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

In typical tilt-up construction, the slab-on-grade is the working surface for the expectancy of the building. Certain situations, such as otherwise inaccessible panel casting beds, demand the use of the slab as a course of access for building vehicles favor concrete trucks. These trucks, once full, can exert high loads and also pose a threat to the slab’s serviceability. An acceptable compromise requires filling the truck only partially when driving over the slab-on-grade. The extent to off-load the truck depends on miscellaneous parameters including slab, soil, and vehicle properties.

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For large-scale warehouses and distribution centers, tilt-up construction is the desired option indigenous a cast-and-schedule standpoint. These buildings tend to have actually multiple repeatable elements and also connections, so every stages of a provided project often tend to roll at a brisk pace. Typically, once the footings space poured, the slab-on-grade is cast. The slab is supplied as a spreading bed for wall panels, which kind the covering of the building. Once the panels are poured, they room tilted up making use of cranes and also placed right into their positions approximately the building perimeter with short-term braces. The roof is erected and also connected come the wall surface panels to complete the structure.

Most contractors setup the pouring of the wall panels in such a method that all the panels can be poured, albeit in stages, from outside the building. However, special problems arise from time-to-time, limiting accessibility. The access to wall panel casts could be blocked because of a selection of reasons, such together closeness come the property line or unforgiving soil. In these cases, the slab is provided as an access path to pour panels. The slab is frequently designed because that a uniform load, rack point load, and forklift fill addressing business conditions. Such a style may not always accommodate a full concrete truck. Each concrete panel can require 30 cubic yards of concrete, depending upon the structure dimensions and also project location. A panel pour of 500 cubic yards is not uncommon, for which lot of truckloads room required. Relying on the slab capacity to take this wheel load, a truck can be partially off-loaded to have minimal effect on the serviceability performance of the slab.

A balance between what the slab deserve to support without visible cracking versus the number of trucks essential to complete a provided pour is required.

American Concrete academy Approach

The slab-on-grade is modeled together a plate supported on a consistent area spring. The bowl is acted top top by a load distributed over a little area representing a wheel. The architecture goal is to store the slab uncracked under the action of wheel loads

Most recognized methods are based on Westergaard’s solutions. These equations assume that the plate dimensions are sufficiently big to protect against edge effects because of the load. The critical location the the wheel for architecture is in the inner of a slab. To avoid the creation of any complimentary edges under wheel loads, and also to help versus the results of curling, enough smooth dowels must be provided at every edges and also corners, which is the existing practice. A variable of safety is employed against the modulus the rupture that concrete for extr assurance.

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

Portland Cement Association method (PCA): released by PCA in Concrete Floors ~ above Ground (2001)Wire reinforcement Institute technique (WRI): published by WRI in Design actions for commercial Slabs (1973)

Both of these techniques are based upon limiting the anxiety on the bottom the the slab result from the used wheel loads. The ACI overview offers style charts because that both of these techniques which contact for comparable inputs and also yield similar results. A third technique using Corps the Engineers’ Charts is likewise suggested through the guide. This method has a far broader scope in state of audit for cumulative passes by different kinds the trucks end the slab’s life, however it is not developed to accommodate the precise inputs and also the particular outputs this article intends to present.

Figure 1. A common concrete truck.

The Concrete Truck

A typical totally loaded truck exerts 66,000 pounds on the slab, 28,000 pounds on every of its behind axles. An north truck weighs 27,000 pounds. Each added cubic garden of concrete to add 4,000 pounds. The distance between the front and also rear axles is typically around 20 feet. The rear axles space separated by approximately 4.5 feet and, on each axle, wheels are separated by approximately 8.5 feet in arrangement view. The rear axles govern design, considering the share of pack they carry and their proximity to each other. As will certainly be displayed in the architecture charts, the proximate wheels have actually a considerable effect top top the slab’s design. The usual tire pressure is 120 psi.

Slab-On-Grade and Soil Properties

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

Geotechnical recommendations frequently include the value of the modulus the subgrade reaction. Soils that are very compressible, and also have low strength, have actually lower subgrade modulus (around 100pci) when moderately more powerful soils have a greater design subgrade modulus (around 200pci).

Figure 2. WRI method.

Design Methods

The WRI method goes through a collection of architecture charts to calculation the style slab thickness because that a offered wheel fill (Figure 2). These charts were used in the reverse direction because that the purpose of this analysis. Rather of computing an allowable slab thickness for a target wheel load, the allowable wheel load for a provided slab thickness and also subgrade modulus needed to it is in calculated. The calculate is tricky together the process now becomes non-linear. One has to fulfill multiple conditions with the favored inputs. The trick lies in start with the entry that are not influenced by the output, and also eliminating them. Express to number 3 because that the algorithm.

Figure 3. WRI method flowchart.

The PCA method simplifies the process, using only one design chart. To estimate the slab thickness, this technique uses rupture anxiety per 1,000 pounds that axle load, the wheel spacing, and also the area of contact. It does not have technique to account because that the existence of a proximate hefty axle. One amplification factor on the axle fill is offered to for sure the inputs are consistent. This amplification element can be the same as the ratio of the additional unit minute to unit moment acquired from WRI.

Figure 4. PCA method.


A constant safety aspect has been used for both methods. The Table presents a summary of the results for common slab thicknesses and subgrade moduli. The result in question is the amount of off-loading important for a common truck. It can be seen that both methods yield equivalent results because that the provided inputs. One 8-inch special slab is nearly always fine because that a fully loaded concrete truck. A 7-inch slab can permit for roughly 60 to 80% the a full truck, vice versa, a 6-inch special slab can only enable for 25 come 35% of a complete truck.

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Table showing readjust per truck.


Driving concrete trucks on slabs need to not be the contractor’s first choice but rather taken into consideration with good caution after every other choices are exhausted. When the study depends on values of modulus the subgrade reaction, certified pads may have soft spots. If it happens that trucks drive over those spots, the slab will certainly be damaged. In the authors’ experience, few of this damage may not appear for years after construction. Additionally, possible slab surface damage from rocks, mud, and also debris, must be considered and planned because that before permitting trucks to walk on the slab.▪