Concrete Slabs on Grade Analysis, Design, and Detailing
By Otto J. Schwarz, P.E., S.E.
Ryan Biggs | Clark Davis Engineering and Surveying, P.C.
SE University, March, 2015 www.LearnWithSEU.com
Concrete Slabs on Grade Analysis, Design, and Detailing
I. Slabs on Grade and the Building Code
II. Types of Slabs on Grade
III. Design Parameters
IV. Methods of Analysis
V. Crack-Control, Vapor Retarders, and Subgrade
Concrete Slabs on Grade Referenced Codes / Texts
2009 International Building Code
ACI 318 08 Building Code Requirements for Structural Concrete
ACI 360R 06 (2010 ACI MCP)
Designing Floor Slabs on Grade, Ringo, Boyd C., and Anderson,
Robert B., 1992
Concrete Floors on Ground, PCA, 2001. (2008 expanded available)
ARMY TM 5-809-12, Concrete Floor Slabs on Grade Subjected to
Heavy Loads, Departments of the Army and the Air force Technical
Wire Reinforcement Institute, 1989.
ACI 360 Definition:
A slab, supported by ground, whose main purpose is to support the applied loads by bearing on the ground.
(1992 whose total loading when uniformly distributed would impart a
pressure to the grade or soil that is less than 50% of the allowable bearing capacity thereof.)
A Slab on Grade is
Concrete Slabs on Grade Type 1 Engineering
Sufficient for light duty projects. Offices, light commercial, residential, etc. Slab on grade is primarily a walking surface.
Standard concrete specification and a note on the building plan.
Type 2 Engineering Projects where grade slabs perform heavy lifting
Industrial applications (forklifts, machinery loads, etc.) Warehouses (critical joint detailing, storage racks, etc.) Expansive soils / significant frost depths Exterior / Aggressive Environments Brittle low-crack-tolerance flooring
Concrete Slabs on Grade First Project Questions
What should the concrete strength be? What thickness of slab should I specify? How do I determine the required slab strength? How should the slab be reinforced? Where do I place contraction / control joints?
Occupancy** Min. Slab Thickness
Sub-Slabs under other slabs 2 None
Domestic or light commercial (loaded less than 100 psf)
4 One layer 6x6 W1.4xW1.4 WWF, min. ideal conditions; 6x6 W2.1xW2.1 WWF for avg. conditions
Commercial institutional barns (loaded 100-200 psf)
5 One layer 6x6 W2.1xW2.1 WWF or one layer 6x6 W2.9xW2.9 WWF
Industrial (loaded not over 400-500 psf) and pavements for industrial plants, gas stations, and garages
6 One layer 6x6 W2.9xW2.9 WWF or one layer 6x6 W4xW4 WWF
Industrial (loaded not over 600-800 psf) and heavy pavements for industrial plants, gas stations, and garages
7 Two layers 6x6 W2.9xW2.9 WWF or two layers 6x6 W4xW4 WWF
Industrial (loaded 1500 psf) 8 Two mats of bars (one each, T & B) each of #4 bars @ 12 c/c, E.W.
Industrial (loaded 2500 psf) 9 Two mats of bars (one each, T & B) each of #5 bars @ 12 c/c, E.W.
Industrial (loaded 3000-3500 psf) 10 Two mats of bars (one each, T & B) each of #5 bars @ 8 to 12 c/c, E.W.
Notes on General Guidelines
** For loads in excess of 500 psf, use at least 3000 psi quality controlled concrete, and investigate subsoil conditions with extra care. Fill material and compaction should be equivalent to ordinary highway practice. If laboratory control of compaction is available, the load capacities can be increased in the ratio of the actual compaction coefficient, k, to 100.
For loads in excess of 1500 psf, the subsoil conditions should be investigated
with extra care and subbase should provide k 200. Place first layer 2 inches below top of slab; second layer 2 inches above
bottom of slab. Information taken from Concrete Reinforcing Steel Institute Handbook
(CRSI), 1984, pg 13-61.
Concrete Slabs on Grade confession
I was a successful Type 1 Engineer for many years.
Stable subgrade materials Uniform loading criteria Light service point loads (< 500lb) Tolerant Floor Coverings
Carpet Vinyl Tile
General Guidelines and Rules of Thumb Worked!
International Building Code What Applies?
non-structural and structural concrete
Non-structural the design and construction of slabs on grade shall not be governed by this chapter except Section 1910 Minimum Slab Provisions Section 1904 Durability Structural Slabs on Grade: transmitting vertical loads or lateral
forces from other parts of the structure to the soil.
Structural concrete shall be designed and constructed in accordance with IBC Chapter 19 Concrete ACI 318 as amended by section 1908.
International Building Code Section 1910 Minimum Slab Provisions
Thickness not less than 3.5 inches, and
Either 6-mil minimum polyethylene vapor retarder
Joints lapped 6 inches Between base course or subgrade and floor slab
Or Other approved equivalent method or materials to
control vapor transmission
Type 1 Engineering check!
International Building Code Section 1904 Durability Requirements
Concrete Strength and W/C Ratio Must Conform to ACI 318 Based on Exposure
Exposure to Freeze/Thaw in moist condition or deicers Exposure to Sulfates in water or soil Exposure to water when low permeability is required Exposure to chlorides when concrete is reinforced:
Deicing chemicals Salt Saltwater Brackish water Seawater
No Problem for most Type 1 projects! 12
This Code does not govern design and construction of slabs-
on-ground, unless the slab transmits vertical loads or lateral
forces from other portions of the structure to the soil.
ACI 318 Section 1.1.7
IBC 2009 The Construction Documents for structural concrete construction shall include:
A statement if slab on grade is designed as a structural diaphragm for structures assigned to Seismic Design Category D, E, or F.
ACI 318 - Drawings, Details, and specifications shall show: Statement if slab-on-ground is designed as a structural
The Construction Documents Concrete Slabs on Grade - Seismic
Although not mandated by the code, indication of slab-on-grade function on drawings and details for: Structural bracing for basement or retaining walls, Transmission of lateral forces at bases of shear walls
and frames for MWFRS and Low Seismic Zones, Support of posts, racks, machinery, or vehicles. Others
The Construction Documents Concrete Slabs on Grade
ACI 360 The Slab on Grade...
May be of uniform or variable thickness,
May include stiffening elements such as ribs or beams,
May be constructed of plain unreinforced concrete,
May be conventionally reinforced or prestressed for
the effects of shrinkage and temperature and/or
(The slab on grade may be all of these things; Structural or Non-Structural.)
Slabs on Grade Types ACI 360 A Plain (unreinforced) concrete slabs B Reinforced to limit crack widths due to Shrinkage,
Temperature, and Applied Loads Mild Steel Bars Wire Reinforcement (WWR or WWF) Fiber Reinforcement
C Reinforced to prevent cracking due to Shrinkage,
Temperature, and Applied Loads Shrinkage compensating concrete Post Tensioned
D Structural Slabs (ACI 318) (Inclusive of all types!)
Type A the Plain Concrete Slab
No Reinforcement Simple to Construct Designed to Remain Uncracked in Service Cement Type I or II Close Joint Spacing (2 to 3 per inch thick at limited aspect ratio.)
More opportunity for curling and joint deterioration Flatness and Levelness may decrease over time
Subgrade Uniformity and Drying Shrinkage Extremely Critical
Type B Reinforced to Limit Cracking Thickness Design Can be the Same as for Unreinforced
Reinforcing Does Not Add Significantly to the Load Carrying Capacity of the Type B Slab
Reinforcement (Bars or WWF) Placed in Upper of Slab
Thickness Nominal reinforcement to limit crack widths between joints Reinforced for structural capacity at a cracked section
Assumed to Remain Uncracked in Service May have many, closely spaced, fine cracks.
Cement Type I or II
Joint Spacing Greater than Type A. Based on Thickness and Subgrade Computed using Subgrade Drag Equation
Type C Reinforced to Prevent Cracking
Shrinkage Compensating Concrete (ACI 223)
ASTM C 845 Type K Cement or Separate Expansive Admixture
Reinforcement of 0.15% to 0.2% in Upper of Thickness of
Slab to Limit Initial Slab Expansion and to Restrain the Subsequent Drying Shrinkage
Detailed to be isolated from fixed portions of the structure for
both initial expansion and drying / temperature shrinkage Wider Construction Joint Spacing than Type A Slab (40 to
Type C Reinforced to Prevent Cracking Post Tensioned Control Cracking with Minimum
Precompression, or Prov