Many new concrete structures are designed to include long spans and carry heavy loads. The sizes of the necessary components—drilled shafts, foundations, footings, and columns—often push the envelope of standard construction practices. A greater emphasis on durability also has led to higher cementitious material contents, lower water-to-cementitious-material ratios, and deeper cover over reinforcing steel. These requirements have resulted in more concrete placements that are subject to high internal temperatures.
The problem with high internal temperatures is the increase in the potential for thermal cracking that can decrease concrete's long-term durability and ultimate strength. Thermal cracking negates the benefits of less permeable concrete and deeper cover by providing a direct path for corrosion-causing agents to reach the reinforcing steel. To deal with these issues, designers and owners are treating more concrete placements as “mass concrete,” traditionally defined as any placement greater than 3 feet thick. While there are numerous ways to control the internal temperature in mass concrete placements, using liquid nitrogen is one of the most effective.
Specifications typically limit concrete temperatures to a maximum of 160° F due to durability concerns related to delayed ettringite formation (DEF). DEF is an uncommon form of internal sulfate attack that can result from concrete being cured at high temperatures. Precooling the concrete can reduce the peak temperature.
Maximum temperature differences within the concrete are also usually specified to minimize the likelihood of thermal cracking. Historically, a maximum difference of 35° F (from the center of the concrete placement to the face nearest to the form) has been specified based on experience with unreinforced concrete dams. Insulation of the forms to keep the concrete from cooling too rapidly can limit the temperature difference. Other specifications, such as a maximum initial concrete temperature or cooling rate, can indirectly limit the maximum temperature or temperature difference.
The easiest and most cost-effective way to limit concrete temperatures is often with a properly engineered low-heat concrete. But when mix design changes are not practical, the concrete can be precooled. There are several ways to precool concrete.
- Evaporative cooling: Often overlooked, evaporative cooling of coarse aggregate (sprinkling) is the most economical, cooling method. Sprinkling water can reduce the stockpile temperature by 10° F or more. Use only enough water to keep the stockpile wet, not saturated. Using chilled water is unnecessary, as the heat loss is simply a result of evaporation.
- Ice: The most common, yet perhaps least understood, cooling method is replacing mix water with ice. This cools concrete in two ways: It first lowers the mix-water temperature and then lowers the mix temperature by extracting heat during the phase change from ice to water. This distinction is important, because ice is five times as effective for cooling as water chilled to 40° F. Ice can be added directly to the ready-mix truck or pre-mixer. When a large project requires large quantities of ice, a dedicated ice production plant may be justified. Ice can be substituted for about 80% of the batch water, which limits the cooling that can be achieved with ice to about 20° F.
- Chilled water: Chilled water can be used in the concrete mix to lower the temperature. Chilled batch water alone generally will not lower the concrete more than 8° F, depending on the water temperature and the water/aggregate ratio. If ice is used, then the reduction of mix water makes the use of chilled water ineffective. Where an ample supply of chilled or cool water and a way to drain the pile is available, inundation can be an economical way to cool coarse aggregates.
- Liquid nitrogen (LN2): When concrete needs to be precooled more than about 20° F, or where ice is not available, the most effective method usually is liquid nitrogen.
Advantages of liquid nitrogen
Liquid nitrogen is produced by compressing and cooling nitrogen gas below its evaporation point of about –320° F. The main advantage of LN2 is its versatility. LN2 can be used to chill the aggregates or mix water but most commonly is injected directly into the ready-mix truck drum.
LN2 can cool concrete as little as 1° F or as much as is needed. LN2 equipment runs on power from small field generators, so neither an external power connection nor a large-capacity generator is required. When injected directly into the concrete, LN2 has the advantage of changing the concrete temperature directly. All other concrete cooling methods rely on cooling materials before or during batching.
LN2 is the only cooling method that allows field readjustment of the concrete. It requires fewer workers than are needed to add ice at the batch plant. LN2 injection does not require any plant or truck modifications. Mix proportioning and batch procedures are not affected by LN2 use in the field.
The potential drawbacks of LN2 primarily relate to cost. Equipment and unit materials costs can be higher than for other cooling methods, depending on material availability and related local costs. Setting up and administering the field operation require extreme care and add to the LN2 costs. Liquid nitrogen is extremely cold—at normal atmospheric pressure it reverts from liquid to gas, and its temperature is about –320° F. Therefore, operators and maintenance personnel must wear proper protective clothing and safety gear and must be trained in its safe use and handling.
On the other hand, unless the project is large enough to justify aggregate inundation, LN2 is the only practical way to cool concrete by more than 20° F. On some projects, LN2 has been used to precool the concrete mix to 35° F.
How it's done
The liquid nitrogen supply system consists of one 11,000- or 13,000-gallon cryogenic vessel, one optional additional small cryogenic vessel, and a vaporizer. The small cryogenic vessel supplies nitrogen gas for pneumatic controls and maintains constant pressure in the large cryogenic vessel throughout the cooling process. The need for it depends on the withdrawal rate of LN2 from the supply vessel. The pressure of the liquid nitrogen supply vessel and the size of the nozzle determine the flow rate of the liquid nitrogen. A variable timer allows for close control of the final concrete temperature.
The liquid nitrogen injection station consists of a liquid nitrogen lance, which is moved in and out of the ready-mix truck drum using a pneumatic cylinder. From the control panel, the lance can be moved right or left and up and down, allowing the operator to position the lance correctly.
A pneumatically operated ball valve controls the flow of liquid nitrogen through each lance, and the entire process is sequenced from a pushbutton control panel. A bank of lights helps the driver and operator to position the truck properly and lets them know when cooling is complete.