Shotcrete Core Evaluation

Posted on April 15, 2021 in: Technical

Jim Klinger, Concrete Construction Specialist, The Voice Newsletter, April 2021

Full disclosure: This writer’s first experience with shotcrete was in 1987; when the general level of industry understanding of structural shotcrete was evolving, shotcrete wall reinforcement was typically limited to a maximum #5 bar size, and only a minimal use of reinforcing steel contact lap splices was allowed. Now, here we are some 34 years later, and some truly amazing structures are routinely being placed running structural concrete through a nozzle. Stragglers are finally grasping the fact that shotcrete really is cast-in-place concrete, seismic zone shear walls featuring #14 and even #18 bars are successfully being shot, and ironworkers seem to be earnestlly tied up completing more laps than Michael Phelps in an old gunite pool).

When specified in the construction documents, the process of qualifying shotcrete nozzlemen for a specific project requires close collaboration between the concrete contractor and the project structural engineer (LDP). It is up to the concrete contractor to study the construction documents, then prepare a shotcrete mixture design and reinforcing steel shop drawings for each preconstruction mockup panel intended to simulate the most congested conditions that could be encountered in the structural walls at a forthcoming project. Typically, the most congested (and therefore most challenging) shotcrete wall placement conditions are likely to occur near large-diameter bars, in shear wall boundary elements, at corners, around large openings, and at pilasters. Once the LDP reviews the “worst-case” conditions and approves the shop drawings, preconstruction mockup activities in the field can begin and the proposed shotcrete nozzlemen’s skills to properly encase the congested reinforcing with no significant voids can be tested and confirmed. After the mockup panel concrete has been shot into place, two types of cores are typically extracted from each panel by the owner’s test agency: one set of compressive strength test specimens, and a separate set of cores taken to evaluate shotcrete quality. For compressive strength testing, cores must be taken without containing any embedded reinforcing steel. But to assess quality, three cores for each proposed nozzleman are actually drilled right through the most congested rebar zones so an evaluation of the degree of reinforcing steel encapsulation and other imperfections can be conducted. And it is at this step in the process where misunderstandings, conflicts and costly delays can introduce an unsavory ambience to your project.

Consider the following hypothetical shotcrete mockup scenario, for example, based on recent accounts shared by ASCC members. Two weeks ago your field crew erected the backforms for several shotcrete mockup panels at your company’s offsite yard. Based on shop drawing negotiation with the LDP, installation of reinforcing steel in the agreed-upon, most challenging configurations soon followed. About one week ago, your highly experienced shotcrete
nozzlemen completed their preconstruction shoots. The next day, the test agency extracted quality core samples from all of the mockup panels, each core drilled right through the congested zones of reinforcing steel. This morning, you received a test report indicating the quality cores have been evaluated visually by the test agency using the subjective “core grading” procedure described in ACI 506.2-95 (now obsolete, but nevertheless referenced in
your current project specifications). The agency report concludes that some or all of your expert nozzlemen have failed to qualify to install shotcrete on your project. Your nozzlemen are all ACI-certified and enjoy a good reputation as reliable craftsmen in your local market. You have examined photographs of the suspect cores attached to the agency report. There are indeed small voids visible near some of the bars, but in no way do they appear rejectable. The general contractor’s cover letter includes a friendly reminder that your first shoot date is scheduled on the near horizon and closing fast. Running a second set of mockup panels will be costly in both time and money. It is a classic Karl Malden moment…what will you do?

Well, thanks to joint collaborative efforts among ASCC, The American Shotcrete Association (ASA), and several American Concrete Institute (ACI) committees, significant updates have recently been made to the codes, specifications, and guides that govern the structural shotcrete scope of work. One notable milestone was maneuvering shotcrete provisions out of the IBC code and into ACI 318-19 during the last code cycle. Another major achievement was the revision of ACI 506.4R-19, Guide for the Evaluation of Shotcrete, a comprehensive document aimed not only at the concrete contractor, but also intended to guide the design architect/engineer (A/E) in adequately identifying the scope of the shotcrete project and the associated effort required in testing and control.

The updated Guide presents to the A/E (LDP) three possible levels of increasing shotcrete application difficulty, and offers examples of testing and acceptance criteria associated with each level that could be specified in construction documents. An Application Difficulty Level 1 example is given in the Guide as an 8-inch thick concrete non-load-bearing wall reinforced with a single curtain of #4 bars at 12-inch spacing, each way. An Application Difficulty Level 2 example is given as a 12-inch thick load-bearing wall, reinforced with two curtains of #6 bars spaced at 8 inches each way, featuring frequent lap splices. And an Application Difficulty Level 3 example is given as a high-rise shear wall with highly congested double curtains of #11 bars.

In the hypothetical member predicament described above, the issue was that an outdated reference specification had been cited that required the testing agency to follow an obsolete core evaluation procedure that has often proved to be subjectively used with potentially unequitable results for the concrete contractor in terms of time and unnecessary cost. The updated ACI 506 documents effectively solved this problem by shifting the visual core evaluation scope away from the project inspector and into the hands of the experienced LDP. General core visual evaluation guidelines and illustrated reinforcing steel encasement details with suggested acceptance criteria are presented for use by the LDP in ACI Tech Note 506.6T-17; intended to be combined with engineering judgement to arrive at a reasonable evaluation of core quality based on the Difficulty Level and project-specific design intent. (Tech Note ACI
506.6T-17 is also included by reference in the updated ACI 506 Guides and Specifications). The ASCC member was able to convince the project LDP to acknowledge his specifications were indeed outdated. Using the ACI Tech Note criteria, the project LDP reviewed the cores, overruled the test agency and allowed the shotcrete work to proceed on schedule. Further guidance to the LDP (and valuable information for ASCC members) appears in ACI 506.4R-19, section 10.3.3 as follows: “There is no standard that will cover every possible condition, as it is not possible to present an absolute set of values for allowable defects in a core or set of cores…In mockup panels with large-diameter reinforcing steel or close spacing of reinforcement or other significant obstructions, it should be expected that encapsulation of the reinforcing steel may not be perfect and that there may be some defects that do not materially affect the structural integrity of the concrete section…ACI 506.6T provides visual evaluation information that should be used as guidance in specifying acceptance criteria for acceptable defects and reinforcement encapsulation quality for a specific project”.

Unfortunately, there seems to be a perpetual time lag effect that prevents even the best designers, specification writers, and test agencies from incorporating the latest codes, specifications and guides into their work product. Based on ASCC hotline calls and review of the updated shotcrete documents prepared by ACI, we recommend the following:

  • During the pre-bid phase, carefully study the structural drawings (especially any shotcrete general notes) and all Div. 3 specifications. Check Div. 1 specifications for any additional wall mockup requirements. Confirm that all reference documents are current (see list below).
  • Depending on the type of Ownership, pre-bid RFIs (public works jobs) or bid proposal qualifications and exclusions (private jobs) should be submitted if the bid documents are not clear or complete. Insist that the structural shotcrete scope be inspected, tested and evaluated using guidance and acceptance criteria consistent with the current ACI and industry-standard reference documents.
  • Remember to examine quality cores immediately after they have been extracted. Request the test agency include photographs of any cores in their field reports before the cores are submitted to the project LDP for review. Insist the chain of custody and protection of all quality cores be maintained until all nozzlemen have been qualified.

The most current industry codes, specifications and guides are:

  • ACI 318-19: Building Code Requirements for Structural Concrete
  • IBC (2021): International Building Code
  • ACI 301-20: Specifications for Concrete Construction
  • ACI 506R-16: Guide to Shotcrete
  • ACI 506.2-13(18): Specification for Shotcrete
  • ACI 506.4R-19: Guide for the Evaluation of Shotcrete
  • ACI 506.6T-17 Tech Note: Visual Shotcrete Core Quality Evaluation

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