Tilt-Up Repairs and Connections: Your Guide to Repair of Common Field Challenges
By: Jim Baty, Technical Director of the Tilt-Up Concrete Association
and Jim Ness, P.E., Monroe & Newell Engineers, Inc. jness@monroe-newell.com.
“If you fail to plan, you plan to fail” is certainly an appropriate mantra for site-cast Tilt-Up construction. While Tilt-Up is known for its streamlined efficiencies and use of non-skilled labor, the investment of time in the pre-planning process can eliminate costly delays during the erection process. Proper planning and preparation also lay the groundwork for creating an efficient panel layout and forming process, as well as facilitating a fast construction process — one of the primary benefits of Tilt-Up construction. Foundations or embeds that are mislocated, settlement issues, panels with incorrect reinforcing and other common scenarios can cause huge problems. However, with a basic understanding of typical challenges, you can eliminate issues on your next job.
DETERMINING THE ROOT CAUSE
The growth in complexity of architectural treatments for Tilt-Up facilities during the last decade has mandated strong planning skills as a key element for project success. Although Tilt-Up allows you the flexibility to make changes in the design right up until two days before the panels are cast — allowing you more flexibility than dealing with a precast factory — once the panels are cast, the strength of concrete makes you live with your decisions. Good Tilt-Up design that responds to today’s architectural trends is an economic balance of panel thickness, reinforcing steel, concrete strength and panel width as related to addressing design loads for service and construction conditions. As such, planning and coordination with the general contractor are key. The design engineers and architects must have a working knowledge of how Tilt-Up construction is completed.
From a field perspective, planning is also crucial if the crews are to take full advantage of the speed and ease of construction enjoyed by the method. Simply taking the architect’s thoughts and putting panels around it can have drastic effects on price, as well as the contractor’s ability to construct the facility. Rather, designing for ease in the field is important. Even the most experienced of Tilt-Up contractors will come across situations that extend them out of the comfort zone and into situations that require increased or improved planning and deeper interaction with design professionals. Simply, the industry is expanding so rapidly, both in volume and in project types, that there are very few ‘typical’ projects—ones that contractors can take lightly or in the normal course of business.
COMMON CHALLENGES
So what are these modern challenges and how do they impact today’s Tilt-Up project? There are few projects of any type or any construction method that are not met with challenges, changes or problems. The key is how you address these challenges and turn them into project success. With some insight, many of these challenges can be easily addressed to ensure minimal impact on the schedule and budget.
Following is a listing of some common problems encountered on Tilt-Up projects, as well as recommended solutions. Be sure to note that this quick guide by no means replaces the importance of having the engineer-of-record and the general contractor involved in evaluating any challenges in your project and determining the appropriate solution.
Challenge: The footing for a Tilt-Up panel is mislocated. If left unaddressed, this can cause misalignment in the Tilt-Up panel set on the footing, resulting in an induced eccentricity in the load path between the building structure and the ground. This may be caused by something that was missed in the plan check process or a failure in the layout check prior to forming and/or concrete placement.
Solution: Once it is determined that a footing is mislocated, the first step is having an engineer check the footing with the eccentric load from the panel and roof framing above. The outcome may be that the footing can accommodate the eccentricity, but more often than not, the footing will need to either be removed and replaced or an additional footing cast adjacent to and doweled into the existing footing.
If the mislocated foundation is a drilled pier, the engineer should again review the load eccentricity and may propose the installation of a large pier cap. If panels have not yet been installed, the best solution may be to install two additional piers, one on each side of the in-place pier. If panels have been installed, cast additional piers on the inside and outside with a concrete beam between them. Be sure to allow time for the concrete in the piers and beam to cure before setting the panels.
Challenge: The tolerance for the top of the footing elevation is as stated by ACI, “+ 0.5-inch to 2 inches.” The grout joint between the Tilt-Up panel is ideal between 1 and 2 inches. The top of a portion of the footing ends up greater than the tolerance and is too high to maintain an effective leveling process and grout base for the panel. This is typically caused by finishing or striking accuracy along the top of the footing or an incorrect footing elevation was established during the forming process.
Solution: There are three solutions based on when the issue is identified. If it is caught prior to forming the panels, the overall panel height can be adjusted upward to accommodate the out-of-tolerance footing if the bottom of panel elevation is not a design element. If it is caught during the placement of shim-stacks prior to erecting the panels, it may be possible to remove some of the panel by sawing. In some extreme cases, the footings may need to be removed and replaced. In any case, the engineer-of-record should verify that the required frost depth has been achieved. If the footing is too low, additional shims, up to a maximum height of 4 inches, can be used to support the panel. Beyond the 4 inches of shim height, additional concrete can be applied to correct the top of footing elevation per the direction of the engineer.
Challenge: Determining the soil bearing capacity is a critical step in the design and construction process. Settlement of the soil may, however, occur during site preparation due to the influence of weather such as saturation or freezing. There are also extreme cases when settlement may occur when the panel load is not evenly distributed to the footings or after the project is finished. These extreme cases occur when the soil capacity differs between areas of sampling or when the temporary point load of the panel and building structure is greater than the designed condition. The result in changes in the footing and panel elevation can cause panels to lean outward or inward, adding eccentricity to the load path or lean panels toward each other, creating deviation in panel-to-panel joint width.
Solution: When the site condition is identified during preparation, contact the structural engineer and soils engineer immediately to determine how much settlement has occurred. The soils engineer should observe the soils and verify the changes in capacity. The structural engineer should check the foundation design to see if it can accommodate the lower soil capacity or change in condition. The general contractor should review the panel erection sequencing to discern the impact of sequence changes, allowing some areas of the project to proceed, while remedies are considered for those affected areas. Be careful to exercise caution and thoroughly investigate each solution. Further, when looking at alternative foundation solutions, be cognizant of the heavy loads on a Tilt-Up project. Options include:
- Wider or additional footings
- Revise foundation system: When the site condition is identified, as the footings are loaded or after the building structure is in place, more extreme solutions must be investigated including removing and replacing footings or mud jacking or soil “jetting” consolidation under existing footings.
For settlement issues, when the foundation consists of drilled piers, options include:
- Additional drilled piers
- Add footings (possible combined system)
- Shift the building
- Alternate the foundation systems
And, for drilled piers with spalled top, usually caused by high concentrated loads, options include:
- Install concrete “collar” or pier cap similar to mislocated pier
- Cut down top of pier and install pier cap
- Recommend minimum of 18-inch diameter piers for typical panels and pier caps where loads are high
Challenge: The slab quality is one of the major influences on the quality of the Tilt-Up panel. Quality slabs begin with quality sub-base preparation and finishes with placing the slab over the sub-base as soon as possible after compaction. On rare occasions, the contractor may observe slab movement prior to casting panels where changes in the sub-base or the soils have allowed areas of the slab to settle. This is observed by cracks or depressions in the slab and may be in areas where no load has been applied. This is typically caused by the subgrade heaving or settling or the sub-base becoming saturated prior to the placement of the concrete slab.Solution: The first step after determining what caused the movement is to relocate the panels on the slab to miss damaged areas, and then remove/replace sections of the slab (dowel slabs together). For slabs undermined due to subgrade sloughing:
- Core drill at several locations along saw cut joints
- Fill with “flowfill” concrete (1,500 psi minimum compressive strength)
- Contractor should use caution to not impact the panel finish
If the dowels across the slab joints were not installed:
- Check with the engineer-of-record regarding whether the panel design requires that slabs be doweled across joints
- If yes, activate the slab with dowels embedded into the slab or provide “keyed” slab.
Challenge: Part of the quality control in the Tilt-Up process is verification checks at each stage. One critical stage is the check of the panel reinforcing. Today’s Tilt-Up panels have complex reinforcement requirements given the size of the panels, openings and the loads applied to them as bearing elements. Occasionally, the reinforcement stage is completed incorrectly with either too little reinforcement or bars placed in the wrong locations. This may be due to misinterpretation of the construction drawings or discrepancies between the engineering drawings and the panel construction drawings.
Solution: In this scenario, the easiest fix is the addition of rebar adjacent to the reinforcing as installed. Unfortunately though, sometimes it is best to simply remove and reinstall. Be sure to work with the readily available materials on the site. Fortunately, if no concrete has been placed, nothing has been “set in stone.” This can be avoided through an effective implementation of drawing and process check procedures. Should the panel reinforcing be identified as incorrect after the concrete has been placed, the structural engineer will need to check the design and the panel may require stiffening to the wall panel with channels or tube steel (strongbacks) during erection. These may or may not be retained for long-term loads based on the structural engineer’s findings.
Challenge: Two hard and fast rules about concrete are it gets hard and it cracks. Tilt-Up uses a considerable amount of steel reinforcement and the cracks that occur to engage the steel are ideally microscopic in nature. When visual cracks in the panels occur, they are most often related to shrinkage in the concrete. There are some cracks that occur during the erection procedure due to flexural stress and these are most often not visible once the stress is relieved in the vertical position. Exposure of cracking is accentuated the most during times of surface moisture (rain) and when advanced finishes such as sand-blasting or acid-etching/washing are used.
Solution: Typically, no remedial work is required. To alleviate the problem in the first place, be sure
to check weather conditions during concrete placement and make sure the concrete slump is not too high. Be sure to have a structural engineer observe the cracks to verify that there is not structural implication to the observed cracking. Consult with the TCA for ways to minimize the aesthetic impact of cracking.
If the cracking is presumed to have occurred during erection, the structural engineer should be consulted to confirm the cause and impact. The design will be checked to see if the result might require a structural solution such as epoxy injection. If the cracking is significant enough aesthetically or if there is strong concern for the performance of the panel, recasting may be required.
Challenge: Like the mislocation of footings or piers, elements of the panel are subject to mislocation. Such items include openings, embeds for structural connections and inserts for panel construction. Typically, these items are mislocated because of drawing revisions or missed checks during the construction process.
Solution: If the issue is a window mislocation, there are two professionals to consult. The architect should confirm the impact to the design and whether a change must be made, and most often it will need to be. The structural engineer should check the panel design for modifications that can be made to the opening by sawing and filling. The new opening will need to be reinforced with channels or tube steel, similar to the retrofit of existing panels.
If the issue is a mislocated structural embed, longer pieces of connecting angles or plates will be required
as well as an alternate connection with expansion bolts. Be sure to maintain proper clearances and edge distances, as well as be careful to maintain structural integrity of the connection. If the embed is for roof framing, and is only slightly mislocated, shift the seat angle or joist and check with the structural engineer for design of the seat angle and embed plate. If the embed is not close, check with the structural engineer for use of continuous angles or a plate with expansion bolts. If the embed is a connection for a spandrel panel, you have a tough situation. First and foremost, be sure to talk with your structural engineer. Determine whether remedial connections can be exposed and look at possible external reinforcement with channels, plates or tube steel. Then remove the embed with a saw-cutter and reinstall the connection. Be sure to maintain structural integrity of the connection.
Challenge: One additional common concern is a “popped” embed. This typically occurs because of expansion/contraction of the panels, wet-set embeds poorly consolidated or an undersized embed.
Solution: After determining the cause, remove the source of distress. One remedial option is to epoxy-inject around the embed. If the embed has “failed,” install new connection(s) adjacent to the embed.
