Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/276457
RoadScience 8 March 2014 Better Roads The Kansas DOT has been evaluating performance of Super- pave mix designs with higher percentages of RAP. Hamburg Wheel Tracking Device (HWTD) tests were done on Superpave mixes with high RAP content sampled from a number projects across the state. Each mixture was subjected to 20,000 repeti- tions or 20-mm rut depth, whichever came first. Tests for predicting moisture damage and rutting potential of the Superpave mixes with RAP and temperature stress re- strained specimen tests (TSRSTs) for predicting low tempera- ture cracking susceptibility also were done on selected mixes. Based on this study, the authors articulated these conclu- sions: • The number of wheel passes and rut depth from Hamburg wheel tracking device tests are significantly different for the Superpave mixes with different RAP contents, though the trend of this variation is inconclusive. • Recycled Superpave mixtures with crushed gravel aggre- gates and sand significantly improve the overall rutting performance, compared to the crushed stone or crushed stone/gravel combination in the mix. • Recycled mixes with higher PG binder grade had higher number of wheel passes with lesser accumulation of rut- ting. However, the interaction study between aggregate type and PG binder grade is inconclusive. • Rutting performance of Superpave mixes with RAP is significantly affected by the binder source, regardless of the performance grade of the binder. However, higher PG binder grade improves the rut resistance. • Statistical analysis proved the total number of wheel pass, creep slope and stripping slope of the Superpave mixes with RAP in the HWTD tests are significantly affected by the RAP content, binder grade and asphalt sources at 90-percent confidence interval. • RAP percentage in the mix, aggregate type and interaction between RAP content and aggregate type also affect the pure stripping failure phase (stripping slope) and the total wheel passes at the stripping inflection point. • The analysis of variance (ANOVA) on Superpave mixtures with RAP shows the number of wheel passes at stripping inflection point and stripping slope are significantly af- fected by mix type and binder source. • ANOVA analysis also shows the rutting performance is highly influenced by the voids in mineral aggregate and RAP asphalt content. However, performance parameters are insensitive to the virgin asphalt content, voids filled with asphalt and dust-to-binder ratio of the mix. • TSR values are significantly correlated (negatively) with the percent RAP in the mixture at 90-percent level of confidence or in other words, the mixtures become more susceptible to moisture damages the RAP content increases. • Superpave recycled mixtures with higher RAP contents tend to show lower fracture temperatures, indicating these mix- tures are more vulnerable to low temperature cracking. Sustainable concrete fines from dredged materials Remediated dredged materials may play a role as extremely fine material added to concrete, say two researchers from Florida. Dredging spoil usually is dumped at sea, but its fine nature makes it eligible for addition to portland cement concrete as a reclaimed material, say Nemmi Cole and Doreen Kobelo Ph.D., Florida A & M University, Tallahassee in their paper, Sustainable Use of Dredged Materials in Roadway Construction. Dredging of waterways produces large volumes of dredged material, containing various contaminants – from heavy met- als to pesticides – making its disposal increasingly problemat- ic, they write. "Major ports are in need of an environmentally sound and economically feasible solution for this potentially hazardous material," Cole and Kobelo say. "Dredged material is a valuable resource that can be used as filler, which is useful for the applications aside cement composites. "Fillers are widely used in road construction and various other industries, due to its small average particle size and chemically active silt/clay fraction," they note. "After proper treatment, dredged material has the potential to become a major contribution toward sustainable development and can reduce the quantities of primary resources needed for activi- ties such as construction." Dredge material is seen as a valuable resource, and innova- tive methodologies for its beneficial use are in the infancy stage of development worldwide, Cole and Kobelo write. Web Exclusive: Read "Dredged Material Mostly Clay, Sand," which explains how the material can be used as filler, at betterroads.com/dredged-material. CORRECTION: In the February 2014 Road Science article, "Materials that Make a Difference," reference is made to "high-density polypropylene (HDPE)" pipe. That should have read as "high-density polyeth- ylene (HDPE)" pipe. References later in the article to polypropylene fibers for concrete are correct.