INTRODUCTION
HOME FOUNDATION DESIGN DETAILS - SLAB-ON-GRADE, CORNER REINFORCEMENT DETAILS
Comment: Over the years, this designer’s observed that some better builders regularly place 5-6 linear inches for an interior slab-on-grade, a practice which enables this detail by thickening the placements at least in the corner reinforcement sites.
Let’s do the addition math in linear inches:
0.625 for two rebars side-by-side for corner reinforcement
1.250 for 2-#5 rebar atop one another in grid for slab reinforcement
1.250 for 5/8 linear inch aggregate either side of the slab’s bars
1.500 for 2x¾ linear inch cover including the aggregate (worst case)
4.625, or 5 linear inches of thickened slab-on-grade should do it.
Home Foundation Plan, Corner Reinforcements, Plan View
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Key to abbreviations: APX=APproXimately; BEL=BELow; CL=CenterLine; CONC=CONCrete; CONT=CONTinuous; FDN=FounDatioN; FTG=FooTinG; GBM=Grade BeaM; L=Length; RR=RebaR; SOG=Slab-On-Grade; TYP=TYPical; WL=WaLl
Comment: The illustration Home Foundation Plan, Corner Reinforcements, Plan View is a circled part from one of a group of foundation plans for a slab-on-grade, single-story design, the purpose of which illustration is to offer several solutions to corner reinforcements, some of which solutions are proscribed by control joint layouts depicted. The angled features are the rebars. The squared lines on the interior are contraction joints.
CONCRETE FLOOR JOINTS
Comment: The Foundation Plan, Plan View, Partial is also part of a larger plan view to one of a group of foundation plan drawings. This segment is meant to highlight two points, one just raised - corner reinforcement - and the other about to be - contraction joint layout.
Comment: Note the extents to which one must go in order to conform to Before The Architect’s control joint prescriptions (dashed lines)…the moral of which story is that the simpler the slab-on-grade configuration – the fewer jogs and juts – the better.
Comment: Note, please, the smallish sections of control-jointed slabs toward the middle of the illustration…all in conformance with our objective of minimizing slab cracking except where they’re somewhat less than completely undesirable.
Home Foundation Plan, Plan View, Partial
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Key to abbreviations: CONC=Concrete; CVR=Cover; EL=Elevation; FOH=Front Of House; FIN=FINish; LF=Linear Foot; LI=Linear Inch; REINF=Reinforced; RR=Rebar; SOG=Slab-On-Grade; TYP=TYPical
Comment: Please note, too, other aspects of home foundation design details, including but not limited to interior and exterior corner rebars and notably the one pair of bars on the left truncated at the control joint, grade beams and footings, and the pilasters (dark, little rectangles) at corners, joints, and grade beam intersections with home foundation wall.
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.
Comment: The AG’s seen this number as high as 300 square feet. See illustration immediately above for a sample layout of control joints which layout conforms to the strictures herein.
Comment: This width-to-length rule combined with the total area rule immediately preceding effectively limits a given slab-on-grade face to not greater than about 13 linear feet-6 linear inches x 20 linear feet-6 linear inches.
Comment: Tricky. You’d think that this wasn’t helpful given the first two rules, but think again. This tells us that a short side can be as long as 15 linear feet which leaves us about 18 linear feet-2 linear inches on the longer side, still staying within the length-to-width rule.
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Shall be formed
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As nearly square as possible (though not at all necessarily same-sized or symmetrical in layout)
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At 90 degrees intersection ideally
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But not less than 60 degrees if at all possible
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To intersect at floor drains and other penetrations of a slab-on-grade
Comment: This square rule can be toughest of all, if you don’t give up symmetry. The lesson: thou shalt not offset slab-on-grade wall lines with abandon.
Comment: The inch-to-feet rule restated practically is: a 4 linear inches slab-on-grade shall have a side dimension not greater than 12 linear feet, and a 6 linear inches slab, 18 linear feet.
Comment: When you combine this rectilinear rule with the others, you get the sense that simple slab-on-grade layouts are downright virtuous.
Comment: Perpendicularity herewith is crucial to focus, because elsewhere herein there is a prescript for rebar reinforcement of open, interior corners to slabs-on-grade, which prescript has been amended by placing 2-#5, not less than 6 linear feet long rebar in-line with and perpendicular to the control joint and not at a 45° angle to the joint, the latter angularity being at odds with clean, evenly tensioned contraction.
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Welded wire mesh shall be severed not less than every other wire line
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Isolation joints, or expansion joints
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Shall involve separating concrete elements to permit independent movement over time on the horizontal or the vertical, notably, but not exclusively, applied at the joining of exterior slabs-on-grade to Perimeter foundations
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Shall rim a slab-on-grade
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At its perimeter joints to walls and
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At its perimeter and interior joints to piers and other protrusions and obstructions, as columns, for example and
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Shall run continuous and without interruption from slab-on-grade top of face to slab-on-grade bottom of face at slab-on-grade sides of face
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Shall not be bridged by
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Control and isolation joints
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At perimeter abuts overall to structure
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Shall be cleaned (before and after backer rod insertion, primed well and
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Shall be sealed
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With backer rod or equivalent of appropriate size below, to ½ linear inches depth
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Then filled in on top smooth and to full depth with
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An elastomeric joint sealant, i.e., silicones, urethanes, and polysulfides – each in conformance with not less than ASTM C-920 (a/k/a American Society for Testing & Materials, “Standard Specification for Elastomeric Joint Sealants")
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Of lesser likelihood, a bituminous-based, hot-poured sealant
Comment: Sealants for consideration may include Sonneborn NP1 for large gap filler and Sonneborn SL1 (a self-leveler) for above-rod application.
NOTE: Oil-based sealants shall be a prohibited material in masonry applications
Joints subject to smaller movement, e.g., masonry abuts to door and window casings and frames
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Shall be sealed with a solvent-based sealant, e.g., an acrylic or butyl caulk
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Oil-based sealant shall be a prohibited material in masonry applications
Comment: This designer respectfully notes in summary that there are, in fact, three types of concrete joints:
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Control, or contraction,
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Isolation, or expansion,
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Construction.
While the literature provides direction on construction joints in slab-on-grade applications, construction joints generally beyond common building practice in residential concrete construction, except in slab-on-grade, wall, and foundation extensions from a cold joint. In such instance, all effort shall be taken to extend with materials and methods consistent with existing, including but not limited to substrates, barriers, reinforcements, and mix.
The joints themselves shall be mended with not less than doweling in accord with Simpson Strong-Tie materials and methods or similar and, as needed, engineering latitude. Additionally, slab-on-grade extensions shall be undermined at existing not less than 2x existing slab-on-grade depth for subsequent, reinforced placement, again, with methods and materials consistent with existing structure.
The form of construction joint in residential work that is quite common arises in non-monolithic placements, as, for example, in a keyed and reinforced concrete footing placed and initially cured before a continuous concrete stem wall is placed atop it.
In this section of the monograph, the latter form of construction joint – cold placements with essentially horizontal joints - is addressed variously, overall noting the application of HDPE (a/k/a high-density polyethylene sheeting qualifying as Class A in ASTM E-1745 (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") in continuous sheets at cold joints pre-placement.
SLAB-ON-GRADE SLOPES
GAS CURB
Comment: This statement can be easily misunderstood. Easily misunderstood. It is enough to comply by dropping a garage floor 6 linear inches at passage with habitable and think that the 6 linear inches of exposed foundation on the garage side will do you proud. Usually.
This is about heavy fumes’ transfer far as AG gets it.
You’re free to aim for not less than 6 linear inches of common wall, i.e., to continue with our example, you’ll need to raise up the foundation wall another 6 linear inches over interior floor level (thereby shortening common wall studs 6 linear inches) to comply with this gas curb prescript. Yes, yes, yes. |
