Koster Vacuum Injection used at Second Severn Crossing
The Second Severn crossing is a 5128m long bridge between England and Wales, the centre portion is a cable stay bridge with two approach viaducts. The bridge was completed in 1996 and is now the main entry point into Wales.
The inside of the bridge
The above picture shows the inside of the bridge deck (the roadway sits on top of the upper concrete sections shown in the picture). The main carriageway of the bridge is made up of 2300 pre-cast concrete sections. Towards the top of the picture you can see the joints running horizontally between the concrete sections.
The concrete sections are bonded together, but more importantly they are pulled together and kept under tension by steel cables running between the concrete anchor blocks. During routine maintenance a large number of very small cracks were found in these blocks. As they are a vital part of the bridges structural integrity, it was decided that a structural repair was necessary.
The maintenance contractor for this bridge had previously used Koster’s Injection Resins, so approached us for a technical solution. We arranged for our injection expert to visit the site and conduct some tests.
After the tests were completed a full report was presented to the main contractors and The Second Severn Crossing Authority. We recommended that a bespoke vacuum injection technique was the best way to proceed.
Smoke testing had confirmed that the crack network within each block was interconnected. In some areas the cracks completely surrounded a section of concrete, forming concrete islands. The object of the repair was to bond the structure together, not break it apart, so we had to proceed with extreme caution. The vibration and stress that would have been caused by drilling of boreholes could have been enough to break out a loose island of concrete. Another factor to consider was injection pressure. During conventional injection the higher the pressure the more likelihood of a section of concrete cracking loose. By designing a vacuum injection solution, with surface mounted packers, we were able to avoid any stressing of the concrete. Our process required no drilling and used only extremely low pressure.
In the picture below you can see the set-up for the trial application.
Vacuum injection is a very important tool where a structural repair is required: bonding the flanks of the cracks together with a strong resin to restore the original strength of the structure. To achieve this all the cracks have to be filled with the resin, avoiding foam formation, water and air entrapment. It is a basic law of physics that no two substances can occupy the same space at the same time. Therefore, if you already have air or moisture in a dead end crack, the only possible way of fully filling that crack with resin is to remove the existing matter first, by either sucking it out, or by pushing it out through pressure. Although concrete is porous, air and water do not pass freely through its structure, in practice it would be virtually impossible to force out trapped air or moisture by using high pressure injection over a short time period.
The main pieces of plant required are shown in this diagram above, they were:-
A Vacuum Pump such as a Gast Pump
Created the suction for the vacuum injection.
2 x Pressure Pots
These were very important, they are basically airtight drums in which a vacuum or pressure is created, keeping the injection resin away from the vacuum pump. If any resin is pulled up the tubing into the pressure pot it will not flow readily into the pump.
Small Compressor
On the input side the resin was poured into the pressure pot and the pressure from the compressor slowly increased to gently introduce the resin into the crack network.
Injection Plates, Plastic Tubing, Ball Valves and Push-Fit Connectors
The injection plates were fabricated by an engineering workshop specifically for this project. They were produced from a nylon material with no valve on the inlet / outlet points. It is best to not use a valve at the actual connection point, but to include a manually controllable valve in each line. The injection lines were made from clear plastic tubing, fixed together with push-fit connectors.
The picture above shows a close up of the injection plate. We were using ultra low pressure so making the connections was very easy with push-fit fittings. The plates were stuck on with a fast setting / high strength gun grade adhesive which was also used as a temporary seal to the visible crack openings.
Injection Material
Koster KB Pox IN.
The material selected for these works was the Koster KB Pox IN. It has a very low viscosity at 120mPa.S (120 Centipoise), which is less than half the viscosity of many competitors “low viscosity” structural repair resin. The main properties can be seen below.
Low Viscosity = yes, 120mPa.S
100% solids (solvent free) = Yes
No shrinkage = Yes
Structural Repair = Compressive Strength >50N/mm2
Non- foaming = Yes
Good adhesion even to moist concrete = Yes > 4N/mm2 dry, 2N/mm2 damp
Non-harmful and easy to use = non explosive, mild smell, long pot life
The image above shows an area under injection, some small amounts of resin can be seen running down, mainly from the intersection between the anchor block and the main concrete wall. It can also be seen that the KB Pox IN has filled tiny cracks that were not even visible prior to the injection starting.
The injection trials were carried out by the main contractor, under supervision by Koster Aquatecnic ltd. The trials were extremely successful. A core sample was taken from the anchor block to confirm that the cracks were fully filled and that the adhesion to the concrete was good.
After the core samples were inspected by the client they were sent to Koster Germany for further testing (see microscope image above).
The tests under electron microscope revealed that all cracks that could be found in the specimen were fully filled with the KB Pox IN.
The larger crack at the top of the image is 340 microns, the two smaller cracks were 74 microns and 17 microns. This is exceptional for an Epoxy Injection Resin.
At the time we really didn’t know how much finer we could go with this technique, as we couldn’t find any crack not filled with the resin.
Following the successful trials, the injection works continued smoothly. We have undertaken further core tests for the client on this project by way of Quality Control for the on-going works. So far the smallest crack we have found was approx. 8 microns, this crack was fully bonded and filled with the KB Pox IN.
Both the client and contractors are very happy with the works to date. The technical solution, the training and the injection material provided by Koster Aquatecnic Ltd have ensured that long lasting, structural repairs can be made, with the minimum of disruption.
Further information on Koster’s range on injection materials can be found on our website:-