Caerphilly CBC Culvert Engineering
After carrying out numerous culvert repairs for Caerphilly County Borough Council, EDS was appointed to complete concrete relining works to the invert of St Philips & St James Culvert, which had become severely corroded with areas of washout.
Scope of Work:
Renew the lower half of the circumference of St Philips & St James Culvert with a 100mm-thick concrete pour.
The purpose of the work was to repair the lower half of the circumference of the Armco pipe culvert due to the highly corroded condition of the existing invert. The specification supplied by Caerphilly County Borough Council was that the concrete should be 100mm thick, reinforced with A393 mesh reinforcing bar. An underwater concrete mix containing Sika UCS was used for the concrete repair.
St Philips & St James Culvert is constructed from 1.5m internal diameter Armco pipe sections helically wound. To achieve the 100mm-thick pour on the lower half of the circumference, EDS used 1.3m external diameter Weholite pipe sections cut in half as formwork.
The steel reinforcing mesh was rolled at the EDS workshop and transported to site. It was fixed into place using steel brackets attached to the culvert abutments. The formwork was installed on 100mm-high packers and fixed to the abutments, with Acrow props used to anchor it and prevent lifting under the weight of the concrete. Either end of the formwork was sealed to contain the pour. Each section measured around 12m in length, and expansion joints were created at the start and end of each pour by cutting back the mesh by 100mm. The downstream end of the culvert was poured first.
Mesh reinforcement fixed to the lower half of the culvert.
Mesh reinforcement fixing point (left) – Formwork preparation (right).
The use of the Weholite pipe allowed the water to flow over the top of the formwork during the concrete pour without the risk of the waterway becoming contaminated, eliminating the need for over-pumping of the watercourse. This reduced the cost of the repair work as the fast-flowing water would have required the use of large diameter pumps and potentially road closures during concrete pumping operations.
Prior to the concrete arriving on site, hay bales were placed downstream to absorb any potential spillage or washout. On the day of the pour, a concrete pump with flexible bagging was used, enabling the pipework to be moved along the length of the formwork. The pump outlet was placed at the top of the formwork and moved along once full. A 110V poker was used to compact the concrete, ensure full settlement, and remove air bubbles to prevent honeycombing. Extra curing time was allowed due to cold temperatures. The pour was set with a slope on the top surface to prevent water pooling, in line with CCBC’s requirements.
Formwork with reinforcement and packers, held in place by acrow props.
Formwork in place with water freely flowing.
After curing, the 12m-long formwork was moved upstream to begin the next section, overlapping the previous repair. This process was repeated until the full 32m length of the culvert was relined.
The final stage was the application of bitumen paint to the top of the repair, overlapping the existing Armco pipe construction by 100mm. This was applied once the concrete had fully cured.
Due to deformation in the culvert, which had become almost elliptical in sections, the formwork sometimes sat up to 200mm away from the culvert side. The flexible nature of EDS’ formwork design accommodated this without issue.
Concrete pour behind the formwork and being finished.
Finished concrete surface.
