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Dry-cast production can significantly increase production efficiency.
If you have pondered but not yet made the jump to dry-cast concrete production, chances are you have uncovered more questions than answers. Here are some dry-cast basics that may make your jump a little easier.
Dry-cast concrete mixes, also known as no-slump mixes, are used in precast plants to produce essentially any precast-concrete product. They can significantly increase production volume and lower both material and labor costs. Other benefits, such as increased strength and durability, can be realized with the low water-cementitious material ratio mixes.
Of particular interest to precasters is the increased production efficiency due to the very early product handling strengths and the ability to immediately remove and reuse forms. However, proper manufacturing equipment and techniques must be used to achieve these benefits.
When stationary, dry-cast mixes have the consistency of slightly wetted soil. By ordinary standards, these mixes would be considered harsh and unworkable. However, dry-cast mixes become useable through the use of specialized consolidation techniques, such as heavy-duty vibration, packing, and/or spinning.
The use of sophisticated vibration equipment is a principal method of dry-cast mix consolidation for a wide variety of products, such as manholes, pipes, grade rings, utility boxes, box culverts, septic tanks, blocks, burial vaults, curbs, and other products.
Processes that use packing, pressing, and extrusion of fresh concrete are used in the production of concrete blocks, curbs, cast stone, pipe, roof tiles, and slabs. In these processes, pressures applied to the fresh concrete consolidate the mix and, in some cases, press out excess water.
Spinning is a process primarily used in the production of concrete pipe and light poles in which centrifugal forces consolidate the freshly placed concrete. Coarse aggregate mix proportions must be adjusted carefully to avoid adverse segregation during the spinning process. Consequently, it is common to use two or more blends of coarse aggregate to ensure a uniformly graded aggregate.
The effective use of dry-cast mixes is limited only by the mix’s ability to consolidate adequately, such that the freshly stripped products neither slump nor crack when stripping their forms.
Dry-cast mixes are less workable than conventional concrete mixes because of the lower water content and larger proportions of coarse aggregates. This requires the use of special equipment and handling techniques.
Either pan-type mixers or spiral-blade mixers are required for dry-cast mixes to ensure thorough blending.
Pan-type mixers can discharge a dry-cast mix every 2-1/2 to four minutes depending on the type of mixer.
Spiral-blade or ribbon mixers usually have slightly longer mixing times with discharge cycles of three to five minutes. Both are available in capacities from one-half cubic-yard (cy) to four cy, with the most common ranging from one cy to three cy. Shorter mixing times, of course, result in faster production.
Full consolidation is the key to successful dry-cast manufacturing. Added consolidation effort can be achieved through specialized equipment such as vibration tables, compaction or drop tables, continuous-duty form vibrators, pipe machines, and extrusion presses. Optimized vibration frequencies and amplitudes can achieve even greater consolidation.
The effectiveness of vibratory consolidation equipment depends on three primary variables:
Under typical conditions, fresh concrete should be fed slowly and continuously into the formwork and vibrated as recommended by the equipment manufacturer.
Depending on the consistency of a mix, impact forces from one to two times the weight of concrete and formwork are needed to effectively consolidate a dry-cast mix. Vibration equipment, with frequencies varying from approximately 3,000 to 16,000 vibrations per minute, is available for dry-cast mixes.
Forms or Molds
More rigid formwork is necessary for dry-cast manufacturing than for normal-slump concrete production. Rigid forms and accessories will transfer vibrations more effectively and can withstand the continuous impacts of the consolidation process. Otherwise, the forms will likely wear or become damaged prematurely.
The curing process is particularly important with dry-cast mixes. Adequate curing enhances the desirable properties of concrete, such as strength, impermeability, surface hardness, and crack resistance. Early curing periods are most critical to ensure protection from extreme temperatures and dryness. Dry-cast products must be protected from drafts to prevent cracking due to surface moisture loss.
Concrete cannot cure properly without an adequate amount of water in the mix. And, because the forms are removed immediately, dry-cast products have an undesirable tendency to dry too quickly. Consequently, a dedicated curing area, such as an insulated enclosure with misters, is generally required.
A normal curing cycle for dry-cast products includes a preset period of about two hours, a ramp period (to raise ambient temperature to the desired curing temperature) of two to three hours, and a hold period (at the target temperature) of four to six hours. These curing cycles will vary with the type of product being cured.
Curing in a controlled, moist environment dramatically increases the concrete’s rate of strength development. In many cases, full design strengths can be achieved in one day.
Material handling equipment in dry-cast production is generally different from that required for wet-cast production. Off-bearing and stripping are usually performed with overhead cranes or fork trucks, although automation is becoming increasingly common in precast plants producing dry-cast products.
Dry-Cast Mix Proportioning
Proportioning methods for dry-cast mixes are similar to the methods used for normal concrete as defined in ACI 211.1, “Standard Practice Selecting Proportions for Normal, Heavyweight, and Mass Concrete.” Water-cementitious material ratios and coarse-aggregate contents are selected on the basis of strength or durability requirements and are then modified to produce very stiff concrete.
Generally, the percentage of coarse aggregate varies from 50 percent to 60 percent, and fine aggregate varies between 40 percent and 60 percent. No single combination of ingredients or method of proportioning is ideal for all dry-cast concrete applications. The actual proportions of coarse and fine aggregate ultimately depend on the type of product being manufactured and the equipment being used.
When using dry-cast mixes, formwork can be removed immediately after placing and consolidating the concrete. To accomplish this, sufficient amounts of fine aggregates must be included in the mix to ensure its cohesiveness.
Here are some other considerations for precast dry-cast mix proportioning:
Dry-cast and wet-cast production both have inherent advantages and disadvantages. And proponents of each generally stand by their method of choice. Nonetheless, making the jump to dry-cast production calls for critical production techniques that require careful study.