Jan, 2017 through Rafik R. Gerges, P.Eng, Ph.D., S.E., SECB, LEED AP, BSCP and Harsh K. Nisar, MSCE In Articles, Engineer"s Notebook comment 3

In typical tilt-up construction, the slab-on-grade is the working surface for the expectation of the building. Certain situations, such as otherwise inaccessible panel spreading beds, demand the use of the slab as a path of access for building vehicles like concrete trucks. These trucks, as soon as full, can exert high loads and pose a hazard to the slab’s serviceability. An acceptable compromise requires filling the truck just partially when driving end the slab-on-grade. The degree to off-load the truck depends on various parameters consisting of slab, soil, and also vehicle properties.

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For large warehouses and distribution centers, tilt-up building is the desired option indigenous a cast-and-schedule standpoint. These buildings tend to have multiple repeatable elements and connections, so every stages of a given project often tend to roll at a fast pace. Typically, once the footings room poured, the slab-on-grade is cast. The slab is provided as a casting bed for wall panels, which type the covering of the building. When the panels are poured, they room tilted up using cranes and also placed into their positions about the building perimeter with temporary braces. The roof is erected and connected come the wall surface panels to complete the structure.

Most contractors arrangement the putting of the wall surface panels in such a means that every the panels have the right to be poured, albeit in stages, from outside the building. However, special conditions arise from time-to-time, limiting accessibility. The accessibility to wall panel casts can be blocked as result of a selection of reasons, such as closeness to the property line or unforgiving soil. In these cases, the slab is supplied as an accessibility path to pour panels. The slab is typically designed because that a uniform load, rack allude load, and forklift fill addressing organization conditions. Together a style may not constantly accommodate a full concrete truck. Each concrete panel deserve to require 30 cubic yards that concrete, depending on the building dimensions and project location. A panel to water of 500 cubic yards is no uncommon, because that which multiple truckloads space required. Depending on the slab volume to take this wheel load, a truck have the right to be partially off-loaded to have actually minimal effect on the serviceability performance of the slab.

A balance between what the slab have the right to support there is no visible cracking versus the variety of trucks necessary to finish a offered pour is required.

American Concrete academy Approach

The slab-on-grade is modeled together a plate sustained on a constant area spring. The plate is acted top top by a load distributed over a tiny area representing a wheel. The style goal is to keep the slab uncracked under the action of wheel loads

Most recognized methods are based on Westergaard’s solutions. These equations assume the the bowl dimensions room sufficiently large to avoid edge effects due to the load. The critical location of the wheel for design is in the internal of a slab. To avoid the development of any free edges under wheel loads, and to help against the results of curling, sufficient smooth dowels should be noted at every edges and corners, which is the present practice. A factor of safety and security is employed against the modulus that rupture that concrete for extr assurance.

ACI 360R-10, Guide to architecture of Slabs-on-Grade, suggests the following methods for determining the thickness of a concrete slab-on-grade under wheel loads.

Portland Cement Association an approach (PCA): released by PCA in Concrete Floors ~ above Ground (2001)Wire combine Institute an approach (WRI): released by WRI in Design actions for commercial Slabs (1973)

Both that these methods are based upon limiting the tension on the bottom that the slab resulting from the used wheel loads. The ACI guide offers design charts for both the these approaches which contact for similar inputs and also yield similar results. A third an approach using Corps of Engineers’ Charts is also suggested through the guide. This strategy has a far wider scope in state of bookkeeping for cumulative overcome by various kinds that trucks end the slab’s life, however it is not emerged to accommodate the an accurate inputs and also the particular outputs this article intends to present.

Figure 1. A typical concrete truck.

The Concrete Truck

A typical fully loaded truck exerts 66,000 pounds on the slab, 28,000 pounds on every of its rear axles. An empty truck weighs 27,000 pounds. Each additional cubic yard of concrete to add 4,000 pounds. The distance in between the front and rear axles is typically around 20 feet. The behind axles room separated by roughly 4.5 feet and, on each axle, wheels are separated by roughly 8.5 feet in plan view. The behind axles administer design, considering the re-publishing of fill they carry and also their proximity to every other. As will certainly be displayed in the architecture charts, the proximate wheels have actually a significant effect ~ above the slab’s design. The usual tire push is 120 psi.

Slab-On-Grade and also Soil Properties

On most industrial warehouses and also distribution centers, 4,000 psi concrete is used for slabs. The slab thickness is 6 to 7 inches for smaller scale structures and also 7 come 8 inch for larger ones. The crucial property is the modulus of rupture. The ACI architecture guide says using 9√fc times a security factor. Based on ACI-360 recommendations, a factor-of-safety of 1.7 has been provided in the analysis. A higher factor-of-safety of 2 may be made use of for additional assurance depending on the engineer’s judgment.

Geotechnical recommendations typically include the value of the modulus that subgrade reaction. Soils that are very compressible, and have short strength, have actually lower subgrade modulus (around 100pci) while moderately stronger soils have a higher design subgrade modulus (around 200pci).

Figure 2. WRI method.

Design Methods

The WRI technique goes v a series of architecture charts to estimate the design slab thickness for a provided wheel fill (Figure 2). These charts were provided in the reverse direction because that the function of this analysis. Instead of computing an allowable slab thickness because that a target wheel load, the allowable wheel fill for a given slab thickness and also subgrade modulus essential to be calculated. The calculation is tricky together the process now i do not care non-linear. One has actually to satisfy multiple conditions with the liked inputs. The trick lies in start with the input that room not influenced by the output, and also eliminating them. Express to number 3 because that the algorithm.

Figure 3. WRI technique flowchart.

The PCA technique simplifies the process, using only one design chart. To calculation the slab thickness, this an approach uses rupture stress per 1,000 pounds the axle load, the wheel spacing, and also the area of contact. The does not have an approach to account because that the existence of a proximate heavy axle. One amplification variable on the axle pack is used to for sure the inputs are consistent. This amplification element can be the exact same as the ratio of the additional unit minute to unit moment acquired from WRI.

Figure 4. PCA method.


A continuous safety variable has been supplied for both methods. The Table presents a summary of the outcomes for common slab thicknesses and subgrade moduli. The an outcome in concern is the quantity of off-loading crucial for a typical truck. It have the right to be checked out that both techniques yield comparable results because that the provided inputs. One 8-inch thick slab is nearly always fine for a completely loaded concrete truck. A 7-inch slab can permit for approximately 60 to 80% the a full truck, whereas a 6-inch special slab can only allow for 25 to 35% of a full truck.

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Table showing readjust per truck.


Driving concrete trucks on slabs need to not it is in the contractor’s very first choice however rather taken into consideration with good caution after every other choices are exhausted. If the study counts on worths of modulus that subgrade reaction, certified pads may have actually soft spots. If it happens that trucks drive over those spots, the slab will certainly be damaged. In the authors’ experience, some of this damage may not appear for years after construction. Additionally, possible slab surface damages from rocks, mud, and debris, must be considered and also planned for before enabling trucks to go on the slab.▪