Jim Klinger, Concrete Construction Specialist The Voice Newsletter August 2023

Question: We are building an addition onto a small commercial steel building.  The vertical supports for the elevated work consist of relatively light structural steel columns that sit on concrete footings, each with footing dimensions 2 ft long by 18 inches wide by 12 inches deep. Our submitted mix design is 3000 psi at 28-day concrete with a 0.45 w/cm ratio. The general contractor (GC) is building the project schedule and has asked us "how long after the footings are placed must we wait before the steel columns can be erected?" We have always assumed 70 percent of the 28-day nominal design compressive strength was the rule of thumb everyone used to trigger steel erection. The strength test backup submitted by our ready-mix supplier indicates there should be no problem hitting that 70 percent threshold (2100 psi) in 7 days. We have not found erection wait time addressed in the construction documents or ACI documents, but this is a city job and we want to make sure. What wait time should we advertise to the GC?.

Answer: The erection wait time is going to be determined by several factors, some of which are not in your control. The governing body that actually sets steel erection requirements is OSHA (Occupational Safety and Health Administration). The two OSHA sections that apply are as follows:

1926.752(a) Approval to Begin Steel Erection: "Before authorizing the commencement of steel erection, the controlling contractor shall ensure that the steel erector is provided with the following written notification(s):

1926.752(a)(1) "The concrete in the footings, piers and walls and the mortar in the masonry piers and walls has attained, on the basis of an appropriate ASTM standard test method of field-cured samples, either 75 percent of the intended minimum compressive design strength or sufficient strength to support the loads imposed during steel erection."

Suggest reminding the GC at the preconstruction conference to submit a request for information (RFI) to the structural engineer, who needs to specify concrete compressive strength test criteria that the GC and steel erector can follow. Depending on the erection loads, it appears possible that the engineer could specify a compressive strength less than 2250 psi (75 percent of 3000 psi). At that time, the engineer should also specify what ASTM standard tests are acceptable, number of field-cured cylinders, and so on. All these items are outside of your wheelhouse, since they apply to a follow-on trade.

For your part, your ready-mix supplier should review the RFI response and confirm when their product should be able to meet the compressive strength requirements specified by the design team. The Owner's inspection agency will also be able at that time to determine what tests, if any, they need to arrange. Once the test results are acceptable, it is the GC's responsibility to follow OSHA and advise the steel erector in writing that steel erection may proceed.

Photo credit: CKC Engineers, Seattle

(N.B.: If this was a concrete high-rise job, it is totally conceivable that a slab could be placed in the morning, and by late that same afternoon a lift of columns could be placed on that fresh concrete slab as indicated in the photo, above. Also interesting to note here that ACI 318-19 discusses concrete compressive strengths required to trigger application of prestressing forces, which sometimes can be less...much less...than 2500 psi...see ACI 318-19 excerpt, below).

_____________________________________________________________________________

Question: We are preparing to place a slab-on-grade (SOG) next week in a large, enclosed warehouse setting where construction equipment appears to be creating an atmosphere heavy with carbon dioxide (CO2). The GC has asked us to provide information regarding potential concerns they have of carbonation of the new slab surface in such an environment. The GC has assured us the ambient atmosphere inside the enclosed warehouse on concrete placement day will meet OSHA regulations. Can you offer any guidance regarding the potential for the CO2 to affect the slab?

Answer: Carbonation is the process whereby the surface of a concrete slab can degrade when exposed to certain amounts of CO2 in the air. Since the GC has advised the air quality inside your work area will meet OSHA limits, we need to understand just what that means. According to the OSHA Occupational Chemical Database, the CO2 permissible exposure limits (PEL) are reckoned as a TWA (time-weighted average) of 5,000 parts per million (ppm) per hour, averaged over an 8-hour work day. Since we know what the CO2 content will be, the question then becomes "can that OSHA level of CO2 of 5,000 ppm potentially affect the surface of the concrete?"

And the answer is: not likely. According to one industry article, test agencies set the controlled "ambient" air CO2 content for concrete carbonation test purposes starting at between 30,000 ppm to 40,000 ppm, much higher than what will be ambient at your jobsite. In other words, if the humans will be OK, the concrete slab should be plenty fine as well.

Note: Normal ambient air CO2 content can run 300 ppm to 400 ppm outdoors all the way up to between 600 ppm to 900 ppm in urban areas. A link to the OSHA Occupational Chemical Database cited above is here.
_____________________________________________________________________________

Jim Klinger Joins Baker Construction’s Podcast

ASCC member Seth Tandett of Baker Construction hosts a podcast called "concrete logic" aimed at various stakeholders in the concrete construction business...contractors, engineers, architects, suppliers, educators and so on. By the time you read this, the podcast will be approaching its 60th episode. We visited Seth's podcast here a few weeks ago to tape Episode #54 and discuss the "Concrete Contractor's 12 Most Essential Books". To oversimplify enormously, the cost to purchase these publications is less than $600, the smallest investment with the biggest return a concrete contractor or structural engineer can ever make. We explain why during the podcast. Here's the list:

• ACI 301-20 Specifications for Concrete Construction
• ACI 117-10 Specification for Tolerances for Concrete Construction and Materials
• ACI 318-19 Building Code Requirements for Structural Concrete
• Field Reference Manual ACI MNL-15(20)
• ACI-ASCC Contractor's Guide to Quality Concrete Construction, 4th ed., MNL-5(19)
• Tolerances for Cast-in-Place Concrete Buildings: A Guide for Specifiers, Contractors, and Inspectors
• Guide to the Design and Construction of Concrete Toppings for Buildings
• User's Guide to ASTM Specification C94/C94M on Ready-Mixed Concrete, 2nd ed., ASTM MNL49-2ND
• Formwork for Concrete, 8th ed. ACI SP-4(14)
• ACI 302.1R-15 Guide to Concrete Floor and Slab Construction
• Significance of Tests and Properties of Concrete & Concrete-Making Materials ASTM STP 169D
• ACI 117.1R-14 Guide for Tolerance Compatibility in Concrete Construction

During the podcast, there is ample discussion describing why ASCC members (and other stakeholders) should have each and every one of these publications on their desktops. We stopped at 12 publications due to time constraints. Of course, there are other high-value documents that belong on your library shelf, but these 12 are a fantastic start.

Here's a link to Seth Tandett's podcast "concrete logic", episode #54. Check it out.