Slab thickness is the greatest single factor controlling the load-carry capacity of a slab-on-ground. While other factors including concrete compressive strength, the thickness and quality of the base material and sub-grade stiffness are important, they are minor players as compared to the slab thickness. For this reason, slab thickness and base tolerances are critical and typically specified in the contract documents.
The support system consists of the base, sub-base and sub-grade. The base course, directly beneath the slab, typically consists of crushed rocks and gravels and directly supports the slab. Sometimes the designer may specify a sub-base consisting of crushed rock, gravels, select or stabilized soils to help stiffen the support system, especially if the sub-grade or existing soils are of poor quality.
Since the base material is in direct contact with the slab, it must be smooth, hard and well compacted. Additionally, the contractor must grade the base material to the proper elevation. Otherwise, the slab thickness may be thicker or thinner than the specified thickness. Assuming the top of slab elevation is correct, the slab will be thicker if the base elevation is low and thinner if the base elevation is high.
ACI 117-10 states1:
4.4.5 Fine grade of soil immediately below slabs-on-ground ……………… ±3/4 in.
4.5.4 Thickness of slabs-on-ground
Average of all samples ……………………..-3/8 in.
Individual sample ………………………………-3/4 in.
The slab-on-ground thickness tolerance sets both an average thickness for all samples measured and a minimum thickness for individual samples. ACI 117-10 does not specify a plus slab thickness tolerance but does specify a ±3/4 in. top of slab elevation tolerance.
The ±3/4 in. fine grade elevation tolerance for the base or the soil immediately below the slab-on-ground corresponds directly with the -3/4 in. slab thickness tolerance for individual samples. For a more stringent fine grade tolerance, ACI 117-10 recommends a ±1/2 in. tolerance due to the sophisticated equipment available for establishing the fine grade elevation.
Actual slab thickness
Measured slab thicknesses indicate most slabs are much thinner than specified. Based on 30,000 measured data points reported by Suprenant and Malisch, the average slab thickness was about 3/8 in. less than the specified thickness and the average standard deviation for the slab thickness was about 1/2 in.2 The average standard deviation reported was centered on the -3/8 in. value and not on the specified thickness.
Suprenant and Malisch concluded the -3/8 in. average measured value appeared to agree with ACI’s -3/8 in. tolerance for the average of all samples. However, they took exception with ACI’s -3/4 in. tolerance for an individual sample due to the 1/2-in. average standard deviation for the measured slab thicknesses.
Using a tolerance value of three standard deviations indicates a thickness variation of ±1½ in. Therefore, Suprenant and Malisch reported an actual plus tolerance of +1⅛ in. (-3/8 + 1½ = +1⅛ in.) and a minus tolerance of -1⅞ in. (3/8 – 1½ = -1⅞ in.) for the slabs tested. Three standard deviations on a Bell Curve (standard normal distribution) show 99.7 percent of slab thicknesses would fall within the ±1½ in. tolerance. For 2.0 standard deviations, 95 percent of the thicknesses would fall within ±1 in. For 1.5 standard deviations, 87 percent would fall within a tolerance of ±3/4 in. and 68 percent thickness would fall within a ±1/2 in. tolerance for a 1.0 standard deviation.
The computed minus tolerance of -1⅞ in. is significantly larger than ACI’s -3/4 in. thickness tolerance. Based on the ½ in. average standard deviation for the measured slab thicknesses, Suprenant and Malisch state a ±1½ in. tolerance would be more realistic for slabs-on-ground.
What does all this mean? In reality, ACI’s -3/4 in. thickness tolerance may not be achievable for 100 percent of the slab. Using the test data reported by Suprenant and Malisch, only 87 percent of the slab will fall within ACI’s thickness tolerance. On your next job, use special care establishing and maintaining the fine grade elevation. Also, to ensure compliance with ACI’s -3/4 in. thickness tolerance, you’ll need better quality control than the contractors that installed the slabs where 30,000 date points were measured.
Costs of thick or thin slabs
Slabs thicker than the specified thickness will cost more money. Thicker slabs increase concrete costs. For every 1/8-inch the grade is low, the volume of concrete increases about 0.39 cubic yards per 1,000 square feet. This may not seem like a lot of extra concrete but for a 30,000 square foot placement and 1/8-inch low grade, you’ll need about 11.6 cubic yards of extra concrete. If the grade is 1/4 or 3/8-inch low, then you’ll need about 23 or 35 cubic yards of extra concrete for a 30,000 square foot placement. This can really add up for a large slab. Purchasing extra concrete will please the concrete supplier but will reduce your profits.
Slabs thinner than the specified minimum thickness may also increase costs, especially if thickness samples or measurements are part of the acceptance criteria for the slab. Slabs that fail to satisfy the specified thickness requirements may be subject to monetary penalties or rejection even though there are few reported cases of slab failures caused by out-of-tolerance or thin slabs.2
Building owners are sensitive about paying for slabs that are thinner than the specified thicknesses for several reasons including contract requirements and potential reductions in the load-carrying capacity of the slabs. A change in the actual thickness of the floor does affect the load-carrying capacity of the slab.
For example, reducing a six inch thick floor slab to five and one-quarter inches reduces the load-carrying capacity of the slab by roughly 23 percent assuming all other factors are constant.3 However, this reduction should not be a serious concern because the concrete strength of the as-built slab and stiffness of the support system beneath the slab typically exceed the design values used to establish the slab thickness.
On your next job, be sure to give the fine grading the attention it requires to ensure a successful and profitable project.
- ACI117-10 Specification for Tolerances for Concrete Construction and Materials (ACI 117-10) and Commentary, An ACI Standard, Reported by ACI Committee 117, American Concrete Institute, www.concrete.org
- Suprenant, B.A. and Malisch, W.R., Tolerance for Cast-in-Place Concrete Buildings - A Guide for Specifiers, Contractors, and Inspectors, American Society of Concrete Contractors, 2009, pgs. 61 & 62, www.ascconline.org
- Ringo, B.C. and Anderson, R. A., Designing Floor Slabs on Grade, 2nd Edition, The Aberdeen Group, 1996, pg. 184