June 2002
Industry research project successfully replicates South African Roads Board design
By Gregory E. Halsted
An innovative technology borrowed from the South African Roads Board (SARB) may prove to be a cost-effective way to produce long lasting pavements, according to recent research.
An inverted pavement system, or inverted base as it is commonly referred to, is a deep roadway structure where the thickness and stiffness of the supporting layers is greater than that of the top structural layers. This system consists of a portland cement-treated base (CTB) layer that acts as a platform for a graded aggregate base (GAB) layer which, in turn, is covered by a thin asphaltic concrete layer.
Just how thin is this asphalt riding surface? According to information supplied by SARB, an inverted base provides enough structural performance that a maximum 2-in. asphalt riding course is more than adequate to support traffic loadings up to 100 million Equivalent Single-Axle Loads (ESALs). This is the same type of loading that the state of Georgia experiences on large portions of its Interstate Highway System.
Following is an overview of how inverted pavement structure is designed and how it compares to a conventional pavement structure.
A conventional asphalt (flexible) pavement structure like those found on Georgia’s Interstates typically consists of 12.25 in. of asphaltic concrete paving on top of 12.00 in. of GAB. This design causes the critical stress/strain plane to be located at the interface of the asphalt and GAB layers (see Figure 1).
| Figure 1. Conventional pavement structure |
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This is why a traditional flexible pavement has such deep asphalt layers—to help dissipate the loads caused by traffic before it reaches the critical stress/strain area. In short, the top portion of the pavement structure is absorbing the loads caused by traffic.
To compare, an inverted base capable of supporting the same Interstate-type traffic would consist of 2.00 in. of asphaltic concrete paving on top of 6.00 in. of super-densified GAB. These two layers would be supported by 10.00 in. of CTB. This design causes the critical stress/strain plane to be located at the very bottom of the pavement structure where it comes into contact with the subgrade (see Figure 2). The support supplied by the CTB course acts as a sound working platform for the construction of the GAB layer and also reduces the stresses in the GAB and asphalt layers. In this design, the bottom of the pavement structure absorbs the majority of the loads caused by traffic.
To see if an inverted base could be successfully constructed, material specifications and construction requirements were obtained from SARB and a cooperative effort between the Georgia Department of Transportation (GDOT), the Georgia Crushed Stone Association (GCSA), Blue Circle Aggregates Inc. (now Lafarge) and the Portland Cement Association (PCA) ensued.
| Figure 2. Inverted pavement structure |
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An entrance road at a new Blue Circle quarry in Morgan County, Ga., was selected to be the candidate for the construction of the inverted base. The primary purpose for this particular exercise was to determine if the high density requirements mandated by SARB for the GAB layer could be achieved with typical construction aggregates and practices used in Georgia. It was ultimately decided that two 400-ft. test sections would be constructed—one to see if density could be obtained following SARB methodology and another to see if this density requirement could be obtained following standard/conventional GDOT construction procedures. The quarry entrance location would allow for the easy and accurate recording of traffic volumes and loads passing over the test sections, as weight tickets would be issued to all loaded trucks leaving the facility.
From subgrade to paving, what follows next are the SARB specifications for constructing an inverted base and how construction processes were carried out in order to best comply with their requirements.
SARB required that the subgrade have a minimum California Bearing Ratio (CBR) of 15 (see Figure 3). Most soils in Georgia do not even come close to having a CBR value of 15, but they can be economically strengthened either mechanically through the addition of aggregates and their fines, or chemically using stabilizers such as portland cement. All excavation, embankment and subsequent subgrade construction was completed using the in-situ soils found on the quarry site. The subgrade was constructed using only a bulldozer and a motor grader, and then compacted with a smooth drum steel wheel roller. Stationing for the test sections was established in the field, samples of the subgrade were obtained every 200 ft., and the GDOT Materials Lab determined that the average CBR for the test sections was only 5.4.
| Figure 3. Inverted Base Specifications |
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Blue Circle and GDOT decided that the most economical way to strengthen the subgrade was to incorporate aggregate into the in-situ soils while maintaining the roadway under traffic. Not only did this action strengthen the subgrade and bring it to the correct line, grade and cross-section, it kept quarry trucks from becoming stuck in mud after heavy rains. After the subgrade was completed, CBR tests were performed in the field using a hand-held penetrometer. Using the probe of this device, CBR values were determined through a standard penetration shear test in conjunction with empirical curves. The average CBR value for the subgrade after the addition of the aggregate was 48.4.
In December 2001, the remainder of the inverted pavement system was constructed. The next layer, the CTB, was stabilized with Type I portland cement with the intent of achieving a seven-day unconfined compressive strength between 145 psi and 435 psi as required by SARB. Samples of the granular material to be used for the CTB layer were obtained at the quarry and a mix design was performed by the GDOT lab to determine the percent portland cement by weight that would be required to achieve the desired strength. The lab determined that 4.5 percent portland cement would be adequate; however, because this layer was to be constructed using a mixed-in-place method, the percent cement was increased to 5.0 percent cement in accordance with GDOT standard construction guidelines.
The granular material was placed on the subgrade by means of a Jersey
spreader box to ensure that it was placed to the proper width and depth across the roadway with a minimum of segregation. Portland cement was then placed onto the loose granular material through blow tubes mounted on the back of cement tankers. Three tankers—for a total of 77 tons—were required to place the required amount of portland cement.
The portland cement was next mixed dry into the loose granular material by means of a rotary mixer. Two passes were made with the rotary mixer to ensure that the dry portland cement was adequately incorporated into the loose granular material.
Water was then added to the granular material and portland cement mixture by means of a water truck until tests showed that it reached the optimum moisture for the material as established by the mix design. The wetted mixture was blended again by two passes of the rotary mixer and shaped to line, grade and cross-section by a motor grader.
Compaction was achieved through several passes with smooth drum steel wheel rollers. After the final rolling of the CTB, another application of water was added to dampen the surface and a sealing coat of bituminous prime was applied to ensure proper curing of the portland cement. After seven days of curing, core samples of the completed CTB layer were obtained and tested and found to have an average unconfined compressive strength of 491 psi.
According to SARB, the subsequent GAB layer would be the key to the whole structural integrity of the inverted base. GDOT monitors its individual Group I (limestone) and Group II (granite) aggregate densities by performing Test Procedure GDT 49 where a sample of material from a source is obtained and the lab determines a theoretical density using a Proctor test. SARB calculates densities in a different manner by simply taking the solid weight of stone based upon the apparent specific gravity for that source.
According to SARB, the successful construction of this GAB layer would be its compaction to a minimum of 86 percent of this apparent, or solid particle density (see Table 1). GDOT discovered that while its density determinations closely matched those obtained by SARB for limestone materials, it did not fair so well with the granite materials. In fact, for granite aggregates like the well-graded granite gneiss used on the inverted base test sections, SARB procedures showed substantially higher densities than what was determined in the lab by GDOT methods. In the field, SARB has a special construction process for achieving higher densities in order to ensure a super-densified GAB layer. This process will be described a little later in this article.
| Table 1. Apparent density or solid particle density | ||
| Actual Density as Determined From GDT (PCF) |
86% Solid Mass Based on GDOT Apparent Specific Gravity (PCF) |
% of GDT 49 Required for 86% Density (PCF) |
| 145.2 149.9 142.3 134.3 |
147.1 152.7 150.9 144.1 |
101.3 101.9 106.0 107.3 |
Before any loose GAB for this layer was placed, it was important that the shoulder notch-outs were in place to restrict the layer from shifting laterally. Again, the GAB was placed using the Jersey spreader box to ensure proper placement and to minimize segregation. The GAB was placed to a depth of between 8 and 9 in. loose in order to achieve a depth of 6 in. after compaction.
The first 400-ft. test section constructed was the one to see if the SARB density requirement could be achieved through standard GDOT construction procedures. The loose GAB was brought to optimum moisture and then compacted by means of a vibratory smooth drum roller. The GAB was then clipped and shaped to the required line, grade and cross-sections by means of a motor grader, and the surface was sealed with several passes of a pneumatic tire roller.
The second 400-ft. test section was constructed in accordance with SARB procedures by first wetting the loose GAB and obtaining initial compaction with three passes of a sheepsfoot roller in the static mode. A medley of compaction modes, chosen to best match SARB construction requirements, was performed as follows:
• One pass using a steel wheel roller in the static mode;
• Three passes using a steel wheel roller at high amplitude and low frequency;
• Three passes using a steel wheel roller at low amplitude and high frequency; and
• Multiple passes with a pneumatic tire roller.
The GAB was then shaped to the required line, grade and cross-sections by means of a motor grader and then sealed through several passes of a pneumatic tire roller. This process allowed GDOT to achieve approximately 85 percent of the solid particle density. In order to get that last 1 percent of compaction, GDOT tried the SARB special construction process—a process referred to as “slushing.”
What exactly is slushing? Slushing is the process whereby the GAB layer is essentially flooded in order to remove the excess fines and super-densify the lift. According to SARB, after completion of the standard compaction process, short sections of the GAB surface are to be thoroughly watered, rolled and slushed by means of steel-wheeled rollers or with pneumatic-tired rollers. This process is to continue until all excess fines are brought to the surface.
To begin slushing, a heavy application of water was added to the compacted GAB layer and then it was rolled with several passes by steel wheel rollers in the static mode. During these slushing operations, care was taken not to roll the surface out of shape. Water was continually added to the compacted GAB during slushing which was carried out in one continuous process covering the entire 400-ft. test section.
It was discovered that the pneumatic tire roller was more effective in bringing the fines to the surface of the compacted GAB. While water was essential to this slushing operation, it is important to note that the only reason the application of this much water was possible in the first place was because of the rigid, portland cement-treated layer underneath. Without the CTB layer, the underlying subgrade would have become flooded and turned to mud during compaction.
An inspection of the GAB layer during slushing showed air bubbles coming up through the water and fines—indicating the presence of further air voids in the GAB layer. According to SARB, slushing is basically completed when no more air bubbles are present. As the fines were brought to the surface, they formed a thin paste. This grout was uniformly broomed over the surface of the GAB to correct any areas still deficient in fines, whereupon the excess fines were broomed from the surface of the layer. This process continued until all excess fines had been brought to the surface of the GAB layer and its specified density had been reached. The excess fines and loose aggregate were then swept from the surface while the surface was still damp, and the GAB layer was then allowed to dry out.
In accordance with SARB descriptions, the completed GAB layer was firm and stable with a closely-knit surface of aggregate exposed in mosaic and free from nests of segregated material, laminations or corrugations.
GDOT engineers speculate that, in the presence of water, the smoother fines found in limestone GAB act as a lubricant that allows the particles to be vibrated into a dense mass more easily. There are also a lot more fines in limestone GAB to fill all the voids. Assuming that they do serve as a lubricant, then the SARB process of slushing out the fines makes sense. The fines are used solely as a lubricant and then flushed out of the layer so that all that remains is particle-to-particle contact of the coarse aggregate particles with just enough fines to occupy the voids, but not enough to support the coarse aggregate particles (see Figure 4).
| Figure 4. Stone contact |
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GDOT also notes that it may be the fact that granite materials, with their sandpaper-like texture, lock down in lab testing and cannot be beat into place, but must rather be slushed into place as performed by SARB with its heavy applications of water and rolling. Compactions on both the conventional section and the slushed section were measured using both a nuclear density gauge and by the sand cone method. Both testing methods showed that 86 percent of solid particle density could indeed be obtained on the granite material from this quarry using conventional GDOT construction procedures.
The last layer of the inverted base test sections was the riding course. Again, the function of the asphalt layer on the inverted base was primarily to protect the underlying GAB layer from the elements, so one thin layer of 19-mm Superpave was placed. The slopes and shoulders were dressed and grassed, and today the roadway looks like any other paved road from the surface. However, the inverted base is unique and has drawn a lot of attention. For instance, Georgia’s inverted base project won the National Stone, Sand and Gravel Association’s (NSSGA) 2001 Gold Capstone Award for Market Development.
Not only is the inverted base concept a good one in theory, it makes good economic sense as well. Cost estimates provided by GDOT showed that the inverted base section cost 27 percent less than the conventional section. This difference is primarily found in the cost of the extremely thick asphalt layer that is required to construct a conventional flexible pavement structure.
Gregory E. Halsted is a pavements engineer with the Portland Cement Association.