Thicker pavement layers are known not only to help spread and reduce the strain of traffic loads but also reduce the strain below the pavement due to soil heave. The Roman roadbuilders understood this and implemented multiple layers of pavement support for roads that can still be seen today. The Texas Department of Transportation has adopted this concept for years in our era of modern road building by using an underlayment layer on all concrete pavements. The scientists and engineers who have developed the pavement design programs in use today, all recognize the benefits of an improved pavement underlayment layer under concrete pavements. More and more concrete pavement is being built without reinforcing steel and a strong underlayment that lasts multiple decades without maintenance. Field performance evaluations of concrete pavements have revealed that a durable, stabilized, non-erodible base is essential to the long-term performance of concrete pavements. If the base does not provide good support, the concrete pavement will be compromised, and long-term performance will be compromised. So, I am suggesting and recommending that it is beneficial to support concrete pavement with an intermediate strength layer below it.
One of the issues for city street pavement is that there is simply not the depth available for a deeper underlayment support layer because multiple utilities cross shallowly below the pavement. My recommendation for this case, due to the tremendous benefit of a good support underlayment, is to require stricter depth requirements on utility crossings or stop the underlayment short where shallow utility crossings are determined to be necessary. A supportive underlayment layer or cement-stabilized soil would be beneficial even with leave-outs.
In the 1970’s the Texas Department of Transportation (TxDOT) leaders recognized pre-mature failures in concrete pavement placed on untreated flexible base across the state. Increasing the pavement thickness helped but it didn’t solve the problem and the expense was significant. As a practical matter, the department’s pavement engineers suggested an underlayment layer that would better support the concrete pavement as well as serve as a great working platform for it. Those districts with better concrete pavement used cement-treated base or asphalt-stabilized base as an underlying layer. It was not scientifically derived but as a noted performance measure, two suggested options for underlayment were suggested. TxDOT recognizes one of the following layers for concrete slab support:
- 4 in of hot-mix asphalt (HMA) or asphalt stabilized base (ASB)
- Or a minimum 1 in hot-mix asphalt bond breaker over 6 in. of cement-treated base (CTB)
Some TxDOT districts choose one approach, and some districts choose the other. Falling Weight Deflectometer Data taken by TxDOT on multiple pavement sections to see which approach performed better. The data consistently showed that for equivalent pavement sections using Cement Treated Base (CTB) as an underlayment resulted in lower deflections and thus damage to the pavement. Additionally, the TxDOT Pavement Management System data demonstrates that concrete pavements with a Cement Treated Base perform significantly better, especially in areas with high rainfall or are subject to flooding. Many TxDOT Districts are now recognizing this difference and are adopting increased use of Cement Treated Base as the preferred underlayment for Concrete Pavement.
In 2017 Hurricane Harvey put Cement Treated Base to the test. Cement Treated Base underlayment used to support the concrete pavement in the Houston and Beaumont Districts maintained their resilience despite the massive flood waters inundating the coastal area roadways. Cement Treated Base Underlayment is not only resilient, but it is economical with a significantly lower cost than stabilization with asphalt. It is also recognized that Cement Treated Base provides better freeze-thaw protection as an underlying layer in colder climates.
Representative sample concrete section with 4 in HMA showing 23 mils of deflection
(Source: TxDOT)
Representative sample concrete section with 6 in Cement Treated Base showing 15 mils of deflection
(Source: TxDOT)
Note, that the critical stresses and strains are significantly lower in the thicker cement-treated base pavement layer and subgrade layer than in the thick asphalt pavement layer and its subgrade layer.
In summary:
- Improved Stability and Durability with Cement Treated Base
- Improved Resilience with Cement Treated Base
- Concrete Pavement thickness could be reduced with increased use of CTB
- Agencies should review current practices and better correlation with their policies for Concrete Slab Support if they want longer-lasting pavements