Authors: Alan Kavanagh, technical manager, Atlantic Bitumen Ltd; Gearóid Lohan, general manager, Atlantic Bitumen Ltd; and Ali Salih, technical manager, Casey Enterprises Ltd
Ground granulated blastfurnace slag (GGBS) is a by-product slag material from the iron and steel industry. For many years now, GGBS has been used as a partial replacement of conventional Portland cement in concrete to enhance the performance of concrete. This replacement brings advantages including increased long-term strength, protection from aggressive chemical attack and reduced heat of hydration, which leads to a reduced risk of thermal cracking in large pours and a reduction in thermal stresses during curing.
It also provides better workability, making placing and compacting easier. The resulting concrete is denser, less permeable and more durable, meaning longer service life and value for money. GGBS has a carbon footprint of 42kg CO2/tonne compared to 760kg CO2/tonne for Portland cement and has a huge influence in reducing the carbon content per tonne of the concrete.
In more recent years, the use of GGBS for the production of bituminous mixtures or ‘asphalt’ has been shown to offer similar advantages. In this article, the benefit of replacing 50 per cent of conventional filler with GGBS is investigated and described.
GGBS has been used since the 1800s to make durable concrete structures in combination with ordinary Portland cement and/or other pozzolanic materials. For this pilot study, the effect of using GGBS in asphalt was assessed at Atlantic Bitumen’s Asphalt Laboratory in Oranmore, Co. Galway. The aggregate used to make the asphalt was supplied by Casey Enterprises Ltd, while the GGBS was supplied by Ecocem Ireland in Dublin. The results of the laboratory tests are presented below.
MIX RECIPES AND TARGET GRADINGS
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Fig 1: Aggregate grading for the reference SMA 10 mix (click to enlarge)[/caption]
For this study, it was decided to use an SMA 10 surf PMB 65/105-60 mix that complied with the requirements of Clause 942 of the National Roads Authority’s Specification for Road Works. The constituent aggregate fractions used were the 6.3/10mm, 0/4mm, Casey’s filler and Ecocem GGBS.
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Fig 2: Aggregate grading for the SMA 10 mix containing GGBS (click to enlarge)[/caption]
The recipe and resulting mix grading for the reference mix and for the mix containing the GGBS are shown in Figures 1 and 2 below, respectively. Ecocem GGBS is produced to a specific particle size distribution, which allows material savings in the mix. For the GGBS mix, it was possible to increase the 0/4mm dust content from 36 per cent to 38 per cent and to reduce the filler content from 10 per cent to 8 per cent. This 8 per cent was split as follows: 4 per cent Casey’s reclaimed filler and 4 per cent GGBS.
- Rut-resistance test results
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Fig 3: Slab compactor[/caption]
In order to compare the rut-resistance of the SMA 10 surf mix with and without the use of GGBS, two slabs of each mix were made in the laboratory for wheel tracker testing in accordance with BS 598-110. The equipment used are shown in Figures 3 and 4, respectively.
BS 598-110 specifies that the wheel tracker test should be carried out at 45°C for ‘moderate to heavily stressed sites requiring high rut resistance’ or at 60°C for ‘very heavily stressed sites requiring very high rut resistance’. For this study, the test was performed at 60°C. The results of the tests are presented in Table 1.
As can be seen in Table 1, the rut resistance of the mix, in terms of both rut depth (9 per cent improvement) and rut rate (19 per cent improvement) was improved thanks to use of 4.0 per cent GGBS in the mix.
The water sensitivity of an asphalt mix will determine how resistant it is to damage caused by water (i.e. its durability). A mix that is water sensitive will allow water to ingress between the aggregate and the binder and cause binder ‘stripping’. This leads to further water ingress and gradual weakening and failure of the asphalt mix.
The water sensitivity of a mix depends on the binder content of the mix, its aggregate grading and, most importantly, the degree of affinity between the aggregate and the binder. If there is a poor binder-aggregate affinity, an adhesion agent may need to be added to the bitumen to improve the degree of affinity.
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Fig 4: Wheel tracker device[/caption]
In order to assess the effect of the GGBS on the water sensitivity of SMA 10 surf PMB 65/105-60 des, the water sensitivity test was performed in accordance with Method B of EN 12697-12 (i.e. the Duriez Test method). A mix with a compressive strength ratio of below 80 per cent is considered to be not water sensitive. As can be seen in Table 2, the compressive strength ratio of the mix was improved thanks to use of 4.0 per cent GGBS in the mix.
CONCLUSIONS
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Table 1: Rut-resistance test results (click to enlarge)[/caption]
In this pilot study into the effect of using GGBS as a partial replacement for the filler in an SMA 10 asphalt mix, it has been shown that the GGBS had the effect of improving both the rut-resistance and water sensitivity of the mix tested. Material savings using GGBS were also realised by reducing the filler content by 2 per cent and increasing the 0/4mm dust content.
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Table 2: Results of water sensitivity tests (click to enlarge)[/caption]
As the GGBS mix is outperforming the reference mix, further material savings could be achieved if equal performance to the reference is the objective. As the results are positive, a fuller, more comprehensive study into the effect of GGBS on asphalt mixes is recommended.