BTS/CONCRETE | TECHNICAL


three new Herrenknecht earth pressure balance (EPB) machines in the mixed ground of the central section and the Chalk of the East. As of the end of March 2023 (time of the BTS talk)


progress on Package 2 saw 100% of the easternmost and two westernmost drives complete, with the drive from New Cross to Kings Avenue close to (93%) being complete, and 61% of the drive from New Cross to Eltham finished.


Why reduce carbon? Reducing carbon is the biggest challenge for ensuring the viability of infrastructure projects, other than funding. It is fundamentally the right thing to do. Doing so aligns to national government policies and


the UN Sustainable Development Goals (UN SDGs). Most companies have policies to reduce carbon and the project client on LPT is particularly strong with a commitment to achieve carbon-neutral construction by 2025/26. One of NG’s Working Groups is on Low Carbon


Concrete and HMJVs proposal to use EFC aligned well to this strategic objective. A breakdown of the carbon footprint of LPT-2’s


Package 2 is shown in Figure 4 with the embodied carbon of materials and transportation of materials contributing 61% and 15%, respectively, of the project’s total. Looking at materials alone, the pie-chart in Figure 5


shows 55% of the carbon footprint relates to concrete and a further 34% relates to other uses of cement. HMJV and NG worked collaboratively from the


tender stage on reducing carbon on the LPT-2 project. At tender, HMJV optimised the tunnel diameter and construction methodologies saving 30% of the baseline (50,000 tonne of CO2


equivalent (CO2 e)). Post award,


shaft diameters were reduced, alignment changes made, temporary works and rebar were optimised, and hydrotreated vegetable oil (HVO) and hydrogen were used in some areas as fuel. On the materials front, there was adoption of high


GGBS and EFC, and tunnel arisings were re-used wherever possible. These measures saved a further 17% on the baseline (18,130 tCO2


e). Elsewhere, there are several measures to reduce


environmental impact, such as a new 400kV substation at the heart of the project in Bengeworth Road, Lambeth, being built using SF6-free, gas insulated switchgear technology, in a UK first.


EFC trials In early 2021, HMJV sought approval to trial EFC with a goal for use in both temporary and permanent works. This process commenced with a visit to Capital


Concrete’s Silvertown Plant, in London, for an EFC batching demonstration. A C32/40 mix was selected to trial and the


Bengeworth Rd site was chosen to be the initial location. NG accepted the proposals in May 2021.


The testing proposed was:


Fresh Concrete - BS EN 12350:2019 ● Slump Testing and Slump Retention ● Fresh Density ● Bleed Testing


Hardened Testing - BS EN 12390:2019 ● Compressive Strength ● Density (saturated/oven dry) ● Flexural Strength ● Tensile Splitting ● Autogenous Shrinkage – Modified ASTM C157 ● Total Shrinkage – ASTM C157 ● Hot Box – Temperature Rise/Temperature Differential


There was a challenge during the trials associated with the distance to the one Capital Concrete plant able to batch EFC at the time, with loads arriving 1hr 40 mins after batching. This may have contributed to some difficulty experienced with the workability and placement of the concrete mix on one occasion. The gang also slipped into existing habits at another trial, such as adding water to the surface to aid curing – which has a detrimental impact on EFC as it dilutes the surface alkali activator. However, even with the logistical challenges every


load arrived on site to be within the S4 slump range. Slump retention was good, with the slump class maintained for 2hrs after arrival. More trialling, usage and workshops with Wagners will help engineers and site supervisors gauge EFCs behaviour, and the use of the product at more plants will of course help. Results were very positive overall. Compressive


strength and tensile strengths were in excess of the design requirements and the teams became more familiar with the product with use. Shrinkage testing was performed at BRE, in


Watford, and yielded very positive results. Both total and autogenous shrinkage values were comfortably within the limits required by the Aecom design team: autogenous (sealed) at 62 microstrain compared to a max target of 250 microstrain; and, total (unsealed) at 297 microstrain compared to a maximum target of 550 microstrain. For the Hot Box test, a 1.0m3


insulated box was


constructed by the HMJV site team as per instruction from our subcontract partner, OTB Concrete. Thermocouples were installed in the box. The results backed up existing data regarding EFC’s low reaction temperature – the maximum temperature rise was only 13.9°C and a peak temperature was 39.9°C. Combined with the minimal temperature differential,


these data prove EFC and the use of AACM could be very beneficial for reducing or eliminating the risks associated with early age thermal cracking in large pours. One key factor in the approval of EFC for LPT-2


project was the first-year durability results published from BRE, in October 2022. At the one-year point, the EFC samples in the test were outperforming traditional DC-4 concrete in the most onerous Class 5 sulphate


July 2023 | 21


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