By John W. Dutschmann
Choose a hole-forming process that is right for your precast concrete operation.
Constructing holes in precast concrete manhole sections for pipe connections can be achieved by several methods. Each requires careful consideration before making the investment in training and equipment. Whether you are building a new plant and trying to decide which production method to use or you have an existing plant and want to determine if your current method is the most efficient, it’s important to make an informed decision on which practice works best for your business.
Pipe-to-manhole connections can be broken into two categories: resilient connectors (flexible mechanical, compression or cast-in boot connections) and grout-in or non-booted connections.
Flexible mechanical connectors use expansion rings or tension bands to compress and seal the resilient rubber material. A compression seal, on the other hand, uses pressure between the outside pipe diameter and manhole opening to confine and seal the connector. A cast-in boot connector is embedded into the manhole wall during forming and casting operations and can have a compression or mechanical seal. Lastly, a grout-in connection is achieved by filling the annulus between the pipe and manhole opening with a cementitious grout.
Prior to placing a connector, a hole must be constructed in the manhole section. The following are descriptions of the manufacturing process a precaster can choose for the specific connections described.
For dry-cast manhole operations, holes may be scored or cut using high-pressure water or air, or by hand-chiseling the stiff, uncured concrete immediately after form stripping and before initial set. After curing either at the plant or job site, the steel reinforcement is cut and the remaining concrete blank in the center of the hole is removed. These are non-precision holes and should be limited to storm structures with grouted pipe connections and soil-tight requirements. When watertight penetrations or structure vacuum testing are required on a dry-cast product, the use of a resilient connector is required.
A cored hole is created by cutting a precision opening in a hardened concrete manhole using diamond-tipped core bits. The diameter of cored openings can be as large as 48 inches. Perhaps the biggest benefit of coring holes is the reduction in production lead times of manhole sections having unique hole placements. The ability to core holes in blank stock bases and riser sections to achieve one- or two-day turnaround times can also be an advantage.
Initial setup of a coring machine and core bits can be expensive. Aggregate type and hardness may be a factor in the life expectancy of your core bits and consideration should be given to the type of aggregates used in your geographic area. Once a precaster has acquired all the bit sizes needed, they can be used for any wall thickness and for both round and flat wall products. For mechanical or compression seals to work properly, tight cored hole tolerances are critical. To maintain uniform roundness of the large core bits and the corresponding holes they create, it is imperative for the precaster to implement specific care and maintenance procedures to ensure proper handling and storage for long-term use. An important decision to consider before spending the capital on coring equipment is whether it is critical to turn product around in one day. Due diligence by the designer, contractor and precaster can eliminate the need for short lead times. However, there will always be situations in which coring holes can be an advantage.
Following coring operations – if required – the precaster places the resilient mechanical or compression connector in accordance with the manufacturer’s recommendations to provide a leak-resistant seal. Lastly, a coring operation should implement a sustainability plan to handle the blank concrete cores and wastewater. To determine the actual quantity of concrete used within a cylindrical opening, refer to the Precast Inc. July-August 2012 article, “Precast Math Tools: Calculate Manhole Blockout Volumes.”1
A formed hole is created by placing a specific piece of hardware into the manhole form prior to casting to create the required opening. Hole formers are available in various materials, such as steel or fiberglass. They are also available in all shapes to accommodate arched, elliptical and box connections, as well as parallel formers to provide for offset connections in circular lift station applications.
The initial cost for forming holes is typically less than coring equipment and core bits. However, hole formers are required for each manhole diameter and hole size. Proper placement and securing of the hole formers is critical to ensure they do not move or float during concrete placement. A post-pour inspection is also needed after form removal to ensure all cast openings are still in their proper design position.
Mechanical resilient connectors require the use of a precision-molded, low-draft tapered hole former while a two-piece mandrel is used to hold the cast-in connector in place during concrete placement. Mechanical connector placement is critical and requires proper production and quality control attention. When using a low-draft tapered hole, placement of the connector in the opening is sensitive and can impact the sealing capability of the expansion band with too little or too much pressure. Most manufacturers offer an optional, ratchet-style expansion band that allows unlimited adjustments when installed to a specified torque value.
Forming holes during the casting operation eliminates the residual waste of coring. In addition, if a cast-in resilient connector is employed, the secondary operation and QC procedures of installing the mechanical-style connector within the hole are eliminated. All hole formers should be inspected on a regular basis and replaced or repaired as necessary. The NPCA Quality Control Manual, Section 6.2.4, “Dimensional Checks,” is a good reference to use for all equipment checks. Attempting to use a hole former beyond its useful life could lead to unnecessary repairs and/or the product not meeting specifications.
Considerations and suggested practices
When manufacturers have the capability of both coring and forming, they might consider forming the hole for the outlet pipe in all base sections. They can stock the blank bases with holes formed for the more common pipe sizes. If a larger connection opening is needed, they can simply core a larger hole. This will add to the life of the core bits.
General production and handling considerations
Remember when handling manhole products, daily wear and tear on equipment, labor hours involved and the potential for product damage need to be considered. With forming, the base or riser section is ready for shipment and may only require one pick. With coring, movement to and from production, storage, coring, and possible pouring and forming of the base invert may require three or four picks. Plant and yard size and layout should be considered as well.
Some manufacturers may wait to core the holes and invert the base until the contractor is ready. When managed properly, this can reduce boneyard inventory and avoid confrontation about unused product.
Also consider that some specifying agencies or special load conditions require additional steel be placed around the hole. If coring is used, this eliminates precasters’ ability to stock blank bases or risers. When using hoop steel in risers or cones, design requirements under ASTM C478, “Standard Specific for Circular Precast Reinforced Concrete Manhole Sections,” Section 14.5, must be followed and a second inventory with wire fabric cage or other acceptable reinforcement is needed.
Each decision is unique
The decision on how to create openings in precast manholes is unique for each plant and requires consideration of the variables, specific equipment and attention to detail to ensure a quality product. Whichever process is chosen, QC procedures should always dictate that holes are the right size, free of burrs or defects, and in the correct location.
John Dutschmann is the technical services manager at Forterra Pipe & Precast in Lorena, Texas, and is the NPCA Manhole Product Committee chairman.