By Kayla Hanson, P.E.
Editor’s Note: This is the fourth article in a year-long series that focuses on the details and more technical aspects of one common thing precast concrete producers do on a daily basis.
Imagine you’re restoring a classic car. You spent years carefully taking each component apart and tirelessly searching for authentic parts. You applied advice from countless hours of research, used tips from others who were embarking on similar projects and learned lessons from other gurus to create something you’re truly proud of. Although each part you selected is crucial to the overall finished product and its performance, all the time, effort and money spent on the project would be diminished unless you used the proper techniques, careful placement of each part and the right tool for each job from start to finish.
In any industry or job where end-product quality is key, every step of the production process requires significant attention to detail. This is true for classic car restoration, and especially true for precast concrete. During placement and finishing, excessive free-fall heights, improper chute angles or troweling too soon can all impact fresh and hardened concrete quality. ACI 304, “Guide for Measuring, Mixing, Transporting, and Placing Concrete,” outlines best practices to help maintain fresh concrete quality and consistency from the mixer to the finished product.
The homogeneity and consistency of fresh concrete must be maintained during its travels from the mixer, down the chute, into the transport bucket and around the plant. This includes travel into the formwork and around the reinforcement and embedded items during consolidation. Other characteristics like w/c, slump or slump flow, air content, temperature and unit weight must also be preserved throughout these steps.
Handle fresh concrete as little as possible
With so many steps involved in the placement process, it’s easy to see how each one could potentially threaten the concrete’s quality and consistency. Best practices must be followed. Fresh concrete should be handled as minimally as possible and transferred from one receptacle to another as few times as possible. Reducing the amount of handling can help maintain fresh concrete’s characteristics and lessen the likelihood of segregation.
Strategically placing forms near the mixing equipment is a good idea. Reducing the duration and distance the concrete bucket travels around the plant can also help preserve the homogeneity of the mix.
Find the right chute angle
Fresh concrete should flow freely and steadily out of the mixer and down the chute into the receptacle or bucket used to transport concrete around the plant. Adjust the chute’s angle to improve the concrete flow into the bucket.
Use transport bucket best practices
Do not apply form release agent inside the transport bucket to aid concrete discharge or to expedite cleaning later. Form oil in the bucket or on the chute into the fresh concrete and affect its performance.
Using bottom-dump buckets enables placement of concrete at the lowest practical slump while still allowing proper and sufficient consolidation. The bucket’s side slopes should be at least 60 degrees from the horizontal, and the walls of the bucket should be smooth and well-maintained so the bucket is free of concrete upon emptying. Be sure to clean the bucket thoroughly at the end of each shift.
Position the concrete transport bucket below the chute so the concrete is deposited into the bucket directly over the bucket’s gate opening. The bucket should be filled from the center and the concrete depth in the bucket should rise consistently. Concrete should not be deposited into the bucket off-center from the gate opening by flowing down the sides of the bucket or by hitting a wall of the bucket.
Concrete flow should start as soon as the discharge gate is opened. During discharge, ensure the gate opening is at least five times the maximum aggregate size in the mix being poured.
Deposit concrete as near to its final location as possible
Discharging concrete while the bucket is moving can cause concrete segregation. Concrete free-fall heights – whether from the mixer chute to the transport bucket or from the bucket to the form – should be limited to 6-8 feet; however, a shorter drop height is preferred. If greater drop heights are required due to the formwork or conditions in the plant, use a tube, tremie or chute attachment on the bucket to guide the concrete on a slope from the bucket to its final location in the form. Ensure the tube attachment is not bent or kinked, as coarse aggregate can get caught and separate from the rest of the mix.
Control the flow
With any form – but particularly those with curved faces, sloped sections and narrow walls – allow concrete to steadily fill the forms in a controlled manner. Open the bucket gates carefully to avoid excessive initial flow rates which can cause splashing and aggregate segregation from paste.
Fresh concrete should not be allowed to deflect off one form face to the other as it makes its way through the form or to the bottom of the form. There is an increased likelihood for segregation when flows of fresh concrete are redirected, disrupted by reinforcement or ricochet off form walls. Self-consolidating concrete should be deposited behind the leading edge of its flow, allowing it to freely spread and fill the form. Converging flows should be avoided as this can entrap air between the two flows.
Avoid moving concrete laterally
Moving conventional wet-cast concrete laterally and scraping it off or across surfaces can result in segregation. Pay careful attention to the positioning of the concrete bucket over the form during placement and discharge the concrete strategically to fill the form completely while preventing the need to scoop excess concrete from one area to deposit in another area. To avoid introducing contaminants into the mix, do not scoop spilled concrete into forms or back into the bucket.
A plant’s equipment selection should be appropriate for its intended use and based on its ability to efficiently and effectively complete the task at hand. Ensure the forms have received and passed their pre-pour inspections prior to batching to avoid unnecessary delays in placing the concrete.
Fresh concrete should have the lowest practical slump while still allowing for proper and sufficient consolidation. Finishing practices should be performed such that the fresh concrete is worked and manipulated as little as possible to achieve the desired result.
Select the right tool for the job
Precasters may first use a float on the unformed concrete surface to fill low spots and smooth out high spots. After floating, the concrete surfaces should then be troweled to achieve the desired finish.
Troweling densifies and seals the concrete surface, which emphasizes the need for troweling to take place only after the majority of the bleed water has evaporated.
Floating surfaces prior to troweling is not necessary and does require additional time; however it can make the troweling process faster and easier. Floats are like coarse, lower-grit sandpaper while trowels could be compared to higher-grit sandpaper used for final finishing touches.
- Wood trowels are usually the most economical option up-front. However, the absorptive nature of wood means they can absorb mix water, warp and must be replaced frequently. They also tend to drag the concrete, rather than glide over it, resulting in rougher finishes.
- Resin trowels are usually made of canvas laminated in resin. They tend to provide a slightly rough finished texture, although smoother than wooden trowels. Resin trowels are more durable than wooden trowels.
- Stainless steel trowels are typically recommended when using white cement or colored concrete, as they are unlikely to discolor the concrete.
- Standard steel trowels are strong, rigid and long-lasting. However, proper care is needed to prevent them from rusting.
- Blue steel trowels usually have a thinner blade than standard steel trowels, making them more flexible and ideal for unique jobs like manhole inverts.
- Magnesium trowels are strong and lightweight. They tend to offer the most consistent blade profile due to their manufacturing process, which helps achieve a level surface with greater ease. Magnesium trowels glide across the concrete surface easier as well, helping create a smoother finish with fewer passes.
- Aluminum trowels are more rigid and heavier than magnesium trowels, yet still provide smooth, consistent finishes.
Avoid overworking the concrete
Overworking or overmanipulating concrete during finishing can draw fines and water to the unformed concrete surface and can cause coarse aggregates to be pushed deeper below the surface. This can result in the concrete surface having a higher w/c than the rest of the product and could lead to drastically decreased strength and durability and increased permeability. Some signs to watch for after the concrete has cured are dusting or delamination, both of which point to an excessive w/c at the concrete surface. Proper care when finishing is especially critical to ensure the concrete at unformed surfaces remains consistent with the entire cast product. This is particularly important for unformed surfaces that will be exposed to aggressive environments, such as wastewater structures or paving slabs.
Timing is important, but can vary
ACI 304 recommends delaying each step in the finishing process as long as practical while still enabling the concrete to achieve the desired results. Although it may seem counterintuitive to wait to trowel concrete until it’s just begun to set, this can help improve the quality and durability of the final product. If an unformed concrete surface requires multiple steps in its finishing process (perhaps an initial troweled finish followed by a grooved or textured finish), allowing the concrete to achieve a certain level of stiffness prior to finishing can help prevent the coarse aggregates or other denser materials from settling, and causing excessive water to rise to the product surface.
However, waiting too long can be just as problematic as not waiting long enough. Allowing the concrete surface to set before performing the initial trowel will disrupt the concrete, causing the trowel to drag sticky concrete across the product surface.
What about defining initial and final set times?
Since concrete setting is a gradual process, ASTM International states that any definition of setting time must remain arbitrary. Additionally, even with minor variations in proportions from one mix to the next, each mix is defined by specific targets for fresh and hardened concrete characteristics. As a result, they can behave differently and require specific steps or precautions depending on the time of year.
Therefore, it’s important to be alert and observant, and adapt production practices and finishing timing to the specific needs of each mix and the given conditions of the day. Defining a fixed initial set time for all mixes and always using that set time before allowing initial troweling can result in challenges for each mix, particularly if set time is not adjusted at different times of the year. A particular initial set time that is accurate in July may need to be extended in December. Using general set times as a gauge or starting point can help fine-tune a more accurate window of finishability, which must be defined through experience with the mix design.
Manage bleed water
Bleeding in fresh concrete refers to the process where free water in the mix is pushed upward to the surface due to the natural settlement of heavier solid particles such as cement and aggregates. Some bleeding is normal but excessive bleeding that pools on the surface instead of evaporating is excessive and can be problematic.
Under no circumstances should bleed water be mixed into the concrete. Bleed water which doesn’t evaporate after an appropriate amount of time following placement should be manually removed prior to troweling. Mixing bleed water into the concrete will increase the w/c of the affected concrete and can decrease durability and strength while increasing permeability.
Is the fuzzy fiber texture OK?
When working with steel or synthetic fiber reinforcement, particularly larger macrofibers, the ends of some fibers may poke out of the unformed surfaces after finishing and remain slightly exposed after curing. This can give the appearance of a slightly rough or fuzzy texture to the concrete, but is unlikely to affect product performance.
To help prevent this rough texture, consider trowel finishing the surface as late as possible in the window of finishability. By that point, the fresh concrete will have stiffened to a point where the tips of the fibers are more likely to remain completely embedded after the pass of the trowel.
What about adding water to the concrete surface?
Water should never be added to the fresh, unformed concrete surface to help achieve a smoother finish or to speed up the finishing process. Adding water to fresh concrete surfaces to aid finishing will significantly increase the w/c of the concrete surface and reduce its quality, strength and durability.
Bring it into the plant
Classic car restoration is a multi-step process where care is taken at every step in order to arrive at a great finished product. Consistently achieving the end-product performance of concrete requires a high quality, well-proportioned mix design; proper batching and mixing procedures; careful transportation; attention to detail during placement; proper consolidation; and the right tools, timing and technique when finishing.
Take this opportunity to review placement and finishing best practices during your next toolbox talk and stress the importance of attention to detail in every step of the precast production process.
Kayla Hanson, P.E. is NPCA’s director of technical services.