The state of Kansas has a climate that goes from being hot and humid during the summer to cold and wet during the winter. Because of this, the concrete paving needs to be durable and resilient enough to weather both extremes. A critical step in that process is the proper curing of the concrete pavement. Thisis commonly done with a spray-on compound that is applied at the end of the paving operation. The question that the Kansas Department of Transportation (KDOT) had was, “is that enough?”
Dave Meggers, the state Research Development Engineer, didn’t think so. Dave wanted KDOT to look at the value of using internal curing in their pavements to improve the concrete durability, especially as it relates to curling and warping. Internally cured concrete is nothing new. It has been used successfully in a number of bridge decks throughout the country and that work has been extensively documented. However, there has been little documentation on its use in concrete paving.
Internal curing is achieved using prewetted lightweight aggregates as a replacement for a small portion of the sand. The absorbed water in the lightweight aggregate is released into the concrete matrix after final set. This additional curing provides an increased hydration of the cementitious materials, resulting in less shrinkage, and reduced cracking, of the concrete.
The city of Iola is located on the junction of US Hwy 54 and US Hwy 169, about 1 ½ hours southwest of the Kansas City metro area. As the main run through the small farm town, US Hwy 54 has a heavy amount of traffic and has been the focus of a multi-year improvement project. KDOT selected this highway as the location for their internal curing demonstration project.
With the help of the paving contractor, Koss Construction, KDOT located a portion of US Hwy 54 to place a 500’ test section of internally cured concrete. This section, along with an adjacent control section, would be instrumented with multiple vibrating-wire strain gauges and internal moisture sensors to monitor the behavior of the pavement. This instrumentation, being connected to a remote data logger, would collect real-time data of the pavement once it was placed. The research plan also called for using a dipstick to monitor the deflection of the pavement.
In May of 2014, after almost 20 months of planning, the internal curing test section was placed. Within 24 hours, members of the KDOT Bureau of Research were on the pavement measuring the deflection of each panel of the pavement. This was done repeatedly over the first 30 days. With these measurements, KDOT was able to provide a profile to show the curling and warping of the pavement.
All of this was repeated in July of 2014 with the placement of the control slab. Due to inevitable project variability, the control section was not able to be placed in conjunction with the internal curing section as planned.
With rising industry interest, KDOT welcomed members of research teams from the FHWA, the University of Kansas and Kansas State University to perform their own testing of the concrete. The FHWA responded by sending their mobile laboratory to the project site.
All of the data has been collected and the analysis of the values comparing the internal curing test section to the control is underway. While strain and moisture data is still being analyzed, preliminary hardened concrete results show that the internally cured concrete pavement has slightly lower shrinkage, lower Modulus of Elasticity and higher Poisson’s ratio all of which could lead to less curling and warping and extended service life. KDOT plans to issue a final report by the end of 2015. KDOT will also continue to monitor the test sections for future performance.