Tolerances are an important component of designing and manufacturing precast concrete structures.
There are different types of tolerances for precast, but the three main classifications are interfacing tolerances, erection tolerances and product tolerances.
Using the NPCA and PCI websites as resources, it is important to identify both the differences and commonality for industry tolerance standards. Once in hand, the math takes over and projects move forward.
What is tolerance?
Tolerance is the allotted or allowable variation that a structure can be produced based on the specified dimensions of the structure. For example, the industry-standard tolerance height for an architectural wall panel up to 10 feet may have a plus-minus one-eighth of an inch. The plus-minus means that the allowed tolerance for the height of the wall can be one-eighth of an inch over 10 feet or an one-eighth of an inch under 10 feet.
Product tolerances describe the dimensional relationships of each precast component that make up a whole structure. Product tolerances are necessary to ensure that all parts of the structure fit with one another.
It is crucial that the form’s dimensions are correct so that the manufactured product has the correct tolerances. It also is important to have a clear understanding of the appropriate product tolerances before a project begins, along with the correct interfacing tolerances and erection tolerances.
This creates clear expectations for the project, and it ensures that everyone is working within the same agreed tolerances.
Form flexibility is important so that precise adjustments needed for the structure’s tolerances are made and eliminate the chance for rework on the structure.
Quality control plays an important role in ensuring that a product is being produced correctly and to identify if any of the forms are becoming warped or bowed past allowed tolerance. If so, adjustments must be made to correct this.
There also are times when a project requires extremely tight tolerances, such as the trough pictured below that has a required tolerance of plus-minus one-sixteenth of an inch. These types of tolerances can be challenging as this only allows for an extremely small deviation from the specified dimensions.
Culverts and pipe
For example, box culvert industry tolerances require:
- Internal dimensions to remain within 1% from the design dimensions, and haunch dimensions are not allowed to vary more than one quarter of an inch.
- Variations of laying two opposite surfaces of the box section must not be more than one-eighth inch per foot of internal span and may only have a maximum of five-eighths inches for sizing of internal spans through 7 feet and a maximum of three-quarters inch for internal spans that exceed 7 feet with the exception where beveled ends for laying of curves are specified by the purchaser.
- Slab and wall thickness are not allowed to vary less than 5% or three-sixteenths of an inch
- The length of the underrun of a section is not allowed to be more than one-eighth inch per foot of length with no more than a half inch in any area of the box section
- The maximum variation for the position of the reinforcement of 5 inches or less for slab and wall thickness should be plus-minus three-eighths inch or plus-minus a half inch for greater than 5 inches.
- A cover over the reinforcement should never be less than five-eighths of an inch.
For precast concrete pipe, allowed tolerances include:
- Wall thickness should not vary more than plus-minus 5% or three-sixteenths of an inch.
- The underrun portion of the pipe should not exceed more than one-eighth inch per foot and should not exceed more than a half inch for any pipe length.
- Variations of laying the two opposite ends of the pipe should not be more than one quarter of an inch. This applies to all sizes through a 24-inch internal diameter.
- Variations of laying the two opposite ends of the pipe should not be more than an one-eighth of an inch per foot for any size with a maximum tolerance of five-eighths inch for any length of pipe through 84 inches.
- Three-quarters of an inch is the max length variation for a pipe with a 90-inch or greater internal diameter unless when the structure has a beveled end pipe designed to lay on curves
- The maximum allowed variation for the circumferential reinforcement is plus-minus 10% of the wall thickness or plus-minus a half inch
- The area of reinforcements is considered acceptable as long as the nominal area of the wire or bars exceeds or equals the required specifications.
Underground utility structures
For the dimensional tolerances measured on the inner surfaces including width, height and length:
- 0 to 5 feet has a tolerance of plus-minus a quarter inch.
- 5 to 10 feet has a tolerance of plus-minus three-eighths inch.
- 10 to 20 feet has a tolerance of plus-minus a half inch.
- 20 feet and longer has a tolerance that is understood and agreed between the customer and supplier.
- 0 to 10 feet has a tolerance of plus-minus a quarter inch.
- 10 to 20 feet has a tolerance of three-quarters inch.
- More than 20 feet has a tolerance that is understood and agreed between the customer and supplier.
Joint surface tolerances or the distance between joints without joint sealant:
- A tolerance of three-eighths inch.
- Each component should not deviate more than plus-minus one-eighths inch from specifications.
- Reinforcement should be within plus-minus a quarter inch of the specifications and the cover should not be less than three-fourths of an inch.
- Spacing for the reinforcements should not vary more than 1½ inch or deviate more than one-tenth of the specifications for the bar spacing.
Slab and wall thickness:
- The thickness of a wall and slab should not be less than the specifications, nor should it be greater than 5% or three-sixteenths of an inch.
Warping and bowing
Warping and bowing tolerances are especially important when aligning precast wall panels. These tolerances influence the way that the panels’ edges align with one another during erection.
Bowing is when the two opposite ends of a panel are parallel, and the section of the panel between the two edges becomes out of plane. It is more common that thinner panels will bow before thick panels bow.
Because of this, tolerances for thinner panels are usually more lenient.
Certain structures also require corbels or haunches, which are components that are anchored on a wall to support the weight of an attached structure.
These components also have tolerances. Typical haunch-to-haunch tolerance is plus-minus one-eighth to one-quarter of an inch.
Erection tolerances describe when precast structures are set in place. While all tolerances are important when designing and building precast structures, erection tolerances give the allowable tolerances of how a precast structure should be set in its final place. This also influences the way that adjacent structure components are set in place.
Equipment plays a significant role with erection tolerances. Because of this, variation of some of the tolerances may be necessary, especially for more custom products.
If erection tolerances are too constraining, this can present challenges for a precaster and installation costs can become more expensive due to the special equipment that may be needed.
To help ensure that the structure is erected properly, all structure dimensions should be checked prior to starting erection. After this takes place, the structure or precast components should be checked to ensure that they are within tolerances.
Here are a few examples of common industry standards for erection tolerances:
- A precast concrete component to another precast concrete component has a half inch (1 inch preferred) recommended clearance.
- A precast concrete component to a steel component has a 1 inch (2 inch preferred) recommended clearance.
- A precast concrete column cover has a 1½ inch (3 inch preferred for taller structures) recommended clearance.
Interfacing tolerances are associated with other systems or materials that interface with precast components. They are needed prior to or after the interfacing system is erected.
For systems that have multiple precast components, it is important to understand the allowable variations in the dimensions associated with other materials or systems that are in contact with or that are near the precast concrete.
Interface tolerances can involve structural steel, and even windows, doors, and ventilating systems and heating systems.
Tolerances play a key role in the design and function of precast structures. It is important to remember that the connection details must be considered when specifying erection tolerances and adequate spacing must be provided to perform the tasks necessary to complete the connection.
An engineer may choose to specify a minimum bearing size for various precast concrete products. It is important to remember that precast structures that do not meet tolerance specifications can be rejected.
To prevent this from happening, ensure that the customer, the engineer and the precaster are all on the same page.
Madison Longoria is the marketing manager for Locke Solutions.