The building comprises a four-storey splitlevel car park and bus station in the centre of Rotherham located between Frederick Street to the east and the River Don to the west. The car park is located over the bus interchange and is accessed via a long elevated concrete ramp that spans the River Don.
The structure is approximately 120m long (north–south) by 40m wide (east–west) with an overall height of approximately 18m. Its form comprises in-situ reinforced concrete frames at approximately 10m centres, with precast prestressed floor units spanning between. The floor units are either single or double T-beams bearing onto a boot nib on the sides of the in-situ concrete floor beams.
A reinforced concrete topping is present on all floor levels. Consistent with many similar structures built in the 1960s, the car park’s concrete elements have experienced significant levels of concrete degradation due to continual exposure to chlorides (primarily from de-icing salts), carbon dioxide and water penetration. Over the past ten years, regular structural inspections had revealed a number of defects to columns, beams and slabs, and a number of smaller repair schemes had taken place to maintain the car park’s structural integrity and safety for its users.
These smaller schemes involved strengthening a number of severely spalled columns with a 150mm reinforced concrete surround. Additionally, patch repairs had been undertaken at numerous locations, but unfortunately many of these were now showing signs of failure, largely due to the incipient anode effect. South Yorkshire Passenger and Transport Executive (SYPTE) considered a number of options for the interchange including demolition, but in August 2016 it was decided the best use of funding would be to redevelop the eye-catching structure of the Rotherham skyline. Therefore it was agreed to progress with a refurbishment of the interchange, comprising major structural repairs and an overhaul to the car park itself, as well as a comprehensive refurbishment of the transport interchange below.
“The car park requires life-extending repairs and improvements and internal repairs were needed in the interchange after the bus fire, in May 2016. All this gave us an opportunity to refresh the whole site,” says Ben Gilligan, director of public transport at SYPTE. Currently, the car park contains 678 spaces. Unfortunately, the parking bays were tight, designed for cars from the 1960s and quite unsuitable for many modern vehicles, resulting in difficulty in manoeuvring in and out due to the close proximity of the existing structure.
SYPTE research suggested this was a key factor on why car park usage had dwindled over the years. Therefore the new design involves replacing the existing four-bay parking module with a three-bay parking module. This will free up significant space for manoeuvring, as well as allowing people to get in and out of their vehicle with greater ease. SYPTE developed a £12.5m plan to reinvent the bus interchange and refurbish the car park, with the value of the works split approximately 50/50 between car park repairs and the interchange refurbishment.
As with any scheme of this nature, comprehensive testing and investigation is critically important to understand the structure and to ensure any repair strategy is fit for purpose. Structural engineer Arup has been involved in structural inspections of the car park since 2003. A suite of half-cell, chloride and cover meter testing of the car park decks was completed by Birmingham City Laboratories in October and November 2017.
• a visual examination identifying readily apparent defects
• extensive chloride testing of the structural concrete topping and limited chloride testing of other concrete elements
• carbonation depth testing
• hammer testing to determine extent of spalled concrete
• extensive half-cell testing of the structural concrete topping and limited half-cell testing of beams and columns
• cover meter testing.
A summary of the findings concluded:
• Embedded reinforcement in in-situ columns, beams, upstands and kerbs had corroded and resulted in areas of local concrete spalling.
• Embedded reinforcement to decks was at significant risk of chloride-induced corrosion with very high levels of chloride contamination and extremely negative half-cell potential readings.
• Steel tie bars between precast planks had corroded badly and many needed to be removed/replaced.
• Deck coatings, where previously applied, had failed in multiple locations and needed replacing.
The condition of the parking decks varied because only those decks exposed to the elements or directly above habitable space had been coated. With defect drawings provided by Arup, the USL StructureCare team calculated that 100m3 of concrete was defective and needed to be removed and replaced.
The majority of this was in the decks and due to this vast amount of concrete removal the decision was made to use hydro-demolition techniques. This represented a safer and faster form of removal, reducing the amount of hand–arm vibration activity on-site, as well as ensuring the prestressed elements of the car park would not be disturbed. A key element of the planned works was also the removal of the steel tie bars/straps between precast ‘T’-units. These steel ties had corroded in numerous locations, leading to local – and in some cases more extensive – concrete spalling of the precast units.
A repair detail to alleviate this problem was proposed by Arup and subsequently modified when on-site, consisting of a 200mm-diameter diamond cored hole through the deck to remove the tie. A 700 . 700 square was then marked and removed around the core hole into the concrete topping screed. An arrangement of H10 reinforcing bars and D49 fabric was then bent in place on each detail to form a plug to hold the concrete in place. Each core hole was then shuttered from below to receive new concrete. Over 1050 steel ties were identified as equiring removal on the car park, with almost 600 of these on the upper decks (leading to the creation of a ‘pepperpot’ effect).
Corrosion management strategy
The tender required a corrosion management strategy with a life expectancy of 20 years. Upon review of the testing information it was established that high chlorides were present throughout the structure but concentrated in hotspot areas mainly in the decks. In certain areas, chlorides were also at a very high level of concentration at the depth of the steel reinforcement.
The innovative corrosion management strategy proposed, sought to offer the client a value-for-money solution while simultaneously delivering complete protection to all decks, especially in the hotspot areas. While Nufins MCI2020 migrating corrosion inhibitor was used to reduce the risk of steel corrosion in lowerrisk areas (some 14,000m2), sound areas of concrete surrounding patch repairs were protected using PatchGuard sacrificial anodes by Concrete Preservation Technologies.
These anodes, placed into holes surrounding the patch repair, corrode in preference to the steel, thereby combatting any incipient anode reactions where corrosion risk has been identified as highly likely. Arguably however, the most important factor during the refurbishment was how to deal with deck areas with very high levels of chloride contamination and showing a very high risk of likely corrosion activity. In these areas where spalling has not yet occurred, PatchGuard Connect sacrificial anodes are being used to protect the steel from corrosion.
These anodes provide a protective current to the reinforcing steel as a result of the sacrificial activity of the PatchGuard anode. Once installed, the PatchGuard Connect anode will corrode preferentially to the surrounding steel, offering protection against corrosion damage. Areas of sound concrete that are suffering from corrosion and from chloride contamination can be protected with a minimal amount of connections to the steel reinforcement – typically two to three per string of up to 50 anodes.
On completion of all corrosion management works and concrete repairs, the parking levels located directly above the bus depot and the exposed roof decks are being protected with the DeckProtect+ Blueshield car park coating system. All internal decks are being treated with the DeckProtect+ Rapidflex system. All remaining concrete surfaces will be coated with Nufins Covercrete, a protective and decorative anti-carbonation paint.
The project also includes a new impact barrier system throughout all levels, decoration throughout of all ceilings, walls and columns, replacement of all movement joints, LED lighting and a new aesthetically pleasing cladding system replacing the existing large unsightly ‘fin’ design. The proposed cladding material is an anodised expanded mesh panel. This provides a high-quality semi-transparent facade that will transform the car park into what will appear to be a new structure. The colours selected for the cladding blocks have been chosen to pay homage to the industrial heritage of Rotherham.
This has been achieved by selecting three tones of brown/gold that mimic the colour of iron before it has been manufactured into steel. The renovation of this Rotherham car park offers a considerable opportunity to integrate the building more appropriately into the context and the history of the area, as well as breaking down its mass. It also offers the opportunity to improve the internal environment, delivering essential structural repairs, while providing a safe, modern and more desirable environment for customers.
For more information please email firstname.lastname@example.org