Merchant Square, London

Solution

PUDLO waterproofing is intrinsic to the concrete, eliminating the time, labour, deliveries and additional equipment involved in laying a waterproof membrane. The basement construction would have involved cutting and fitting membrane material around numerous pile caps and drainage pop-ups, leading to genuine concerns about creating possible areas of weakness. Space was restricted, so eliminating the installation of a membrane eased the complex logistics of managing the basement programme. Plant and equipment could track around freely without risk of damage to the material. Just a piece of rebar, if dropped, can tear membrane material, resulting in a water path and potential defects in time.

Although PUDLO was used primarily for its waterproofing properties, its strength characteristics provided an added reassurance. As well as lining and filling pores, PUDLO alters the micro-structure of concrete to provide significantly increased factors of dimensional stability, waterproofing and strength. It provides over 20 times greater water protection than the next best competing waterproofing additive, and improved 7 and 28 day concrete strengths by almost 50%. Pouring conditions were difficult in some areas, with concrete being pumped over 100m through twists and turns and down several levels. But thanks to PUDLO’s fast-forming strength properties, as well as its excellent workability, texture, plasticity and flow characteristics, early access was gained onto the slab.

As well as incorporating some of the largest floorplates in the West End, building E, at Merchant Square has used ‘top-down’ construction. This involved a slightly different progression of structural detail. PUDLO’s site support team worked with the contractor and ready-mix concrete supplier to ensure that the design detail of the concrete mix and delivery schedule worked with the construction programme.

Firstly, the ground floor slab was constructed over the fully secanted piled basement. With the piles having the capacity to hold at that height, a 10m wide ring slab was constructed around the perimeter to act as a propping slab giving temporary support to the secant until construction could be tied into the ground floor.

Effectively, it creates a solid box around basement construction. Top-down construction was preferred because traditional construction would have involved a mass of sacrificial propping. By putting in the concrete slab, progress was made simultaneously above and below, and the slab was retained as a structure, resulting in less wastage. The sequence of construction involved completion of the pile caps to the three lift and stair cores first, so work on these could progress to upper levels while basement work got underway. As a result, the cores were completed at the same time as the basement.

Superior water-resistance and durability, significant time and cost savings, plus an unrivalled 20 year project warranty provided a convincing argument for using PUDLO modified concrete for the basement of building E at Merchant Square in Paddington, London. A supplier and product that will share, if not absolve the contractor of a large part of the risk of achieving critical structures while providing unrivalled client protection is a rare thing. When it comes to waterproof concrete, it’s practically unheard of but this is precisely the construction and engineering proposition made by PUDLO and its range of BBA approved additives for waterproofing, strengthening and extending the life of concrete. The case for using PUDLO for Building E at Merchant Square proved more than watertight for the foundation services specialist, Expanded Structures Ltd, part of Laing O’Rourke.