By Eric Carleton, P.E
Editor’s Note: This is the final article in a series that focused on the details and more technical aspects of one common thing precast concrete producers do on a daily basis.
The American Concrete Institute and other industry organizations define concrete transport as the movement of fresh concrete. Though this is an important aspect of production and was highlighted in the second article of this series, precasters should also think of transport as it relates to the movement of hardened concrete products. From the time a product is stripped from its form to when it arrives and is placed at the job site, there are three specific transport movements that require careful consideration: plant to yard, yard to truck and truck to installation site. Abiding by best practices throughout each step will ensure a product’s quality and appearance are preserved.
Plant to yard
Efficient and profitable precast operations turn forms at least once, sometimes twice, within a 24-hour period. Movement of hardened precast can be a precarious process, particularly just after stripping and during other early-age transportation processes.
Concrete strength development
The product movements immediately after stripping are critical in the life of every product. The concrete has attained sufficient strength to strip the form, but the concrete is still in the early stages of the hydration process and has likely not reached design strength. Every plant-specific quality control manual should outline a minimum stripping strength for each product that takes resistance to transport stresses into consideration. Products cast with typical wet-cast or self-consolidating concrete mixes are usually considered ready to be transported to the yard for storage after the product has been stripped. It is critical to know the concrete strength exhibited at this stage is only a function of hydration of the cementitious paste. The bond between the paste and aggregate may be minimal. Consequently, though this concrete may have met the minimum stripping and yarding compressive strength, it still is unlikely to possess its design tensile and flexural strengths. Lifting and transporting a product too soon – before allowing it to develop greater resistance to tensile and flexural stresses – can lead to unexpected spalling or tensile cracking.
If initial product transport will rely on embedded lifters, evaluate the individual capacity of the lifting inserts at this early stage of concrete strength development, not the anticipated 28-day strength. In some cases, use of additional lifting inserts may be required to reduce the stress at each pick point.
Minimum steel requirements
Minimum reinforcing steel area requirements for many precast products fabricated under ASTM or DOT standards are based on the installed conditions rather than the transport or handling loads. Handling loads are a consideration for the precaster or contractor, typically not the project engineer. Thus, it is the responsibility of those parties to ensure adequate structural capacity for product lifting, handling and transport is built into the precast product’s design.
For example, within ASTM C1577, “Standard Specification for Precast Reinforced Concrete Monolithic Box Sections for Culverts, Storm Drains, and Sewers Designed According to AASHTO LRFD,” there is a series of tables showing the steel area requirements for various standard precast concrete box culvert section spans, rises and wall dimensions. The assumed loading conditions that were used to develop these tables are described within the appendix at the end of the standard. Figure 1 shows a free body diagram of the assumed soil loads acting on a backfilled box section. Figure 2 shows the same box loaded during transport, either with lift truck forks placed on the inside or with lifting inserts placed in the top slab. Alternately, Figure 2 also shows the loading condition with lifting inserts placed in the top slab. The loading conditions within Figure 2 vary greatly from the assumed design condition in Figure 1. The conditions shown in Figure 2 could cause the product to experience loads during transport that may not be included in the minimum steel requirements of ASTM C1577 for the installed condition in Figure 1. Without additional handling steel, cracking could occur within the top slab near the haunch, outer crown surface or other places. A similar analysis can be seen for the precast manhole riser sections within Figures 3 and 4. Precasters must be cognizant of these unique transport loads, particularly for newly cast concrete. Certain lifting and handling conditions may require additional steel or a very specific placement policy for lifting forks or embedded lifting inserts. Proper lifting practices must adhere to the lifting plan for which the product was reinforced. When training plant personnel, it’s important to not only teach the correct procedures but also explain the reasons behind them.
Careful consideration should also be given to dunnage selection and positioning, since proper dunnage is responsible for maintaining product quality while in storage. For more information about dunnage read the 2018 Precast Inc. July-August article, “You’re Never Done with Dunnage,” or view The Precast Show 2017 presentation, “Dunnage, Loading & Shipping,” located at precast.org/dunnage.
Yard to truck
After spending some time in the storage yard, the product will be transported to a truck or trailer for delivery to the installation site. By then, the product has had additional opportunity to cure and increase its compressive strength closer to the design levels. With improved aggregate-cement bonding, the product should be more resistant to spalling and cracking.
This is the last opportunity to ensure the product is ready for contractor handling. Many precast plants use forklifts or special hydraulic clamps to transport product within the plant or in the yard, but the contractor will typically use an embedded lifting device with appropriate connecting hardware, specific lift holes with lifting pins, or an embedded lifting cable and chains with hooks. If the contractor will use a lifting method that is different from the plant’s method, problems could occur in the field.
Though the lifting insert and positioning are verified during pre- and post-pour inspections, lifting procedures should also be carefully monitored to ensure products are being lifted safely and uniformly.
Consider the following critical items during each inspection:
- Embedded lifting inserts: Is the correct lifting insert being used? For example, was the 5-ton capacity lifting insert embedded rather than the 8-ton capacity lifting insert? Is the recessed area free of debris or ice? Is the insert installed properly or will it prohibit easy use in the field?
- Lift holes: Are the cored or cast holes adequately sized to accept the contractor’s lift pin? Are there any obstructions in the opening that would prohibit use?
- Embedded lift cable: Is the cable kinked or damaged? Is it corroding?
In order to conduct these inspections properly, it’s important to confirm the receiving contractor has the proper lifting and handling hardware on site to correctly transport the product off the truck and throughout the project site. Many precasters choose to provide appropriate lifting devices either as a service or for a fee. If this is the case, ensure there is a policy that all provided items are inspected, in excellent condition and comply with current OSHA or regulatory provisions. Lastly, if these items are furnished by the precaster, make sure they are loaded on the first truck arriving at the job site.
Truck to job site
The driver plays a critical role in ensuring the product arrives undamaged to the job site. The driver must be well-informed. Whether the driver works for the precaster or an outside company, they need to know the DOT requirements for proper load tie down and should be trained to understand the basics of concrete loads, lifting and handling. A well-informed and trained driver may provide guidance to the contractor on proper handling and would be able to report back to the precast company if poor on-site product handling is observed.
Precasters should consider implementing written requirements within their QC plan for the use of chains, cables or strap guards on the precast products whenever these items encounter concrete corners, edges or joints. Even the most carefully secured product is subject to dynamic loading as the delivery truck bounces and flexes during the trip to the job site. The use of guards distributes these loads to reduce spalling or broken components and could be the difference between product acceptance or rejection on site. For more information about product load securement read the 2019 Precast Inc. July-August article, “Maintaining Load Securement Across Your Fleet.”
Precast concrete product transport is comprised of three distinct movements, each having specific considerations and challenges that need to be analyzed and addressed to furnish the best quality precast concrete product possible at the job site. Like most aspects of precast production, transport best practices boil down to having two things: knowledge of what is required and continuous training.
Eric Carleton, P.E., is NPCA’s director of codes and standards. He is an ASTM Award of Merit recipient and currently serves as vice-chairman of ASTM C13, Concrete Pipe.