Post-Tensioned Floor SlabsAs you know, post-tensioned slabs have been around for a long time and have recently become more popular. Concrete institutes and associations have established allowable variations (tolerances) for post-tensioned slabs that do not coincide with other allowable tolerances to which you or I may be held.
The age-old problem with concrete is that once it’s poured you pretty much have to live with whatever you get. My knowledge of concrete is very limited, but I don’t believe there is much you can do if a concrete slab does not align with the slab below or above or if the slabs roll up and down 1/2 inch to 1 inch in variation. I do know that those organizations that establish tolerances for concrete, in so many words, suggest that before an architect, owner or general contractor selects a particular concrete system, they must first determine if the established allowable concrete tolerances will coincide with the allowable tolerances of the overall project.
I’m also aware that the concrete institutes advise installers of concrete systems to consider the allowable tolerances for the overall project and not just their scope of work. During bid preparation and installation, the concrete installer must consider the tolerances other trades are contractually obligated to follow.
Allowable TolerancesHere is an example: If your company is installing acoustical ceilings in a post-tensioned concrete building and your ceiling must be level within 1/4 inch in 10 feet, you can surely put the ceiling in level. However, if the ceiling height requirement was exactly 9 feet off the floor slab, your ceiling could vary anywhere from 1/2 inch to 1 inch in relation to the floor slab.
If your company is installing metal studs and drywall in a post-tensioned concrete building and you are required to hold a 3/8-inch gap at the top and bottom of the wall for sound caulking, you can surely do this also – at a great cost to your bottom line. This detail requires you to keep a 3/8-inch gap in relation to the floor slab only. How do you keep a 3/8-inch gap at the top and bottom of your drywall when the concrete slabs vary 1/2 inch to 1 inch? To my knowledge there is only one way to do this: Cut the top and bottom of your drywall to 3/8 inch from the concrete.
Since the architect drew the detail in relation to the floor slabs, you are in a difficult position because you signed a contract in which you agreed to give them a 3/8-inch gap at the top and bottom of your walls in relation to the concrete.
In the case of the acoustical tile ceiling height varying from 9 feet to 9 feet, 1 inch due to the floor slab variation there is little if any cost to productivity. However, the ceiling contractor does have exposure if the owner’s quality controller says the ceiling heights are unacceptable.
In the case of the drywall contractor, productivity is greatly impacted because being required to hold a 3/8-inch gap at the top and bottom of his walls means each or many of the sheets will have to be trimmed to hold the gap. The additional risk to the drywall contractor is the fact that the wall is sound rated and that the gap must be filled with sound sealant to insure the sound transmission is within specification.
Not Just DrywallSubcontractors who install their products in concrete structures where the concrete tolerances are larger than the allowable tolerances allowed other subcontractors are at risk. You are at risk if your company is required to adhere to tolerances that are less than the allowable concrete tolerances.
Doorframes, handicap accessories, reveal moldings, expansion joints, cabinetry, electrical and plumbing fixtures are just a few of the products that have smaller tolerances than concrete.
Another interesting thought about concrete structures is the willingness of owner, architect and GC to accept or allow concrete structure builders greater tolerances than they will allow the trades who finish the inside of their building. I know that an architect will reject a ceiling or wall that is 1 inch out of plumb or level, yet a concrete floor slab that has a 1-inch variation will be overlooked “because it’s concrete,” or maybe because concrete was the most cost-effective system to use even though the tolerances do not coincide with the overall project.
Finishing Concrete CeilingsThe bottoms of concrete ceilings do not have to be unsightly, but in most cases they are due to improper forming installation, worn out form material or both. Because it is more expensive to have concrete sacked and ground, drywall contractors are often asked to skim the bottom of concrete ceilings to make them look attractive.
The problem is that drywall contractors really don’t know how bad the concrete will be until they see it. The concrete finish may be much worse than the drywall contractor expected but still within tolerance.
The ArchitectAllowable tolerances have been set by most subcontracting associations including concrete tolerances. Can someone tell me why concrete tolerances are greater than most other tolerances? Why would an architect choose a system that has larger tolerances than all other trades must work within? Why would a quality controller allow a floor slab to vary 1 inch and hold a drywall or ceiling subcontractor to a more rigid tolerance?
Personally, I have never seen a concrete structure torn down because it was not in tolerance, although I can assure you that walls and ceilings have been removed because they were out of tolerance.
An architect may show a grab bar in a bathroom 32 inches above a finished floor that varies 1 inch. Subcontractors ask for benchmarks, or an elevation mark, to work off to keep all elevations constant in relation to the mark rather than the floor. However, the building inspector doesn’t measure off the benchmark; he measure off the floor to make sure the grab bar is at the correct height under the Americans with Disabilities Act. Does the architect care that the floors vary 1 inch? Does the architect measure off the floor to see if the lower or upper cabinet was set at the right height?
How do we build a building accurately when a floor slab varies 1 inch? What does it cost us to work within tolerances that don’t coincide with each other?
General ContractorsA general contractor asked me why the sheetrock hanging was going so slow. Actually, he really asked me to get more men because the hanging was going slow. Instead, I decided to meet with him to explain why things were going slow.
I told him that we had to cut or rasp every sheet in order to hold the required 3/8-inch gap for caulking because the floor and ceiling slabs varied so much. I explained that our production was cut in half due to the floor slab variation while still having to hold the 3/8-inch caulk joint. The GC said he would talk to the architect about allowing us a greater tolerance or allowing us to just caulk the top and bottom track to the slabs and eliminate the full 3/8 inch sound caulking. The architect refused based on the fact that the 3/8-inch gap at the top and bottom was needed to meet the sound transmission requirements.
The GC then asked if the gap could be larger than 3/8 inch if the entire gap was filled with sound caulk. The architect said it would be acceptable at the bottom of the wall but not the top. The bottom of the wall is the most difficult to caulk and the top of the wall must be level in relation to the benchmark and not the floor. In order for our sheets to go up plumb the tops and bottoms of the drywall had to be cut or rasped no matter the gap size.
Is there a better way to solve this problem or is it more complicated than splitting an atom?
SuggestionsI’ve found that it costs more money to work in a concrete structure that has greater tolerances than what we are allowed. We do have the option of going to the job site after the first floor slab is poured to measure slab variations to see if they coincide with the tolerances we are obligated to work within. If the slabs exceed our tolerances, we need to notify the GC of the problem.
Most likely, the concrete tolerances will be within industry standards but exceed the tolerances we are required to hold. The GC will reply that the floor slabs are within industry standards. The way I read the industry standards for concrete tolerances indicate that the concrete installer must consider the overall tolerances for the entire project rather than just the industry standard tolerances for concrete. (This is a hint to get a copy of the concrete institutes tolerances.)
Another suggestion is to incorporate wording into your proposal and contract that would protect you from costs or liability if the concrete tolerances were greater than your tolerances.
The final solution, and likely the best solution, is to charge more for your work when you know that the building structure tolerances are greater than the tolerances you are obligated to work within. I’d rather pay for my own mistakes than pay for the mistakes of others.
Remember: Teamwork begins with a fair contract.