Ensuring the right dunnage is used can help protect the quality and appearance of your precast concrete product.
By Eric Carleton, P.E.
How many times have you had to step over pieces of dunnage left behind after structures have been moved or loaded and thought to yourself, “I wish this stuff was cleaned up?”
Well, dunnage may not be glamorous, but it’s much more than just “stuff.” If used properly, you’ll never need to give it a second thought. But if it’s not, it can create issues that will cost you both time and money.
Quality dunnage required
A tremendous amount of time, effort and money goes into making the best and highest quality precast product possible. Amazingly, that can all be lost due to a few scraps of wood. Dunnage, sometimes called blocking, is often a secondary consideration within the full spectrum of a complex precast concrete plant’s production. However, proper application needs to be a vital component and included within the ongoing training regime of the precast workforce.
Dunnage comes in a variety of materials and shapes to meet the specific needs of the product it is designed to protect. Common materials used for precast concrete are wood, plastic, Styrofoam and rubber. While this can be a good application for reuse of discarded timber, careful inspection of the materials is necessary. This should include removing any hardware, looking for hard knots and ensuring the material will not adversely stain or damage the product or create an unlevel stacking situation.
Quality dunnage will provide many safety, quality and economic benefits including:
- easy access for lift truck fork
- placement or lifting slings
- uniform support for products
- separation from the storage yard surface
- protection from staining
- stability on the truck trailer
Providing easy access
Although most precast products are cast with various lifting inserts, for speed and convenience, most product handling in the yard is done by large fork trucks. Attempting to slide a steel fork under a precast product lying flush on the ground can lead to chipping or more severe damage. However, appropriate placement of dunnage provides easy clearance for the forks which can apply uniform bearing on the precast product when lifted and substantially reduce the chance of a spall. Additionally, products stacked with dunnage provide adequate openings for workers to safely slip lifting slings around the product.
Section 4.8 of the NPCA Quality Control Manual for Precast Plants states products are to be stored on firm and level foundations to avoid or minimize product damage. Over time, these surfaces may become uneven or develop hard spots that could introduce an unexpected concentrated load on the precast structure. In turn, this could result in a spall or crack. Properly placed dunnage can easily be checked for levelness and provides a uniform surface to distribute the load of the precast structure. If products can be stacked, dunnage could possibly distribute the weight of other structures as well.
The following general equation provides a minimum bearing area of the dunnage:
A = W/(0.3 x f’c),
W = load on dunnage, lbs.
f’c= concrete compressive strength, psi
A = dunnage contact area, in.
For example, some precast wall panels will be stored by stacking them horizontally and parallel to the ground. The panels have a compressive strength of 4,000 psi and the calculated total weight at the bottom of the precast panel stack is 40 tons. You plan to use two lengths of dunnage boards to support the stack. What is the minimum required contact area for each of the two dunnage boards?
Total precast load = 40 tons x 2,000 lbs./ton = 80,000 lbs.
W = 80,000 lbs./ 2 boards = 40,000 lbs.
A = W/(0.3 x f’c)
A = 40,000 lbs. / 0.3 x 4,000 psi
A = 33.3 in.2 contact area per board
Next, find the required length of each dunnage board based on the required contact area and board’s width. Remember, lumber’s actual dimensions are less than those portrayed by its nominal size. For example, a 2×4 measures 1.5 in. x 3.5 in. If the dunnage selected in our example is a hardwood 2×4, the minimum length of the dunnage board is to be 33.3 in.2/ 3.5 in. = 9.5 in. It is general practice for the dunnage length to run the entire length or width, depending on orientation, of the supported product. This provides easier placement with better assurance of proper dunnage location rather than smaller sections of dunnage carefully placed in specific spots. Smaller sections of dunnage, which provide adequate contact area can be used, but may require a higher level of worker training and QC inspection, or the development of a dunnage placement template to assist with correct and consistent dunnage placement.
Dunnage is typically not required by product specification, but its use should be considered a best management practice. In cold climates, precast components stored directly on the ground can literally become frozen to it, requiring careful effort to not damage the product when attempting to free it. More importantly, precast drainage products employing bell and spigot jointing systems require the joint surfaces to be very clean to function properly. The proper use of dunnage provides the best method to keep joints ready to ship without additional time and effort required to scrape and clean the joint surfaces. It is equally important for the contractor to store the material properly on-site to keep these precast joints out of the dirt and free of contamination. To encourage good storage practices, a precast company could offer a toolbox talk and furnish the necessary storage dunnage for the project. Upon completion of the precast installation, the dunnage could go back to the precaster or the contractor could keep it for the next project.
Not just for precast
Many precasters store important steel items like forms, pallets and reinforcement outside. Steel components kept outside should be stored on dunnage to avoid contact with the ground. The dunnage should be made from a nonconducting material and be high enough to ensure the stored product will not come in contact with soil or standing water. Any dirt or debris on the steel will compromise its bond strength with the concrete, and steel stored in contact with the ground for prolonged periods can be more susceptible to corrosion due to its continued contact with moisture. For the reinforcing steel, dunnage spacing must account for potential sagging of the steel between dunnage supports. The American Concrete Institute concisely states this within ACI 318, section 9.1.6, “Product delivery, handling, and storage.”
Protection from staining
For some precast products, small visual anomalies caused by dunnage staining are not a primary concern. However, for architectural precast products, looking good is a primary attribute, and proper dunnage practices need to be used. Traditional dunnage materials may leave staining on the contact surface. Even plastic-coated wood will create color differences by trapping moisture between itself and the concrete surface during the curing process and yarding of the product. Consequently, specialized architectural concrete dunnage products have been developed to answer these needs.
One such product is a high-density multimonomer – a non-leaching plastic bubble surface sheet which limits the dunnage contact area to small surfaces spread at uniform spacing. This configuration permits increased air flow and reduces discoloration. However, calculations are important to ensure an adequate area of bubble dunnage is used to not exceed the allowable bearing on the concrete section. The precaster should be able to obtain dunnage information for this calculation from the dunnage manufacturer.
Stabilizing the load
For many precast concrete products, the most severe loading is not the final installed condition, but rather loading, unloading, contractor handling during installation, and jostling and dynamic loading during shipment. ACI 318-14 states, “Design of precast members shall consider all loading conditions from initial fabrication to completion of the structure, including form removal, storage, transportation, and erection.”
Going back to its original cargo shipping roots, good dunnage used during truck transport can assist in reducing damage or cracking. One option for truck dunnage is stiff Styrofoam. This material will slightly compress under load and will absorb some of the road shock transmitted to the precast component. Similarly, rubber dunnage planks can be used.
Dunnage can also be applied to the sides or top of products to prevent products from rubbing against each other and fill any voids to protect them from tie-down chains or cables.
Placement and stacking
The ability to stack precast inventory without damage frees up valuable storage space for more production and is one of the greatest economic paybacks with good dunnage practices. The correct placement of dunnage requires careful analysis whether it is an individual product or a stack of products. In most cases, the best location to place dunnage is at or very near the position of the designed embedded lifting device inserts. For many precast products, dunnage placed at this location results in similar stresses on the product to those anticipated during lifting and installation.
The most common dunnage configuration is to provide four dunnage pads in the corners of a rectangular section or two full-length dunnage boards creating a simple span for the stacked products. Though rules of thumb need to be reviewed to ensure applicability for stacking precast panels, the “fifth-point rule” is a common placement method of full-length dunnage. This rule places the dunnage approximately one fifth of the section length or width in from the edge on both sides, leaving 3/5 the product length or width between the two boards. Should a third dunnage member need to be applied, careful attention should be given to ensure all the dunnage boards are equally level and at the same elevation to eliminate any high or low spots.
Similarly, stacked component dunnage should be carefully aligned with each preceding level of dunnage, and the components themselves should be in alignment without individual components overhanging the others.
The NPCA QC Manual, section 4.8.3 reads, “Products shall be stored in a manner that will minimize damage caused by uneven bearing, improperly located dunnage blocks, stacking products too high or difficulty in handling.” The permissible stacking height is to be determined by the QC personnel or engineer after analyzing the dunnage bearing area requirements and soil bearing capacity described earlier, in addition to forklift height safe accessibility and stack stability. The stack stability will require additional analysis. In general, the stack height should not be greater than twice the component base width, unless specifically designed and documented. It is important to have a written stacking policy and train plant personnel on proper stacking and dunnage placement and related safety hazards of improper stacking or changing conditions in the stacking area.
The final word on dunnage
While you may not give much consideration to dunnage on a day-to-day basis, as long as you are manufacturing, storing and shipping products, you are truly never done with dunnage. Review your current precast concrete product storage plans to make sure you are employing the best methods for safe, damage free, and easily accessible storage of finished and received products. And ensure your workforce is trained to properly apply those methods each and every day.
Eric Carleton, P.E., is NPCA’s director of codes and standards. He is also an ASTM Award Merit recipient and currently serves as vice-chairman of ASTM C13, Concrete Pipe.
1 American Concrete Institute ACI 318-14 Building Code Requirement for Structural Concrete
2 Recommended Practice for Precast Prestressed Concrete Composite Bridge Deck Panels, Ross Bryan Associates, March/April 1988 PCI Journal
3 NPCA Quality Control Manual for Precast Concrete Plants, 13th Edition