sheet of high puncture-resistant EPDM (Ethylene Propylene Diene Monomer)
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Set at all cold joints between footing and wall
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Before each successive placement
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When between slab on grade and pilaster top of face and slab on grade and grade beam top of face
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There shall be a continuous sheet of high puncture-resistant EPDM (Ethylene Propylene Diene Monomer)
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Set at all cold joints between both pilaster and grade beam and the slab
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Before each successive slab placement
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A slab-on-grade
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May have a continuous 6 linear inches x 6 linear inches 10/10 welded wire fabric
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Which mesh shall conform to not less than ASTM A-185 (a/k/a American Society for Testing and Materials, "Standard Specification for Steel Welded Wire Reinforcement, Plain, for Concrete")
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In lieu, may have fiber (but not steel fiber) reinforcement
Comment: This home designer personally objects to seeing little fibers sticking from a curing slab, and is entertained by the notion read somewhere of burning them off with a torch once initial curing is finished.
Comment: Welded wire fabric, or welded wire mesh, or wire fabric, or electric welded wire mesh (a/k/a EWWM), shall not be a sole form of reinforcement. Indeed, many sources identify applications of mesh to slabs-on-grade as assisting against pad shrinking and resultant cracking and otherwise as being without significant reinforcement quality, if any at all. In other words, the mesh won't keep a slab from cracking; it'll only keep it from going to pieces.
Comment: Importantly - 6 linear inches x 6 linear inches 10/10 welded wire mesh or fiber mesh lifted into place approximately 1 linear inch below slab-on-grade top of face and not supported, in flat sheets and not rolls with sheets lapped not less than 1 square, or fiber, or both.
Comment: It's the placement that gets botched more often than not when mechanics stomp on the mesh to the slab-on-grade bottom, which is why, among reasons, that this home designer prefers fiber reinforcement wherewith no amount of stomping can botch the job.
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Shall float to house foundation walls and footings, i.e., shall not be barred or otherwise tied to foundation structure
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Shall have a bond break between it and foundation structure –
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For garage floors
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Shall not be less than 6 linear inches thick
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Shall be not less than 4000 pounds compressive strength 28 days after placement
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Shall be reinforced with
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Which rebar shall be deformed in the field
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Which rebar when crossing a contraction joint–
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Shall float
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For exterior slabs on grade other than driveways
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Shall not be less than 4 linear inches thick
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Shall be reinforced with
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Which rebar shall be deformed in the field
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Which rebar shall be smooth or sleeved when crossing a contraction, or control, joint
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Shall float
- For a concrete driveway
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Shall not be less than 4 linear inches depth in the field
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Shall be at its edges
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Shall be reinforced with
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Which rebar shall be deformed in the field
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Which rebar may be
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Severed every other one at a joint or
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Sleeved or smooth when crossing a contraction, or control, joint
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Shall be placed
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Shall have cover on the horizontal not less than 3/4 linear inch plus aggregate size
Reinforced Concrete Slab-On-Grade with Substrates, Section.jpg)
Comment: Note well that this order of layering may not go well with some, particularly in the upper-end arrangement of the cross-laminated poly, the application of it and not regular poly sheeting, the membrane's unbonding to placement by crusher run cover, and sand below the membrane. Both materials and methods are equally important in this designer's opinion.
Keys: a non-wicking pad on which the slab can crawl; tougher membrane than common; deeper sub-base of crushed gravel; all the tamping.
More commonly, substrate application involves one or more of these substandard materials and methods in this designer's opinion: thinner subbase of large, crushed rock, thereby reducing the drainage plane; sand in lieu of the crusher run or equivalent, thereby permitting constant wicking and holding of moisture sub-slab and offering a much softer base for puncturing the membrane; a 6-millimeter or so polyethylene in lieu of the high-density polyethylene, thereby laying down a moisture impermeable membrane much more vulnerable to puncture during both setup and placement; closed-cell polystyrene panels in lieu of InsulTarp, thereby offering crawly critters lifetime lodging that deteriorate the thermal barrier, presents no reflectivity, and can't come close to the InsulTarp's PERM rating for moisture impermeability.
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A house foundation slab-on-grade
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For interior slab-on-grade, exterior patios and paths or
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For driveways
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Over not less than 4 linear inches of stone dust (a/k/a crusher run, dense grade aggregate, DGA) to 5/8 linear inch diameter
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Over membrane of cross-laminated high-density polyethylene sheeting (a/k/a high-density polyethylene, or HDPE)
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Lapped at seams, penetrations, terminations and then sealed to not Less than manufacturer's specifications
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Where the membrane is
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Either carried across footings, pile caps, grade beams, and foundations
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Or turned up to the top of the slab-on-grade or similar and then sealed to manufacturer's specifications
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Meeting or exceeding the "Class A" performance standards in ASTM E-1745-97 (a/k/a American Society for Testing and Materials, "Standard Specification for Plastic Retarders Used in Contact with Soil Or Granular Fill Under Concrete Slabs")
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Applied according to the more rigorous of manufacturer's specifications or ASTM E-1643 (a/k/a American Society for Testing & Materials, "Standard Practice of Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs"), or, if thermal insulation is desired, over Insul-Tarp (preferred)
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Over not less than 2 linear inches continuous layer of coarsest sand
- Over not less than 8 linear inches continuous layer of crushed gravel to 3 linear inches diameter
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The top of face of which is not less than flush with the footing top of face
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Tamped the greater compaction of
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Not less than 50 beats per square foot and
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95% density, modified proctor in conformance with not less than ASTM D-1557 (a/k/a American Society for Testing and Materials, "Substrate Tests for Moisture-Density Relations of Soils and Soil Aggregate Mixtures Using 10-Pound Rammer and 18-inch Drop")
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For each layer, except the 8 linear inch of crushed gravel which shall be tamped in lifts of approximately 4 linear inches each
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Over earth substrate tamped damp (not wet) the greater compaction of these two
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Not less than 50 beats per square foot and
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95% density, modified proctor in conformance with not less than ASTM D-1557 (a/k/a American Society for Testing and Materials, "Substrate Tests for Moisture-Density Relations of Soils and Soil Aggregate Mixtures Using 10-Pound Rammer and 18-inch Drop")
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In layers not greater than 6 linear inches depth
Comment: The application of moisture and vapor barrier to exterior slabs-on-grade may appear wasted effort. And it will be if a drainage plane is not applied in regard to slab pitch, slab substrates, and other subgrade drainage applications.
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A slab-on-grade exposed to weather and abutting structure shall slope down and away from that structure at not less than ¼ linear inches/linear foot overall
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Slab-on-grade applications shall involve both control joints, or contraction, joints, and isolation, or expansion, joints
Comment: The dear reader is encouraged to keep these two types of joints clearly and distinctly defined, since there is considerable cross-over or muddling of definitions in the literature, particularly in variously defining expansion joints as control joints and confusing construction joints with contraction joints.
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A slab-on-grade shall be thickened
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To not less than 12 linear inches and
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#5 rebar reinforcement
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At not greater than 16 linear inches on center grids and
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Not less than 2 rebars continuous at top and bottom
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In the following sites, among others
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Below a fireplace, including chimney and hearth
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Below a staircase foot
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Below an interior bearing wall or column
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Below an island, e.g., as in a kitchen island
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Below a bathtub or whirlpool
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A pitched slab-on-grade shall not be of excess volume in order to level its top of face;
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Instead, the pitched slab-on-grade shall be demolished and
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Removed from the site and
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Replaced with a new slab-on-grade in a new-construction mode
Comment: Please note that this last prescript does not prohibit applying an array of self-leveling products in conditions of out-of-true at margins.
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Slab-on-grade
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Compressive strength shall not be less than that of the
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Principal footing and
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Foundation stem wall and
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May be more, e.g., as a garage slab-on-grade
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Compressive strength shall not be greater than that of the slab-on-grade it extends or repairs
SCORED CONCRETE
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This home designer's giving almost all this subject – which he regards as inclusive of almost all forms of decoratively finished functional concrete flooring which involve staining and stamping – to 2 books:
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Guide to Stained Concrete Interior Floors: The Bob Harris Decorative Concrete Collection by Bob Harris, Decorative Concrete Institute, Inc. and ConcreteNetwork.com, 2004 and
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Guide to Stamped Concrete: The Bob Harris Decorative Concrete Collection by Bob Harris, Decorative Concrete Institute, Inc. and ConcreteNetwork.com, 2004
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What's to follow is gleaned from a wealth of other information, these entries chosen for their vital importance in both the art and science of scoring concrete
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Scored concrete contractor shall have achieved American Concrete Institute Certified ACI Concrete Flatwork Finisher & Technician
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Tamp substrates as slab-on-grade except with not greater than 4 linear inch lifts (as opposed to 6 linear inches otherwise)
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Apply as monolithic placement and without cold joints
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Slump shall be 4-5 and preferably toward the lower end of range
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Water content shall be .45 maximum
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May amend with synthetic fiber unless planning to apply dry-shake color hardener
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Cure wet and unloaded 7 days
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Protect from casual, work-related staining
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