By Eric Carleton, P.E.
Concrete is one of the oldest and most widely used building materials in the world. Some of the earliest concrete structures were built by tradesmen in the Middle East more than 8,000 years ago. When constructing the underground concrete water cisterns, they had already learned that the key secret of quality concrete is to keep the water-cement ratio as low as possible. Today, precast manufacturers continue to rely on this secret to produce quality concrete. One such product that depends on a low w/c ratio is dry-cast concrete. A dry-cast mixture has just enough water to initiate cement hydration within the range of 0.30 to 0.38 without the use of admixtures. Below, several important facts regarding dry-cast concrete are outlined.
1. Increased production
One benefit of using dry cast is it can be consolidated within a form. The stiffness of the mix allows the form to be immediately stripped. The product will stand under its own weight to complete the curing process and the form can be returned to cast another product.
2. Reduced shrinkage and creep, rapid strength increase
In 1969, The Aberdeen Group published an article titled, “An Introduction to Dry-cast Concrete,” that introduced the process. All the concrete components are placed into the formwork dry, then compacted and lastly, infused water is added into the mix via capillary action to start hydration. Though this process of dry casting didn’t catch on commercially, the article described the reasons for the positive attributes of dry-cast concrete: “Both shrinkage and creep are reduced because all of the aggregate particles are in contact with each other, instead of being separated by the cement-water paste formed in the wet-mix process of producing concrete. The already compacted aggregate particles must of necessity resist the shrinkage of the cement, with the result that the particles are preloaded to increase the compressive strength of the concrete.”
Modern dry-cast concrete processes introduce water during mixing prior to vibration. The low amount of water and corresponding paste combined with consolidation from external vibration provides the same reduced separation of aggregates. Consequently, the same attributes described include a rapid increase in compressive strength. In some cases, dry-cast products can comply with specified material strength requirements in 24 hours.
3. Dry-cast concrete ≈ roller-compacting concrete
Although dry-cast concrete mix designs and applications have been used for decades within the precast concrete industry, the batching method is rarely discussed in concrete engineering classes. In fact, zero-slump concrete testing is not included in ACI Level 1 technician certification. With the expanded use of roller-compacted concrete for paved surfaces, dry-cast methods are now becoming more popular with engineers. Many of the same attributes associated with dry-cast concrete are present with RCC.
4. Vibration: not one size fits all
One of the primary keys to quality dry-cast concrete products is consolidating the mix with intensive external vibration, because the stiffness of the mix makes traditional internal vibration impossible. Dry-cast products need to be consolidated through a variety of methods. One is the use of centrifugal force and mechanically packing the mix into the outside form jacket with the use of spinning cylindrical heads. The method is similar to the packer-head pipe-making process. The other more conventional methods are via individual external vibrators attached to the product form or the entire form attached to a vibrating table. Both vibration processes are designed to consolidate the mix to the maximum density.
For optimal use, precast concrete producers need to be aware that the energy provided by vibration technology needs to be directed to the concrete mix. Additionally, different products may require different vibration settings to insure the consolidation energy is adequate for the product being cast. For example, a small short inlet section may use a vibration with high frequency but low amplitude, while a large 8-foot-tall manhole section may need a higher amplitude setting and lower frequency to evenly distribute the energy throughout the product mass.
5. A heavy-duty form, all day long
The need for heavy-duty forms is twofold. Due to the need to transfer intense external vibration energy to the mixture through the formwork, dry-cast forms need to be very rigid. This does not simply mean thicker skin, as increased jacket thickness can lead to dampening as a result of vibration energy. Similarly, simply adding structural steel stiffeners can also be detrimental. Adding more weight to a form reduces the effectiveness of the vibration methods employed.
Heavy-duty forms are typically used continuously throughout the day’s production. The primary benefit of using a dry-cast process is the ability to produce multiple precast concrete sections using a singleform system. Consequently, that single form is handled repeatedly throughout the day with setup, pouring, vibration, transportation to curing and setup areas and product stripping. A dry-cast form must be designed to handle all that use, yet not be so bulky to reduce the consolidation effectiveness. Proper form design is an engineering art optimizing the highest form stiffness and ruggedness with the lowest form weight.
6. Let a little out at the bottom
It is not always recognized or understood that most dry-cast products’ wall surfaces are not perfectly vertical. A slight taper of the product is required to accommodate immediate stripping. For traditional wet-cast precast operations, the concrete mix is cast and internally vibrated into the forms and cured overnight. The product is then hardened when the forms are removed.
Typically, to assist with stripping, the forms are hinged on the outside with latches to open and are collapsible on the inside. However, many dry-cast outside jackets or inside cores are fabricated as a single-steel section. Consequently, to physically pull out either the form from the cast product or the cast product from an anchored core or jacket, the form must include a slight taper. This reduces the continuous skin friction of the lift and the potential vacuum suction that would occur with vertical-facing forms. This taper is typically only 1/8 to 1/4 inch over the length of the wall and has minimal effect on product function. However, this small variation should be noted if the exact product perimeter or circumference is needed.
7. Leave the slump cone in the closet
When speaking to a precast manufacturing friend who lives in the Chicagoland area on this subject of dry-cast facts, he said, “Unlike the Cubs, dry-cast concrete doesn’t slump.”
Dry cast is known as zero-slump or negative-slump concrete because with typical mix designs, additional water could be added and the slump would still be zero if tested in accordance to ASTM C143, “Standard Test Method for Slump of Hydraulic-Cement Concrete.” In fact, the mix is so stiff that the tamping rod simply would leave a hole when extracted.
A true story regarding slump testing and dry-cast concrete occurred at a concrete pipe plant in the Midwest. During an unannounced inspection by the owner’s engineer, a slump test was requested. When the plant manager said they did not have a slump cone, the engineer lambasted him about the company being inept by not conducting daily slump testing. After accepting a little more abuse, the plant manager answered while pointing at the 2 full kilns of 24-inch-diameter-by-8-feet-tall sections of pipe made that day, “Look, I just did 300 slump tests today, if the mix is off, they all fall down!” The engineer learned something new about dry-cast concrete that day.
8. Leave the air tester
It is extremely difficult, if not impossible, to use air entrainment admixtures to produce a controlled air content of a finished dry-cast product. It is believed the high-intensity vibration and consolidation of the mix collapses the entrained air bubbles. However, precast concrete products that use dry-cast processes have shown complete resilience to freeze-thaw degradation, which can occur with mixes of higher w/c ratios. Thousands of precast concrete pipes and box culverts project out of roadway embankments in highways and are exposed to severe freeze-thaw environments, yet they hold up to abuse. Canadian testing company, Service d ’Expertise en Materiaux Inc. confirmed this through formal research. The study conducted an extensive literature review that showed properly made dry-cast concrete resisted frost-induced cracking. Additionally, dry-cast pipe and box culvert products proved to meet the requirements of ASTM C1262-97, “Standard Test Method for Freeze-Thaw Durability of Manufactured Concrete Masonry Units and Related Products.”
9. Running on all cylinders
Though dry cast is not conducive to testing for slump or air, a producer still needs to make and test concrete cylinders for testing in accordance to the product specification. However, the method of fabrication for these cylinders varies greatly from the traditional wet-cast concrete mix means to make a test cylinder described in ASTM C33, “Standard Practice for Making and Curing Concrete Test Specimens in the Field.” It is important to know these differences. More information can be found in the July-Aug. 2014 Precast Inc. article “Compress for Success.”
10. Production Savings & Speed of Fabrication
If the market for the precast product is great and the product to be made can be standardized, then the initial expense of the dry-cast forms and production equipment can be justified. As described by Mark Wilson, plant manager at Cretex Concrete Products, “The economic factor (of dry-cast production) can’t be ignored. Man-hours per ton and form costs over time can be attractive, as well as, a quick response time in serving the customer.”
As discussed, dry-cast mix and production methods provide many advantages. New admixture technology has also introduced game-changing properties to wet-cast concrete, including the rapid application of self-consolidating concrete within the precast industry. However, regardless of your production methods, always make sure it is quality concrete.
Eric Carleton, P.E., is NPCA’s vice president of Technical Services.