We live and work in an ever increasing number and variety of spaces where concrete is used as the finished medium and visual surface; hardly surprising given the material’s sculptural versatility and thermal mass qualities. This places an onus on the project team to produce clearly defined specifications, set expectations and present procurement practices that achieve the original aesthetic design intent. Post-finishing programmes can offer valuable support that’s missing from specifications with scant “making good” clauses.
What is post-finishing?
Post-finishing is any controlled intervention or mechanism that improves concrete’s aesthetic quality after the concreting process is complete, allowing for future maintenance provision and planning for the end user. Unlike other post-finishing techniques that alter the surface texture of concrete, such as shot blasting, this article will focus on the planning and delivery associated with remediation, or improving the overall appearance of newly formed concrete.
Communication, discussion and planning is key to controlling subjectivity when making aesthetic judgments. Concrete is a natural material, and will always have some tonal and surface variation. Some so-called blemishes are inevitable and often acceptable, depending upon the finish specified.
The objective of integrated post-finishing is to keep the scope of remediation work to a minimum, celebrating concrete itself while improving the aesthetic. Fundamentally, any work undertaken to the as-struck finish must maintain the inherent character and nature of the concrete, unless an applied finish is required.
Planned or integrated post-finishing reduces risk and improves quality under a clear protocol set out at the start of a project. A strategy is established for the following processes:
- Curing and protection
- Remedial works
- Handover and content for operation and maintenance (O&M) manuals
The flowchart in figure 1 shows the ideal order, process and roles involved in successful post-finishing procurement.
Curing and protection
Curing provides adequate moisture, temperature and time to allow the concrete to achieve the desired properties for its intended use. Maintaining similar levels of moisture in the concrete while keeping an air flow around the surface will prevent permanent discolouration. It is useful to combine protection and curing as one operation by covering or boxing in surfaces with an air gap until the concrete is ready for handover.
Water ingress, from rain for example, can also create instances of blemishes that cleaning will not resolve. Any protection should be designed to prevent this. Although this is not always practical, it should be noted that such protection can greatly reduce the impact of these blemishes.
The benefits of a consistent cleaning regime cannot be underestimated when it comes to improving aesthetic value. Judging concrete quality is only viable after cleaning, but it can also permanently change the surface quality of concrete.
Methods should be trialled early on depending on the type of surface finish and whether the cleaning is likely to abrade the surface. Soffits often benefit from a different approach to walls on the same project but, whichever method is used, cleaning should start with the least invasive approach. Dry abrasive cleaning will temporarily lighten the tone and wet cleaning will temporarily darken the tone. Only when dust free and dry will the true underlying tone and variation be exposed. Chemical cleaning is not usually practical for in-situ concrete.
“Active” cleaning regimes should also be maintained during pours. This is key for reducing or removing the risk of grout-loss striations at construction joints on walls and columns. Because grout-tight joints are essential to producing visual concrete finishes, continual inspection during upper pour and compaction is vital. Gentle sponging and buffing of fresh grout loss reduces staining and the need for future cleaning. This is especially true of board-marked concrete where the biggest issue can be unchecked grout stains running from the pour above.
Even with the greatest care and professionalism, some degree of remediation works to visual concrete is inevitable and should be planned for. The biggest potential stumbling block is the failure to to put in place a procedure for how and when to undertake these works and what quality to expect.
Remedial works are intrinsically linked to curing, protection, cleaning and sealing activities, which should be documented and woven into the manual for the O&M programme.
Remedial works should take the following steps, in this order:
- Identifying “categories of repair” as soon as the concrete is clean
- Trialling methods and quality-testing on the mock-up in combination with sealing
- Producing a benchmark sample in situ, treating each category of repair
- Creating a full schedule of works as agreed on site after cleaning is complete
- Undertaking the aforementioned schedule once the protection is removed, the building is weathertight, with glazing, heating and, if possible, lighting installed
- Sealing (if specified) should be implemented as part of the post-finishing package alongside the remedial works programme.
It is very useful to compile a list of repair types early in the contract. This assists in identification and also probable cause, allowing for adjustments in working practices to reduce future issues. Below is a list of common categories of repair:
- Construction joint deflection
- Grout-loss-hydration staining
- Construction damage
- Sand run segregation
- Construction oversights
- Surface discolouration
- Unplanned repairs
Benchmark samples for approval
Producing benchmark samples of remedial processes is key to allowing projects to progress in the knowledge that a full system is in place to deliver the required quality. It is important to produce a full set of samples on the actual concrete for each repair type and for this work to be undertaken as soon as the concrete is clean. Although trials can be undertaken on the mock-up, it is more useful for benchmark samples to be carried out in situ and for these repairs to be inspected from an agreed viewing distance.
It is not particularly useful to present a specific area for approval as natural variations in the material may make this sample non-representative elsewhere. It is more beneficial for each sample to treat each category of repair independently so as to demonstrate that individual repairs will not “interrupt the enjoyment of the design” and will “support the original design intent”. Benchmark samples should also be sealed as per the specification so as to make the sample process as comprehensive and informative as possible.
Schedule of works
Following the process of approved benchmark remedial works, a full schedule of works should be collated prior to protecting or re-covering surfaces. Projects that commit to a fully agreed schedule of works tend to support the concrete aesthetic more in the long run, limiting “mission creep” that can lead to over-repairing.
Schedules of work detail position, type and location including a “before” image of the concrete. The post-finishing specialist is best placed to collate all before, during and after imagery, compiling reports for handover. This can also be used as a record of concrete condition, limiting liability should further damage be caused by others.
Sealing, handover and O&M manual
Sealers may be used as dust binders, for lessening maintenance regimes or as anti-graffiti protection. Although not essential, sealers provide a degree of protection especially in the public realm. Proposed types should be trialled on the mock-up and tested for suitability, with the following key characteristics noted: maintained or enhanced surface appearance, performance levels in terms of penetration, breathability, dust suppression, hydrophobic nature and light reflectivity.
Jonathan Reid is a remediation consultant and practitioner at GreyMatter Concrete