Louisiana, USA, looks at lighter layer of roller-compacted concrete
Roads and Bridges online Magazine recently reported on a unique, thinner roller-compacted concrete pavement (RCC). As a durable, economical and low-maintenance concrete material, roller-compacted concrete (RCC) is steadily becoming the choice for many pavement applications. And researchers in Louisiana are testing just how thin those applications can go.
Traditionally, RCC pavements have been built with an 8- to 12-in. slab thickness and (because of its relatively coarse surface) used for pavements carrying heavy loads in low-speed areas, such as warehouses, truck terminals, distribution centers, and other industrial pavements. Now, a group of Louisiana researchers are exploring even thinner RCC options: 4- to 8 in. slab thickness—hoping to provide cost-effective pavement design alternatives for low-volume roadways where these types of heavy loads are encountered.
Low maintenance for the low volume?
In recent years, the Louisiana Department of Transportation and Development (LaDOTD) has seen a rapid pavement serviceability decline on rural, low-volume roadways due to the increasing use of ports, intermodal facilities, shale gas exploration, agricultural activities, and logging activities in the state. A low-volume pavement structure in Louisiana typically consists of a thin asphalt concrete over a cement-treated/stabilized soil base. Due to an increase in heavy and/or overloaded truck trafficking, those low-volume asphalt-surfaced roadways require frequent maintenance and deteriorate rapidly as a load-induced, fatigue-damaged pavement.
However, because of the proven outstanding load-carrying capacity and low-maintenance features of RCC pavements, LaDOTD requested a research study from the Louisiana Transportation Research Center (LTRC) to determine how thinner applications of RCC would withstand the wear and tear such heavy loads typically bring.
Running some test lanes
Spearheaded by LTRC’s Dr. Zhong Wu and Dr. Tyson Rupnow, researchers launched a study titled, “Performance of Thin RCC Pavements Under Accelerated Loading”. Using the Pavement Research Facility (PRF) in Port Allen, La., researchers hoped to create an appropriate RCC mixture proportion for constructing the test lanes (including density and strength characteristics). They sought to determine the structural performance with failure mechanism and the load-carrying capacity of thin RCC pavements that may be used as a design option for low-volume pavement design in Louisiana.
Mixture materials
Concrete samples were produced in laboratory conditions from four different cement contents to show the effect of differing cement contents on the moisture density and strength relationships. Compressive strength specimens were produced at the maximum densities for each corresponding mixture and tested at seven and 28 days of age. Figure 1: Final gradation recommended in the PRF pavement lanes construction. Note that the recommended RCC mix contains 11.4% of portland cement with an optimum moisture content of 6.5%.
Construction
Figure 2: Six RCC test sections were designed to construct at PRF. The 10-in. cement-treated subgrade contained a cement content of about 4%—just enough to provide a dried stable working platform in which to build the stronger base. Figure 3: Two finished lanes consisting of six RCC test sections. Each section is about 72 ft long and 13 ft wide.
RCC production was accomplished by using a twin-shaft pugmill operation on the paving site. A high-density asphalt paver was used to place the RCC and achieve initial density, and a 10-ton steel-drum roller was used for final compaction of the RCC pavement structure. Each lane included three test sections with RCC layer thicknesses of 4, 6 and 8 in. Finally, a white-pigmented curing compound was sprayed on the finished RCC surfaces for curing.
Accelerated loading test
A heavy vehicle load-simulation device, ATLaS30, was used to load the constructed RCC test sections. The accelerated loading test was in a time-sequence order of Sections 4, 5, 6, 3, 2 and 1. In the end, four RCC sections 2, 3, 5 and 6 loaded to a cracking failure, as the final pavement surfaces.
The overall accelerated loading results showed that all thin RCC test sections had a high load-carrying capacity under a typical southern Louisiana pavement condition. Since each RCC section had endured a certain number of extremely high-axle loads before a fatigue failure, such high RCC fatigue lives proved the feasibility and suitability of using a relatively thin RCC pavement on low-volume roadways where heavy/overloaded trucks are often encountered.
The development of surface cracks on a thin RCC test section under the ATLaS30 wheel loading are illustrated through crack mapping. A post-mortem trench evaluation was performed at the end of APT testing on the four failed RCC pavement sections.
According to the results from the crack mapping and post-mortem trenches, the cracking failure mechanism for a thin RCC over a soil cement or cement-treated soil base pavement may be summarized as the following:
- The repeated heavy axle loads would first crack through saw-cutting joints of a thin RCC layer (possibly due to the bottom bending), and subsequently create pumping actions at the cracked joints under a wet condition
- The pumping-out of fine materials would gradually weaken the overall pavement strength by forming voids in the base under or near the joints
- Continuously heavy wheel loading over a weak subgrade and/or voided base locations would start to break a thin RCC slab longitudinally and eventually result in an overall fracture failure due to the repetitive fatigue bending as well as the temperature-induced slab curling, especially under a naturally warm and wet southern Louisiana pavement condition.
Thin and strong
To illustrate a potential benefit of using a thin RCC pavement in lieu of an asphalt pavement alternative for low-volume roadways in Louisiana, a construction cost analysis was performed on two pavement structure alternatives.
- Pavement A consisted of a 5-in. RCC over an 8.5-in. soil cement
- Pavement B had a similar base and subgrade structure as Pavement A, but it used a 7-in. hot-mix asphalt (HMA) as the surface layer
- According to the 1993 AASHTO pavement design guide, as well as the loading results from this study, both pavement structures would be expected to have at least a pavement life of 1 million of 18-kip ESALs.
The construction costs of two pavement alternatives were determined from the previous construction costs in Louisiana experiments. The quantities were calculated based on a 1-mile-long, 13-ft-wide lane. Applying the estimated cost benefits to a typical two-lane, 10-mile-long roadway project, the use of a 5-in. RCC layer in lieu of a 7-in. HMA layer results in a total construction cost savings up to $2,261,740.
Overall, the results showed that a thin RCC over soil cement-pavement structure, in lieu of an HMA layer for a low-volume pavement (where heavy or overloaded trucks frequently travel in Louisiana), has a superior load-carrying capability … can be cost-effective in construction and could extend pavement service life with less maintenance.
For the entire article, including Mixture materials; Many more construction details; Accelerated loading test details; Additional photos, Tests, and Charts; and More, please go to: https://www.roadsbridges.com/wanting-be-thinner
HOME PAGE PHOTO: A high-density asphalt paver was
used to place the RCC and achieve initial density.