Binder content

Although the binder content was not an experimental variable in this study, not all mixes were tested at the same binder content. The mixes with a higher percentage of RAP (i.e. 75 % in case of the high RAP mixes used in this study) were stabilised using a lower residual binder content (2.4% vs. 3.6% for the mixes with 25 % RAP). The binder content of the mixes with a high percentage of RAP was chosen lower because the existing bitumen present in the RAP. The binder contents adopted in this study were chosen arbitrarily in consultation with the sponsor of the research. It is believed that similar mix properties may be achieved with the lower binder content for mixes with a high percentage of RAP for reasons stated in Section 2.2.1.

As also mentioned in Section 2.2.1 the RAP aggregate in a BSM should be regarded as “black rock” and the existing bitumen in the RAP has little “binding” effect of its own.

In evaluating the results of the mixes with high and low percentages of RAP it needs to be kept in mind that the binder contents of these mixes differ. It is therefore difficult to conclude with confidence if the difference in performance is the effect of the percentage of RAP or the binder content.

The range of practical binder contents in BSM’s for application in CIPR is fairly limited. Minimum flow rates and particularly pumping pressures that need to be maintained in the recycling machine result in minimum application rates in the order of 1.5 % (lower application rates down to 1.0% have been achieved successfully in cases where the standard nozzles have been replaced with smaller nozzles in order to maintain sufficient pressure). It is particularly with thinner layers (100 – 150 mm) and low binder contents that the minimum application rate becomes critical. With bitumen emulsion the minimum application rate results in a minimum residual binder content in the order of 1.0 % (35 – 40 % of the application is emulsion water). For emulsions the application rate could be further reduced by using a diluted emulsion,

however, at such low residual binder contents the effect of the bitumen is more as compaction aid and improvement to moisture susceptibility then as stabilising agent. At the upper application range of bitumen emulsion the amount of water in the mix plays a role. Depending on the in-situ moisture content, high application rates of bitumen emulsion may lead to BSM’s after CIPR with over-optimum moisture contents. For foamed bitumen and otherwise also for bitumen emulsion, the upper limit of application rates is more based on economics. High residual binder contents are expensive and results in the use of BSM being less favourable from an economical perspective. Practically, binder contents in excess of 3 % are seldom used for BSM’s in Southern Africa.

Within the practical binder content range of 1.5% - 3.0%, the following effects on material properties and behaviour as tested in this study is to be expected:

Table 11: Effect of increasing binder content on material properties Material property or behaviour Effect

Cohesion Friction angle Shear strength

Stiffness

Time-temperature dependency Permanent deformation resistance

Strain-at-break Fatigue life increase decrease variable variable increase variable increase increase

It can furthermore be expected that the durability of BSM’s improves with increasing binder contents. This durability, which may exhibit itself inter alia in improved moisture susceptibility, improved resistance to erosion and reduced permeability, could be an important BSM property that currently receives little attention in the mix design.

2.4.5 Curing

The curing protocol followed in this study is discussed in Chapter 4 and in more detail specified in Appendix D. Save for the step of changing after the first 24 hours the plastic bag used to seal the specimen, the curing procedure is identical to the one developed by Houston and Long (2004). It was found in this study that the reduction in the moisture content of the specimens in the last 24 hours of the curing process is very limited. It would therefore make little difference if the bag is changed after 24 of the 48 hours at 40ºC sealed in a plastic bag or not. The process of changing the bag after 24 hours is cumbersome and unpractical. This step could therefore be omitted, which effectively means that the protocol as developed by Houston and Long is recommended.

There is however a number of factors that are not taken into account in this recommend curing protocol or the protocol adopted in this study. These are:

• The effect of the relative humidity during curing;

• The type of oven used (ventilated vs. non-ventilated, this would have an effect on the first bulleted factor above);

• The stiffness development during curing.

It is postulated here that it is not as much the temperature, but more the relative humidity at time of curing that influences the rate of moisture reduction in the specimens. It is therefore proposed that this parameter is monitored (or specified) in combination with the curing temperature. Also, the draft of air over the specimens has a considerable effect on the evaporation rate of moisture out of the specimens. This aspect requires further attention.

Eventually it is the strength and stiffness of the laboratory specimens at time of testing that should be comparable to the strength and stiffness of the mix in the field. This strength and stiffness is influenced by the moisture content in the laboratory specimens as well as the curing time in case active fillers are used (i.e. cement or lime). This is along the lines of the proposal by Kekwick (2005). It would imply that the curing time could be variable and that the stiffness of the specimens needs to be monitored during curing. It would also require knowledge of the expected field stiffness of the BSM laboratory mix, which in itself is a challenge.

A final consideration with regard to curing is the type of curing term that is critical. This may be a short-term cure to simulate the BSM properties immediately after construction and opening to traffic and up to a week’s time, a medium-term cure to simulate the properties up to a month after construction and a long-term cure to simulate the BSM properties at equilibrium moisture content (EMC). The time required in the field to achieve this EMC is variable and can in some cases last up to 18 months. The field strength and stiffness of the BSM continuously increases during this time.

The curing protocol adopted in this study aims to simulate a long-term cure to arrive at the EMC. Often short-term curing properties are not critical in practice. When, however, immediate opening to heavy traffic and high traffic volumes is required short-term curing properties may become critical and should be verified.

2.5 The advantages of Cold In-Place Recycling