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

In typical tilt-up construction, the slab-on-grade is the working surchallenge for the lifeexpectancy of the building. Certain instances, such as otherwise inavailable panel casting beds, demand the usage of the slab as a path of access for building and construction vehicles favor concrete trucks. These trucks, when complete, can exert high tons and also pose a risk to the slab’s serviceability. An acceptable deteriorate requires filling the truck just partly once driving over the slab-on-grade. The extent to off-fill the truck counts on miscellaneous parameters consisting of slab, soil, and also vehicle properties.

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For massive warehomes and also circulation centers, tilt-up construction is the preferred choice from a cast-and-schedule standpoint. These buildings tfinish to have multiple repeatable elements and connections, so all steras of a given project tend to roll at a bhazard pace. Typically, as soon as the footings are poured, the slab-on-grade is actors. The slab is provided as a casting bed for wall panels, which develop the shell of the structure. Once the panels are poured, they are tilted up utilizing cranes and placed right into their positions approximately the building perimeter via short-lived braces. The roof is put up and also connected to the wall panels to complete the structure.

Many home builders arrangement the putting of the wall panels in such a method that all the panels have the right to be poured, albeit in stperiods, from exterior the structure. However before, unique conditions arise from time-to-time, limiting access. The access to wall panel casts might be blocked as a result of a variety of factors, such as closeness to the residential or commercial property line or unforoffering soil. In these cases, the slab is used as an accessibility route to pour panels. The slab is generally designed for a uniform fill, rack suggest load, and forklift load addressing company problems. Such a architecture may not constantly accommoday a complete concrete truck. Each concrete panel have the right to require 30 cubic yards of concrete, depending on the building dimensions and also task location. A panel pour of 500 cubic yards is not unwidespread, for which multiple truckloads are forced. Depfinishing on the slab capacity to take this wheel load, a truck have the right to be partly 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 offered pour is required.

Amerideserve to Concrete Institute Approach

The slab-on-grade is modeled as a plate sustained on a constant area spring. The plate is acted upon by a pack dispersed over a tiny location representing a wheel. The design goal is to keep the slab uncracked under the action of wheel loads

Many well-known techniques are based upon Westergaard’s remedies. These equations assume that the plate dimensions are sufficiently huge to avoid edge effects as a result of the pack. The crucial area of the wheel for design is in the internal of a slab. To protect against the creation of any type of cost-free edges under wheel tons, and to help versus the effects of curling, adequate smooth dowels should be gave at all edges and also corners, which is the present exercise. A aspect of safety is employed against the modulus of rupture of concrete for extra assurance.

ACI 360R-10, Guide to Deauthorize of Slabs-on-Grade, argues the complying with techniques for determining the thickness of a concrete slab-on-grade under wheel lots.

Portland also Cement Association Method (PCA): published by PCA in Concrete Floors on Ground (2001)Wire Reinforcement Institute Method (WRI): publimelted by WRI in Deauthorize Procedures for Industrial Slabs (1973)

Both of these methods are based upon limiting the stress on the bottom of the slab resulting from the used wheel lots. The ACI overview uses architecture charts for both of these approaches which contact for equivalent inputs and also yield equivalent results. A 3rd approach using Corps of Engineers’ Charts is likewise argued by the overview. This method has a much bigger scope in terms of audit for cumulative passes by different kinds of trucks over the slab’s life, yet it is not arisen to accommodate the specific inputs and the particular outputs this write-up intends to present.

Figure 1. A typical concrete truck.

The Concrete Truck

A typical totally loaded truck exerts 66,000 pounds on the slab, 28,000 pounds on each of its rear axles. An empty truck weighs 27,000 pounds. Each added cubic yard of concrete adds 4,000 pounds. The distance in between the front and rear axles is frequently around 20 feet. The rear axles are separated by about 4.5 feet and, on each axle, wheels are separated by approximately 8.5 feet in plan check out. The rear axles govern style, considering the share of pack they bring and their proximity to each other. As will certainly be shown in the style charts, the proximate wheels have a significant effect on the slab’s style. The typical tire push is 120 psi.

Slab-On-Grade and Soil Properties

On the majority of industrial warehouses and distribution centers, 4,000 psi concrete is supplied for slabs. The slab thickness is 6 to 7 inches for smaller sized scale structures and 7 to 8 inches for larger ones. The essential residential property is the modulus of rupture. The ACI architecture guide argues utilizing 9√fc times a security factor. Based on ACI-360 referrals, a factor-of-safety and security of 1.7 has been provided in the evaluation. A better factor-of-safety of 2 may be made use of for additional assurance depending upon the engineer’s judgment.

Geotechnological recommendations frequently incorporate the value of the modulus of subgrade reactivity. Soils that are extremely compressible, and also have low toughness, have reduced subgrade modulus (about 100pci) while moderately stronger soils have a higher style subgrade modulus (about 200pci).

Figure 2. WRI strategy.

Design Methods

The WRI approach goes through a series of architecture charts to estimate the design slab thickness for a given wheel load (Figure 2). These charts were used in the reverse direction for the purpose of this analysis. Instead of computing an allowable slab thickness for a taracquire wheel load, the allowable wheel load for a given slab thickness and also subgrade modulus necessary to be calculated. The calculation is tricky as the procedure now becomes non-direct. One has to satisfy multiple conditions through the chosen inputs. The trick lies in start through the inputs that are not affected by the output, and also eliminating them. Refer to Figure 3 for the algorithm.

Figure 3. WRI technique flowchart.

The PCA approach simplifies the procedure, making use of just one design chart. To estimate the slab thickness, this strategy provides rupture tension per 1,000 pounds of axle pack, the wheel spacing, and also the location of call. It does not have actually a method to account for the existence of a proximate heavy axle. An amplification factor on the axle fill is offered to ensure the inputs are consistent. This amplification aspect deserve to be the same as the ratio of the additional unit moment to unit moment obtained from WRI.

Figure 4. PCA method.


A continuous safety and security variable has actually been provided for both approaches. The Table presents a summary of the results for typical slab thicknesses and subgrade moduli. The result in question is the amount of off-loading necessary for a typical truck. It can be watched that both methods yield similar outcomes for the given inputs. An 8-inch thick slab is virtually always fine for a completely loaded concrete truck. A 7-inch slab can enable for roughly 60 to 80% of a complete truck, whereas a 6-inch thick slab can only enable for 25 to 35% of a full truck.

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Table showing readjust per truck.


Driving concrete trucks on slabs must not be the contractor’s initially alternative but fairly taken into consideration via good caution after all various other alternatives are exhausted. While the research counts on worths of modulus of 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’ suffer, some of this damage may not show up for years after building and construction. Furthermore, feasible slab surface damages from rocks, mud, and also debris, have to be considered and planned for prior to enabling trucks to go on the slab.▪