The Effect of mixing time on the compactability and stiffness properties of CBEMs

As earlier mentioned in this chapter the need to change to the Sun and Planet Mixer was inevitable due to the inability of the Hobart Mixer to cope with the gradation for the design mix. The Sun and Planet Mixer was thus investigated for the appropriate mixing times for both the pre-wetting water and bitumen emulsion addition to the mix. Although temperature could be controlled on this mixer, it has only one speed. The experience of Ibrahim (1998) on this mixer with similar mixtures gave a head start for the investigation. He generally used a mixing time of 120s for pre-wetting water while he used times ranging from 90s to 120s for mixing bitumen emulsion (depending on the nature of the emulsion).

Therefore, pre-wetting water mixing times paired with the following corresponding mixing times for bitumen emulsion were used respectively: 90s, 90s; 90s, 120s and; 120s, 120s.

The pre-wetting water was added to the mix from the onset of mixing. After

157 Table 5.15a: Maximum Density for the RAP CBEMs

Maximum Density kg/m³

Test 1 Test 2 Test 3 Test 4 Average

2529.76 2509.28 2508.13 2515.23 2516

Table 5.15b: Maximum Density for the Virgin Aggregate CBEMs

Maximum Density kg/m³

Test 1 Test 2 Average

2627.99 2601.25 2615

158 mixing halfway into each specified time (i.e. pre-wetting water and bitumen emulsion mixing times) the mixer was stopped to allow for manual stirring using spatulas before the mixing was completed. This further ensured uniform distribution of the fluids.

Except for mixing time, and the target number of gyrations of 200, the cylindrical cores were produced and cured similarly as in Section 5.11 of this chapter. Figure 5.19 and Table 5.16 both detail the results of the investigation. The results indicate that the mixing time pair of 120s, 120s for pre-wetting water and bitumen emulsion respectively gave the best stiffness response and the lowest air void content whether in dry (cured) condition or wet condition. The pair of 90s, 90s for mixing the pre-wetting water and bitumen emulsion with aggregates gave the lowest stiffness value and highest air voids content. However the moisture loss trend throughout the range upon curing was observed to be similar.

The difference in stiffness values for the 90s, 90s pair and the 90s, 120s pair was quite large i.e. 568MPa (which also represents a 36.1% improvement over the stiffness of the former) compared to that between 90s, 120s pair and the 120s, 120s pair, i.e. 142MPa (which similarly represents a 6.6% improvement over the stiffness of the former). While the results indicate that a paired mixing time of 90s, 90s for both pre-wetting water and bitumen emulsion respectively was inadequate, it was obvious that increasing the paired mixing time beyond 120s, 120s might not be beneficial as the difference between the stiffness values at this point and those of the 90s, 120s pair were not that significant. It is thus assumed that the 120s, 120s pair is the optimum (or very close to the optimum value).

Going beyond this level could have induced emulsion breaking and subsequent inhibition of proper packing during compaction.

This investigation shows that CBEMs are very sensitive materials as a difference of 30s during mixing could make a marked difference to their stiffness and compactability properties especially in this case where the breaking of the bitumen emulsion is a physico-chemical process. Although this is fully investigated using statistical tools later in Chapter 6 of this thesis, there is an indication here that stiffness is influenced by air voids content. As air voids increase, stiffness modulus decreases and vice versa. The investigation summarily informs of the need to apply just enough mixing time for both the pre-wetting water and bitumen emulsion in order to enhance the performance of CBEMs. At such an optimum level, it is believed that the materials have been properly coated (optimum degree of aggregate wetness achieved) and also the emulsion in the mix has not started breaking which are desirable for performance.

159 Figure 5.19: The Effect of Mixing Time on the Stiffness Modulus of CBEMs

Table 5.16: The Effect of Mixing Time on the Stiffness Modulus of CBEMs MTPE

MTPW= Mixing Time for Pre-wetting Water A = Aggregates MTBE = Mixing Time for Bitumen Emulsion

0.0 MTBE = Mixing Time for Bitumen Emulsion

ITSM (MPa) After Curing Air Voids (%) After Compaction Air Voids (%) After Curing Difference in Voids(%)

160 5.13.3 The effect of filler type, and mixing and compaction temperature

on compactability and stiffness properties of CBEMs

Up till this point in this study, only limestone filler was used due to the fact that it was the only filler type that was available when the laboratory work started. Also, mixing and compaction activities have both been carried out at 20 C (ambient temperature) since temperature could not be controlled on the Hobart Mixer which was used up till this point. However, replacement of the limestone filler with granite as soon as it was made available was considered necessary as granite fillers are always used for asphalt mixtures in Nigeria since they are abundantly available. Similarly, introducing a slightly elevated temperature of 32 C for the mixing and compaction processes was thought necessary since the average maximum air temperature for Ibadan a typical tropical City in Nigeria has been selected for this research work, which is 32 C (BBC Weather, 2008). As earlier noted, the Sun and Planet mixer had an advantage of temperature control while in operation. It was envisaged that the introduction of these new measures should simulate road pavement construction practice in Nigeria. Except for a fixed target number of gyrations of 200 applied and adoption of a mixing time of 120s for pre-wetting water and 120s for bitumen emulsion addition, all other parameters used in the previous section remained the same here.

The graph in Figure 5.20 and details in Table 5.17 clearly indicate that the CBEMs containing granite filler generally had higher dry stiffness properties than those with limestone filler although the latter performed much better both in wet stiffness and compactability properties. These observations were irrespective of the compaction and mixing temperatures used. The CBEMs with limestone filler demonstrated a good ITSMR with the observed values clearly above average compared to those containing granite filler which had values generally below average. This observation for ITSMR is in conformity with other researcher‟s findings on the good anti-stripping properties of limestone filler in hot asphalt mixtures (Airey et al, 2005). However, it is worth noting that irrespective of the filler type used and the temperature of mixing and compaction, moisture loss was similar just as observed in the preceding section. This implies that the degree of curing is not affected by filler type in the CBEMs studied.

Similarly, the effect of mixing and compaction temperature on the compactability and stiffness responses of CBEMs irrespective of the filler type used were studied just as indicated in Figure 5.20 and the accompanying Table 5.17. Mixing and compaction of CBEMs at 32 C gave better responses in stiffness and compactability compared to 20 C. For example, the granite filler CBEMs prepared

161 Table 5.17: Effect of Filler Type, Compaction and Mixing

Temperature on Stiffness Properties of CBEMs Filler

Figure 5.20: Effect of Compaction/Mixing Temperature and Filler Type (Note: same compactive effort of 200gyrations @ 600kpa pressure was applied)

Compaction and Mixing Protocol and Filler Type ITSM dry ITSM wet ITSMR (%) Difference in Voids (%)

162 at 32 C gave improvements of 26.5%, 38.8% and 9.7% for dry stiffness modulus, wet stiffness modulus and ITSMR values respectively over those prepared at 20 C.

The observed values for limestone filler CBEMs were similar as improvements of 15.6%, 39.5% and 11.6% for dry stiffness modulus, wet stiffness modulus and ITSMR values respectively were noted for those prepared at 32 C over those prepared at 20 C. Irrespective of the temperature of mixing and compaction, moisture loss was similar just as observed in the preceding section. This also implies that the curing protocol used here was not affected by the temperature at which the CBEMs studied were prepared.

These performances were also observed for compactability of the CBEMs although a reduction here implies enhancement. For limestone filler CBEMs there were reductions of 40.9% and 31.6% reduction in wet and dry air voids contents respectively for those CBEMs prepared at 32 C compared to those at 20 C. In the case of granite filler CBEMs, the reductions observed were 38.8% and 31.0%

reduction in wet and dry air voids contents respectively for those CBEMs prepared at 32 C compared to those at 20 C. These reductions could have been a result of the residual bitumen in the bitumen emulsion becoming less viscous at 32 C compared to 20 C as noted in section 5.5.1 of this chapter. Apart from enhancing the lubricating effect of the bitumen emulsion which subsequently facilitates good packing of the substrates (aggregates), it is also believed that at such temperature, the residual bitumen in the RAP is getting excited (active) and possibly rejuvenated by the fairly soft residual bitumen in the emulsion. Although the extent of this possible rejuvenation cannot be ascertained, Karlsson (2002) observed in his work that, diffusion which is enhanced by temperature is one of the key factors that facilitate the rejuvenation of aged bituminous binders.

Possible changes in the volumetrics which subsequently enhanced compactability of the mixtures could also have been responsible for these observations.

Subsequent to these observations, the decision to use granite filler for producing CBEMs was adopted for the detailed investigation reported in Chapter 6 in order to properly represent what obtains in the road construction industry in Nigeria although it was obvious from this study that limestone fillers performed better.

Also these investigations showed that slightly elevated temperature of 32 C for mixing and the compaction of CBEMs resulted in better mixtures compared to those at 20 C. This is expected to be a plus for cold recycling practice if adopted in a hot tropical country such as Nigeria. It similarly gives an indication that it is beneficial if such mixtures are prepared and laid at such temperatures in the temperate climates where cold recycling is already being practiced.

163 5.13.4 The effect of applied compactive effort on stiffness properties of

CBEMs

CBEMs have been observed by researchers (Ebels, 2008; Ibrahim, 2008; Jenkins, 2000; Sunarjono, 2008; Thanaya, 2003; and Twagira, 2010) as materials that are difficult to compact compared to hot mixtures. These researchers are however of the opinion that when either the pressure or the gyration angle are increased individually or combined, the specimen reaches the desired density quicker.

However, there is the possibility that some other important properties of the mixture could have been compromised in the process. An effort was thus made to study the effect of changing these two parameters on the stiffness (wet and dry) of the CBEMs. Along with CBEMs containing RAPs with residual binders of 20dmm penetration, the virgin aggregate mixture was studied here. While every other parameter holds for the virgin aggregate mixture, the aggregate proportion changed to 95% virgin aggregate and 5% filler.

It is worth restating here that the following parameters have been used on the Cooper Research Gyratory Compactor up to this point:

 Pressure – 600kPa

 Gyration angle – 1.25

 Rate of Gyration - 30 rev/min

 Mould Size – 100mm diameter

 Target Number of Gyration – 50 (for design stage), 200

A pressure of 800kPa (maximum attainable) and gyration angles of 1.5 and 2.0 were thus explored along with those previously used. The effect of similar compactive effort on the stiffness and moisture susceptibility of CBEMs are shown in Figure 5.21 and Table 5.18. From Figure 5.21, it is obvious that the virgin aggregate CBEMs were only able to attain better ITSMdry values than the RAP CBEMs at a greater angle of gyration (1.5 ) and later at a higher compactive pressure (800kPa). Irrespective of the property of the compactive effort (angle of gyration and pressure), the RAP CBEMs had better ITSMwet which is most desired for pavement materials being a major indicator of durability. This logically enhanced the ITSMR of the RAP CBEMs, almost doubling that of the virgin aggregate CBEMs at some points. Similarly, the result shows the superior positive effect of compaction at 600kPa and 1.25 on the ITSMR over those of higher compactive effort (higher angle of gyration and pressure). More importantly, having been treated similarly, the RAP CBEMs had lower void content than the virgin aggregate CBEMs. Low void content (porosity) has been widely regarded as

164 Figure 5.21: The Effect of Similar Compactive Effort on the Stiffness and Moisture Susceptibility of CBEMs

Table 5.18: Properties of CBEMs with Similar Compactive Effort Material

ITSM dry (MPa) ITSM wet (MPa) ITSMR (%) Void Content (%)