Anti-Corrosion Prep: Laser Cleaning Structural Steel Bridges with High-Power Lasers

Abstract

Bridge maintenance for infrastructure contractors, Department of Transportation (DOT) officials, and project managers is a delicate balance of cost, time, and specifications. In almost every bridge maintenance project, surface preparation for paint is the most costly and time-consuming activity. This paper provides a data-driven, operations-focused analysis of laser cleaning structural steel, without delving into complicated thermodynamic theory. What is important to bridge maintenance operations will be covered, such as processing speed, surface preparation quality (less than +/- 0.05 mm), compliance with EPA and OSHA regulations, and return on investment compared to grit blasting.

The Commercial Reality of Bridge Surface Preparation

Steel bridges are a common type of structure, and it is generally common knowledge that the anti-corrosion paint system is the structural part of the bridge. This means that the paint surface has to be applied to a suitable surface. Thus, the paint reliability can be compromised by a very simple parameter: the surface preparation. Indeed, when a bridge has to be recoated due to a failure of the epoxy coating layer or due to rusting, the steel has to be blasted back to its clean and smooth surface, free from any contamination, to the Sa 2.5 ISO 8501-1 or at least to the SSPC-SP 10 level.

For decades, the industry’s conventional method has been abrasive grit blasting. Although effective at surface preparation, the process creates numerous logistical and very costly challenges in today’s infrastructure maintenance and expansion projects:

Containment Costs The costs to contain the effects of abrasive media and toxic lead-based paint are enormous, to the extent that they must be contained from entering the sewer system or public airspace. The cost is often in the range of 30% to 40% of the project’s overall cost.

Media Logistics: Abrasive blasting requires the purchase, transportation, and storage of massive quantities of silica, garnet, or steel grit.

Hazardous Waste Disposal:l The grit mixes with the old paint and rust. Turns out our whole mess of paint and rust is now hazardous waste. We’re talking about hundreds of tons of gritty waste here, and disposal is a very expensive process.

Labour Fatigue Because of the high pressure in the blast nozzle and the weight of the protective suit, it is a heavy, tiring job. This means that the welder has to have regular breaks. This reduces their coverage rate in an hour.

According to industry insiders, the construction business is limited by a range of constraints. High-power laser cleaning of structural steel can effectively solve these limitations and help achieve the goal of operation, using a light-based subtractive processing method to remove abrasive media transport and other difficulties in the preparatory phase of construction work. Ultimately, this method can achieve the necessary degree of cleanliness for painting on the surface of structural steel.

How the Technology Works (The Contractor’s Perspective)

A Phd in Optical Physics is not required to understand why a Rust Cleaner Laser is an effective tool for bridge repair. It is a straightforward case of differential light absorption.

When the laser equipment is shone with a high-intensity near-infrared light beam, the varying composition of the materials on the girder bridge reacts very differently:

The contaminant is generally in the form of rust, paint, or mill scale and is black and porous. It rapidly absorbs the laser light, and this absorption is accompanied by a rapid temperature rise until the material is vaporised in a process known as ablation.

The Substrate (Bare Carbon Steel). As we noted above, the rust on the steel is highly reflective. Once the laser fends off the rust, it then impacts the bare metal. The bare metal does not actually heat, melt, or vaporize, simply because most of the laser energy is reflected by the steel, rather than being absorbed.

This creates a self-limiting process, which means that the laser removes the contamination, but stops removing material the instant it encounters the bare steel surface. So, rather than having to worry about the potential risk of burning through to the structural surface of the bridge, a contractor’s laser cleaning operators can simply use the laser to remove the contamination directly from the surface of the bridge, without having to worry about potentially causing damage to the underlying structure for as long as they position the laser over a particular area.

Substrate Integrity and the ±0.05 mm Tolerance

There’s a world of difference between suggestions and requirements when it comes to structural engineering, and dimensional accuracy is required, not suggested. For instance, in bridges, the exact thickness of the sections in the vicinity of slip-critical bolted connections and welds can dictate whether the joint will carry a certain load and whether the part of the joint that experiences the most cyclical loading can safely exist.

The Danger of Mechanical Methods:
The common practice of using mechanical grinders, needle scalers, or steel grit to de-rust bridges is causing microscopic to macroscopic thinning of the underlying healthy steel. This thinning of the steel occurs over the course of many maintenance cycles and serves to gradually move the bridge components outside of their design tolerances, and in so doing, reduces the safety factor of the structure.

The Laser Advantage:
Laser cleaning is a non-contact method, relying on the self-limiting reflection principle, and thus retains the exact contours of  the steel structure Surface. When run to optimal performance, a trained laser cleaner will operate within a dimension of no more than ±0.05 mm.

Our magna-blasting process removes wüstite and hematite (rust layers) down to the absolute base metal without the need for etching, gouging, or compressing the healthy steel lattice. This provides reassurance to DOT inspectors and structural engineers that the bridge has not lost any of its specified thickness and still maintains its original design load capacity.

Surface Profiling and Coating Adhesion

We get asked a lot by painting contractors whether our laser will damage the steel and if the pattern will provide enough anchor surface for the paint to hold on to.

There is much confusion surrounding the effect of laser cleaning on structural steel surfaces and, in particular, whether or not the laser will create an adequate anchor profile. A laser will not create an anchor profile similar to grit blasting, which will forcefully key the surface with peaks and troughs. The laser simply highlights and removes rust and corrosion naturally lying on the surface of the steel.

However, for bridge maintenance, this is actually a significant advantage:

The steel on an existing structure (in this case, a bridge) had already been profiled (or blasted) prior to the first paint layer being applied.

The laser burns away the old paint and rust that have accumulated in the valleys of the original surface profile.

The result is a perfectly clean, deep, original anchor profile ready for coating with a new layer of zinc-rich epoxy or polyurethane.

Another key benefit of laser cleaning is the removal of molecular moisture and chlorides from the steel surface, thereby reducing the risk of “flash rusting” that can be a major concern for contractors operating in high humidity or coastal environments.

Equipment Selection for Heavy Infrastructure

Bridges and other structures have to last for decades and can be impacted by a wide variety of corrosive elements in the environment in which they are located. Using a 50W handheld gun suitable for restoration of automobiles won’t do the job in the heavy civil construction industry. The selection of tools required a lot of thought in terms of production rate and film build.

High-Power Pulsed Lasers (500W to 2000W)
The strict ±0.05 mm tolerance on modern bridge girders, as well as the requirement for zero thermal distortion, are both met with high-powered pulsed lasers.

How they work: These types of devices deliver a huge burst of energy in nanoseconds, which instantly vaporises the rust. The heat from the arc does not have time to actually transfer into the structural steel beam.

Removal of rust on structures or other metallic surfaces can be done quickly and effectively with lasers. An average 1000W pulsed laser can strip standard atmospheric corrosion at rates of over 5 to 8 square metres per hour, leaving a surface that can withstand the most stringent adhesion test.

Continuous Wave (CW) Lasers (1500W to 3000W)
CW lasers produce a continuous beam of light and are also less expensive than high-power pulsed lasers.

Application: The Operational capability of this product is very good and can be applied for the removal of thick, heavy marine rust and scale from thick structural steel.

The Trade-off The continuous heat source requires careful operation to prevent local heating of the steel. They are best used on large structural sections where the mass of the steel can act as a heat sink and prevent excessive heating of thin web girder sections.

The majority of bridges today use a hybrid approach, using high-power pulsed units for precision joints, bolt clusters, and focused areas of load, while using continuous wave systems for broad, thick, and flat structural surfaces.

Environmental Compliance and Safety

Bridges have to be maintained regularly. Over the years, the regulations have become more stringent, and the financial penalties that can be imposed for non-compliance are becoming a major concern. EPA and OSHA fines for excessive heavy metals in soil or for silica in the air can severely impact the budget of a bridge maintenance project.

Laser cleaning structural steel drastically simplifies site compliance:

Zero Media Containment: There is no grit, so no dust is produced. What is released is vapor rust and paint.

Fume Extraction The vapor is then removed directly at the point of origin, using a fume extraction system (a heavy-duty HEPA vacuum located at the laser nozzle). Normally, a full day’s production of vapor can be contained in a single filter, rather than being poured into hazardous waste trucks by a whole fleet of vehicles.

The Operator Safety benefit is that the user does not have to wear supplied-air respirators or heavy blast suits. Instead, they just need to don the laser-safety eyewear, a basic respirator mask, and normal work clothing. This gives them a lot less fatigue and eliminates the threat of silicosis.

Acoustic footprint: The noise produced from grit blasting is typically so loud that it infringes on local noise restrictions and working hours in towns. Laser cleaning produces significantly less noise, and in most cases, it is possible to work around the clock, even in the most noise-sensitive areas of a residential district.

The True ROI (Return on Investment)

Although the initial capital cost for a high-power system is naturally much higher, the capital comparison does not end there because such a system is costed against a relatively low capital cost standard diesel air compressor and blast pot. In other words, the Capital Expenditure (CapEx) cost savings or reduction lead to a very compelling economic argument for considering lasers in terms of operational expenditure (OpEx).

When calculating the ROI for a bridge maintenance project, project managers must factor in the eliminated costs:

Consumables: The price for abrasive grit is £0 / $0. The laser is powered by electricity.

Waste Disposal: The amount of hazardous waste disposed of is significantly decreased up to 98%, lowering disposal costs from thousands of dollars to almost nothing.

Containment: Some of the stringent requirements for negative pressure containment for Class 1 projects have been eliminated or simplified. Weeks of scheduling and tens of thousands of dollars of capital have been saved as a result.

Setup and Teardown: Our laser system is very quick to deploy. One can roll it out of the van, hook it up to a genset, and be up and running and cleaning in as little as 15 minutes.

Multiple annual municipal projects per year translates into a payback period for the 1000W pulsed laser system of less than 14 months, based solely on eliminating courier costs of media and environmental handling costs.

Real-World Implementation on the Job Site

Laser processing of a bridge requires very little site preparation. The processing unit is made up of a laser source and a chiller. This unit is mounted to a fiber optic delivery cable that can be anywhere from 10 to 50 metres long. This cable allows for easy movement around the scaffold and allows the operator to reach all areas.

Key operational considerations include:

Power Supply: High-power systems often require 3-phase power, and this is readily provided by any standard towable site generator.

Access: The lightweight of the handheld optical head (usually under 2kg) makes access to difficult-to-reach areas on bridges, including individual nodes, gusset plates, and even rivet clusters, much simpler than using a high-recoil blast hose.

In automation applications, the laser head can be mounted on a magnetic crawler, which is often used for large, flat surfaces such as bridge decks. The crawler then self-propels along the steel beam at a constant speed, allowing for a uniform speed and overlap, and hence higher production rates. This also enables the operator to be on “lookout” and do little to no physical work.

Conclusion

Moving from abrasive blasting to laser cleaning structural steel is not simply a matter of upgrading. It is a paradigm shift that dramatically changes the economics of a project. The ability to remove metal and produce a metallurgically clean surface, within the tight ±0.05 mm geometric tolerances required to ensure the long term safety of the structure, has been demonstrated as being possible with high-power lasers.

Perhaps most importantly, laser cleaning reduces maintenance downtime and eliminates operational expense associated with the use of abrasives, containment of chemicals, and disposal of hazardous waste. As civil structures get older, regulations pertaining to structure cleanliness become more stringent, and laser cleaning offers a fast and environmentally friendly solution for civil contractors.

References

ISO 8501-1 defines a method for visually assessing the cleanliness of steel surfaces before they are coated or treated with paints or products.

SSPC-SP 10 / NACE No. 2: Near-White Metal Blast Cleaning standard (Benchmark for achieving equivalent surface preparation result with laser cleaning).

The American Institute of Steel Construction (AISC) has issued a technical bulletin discussing tolerances for slip-critical connections in the AISC Specification for Structural Steel Buildings.

The OSHA Standard 1926.1153 protects Construction workers from silicosis and lung cancer by limiting exposure to respirable Crystalline Silica. By eliminating abrasive processes with non-abrasive alternatives, the additional regulatory burden can be avoided, as abrasive processes must have air monitors present to track silica levels if actions exceed the permissible exposure limit.

Frequently Asked Questions (FAQs)

Does laser cleaning structural steel remove the original mill scale from new steel?

1. No, laser cleaning does not remove the original mill scale from new steel. The mill scale on new steel is strongly bonded to the steel surface and cannot be removed by laser cleaning. However, mill scale on existing steel can be removed effectively.
   Yes. High-power lasers can be very effective in blasting off mill scale. The mill scale is an iron oxide layer on the steel surface (wüstite, magnetite, hematite) and is very absorptive for the near-infrared laser wavelengths used. The iron oxide will be vaporized very quickly and thus strips off the mill scale without affecting the dimensions of the carbon steel surface.
2. Can the laser remove thick, multi-layered epoxy paints on the surface of old structures like bridges?
   Yes, but coating performance will vary depending on the composition of the coating. Darker, older paint finishes absorb the laser energy very effectively and are easily stripped away. Extremely thick elastomeric coatings and heavily rubberized paints may require a laser stripping process to strip them down layer by layer. In a few unusual applications, the contractor may be using a combination approach to remove very thick rubberized paint, where the majority is physically removed using surface preparation grinding wheels and scrapers, and then using the laser for the finer cleaning out of the primer and rust at the surface of the substrate.
3. Yes. Is the equipment safe to use in wet or highly humid coastal environments?
   The laser beam can tolerate relatively high humidity and actually helps to drive out moisture from the steel pores, but the base equipment needs protection in the same way that you would protect a high-end industrial welding power supply from the rain. But since this process does not involve any water, there is no need to contend with the hassle of removing and cleaning out the slurry left behind from UHP water jetting.
4. How does the production rate of a rust cleaner laser compare to a standard grit blasting nozzle?
   A single operator with a large abrasive blast nozzle can cover an area faster than a single laser operator on flat, easily accessible surfaces. However, when you factor in containment setup and take down, shovelling out abrasive grit and the need to shut off and restart the system for visibility, the net daily production of the laser system is comparable to, and often greater than, the conventional processes – particularly in the more complex surface features such as gusset plates and bolt clusters.
5. Will the heat from a 2000W laser warp the bridge girders?
   No, if appropriate laser technology is used. In a Pulsed Laser system, the energy is delivered in the form of nanosecond pulses which remove the rust in such a way that the heat is not conducted sufficiently into the steel to cause any distortion or damage to the material, this means that the substrate remains within critical temperature tolerances and does not distort or suffer any metallurgical changes and therefore the structural tolerance is maintained to within ±0.05 mm.