The leadership in charge of making pavement decisions in the Dallas District of the Texas Department of Transportation is making the move towards longer-lasting and economical pavements. Building on the concept of using recycled materials that are treated with cement for strength, which was introduced by design-build contractors for major projects, the Dallas team adopted the concept for more and more of its freeway projects. It’s easy to understand when the reasons for this move are explained.
A history of success
Making the switch is not a hard discussion when you consider the tried-and-true success of cement-treated base. Used in the United States for almost a century, cement-treated base has a well-known history of success for strong and resilient pavements. The Texas Department of Transportation has had a long history of successful pavement building cement treated base as both the primary roadbed material and a supporting layer for concrete pavement for higher traffic volume roadways.
In the 1970’s the Texas State Department of Highways and Public Transportation 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 inches of hot-mix asphalt (HMA) or asphalt stabilized base (ASB) or a minimum 1 inch hot-mix asphalt bond breaker over 6 inches of cement-treated base (CTB)
Some TxDOT districts choose one approach, and some districts choose the other. In recent years, 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. The Dallas District of TxDOT recognized this difference and has adopted increased use of cement-treated base as the preferred underlayment for concrete pavement.
The importance of supporting pavement layers is recognized
The Cement Council of Texas technical staff makes many visits to the owner/operators of our roadway network each year to assist them with information to build longer-lasting pavements. After a visit to the Dallas District, our own Dr. Nancy Beltran, Ph.D. supplied an analysis of a freeway section of pavement that analyzed the design using the Rigid Pavement Analysis System (RPAS), a finite element analysis tool developed by researchers at the University of Texas at El Paso. Her analysis showed that incorporating a cement-treated base (CTB) layer in their concrete pavement design significantly enhanced support, enabling a shift away from much thicker concrete pavements. In her post this month she discusses how the AASHTO concrete pavement design program may be overly conservative and may fail to recognize the significant contributions of the supporting pavement layers. The leaders of the Dallas District took this analysis into account when making their decision.
Simplifying the construction of pavement layers makes economic sense
Due to the more complex nature of building Continuously Reinforced Pavements with thicknesses that require two mats of reinforcing steel, TxDOT policy requires written justification to do this. To avoid this more complicated and thus more expensive option, the Dallas District prefers to improve and simplify the supporting layers to build long-lasting and economical pavement.
It is always simpler and more economical to build supporting layers in single lifts if viable. In some cases, the subgrade improvement could be reduced to a single lift by increasing the depth of the single lift of cement-treated base underlayment to 8 inches instead of 6 inches. It should also be noted that more economical increased thicknesses of subgrade or the cement-treated base underlayment helps with the issue of potential vertical rise (PVR) which once helps again on our quest to build longer-lasting pavements.
Final thoughts
Cement Treated Base Underlayment is not only resilient, but it is economical with a significantly lower cost than full-depth underlayment with asphalt 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. It is also recognized that cement-treated base provides better freeze-thaw protection as an underlying layer in colder climates. The leaders of the Dallas District are enthusiastic about making the move to using more and more cement-treated base underlayment and having longer last roads.