How to Achieve Quality Corners and Edges in Precast Concrete Products

By Claude Goguen, P.E., LEED AP

Precast concrete manufacturers strive to achieve the best finish on all structures, whether products are destined for a downtown building veneer or buried in a backyard.

Concrete surface imperfections not only can be unsightly, but they also can affect the structure’s performance and durability.

Achieving a good surface finish can be especially challenging at the bottoms or corners of precast products, where the formwork changes direction and may be discontinuous.

ROOTS OF ROUGH EDGES

Concrete is a homogenous composite material containing water, cementitious materials, admixtures and fine and coarse aggregates. Placing this mass of fluids and solids into formwork and achieving a smooth finish is both an art and a science.

The fresh concrete’s final position invariably includes narrowing areas of formwork where the concrete meets adjoining right angles at formed surfaces. This could be the bottom of a manhole where the pallet meets the outside jacket or the vertical outside corner of a catch basin.

Also susceptible to imperfections are exposed or unformed precast product edges such as the top of a structure where the horizontal finished surface meets the vertical formed surface.

Let’s start with edges that are confined on multiple sides by formwork.

• Leaky Forms

The No. 1 culprit of rough edges on precast concrete structures are slight openings in form joints that allow the passage of paste, mortar or concrete out of the form.

Paste is the mixture of cementitious materials, admixtures and water. As a liquid, it can escape through very narrow gaps in formwork, especially when under pressure. Mortar is paste plus sand. Because mortar includes sand, it has a higher viscosity than paste, which means it needs a slightly larger gap to escape formwork.

Concrete is mortar plus coarse aggregates, which generally has a higher viscosity and contains larger constituent materials than mortar, so it needs a substantially larger gap to pass through.

The nature of the escaping paste, mortar or concrete can be revealed by examining the rough corner. For example, if it is a type of honeycombing where there mostly are large aggregates left, mortar may be leaking.

Gaps in formwork are caused by improper use, cleaning and maintenance. If two adjoining form surfaces are not cleaned properly, small chunks of mortar or concrete may be left behind on surfaces. The next time the form is used, a worker may need to use force to close a form clasp, which can damage the form, and thus begins the formation of a gap, which can continue to expand from there.
Sometimes, the formwork itself is not maintained, and the closing systems are not operating at full capacity, leaving a slight opening.

• Form Release Overapplication

When it comes to improperly applying form release agent, the No. 1 offense is overapplication.

Depending on the type of form release agent, this can lead to precast product surface issues that include bugholes and staining. Overapplying form release on a vertical or horizontal piece of formwork can further aggravate corners.

The solution comes down to simple geometry. Nozzles for sprayed-on form release agents spray in the shape of a cone or fan, so when workers arrive at a form’s corner and turn the sprayer, they hit the same surfaces in an attempt to apply form release agent on the corner (Figure 1). This results in overapplication.

Gravity also magnifies this issue at bottom edges. Overapplied form release will run down the form surface and puddle at the bottom corners. This can be detected and remedied during prepour inspections.

To compare thickness of release agent application in different locations on a single form, run a finger along vertical or horizontal form surfaces that have been sprayed, and notice the thickness of the release agent application.

Do the same in vertical or horizontal corners, and take note of the release agent thickness. If there is a noticeable difference and the corners have a heavier application, use this opportunity to train employees on proper release agent application and why it’s so important.

• Improper Consolidation

All concrete needs to consolidate. Proper consolidation is key to optimizing concrete’s density, strength and enhancing durability. Self-consolidating concrete (SCC) does this on its own, while conventional and dry cast concrete need help from internal and external vibration.

When an internal vibrator head enters concrete, its zone of influence depends on many factors, including the vibrator amplitude, head size and fresh concrete properties. Picture an invisible sphere surrounding the tip of that vibrator head. Any placed concrete that is missed by that sphere experiences a much different level of consolidation.

Form corners, both vertical and horizontal, are prime targets for missed areas of vibrator influence. For bottom corners, an operator may not be lowering the head far enough down into the form. For vertical corners, the operator may be overestimating the radius of influence and fail to insert the vibrator near the form corner one more time.

This issue also can occur with external vibrators affixed to the formwork’s outside. External vibrators also have a zone of influence that depends on vibrator amplitude, the manner in which it is attached to the formwork and fresh concrete properties. If not sized, installed, placed and used properly, a vibrators’ spheres of influence could miss some corners as well. Undervibrated concrete in form corners may exhibit an area of large irregular-shaped bug holes and honeycombing.

It is not just lack of or insufficient vibration that can cause rough corners. Overvibration also can cause serious issues. An operator could be using the wrong equipment or leave it inserted in the concrete too long. This can result in the heavier particles – in this case coarse aggregates – sinking toward the bottom of the form and pushing mortar and paste to the top.

The concrete finish associated with overvibration typically resembles honeycombing, where large aggregates are visible or exposed with an absence of paste surrounding them.

• It Could Be the Concrete

Concrete itself could be a causal factor in rough corner edges. Proper raw material proportions, batch sequencing and mixing ensure the fresh concrete moves homogeneously within the formwork as intended.

For example, insufficient mix water due to absorption by dry or dirty aggregates can lead to diminished workability that cannot be overcome by typical placing practices. SCC that has lost some stability because of longer-than-usual horizontal flow along a longer piece of formwork also can lead to hitches in corner finishes.

• It Could be the Forms or the Plant Environment

Placing concrete in hot or cold conditions can cause issues in how the concrete moves and sets, and this can result in edge finish issues.

When placing concrete in higher temperatures, concrete could experience a higher loss of slump or spread, and this affects its ability to be moved and consolidated. Colder temperatures could delay setting and bleed water migration, which can lead to premature finishing and potential issues at top or unformed edges of precast products.

Avoid placing fresh concrete against cold or hot form surfaces. This also can result in finish issues.

ACI 307, “Guide to Cold Weather Concreting,” recommends that form surface temperatures should differ no more than 10°F (5°C) greater or 15°F (8°C) less than that of the fresh concrete at the time of placing to avoid inconsistent setting, rapid moisture loss and plastic shrinkage cracking.

SOLUTIONS

There is light on the rough horizon for anyone struggling with these issues.

Imperfect precast product edges and corners easily can be fixed and become a thing of the past. The key is to diagnose the issue and address it at the source.

• Formwork

When two mating surfaces need to be watertight, there usually is a sealant or gasket material involved. Sealants are applied in buildings around outer doors and windows. Caulking is used around tubs and showers. Pipes are sealed together. Two surfaces meeting and expected to stop low viscosity fluid from escaping is a challenge, but we expect precast formwork to accomplish this feat.

Consider the following scenario: It is determined that the formwork is leaking, and paste, mortar or concrete is seeping out. The first thing to do is make sure everything is being used properly. Are engineered form panels installed and fastened correctly? Are form jackets bent or out of square/round, or are moving parts such as hinges and clasps loose?

Remember: Before attempting to fix formwork, contact the form manufacturer to determine best maintenance practices. Also be sure to review the plant’s form maintenance program.

The most important steps to avoiding leaky forms are proper cleaning, routine inspections and proper maintenance.

Garrett Hoffman, production supervisor at Creter Vault in Flemington, N.J., recalled when his crew was dealing with rough looking concrete edges. They investigated and found leaks in the forms.

“We set up a rotating schedule for cleaning and maintaining forms,” Hoffman said. “I can personally vouch for the difference it made in producing tighter edges. Taking the time to clean your forms allows you to check any welds or joints that have loosened up and settled over time. You can make necessary repairs to see where your leaks might be.”

Periodically inspecting clamps, latches and hinges is important. Train employees by demonstrating how to do inspections and how to properly clean forms. A noise that raises hairs on the backs of the necks of many plant managers and owners is hearing the distinct, resonating bang of a hammer hitting a form. Training plant personnel to use the right tools and procedures can prolong form life significantly.

Older forms may need to be upgraded or replaced to accommodate changes in manufacturing. For example, a form may work great for years when used with a conventional concrete mix. Then, after switching to SCC, the difference in fresh concrete properties can create a higher hydraulic pressure, which may be just enough to create form leaks where there were none before. All of a sudden, it is time to tighten bolts and clamps and perhaps make other modifications.

Greg Stratis, president of Shea Concrete Products in Amesbury, Mass., experienced this.

“We have found on older forms there just wasn’t enough clamps going up the corner especially when using SCC,” Stratis said. “Also, we have found that the way the clamps are attached to the form in that area, we needed to reinforce the doors in order to take the pressure off the clamps.”

Again, it always is a good idea to consult with the form manufacturer prior to making modifications.

Another option to resolve leaky form joints is to add material to seal the opening. Chamfers are an ideal way not only to seal the open gap but also to produce precast product corners that are harder to chip.

“One of the most successful methods I have experienced is by adding a 45-degree metal chamfer, which helps with leakage and produces a better-looking edge on the finished product,” said Joe Marin, quality control director at Western Precast Concrete Inc. in El Paso, Texas.

Jason Bartlett of H2 Precast in East Wenatchee, Wash., said they also use chamfers.

“It makes a nice-looking corner and stops the leaks, said Bartlett, the purchasing manager at H2. “Also, it helps on the bottom of the product when the forklifts are picking them up to stop the chipping.”

Frank Bowen, business development manager at Jarrett Concrete in Ashland City, Tenn., uses magnetic chamfers.

“Most of my problems are fixed with magnetic chamfers – one of my favorite quick-fix tools,” Bowen said. “We buy 10-foot strips of the steel magnetic chamfers and 8-foot strips of the urethane magnetic chamfers. The urethane magnetic chamfers allow for flexibility and can be used in round structures, which has its advantages for improved resistance to chipping and spalling from jobsite handling.”

Some use rubber gaskets and waterstops in joint corners to prevent leaking. These chamfers and gaskets may require additional cleaning and maintenance and periodic replacement, but it may constitute a better option in the short term.

More temporary fixes include running tape in the form corners and applying a thin layer of grease or a bead of silicone.

If it is time to look at new forms, consult the manufacturer about options to make the joints tighter. Some forms on the market can be purchased with factory-installed gaskets to ensure a tight seal. It may cost more up front but pay for itself in reduced labor and higher quality.

• Form Release Agent

It always is a good idea to review the type of form release agent used and explore its compatibility with different concrete mixes.

Heavy petroleum-based form release agents, for example, resist entrapped air migration more than lighter alternatives. Review the application equipment. Is the spray tip adequate for the intended application?

Remember that in colder weather, workers may have to change spray tips to achieve the same rate of coverage. Try applying form release with a rag or mop in form corners. The most important thing is to train employees on proper application. Visually demonstrate how much agent is sufficient.

• Consolidation

Check with vibrator manufacturers on proper use of equipment. Is the vibrator suited for this specific application? When internal vibrators are used in taller forms, have operators lower the head to the form bottom first before turning it on. Once they know where the bottom is, have them lift it 3-5 inches. At that point, grasp the vibrator shaft where it meets the top of the form or mark it as a reference point to know how deep to lower the head when concrete has been placed.

Train employees on proper vibration, demonstrating how to lower it under its own weight, and raise it a little slower (3-4 inches per second) than it is lowered.

Make sure external vibrators are positioned correctly. Ensure operators know when to turn them on as fresh concrete is being placed and how long to let them run.

• Concrete Mix and Placing Environment

Train personnel to recognize changes in fresh concrete properties and how this may affect final finish in corners.

“The slump is very important since it will dictate the flow and workability of the concrete,” Marin said. “Too dry causes bugholes and honeycomb. Too wet causes the materials to separate from each other and result in leakage or unconsolidated concrete.”

Also, keep close watch on concrete and ambient temperatures. Keep form temperatures as close to concrete temperatures as possible. During colder months, move forms inside the night before they are to be used so the form temperature can acclimate.

• Prepour and Postpour Inspections

The key to identifying and preventing edge and corner finish issues are the prepour and postpour inspections. During the prepour inspection, make sure to:

• Check for gaps at form joints. Check for hardened concrete debris left from a previous pour that might block full closure.
• Check form release application or accumulation in corners and bottom edges. Look for form release leaking out of forms.
• Check to make sure clasps and latches are fully closed and there are no loose parts.
• Remedy any of the instances listed above prior to pouring.

The postpour inspections are critical in identifying and diagnosing these edge issues. When a rough edge or corner is found, immediately:

• Mark it for repair or disposal if applicable.
• Photograph the imperfection – up close and farther back – for reference as to where on the structure it occurred. Use a coin or something else to show scale at close-ups.
• Identify what date the structure was cast.
• Identify what form the structure was cast in. If unknown, inspect all forms used that day. Ideally, paperwork accompanying the structure identifies a form number and orientation within that form that will drastically simplify the investigation. If known, locate the form and mark it for immediate inspection and repair, or remove it from production.

Closely examine the imperfection. Check for staining, bug holes including their size and shape, larger voids and honeycombing, presence of mortar around larger aggregates and proximity of those larger aggregates. (See Diagnosis Guide in Table 1)

No matter the end use, the precast product is a reflection of a commitment to quality. Ugly, rough corners on structures may lead some to believe that a facility is cutting corners on commitment to quality. This is why the \

NPCA Quality Control Manual requires that every piece of formwork is inspected on an annual basis. If the measurements or squareness are out of tolerance, the form must be taken out of production until it can be fixed.

The commitment to mitigating these imperfections can involve a lot of work but will more than pay for itself down the road.

Claude Goguen is director of outreach and technical education at NPCA.