On October 29th 2008, Roman Stone Construction Company conducted a trial installation in their yard of a new concrete road repair method. The purpose of the demonstration was to gain approval for the method under the guidelines of §704-15 Precast Concrete Pavement Slab Systems (PCCPS). This standard specification first appeared under Engineering Instruction 05-041, EI 05-042, and EI 05-043.
PCCP repairs are finding increased use in high traffic areas where the DOT wants to limit the amount of downtime and inconvenience to the travelling public. With the volume of traffic on the roads around metropolitan areas increasing every year the amount of time important roadways are shut down for repairs is becoming a critical concern of everyone. Due to the variable quality of rapid set concrete patches a precast panel was considered. Precast has the obvious advantage of being fabricated in near laboratory conditions with strict quality control of materials, moisture, temperature and curing conditions.
However, how can you match the grade of the existing road surface? The answer that Roman Stone has come up with is to build a slightly thinner slab than the original roadway then placing the slab in the excavated roadway and then raising it and leveling it by injecting polyurethane foam through the slab. This accomplishes several key things; one is that it has now become possible to match the existing road profile and two the polyurethane “fixes” any problems to the sub base that caused the road to fail in the first place.
The purpose of the trial installation was to perform a complete repair as if it was being done on a highway. So that each step of the process could be reviewed safely by DOT Inspectors, the entire demonstration took place in Roman Stone’s backyard in Bay Shore, Long Island. Roman Stone actually formed out the slab with a missing 24’ x 24’ section in the middle which is where the lift out of the old, damaged road surface was supposed to be.
Roman Stone had to submit Fabricator Standard Drawings that included all of the details of fabrication of the slabs and installation of the slabs. They ended up with a page one for the fabrication guidelines and a page two with the installation instructions. Page one is used for inspection purposes in the precast plant to insure that the material requirements are met. The second page is an installation manual that the EIC on the job and the contractor can follow to ensure that the installation is conducted properly in the field. Page two details what kind of equipment is needed by the contractor, who is responsible for doing what and how items are going to be paid for. All of this is necessary in order to ensure that the installation goes smoothly and that there will be no finger pointing as to who is responsible once the road is shut down and the area needing repair is lifted out.
The trial took place under the watchful eyes of three of NYSDOT’s pavement engineers, Michael Brinkman, Julian Bendana and Bill Cuerdon. First we had to demonstrate how the slabs are brought to the job site and unloaded and put into the excavated hole. We learned that it is very important to have the proper lifting devices and that the slab must be completely level otherwise it becomes very difficult to get the slab in the hole. The slabs were fitted utilizing plastic shims to maintain the required ¼” – 3/8“spacing. The 8” thick slabs were 1” thinner than the existing pavement so they sat 1” below the grade.
Installing a PCCP slab, the sawcutting operation is critical to success.
After the slabs were installed a truck containing the polyurethane slab jacking material moved into place and the crew began injecting the material through the 5/8” diameter injection ports. The height of the slab is controlled by the amount of foam injected. As the foam is injected it spreads out under the slab filling every small void in the slab as well as any loose pocket in the soil providing an even firmer base for the slab to rest on. The foam can be seen coming up through the joints in places which is evidence that is has traveled underneath the entire slab. Any material that comes up on the surface is allowed to harden into a “road apple” and then it is shoveled off flush to the surface. Once the slab is brought up to level with the existing pavement the road can sustain traffic on a temporary basis until the dowel bar retrofit can be accomplished. The foam cures completely in 15 minutes so by the time the TP&E is removed the surface is ready to drive on. One advantage of this method is that the foam can be installed even if it is raining. This is because the foam is hydro-insensitive, meaning that it is unaffected by water. The foam can also be installed in temperatures where the sub base is above 32 degrees although it might take a little longer to work or require preheating of the foam and can also be injected in ultra high temperatures such as 105 degree Fahrenheit. These two factors allow a greater period of the year for contractors to work and get the roads repaired in comparison to rapid set concrete.
Injecting the polyurethane foam to bring the slab up to grade. Note road apples
forming where foam has travelled under the slab and seeped up through joint.
Load transfer devices need to be installed next but they do not necessarily have to be installed immediately. So any time the weather is conducive the next step would be to install dowel bars and tie bars as needed. The State’s dowel bar retrofit method 18502.70001 is utilized to accomplish this.
Installing Load Transfer Devices
The use of a precast slab to achieve a repair on a roadway is still a developing technology. Contractors will no doubt have some good ideas and be able to develop their own installation methods that will make it easier and faster to install. While the use of a precast slab is not the fix for all applications it will become a useful tool where situations warrant a long lasting repair under extreme traffic conditions and within the smallest time window.
Completed trial with LTD's backfilled
