Location, materials, structures and construction of the trial road

Six different AC pavement structures, each about 150 m long, were constructed in 2010 as a part of an access road to a sand-washing plant in the south of Qatar. The location, shown in Figure 25, is 60 km away from the capital, Doha and was selected for construction of the trial road due to its high traffic loading.

71 The asphalt pavement structures of the full-scale trial road were designed as perpetual pavements to examine the effect of bitumen grade/type, aggregate gradation/type and mix design against different surface distresses and deteriorations under the same traffic condition. The aim was to examine the performance of perpetual pavement structures in Qatar and identify their main advantages and problems. The transverse profile of the trial road is not available, unfortunately, but the layers and materials used in each trial section are shown in Figure 26. The aggregate gradation for the surface and base courses of the full-scale trial sections is shown in Table 13 and Table 14, respectively.

Figure 26. Layers and materials’ properties for all trial sections (TRL Client Project Report 282, Phase D, 2010).

Table 13. Aggregate gradation for surface course of the trial sections. BS sieve size

(mm)

Cumulative passing (%)

Section 1 Section 2 & 3A Section 3B & 4 Section 5 & 6

20.0 100.0 100.0 100.0 100.0 14.0 84.0 82.1 90.7 82.0 10.0 72.4 68.9 70.4 68.6 5.00 48.9 51.8 54.9 51.7 2.36 36.1 34.4 33.1 32.9 0.30 13.0 11.4 12.1 11.0 0.150 9.1 8.4 8.2 7.0 0.075 5.7 5.0 4.2 5.1

72

Table 14. Aggregate gradation for base course of the trial sections. BS sieve size

(mm)

Cumulative passing (%)

Section 1 Section 2 Section 3 Section 4 Section 5 Section 6

37.5 100.0 100.0 100.0 100.0 100.0 100.0 28.0 99.0 98.0 96.0 98.0 97.9 97.0 20.0 90.0 88.9 93.2 86.1 84.2 90.1 10.0 47.1 51.2 49.3 60.2 60.1 49.8 5.00 34.5 34.8 40.3 42.8 41.8 31.6 2.36 27.9 30.0 28.3 29.0 26.9 26.8 0.30 12.1 13.1 13.0 10.0 11.1 13.9 0.150 8.4 9.2 9.8 7.6 7.9 8.1 0.075 6.3 5.6 6.7 4.6 4.1 5.2

As shown in Figure 26, the Percentage Refusal Density (PRD) design method (BS EN 12697-32:2003) was used in most of the mixtures of the trial pavement sections. The PRD design method is a Marshall method but with more blows to simulate the field compaction and loading until the density becomes refusal. The PRD value is defined as the ratio of the initial dried bulk density of the sample to the final density (refusal density) expressed as a percentage. In the PRD design method, a test is performed to obtain the design’s optimum bitumen content at which the voids in the mix (VIM) at refusal density are only 3%. This requirement is supposed to reduce the chance of plastic deformation, which is associated with VIM in the field lower than 3%, occurring.

Unmodified 40-50 Pen bitumen was used in trial section 1 (surface and base courses), while unmodified 60-70 Pen bitumen was used in all other trial sections except sections 5 and 6. Trial section 4 is the control pavement structure that was designed and constructed following the standards of Qatar Construction Specifications (QCS-2010), which essentially follow the Marshall method. Trial section 5 involved the use of a sulphur-extended bitumen, Shell Thiopave, while trial section 6 comprised polymer-modified bitumen (PMB) with a Styrene- Butadiene-Styrene (SBS) modifier that was graded as PG76-22. Unmodified 60-70 Pen base bitumen was used to produce the PMB and its fresh characteristics can be found in Appendix A. Unfortunately, no attempts were made in recovery and testing the bitumen (DSR, Pen,

73 TR&B) of the mixtures as laid. Such an effort, however, is highly recommended for future projects since it will allow a much better analysis of pavement performance.

The aggregate used in the surface course for all trial sections was Gabbro, which was imported from the United Arab Emirates. This aggregate is an igneous rock that has been used in road construction for a long time in the Arabian Gulf region. A local aggregate in Qatar, Limestone, was used only in the base course of trial section 3 to compare it with the performance of trial sections in which Gabbro was used. The aggregate gradations shown in Table 13 and Table 14 are all within the QCS limits; no big differences between them were observed.

The same granular sub-base with Limestone aggregate was used for all trial sections with an estimated design modulus of 450 MPa, and the subgrade was weathered Limestone with a design modulus of 200 MPa (TRL Client Project Report 282, Phase C (2010) and TRL Client Project Report 282, Phase D (2010)).

For the construction of the trial road, the weathered Limestone subgrade was excavated to a depth of 540 mm to allow the top of the trial road to be at the same level as that of the old carriageway. The 200 mm sub-base material was laid over the entire area and then sprayed with a bituminous prime coat of cut-back 60-70 Pen bitumen, ready for the first AC layer to be constructed (TRL Client Project Report 282, Phase D, 2010). The compaction was undertaken by two vibrating rollers of 10-tonnes deadweight, two 18-tonne deadweight pneumatic-tyred and two 20-tonne deadweight pneumatic-tyred rollers together with a 3-tonne vibrating roller for the transverse joints. A K1-40 emulsion bitumen tack coat was applied to each AC layer using a tanker before it was overlaid with the next layer. The asphalt concrete (AC) base layers were constructed in two layers, about 135 mm each, which is not the typical compaction thickness in Qatar (usually 70-90 mm each). The 135 mm thickness was used for the purpose of obtaining field cores later from these base courses. In order to resist rutting, the current

74 practice in Qatar is to over-compact the AC layers at a relatively low temperature for up to 4 or 5 hours to achieve the required low air void content (≈ 3.0%) in spite of the low bitumen content. This might cause grinding of the hard aggregate (Gabbro) used in the constructed roads and might even cause micro-cracking.

A day after paving the trial sections, three pairs of field cores were extracted from each trial section and tested for compositional analysis, and the results are summarised in Table 15.

Table 15. Compositional analysis summary for all trial sections (TRL Client Project Report 282, Phase D, 2010).

Layer Section # (mix)

Bitumen content (%) VIM (%) VMA (%) VFB (%) Stability (kN) Flow (mm) Stiffness (kN/mm2₎ By weight By volume Surface course 1 (PRD) 3.9 10.2 4.0 14.2 71.7 14.8 2.6 6.2 2 & 3A (PRD) 3.8 9.4 4.9 14.3 69.1 14.7 2.6 5.7 3B & 4 (QCS) 3.8 9.5 5.3 14.8 64.4 13.4 2.7 5.2 5 & 6 (PRD) 3.8 9.4 4.7 14.1 68.5 14.4 2.5 5.8 Base course 1 (PRD) 3.6 9.2 4.1 13.3 69.0 15.2 3.1 4.9 2 (PRD) 3.4 8.7 4.5 13.2 65.8 14.1 2.6 5.5 3 (PRD) 4.4 10.0 4.2 14.2 71.2 11.5 2.9 4.0 4 (QCS) 3.5 8.9 4.8 13.7 64.9 14.1 2.6 5.4 5 (QCS) 3.9 9.5 4.2 13.7 69.4 18.1 2.6 7.1 6 (PRD) 3.5 9.5 4.2 13.2 67.9 15.2 2.6 5.9

According to the compositional analysis summary table, the average bitumen content by weight for the surface course and the base course is 3.8% and 3.7%, respectively. These bitumen content values are low enough to make the mixtures too stiff, as shown in the stiffness results in the table, which raises a concern about the durability and fatigue resistance of these mixtures.