Contractors Make Recommendations Based on Experience

Contractors citing previous experience should make their recommendations with the understanding the final decision will be made by the designer and owner.

We sometimes get calls from knowledgeable contractors who ask us questions similar to the following:

I have a project in a northern state with 100-foot-long, eight-foot-high retaining wall, 50 feet of which is outside a building, with the other 50 feet inside the building. I’m thinking of telling the engineer that the wall should contain a thermal break to separate the inside and outside because, the differing exposure to temperature changes may cause problems. What’s your opinion?

I’m going to be placing a five-inch-thick floor slab. The plans call for #4 reinforcing bars spaced at 18 inches center to center, and on three-inch-high chairs. Plans show that all the bars pass through sawed contraction joints with none of them being cut at the joint. I believe this will cause out-of-joint cracking because the contraction joints won’t open. Should I suggest that half of the bars be cut at each of the contraction joints?

We’ll be placing a four-inch-thick floor for a library, for which an architect has designed a decorative pattern for sawed construction joints. The pattern results in some panels that are triangular in shape, with sharp acute angles. We believe the triangular panels will crack and want to suggest a normal jointing pattern with joints spaced at eight to 12 feet apart to create rectangular or square panels. What’s your opinion?

We’re bidding a job that includes placing a concrete topping on an existing slab. The specifications require us, the contractor, to design the topping slab. That doesn’t seem right. Is it common for contractors to design part of the work they’re performing on a job?  

Recommendations based on experience            

Yes, contractors should share their experiences, especially the negative ones. But make sure, when providing recommendations, that they are made with the understanding that you are not an expert in Code requirements and have not had the benefit of meeting with owners to understand their criteria. There are three different ways to communicate your concern:

  1. State your concern based on your previous experience with the design. Nothing more.
  2. State your concern and offer an ACI or ASCC document that indicates recommendations. Then it is up to the designer and owner to consider those recommendations.
  3. State your concern and provide recommendations based on your experience, but with the understanding that the final decision will be made by the designer and owner.

An approach for the four examples

If contractors see something in the plans or specifications that they believe will result in a concrete performance problem, either during or after construction, shouldn’t they call attention to it? That’s true. But the contractors don’t have to claim engineering knowledge to handle such cases. Let’s outline an approach for each of the four examples we cited.

The inside-outside wall

In this case, having no thermal break is not an obvious error in the plans. The structural engineer may have taken into account the temperature differences between the inside and outside portion of the wall and the possible consequences. Or the architect may have decided the difference in temperature wasn’t going to create problems within the building. Our advice, bring your concerns to the engineer or architect in writing—so you have a record. Then let the design professionals make this decision.

Rebar passing through a contraction joints

You’re correct that restraint from the rebar may not allow the joint to open and cause out-of-joint cracking. But that may be the designer’s plan. This floor has an area of steel/gross area of concrete equal to 0.22 percent. If more than about 0.1 percent passes through the joint, cracking at other locations is likely.

As a general rule, the continuation of larger percentages of deformed reinforcing bars should not be used across saw cut contraction joints or construction joints because they restrain joints from opening as the slab shrinks during drying, and this increases the probability of out-of-joint random cracking (ACI 360R-10 p.28). Some engineers, however, increase the amount of steel so there are more cracks, but the cracks are narrower and less likely to result in spalling. If that’s the case, however, sawing the joints is a needless expense. We’d suggest sending an RFI and noting that, in your experience, having that much rebar pass through the joints results in out-of-joint cracking. Then ask if that is the intent. Attach page 28 of ACI 360R-10 and let the engineer confirm his original design or make changes.

Triangular floor panels created by jointing pattern

We suggest an RFI noting that triangular-shaped panels are more likely to crack near the sharp acute angles, and note your concern for the increased cracking due to the jointing layout suggested. To support your concern you can cite excerpts from ACI 302.1R-15 (“Contraction joints should be continuous, not staggered or offset.”), PCA’s EB075 4th ed. (“Joints should create panels as nearly square as possible…”), and ACI CCS-1(10) (“When slabs are formed with very sharp corners, designers sometimes locate contraction joints at locations where concrete is most likely to crack.” (Fig. 8.6).

Contractor required to design a topping slab

Concrete contractors have no responsibility for engineering design unless such responsibility is required by the contract documents. Contract documents can require licensed design professionals to design formwork and shoring, or to have responsibility for design and construction of precast concrete elements or of a cast-in-place post-tensioned floor system. Any other design requirements in specifications that shift design to the contractor should be carefully considered during the bid, and excluded if they make the contractor uncomfortable.

The final words: Concern and Confirm

The designer has more information about the specific project, and more expertise in design than contractors do. So if contractors see construction requirements that seem incorrect, their role is to state their concern and then confirm the designer’s intent in writing. By doing this they can help the owner or design professional, but are not providing design services or accepting any design liability.

 

 Ed. Note: ACI 302.1R-16, ACI 360R-10, and CCS1 -10 can be purchased at www.concrete.org

                EB075, published by the Portland Cement Association, can be purchased at www.cement.org

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