Preliminary Assessment of the Structural Properties of Pavements with Foamed Bitumen Base Layers
The use of foamed bitumen is increasing, especially in the Kwazulu-Natal region. Two aspects need to be addressed for the design engineer to be able to use a fairly unknown material like this with confidence:
The confidence in these procedures will be boosted further if the expected behaviour and performance of the pavement designs predicted from the material and structural design procedures, are confirmed by the performance of these designs under traffic or accelerated pavement testing.
This report contains a preliminary assessment of the structural properties of pavements with foamed bitumen base layers and should serve as a starting point in the process of developing a structural design procedure for these pavement types. The aim of the research reported here is to try and establish the relative position of material treated with foamed bitumen compared to other bound road building materials in terms of its strength and structural properties. The mix design procedure will not be addressed in this report.
Field test sections were identified for field testing and obtaining material samples for laboratory testing. The data obtained from the field test sections serve as the basis for this study.
Three projects were identified by the Kwazulu-Natal Department of Transport for inclusion in this investigation. A number of sub-sections were identified on two of these projects and the sections tested therefore included:
It is not the intention of this report to trace the development of foamed bitumen nor to describe the process of producing foamed bitumen in great detail. Detail information in this regard is available elsewhere(1,2). It is, however, necessary to describe the general characteristics of the materials investigated during this project.
Foamed bitumen is produced by injecting cold water into hot bitumen, thereby forming a foam. The foaming process increases the volume of the binder and reduces the viscosity of the binder allowing the foamed bitumen to be mixed with cold, damp aggregate. The aim is to have the maximum increase in volume for as long as possible to assist in the mixing process. Two concepts are therefore used to describe the amount of foaming and the duration of the foamed state:
Both these parameters are largely controlled by the amount of water injected into the binder but are unfortunately, inversely proportional to each other. The larger the amount of water added to the bitumen the greater the increase in volume will be but the shorter the duration of the foamed state will be.
Some of the major advantages of using this process as compared to conventional road building materials are:
Sand stabilisation with foamed bitumen is a well established application of foamed bitumen and one such section at Sodwana is included in this study. The material that was stabilised on the other sections mainly consisted of the in-situ natural gravel at the particular site except for the milled asphalt that was used on P504.
Before attempting to quantify the properties of foam treated material, a general description of the material based on physical appearance may prove worthwhile. It was evident that the foam treated material differs from asphalt concrete in the sense that all the granular particles are not covered with binder. The binder seems to attach to the fine material, forming mortar globules that keep the granular matrix intact. The material is therefore still quite brittle, unlike asphalt and the voids are also far from being filled with binder. In terms of physical appearance and feel, the material seems to be closer to emulsion treated natural gravel and cement stabilised material than asphalt.
As mentioned earlier, the approach of the project was to evaluate the field and laboratory properties of existing trial sections where foamed bitumen has been used. Background information was obtained on the three projects identified by the client for investigation. These projects were then subdivided into units of similar design and these units were tested. The project therefore consisted of two main components namely field testing and sampling of the identified sections followed by laboratory testing of the samples removed during field testing.
The following field tests were done during an initial field testing phase:
Sampling done during the initial field test phase included:
The laboratory tests that were done on the cores from the foam treated base layers included:
The final field testing of the sections consisted of the instrumentation of P504 at Shongweni with Multi-depth Deflectometers (MDDs) and taking in-depth deflection readings on this section with a truck loaded to the standard axle load of 80 kN. Falling Weight Deflectometer tests were also done on all the sections and stiffness moduli were calculated from the deflection measurement data.
CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK
The structural properties of natural gravel and recycled asphalt material were tested on six test sections on three trial projects of the Kwazulu-Natal Department of Transport. A preliminary classification of the pavement designs on the trial sections was also done based on a traffic count and rut measurements for three of the sections and DCP test results for all the sections.
The use of foam treated material has proven to be very effective in the construction of roads in difficult terrain under heavy traffic in Kwazulu-Natal. Some of the major advantages of using this process as compared to conventional road building materials are:
Tables 6.1 and 6.3 give summaries of selected test results and back-calculated stiffness modulus values for all the test sections. The test section on P504 was divided into four sub-sections. In general, the following conclusions are made regarding the volumetric properties and strength parameters of the foam treated material:
It is believed that foam treated material should not be treated similarly to conventional asphalt concrete during the material and pavement design process. Material test methods for the foam treated material should not necessarily be the same as for conventional asphalt. The reasons for these statements are the following:
The foam treated material seems similar in nature to a cement or emulsion treated material (higher binder content emulsion mixes or GEMS) in terms of volumetric properties and strength parameters. The difference between a foam treated material and a cement stabilised material seems to be that the foam treated material does not crack and break down to the original strength properties of the parent material as quickly as a cement stabilised material.
It is believed that the material structure of foam and emulsion treated material are very similar and that these are two different vehicles for getting the binder well dispersed in and mixed with the granular material.
The diagram in Figure 6.1 illustrates the relation between different types of road building materials.
Foam treatment definitely has the advantage of increasing the strength of a G6/G7 material which is below base and sub-base quality to enable the material to be used in a base or sub-base layer.
The pavement structures investigated during this project seem to provide a range of bearing capacities from 300 000 E80s to 3 million E80s. A preliminary classification of the structural bearing capacity of the tests sections was done in Section 5.3 and is shown in Table 6.3. This classification is based on very little performance data and DCP bearing capacity predictions and should only be regarded as an interim guideline.
The mix design procedure for foamed bitumen should be addressed in detail. Test methods other than the test methods for conventional asphalt should be investigated for material design purposes.
The performance of the test sections used in this study should be assessed on a regular basis, at least once a year. A limited number of field tests should be included in this process.
7-day traffic counts should be done on the test sections used in this study.
The bearing capacity classification of the test sections should be updated once information from the above list of recommended work becomes available.