Preliminary Assessment of the Structural Properties of Pavements with Foamed Bitumen Base Layers

H L THEYSE

INTRODUCTION

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:

Proper mix design procedures should be established for optimising the strength properties of the material

A structural design method should be available to provide an appropriate pavement design for the traffic and environmental conditions under which the road will perform.

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:

A foam bitumen treated sand section on P466-2 near Sodwana.

A foam bitumen treated weathered granite section on P423-0 near Nagle dam.

A foam bitumen treated weathered sandstone section on P423-0.

A foam bitumen treated weathered granite section on P504-0 near Shongweni interchange.

A milled asphalt section on P504-0 with the asphalt recovered from a rehabilitation project on the N3.

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:

The expansion ratio expresses the maximum foam volume as a ratio of the volume of the bitumen once the foam has subsided completely.

The half-life is the time period in seconds during which the volume of the foamed bitumen is reduced to a half of the original, maximum foam volume.

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:

The material may be prepared at a plant, stockpiled and used when required. No mixing of the material is necessary on the road. The time required for construction and hence the closing of lanes during rehabilitation is therefore reduced and the effort of working the material on the steep, winding slopes is reduced.

The process is ideally suited to deep in place recycling.

The process allows the use of locally available gravels that may not be regarded as base quality material.

The initial strength gain of foam treated material is rapid and the road may be opened to traffic within a few hours on the same day of construction.

Due to the bound nature of the material, it should not be as susceptible to moisture as granular material.

The foam treated material does not crack due to shrinkage as does cement treated material.

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:

Dynamic Cone Penetrometer (DCP) tests

Rapid Compaction Control Device (RCCD) tests

Nuclear density tests (Strata and surface gauge)

Straight-edge maximum rut measurements

Sampling done during the initial field test phase included:

A test pit for taking bulk samples from at least two layers below the foam treated base layer and a slab cut from the foam treated base layer.

Cores taken from the foam treated base layer.

The laboratory tests that were done on the cores from the foam treated base layers included:

Maximum Theoretical Rice Density (MTRD)

Bulk Density

Geometric Bulk Density

Binder recovery to obtain the binder content

Grading analyses

Voids calculations

Indirect Tensile Tests (ITT) to obtain the Indirect Tensile Strength (ITS), strain at break and the Stiffness Modulus

Dynamic creep tests

Unconfined Compression Tests

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:

The material may be prepared at a plant, stockpiled and used when required. No mixing of the material is necessary on the road. The time required for construction and hence the closing of lanes during rehabilitation is therefore reduced and the effort of working the material on the steep, winding slopes is reduced.

The process is ideally suited to deep in place recycling.

The process allows the use of locally available gravels that may not be regarded as base quality material.

The initial strength gain of the foam treated material is rapid and the road may be opened to traffic within a few hours on the same day of construction.

Due to the bound nature of the material, it should not be as susceptible to moisture as granular material.

The foam treated sections did not crack due to shrinkage except for the section at Sodwana where some cement was added to increase the fines in the sand.

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:

The binder content of foam treated material is lower than that of conventional asphalt concrete and in the same range as that of Granular Emulsion Mixes (GEMS).

The average void content for foam treated material is much higher than the void content of conventional asphalt concrete.

The indirect tensile strength and ITT stiffness modulus of foam treated material is lower than that of conventional asphalt concrete.

The UCS-value of foam treated material is in the lower half of the range of UCS-values for cement 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 appearance of natural gravel which is treated with foamed bitumen is not at all similar to conventional asphalt. The binder tends to form a mortar with the fines leaving globules of black mortar throughout the material. Very little of the larger aggregate particles are therefore actually covered with binder.

A compacted sample of the material, although intact, is very brittle compared to conventional asphalt. Granular particles or lumps of material may be removed from the side of the sample without much effort. This makes the glue-ing of sensors such as that required by ITT testing and dynamic creep testing difficult and samples are lost in the process.

The void content of foam treated material is far above the void content of conventional asphalt.

The values of the conventional asphalt strength parameters measured for the foam treated material during this project were much lower than that of conventional asphalt.

UCS testing seems more appropriate for foam treated material than conventional asphalt tests. The UCS-test is a "robust" test that does not need complicated instrumentation and is appropriate for brittle samples.

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.

Figure 6.1

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.

Table 6.3: Preliminary classification of pavement designs on the test sections investigated
Design traffic classes (E80s)	Pavement structure
100 000 - 300 000		100 mm weathered granite or sandstone; 3% binder In situ weathered granite or sandstone
300 000 - 1 million		175 mm weathered granite; 1% roadlime; 4.5% - 5% binder                                            100 mm weathered granite; 95% Mod AASHTO; CBR = 27% In situ weathered granite; 93% Mod AASHTO; CBR = 23%     75 mm Aeolian sand; 5% binder 125 mm Aeolian sand; 4% binder In situ Aeolian sand; CBR = 17%
1 million - 3 million		175 mm 80% recycled asphalt; 20% crusher dust; 5.5% - 6% binder 100 mm weathered granite; 95% Mod AASHTO, CBR = 27% In situ weathered granite; 93% Mod AASHTO; CBR = 23%