The minimum requirements for joints in precast structures

Wells Concrete BlogUncategorized

Credit: Original article published here.

What is a joint?

Figure 1: Block 9, Fargo, ND

A joint is an intentional space that typically creates a tolerance for clearance between adjacent construction elements and provides a buffer area where production, erection and interfacing tolerance deviations can be absorbed.  Product tolerance relate to the dimensions and dimensional relationships of the individual precast components.  Erection tolerance are required for the acceptable matching of the precast members after erection.  Interfacing tolerance are associated with other materials or building systems in contact with or in close proximity to precast concrete, both before, during and after precast erection.  Joints may also be required to accommodate changes in wall panel or structure dimensions caused by changes in temperature, moisture content, or deflection from applied design loads.  Born is the purpose of a joint for precast structures.  Figure 1 and 2 show some examples of needed joints.

Widths of Joints

Figure 2: US Bank Stadium, Minneapolis, MN

The architect establishes the tolerances required to make the building concept work and must temper the desire for close tolerances with the knowledge of what can be practically achievable in the plant during construction, and in the field during erection.  It is also worth noting that precast products can attain a much lower tolerance than cast in place, being constructed in plant conditions in a controlled environment.

Joint width must not only accommodate variations in the panel dimensions and the erection tolerances for the panel, but must also provide a good visual line and sufficient width to allow for effective sealing.  The performance characteristics of the joint sealant should be taken into account when selecting a joint size.  Joints between precast concrete units must be wide enough to accommodate anticipated thermal expansion, as well as other building movements and proper sealant installation. When joints are too narrow, bond or tensile failure of the joint sealant may occur and/or adjacent precast units may come in contact and be subjected to unanticipated loading, distortion, cracking, and local crushing (spalling).

Joint widths should not be chosen for reasons of appearance alone, but must relate to panel size, building tolerances, joint sealant materials, and adjacent surfaces.  The required width of the joint is determined by the temperature extremes anticipated at the project location, the movement capability of the sealant type, the temperature at which the sealant is initially applied, panel size, fabrication tolerances of the precast concrete units and panel installation methods.  Tolerances in overall building width and length are consumed in the panel joints.  Take a plan dimension of a building, out to out, being 2” too small over 300’.  The joints widths are slightly decreased over most the wall during erection to properly locate corners at a slightly decreased grid out to out.

To provide optimum quality for the installation and performance of sealants, the architect should specify a minimum panel joint width of no less than ¾”, (Architectural Precast Concrete MNL-122).  Wells Concrete uses 5/8” to cheat it down a bit more.  This is the minimum nominal joint width needed to adequately account for production and erection tolerances, and still maintain an effective minimum joint width that can be caulked properly.

Consider the following items when determining the appropriate joint size:

  • Product tolerance
  • Type of component
  • Size of component
  • Type of abutting construction
  • Location of component
  • Component movement
  • Function of component
  • Erection tolerance
  • Space required for fireproofing
  • Thickness of plates, bolt heads, and other projecting elements
  • Deflections with elements directly below
  • Roof or floor member elastic shortening and camber can shrink the length, requiring an increase in pour length to accommodate
  • Shrinkage of columns under heavy loads from levels above
  • Volume-change deformations (temperature changes, shrinkage, and creep) which cause movements (volume change) of precast components
  • Windows and doors
  • Mechanical and electrical equipment, both post installed items, and large equipment installed after precast erection
  • Elevators and escalators
  • Architectural cladding
  • Structural and miscellaneous steel
  • Masonry
  • Roofing
  • Waterproofing
  • Fireproofing
  • Interior walls partitions and finishes

One advantage of jointed construction is the ease of defining load path through connections, at the joints.  Connections can be designed for specific directional resistance, while maintaining flexibility in one or more other directions.  These connections can also allow for thermal expansion and contraction of the building or members.  They can be designed to yield, or give, allowing the concrete stress relief, preventing cracking!  Precast components connected at their joints can create construction that either is monolithic at the critical joint or provides connections allow the precast components to act monolithically.  The code allows this provided they are connected so they will perform essentially the same as a cast in place concrete unit, (Precast and Prestressed Concrete MNL-120).

Type of Joints

Figure 3: Single Stage Joint

There are three basic precast wall unit joint types: one-stage, two stage, and expansion joints.

The one-stage (face sealed) joint has a single line of caulking for weatherproofing.  This is normally in the form of a gun-applied sealant close to the exterior surface of the precast member.  Figure 3 shows a typical single state joint.

Water tightness of sealant joints can be improved by installing a second line of sealant in each joint, the two-stage joint.  This layer provides redundancy in the system, as it is fully protected from weather and UV exposure by the outer layer of the sealant.  This approach may require the installation of weep openings in the exterior seal to allow water contained by the inner seal to exit the cavity between joint seals.  This requires care in detailing and construction.  Failure to provide these weep openings may result in trapped water within the joint and ponding against both seals.  This accelerates deterioration of the sealant material and its bond to the substrate. Figure 4 shows two stage vertical joints.

Figure 4: Two Stage Vertical Joint

Expansion joints are placed in structures to limit the magnitude of forces, which result from volume-change deformations (temperature changes, shrinkage, and creep), and to permit movements (volume change and seismic) of precast components.  Joints that permit contraction of the structure are needed to relieve the strains typically cause by temperature drop, creep, and shrinkage.  For expansion joints to be effective, they are required to be watertight, provide a full range of movement, complement the durability of the deck or structure, and be low maintenance.  They may be especially needed for non-rectangular structures and should be located at places where the plan or elevation dimensions change radically.  Their location should also consider gravity load paths, and be ideally located parallel to nonloadbearing precast joints in roof and floor members, as members could slip and lose bearing.  The width of an expansion joints depends on many variables like building shape and size, member shapes and sizes, as well as the spacing and number of multiple expansion joints that may be required.  Expansion joints must accommodate both vertical and horizontal movements.  Typical expansion joint sizes can vary from 2” to 12” depending upon its use, a may be required in building footprints from 250’ to 350’ or larger.  Figure 45 is an example of an expansion joint detail.

Figure 5: Expansion Seal in Vertical Joint System

Erect-ability

Erection tolerance must be provided to perform the tasks necessary to complete any connection across the joint and must account for the most adverse combination of tolerances to supply space for precast member adjustments to attain the alignment required.  Below are the production tolerances, taken from PCI tolerance manual 135, for an architectural panel, that joints must allow for during erection.

Shims play an important role in the construction of architectural precast concrete.  A horizontal joint is required for the placement of shims.  They are used to transfer gravity loads to the foundation.  A typical joint at the bottom of a non-exposed wall panel to foundation is 1 1/2” to allow for extra adjustments.  An exposed joint is typically 1”, allowing room for a bead of caulk, grout and shim space.  Shims also enable the erector to adjust precast elements to achieve proper joint widths and alignment by adding shims at one side of the wall panel while removing them from the other side.  This will tip the panel plumb, ahead or behind, parallel to the wall line, depending upon what is required. Typical vertical wall jointing usually reflects roughly the width of any reveals, or false joints, used on the project, or other architectural features.  Joints should not be too wide or small, generally less than 1” and greater than ½”, to allow for a good caulk seal. Too small may not allow for precast adjustment.  We want brick, reveals, formliners, buildups, cornices and all architectural precast elements to align to achieve the most desirable look possible, and the shim space makes that possible. Figure 6 are some product tolerances and 7 are some of the erection tolerances of joints, (Tolerance Manual for Precast and Prestressed Concrete Construction MNL-135).

Now you can see that there are many items to consider when sizing the joints of a precast building system. Figures 8, 9, 10 and 11 demonstrate plant and erection tolerance for a particular arch member, taken from Rochester Parking Ramp #6, Rochester MN.

Dustin Jones, P.E.

References

PCI, Architectural Precast Concrete MNL-122
PCI, Tolerance Manual for Precast and Prestressed Concrete Construction MNL-135
PCI, PCI Design Handbook, Precast and Prestressed Concrete MNL-120