Dry cast offers efficiency and economic benefits.

The engineering and construction community knows a lot about concrete. Concrete is, after all, the most-used construction material in the world. Mechanical properties of wet concrete, such as its workability, density, volumetric yield and air content, are well known throughout the concrete industry. And yet, while dry cast concrete has been used to manufacture precast for more than 100 years, there are still many in the industry that are not familiar with it.

Differences Between Dry Cast Concrete and Conventional Concrete

Dry cast concrete also is referred to as zero-slump or negative-slump concrete. These terms describe concrete that is sufficiently hydrated and thoroughly mixed but does not flow and cannot be poured into molds by conventional means. Dry cast concrete refers to a consistency that is even stiffer, or drier, than “no-slump” concrete. Zero-slump means that the concrete can hold the shape of an ASTM C143 slump cone without any measurable change in height after the cone is removed, and negative slump refers to the fact that additional water can be added to the mix without the free-standing sample of concrete exhibiting any measurable drop in height.

Because it is too dry to flow, dry cast concrete must be mechanically consolidated through physical means, either through high-amplitude vibration, centrifugal force, tamping or pressing – often by two or more of these methods in combination. Form equipment for producing dry cast concrete products must be both durable enough to supply the necessary mechanical energy for consolidation and resilient enough to withstand the same repeated and constant consolidation energy without breaking or losing shape over time.

Due to the amount of energy needed for consolidation, larger coarse aggregates can cause excess wear on dry cast forming equipment. Accordingly, most dry cast concrete mixes use smaller, more rounded coarse aggregates, usually pea gravel. Larger coarse aggregates, if used, tend to only be used in combination with smaller aggregates and only in limited quantities.

Benefits of Dry Cast Concrete

The amount of consolidation effort applied by dry cast manufacturing equipment, usually vibratory energy, often produces sound levels that exceed the occupational noise exposure levels allowed by OSHA, and workers must use hearing protection when working around such equipment. However, once thoroughly consolidated, dry cast concrete holds its molded shape, and formwork can be removed almost immediately and reused to cast another product. The economic advantage of being able to use a single form 20 to more than 100 times a day rather than once or twice a day is readily apparent, and openings or leave-outs can be etched immediately into the walls of the finished product rather than having to undergo the labor-intensive process of saw-cutting the product after the concrete has hardened.

Another advantage is its improved strength and durability owed to the reduction in amount of mixing water used. Since dry cast concrete is mechanically consolidated, it naturally does not require much water, allowing for lower water to cementitious materials (w/cm) ratios that are proven to enhance both the strength and durability of hardened concrete.

Once cured, lower w/cm ratios also help make the hardened concrete less permeable, a welcome benefit in harsher environments such as sanitary sewers and conditions such as freezing temperatures. Additionally, there are other key differences between dry cast concrete and other types of concrete regarding development of mix designs, moisture control and density, as well as freeze-thaw resistance.

Mix Designs

With conventional and self-consolidating concrete (SCC), flowability is a key factor in designing concrete mixes. As more water is added to the mix, more cement is needed to compensate, lowering the w/cm ratio to more favorable values. Due to the mechanical consolidation used in dry cast, there is no need to add additional water and cement to help the concrete flow. Only the necessary amount of cement to bind all the constituents together is needed; however, be sure to check with local standards because most construction specify a lower limit of 470 pounds of cementitious powder per cubic yard of concrete. Even so, not only are w/cm ratios typically lower for dry cast concrete, but total cement usage is also typically lower as well.

Although the American Concrete Institute (ACI) once published a guide for selecting proportions for “no-slump” concrete, it is no longer available as of this writing. Even so, most dry cast machine manufacturers prefer to provide their own suggestions to producers about where to start when developing mix designs for dry cast concrete. Emphasis is placed here on the fact that these are merely recommendations for where to start, as every dry cast mix design will be different depending on several factors, starting with the sources of raw materials.

Producers will need to experiment with each machine and product to develop an optimal mix design for each combination. For example, a circular manhole section that is vibrated may require a higher proportion of coarse aggregate than the same manhole section produced via centrifugal force, or a rectangular product may require a lower w/cm ratio than a circular product. It may take a considerable amount of time to go through all the variables to arrive at the perfect combination for each type of product.

Aggregate Moisture Control

An experienced batch plant operator that mixes dry cast concrete on a daily basis knows exactly how it looks in the mixer, how it sounds as the mixer blades push their way through the batch, even how the mixer motor sounds as it is turning the perfect batch of dry cast concrete. However, it’s important to know for sure whether the w/cm ratio is within specified limits, and that can only be determined through accurate measurement of the total water that is in each batch.

Making this task more difficult, the amount of required mixing water can vary greatly based on how much water is already clinging to the aggregates or even how much residual wash water remains in a clean mixer.

Installing several moisture probes at strategic points along the batching process is an ideal way to constantly monitor aggregate surface moisture throughout the day. Without probes, checking the aggregate surface moisture content once per day in accordance with most specifications may not be often enough to account for changes in the condition of batching equipment or changes in material moisture throughout the day. As more aggregates are consumed from the storage bins, the moisture levels are constantly changing, making it difficult to rely on a single moisture calculation for multiple batches. Using moisture probes allows the batching equipment to sense changes in surface moisture as they occur, helping ensure w/cm ratios always remain within allowable tolerances.

To properly calibrate a moisture probe, the widest possible range should be used. In other words, the probe should be calibrated for dry or nearly dry aggregate, then calibrated again for saturated conditions. The smaller the aggregate, the higher the possible variation in aggregate moisture. A well-thought-out calibration procedure is necessary for accurate calibration.

Moisture probes for coarse aggregates should also be shielded against possible damage when filling the bins, and aggregate bins should never be emptied to the point where probes become uncovered. Moisture probes should always remain covered to ensure consistently accurate readings.
For most concrete batching systems that proportion materials by weight, a mixer probe is essential. A mixer probe essentially detects total moisture from all sources, whether it be from the aggregates, residual water in the mixer or any other source of moisture. The mixer probe reads the amount of moisture in all the materials that have been added to the mixer as a percentage of total weight, and the amount of mixing water is adjusted accordingly. Since, by definition, the w/cm ratio is based on weight, this ensures accurate control of the w/cm ratio when weigh batching.

If the aggregates are proportioned volumetrically, the percentage of moisture of each aggregate will need to be precisely known to calculate the amount of the water in each, which would then be combined with the amount of added mixing water to determine the actual w/cm ratio. Similarly, only by knowing the amount of moisture in each aggregate can the total volume of the concrete batch be determined, which is why it is important to use properly calibrated moisture probes on both the aggregate bins and the mixer when batching volumetrically.

Higher Density

One thing that may be surprising to learn about dry cast concrete is that it typically weighs more than conventional concrete. By adding less water and using less cementitious materials, fine and coarse aggregates comprise a greater percentage of the total volume of dry cast concrete. Since the unit weights of most aggregates are higher than those of water and cement, the total unit weight of dry cast concrete might be 4% to 6% higher than the same materials proportioned to produce conventional concrete. If not properly accounted for in design, this could result in lifting equipment or perhaps a delivery truck being overloaded, presuming the actual unit weight exceeds the 150 lb./cu. ft. typically used to calculate the weight of reinforced concrete.

Further contributing to differences in unit weight is the amount of compaction resulting from the dry cast manufacturing process. Physical compaction typically results in very dense concrete. To properly account for this level of compaction, both vibration and a weighted cylindrical hammer must be used when making test cylinders to ensure the level of compaction in the test cylinders properly matches that of the actual product.

Resistance to Freeze-Thaw Conditions

The lack of flowability of dry cast concrete precludes the use of air entraining admixtures, and yet, precast structures constructed of dry cast concrete historically have had very little trouble withstanding the rigors of cold weather. As referenced in a previous article, a research study conducted by the firm Service d ’Expertise en Materiaux Inc. (SEM) concluded that dry cast concrete does not need to be air entrained in the same way as conventional concrete to be frost durable1. Ample installations in the upper regions of North America attest to the ability of dry cast concrete to withstand freezing conditions.

Additionally, the same properties that allow dry cast concrete to hold its shape immediately after casting also make it more resistant to freeze-thaw conditions. Higher compressive strength and lower permeability are both positive benefits of a low w/cm ratio that are proven to enhance the freeze-thaw durability of concrete2.

In Conclusion

Precast concrete can take on many forms, from the consistency of wetted soil in the case of dry cast concrete to flowing freely almost like water in the case of SCC. Regardless of the concrete’s consistency, the result is a durable, high-quality precast product that will last a lifetime.


  1. Carleton, Eric. “Top 10 Facts about Dry-Cast Concrete.” NPCA, 10 Sept. 2015, precast.org/blog/top-10-facts-about-dry-cast-concrete/.
  2. Mindess, Sidney, et al, Concrete, Pearson Education Taiwan, 2008.