Building with formed concrete: How good is ‘good’?
The problems became obvious when the forms came off the 70-year-old public structure undergoing renovation. It was a beautiful, high-profile, art-deco-style structure made of cast-in-place concrete. But there were rock pockets, sand streaks, and form offsets marring the appearance of the addition intended to match it. Furthermore, the texture was inconsistent, ranging from what the owner called “barely acceptable” to worse. Combined with variations in the color and alignment of the concrete at joints and corners, these flaws left the building owners shaking their heads—and the contractors with an expensive dispute on their hands.
While exposed cast-in-place concrete is a flexible, cost-effective, and popular construction method, it comes with a unique set of challenges. The most apparent of these is achieving the desired finish, especially one that will be exposed to public view. In this case, the finish was unacceptable to the building's owners, so the prime contractor and concrete subcontractor were beaten around the head and shoulders regarding improper construction tolerances and the unsightly formed finish and texture. The owners insisted that the remodeled structure retain the attractive appearance of the original and not be marred by cosmetic inconsistencies.
The contractor agreed to repair much of the larger problem but argued that some degree of blemishes and surface irregularities was acceptable under the contract and by various accepted standards of workmanship. This was concrete, after all. The owner disagreed; each side geared up for battle, and the subcontractor hired me to consult, presuming future litigation and the need for an “expert witness.”
It turned out that the fundamental problem was not in the concrete. It was in the abstract and, at times, vague concrete specifications (as outlined in Section 3300 of the Standard Construction Specifications) defining the finished product. “Acceptable” and “unacceptable” were not unambiguously defined. Worse, the fixes mandated in the specifications were not allowed by the powers enforcing the contract!
Similar challenges arose with a 20-plus-story high-rise and a healthcare facility. Both featured cast-in-place concrete using a variety of form types and styles, including single- and multiple-use metal and wood products. To ward off litigation, I was retained to foster a compromise by discussing ranges of “reasonable remediation” based on the contract documents. Contract documents are the sum total of all provisions of the written construction agreement; they contain the contract terms, incorporate the blueprints or plans, elevations, sections, and details; and reference (hopefully) appropriate standardized construction specifications for the project chosen by the designer, usually including a list of select consensus documents by independent standard writing organizations like ASTM and ACI. The specifications usually include many references to industry written standards and other applicable criteria the design professionals believe are important to the project's success.
Many times, the situation is that more reinforcing steel is stipulated for today's concrete members so the structures can withstand extraordinary lateral loads, especially in zones associated with high seismic activity or powerful wind loading. Compounding this is the desire to maximize useable floor space by limiting column dimensions and lengthening spans. The result: More rebar for column and beam members.
Design teams and builders alike are challenged when constructing such concrete structures, which are common in healthcare. More steel in a smaller space means less room for the concrete—it doesn't flow well around the congested steel, especially at connections, laps, and between bars and form surfaces. The rebar cages and the embeds in columns and beams (among other areas) often resemble bird cages, but canaries can hardly pass through these narrow spaces, let alone coarse aggregate (figure 1). Not only does rebar congestion increase the cost and difficulty of the job, but it also may lead to problems down the road. Usually, structural deficiencies are immediately obvious; e.g., large rock pockets. These must be fixed, and it is best to repair them while the concrete is young. Rebar congestion also increases the likelihood of variations in surface texture, excessive form or line offsets, blemishes, and poor color uniformity. These require good cosmetic repairs.
Rebar congestion can greatly restrict the mobility of concrete around the rebar and into the forms, leading to poor consolidation, especially at column to beam connections such as this one. Note the blue writing pen for scale
Larger aggregate sizes, insufficient or variable concrete workability, rebar congestion, and improper vibration-control practices are all precursors to possible surface defects. Greater lengths, heights, and depths of expected concrete travel during placement may also signal the potential for surface defects. One possible solution: In addition to ensuring sufficient distance between the bars, make sure the workability (known as “slump”) of the concrete mix itself is beneficial. Another solution might be using smaller maximum-size aggregate particles. A variety of concrete additives can help, as well. These practices can accommodate rebar congestion and provide better cohesion to reduce the segregation potential arising when greater concrete fall and lift heights are contemplated.
Unfortunately, even these steps won't always solve the problems, and once those forms come off, all too often the gloves come off, as well.
The question is: How good is “good”? How much variation in the surface appearance is acceptable? If the answer is not spelled out in the concrete specifications, disputes and litigation may arise. That's when folks like me are called in to help the parties resolve the situation—or to explain it to the owners' lawyers and the judge. To avoid this headache and expense, it behooves all those involved to address these potential problems well before the first form is ordered.
If expectations are clearly defined prior to bidding, the concrete subcontractor can make a fair assessment of how difficult it will be to meet a particular visual standard. If exceptional results are desired and correctly specified, the contractor should demand a higher price, and the owner should expect to pay more. Laying a good foundation in your specs and following the appropriate construction practices will help to avoid disputes and litigation once the forms are stripped away.
In short, most projects could use a spec checkup.
Many disputes over the appearance of concrete surfaces would not exist, or would be less severe, if the desired level of finish quality had been better defined at the outset. In the case of the high-profile structure mentioned earlier, the contractor was accused of providing concrete that was not uniform in color or texture. In addition, form leakage was evident, leading to regions of exposed sand around corners and other form joints. These problems and others—including bug holes, rock pockets, voids, honeycombed areas, and poor alignment—occur, to some extent, in all cast-in-place concrete.
For the contractor in our story, a common remedy was mandated by the specifications—a fix called “sacking” or “sack rubbing”—a process in which the visual defects or blemishes on a concrete surface are removed by applying a mixture of sand and cement to the moistened surface and rubbing it with a coarse material, such as burlap. However, the owner's agent refused to allow this. He arbitrarily demanded removal and replacement, prohibiting the contractor from doing anything else. This approach ignored the fact that, as mentioned earlier, cosmetic repairs are easier and achieve a better result when concrete is young; therefore, specs should always include a provision for early sacking.
Sacking blends a repair with the original texture and color. Many materials and methods are acceptable for most specifications, and some project specs make a low-tech requirement for sacking material. For example, “Use two or three parts sand to one part cement,” or “Use material proportions similar to those of the specified concrete but excluding the coarse material, and add white cement sufficient to match color.” Other specs specify use of a certain manufacturer's prepackaged repair materials.
Owners, architects, and engineers must realize that concrete construction is frequently done on a fast track. This can lead to a greater possibility for rebar congestion—more specifically, for insufficient room for the concrete to flow and encase the rebar and flow to the forms and produce a smooth finish. The structural engineer's final design and rebar shop drawings identify the amount of steel, but occasionally no one has determined whether there is sufficient space for all of the steel and concrete as designed, especially at column-beam connections and where laps occur. Because of the varying costs for different finishes and the potential for steel congestion complicating the work, the owner should ask the design team:
In short, how good is “good,” if your final product is formed concrete?
Mr. Contractor, you should ask whether the contract price is reasonable in terms of what you, as a competent tradesman, can accomplish. For the concrete subcontractor, overcoming constructability concerns is paramount to a successful project; these include prevailing over rebar congestion and ensuring form adequacy. Proper planning for concrete pumpability, flowability, consolidation, and placeability can prevent problems, especially of finish and alignment.
In a typical cast-in-place project, specifications should cite, at minimum, several American Concrete Institute (ACI) standards, either wholly or in part. These include direct references to ACI 117, “Standard Tolerances for Concrete Construction and Materials,” and ACI 301, “Specifications for Structural Concrete for Buildings,” as well as extensive excerpts from ACI 347, “Guide to Formwork for Concrete.” ACI 116R, “Cement and Concrete Terminology,” is beneficial to anyone interested in the strange vocabulary of this trade.
ACI 347 and ACI 117 include requirements for Classes A through D surfaces and form offset amounts: A—“For surfaces prominently exposed to public view where appearance is of special importance”; B—“Coarse-textured concrete-formed surfaces intended to receive plaster, stucco, or wainscoting”; C—“General standard for permanently exposed surfaces where other finishes are not specified”; and D—“Minimum quality surface where roughness is not objectionable, usually applied where surfaces will be concealed.” When the contract documents are silent about the requirements for surface quality, Class C is the assigned mandate; Class C permits larger form offsets and greater amounts and sizes of irregularities, bug holes, and blemishes.
For larger or more critical work, the contract can dictate that mock-ups are made so that the surface quality can be memorialized as a contract requirement (figure 2). Mock-ups establish an agreed-upon comparison for reference, leading to acceptance or rejection of the work. Ideally, mock-ups should be composed of section sizes, reinforcing, ties, embeds, formwork, concrete mixtures, multiple panels, rustication strips, chamfers, line, edge and corner quality, etc., as well as feature finishes envisioned by the designer. The contractor should vigorously fight the temptation to use techniques different from those shown in the mock-up, and strive to achieve a higher standard of work, since the mock-up, when accepted for color, texture, offsets, etc., becomes the mutually accepted standard by which his future work will be judged, costs established, and payment made.
The construction of full scale mock-up samples can establish the finish quality the owner desires and for which the contactor should strive
Today, as I've mentioned, more rebar is required in concrete members for the greater strength needed to resist lateral loads. But we're still trying to make the members as small as possible. For constructability, the design team must make sure there is sufficient space between reinforcing bars. The team needs to verify that specifications mandate that the concrete be workable around and through all steel and within the forms. Finally, they need to verify that the forms are capable of supporting and containing the concrete mixture, preventing leakage, and imparting the desired surface, particularly for work exposed to public view.
Sometimes concrete doesn't come out looking perfect, uniform, and unblemished, yet it may be acceptable depending on your project, the owner's needs, and the project's specs. By doing one's homework—doing a “spec checkup” and going into the job with well-prepared contract documents (including plans and proper specifications)— you can hope for a meeting of the minds between the parties regarding concrete finish quality. Thus armed, you can ward off disputes during construction, reduce contract extras and withheld retentions, and avoid possible legal spats, and everyone will be happier in the end.
Well, everyone, that is, except the lawyers. HD