The multi-span composite viaduct is 450 metres long and carries the M25 motorway between junctions 20 and 21. It comprises twin decks, each of which is formed from 4 open topped steel box girders within an insitu concrete slab.

Osborne were given the task, by Connect Plus and Highways England, of carrying out strengthening repairs following the discovery of a lack of fatigue strength in the bottom flange of this structure. Jack Tighe Ltd is a longstanding member of the Osborne supply chain and successfully tendered for the surface preparation and painting works associated with the repair programme in 2016.

Following some minor preparatory work late in 2016, we set up our site compound in January 2017. The contract was run from our Portsmouth office with additional QA and commercial support from our Head Office in Lincolnshire. A team of 12 ICATS certified operatives and 2 full-time SSSTS accredited supervisors have worked continuously on this project, with additional resources being made available to accommodate peaks in the programme as required. One of our supervisors, Grant Wills, was awarded a monthly Smart Safety Award by Connect Plus for having positively intervened to help increase safety on this project.

Specific risks associated with this project include working at heights, working in confined spaces, abrasive grit blasting, ultra high pressure jet washing and paint application by spray methods. All of our site personnel were selected to take account of these particular skill set requirements. Access to the work areas is via fully fixed scaffold supplied by Geoffrey Osborne Ltd who have also supplied a blast booth integral to the scaffold. New plates required for the strengthening process are lifted up to the blast booth by means of a mobile scissor lift trolley and positioned using a lifting magnet. The plates are then grit blasted, removed from the blast booth and placed where needed in the structure, ready for painting.

The specifications are as follows:

Stiffener Plate Installation – Designated External Areas

  • Debris netting and hessian matting erected for containment purposes
  • All surfaces cleaned to remove any traces of grease and contamination
  • Burrs ground down to be suitable for primer application
  • Areas of existing coatings fully removed where required by UHP water jetting to expose original SA3 standard with a medium profile
  • All prepared areas visually examined with an illuminated magnifier to ensure no traces of thermal metal spray remain in profile troughs
  • Salt levels checked using Elcometer 130 salt contamination meter
  • New plates checked to ensure they have a radius on all edges and are blast cleaned to SA2.5
  • Existing surfaces flash blast cleaned
  • During plate installation a heavy brush coat of Jotamastic 90 Aluminium is applied around the mating face of the new plate immediately prior to installation
  • Once plates are fitted, apply full coat of Jotamastic 90 Aluminium and check dft readings for QA conformity
  • Apply Jotamastic 90 stripe coat to all edges, welds, fixings, etc and take wft readings
  • Apply Jotamastic 90 intermediate/build coat to all surfaces and check dft readings
  • Apply Hardtop XP finish coat to all surfaces and take dft readings

 

Stiffener Plate Installation – Designated Internal Areas

  • All surfaces cleaned to remove any traces of grease and contamination
  • Preparation by rotary wire brush to provide a key for subsequent coats
  • Apply 2 coats of Jotamastic SF intermediate/build coat to all surfaces, taking dft readings between coats

 

Full Bottom Flange, Webs & External Portion of Top Flange – Designated External Areas

  • Debris netting and hessian matting erected for containment purposes
  • All surfaces cleaned to remove any traces of grease and contamination
  • Areas of existing coatings fully removed where required by UHP water jetting to expose original SA3 standard with a medium profile
  • All prepared areas visually examined with an illuminated magnifier to ensure no traces of thermal metal spray remain in profile troughs
  • Salt levels checked using Elcometer 130 salt contamination meter
  • Apply primer coat of Jotamastic 90 Aluminium and record dft readings
  • Apply Jotamastic 90 stripe coat to all edges, welds, fixings, etc and take wft readings
  • Apply Jotamastic 90 intermediate/build coat to all surfaces and check dft readings
  • Apply Arbokol 1000 Polysulphide sealant to designated areas between the concrete and steel interfaces
  • Apply Hardtop XP finish coat to all surfaces and take dft readings

This is the first contract which Jack Tighe Ltd has been involved in which has made extensive use of Business Information Modelling (BIM). This has been very positive across all trades with the ability to upload all QA data for the vast number of repair areas to a single database and have all information available in real time. Osborne ensured that our supervisors were trained in the use of BIM applications on iPads, and this was a perfect example of the close collaboration evident on site.

Value to date of this contract is approaching £1 million and work is likely to continue into 2019.

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