Jim Klinger, concrete construction specialist The Voice Newsletter April 2022
Full disclosure: A wise man once offered the following bit of advice on how best to navigate the uncertain waters of concrete construction. "It's easy," he said, "you just have to position yourself to get lucky". So consider if you will, the following Hotline call fielded by the ASCC Technical Division last November that showcased the long-standing need for industry guidance for F-number testing of broom and swirl-type textured finishes. As recounted below, what started out as a routine call grew into a three-part test program sponsored by the ASCC Foundation. As you'll see it provides immediate value to ASCC members, and will help bring improvements to our industry. (N.B. Due to VOICE space limitations, there really is no good way to do this the justice it deserves. This writer has romanced certain passages below, but only for purely technical reasons).
"We recently completed concrete placement at level three of a 10-story, post-tensioned (PT) parking garage. The building geometry is based around a single-threaded helix. The typical floor plans show no level spots, meaning the structural frame is really one continuous ramp that slopes from the foundation mat to the roof. Approximate building footprint is 21,000 SF. The Division 3 specifications call for a "non-slip, non-skid, swirl-float" finish at all vehicle traffic and parking areas. Feedback noted in owner/design team site visit reports indicates satisfaction with all work placed to date. The ramp slabs are boldly exposed to rainfall during construction and no ponding of water has been reported, even after the elevated PT slabs have been stressed and the supporting formwork cycled.
Due to misunderstandings at bid time, no one on the project team included ASTM E1155 Flatness and Levelness Testing in their scope. As a result, the top surfaces of the first three floors were never tested for flatness (due to the "all ramp" slope condition, levelness testing does not apply). Since the Owner and Architect visit the jobsite weekly and are happy with the work in place, the lack of flatness testing went unnoticed until someone in the GC food chain scrutinized Section 03 35 00 CONCRETE FINISHING, which arguably assigns ownership of said testing to the concrete contractor. This is a public works project, which did not allow qualifications to be listed on the bid form. We have enjoyed a successful working relationship with the key project stakeholders for many years and have agreed to pick up the flatness testing scope for floors four through the roof; confident that an equitable commercial arrangement will eventually be crafted that will make us whole. That being said...we have examined the project specifications, and are not sure what flatness test criteria we should provide to our newly-hired test agency, especially for slabs featuring such a rough-textured finish.
We have been building concrete parking structures for many years and have never seen flatness tested on swirl finish jobs. Acceptance criteria was usually based on symmetry of the swirl pattern and sometimes water testing to detect ponding or birdbaths. If slab thickness was in question, that could easily be spot-checked at blockouts, sleeves, edgeforms and so on. Please advise how we should proceed with direction to the test agency.”
A review of the Division 3 specifications turned up the following:
"Comply with ACI 117 for local flatness/levelness tolerance measured in accordance with ASTM E1155. Specified Overall Value (SOV) of F/F=30 and Minimum Local Value (MLV) of F/F=25 ".
"Suspended concrete slabs: F/F: SOV=35 MLV=25 "
Based on recommendations made by the ASCC Technical Division, the concrete contractor submitted an RFI requesting that the flatness testing be waived. The rationale for this request is that there has always been confusion in the industry regarding the inherent variability in the surface quality of textured, non-skid applications such as swirl and broom finishes. In addition, no guidance for such testing appears in ACI 117-10 Specification for Tolerances for Concrete Construction and Materials. The design team was advised that based on experience and available ACI and other industry documents, the specified flatness criteria were likely to be unachievable.
A conference call with the designers was held. The design team would not waive the flatness testing but did acknowledge the "ramping slabs as non-critical" and reduced the specified flatness SOV to F/F=20 and MLV to F/F=15. The designers again expressed satisfaction with the work in place and agreed it was fit for purpose based on appearance and lack of ponding. The concrete contractor directed the test agency to start ASTM E1155 Flatness testing using the adjusted SOV=20 and MLV=15 criteria.
After testing two successive placements, the reported F/F combined scores were 17.90 and 15.84, just under the SOV required value of F/F=20. After reviewing this test data, the design team reconsidered the RFI request for waiver and agreed to cancel further testing.
The Rest of the Story (condensed version):
Within a few days of the Hotline call, we learned that the ASCC contractor member was planning to replace a concrete access road in their maintenance yard. Several thousand square feet (SF) of 6-inch thick slab was on the books for replacement. We requested permission to carve out a 20 ft. by 80 ft. section of the area for testing. The test slab was reinforced with #4 bars at 14-inch centers, placed over tightly compacted aggregate base. No vapor barrier was used. Prior to placement of reinforcing steel, the subgrade elevation was recorded with a laser scanner. The test panel was subdivided into four equal sections: bull float finish, machine float finish, broom finish and swirl finish. During concrete placement, the four test areas were separated by tooled, contraction joints. After placement, the top of each test panel slab section was tested for flatness by the test agency using a dipstick. At the same time a companion laser scan was performed by the concrete contractor.
Once the laser scan of the top surface was complete, we were able to use the test slab not only to test flatness of textured finishes, but to collect test data on slab thicknesses using laser scanning, impact echo, ground penetrating radar (GPR), and drilled core measurement methods as described below.
The top surface elevations were subtracted from the bottom base surface elevations to obtain concrete thickness at 1-ft. grid intervals. This resulted in about 1700 thickness values for the test panel. But how good was this data? We could find nothing in the literature comparing laser scan thickness data to any other methods. The contractor owned a ground penetrating radar device which was then used to scan the slab for thickness. About the same time, the ASCC Foundation funded the purchase of an impact echo device to allow members to collect slab thickness data. That device was loaned to the contractor who took impact echo thickness measurements from the test panel.
Finally, with all this data, we felt this investigation would benefit from the “gold standard” in slab thickness measurements—cores. We requested, and the ASCC Foundation funded, the drilling and measuring of 30 cores in accordance with ASTM C174 “Standard Test Method for Measuring Thickness of Concrete Elements Using Drilled Concrete Cores.” But what about measuring cores by ASTM C1542 “Standard Test Method for Measuring Length of Concrete Cores?” Once again, the Foundation funded the measurement of the same 30 cores by both the jaw caliper and ruler procedures in ASTM C1542.
ACI-ASCC 117 Tolerances is updating its tolerance specification and has appointed a new subcommittee on measurement protocols. The slab thickness data was presented to the main 117 committee at the Orlando convention in March 2022. While 117 provides for only core and impact echo thickness measurements, other methods are being used to investigate slab thickness. This presents a most interesting and difficult question—how should nondestructive test methods used to determine slab thickness be compared to a thickness tolerance originally established by core measurements?
Results of the testing and an in-depth discussion titled "F-numbers and Textured Concrete Surface Finishes" will appear in the May 2022 issue of Concrete International as part 1 of a 3 part series. Parts 2 and 3 are being drafted and are slated to be published later this year.
Thanks to ACI Honorary Member Bev Garnant and the ASCC Foundation for supporting the test program, arranging funding on short notice, and helping to position us to get lucky.