SubscribeSelf-levelling, Self-compacting foam concrete subway infill chosen for six subways in Milton Keynes
Location: Milton Keynes, Bedfordshire, UK
Main Contractor: Birse Civils Ltd
Client: English Partnerships
Foam concrete has been used extensively throughout the UK since 1980, with its flowing, self-levelling and self-compacting properties having proved popular with both engineers and contractors looking for a fast-track, effective and competitively priced void-fill solution. With volumetric masses of between 350 and 1600 kgs/m3 and compressive
strengths of between 0.5 and 12.0 N/mm2, foam concrete's rigidity, thermal insulation and water absorption properties in particular, promote its application in tailor-made structures.
Foam Concrete Mobile Batching Plant
This project was part of a scheme to improve public transport access and general traffic management in central Milton Keynes and called for the infilling of six pedestrian subways under the main road at Witan Gate and Avebury Boulevard, with around 1300m3 of light-weight foam concrete to be poured over an eight day period in February 2006.
Access to the subways was difficult and this, combined with strict environmental controls on site, meant that the
mobile batching plant operated by Foam Concrete Limited would provide the most effective solution.
Using a dedicated on-site batching plant allows significant operational flexibility as the machine is able to pump foam concrete over 500 linear metres without needing to be relocated and enables the crew to stop and start pours as required – an important factor in a job of this nature.
Foam concrete is a cement-bonded material produced by blending cement slurry with a pre-formed foam (similar in appearance to shaving foam). Foam concrete has a high level of fluidity in its plastic state and considerable care needs to be taken when constructing shuttering around a pour to avoid the possibility of leaks.
In any mass-fill pour of concrete, the heat of hydration of the cement needs to be taken into account when selecting the powdered material element, in order that the risk of thermal cracking is reduced as much as possible. It is the same when using foam concrete and mainly for these reasons it was decided to use a CEM1 cement with 18% blended limestone, supplied by Castle Cement Limited.
Foam Concrete Subway Infill
The height of the subways to be filled was between 2.4 and 2.6 metres so it was decided that the pour would be completed in three lifts.
Pipelines and cable runs designed for future use by utility companies were inserted beneath the roofline of the subways by the contractor. These pipes would eventually be securely encased by the flowing and self-compacting foam concrete.
Advantages of using foam concrete for subway infill:
Ease and speed of placement
When judged against other more labour/time intensive methods like polystyrene/grout, the ability to produce quantities of up to 600m3 a day means that construction times can be significantly reduced, with consequent cost savings.
Total void-fill
The flowing, self-compacting properties of foam concrete mean that you can be assured that all voids are completely eliminated.
Good energy absorbing qualities
As the foam concrete is compressed during the collapse or subsidence of material above, due to its cellular structure, the resistance of the foam concrete increases, absorbing the kinetic energy.
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Susceptibility to breakdown
Unlike some synthetic lightweight foams (polystyrene for example), hardened foam concrete is not susceptible to breakdown due to the presence of hydrocarbons, bacteria or fungi. It is insect, rodent and fireproof.
Environmentally sound
Using Foam Concrete Ltd''s on-site batching plant means less traffic disruption both on-site and in the surrounding area. Up to 109m3 of 400 kgs/m3 density lightweight foam concrete can be produced from one bulk powder tanker delivery, making it safer for site-workers and residents and kinder to the environment.
| Characteristic | Type of foam concrete (cast density) | Unit | |||||||||
| Cast density | 400 | 500 | 600 | 700 | 800 | 900 | 1000 | 1200 | 1400 | 1600 | kg/m3 |
| Cube compressive strength (28 days) attainable maximum (28 days) | 0.5 1.3 | 1.0 2.0 | 2.0 3.0 | 2.5 3.5 | 3.0 4.5 | 3.5 5.5 | 4.0 6.5 | 6.0 10.0 | 8.0 12.0 | 10.0 16.0 | N/mm2 |
| Tensile strength (28 days) attainable maximum (28 days) | 0.05 0.10 | 0.10 0.20 | 0.20 0.30 | 0.25 0.35 | 0.30 0.45 | 0.35 0.55 | 0.40 0.65 | 0.60 1.1 | 0.80 1.2 | 1.0 1.6 | N/mm2 |
| Flexural strength (28 days) | 0.10 | 0.15 | 0.35 | 0.44 | 0.50 | 0.60 | 0.70 | 1.10 | 1.45 | 1.85 | N/mm2 |
| Modulus of elasticity (compression, 28 days) | 300 | 650 | 1,200 | 1,650 | 2,200 | 2,900 | 3,700 | 5,800 | 8,400 | 11,500 | N/mm2 |
| Shrinkage (laboratory) Shrinkage (in actual practice) | 6.5 1.5 | 5.5 1.3 | 4.5 1.2 | 4.5 1.2 | 4.0 1.2 | 4.0 1.1 | 3.5 1.1 | 2.5 1.0 | 2.0 1.0 | 1.5 1.0 | % % |
| Water absorption* | 75 | 50 | 33 | 22 | 15 | 10 | 7 | 5 | 5 | 5 | kg/m2 |
| Water vapour diffusion resistance factor - between 50% - 100% RH - between 70% - 100% RH | 2.5 5.0 | 3.5 6.0 | 4.0 7.0 | 4.5 8.0 | 5.5 9.0 | 6.0 10.0 | 6.5 12.0 | 9.0 16.0 | 13.0 24.0 | 18.0 34.0 | - - |
| Heat conduction coefficient - absolutely dry material - at 70% RH - at 95% RH | 0.09 0.11 0.14 | 0.10 0.13 0.17 | 0.12 0.15 0.20 | 0.14 0.18 0.23 | 0.17 0.22 0.27 | 0.20 0.26 0.31 | 0.23 0.30 0.35 | 0.30 0.40 0.50 | 0.40 0.55 0.65 | 0.50 0.70 0.80 | W/mk W/mk W/mk |
*The guide value indicates the total quantity of water in kg that permeates a 1m2 foam concrete surface during 10 years, if this surface is constantly exposed to (ground) water with the same pressure as a 1m water column