Quality in, Quality out: a look at steel fabrication, forming and consolidation equipment.
By Adam Neuwald
The key to producing quality precast concrete products is having the right equipment for the job. Going hand in hand with the right equipment are adequate training to operate the equipment and proper inspection and maintenance to ensure that the quality of finished product is not compromised.
The most common production equipment you will need in your business will include reinforcing steel fabrication equipment, forming equipment and consolidation equipment. Coincidentally, this is the same equipment that causes many plants to lose points during their plant certification audits. These plants are cited for not using the appropriate equipment for the job, not using the equipment in an appropriate manner or using equipment that is in need of maintenance.
Reinforcing steel fabrication
Nearly every precast concrete producer uses deformed steel bars for reinforcing precast concrete products. When fabricating reinforcing cages and/or mats, it is important that production personnel have the appropriate equipment for cutting, bending and securing the reinforcing bars within the specified fabrication tolerances. Reinforcing steel fabrication equipment ranges in size and cost from small portable units to fully automated machines capable of fabricating intricately bent reinforcing steel bars.
When purchasing reinforcing steel fabrication equipment, one must step back and take a look at the size and grade of reinforcing steel used, types and number of bends required and volume of daily production. The size, grade and volume of reinforcing steel bars typically used at the plant will determine the required shearing capacity of the equipment. Larger machines will be capable of cutting a higher number of larger diameter bars at the same time.
There are a variety of combined rebar cutting and bending machines available. In addition to the size, grade and volume of reinforcing bars used, one must also consider the type and number of bends required when purchasing bending equipment. Machines are available with various pin configurations, making it possible to perform intricate bending details.
It is extremely important that production personnel are properly trained on how to use the equipment. Bending equipment typically will come with a series of pins of varying size. The size and placement of the pins will be critical in ensuring reinforcing steel is fabricated in accordance with the specified shop drawings. Ensure that reinforcing bars are bent in accordance with standard CRSI fabrication practices as outlined in Chapter 7 of ACI 318. Minimum bend diameters (as measured from the inside of the bend) should not be less than those specified in ACI 318.
For example, if bending a #6 bar, which has a 3/ 4-inch diameter, the minimum diameter the bar can be bent is 6 x 0.75 = 4.5, meaning the pin should be 4.5 inches in diameter or larger. Ensure that the right size pins are available for bending reinforcing steel.
ACI 318 also contains information on bending stirrups and welded-wire reinforcement.
Rolled welded-wire reinforcement should be used only in round products unless it has been mechanically straightened. Straightening and cutting equipment is available for rolled welded-wire reinforcement and may be more efficient than straightening and cutting rolled welded-wire reinforcement by hand, which often leads to contamination of the steel – especially when unrolled in the yard or on the production floor where mud, oils and grease are present. Welded-wire reinforcement typically can be cut with hand tools, while mechanical cutting tools are available for heavier gauge wire.
Automated cage machines are also available for fabricating standardized reinforcing cages. Plants producing a high volume of standardized product may consider investing in such equipment. Although the process is automated, it is still extremely important to perform routine quality control inspections on the cages throughout each production run. At a minimum, the length, spacing, cage diameter and welds should all be inspected.
The majority of reinforcing cages are fabricated by hand. Jigs are commonly used for fabricating standardized reinforcing cages and/or mats, while some cages may need to be constructed within the form. The use of a fabrication jig typically increases productivity and minimizes the potential for erroneous placement of the reinforcing steel.
Reinforcing steel may be secured by a number of means, including mechanical fasteners, tie wire (by hand or tie gun) or welding. When welding, please consult the American Welding Society’s “Structural Welding Code for Reinforcing Steel (D1.4)” for additional information on welding reinforcing steel.
Probably one of the most important investments a precast concrete manufacturer will make is in the forming equipment. Whether purchasing forms from a supplier or fabricating them internally, many factors must be taken into consideration during the design phase. The type of product and production methods used at the plant will determine the best material and design for each form. Commercial forms are available for standardized items like manhole sections or traffic barriers, while wood or panel forms are often used for constructing custom products. Large casting beds with side channels are often utilized for casting prestressed and panel products and are typically constructed from steel and/or wood.
When purchasing standardized forms from a supplier, take into consideration your plant’s production procedures. The first step is to determine the type of concrete that will be used in the form: zero-slump, conventional or self-consolidating. The type of concrete used will determine the required method of vibration, which will ultimately determine the strength and rigidity required for the form. In particular, dry-cast forms for zero-slump concrete are often designed for a particular type of vibrator. This is why it is not recommended to mix and match dry-cast forms and vibrators from various manufacturers.
Wet-cast forms must be rigid enough to handle the daily abuse of concrete placement and consolidation procedures. Steel is often used for the casting surface, which is reinforced with structural supports designed to withstand the internal concrete pressure and vibration forces. Forms used with external form vibrators may have to be structurally reinforced with stiffeners to ensure the vibratory forces are evenly distributed throughout the casting surfaces.
The design of the form will also play a large role in the ease of performing pre-pour and post-pour operations. Form jackets for rectangular structures are often hinged at the bottom or placed on a track or rollers. Depending on the size of the jacket, it may require several workers or an overhead crane to open and close hinged jackets. Properly designed and maintained roller jackets typically can be opened and closed by one individual.
Take safety into consideration as well. Legs are often attached to the exterior of hinged forms for support while in the open position. Forms of considerable height should be fitted with an approved walkway and rail so production personnel can safely reach the top of the form during placing, consolidating and finishing operations.
Various options are also available to assist in stripping operations. All blockouts and knockout pans should have an acceptable taper or draft for easy stripping. Securely fasten blockouts and knockout pans to minimize the potential for grout leakage, which may lead to rock pockets and concrete spalling during stripping operations.
Tapered or collapsible cores are available in addition to hydraulic jacks that evenly lift the bottom pallet to break the bond between the concrete and internal core. When determining whether to use a tapered or collapsible core, consider the versatility of each option. If you plan to cast products of various heights, you may need multiple pallets to fit tightly around a tapered core. Another option is to fill the void between the pallet and tapered core as the height of the product is reduced. Turning trunions are also available for easily flipping and stripping product cast upside down such as manhole bases or septic tanks.
Quality forms are not cheap, but when properly maintained, they should last for a number of years. Production personnel should be properly trained on seasoning, cleaning, applying release agent, stripping and inspecting forms. Measure and inspect all forms and forming equipment prior to initial use and not less than annually. It is suggested to label each form and develop a maintenance log for forming equipment. Some plants will document the actual form a product was cast in, so if a recurring problem exists it can easily be traced to the form for corrective maintenance.
Freshly placed concrete may have as much as 20 percent trapped air. Thus, concrete must be consolidated to ensure it will meet the specified strength, durability, permeability and surface finish requirements. The most commonly used consolidation method is vibration. According to ACI 309, “Guide for Consolidation of Concrete,” vibratory impulses liquefy the mortar, drastically reducing the internal friction between aggregate particles. The mixture becomes unstable, allowing entrapped air to rise to the surface while the heavier aggregate settles into a dense matrix.
Vibrators typically are powered by electric or gas motors, compressed air (pneumatic) or hydraulic gear motors. Electric and pneumatic vibrators are the most widely used within the precast concrete industry, while hydraulic vibrators typically are used on paving equipment.
When discussing vibrators, there a two key terms to be familiar with: frequency and amplitude. The frequency is the number of vibration cycles per minute expressed as revolutions or vibrations per minute (rpm or vpm). Frequency will have an effect on lighter masses, moving sand and slurry around aggregate and ultimately causing the mortar to liquefy. The amplitude is the maximum distance a point on the vibrating head moves from its position of rest. Amplitude will have an effect on heavier masses, moving coarse aggregate and ultimately determining the radius of action or influence. The radius of action is the area of concrete influenced by the vibrator.
The frequency of pneumatic vibrators can be adjusted easily by changing the air pressure, while some electric vibrators have adjustable frequency settings. The amplitude typically can be changed by changing the mass of the eccentric weight and head of internal vibrators. The mass of the eccentric weight can also be changed in certain types of external vibrators to increase or decrease the amplitude.
Other types of consolidation equipment include tampers, vibrating screeds, drop tables and vibrating tables. Selecting the right type of vibration equipment, frequency and amplitude is greatly influenced by the type of product cast, the type of concrete used and the type of formwork. High-frequency low-amplitude vibrators are commonly used for consolidating higher slump concretes, while low-frequency high-amplitude vibrators are preferred for stiffer mixes. Regardless of the frequency and amplitude, vibrators should never be used to move concrete. Place concrete as close to its final position as possible and use vibration equipment only for consolidation.
External form vibrators
Plants will often use external form vibrators because they typically require less time for consolidation compared with internal vibrators. Form vibrators are also the vibrator of choice for consolidating zero-slump concrete and are often attached to the core and jacket of dry-cast forms. As mentioned earlier, dry-cast forms and vibrators are often designed to work as a system, so use caution when attaching a different type of vibrator to a dry-cast form.
When selecting external form vibrators, it is extremely important to provide the vibrator supplier with detailed drawings of the forms as well as information on the workability of the mix design. Depending on the design of the forms, they may need to be retrofitted with stiffeners to ensure the vibratory energy is evenly distributed throughout the form. Form vibrators should never be attached directly to the form skin. This may lead to fatigue stresses around the vibrator. Vibrating the form skin itself can also draw air into the top portion of the form, trapping air between the form and the concrete. Attach form vibrators to a structural stiffener in locations recommend by the vibrator manufacturer. Routinely inspect welds and reinforce as needed.
Few plants permanently attach form vibrators to wet-cast forms. A mounting bracket typically is welded to a stiffener and vibrators are simply transferred to each form. Ensure that vibrators fit snugly into each bracket so the vibrator force transfers evenly throughout the form. The force required for vibrating concrete with a slump greater than 3 inches is typically determined by calculating the combined weight of the form and concrete. This value should be increased by 125 percent to 175 percent for consolidating concrete with a slump between 1 and 2 inches. The vibratory force should also be increased by as much as two to three times when consolidating zero-slump concrete.
If surface defects and bug holes are present in finished products, consider revising your mix designs, increasing the frequency and/or number of vibrators or trying a combination of external and internal vibrators.
When using internal vibrators, the frequency, amplitude and head size should be appropriate for the concrete mix design and product being cast. The head or stick size is typically one-quarter of the wall thickness. Use caution to minimize contact with the reinforcing cage and form surfaces when inserting the vibrator, especially when using epoxy-coated reinforcing steel. In this case, use a rubber or coated vibratory head to minimize the potential for damaging the epoxy coating.
Vibration times will depend on the vibrator’s frequency, and higher frequencies mean less time required for each insertion. An experienced vibrator operator will know when the concrete has been adequately consolidated by changes in the sound and feel of the vibrator in addition to the appearance of the concrete. The vibrator should then be reinserted so the fields of action overlap slightly. The manufacturer provides the theoretical field of action, but it may vary slightly depending on the concrete mix design.
Unfortunately the task of vibration sometimes is not given to experienced employees but rather to the newest production employee. New employees typically have a tendency to simply level the concrete and quickly remove the vibrator. They may drag the vibrator through the concrete at an angle or even stir the concrete with the vibrator. Some may even insert the vibrator at an angle so it hits the form surface with hopes of improving the consolidation process. These are all incorrect consolidation procedures and should be immediately corrected. It is extremely important to train all employees on the proper use of production equipment.
Even experienced precast personnel begin to adopt bad habits. A commonly cited deficiency related to vibration occurs when consolidating thin precast concrete sections like flat slab tops or wall panels. Production personnel have a tendency to drag the vibrator through the concrete at an angle. This is incorrect and may cause mortar pockets throughout the product. Again, internal vibrators should be inserted only in a vertical position. This may mean that a vibrator with a radius of influence of 6 inches would have to be inserted 60 times to properly consolidate a 6-by-10-foot precast concrete panel. Plants casting a large number of flat slab tops or panels may consider the use of other, more efficient consolidation equipment.
Table and surface vibrators
As noted above, the use of internal vibrators may not be the most efficient method for consolidating precast panels. Vibrator manufacturers do, however, supply shorter, wider heads designed for consolidating concrete in thin panel sections. Depending on the required finish, architectural and panel producers often use large surface vibrators or vibrating tables for consolidating concrete.
Surface vibrators are often used to efficiently consolidate concrete in thin products with a large surface area. Vibrating tampers are sometimes used for consolidating stiffer face mixes while vibrating screeds are used to consolidate conventional concrete mixes. Consolidation is achieved from the top down. The leveling effect from vibrating screeds actually assists in finishing operations. Screeds can be moved manually by production personnel or mounted to a track on the casting bed.
Vibrating tables normally consist of a steel table with external form vibrators rigidly mounted to the supporting frame. The table and frame typically are isolated from the base by steel springs or neoprene pads. The table itself can be used as part of the form, or forms can be placed on the table during concrete placement and consolidation.
Unidirectional vibration is often used with vibrating tables to avoid “walking” the concrete. A pair of vibrators are positioned so vibratory forces are directed only in the vertical plane. Check the table for dead spots by running your hand over the surface or by using a vibrograph to determine the frequency and amplitude throughout the table. The required force for a vibration table is typically 1.5 to 2 times larger than the combined weight of the form and concrete.
Having the right equipment for the job and training employees how to properly use and maintain that equipment is an important component of producing quality precast concrete products. Paying close attention to these key areas – steel fabrication, forming and consolidation equipment – will pay dividends toward that goal.