CTR Pavement Researchers Win 2003 TxDOT Innovation Award
Research Highlights
Project 0-1700: Thermochron and Hygro Button Innovation
Research Supervisor Dr. B. Frank McCullough, along with co-Principal Investigator Terry Dossey, Researcher Seong-Min Kim, and graduate students Shantala Ramaiah and Jeong-Hee Nam were recently chosen by the Texas Department of Transportation (TxDOT) for a TxDOT Top Innovations Award. The award was based on their work on TxDOT-sponsored CTR research project 0-1700, "Improving Portland Cement Concrete Pavements."
Each fall, TxDOT selects Top Research Innovations and Findings for the past year based on anticipated or already realized dividends to the department and the state. These dividends may be in terms of saved lives, more efficient operations, improved services, and/or financial savings. Products from the research program may include devices, machines, tools, materials, manuals, and software, while others are less tangible concepts, knowledge, or advice. These products affect virtually every area of TxDOT operations.
Portland Cement Concrete roads are used on highways with high traffic volumes and large trucks due to their load carrying capacity and durability. Unfortunately, concrete pavements constructed during high temperatures and moisture conditions lead to temperature and moisture loss levels in the pavement that produce excessive spalling. Spalling can be so severe that major rehabilitation is required in less than ten years on pavements designed for 30 years. In cases like these, the cost to the public can be large as illustrated by two cases in Houston where a bonded concrete overlay was required on a section of BW-8 with a cost over $6 million dollars, and an asphalt overlay on SH-6 with a price tag over two million required on a section of SH-6. Traffic delay and other user costs due to the premature rehabilitation would add more to the total cost.
Figure 1. Installation of Thermochron buttons in new paving
Engineers have known for some time the detrimental effects of high temperatures and excessive moisture loss, but have not been able to monitor them in an acceptable fashion. The Thermochron (temperature) and Hygrochron probes (moisture) developed under this study provide the engineer a cost effective way to extensively monitor the pavement during construction, potentially saving millions of dollars over the years as well as keeping the goodwill of the traveling public. Figure 1 shows installation of the Thermochron buttons in fresh concrete.
Figure 2. Winter concrete temperatures from an Amarillo pavement
The Thermochron buttons are $8 each, which allows economical monitoring of temperature in new construction, possibly replacing the conventional maturity meter at lower cost and with higher security (buttons are embedded in pavement and store data internally). In addition, the researchers have used Thermochrons to measure minimum temperatures at various depths over two winters (Figure 2). This data indicates that mid depth temperatures in thick pavements are not as low as expected, which means steel designs can be optimized by region at considerable savings.
In a similar manner, the Hygrochron buttons store humidity readings which can be used in fresh concrete to indicate how effective the curing is under any condition. Figure 3 shows results from monitoring concrete humidity during construction in Van Horn. Conditions became severe due to high winds and temperatures the second day, resulting in moisture loss at the pavement surface that could not be controlled by the curing compound. If contractors had ready access to information of this sort, additional measures could be taken during high evaporation periods to avoid strength loss or differential shrinkage conditions that lead to spalling.
FFigure 3. Failure of curing compound during extreme evaporation condition
As a rough estimate of possible statewide savings, the Texas Rigid Pavement Database was examined to determine what percentage of concrete roads fail due to poor temperature or moisture control during construction. The analysis determined that roughly 8.7% fail to reach design life due to close cracking (possible temperature problems) and 7.2% from spalling (possible uncontrolled moisture loss), with some overlap between the two (i.e. some pavements experience both problems). Since 2.5 million cubic yards of paving concrete are used in Texas annually at an estimated cost of $137 million, the failure rates account for about $20 million in replacement cost, allowing for the overlap. If these pavements average a 20 percent reduction in design life, then the savings from correcting the problem might be as much as $4 million / year. For more information on the use of thermochrons in concrete, visit IButton.com.