Coating bridges requires precise execution and inspection
The contractor has to warranty his work anyway, right?
The reason is that certified independent inspection services can save hundreds or even thousands of times their cost by ensuring that the job is done right—which can translate into many years of extra life. Here’s an example of how inspection can save money.
A major bridge in the northeast was repainted at a cost that ran well into the millions. A year later, paint was debonding in large pieces and the contractor offered to perform a touchup at no additional charge. The next year, the problem was even worse so an independent inspector was finally brought in to survey the contractor’s work after-the-fact. The inspector determined that the 12 or so layers of paint that had been previously applied to the bridge were never removed as had been specified in the original contract and recommended that the entire bridge be blast cleaned and repainted again.
How can an independent inspector help avoid pitfalls like this one—admittedly an extreme case—as well as dozens of other potential problems that can subtract years or decades from the life of a bridge painting job—and moreover add significant maintenance costs over the years?
The inspector’s job starts long before painting begins by examining the existing coating and determining the type and thickness of existing coatings, presence of rust and mill scale and any other special conditions that will affect surface preparation and painting requirements. The inspector, as agent for the professional engineer, also meets with the contractor to take an inventory of their equipment, personnel, planned procedures and the coating itself. At this point, the inspector has the opportunity to voice any concerns and also make clear the specified standards that are to be met on the project such as in the containment system, quality of surface preparation, coating methods, etc. This meeting helps to make sure the contractor knows exactly what is expected to comply with the specified bridge painting approach.
Keep it contained
The job normally begins with surface preparation, and in today’s era of tough environmental regulations the containment system is critical. One of the first steps in the inspection process is typically sampling the existing paint to determine whether it contains lead or other harmful materials. The type of enclosure required depends on what method is being used to remove the paint and the original coating condition. For example, if scraping is all that is required, ground covering tarps may be sufficient while for blasting operations more extensive precautions, such as an enclosed structure with a negative pressure ventilation, is usually needed. Enclosures are generally made up of combinations of scaffolds, cover panels, supports, screens and tarps. A majority of state transportation departments reference the Steel Structured Painting Council Guide 6, which is called "Guide for Containing Debris Generated During Paint Removal Operations."
The independent inspector will carefully examine the containment system on a regular basis throughout the period during which it is used to make sure that it meets the requirements of the project. For example, Caltrans requires forced air ventilation to produce negative pressure to contain potential contaminants. The inspector will verify compliance by visually inspecting the flexible containment screens to be sure they have a concave shape indicative of negative pressure.
The Massachusetts DOT requires Class 1 containment for dry blasting and Class 3 containment for vacuum or wet blasting as defined in the guide mentioned above. The inspector will remain on the jobsite to ensure that the appropriate level of containment is provided at each stage in the job. The inspector also will ensure that soil samples are taken in the proper timeframe and, by ensuring a proper containment area, will eliminate or reduce the need for supplemental remediation after the job is completed.
Surface preparation is the most critical aspect of nearly every bridge painting job. It has been estimated that 75% to 80% of all premature coating failures are caused, at least in part, by deficient surface preparation. The presence of the inspector will ensure that the contractor does a thorough job while making reasonable allowances for areas that are difficult to access. The continual presence of an independent inspector brings a level of discipline and accountability that is absent when the only inspectors are provided by the contractor. It’s important to note that the lack of surface preparation won’t be noticeable until long after the contractor has finished the job, been paid and perhaps even gone out of business.
Blast cleaning is a difficult and expensive job and there is considerable room for varying interpretations of visual and written standards. The key is clearly communicating expectations up front, both in the project specifications and verbally, then remaining present on the jobsite to ensure that sound practices are followed at all times. Independent inspectors will typically work with a visual standard and may even post pictures of the type of surface that should be generated around the jobsite to ensure that everyone involved understands the requirement. The most common requirement is an SP-1 finish, which denotes nearly white blast cleaning. The surface should be free of oil, grease, dirt, rust, paint and any foreign matter leaving only slight stains from rust and mill scale. At least 95% of each 150 cubic centimeter area should be free of all visible residue with the remaining 5% limited to slight discoloration.
Roughing it up
Another important part of the surface preparation process is to provide the right level of surface roughness. A surface that has been over-blasted to the point that it’s glassy-smooth will provide poor paint adhesion. On the other hand, the surface can’t be too rough either or difficulties will be experienced in covering the surface. A typical organic zinc primer is designed to be applied in a 4 mm thickness coat. For that reason, surface peaks shouldn’t be any higher than 3.5 mm. The independent inspector will use a micrometer and replica inspection tape to measure surface roughness at regular intervals as blasting proceeds.
Prior to application, the inspector will check the coating materials to be sure they meet the specifications for the job and are within the usable shelf life. Some suppliers indicate shelf life with a special code and the inspector will make sure they can read it, contacting the manufacturer if necessary. The inspector also will track the paint’s pot life, the length of time on the product data sheet that it can safely be used after the manufacturer’s seal was broken. Exceeding the pot life can result in poor adhesion and sagging and is especially critical with two-component paint systems. The inspector will check the conditions under which the paint is being stored, for example, making sure it is not being kept in an unventilated trailer where it could become very hot.
A coat for perfect weather
Ensuring the paint is applied only in the right environmental conditions is critical to good adhesion and long life. Site conditions are continually changing during the day. The contractor is generally paid a fixed price for the job and wants to do get it done as soon as possible. Some contractors have been known to argue that any weather other than rain is acceptable. The independent inspector will ensure that painting takes place only under the right conditions, such as when the surface temperature is 50ûF or higher, relative humidity is below 85% and the surface temperature is at least 5ûF above the dew point. It’s important to note temperature should be measured at the surface of the bridge with a special magnetic thermometer, rather than simply taking an ambient reading. The independent inspector will make sure no one starts until conditions are right and will continually make readings to ensure that conditions have not deteriorated to the point that work should stop.
As the job progresses, the inspector will measure the thickness of each coat, typically using instruments that use magnetic and eddy current principles to measure coating thickness on both ferrous and nonferrous metals. Standard practice is to take three separate readings at each of five locations every 9 square meters. The average of the three readings should not be less than 80% nor greater than 150% of the specified thickness. While the focus is usually on avoiding thin spots, excessively thick application must be avoided because it can cause the coating to sag or not fully cure.
The bottom line is that every penny spent on inspection will typically save dollars or even tens of dollars in longer structural life. With coating costs continually rising due to new safety and environmental regulations, the economic benefits of independent inspection are rising every year. Many paint manufacturers recognize its importance by guaranteeing a higher life for their product if it is independently inspected.
Remember the old commercial where the auto mechanic says that you can "pay me now or pay me later?" Undergoing an expensive bridge painting job without scrutinizing day-to-day work is an open invitation to pay later.