How Admixtures Enabled a 9-Day Continuous Concrete Pour for 24 Slipform Silos

Bay State Milling, a fifth-generation family-owned company, needed a set of new slipform grain silos for their Tolleson, Arizona location. The scope called for 24 silos, each rising 140 feet in the air with a 21-foot diameter. McCormick Construction, an industrial contractor based in Minnesota, was hired to deliver this massive project using a continuous concrete pour spanning nine straight days around the clock. The success of the pour depended heavily on the concrete mix design and the strategic use of admixtures to maintain workability and set time in extreme desert conditions. Understanding how admixtures perform in large-volume continuous pours is essential for any contractor tackling slipform work, just as knowing Concrete 3 Day 7 Day and 28 Day strength requirements helps ensure long-term structural performance.

The Slipform Challenge: 24 Silos, 9 Days, No Interruptions

Slipform construction is one of the most demanding concrete placement methods. The formwork moves continuously upward as concrete is placed, which means the pour cannot stop until the structure reaches its full height. Any interruption risks cold joints, structural weaknesses, or even a complete failure of the slipform operation. For this project, McCormick Construction planned a nonstop pour lasting eight to nine days, operating 24 hours a day, to raise all 24 silos.

Project Overview and Specifications

The project required a carefully engineered mix that could flow easily through congested rebar while setting fast enough to support the concrete above it. Each bucket of concrete had to perform consistently across hundreds of deliveries over nine days. The ready-mix producer, CalPortland Company, brought extensive experience to the table. McCormick Construction had worked with CalPortland on a previous project in Washington and trusted their attention to detail.

Why Continuous Pour Matters in Slipform Work

In slipform construction, the forms are lifted incrementally by hydraulic jacks as the concrete gains enough strength to support itself. The concrete must achieve initial set within a narrow window: too fast, and it becomes unworkable before it is placed; too slow, and the green concrete cannot support the rising formwork or the weight of concrete above it.

For context on the scale of this project, at the time of construction the Guinness World Records recognized the longest continuous concrete pour at five days, four hours, and 11 minutes. The largest continuous concrete pour was recorded at 32,315.5 cubic meters for a dam project in India. The Bay State Milling silo project, with its nine-day schedule and more than 6,000 cubic yards of concrete, stands as a remarkable feat of coordination and mix engineering.

How Admixtures Solved Extreme-Weather Concreting in Arizona

The Tolleson, Arizona location presented a major environmental challenge. Ambient temperatures regularly exceeded 100 degrees Fahrenheit during the day. High heat accelerates the hydration reaction in Portland cement, which can cause the concrete to set prematurely in the truck or in the pump line before it reaches the forms. For a continuous pour spanning nine days, this risk was magnified because every load had to arrive at the forms with consistent workability regardless of whether it was placed at noon or midnight.

Managing Set Time in Extreme Heat

CalPortland Company performed extensive laboratory and field trials before the pour began. The goal was to determine the optimum mix design that provided:

• Controlled set time that could be adjusted throughout the day as temperatures fluctuated
• Slump retention sufficient to keep concrete workable through delivery and placement
• Temperature management to prevent thermal cracking and premature setting
• Pumpability through the slipform system with congested reinforcement

The trials tested dosage rates of each admixture to find the right balance for the specific cement, aggregate, and environmental conditions at the jobsite. As Lauro Rivas, P.E., CalPortland Director of Quality Control, explained, having pre-conditioned and tested aggregates was also critical for cooling the coarse aggregate and maintaining saturated surface-dry (SSD) moisture conditions throughout production.

Temperature Control Measures

To combat the high ambient temperatures, the project team implemented several cooling strategies:

1. Large quantities of chilled water were used in the batching process to lower the initial concrete temperature
2. Ice was added directly to the mix as a partial replacement for mixing water to provide additional cooling
3. Coarse aggregates were pre-conditioned and tested to reduce their temperature before entering the mixer
4. Admixture dosages were adjusted based on real-time temperature readings at the batch plant and jobsite

These measures ensured that every truckload of concrete arrived at the slipform at a consistent temperature and workability, regardless of whether it was batched during the heat of the day or the cool of the night. Maintaining this consistency was essential for a project where a single out-of-spec load could disrupt the entire continuous pour.

The Three Admixtures That Made the 9-Day Continuous Pour Possible

CalPortland selected three admixtures from GCP Applied Technologies for the Bay State Milling project: RECOVER hydration stabilizer, WRDA 64 water-reducing admixture, and DARACEM 55 mid-range water-reducing admixture. Each played a specific role in achieving the performance requirements of the slipform pour. When you compare this approach to standard practice, it is worth reviewing the Effect of Transit Time On Ready Mix Concrete, which shows how extended delivery times affect workability and slump retention in hot weather.

RECOVER Hydration Stabilizer

RECOVER is a chemical compound designed to stabilize the hydration of Portland cement concretes. In practical terms, it retards the initial set without compromising ultimate strength development. For the Bay State Milling project, this admixture was essential because the concrete had to remain workable from the batch plant through delivery, pumping, and into the slipform. In ambient temperatures exceeding 100 degrees Fahrenheit, the hydration reaction accelerates dramatically. RECOVER allowed the mix to retain slump for extended periods, giving the operations team flexibility to adjust delivery schedules without risking set-up in the truck or pump line.

WRDA 64 Water-Reducing Admixture

WRDA 64 improves concrete performance by making it more workable and easier to place and finish. It is a water-reducing admixture that allows the mix to achieve higher compressive and flexural strength at a given water-cement ratio. For the slipform application, where concrete must flow through dense rebar cages and fill narrow form spaces, workability was critical. WRDA 64 reduced the water demand of the mix while maintaining the slump needed for proper placement. This meant the 4,500 psi design strength could be achieved reliably across all 6,000+ cubic yards of concrete placed during the nine-day pour.

DARACEM 55 Mid-Range Water-Reducing Admixture

DARACEM 55 produces concrete with low water content and improved placement properties. It also imparts a slickness to the surface of the concrete, making it especially well-suited for slipform work. When forms slide upward, any roughness or friction at the concrete surface can cause tearing, surface defects, or binding of the formwork. The slickness imparted by DARACEM 55 reduced this friction, allowing the slipform to rise smoothly and continuously over the nine-day period. This admixture also contributed to the low water content needed to achieve early strength gain so that the green concrete could support the rising forms.

Understanding how these admixtures interact with reinforcement and embedments is important. For projects where embedded items cross the slipform path, contractors should study Embedments in Concrete and When It Is Used to plan for proper installation sequencing.

Quality Control and Collaboration Sustained the Continuous Pour

A continuous concrete pour of this magnitude does not succeed on mix design alone. The project required round-the-clock coordination between the contractor, ready-mix producer, admixture supplier, and quality control team. CalPortland stationed a quality control technician at the batch plant and another at the jobsite at all times. These technicians checked every load of concrete and made real-time adjustments to admixture dosages to maintain consistency.

Laboratory and Field Testing Protocol

The testing and quality assurance process included the following steps:

1. Pre-construction laboratory trials to establish baseline admixture dosages and set time curves for the specific cement and aggregate sources
2. Field trials at the jobsite to validate mix performance under actual ambient conditions, including temperature, humidity, and wind exposure
3. Continuous monitoring of concrete temperature, slump, and air content at the batch plant before each load was dispatched
4. Jobsite acceptance testing for every truckload, with adjustments communicated back to the plant for the next batch
5. Set time verification throughout the pour to ensure the slipform could advance at the planned rate without the concrete being too soft or too hard
6. Documentation of all test results and admixture dosage adjustments for quality records and troubleshooting

Paul Marsh, CalPortland Phoenix Ready-Mix Operations Manager, emphasized the team effort required to maintain consistency across a 24-hour, eight-to-nine-day continuous pour. Batch plant operators, drivers, and the quality control team worked together to ensure every load met specifications. For McCormick Construction, the admixtures provided a sense of control over the set time that gave the operations team confidence throughout the duration of the pour. Steve Swanson, Chief Operating Officer of McCormick Construction, described the results as stellar.

Contractors planning similar work on existing slabs or foundations should also understand the procedures for bonding new concrete to old. The Pour New Concrete Over Old Concrete Surface guide explains surface preparation, bonding agents, and joint detailing that apply when slipforms are erected on existing foundations.

Lessons for Contractors Planning Large Continuous Pours

The Bay State Milling silo project demonstrates several principles that apply to any large-volume continuous concrete placement:

• Engage the ready-mix producer and admixture supplier early in the planning process. Laboratory and field trials must be completed well before the pour date to establish baseline performance data.
• Design the mix for the worst-case environmental conditions, not the average. When ambient temperatures can exceed 100 degrees Fahrenheit, the mix must perform at the peak temperature for hours at a time.
• Station QC personnel at both the plant and the jobsite for the entire duration of the pour. Real-time communication between plant and field allows immediate adjustments when conditions change.
• Use multiple admixture types to address separate performance requirements. No single admixture can provide hydration control, water reduction, and surface lubrication simultaneously at the levels needed for a complex slipform project.
• Plan for temperature control from the aggregate stockpile through the delivery truck. Chilled water, ice, and pre-conditioned aggregates are not optional when placing large volumes of concrete in hot weather.

The combination of RECOVER hydration stabilizer, WRDA 64 water reducer, and DARACEM 55 mid-range water reducer allowed CalPortland to deliver a consistent, workable, and pumpable concrete mix across nine days of continuous operation. For McCormick Construction, this translated into smooth slipform advancement, no cold joints, and 24 silos completed to specification. The project stands as a case study in how the right admixture selection, thorough testing, and round-the-clock quality control can turn an extremely challenging continuous pour into a success.