Feature

Perfect Timing

When in-situ concrete is to be left exposed and a high quality of finish is desired, some degree of surface enhancement should be anticipated. If planned from the outset as part of the programme of construction works, the efficiency of the process can be greatly improved, leading to more successful results. The particulars will be project-specific, but establishing a strategy pre-tender will reap rewards in the long run.

This article explores the key considerations and the recommended timing of remediation work. When planning for remediation works, specifiers should initially explore the following aspects.

Quality of finish, potential scope of works and identification of likely issues

The specification should include supplementary information on common issues and best practice to help reduce their occurrence from the outset. Guided by reference projects and subsequently narrowed down to site-specific mock-ups, a framework of quality can be established. Potential project-specific issues will become apparent through mock-up production – this is an extremely useful tool for identifying challenges and adjusting work strategies to reduce the final scope of remedial works. The mock-up will in turn clarify the most appropriate cleaning mechanism, trial proposed sealers and form a benchmark agreement for the quality of any remedial works.

Categorisation of remedial works

Identifying categories of remedial works simplifies identification, and assists in creating a schedule of works when concrete production (or individual phases) have ceased. These can be marked on a plan and cross-referenced to any benchmarks.

Types of access required

Access considerations are likely to be the main factor determining timings and sequencing of post-finishing work, so an access plan is critical to the efficiency of remediation works. If works rely on a mobile elevated work platform (MEWP), such as a scissor lift or cherry picker, then efficient use of these machines will be a priority. Nearly all soffit work and walls in double or triple-height atriums and columns will benefit from using a MEWP.

St Paul’s School, London

A smart new general teaching building designed by Walters & Cohen Architects is being constructed at St Paul’s School in Barnes. It is being delivered in two phases to minimise disruption, with phase 1 completed in October 2017.

Exposed concrete is a significant feature of the interiors, with in-situ soffits, frames and stairs all on show. Concrete contractor Toureen Group, well versed in the provision of high-quality concrete surfaces, was proactive in organising the post-striking improvement works. A two-part approach was established soon after the concrete was struck based on a hierarchy of quality and the extent required.

General cleaning and rubbing down was carried out by Toureen, together with isolated patch repairs in less conspicuous areas, made without colour matching. A separate “VIP area” was identified in the stair core, recognising the specialist skill needed to improve the untidy junction and marks created by the grout check. A schedule of remediation works for the stairs was collated in collaboration with GreyMatter Concrete and subsequently subdivided into remedial works categories. A benchmark sample of each category was then carried out for approval. Undertaking this exercise early on proved worthwhile, for setting quality standards, establishing an overall strategy and programming future works.

On larger-scale projects, MEWPs are the single most valuable tool for safe, efficient working. If there is to be a raised access floor with services underneath, for example, then finishing soffits above this area becomes a priority before services are installed. Once raised access floors are in place, only lightweight mobile scaffold towers or podiums can be used.

Access to stair cores should be carefully timed so as not to inhibit movement of follow-on trades. If possible, works should be completed early on and subsequently fully protected. An ideal scenario would use temporary scaffolding “Haki” type stair towers which fully restrict general access to the stair cores, but allow free-flow of other trades.

Protection

There are two types of protection to consider: during concreting and after concreting. During the works, the concrete contractor is best placed to arrange protection and this is likely to have a dual purpose, to both prevent damage and aid curing. Reusing ply form-face material can be very useful in this instance. Protection will be removed for any remediation works, so follow-on protection against damage by other trades is highly recommended. This may be best undertaken by the main contractor to mitigate against additional damage.

It is also worth mentioning that damage can be effectively reduced by clearly communicating to follow-on trades that the concrete is a finished item. This is best achieved during initial site inductions.

Dust, water and messy operations

There are significant advantages to carrying out dusty, wet and messy work as soon as possible in order to avoid damaging other finished work and to reduce the need, and therefore cost, of protection. This will specifically relate to cleaning, preparation such as chiselling out, and filling operations.

Weather and temperatures

Remediation works are best carried out in warm, dry conditions as this is likely to speed up the process. In warm conditions, proprietary repair mortar can set in 15 minutes whereas it may take up to an hour in cold weather, which can affect its quality. External working in winter months is likely to be impractical and is best left to between spring and autumn in dry weather or under protective cover. Internally it is best to undertake works when the building is weathertight, and preferably glazed to reduce wind chill.

SEQUENCING

Cleaning

Quality judgements cannot be made until the struck concrete has had the opportunity to cure and has been cleaned. Cleaning can greatly elevate the aesthetic quality of the work, and impulsive decision-making should be avoided before it has taken place. Timings for cleaning should allow for a full 28-day curing period undertaken as per the agreed method from mock-up trials. Starting with soffits or areas subject to access restrictions will ease future congestion as the site gets busier.

Inspection and schedule of works

Inspection will follow cleaning, with quality assessed in relation to the approved mock-up, reference project and tender document. The schedule of works should be limited to an agreed scope unless additional damage occurs. As the concrete’s appearance improves, other nuances tend to become more evident and there is often the temptation to add new items to the scope.

This can risk a spiralling of costs and time. Subjective items or those that may improve over time, such as discolouration or tonal variation, should be held in an “abeyance list” for later evaluation. The sequence will be determined by access priorities and the critical path. For efficiency, all works should be grouped and organised by location.

Selecting and sequencing critical and non-critical works

Works might be deemed critical or non-critical where they require differing levels of remediation quality. This is likely to be determined by viewing distances or position – for example, a front-of-house lift lobby versus a fire-escape stair core. Accepting that one size does not fit all will allow for a more flexible and cost-effective strategy. Non-critical repairs, where final colour matching is not essential, may only require generic mortar filling. These can begin as early as possible once the schedule of works is complete.

Critical works are likely to involve specialist input and careful colour matching. Although the first phases of preparation and mortar filling can be carried out early on, as per the non-critical works, a much later phase of colour matching will ideally take place as late as possible in the build. Colour matching is a clean, dust-free process undertaken under the perfect scenario of full access, a weather-tight building with glazing in dust-free dry conditions. Any items previously collated on an abeyance list can now be revisited and treated if they are still thought to be necessary.

Tie-hole filling

Tie-hole filling can be undertaken before or after sealing but should be one of the latter operations in the concrete contractor’s programme. If this is impractical due to access restrictions, it should be remembered that excessive moisture from condensation can darken mortar or tie-hole plugs, especially in stair cores or where large temperature fluctuations might exist. Hence these areas should be left until last if possible.

Sealing

Sealing concrete will again be influenced by access but should be one of the last activities. It is likely to be more efficient if finished work is handed over as part of a decorating or final cleaning package. The surface should be clean and dust-free before application, and those responsible for removing construction dust should also be responsible for the sealant. Walls should always be sealed from bottom to top in a planned sequence with completed areas marked up on a drawing – it is very easy to lose sight of what has and has not been sealed.

Summary

If one considers remediation work to be an essential element of visual concrete production, then developing a straightforward plan will be both simple and highly influential on efficiency and quality. Having a clear understanding of each phase and the best party to undertake it will inevitably lead to a beneficial and integrated strategy.

Jonathan Reid is director of GreyMatter Concrete.

See CQ 259 for his introduction to remediation: concretecentre.com/cqarchive