Over the years we have used various methods and techniques to strengthen and upgrade our concrete structures. The earliest method, jacketing, was simply to dowel in extra reinforcing and encase the existing member, be it a column, beam, or slab, with normal-strength concrete. This works well but the limited capacity of the existing section could lead to a large jacket with impacts on headroom and floor space – not always acceptable to clients. The design must also address the risk of slip along the interface of the concrete cast at different stages.
The next method, introduced in the late 1960s, was to install steel plates, termed epoxy-bonded reinforcing (EBR). Again, this works well, but the plates can prove extremely difficult to install. The plates required are generally heavy and getting the steel and concrete to work ‘compositely’ requires the use of epoxy resin as well as bolts. Bolting the plates in position can also prove problematic because of the high density of reinforcing bars in the bottom of the beam.
In the 1980s, carbon-fibre-reinforced polymer (CFRP) was introduced as a means of strengthening and upgrading structures, with the first project taking place in Germany in 1987. This process involves bonding the CFRP plates or wraps onto the concrete member using specially designed epoxy adhesives. The CFRP plates and wraps are extremely lightweight, do not increase the cross-section of the member and require no bolting. This proved a winner with both consultants and contractors, and the system was quickly adopted in the repair and rehabilitation sector. It is now the default technique for strengthening and upgrading concrete structures.
However, strengthening structures using CFRP plates and wraps also has it limitations, and a new technique using High-performance concrete (HPC), or ultra-high-performance concrete (UHPC) is now being used more frequently.
HPC is characterised by a compressive strength of 60–120MPa and 120–200MPa for UHPC. The inclusion of fibres with carefully selected fines and aggregates allows the HPC to offer superior performance properties such as durability, tensile ductility, toughness, hardness, impermeability, wear resistance, impact resistance, chemical resistance and resistance to repeated cycles of freezing and thawing.
HPC was first developed in 1978 in Denmark by Hans Henrik Bache, who was able to carefully select densified system particles containing homogenously arranged ultrafine particles to produce HPC. It is only recently, however, that concrete professionals have started using HPC in thin layers to repair and strengthen columns, beams, slabs and bridge decks. The practical use of this strengthening technique can be seen in two recently completed projects.
During the redevelopment of a former store in the Birmingham city centre and while assessing the existing structure with the requirement for new loads from the rooftop terrace, it was identified that eight circular columns needed to be strengthened. As the columns were also located in the loading bay on the first floor, space was at a premium to ensure delivery lorries could still safely use the area.
Mapei’s UK structural strengthening team worked in collaboration with the client’s design team to look at the best possible strengthening options. Initially CFRP wrapping was considered, but the required improvement in flexural and axial capacity could not be met due to the columns’ high slenderness. CFRP is an effective method to increase the axial capacity of RC columns by the confining action of wraps; however, it does not induce any enhancement on the inertia of the section, so in this case it was not a viable solution.
Instead, a proposal was put forward where the columns could be jacketed with Planitop HPC, a free-flowing, high-performance and high-ductility micro-concrete with steel fibres that could be used without the need for reinforcing steel, thus limiting the jacket thickness to 40mm. This ensured that the loss of space and the jacket’s extra self-weight were minimal.
Concrete Repairs Ltd (CRL) carried out the works with the repair specification provided by Mapei UK.
The works involved the strengthening of eight 10m-long beams in a previous RAF airmen’s mess being redeveloped as Grade-A offices. Tests on the existing concrete showed compressive strength values below 20MPa. An evaluation by Beta Design Consultants (BDC) quickly ruled out CFRP strengthening due to the low bond strength of the existing concrete. Traditional jacketing would have required a deep heavy section, so HPC was proposed. BDC’s design was based on a 70mm three-sided HPC jacket (fck=70MPa) with additional longitudinal reinforcing bars. Typically, HPC bonds well to roughened concrete but in this case shear links were required to prevent slip along the HPC/existing concrete interface.
CRL was awarded the contract to carry out the strengthening. Recommendations for the substrate preparation, concrete repairs, structural strengthening and protection of the beams were provided by Mapei UK.
Following the removal of render from the beams, the concrete was cleaned and prepared to the required textured finish using Mapemortar HB R3 (Figure 6). All concrete repair materials were specified according to EN 1504(1). Reinforcement was installed to anchor the proposed 70mm three-sided HPC jacket to the existing beam (Figure 7). Bespoke-designed formwork was then installed around the beams using 18mm WBP ply, 100 × 50mm timber sections and supported by Titan props (Figure 8). Slots 300 × 60mm were then cut into the slab above to enable the HPC to be poured into the formwork (Figure 9).
To pour 2m3 of concrete through slots above and keep the concrete flowing along the 10m-long beams within the enclosed formwork was a challenge, to say the least. To maintain the flowable properties of the fibre-reinforced concrete and obtain a good distribution of the fibres within the mix it was critical to keep a constant mixing time of eight minutes and have a continuous flow of concrete going into the formwork (Figure 10).