When designing precast concrete products – such as manholes, utility vaults, water and wastewater structures, retaining walls, short span bridges or other custom precast products that are designed to resist soil loads – civil and structural engineers rely on appropriate codes and standards to match the design to the installation.
A major change was introduced in ACI 318-19: Building Code Requirements for Structural Concrete that affected the design of precast concrete structures. Previously, it was possible to calculate shear capacity based on concrete compressive strength and the geometry of the section. However, ACI 318-19 added requirements for both a size effect and the influence of flexural reinforcement in shear calculations. These revisions drastically reduced the calculated shear capacity by as much as 30 to 40 percent for many precast members, substantially increasing the amount of reinforcing steel needed to meet the same performance criteria.
In response, many designers turned to other codes and standards for design, such as the AASHTO LRFD Bridge Design Specifications, to preserve shear capacity that would otherwise be lost by referencing the ACI 318-19 code. AASHTO LRFD is commonly used to design buried precast concrete structures that experience vehicular traffic live loads, whereas, up until 2019, ACI 318 had been the go-to standard for most installations, having also been the primary reference in ASTM standards for many years.
In actuality, the reduced shear capacity in ACI 318-19 was not supported by real-world evidence. Precast concrete structures have demonstrated decades of safe and reliable performance, with millions of installations in service with no shear issues. This consistent success can be attributed in part to conservative assumptions, especially regarding soil pressure, but also due to the exclusion on beneficial soil-structure interactions such as arching, friction and passive soil pressure, which are often omitted due to the difficulty of quantification in standard design.
Recognizing these factors, ACI introduced major revisions in ACI 318-25, particularly in Sections 13.2.6 and 13.3.7, which apply to structures that bear against or resist pressure from soil. The update restores most of the shear capacity that was lost in 2019, allowing designers to once again calculate shear strength based on cross-sectional geometry alone. As noted in the commentary to section 13.2.6: “… members directly bearing on soil … and designed under previous editions of the Code have exhibited satisfactory behavior without consideration of the influence of size effect or flexural reinforcement effect on shear strength.”
This update is a welcome development for the precast concrete industry. It reaffirms what industry professionals have long observed: precast concrete structures are inherently durable, resilient and capable of delivering long-term performance under demanding conditions.
ACI/PCI 319-25 and its relevance to buried precast structures
Another notable update in ACI 318-25 is a new reference in Section 4.12.1.1 to ACI/PCI 319-25, a companion code developed specifically for the design of precast concrete members and connections. At first glance, this new reference suggests a shift away from the traditional reliance on ACI 318 for precast concrete design. However, in its initial release, ACI/PCI 319-25 has very little impact on the design of precast concrete utility structures.
With respect to shear design, key provisions in Sections 13.2.6 and 13.3.7 continue to reference ACI 318-25 directly. Likewise, shear requirements found in Sections 22.5 and 22.6 of ACI/PCI 319-25 defer to ACI 318-25, except in cases involving prestressed or pretensioned members. For this reason, designers working with mild steel-reinforced precast concrete components can continue to rely solely on ACI 318-25 without the need to consult ACI/PCI 319-25, unless they are engaged in the design of prestressed or pretensioned elements.
While the release of ACI/PCI 319-25 represents a new chapter in the development of precast-specific codes and standards, its current influence on utility structure design is limited. ACI 318-25 remains the primary reference for the design of conventionally reinforced precast concrete structures.