Over the years, Tampa Bay, Florida, area residents have had to contend with high temperatures, rain, and hurricanes in the summer but they have not had to worry about the durability of some of their roadways. Many subdivision streets, parking lots, and some county roads in Hillsborough, Pasco, Pinellas, and Polk Counties have been constructed using a soil-cement mixture, or cement-treated base (CTB), that show absolutely no signs of failure after more than eight years of traffic and Mother Nature.
These counties have roadways that were constructed as far back as November 1996 that today show virtually no rutting or cracking in the thin asphalt surfacing. “The reason for the longevity of these roadways without any indications of distress in the paving is attributed to the Croc Rock that lies underneath them,” says Alan Payne, president of Florida Soil Cement, LLC based in Riverview, whose company supplies the CTB manufactured under the name Croc Rock to the Tampa Bay market.
One of the bad raps that cement-stabilized roadway bases get is the frequent appearance of small cracks in the asphalt surfacing. These types of bases, including CTB, can be the source of shrinkage cracks that can occur in the stabilized base layer during curing, which can reflect up through the asphalt surface. These cracks in the CTB layer can result in stress concentrations and cracking in the asphalt surface layer. The surface cracks tend to follow the same pattern as the cracks in the base, and are commonly referred to as “reflection” cracks. These reflective cracks are typically only cosmetic in nature and can be present for many years without the need for significant and costly maintenance. However, Payne is supplying CTB for roadways that exhibit no indications of reflective cracking at all.
Roadways in these four Tampa Bay area counties are typically constructed from either Florida Department of Transportation (FDOT) approved unconsolidated limerock, crushed concrete, or soil-cement (CTB). In fact, Payne’s Croc Rock is actually FDOT limerock blended with portland cement at a central plant, or pugmill, and delivered to the roadway where it is then spread on the prepared subgrade. “The big advantage to using Croc Rock is that there is an extremely high water table around these parts and CTB utilizing 100 percent Florida Department of Transportation (FDOT) limerock and cement gives us pavements that don’t rut, pothole, or crack,” says Payne. “By mixing in the cement, we take an approved roadway construction material and make it better. With the extremely limited supply of crushed concrete; Croc Rock becomes the natural choice for a durable and economic roadway building material.”
Jeff Joaquin, vice president of Kearney Construction Company, Inc. located in Tampa agrees. “There is not another base material which compares with Croc Rock based on its durability, consistency, availability, and ease of installation. Kearney utilizes Croc Rock on every project where CTB is specified and we have also had projects where we have convinced the owner and the geotechnical firm to switch from the original designed base to this product. From a standpoint of productivity, Croc Rock allows Kearney the ability to finish the product the same day it is installed which greatly reduces the time and energy spent on re-grading and finishing when compared to other base products.”
Due to the high water table and the lack of “hard” rock that does not absorb moisture in these situations, engineers are typically specifying roadway bases of either crushed concrete or soil-cement, manufactured using material that has a minimum limerock bearing ratio (LBR) of 100. The soil-cement is extremely cost competitive with crushed concrete and typically is a better choice when construction and long term maintenance costs are taken into consideration. As municipalities and developers become educated about the benefits of cement-treated limerock, and visually observe the lack of reflective cracking in old cement-treated base roadways, the demand for cement-treated limerock is increasing.
The success of Payne’s CTB roadways is the direct result of the quality control he puts into the testing, production, and construction on each project. Limerock samples are obtained from the mine and blended in the lab with Type I portland cement to prepare specimens for unconfined compressive strength testing after 7 days of curing. A minimum strength of 300 psi is desired and is usually obtained with the addition of only 2 to 2.5 percent cement by dry weight of the limerock. This combination of dense graded limerock and extremely small amounts of cement typically results in CTB that has densities between 110 and 115 pcf with optimum moisture contents between 12 and 15 percent. When homogenously blended in Payne’s ARAN pugmills, approximately 600 tons per hour of this non-cracking base material is produced.
During construction, the CTB is delivered from the mixing plant to the roadway in rear-dump trucks and spread on the subgrade by either a bulldozer or front end loader. It is then shaped to grade and the desired thickness (typically 6 inches compacted) by a motor grader. Initial compaction is achieved through multiple passes of a smooth drum roller in the static mode with finish rolling performed by a pneumatic-tire roller to seal the surface. A minimum field density of 98 percent of the established laboratory modified Proctor test is required for acceptance. After density is obtained, the base is then primed to aid in curing for between 7 and 14 days, depending on the County, before the final application of a thin 1.75-inch hot-mix-asphalt surfacing. It is roadways constructed in precisely this manner that have resulted in crack-free pavements for the better part of a decade!