Urban metamorphosis is underway in Washington, D.C., and the new NoMa district nestled between Union Station and the U.S. Capitol is a hotbed of construction activity, including the new 560,000-square-foot Union Place mixed-use development project.
This 14-story building will include 525 residential apartments, 13,000 square feet of retail space, below-grade parking and a rooftop lounge with Capitol views. It will also feature 1,000 high-quality precast/prestressed concrete foundation piles produced by Atlantic Metrocast Inc. using an advanced self-consolidating concrete.
Fabrication in Maryland
The 20-foot-long, 14-inch-square foundation piles for the Union Place development were produced at Atlantic Metrocast’s PCI-certified facility in La Plata, Md., which is strategically located adjacent to an Aggregate Industries ready-mix concrete plant. The process for producing the precast, prestressed concrete piles started with a thorough cleaning of the formwork, spraying the casting bed with a very light mist of an oil-based release agent to prevent the bonding of concrete, setting the bulkheads to the specified lengths, and stringing the half-inch-diameter steel prestressing strands.
Using a hydraulic jack equipped with pressure gages, workers next applied an initial load to straighten the individual low-relaxation strands, tensioned each strand to achieve a total load of 32,000 pounds, and tied a reinforcing spiral into place per contract plans and specifications. Workers then poured concrete designed to achieve an 18-hour strength of 3,500 psi into the casting bed, and the top surface of the concrete was screeded with a straightedge and finished with a hand float.
The next morning, the production crew released the prestressing strands from their anchorages on the casting bed to transfer the tension as compression into the concrete pile poured the previous day. Workers next removed the concrete pile from the casting form using a Mi-Jack crane, performed a complete inspection, and repaired any surface defects by rubbing, patching bug holes, and dressing up the pile. The finished product was then placed in the designated storage area of the yard until the prestressed concrete foundation pile reached its specified 28-day strength of 6,000 psi before being loaded onto the truck for transport to the Union Place construction site.
Traditionally, Atlantic Metrocast has used a conventional concrete with a slump of 7 to 8 inches. But the workability of the conventional mix was challenging for this type of application, and it was time-consuming to achieve consolidation around all the highly congested reinforcement in the casting forms.
“On average, the conventional concrete would take five hours to pour for each 14-inch by 14-inch pile bed,” says Joe Merry, plant manager at Atlantic Metrocast. “In addition, we normally would see surface defects in the piles after they were stripped from the casting beds even after the considerable vibration to consolidate the concrete.” To meet customers’ high-quality standards, Atlantic Metrocast would have to spend, on average, eight hours of labor time in rubbing, patching bug holes, and dressing up each pile during the back-end finishing phase of production.
Precast concrete producers are continually searching for creative new approaches to work faster, reduce operating expenses and eliminate the production of off-spec products to achieve a competitive advantage. For Atlantic Metrocast, this new approach to improving precast production times and finished product quality was found in making a switch from using conventional concrete to an advanced self-consolidating concrete (SCC) technology.
Called Agilia, this custom-designed concrete product places more quickly, flows easily through congested reinforcement and provides superior non-segregation properties for greater structural integrity. Other advantages of using the LafargeHolcim SCC technology include increased strength, reduced production times and labor costs, higher-quality finished surfaces, and improved jobsite safety due to the elimination of vibration requirements.
Not Your Typical SCC
“What makes the LafargeHolcim SCC technology different from standard SCC is as much about following a rigorously defined process to custom-design an application-specific mix as is it about applying the scientific concepts of granular packing, chemistry and fluid mechanics,” says Jean-Phillipe Thierry, U.S. product technical performance manager for LafargeHolcim. “LafargeHolcim follows a consistent procedure worldwide for setting up a mix in the right proportions based on local raw materials characterization and targeted performance properties specific to the type of application. Quality is just not related to how many chemicals and water are in the mix but to how proportions are determined to obtain a more fluid mix and an ideal solution for a specific horizontal or vertical application.”
Optimizing Viscosity: Unlike most standard SCC mixes, viscosity is a key consideration in the development of an application-specific Agilia mix, since it plays a very important role in optimizing flow characteristics and workability, controlling surface defects in precast product and verifying desired performance properties.
To achieve a very high-end surface finish for walls or columns, for example, the viscosity needs to be low enough to ensure entrapped air will go out of the concrete on its own and not leave bubble defects at the surface. But the viscosity should not be too low, or large particles can drift down over time into the formwork and create other defects.
For structural concrete beams containing heavily congested reinforcement, concrete will take a lot of time to fill the form and will struggle to properly embed the rebar if the viscosity is too high. Therefore, the viscosity needs to be low enough so that the concrete flows around the reinforcement very easily and makes it easy to cast reinforced structures without having to move the pouring point and put good vibration in it.
“Every application has different viscosity requirements, and there is a window in viscosity that needs to be targeted to ensure optimal results,” says Thierry. “Because viscosity plays such an important role in our process, we developed new testing methods to ensure the concrete is at the proper viscosity and the mix is stable enough to provide the right performance. We also had to adjust existing test methods to ensure we are targeting the right properties and could measure them accurately.”
Maintaining the Spread: Strict control of the mix design technology enables optimal application of cement properties and allows Agilia to maintain its spread and workability for up to two hours. With standard SCC mixes, the spread starts to decline tremendously at one hour. The LafargeHolcim SCC technology maintains its flowability properties due to the selection process in place for the chemicals used in the mix.
Structural Integrity: To increase robustness, standard SCC mixes typically require adding a VMA admixture to control segregation. Agilia mixes do not use VMA admixtures, but rather control segregation through proper mix design. “We have spent many years in working on the key parameters and metrics that must be measured to ensure we are selecting the proper proportions to avoid segregation while giving the appropriate flow and viscosity,” said Thierry.
Developing the Optimal Mix
As discussed earlier, there is no single recipe for the LafargeHolcim SCC technology. Every mix is custom-designed for a specific application following a common defined procedure that relies on local raw material characterization and properties that must be targeted.
For the precast pile fabrication application at Atlantic Metrocast, the process for developing a tailor-made solution started with shipping local cementitious and aggregate materials to the LafargeHolcim Materials Performance Center in Romeoville, Ill., for assessment. Raw materials analyzed in the laboratories included cement from the Holcim Hagerstown, Md., plant, sand from the Aggregate Industries La Plata, Md., operations, and aggregate from the Aggregate Industries Millville Quarry in Harpers Ferry, W.Va.
Upon successful completion of the material assessments, the LafargeHolcim quality control team developed, tested and fine-tuned a wide range of mix recipes in the laboratory and in the field for meeting the targeted performance properties. To come up with the highest-quality mix design, extensive lab analysis was conducted for compressive strength, flowability, viscosity, air entrainment, permeability, set time, shrinkage and durability.
The specifications required an early strength of 3,500 psi at 18 hours for releasing the piles from the casting beds, 6,000 psi at seven days for shipping the piles to the Union Place construction site and a final design strength of 6,000 psi at 28 days. In addition to achieving these strength parameters, the SCC had to meet stringent performance criteria for flow through highly congested reinforcement, while adhering to durability requirements. Air content was specified at 3%-7%, and spread was targeted at 22-27 inches.
Upon successful completion of the mix design, which was approved by the general contractor’s engineering team, the LafargeHolcim quality control team started running trial batches at the Aggregate Industries La Plata Ready-Mix plant to fine-tune the performance of the Agilia. As a final step in the process, a demonstration trial was run in the precast forms at Atlantic Metrocast to ensure everything was as required as far as flowing through the heavily congested reinforcement and meeting all the performance requirements.
Quality Control at the Forefront
Training on proper quality control measures at a local level is an important part of the defined mix design process that LafargeHolcim follows with any application. At the La Plata plant, the training program included all personnel on the local project team—from QC and ready-mix production to dispatchers and delivery drivers—to ensure a high degree of consistency, reliability and quality control. Following strict quality control guidelines, the raw materials were mixed in precise proportions, and every batch of concrete was tested to ensure all performance requirements were met.
Each pile pour of 30 cubic yards of concrete was tested onsite for spread, air entrainment, temperature and compressive strength. “The biggest tip for working with SCC is to find the spread that works best for the product you are casting,” says Merry. “Although it is permissible to come in at the top end of a broad spread specification range, it doesn’t mean it is suitable for the product you are making. Keep in touch with the batch plant operators and do not hesitate to ask for adjustments. For this job, the 22- to 24-inch spread we were seeing was perfect—the product maintained excellent flowability characteristics and completely embedded the highly congested reinforcement in the form.”
In cold ambient temperatures, Atlantic Metrocast would cover the bed with insulated tarps prior to the pour, use a steam generator to heat the product and monitor the temperature with infrared thermometers to ensure the form temperature was at least 40° F. If required, Aggregate Industries also would add hot water to the mix during the winter to help control the concrete temperature.
Test cylinders were cast and tested in Atlantic Metrocast’s onsite laboratory to ensure the concrete achieved its required strength at 18 hours, seven days, and 28 days. “We achieved excellent results for all the breaks, and the actual strengths of 5,000 psi within 12 hours and 8,000 psi at 7 days were phenomenal,” says Merry.
For Atlantic Metrocast, the switch to the LafargeHolcim SCC technology was a tremendous success. “There was no need for vibration, and when we released the piles from the forms, the finish was nice and smooth, which was a huge benefit in terms of time and labor cost savings,” commented Merry. “Another significant benefit was the reduced pour times. Conventional concrete usually took us four to five hours to pour, and with the Agilia we cut that time in half to two to two and a half hours.”
All 1,000 precast concrete piles, which required 3,000 cubic yards of the LafargeHolcim SCC technology, are now complete and installed at the Union Place construction site. “The customized Agilia mix was just so much easier to work with than standard concrete, and the actual high early strengths we were attaining allowed us meet our production schedules,” says Merry. “It was a great solution to our production challenges that significantly lowered our labor costs by reducing average pour times by 50%, average finishing times by 90% and overall production times by 35% to 40%.”