Pavement mix is an 'overnight' success on Dublin's streets

Authors: Alan Kavanagh (technical manager) and Gearoid Lohan (general manager), Atlantic Bitumen; Denis Ryan (general manager) and Jennifer Molloy (technical manager), SIAC Bituminous Products Ltd

EME (or enrobé à module élevé in French) is a very dense, binder rich, continuously graded mix that was developed by the LCPC in France in the 1990s. The binder used to make EME is a very hard 10/20 pen grade. The combination of a dense mix, high binder content (typically of the order 5.3%) and the use of 10/20 pen binder results in a very durable asphalt mixture with a very high stiffness modulus. These qualities make EME most suitable for urban pavement renewal work where the existing pavement has to be removed and a new pavement laid in its place.

In August 2011, Dublin City Council (DCC) tendered a 23km Carriageway Renewal Works worth a value of €18 million. In order to reduce the depth of existing pavement that had to be removed, Dublin City Council specified the use of EME for the base and binder course of the new pavement. As EME is such a durable mix and has such a high stiffness modulus, it can be paved to significantly lesser layer thickness compared to the conventional high modulus asphalt concretes, while still achieving the required performance levels and pavement life. As well as saving on valuable resources, this also reduces the time needed to renew street pavements.

In order to limit traffic disruption, all of the work had to be performed at night and, in addition, had to be completed within a 13-week timescale. A resurfacing project of this scale was unprecedented for urban street renewal in Ireland. The project was split into four packages and one of them was awarded to SIAC Bituminous Products Ltd.

It is the use of the 10/20 pen bitumen that sets EME apart from other ‘high modulus’ asphalt mixtures. As 10/20 pen is a very hard grade of bitumen, a high percentage of it can be used in the asphalt mix without the risk of permanent deformation (rutting) of the pavement layer. This high percentage of hard bitumen gives EME its very high stiffness modulus – normally of the order 12,000 to 14,000 MPa when tested at 20°C.

Since its development, EME has performed well in France and in the UK. In order to become more familiar with the material and examine if it could be manufactured successfully using Irish aggregates, Atlantic Bitumen sponsored a student to perform a research study on EME for his Master’s degree in 2008/2009. The study compared the performance of EME versus HDM50. At the time of the study, HDM50 was considered the stiffest base and binder course asphalt mix contained in the NRA’s Specification for Road Works (SRW) [1].

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Table 1: Comparison of EME versus HDM50 (click to enlarge)[/caption]

The results of the research project showed that the rut-resistance and the stiffness modulus of both AC 14 EME2 and AC 20 EME2 were significantly higher than that of the 0/20 HDM50 material. The research project also found that the water sensitivity (Duriez Test) of the EME mixtures was equal to that of the HDM50 mixtures. All tests were performed using aggregate from five quarries in Ireland and the full set of results were presented in a paper published in Volume 42 of the IAT’s Asphalt Professional in January 2010 [2] and are summarised in Table 1.

PAVEMENT DESIGN USING EME

One of the reasons EME is so suited to urban street renewal work is that it can be laid to lesser thicknesses but still provide the required traffic load bearing capacity. This, in part, is due to its much higher stiffness modulus. HD 25/26 of the NRA’s Design Manual for Roads and Bridges is used by pavement designers to calculate the depth of pavement layers that are required to provide the required design life.

Design life is measured in the number of standard axle loads that the pavement can support over a period of normally 20 years before signs of pavement failure will appear. A standard axle is assumed to apply a load of 40kN through the wheels on each end of the axle (i.e. a total of 80kN per axle).

The thickness of asphalt layer needed to provide the required design life depends on the bearing capacity of the subgrade (CBR), strength of foundation material, predicted traffic loading and the stiffness modulus of the asphalt layer. So if everything else is equal, an asphalt mix with a high stiffness modulus will result in a thinner pavement layer being required, compared to a mix with a lower stiffness modulus.

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Fig 1: Determination of asphalt layer thickness for AC 20 HDM50 (click to enlarge)[/caption]

For example, if a pavement designer was considering a new pavement design in which the asphalt layer was to be made using AC 20 HDM50 and the required design life was 20msa, from Figure 4.2 of HD 25/26, the required asphalt layer thickness would be 280mm (this will include the asphalt surface course).

However, if the designer choses to use EME2 for the base and binder course layers, the required asphalt layer thickness would be 220mm (see Figure 3 below). This equates to a difference of 30mm in asphalt layer thickness.

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Fig 2: Determination of asphalt layer thickness for EME2 (click to enlarge)[/caption]

However, if the design life was 100 msa, the difference is even more pronounced at 85mm, i.e. 265mm compared to 350mm. So, in the latter example, not only will the contract require less asphalt to be laid but it would have the additional benefit of requiring less existing material to be removed. It is for this reason that pavement designers are choosing to specify EME2 instead of the conventional HDM50.

LABORATORY DESIGN MIX OF EME

In preparation for the contract, SIAC has asked Atlantic Bitumen to perform a laboratory design of an AC 14 EME2 mixture made using their aggregate. A laboratory design is required to demonstrate that an EME mixture manufactured with a certain combination of aggregates and binder satisfies the requirements of Clause 930 of the NRA’s SRW.

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Fig 3: Determination of mix recipe for AC 14 EME2 (click to enlarge)[/caption]

The laboratory tests are performed to demonstrate that:

the aggregate and binder can satisfy certain property requirements;
the aggregate fractions and the binder can be blended to produce the required grading and binder content;
the compacted mix has an air voids content of ≤6.0%;
the mixed material has a sufficient rut-resistance;
the mixed material has a sufficient stiffness modulus; and that
the mixed material has a sufficient resistance to damage caused by the action of water.

The results of the laboratory tests are detailed in the following sections of this paper and show that an ‘AC 14 EME2 bin/base 10/20 des’ mixture can successfully be produced using the identified constituent materials.

In order to determine the mix recipe, the particle size distribution of the five different aggregate fractions was first determined. To make the AC 14 EME2 mix, the following aggregate fractions were required: 10/14mm, 6,3/10mm, 2/6mm, 0/4mm and filler. A computer program was then used to determine what fraction of each was required to meet the midpoint of the target grading limits, as defined by Table 9/20 of Series 900 of the NRA’s SRW [1].

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Fig 4: Calculation of richness modulus for AC 14 EME2 (click to enlarge)[/caption]

In accordance with Clause 930.22 of the NRA’s SRW, resistance to fatigue testing is not required if the richness modulus of the mix is not less than 3.6. The Richness Modulus of asphalt mixtures is based on its aggregate grading and binder content. It is an indication of how ‘binder rich’ the material is. According to the French designers of EME, if the richness modulus is not less than 3.6, then the material will not fail due to fatigue stress. The Richness Modulus of the AC 14 EME2 mixture described in this paper was calculated as defined in Annex E of BS 594987. The calculation is shown to the right.

NRA Clause 930.14 requires the air void content of AC 14 EME2 specimens compacted in the laboratory using a gyratory compactor to have an air voids content of V_max6.0. The results of these tests are shown in Table 2.

Specimen Number	Air Voids at 100 gyrations (%)
1	2.0
2	2.3
3	2.2
Average	2.2
Spec.	≤ 6

Table 2: Air Void Contents of gyratory compacted specimens

RUT RESISTANCE

NRA Clause 930.19 requires the rut resistance of the material to be tested using both the ‘large’ and ‘small’ wheel tracker devices specified in EN 12697-22. Six 305 x 305 slabs of the AC 14 EME2 mixture were manufactured and their rut-resistance were determined at 60°C using the Small Device in the Atlantic Bitumen Asphalt Laboratory. The results of these tests are shown in Table 3.

Specimen Number	Air Voids (%)	Rut Depth (%)	Rut Rate (µm/hr)
1	2.7	7.0	0.0617
2	3.0	7.6	0.0628
3	3.4	6.7	0.0608
4	2.8	7.1	0.0615
5	3.1	6.8	0.0618
6	3.2	7.4	0.0625
Average	3.0	7.1	0.0619
Spec.	3 to 6	**WRS_{AIR Declared}**	**PRD_{AIR Declared}**

Table 3: Rut Resistance (small device) results

Two 305 x 305 slabs of the AC 14 EME2 mixture were also manufactured and their rut-resistance were determined at 60°C, in accordance with EN 12697-22 (2003) using the large device at the Colas CST laboratory in France. Colas is Atlantic Bitumen’s parent company. The results of these tests are shown in Table 4.

Specimen Number	Air Voids (%)	Proportional Rut Depth (%)
1	3.2	4.21
2	3.1	4.45
Average	3.2	4.33
Spec.	3 to 6	**PRD_{AIR 7.5}**

Table 4: Rut resistance (large device) results

Clause 930.21 of the NRA SRW (2011) requires that core specimens taken from a full-scale trial shall have a minimum Indirect Tensile Stiffness Modulus of 5500MPa. For indicative purposes, test specimens were manufactured and tested as part of the laboratory design. The results of these tests are shown in Table 5.

Specimen Number	Air Voids (%)	Stiffness Modulus (MPa)
1	2.9	13849
2	2.9	13420
3	3.2	14175
Average	3.0	13815
Spec.	≤ 6	≥ 5500

Table 5: Stiffness modulus results

The water sensitivity of the material must also be assessed in the laboratory. Clause 930.18 of the NRA SRW (2011) requires that laboratory compacted specimens of the EME2 mix shall have a Duriez Ratio of not less than 0,75. The test was performed in accordance with the French test method NF P 98-251-1. The results of these tests are shown in Table 6.


	Specimen No.	Mass (g)	H_Avg (mm)	Volume (cm³)	Mix Bulk Density (kg/m³)	Geometric Voids Content (%)	Load at 18 °C (kN)	Strength (MPa)

Dry Subset	2	1000.6	88.7	445.9	2244	9.7	91.4	18.2
	4	999.1	88.1	443.1	2255	9.2	85.4	17.0
	6	1002.2	89.0	447.5	2240	9.9	76.2	15.2

					Average Dry Strength =			16.8

Wet Subset	1	1002.1	89.1	447.9	2237	10.0	85.3	17.0
	3	1002.0	89.6	450.4	2225	10.5	75.1	14.9
	5	1010.9	89.4	449.3	2250	9.5	76.8	15.3

					Average Wet Strength =			15.7

						Duriez Ratio =		0.94

Table 6: Duriez Test results

URBAN STREET RENEWAL CONTRACT

The 2011 Dublin City Urban Street Renewal Contact was the first such contract to specify the use of EME in Ireland. Because lower layer thicknesses are required, compared to conventional HDM50, the time required to reconstruct the street pavements with EME is much shorter. This is a big advantage for urban renewal works when road closures need to be kept to a minimum and laying is mostly carried out at night.

One of the four packages in the €18 million programme was awarded to SIAC Bituminous Products Ltd under a MEAT (Most Economically Advantageous Tender) system with a strong emphasis on quality, resources, experience and ability to complete the project on time. The undertaking of a 23km resurfacing project with a 13-week timeframe was considered ambitious by all parties involved. To minimise the impact on the transport arteries of the city, night work was the only favourable time to carry out the work. All works commenced at 7:30pm and were fully completed by 6:00am.

As Dublin is more than just a commercial and retail centre, it is a home to many and a busy tourist destination, a noise restriction was also enforced, requiring all planing and pneumatic breaking to finish at 11:00pm. Orchestrating temporary lane closures, traffic light switch-offs and diversions for up to 15 streets on any given night required meticulous planning by DCC Traffic, Roadworks Control and the contractor to minimise the impact and risk to the public and workforce. Logistically, SIAC’s work involved the transportation of over 15,000 tons of asphalt to the site and also 75,000 m² of fabric textile, all between 8:00pm and 5:00am.

The renewal work involved the removal by mechanical planer of the old surface to a depth of 120mm (with some areas requiring reconstruction to a depth of 300mm). The road pavement was then rebuilt in three layers; a base and binder course of EME2 and a surface course of SMA 10. A textile membrane was also to be applied in the new pavement to reduce reflective cracking.

The € 18 million programme was completed ahead of schedule in November 2011. During production, on-site density measurements were taken using a nuclear density gauge. The average air voids content was found to be 2.7% with an average in-situ bulk density of 2.41kg/m³.

CONCLUSION

The main purpose for the development of EME asphalts is to reduce asphalt layer thicknesses and to prolong the pavement life. Full scale laboratory studies conducted by Atlantic Bitumen have shown that EME can successfully be produced using Irish aggregates and that the performance of EME significantly surpasses that of the conventional HDM50 alternative. Its improved performance is due to the use of a higher binder content and a very hard 10/20 pen bitumen. Both of these factors combine to produce a very durable asphalt mix with a very high stiffness modulus.

The contract performed by SIAC Bituminous Products for Dublin City Council pavements has further highlighted the advantages of using EME for street renewal work. EME can be laid quickly as it is very dense and has a thinner layer thickness. These characteristics enabled SIAC to complete the project ahead of time, despite be restricted to perform all of the works at night.

The authors would like to thank Ksawery Hession, senior executive engineer of the road-maintenance section and Padraig McNulty, senior resident engineer of the road-construction section of Dublin City Council for their support in conducting this work.

References

[1] National Roads Authority, Specification for Road Works, Dublin, 2005.

[2] Brennan, MJ, O. Ardill, A. Kavanagh and J. Sheahan. 'A Comparison of the Laboratory Performance of EME2 and HDM50.' Asphalt Professional, Vol. 42, January 2010.

[3] National Roads Authority, HD 25/26, Design Manual for Roads and Bridges, Volume 7: Pavement Design & Maintenance. Section 2: Pavement and Foundation Design, 2010.