By Eric Carleton, P.E.
Green Bay Packers Coach Vince Lombardi said, “Perfection is not attainable, but if we chase perfection, we can catch excellence.” Precast manufacturers try to make perfect products, but like most things, attaining complete perfection is elusive if not impossible. While the goal is to attain perfection, precasters recognize the economic and production realities that some product irregularities will occur. The goal is to mitigate imperfections while still maintaining tolerances and meeting owners’ expectations.
Formed surface irregularities include bugholes, cracking, fins, honeycombing, sand streaking, layering marks, cold jointing, surface roughness, efflorescence, staining and color continuity .
Whether these surface irregularities are an acceptable blemish, or an unacceptable defect requiring repair, it depends upon the severity of the issue and the owner’s expectation of the surface finish for that product. With vertical building projects, particularly architectural components, expectations are typically defined and qualified within the project specifications.
Consequently, it is typical for architectural precast components to require mock-up sections for inspection and evaluation by the owner/architect prior to casting. Additionally, general on-site blemish inspection and acceptance criteria must be established as described within ACI 303R-12, “Guide to Cast-in-Place Architectural Concrete Practice.” It states, “Architecturally acceptable concrete surfaces should be aesthetically compatible with minimal color and texture variations and surface defects when viewed at a distance of approximately 20 ft. (6 m) or more, as agreed upon by the architect, owner, and contractor, or as otherwise specified.”
However, for many underground precast concrete products such as water and wastewater or utility products, expectations of acceptable surface irregularity are not clearly described within contract documents or project specifications. This can sometimes lead to confusion on product acceptability. Concrete guide and code language can assist in describing surface finish criteria for concrete product applications within their intended purpose.
The goal should be to consistently manufacture products with the highest possible level of quality. This should include implementing continuous improvement processes to eliminate surface irregularities that can occur on precast concrete surfaces.
For formed concrete surfaces, many problems can be narrowed down to three practices: consolidation or vibration, form set up and maintenance, and form oil/release agent application.
There is little consensus on what is an acceptable criterion for a surface containing bugholes. One document, AASHTO R73, “Standard Practice for Evaluation of Precast Concrete Drainage Products,” evaluates bugholes for certain products and is used as a guide by some agencies. However, there is general consensus on the cause, which is the entrapment of small air and water pockets between the fresh concrete and form wall. Two factors are primarily responsible for the excessive presence of bugholes:
- Poor or improper concrete consolidation. Double check your vibrator settings and consolidation procedures and ensure all of the concrete is within the zone of influence. Watch NPCA’s Precast Learning Lab video, “Proper Consolidation,” to revisit proper consolidation procedures.
Excessive application of form release oil or agent. Thick application beyond the manufacturer’s recommendations can cause air and water to travel along the form wall during consolidation. Use the thinnest application that still provides the release function. Watch NPCA’s Precast Learning Lab video, “Form Oil Application,” to revisit form release application best practices.
Honeycombing is described as an area where greater than half of the coarse aggregate surface area is exposed and is not surrounded by mortar. Honeycombing is also linked to poor consolidation practices. However, poor concrete placement procedures, incorrect mixture proportions, or congested rebar sections can contribute to the phenomenon. Besides causing a poor concrete aesthetic, honeycombing can also pose a service risk due to reduced concrete strength, structural function and possible durability concerns. It is important to identify honeycombing characteristics, a repair remedy when appropriate and guidelines for when a repair remedy is not appropriate. A 2016 AASHTO document (R73-16), “Standard Practice for Evaluation of Precast Concrete Drainage Products,” provides guidance. It states, “Honeycomb or bleed out that extends to a depth greater than the size of the coarse aggregate and exposes reinforcing steel or causes concrete permeability beyond the project specification for leakage, which either occupies a single defect area greater than 4 percent or a cumulative area greater than 10 percent of the internal surface area of the product shall not be repairable.”
Shallow surface cracking typically does not appear readily along the formed surfaces of ordinary or SCC concrete. If this occurs in a precast product, it is typically on the unformed exposed surface of the product, which will be discussed in more detail in the next article.
Fins occur when fresh concrete squeezes out of ill-fitting form joints. Concrete fins can be small and thin and easily remedied, but thick fins may require special consideration for removal to ensure greater damage or spalling doesn’t occur. Forms need to be modified to improve tolerances to eliminate fins. In some cases, concrete fins within the bell or spigot area can interfere with a good joint connection and fins can come off during truck transit.
Concrete form bleed occurs with ill-fitting forms that have a gap at the pallet interface, jacket or core connections. This allows the concrete mix, primarily paste, to leak out. The result is an excess amount of cement slurry on the production floor and a bleed out section on the product. This area has an appearance similar to honeycombing in that it has exposed aggregate with missing cement paste. Typically, this occurs at the bottom of the form where the mix has been dropped through the form opening. Many form bleed areas, depending on the affected size and depth, will require some approved patching remedy. The long-term remedy is to seal the leaking areas prior to casting or correct the gap tolerance with maintenance on the problematic form parts.
Poor Form Fit
Forms must be checked for dimensional tolerance prior to initial use, annually thereafter and each time the form is prepared for casting. That means most precasters are measuring and recording the dimensions of a particular form daily – every time it’s used. This may seem excessive if finning or form bleeding aren’t occurring, however, even minor offsets in the stripped product dimensions could result in a finished product that is out of tolerance.
A sand streak propagates with water along the form wall, removing the cement paste and small fine aggregate and leaving behind elongated veins of exposed aggregate without cementitious paste. According to ACI 309R-19, “harsh, wet mixes that are deficient in cement and contain poorly graded aggregates may cause sand streaking and other problems.” Sand streaking can occur within attached form vibrators, which can create a pumping action that can draw air into the formwork at the joints and force a water stream upward.
Layering marks are a visual anomaly showing where one concrete pour was cast on another and two batches have combined. Typically, these marks will dissipate with a consistent deep vibration technique to better blend the two pours along the form wall. If there is no issue regarding the visual effect and an explanation is needed for interested parties that the imperfection is not a cold joint, a layering mark provides no ill effects to the precast concrete product.
Similar to a layering mark, a cold joint is where two separate concrete batches are poured to complete a casting for a concrete component. However, unlike a layering mark, the two concrete batches have not become homogeneous. Typically, there is some delay in casting the second lift of concrete atop the first. Consequently, the first section has had time to initiate its initial set. This can be very problematic for several reasons. A product with unanticipated cold joints is an indicator of poor production practices. Typically, precast concrete products showing cold joints are not suitable for the intended purpose and cannot be repaired without additional analysis, surface preparation and the owner’s approval.
Staining on formed surfaces is attributed to improper application of form release oil. If new wood forms are being used, pretreatments on the wood surface can leave a concrete stain. If the stains look rusty and the steel forms have been taken out of storage prior to usage, check the interior for rust spots and determine if the form surfaces need to be re-seasoned with the appropriate agent. If the rust staining is intermittent along the formed surface, it’s a best practice to check if the wire ties being used are being bent away from the formed surfaces and are not straight out near the end surfaces.
Concrete colorization is a unique specialty requiring intricacies beyond the scope of this article. However, one simple message provided within the technical literature regarding maintaining concrete color continuity is to use extra consistent production practices. The precaster must be very intentional to use the same mix design and not revising cement, aggregate, admixtures. Additionally, variations with precast product vibration, form stripping time and curing conditions can lead to reduced uniform color conditions on the concrete surface.
As described in ACI 309R-05, “The formed concrete finish should be observed when the form is stripped so that appropriate corrective measures can be expeditiously implemented.” This observation should be made by plant management or QC personnel who can act quickly to correct the next concrete pour and repair the product before it is transported to the yard.
Using ACI criteria and other industry best practices, if no finished formed surface requirements are included within the precast product standards, proactive precast concrete manufacturers can evaluate and designate an appropriate surface standard for their various concrete products. This evaluation and acceptance criteria along with an allowable repair procedure and final acceptance procedure should be included within the plant’s written quality systems manual. This can be shared with the design and inspection community to provide a more uniform and rational on-site evaluation for delivered precast concrete structures.
Regardless of the end use, a well-cast product, free of formed surface irregularities, provides an excellent representation of a quality operation.
Eric Carleton, P.E., is NPCA’s director of codes and standards. He is an ASTM Award of Merit recipient and currently serves as vice-chairman of ASTM C13, Concrete Pipe.