BlastOne

Bridge Blasting Project Sites

Removing the coating and rust and staying high and dry.

Overview of why we blast

Most bridges require blasting due to the steel construction and the structure support for a road or a railway line or some other utility, for example pipelines will often go across a bridge.  The reason we blast is because if the structure is left to corrode it will actually weaken the structure. This could mean that the structure may fall, as with what we’ve seen in Minnesota with the Minneapolis bridge collapse, the structure fails.  A relatively insignificant issue might be that the structure is closed until repair is done which causes exorbitant public works cost and inconvenience. But also in the case of a catastrophic failure, loss of life and surrounding infrastructure damage is a real possibility.  We are blasting to maintain the integrity of the structure.

The cost to maintain these structures is very high.  Bridges by their construction nature are often restricted for Inspection access, an example is the underside area that requires rope access specialties and the access to these structures costs more than the blasting and painting process.  Because the cost of carrying out the activity is high and the access is expensive, you want to minimize the amount to minimise the amount of time required to carry out this form of project work, therefore a first choice in an excellent quality abrasive will ensure the best quality surface. This clean, well profiled surface will extend the coating adhesion and anchorage, extending the coating life. Less maintenance means less cost!

COMMON ABRASIVES USED

The most common types of abrasive are copper slag, coal slag, garnet and steel grit.  Steel grit has been used a lot to reduce the cost of disposal, particularly with hazardous coating removal, lead paint.  It has been found more and more that Australian GMA garnet will actually outperform other abrasives when the entire project is taken into a costed account, in that the consumption of the abrasive and production speed of the abrasive outweighs the cost of disposal and using a bit more with a non-recycling abrasive.  Steel grit and GMA garnet are used more than the slag abrasivesand evidence from years of research proves how much cheaper it is to use Australian garnet than what has been conventionally used, which is steel grit.

What Causes Corrosion?

Often bridges are located over either sea or fresh water and rivers a lot of the time,,,,,,the main issue found with low lying bridges is the forming of condensation. This builds up on the steel, and can cause corrosion particularly in box sections and flange joints. Steel beam supports in lower areas of a bridge are often the main area of corrosion, and if the location is close to the coast or affected by salt de-icing processes, the accumulated moisture and chlorides is a highly corrosive recipe!

EQUIPMENT CONSIDERATIONS

Because we are working with bridges, you may need to be operating off a barge which could be a long way below, or off the road deck, or you may need to even keep your equipment right off the bridge, at one end of the bridge or the other.  Because of that, youll often need to run hoses long distances to reach to area to be worked upon, ... you may need to close traffic lanes or restrict access - all costly and often frustrating for users! Your crews will be desperately aiming to maximise EVERY MINUTE of their shift, the result is sure likely to be an ACCIDENT! Running bigger compressors means more cost, big blast pots, multiple outlets, big vacuum systems that are all trailer mounted towed by a truck.  When you have large bridge projects and we are talking about things across rivers, across other terrain, valleys and other roads, you’ve got multiple operators working so you want something that doesn’t take a lot of time to load.  Because you are on site  you want to maximize your production so a multiple outlet bulk blaster is used. 

The best containment systems are ones that minimize the cost to clean up but also work in with the access system to minimize the cost of access.

Bridge Site Access Contraints

You are often having to work around other traffic movements.  In the instance of a rail bridge you often have to work around other train movements.  In the incidence of a road bridge you have to work when the traffic flow is lower, so that means working at night.  Working at night brings its own challenges because it means that suddenly you’ve got a large visibility issue, so you need to figure out excellent lighting;  helmet lighting, stand lighting, hanging lighting, nozzle lighting, anything to be able to increase visibility and make sure you get a good, thoroughly prepared surface and also a consistent quality coating application.   The other challenges around train movements is you have to stay around their schedules.  If a train comes through and you are applying containment, because of the fact that trucks and trains create a turbulence issue, meaning the sprayed out coating will be damaged by "dry spray". The containment system has to be very well attached to the scaffolding structure, otherwise it can be ripped off the scaffold.  Often train drivers are weary and don’t see the slowdown signs and so trains can come through at very short notice, creating quite a major safety risk.  It is not uncommon to hear of railway bridge projects where the operators can be exposed to hazards such as train movements as a consequence of poor communication or misunderstanding regarding site access.

Another consideration are the distances to be found in projects such as these...You may be working on a bridge that could be a mile long and you may have to operate off a barge, accessing that barge in the middle of the river.  Sending the blasting, abrasive and compressed air out into the middle of the bridge you have complex access arrangements, scaffolds, and containment.  It can often get very hot and humid inside, so you may need dehumidification and cooling.  You may need motor boats to access your barges.  You need to make sure that you don’t get any hazardous coatings falling into the rivers and waterways.  Typically a lot of bridges have hazardous coatings, e.g. lead paint, so you need specialized personnel on site, certain qualifications to do the work, soil testing, blood testing and air testing.  You need very thorough documentation for the whole process and you also need to protect your workers with decontamination trailers and wash facilities.

You also need to think about the weight loading of the bridge, are you adding extra weight to the bridge;  the wind loading on the bridge when you contain it.  You need to think about when you’re designing the containment size, how much you can blast and paint in that day.  If the containment is too big you are not going to be able to provide enough extraction.  You need to provide the right amount of extraction in a hazardous coating area of 100ft per minute in the case of lead paint, which means you need to have your containment size around your dust collector size.  Correctly sizing the duct work to run to the contained area is also a challenge.  You may need to run multiple ducts, you may need to run larger than standard size ducts.  You need to have good quality duct work so that it doesn’t collapse under the suction pressure.  There are a lot of challenges about understanding containment, conveying abrasive over long distances, vacuuming abrasive over long distances and you need to make sure your guys don’t get dehydrated.  There are a lot of things to be thinking about on a bridge project.

COATING TYPES AND EQUIPMENT

If the bridge is visible to the public the finish coat is often quite important, so using a polyurethane to have good gloss retention or a micaceous oxide which combines a good finish for many years with a good corrosion barrier.  You are also seeing epoxy zinc primers and epoxy build coats quite commonly, particularly under the decks of bridges.  The reason micaceous oxides are used quite a bit is because it does have a reasonable finish, but it also is an improved barrier to moisture ingress and helps extend coating life.

Normally a total of 15 mils, for even the three coat systems.  Typically you are going to see 12-18 mil coating thicknesses.

The coating would normally be applied with airless sprayers and on particularly large projects you may get plural component sprayers, but typically because a bridge is being done in stages, as in you can only paint what you’ve blasted and containment size issues, you can only paint in one contained area a day, you are not spraying massive volumes of coating in any one day.  Therefore you will generally find an 60:1 – 80:1 single component airless will be used and often with a heater because you are running extended lines and you’re wanting a bit of extra heat to help reduce the viscosity to improve the atomization of the coating when you’re applying it, to get a nice, consistent finish and a good visual finish.

Be aware of overspray

We have talked about products falling into rivers but even when you are working around trains, truck, cars and traffic the small amount of overspray falling on cars is a very expensive mistake.  You can get someone’s very nice new Jaguar or Rolls Royce, Audi, Lexus covered in some industrial coating overspray and then you can get class action.  You could get overspray on 1,000 cars in a shift just because of one piece of containment being left off or left open – someone has opened it to try and cool the area and all of a sudden you get a lot of overspray exiting.  This is a very expensive problem

INSPECTION NEEDS

Climatics are particularly important and often on a bridge you will be required to do a destructive test, like a crosshatch test, just to ensure that when your coating is all cured you are actually getting a good adhesion with that coating system on that bridge.  Part of the reason is because bridges are often part of an ageing infrastructure, so it’s maintenance work and not new construction work.  The steel quality over that time can vary quite a lot, so often on the older bridges they are often cast iron rather than steel, or a high carbon steel and a crosshatch test gives you the ability to guarantee if you like, the adhesion of that coating system to the bridge substrate.

Want more information?

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