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Testing Film Thickness

Q: In AWCI’s Technical Manual 12-B, Standard Practice for the Testing and Inspection of Field Applied Thin-Film Intumescent Fire-Resistive Materials; an Annotated Guide, the required frequency and location of testing of the film thickness on structural members is causing a heated discussion. Some argue that when testing a beam, the thickness must be tested in nine locations around the beam every 12 inches. Others claim that the instructions mean that the sample beam must be tested 12 inches from the end(s) of the member. What is the intended pattern? How many beams/members must be tested?

A: The wording of the instructions for testing the film thickness of Thin-Film Intumescent Fire-Resistive Materials on structural members in Technical Manual 12-B can indeed be a bit confusing, especially when read without having first read the earlier passages in the manual. A stand-alone reading of section (A) offers the following: “Beams. Thickness determinations shall be made at nine locations around the beam, at each end of a 12-in. (300-mm) length.” (What?!)

When I first encountered this instruction many years ago, I happened to be observing a fire test of a new formulation of an intumescent coating in a small-scale furnace conducted by one of our manufacturer members. After several failed attempts to explain this testing procedure to me (I first took it to mean one must measure near each end of the beam, 12 inches in), the technical sales rep wound up drawing a diagram that finally made it clear. (There are several diagrams in 12-B, but they are cross-sections of a beam, a column, a tube and a truss; the cross section of the beam includes arrows indicating the nine points around the beam where the film thickness of the TFIFRM is to be measured.) The magic drawing was a crude 3-dimensional image of an I-beam showing some of the nine points to be measured and then, 12 inches further along the beam, was another set of points to be tested. That’s it. A pair of measurements 12 inches apart on a given beam in no particular, random location along the beam—the tested area could be near an end or in the middle of the span.

So, one might wonder, how many beams (or other structural members) per test? That’s where reading the earlier material comes in handy. Section Testing Frequency, states the following: “Thickness determinations shall be made on structural elements on a random basis in at least one bay per floor, or for each 10,000 sq. ft. (930 sq. m) of floor but not to exceed one test per 10,000 sq. ft.” The section continues: “Thickness determinations shall be conducted on each of the following structural members present in each randomly selected bay: one column; one primary beam and one secondary beam; and one truss.”

The manual’s foreword offers some insight as to why the testing frequency might seem sparse to some: “The goal, as always, is to promote a higher level of quality and performance of thin-film intumescent fire-resistive materials, and to facilitate the accurate, timely and cost-effective inspection thereof.”

Q: We have a problem with stucco that was applied directly over concrete that is now losing adhesion. What could have caused this, and how do we avoid it recurring?

A: Portland cement plaster, often referred to as “stucco,” should bond adequately to a bare concrete substrate provided that there’s sufficient porosity or texture on the concrete surface and the concrete isn’t/wasn’t so dry as to absorb all the available moisture out of the portland cement plaster/stucco mix. In other words, the concrete can’t be too slick or too dry. If the concrete lacks sufficient “tooth” for the plaster to bond to, it can eventually fail.

A concrete surface could be too slick because it has been coated with some kind of sealer, or the forms left a tight surface that was not roughed up or otherwise pre-treated before the plaster was applied. The concrete could also have been considerably drier than the portland cement plaster when it was applied and absorbed too much of the moisture from the mix to ensure proper curing.

To ensure a concrete surface is clean and rough enough to receive a stucco finish, it may need to be sandblasted or mechanically abraded, a dash coat of the cement plaster mix can be applied before the finish coat, or metal lath can be attached using power-actuated fasteners (implying a two- or three-coat stucco system to follow).

The use of a bonding agent may also improve the bond of portland cement plaster applied over relatively slick concrete, but the literature warns that one should ensure that it is an exterior grade bonding agent because a cheaper interior grade of bonding agent may not provide adequate bond.

In lieu of using a bonding agent to ensure that the concrete does not absorb too much of the moisture out of the portland cement plaster, the concrete can be given a “saturated/surface dry” treatment in which the concrete is saturated with water and then the surface is allowed to dry. Relatively smooth concrete surfaces may only need a finish/texture coat of stucco; rougher or more uneven surfaces may first require an intermediate coat of the portland cement plaster that is between 1/4-inch and 1/2-inch thick. Each layer of portland cement plaster that is applied to the concrete should be damp-cured to prevent drying too quickly.

Lee G. Jones is AWCI’s director of technical services. Send your questions to [email protected], or call him directly at (703) 538.1611.

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