Sprayed concrete for the railway infrastructure

Sprayed Concrete has been a tried and tested method of improving the Railway Infrastructure over many years. However, the drive to improve Health and Safety within both the construction and rail industries has resulted in a variety of methods being developed to meet this goal. Traditionally Sprayed Concrete works have been undertaken by a 'nozzleman' controlling the water addition, and therefore the quality of concrete, to a stream of dry materials to apply Sprayed Concrete to railway tunnels, bridges, retaining walls, arches and other structures.

The skill and experience of operatives is paramount to the successful completion of projects and Certification by ClTB and the UK Sprayed Concrete Association should be a pre-requisite in all cases. The use of prebagged blended materials has given great flexibility in terms of material transportation and storage, and can be of benefit in possession works or where construction work has to be undertaken at geographically remote sites.

St Thomas Viaduct, Exeter
An example of Sprayed Concrete for the railway infrastructure improvement using manual Sprayed Concrete application was the strengthening works undertaken by BAM Ritchies to two brick rail arches in Exeter during Christmas 2006. The brickwork forming the arches had suffered over a number of years from water ingress causing mortar loss, brickwork spalling and loss of individual bricks, giving cause for concern for the long term stability of the arches.

A design was drawn up to reinforce the arches by application of 1S0mm of Sprayed Concrete to the arches incorporating a layer of A393 reinforcing mesh, fixed in place with resin fixed dowel bars, within the concrete layer. The fixing of strips of 'egg box' waterproofing membrane was incorporated to ensure future water ingress through the arch brickwork would be channelled into the existing arch drainage system.

All preparatory works and the construction of a concrete sidewall were
undertaken during normal day shift working in the build up to a 'wheels free' possession starting at 2300 hrs on December 24th. With the potential for low night time temperatures being recognised on site, all works were undertaken in tented and heated areas to ensure early strength gain of the Sprayed Concrete with no possibility of frost damage.

To ensure a high early strength gain, preconstruction testing was undertaken to confirm early strength development of an accelerated pre-bagged Sprayed Concrete material, with the material giving a 10 N/mm2 compressive strength at 24 hrs.

The arch spraying was undertaken within a six hour period using two BAM Ritchies spray teams, allowing the 'wheels free' possession to be lifted following the Christmas break.

Travis Street, Manchester
The introduction of remotely controlled robotic sprayed concrete applicators allows the use of ready mixed concrete materials, with quality control being exercised at the ready mix plant, and in addition has allowed the reduction of risk to the sprayed concrete 'nozzle man'. The operative is physically removed from the area of application, but the process still relies on the skill and experience of the operative to control layer thickness during Sprayed Concrete application.

At Travis Street in Manchester, adjacent to the main line Manchester Piccadilly Station, two brickwork arches required strengthening due to movement through the arches resulting in brick loss and arch stability concerns. The re-instatement design called for a construction of new arch sidewalls including footings, to support a new 400mm thick concrete arch - all in 40 N/mm' strength concrete.

The footings and wall were constructed in traditionally reinforced, shuttered and poured reinforced concrete. The arches were designed to incorporate two layers of 10mm reinforcement matting together with the addition of 40 Kg/m3 of stainless steel fibres in the concrete mix to provide additional load bearing capacity for the arch section. Prior to any reinforcement
fixing, a layer of drainage membrane was fixed to each arch to provide a waterproof barrier between the existing brickwork and the new concrete construction.

To enable an economically viable construction method, a Sprayed Concrete technique was chosen using a 10mm aggregate ready mixed concrete, incorporating the stainless steel fibres, super-plasticiser and hydration control additives. To enable overhead application, an alkali-free accelerator was added at the spray nozzle, dosage being controlled by automatic monitoring of the Sprayed Concrete pump output.

A remotely controlled robotic applicator was employed on site by BAM Ritchies to allow the 400mm Sprayed Concrete to be applied without the need for access to the arch and to remove the 'nozzleman' from the risk area beneath the freshly applied arch concrete.

The movement of the robotic applicator is controlled by joystick operation, the control of which requires a great deal of skill on the part of the operator to guide the Sprayed Concrete jet along the wall whilst optimising the sprayed concrete thickness and minimising waste. The operator must practice a long time to master manual control of the process to a satisfactory standard. Quality control of the material, however, is similar to that exercised in normal ready mixed concrete operations. Both arches were completed during normal day shift working during normal train operations.

The future for Sprayed Concrete application
The recent introduction in Europe of computer controlled robotic nozzle applicators with laser thickness control, has taken the development of sprayed concrete application a step further and is awaiting introduction into the UK rail industry. The laser controlled robotic nozzle uses laser scanning to provide both preapplication profiling of the tunnel or arch to be sprayed and automatic layer thickness control during Sprayed Concrete application, thereby eliminating the need for operator control of the spray applicator.

The laser robot is computer controlled and enables the system to measure the existing profile of a tunnel to within Smm, using a laser scanner over a selected areatypically a three to four metre long section of tunnel. The operator selects an area of tunnel to be scanned using a red dot laser marker integrated in the sensor. The sensor can scan up to 3000 and scans at 200mm centres longitudinally and radially, providing an accurate computer file representation for the robot control processor.

Using the stored profile information the processor controls the sprayed concrete pump operation and nozzle orientation over the selected area. The nozzle orientation and position is automatically updated for each of the pre-measured 200mm centres. This process allows the Sprayed Concrete layer to be applied to the correct thickness over the full tunnel profile. The accuracy of thickness control is both excellent, with tolerances of +j- Smm achievable and, more importantly, repeatable.

The system has been used to line over 28,000 m' of tunnel in the Netherlands and presents an exciting opportunity for its future introduction into the UK, thereby improving not only the quality of Sprayed Concrete within the railway infrastructure works, but also the Health and Safety of the work force. Fires in tunnel do occur, as demonstrated recently in the Channel tunnel. The use of plastic fibres in the sprayed concrete mix increases the durability of sprayed concrete linings in such situations.

Increasing use of steel fibres provides the opportunity to reduce the use of conventional reinforcement, mesh and lattice reinforcement. Modern additives reduce the aggressive nature of cement/additive rich mixes so reducing the risks of alkaline burns to operatives.

This article has been created by Bill Jones, Contracts Manager BAM Ritchies