Compaction

Compaction was not considered an experimental variable in this study. All specimens tested were compacted to the target density of 98% of Modified AASHTO Density, which was a requirement of the sponsor of the research. This is a relatively low density specification by South African standards, where crushed stone base layers are normally compacted to 86% or 88% of bulk relative density of the stone and gravel base layers up to 102% of Modified AASHTO Density. For BSM materials a relative density requirement in terms of Modified AASHTO Density is often specified with a minimum of 102%.

Compaction of soils has been used for as long as mankind had the need to improve the properties of the soil. Properties such as strength and bearing capacity increase with compaction, while the compressibility and permeability reduce (Ebels et al., 2004). This would also apply to bound and stabilised granular materials. Increasing the compaction of a material has proven to be one of the most cost-effective and easiest methods of improving the properties of the compacted material.

Examples of improved material response at higher density levels are plentiful in literature. The effect of compaction on the bearing capacity of a crushed stone material is e.g. shown in Figure 29.

One of the main findings of van Niekerk’s work (2002) was that the degree of compaction ranks as the most important factor influencing mechanical behaviour of unbound and hydraulically bound granular materials. He stated that it improves the shear strength, the resilient response and “has by far the largest effect” on the permanent deformation behaviour.

Although density was not varied on this study, it can safely be assumed that the material properties and behaviour of BSM’s as presented in this study would improve at higher levels of compactions. Furthermore, the response of pavements incorporating BSM’s would improve when the density of the BSM’s increases. Vertical deflections would reduce significantly due to better load spreading ability and shear failure would occur only at higher load levels. This would improve the load carrying capacity of the pavement and therefore impact on the structural design. This is illustrated by the transfer functions for permanent deformation provided in the TG2 Guideline for foamed BSM’s, in which the relative density is the most important variable. This transfer function for a 90% reliability level (South African Road Category B) is shown in Equation 2.

[ 1.9511.938 0.0726 1.628 0.691( / )] 10 30 1 RD PS SR cem bit PD N _{= ⋅} − + + − + ₂

whereby NPD = Structural capacity [No. of load repetitions] RD = Relative density [%-Apparent Density] PS = Plastic strain in the layer [%]

SR = Deviator Stress Ratio

cem/bit = ratio of cement and bitumen contents

For the laboratory compaction of test specimens for mix design purposes the methods used for compaction of BSM’s emerged from an HMA approach to these materials. In Europe and the USA these materials were perceived to be hot-mix asphalt equivalent cold-mixes and Marshall Compaction was adopted as the preferred laboratory compaction method. In developing countries such as Australia and South Africa, the BSM’s were perceived as “modified, slightly bituminous granular materials” and a shift to Modified AASHTO compaction was adopted (as outlined in the TG2 Guideline, Asphalt Academy, 2002).

Problems encountered with the Marshall and Modified AASHTO Compaction methods when used on BSM’s in Southern Africa included inter alia (Ebels and Jenkins, 2007):

• Lack of simulation of the kneading effect of rollers and particle orientation achieved in the layers in the field. Especially Marshall Compaction resulted in specimens with coarse outside texture due to the vertical hammering action not allowing mortar to occupy the voids between large aggregate on the annulus

• Variability in target densities achieved on site for quality control purposes. This is due to variation in material type and grading along the length of a rehabilitation project.

• Delamination of specimens produced using Mod AASHTO Compaction. The individual layers of BSM split during ITS tests, for example, yielding unrepresentative results.

During recent years various institutions in South Africa have used other forms of compaction such as vibratory and gyratory compaction for BSM’s. The test specimens for this study were compacted using a gyratory compactor (tri-axial testing) and a vibratory roller (slab compaction for four-point beam fabrication) These methods have intrinsic disadvantages for commercial application in mix designs, such as cost, moisture damage to equipment (gyratory) and variability of results.

Vibratory compaction is however considered to have the best potential to simulate field compaction. Reducing the variability of results can be achieved using a vibratory hammer in a fixed mechanical set-up with a constant dead weight surcharge, without the variable interference of an operator. Therefore refusal density using vibratory compaction is considered to be the method of laboratory compaction identified as having great potential for BSM’s. Refusal density compaction has been effectively used for HMA in the UK and other countries and also for BSM’s (British Standards, 2004).

The refusal density of a BSM depends on the applied compaction energy. Variables of the compaction energy are mass, frequency and amplitude. Modern compaction equipment has on-board sensors that indicate when refusal density is achieved. Specifying refusal density in the field is however not possible without specifying the type of roller (mass) and roller setting (frequency and amplitude) and a bench-mark for subgrade support. Further research into these variables in relation to the laboratory refusal density using a vibratory hammer is required.