Travelling to a low–carbon future

Materials World magazine
,
13 Oct 2014

Charlton Hawley from Network Rail and Paul Crowther of Cemex UK, talk about their recent combined efforts to develop a low-carbon concrete sleeper. 

Nearly half of the UK’s carbon emissions come from infrastructure and related areas, such as energy. In November 2013, MPs and major industry leaders gathered together to sign a joint initiative to reduce carbon in infrastructure by as much as 24m tonnes by 2050. As they did so, Network Rail and Cemex UK were already working in partnership to develop a lowcarbon concrete sleeper, inspired by the low-carbon work that had been done at the Olympic Village for the 2012 Games in London.

Two materials are widely used in parts of the construction industry for cement replacement. These are pulverised fuel ash (PFA), a waste product from coal fired power stations, and ground granulated blast furnace slag (GGBF), a waste product from the steel industry. These materials are, respectively, pozzolanic and latently hydraulic, and can be substituted for cement in a mix to large degrees (up to 70% in the case of GGBF) while still producing a compliant and strong concrete. Unfortunately, these materials increase the curing time of the concrete in gaining its early strength. This is a key issue for sleeper manufacture, as the sleepers are demoulded and the pre-stressing released 14 hours after pouring. This disadvantage means higher levels of replacement are not possible, as they would adversely affect the existing production method. However, Cemex discovered that substitutions with PFA of up to 28% were possible without adverse effects on the production process.

After a review of the proposed mix designs, in combination with the cradle-togate analysis previously carried out, a mix with around 25% PFA was deemed most suitable for the production trial. Interestingly, although PFA is considered a waste material, it still has a carbon footprint associated with its use, which meant it could not be considered a carbon footprint-free substitution for the cement. However, the carbon reduction was still more than 11.5%.

In November 2012, two lines of the new mix, around 800 sleepers, were cast. Representative samples from this production trial were tested in accordance with the existing specification and were found to exceed the requirements. The new mix is also designed to remove the risk of failure due to alkali silica reaction and delayed ettringite formation, which can be an issue in some older sleepers. Since October 2013, Cemex has been producing to this revised mix.

One of the biggest challenges of this project was ensuring the proposed changes did not impact on production volumes and product long-term durability. An extensive testing regime had to be developed to ensure continued product supply and conformity.

Initially, there was concern over the variety of carbon footprint calculation methods available and the resulting possible spread of values. The Building Research Establishment (BRE) used the Environmental Profiles Methodology calculation, which gave them a well-used and recognised method of calculation and one that was consistent with calculations for the 2012 Olympics. In addition, BRE was commissioned to act as an expert reviewer of the mix designs proposed by Cemex and to carry out chemical analysis of the new mixes and, eventually, the production concrete.

This was just the first step in lowering the carbon footprint of a concrete sleeper, with further adjustments to the mix possible. Presently, the use of recycled materials has been ruled out of the process, as the carbon footprint of transporting the materials outweighs the existing footprint of the sand and aggregate used (which accounts for a very low proportion of the carbon footprint of the final product). By increasing production times and the size of the plant, it will be possible to increase curing times, and thereby increase PFA content and potentially use GGBFs. Cemex sis also working on a zero-cement geoplymer sleeper, capable of withstanding the process of transferring the forces in the prestressing tendons into the concrete. These sleepers are now undergoing static testing, dynamic testing, chemical analysis and mechanical properties. If successful, they could significantly further reduce the carbon footprint of concrete sleepers.  

 

Did you know?

In 2012, the concrete industry consumed 62 times more recovered and waste material than the waste it sent to landfill 

88%
of UK concrete is already responsibly sourced, according to the BRE
Framework Standard for the Responsible Sourcing of Construction
Products 

The industry diverts more than 5 million tonnes of
material from external waste streams, and uses them in place of primary
materials 

28% of virgin aggregates are being replaced by re-used aggregates in the waste stream  

In
2011, Cemex UK produced 534,325 sleepers. Based on an 11.5% CO2
reduction, this equates to a reduction of 17,881 tonnes of CO2 per annum
– equivalent to more than 18,000 economy return flights from London
Heathrow to New York JFK or nearly 1.8 million rail passenger journeys
between London and Milton Keynes. When Network Rail’s second sleeper
supplier reaches full production, this should be applicable to more than
800,000 sleepers a year equivalent to the per capita use of 3,150 UK
residents (based on 2008 figures of 8.5 tonnes CO2 per person, per
annum).