We’ve been receiving a lot of requests lately from engineers wanting to know exactly what the difference is between Simpson Strong-Tie’s relatively new adhesive, SET-3G™, and its predecessor, SET-XP®. Both are epoxy-based adhesives used to anchor threaded rods and reinforcing bars in concrete base material for structural applications. If you perform a live pull test on a ½“-diameter mild steel rod embedded 4“ deep in 3,000 psi uncracked normal-weight concrete, the result will likely be the same; in both cases, the steel rod will break in a ductile manner at around 11 kips. You can see this hourglass-shaped steel failure mode happening in Figure 1. (To learn more about anchorage failure modes and ductility, check out this blog). Yet, the SET-3G design values shown in ESR-4057 come out ahead. But why?
The evaluation report, ESR-4057, was recently updated to allow the design of SET-3G adhesive for post-installed reinforcing bars using the ACI 318 development length provision. This blog has been reposted replacing SET-XP with SET-3G using the original design examples. The SET-XP evaluation report, ESR-2508, currently limits f’c to 2,500 psi for seismic applications located in seismic design category C–F. SET-3G does not carry the same limitation allowing for a considerable reduction in development length at higher values of f’c. In general, a substantially lower installation cost can be expected using SET-3G for seismic applications. Additionally, SET-3G has slightly reduced edge and spacing requirements. Engineers can access a free online calculation tool to easily determine the rebar development or lap splice length for either adhesive product.
I first learned about the application of the ACI 318 development length provision to post-installed reinforcing bars back in 2003 when I read Post-Installed Adhesive-Bonded Splices in Bridge Decks, authored by Ronald A. Cook and Scott D. Beesheim. In their series of experiments, holes were drilled adjacent to cast-in-place bars using a carbide-tipped drill bit, and new bars were secured in these holes using an anchoring adhesive presumed to be of a type commonly used in concrete construction.
Not all anchoring adhesives are created equal. There are important differences between acrylic-based and epoxy-based adhesive systems — differences that affect installation, gel and cure times, and anchoring performance. In the following post, Marlou Rodriguez, S.E., of Simpson Strong-Tie, lays out some of the comparative installation advantages of each system.
There are two common types of adhesives for anchoring threaded rod or rebar into concrete — epoxy-based systems and acrylic-based systems. What’s the difference? When should you specify one rather than the other? This blog post will help you understand the differences and guide you in choosing the best adhesive for your anchoring solution.
Specification of Concrete Anchors
The 2012 IBC and its Referenced Standard, ACI 318-11, is the first to mandate that contract documents specifically address installation, inspections and design parameters of concrete anchorage. For this reason, the specification of anchors in drawing details alone is impractical. To fully and effectively address these code mandates, concrete anchorage is more practically specified in both drawing detail(s) and the General
Structural Notes or specifications of the contract documents. The drawing detail(s) would typically call out the anchor type, material specification, diameter, and embedment depth. The General Structural Notes or specifications would include the name of the qualified anchor(s) and address the installation, inspections and design parameter requirements of ACI 318-11.
The following sections of ACI 318-11 discuss the contract document requirements for concrete anchorage:
The commentary in ACI 318-11, RD.9.1 discusses the sensitivity of anchor performance to proper installation. It emphasizes the importance of qualified installers for all anchors, and compliance with the Manufacturer’s Printed Installation Instructions (MPII) for post-installed anchors. Training is required for adhesive anchor installers per ACI 318-11 D.9.1. Simpson Strong-Tie Co. Inc. provides free installer training by experienced Technical Sales Representatives for our adhesive, mechanical and specialty anchors. Contact us at 1-800-999-5099. Special inspection and proof loading are addressed in ACI 318-11 D.9.2
Per the section above, anchor installation requires inspection per Section D.9.2. In addition, the design parameters for adhesive anchors are required to be specified in the contract documents. An explanation of the design parameters listed in ACI 318-11 D.9.2.1 is provided below:
- Proof loading where required in accordance with ACI 355.4. Proof loading is only required for adhesive anchors loaded in tension in which the inspection level chosen for the adhesive anchor design is “Continuous” (Ref. ACI 355.4 Section 10.4.6). Selecting “Continuous Inspection” can result in a higher “Anchor Category,” which in turn results in a higher strength reduction factor, φ. Reference Section 13.3.4 of ACI 355.4 for the minimum requirements of the proof loading program, where required. The Design Professional is responsible for performing the quantity, the duration of
the applied load, and the proof load to which the anchors will be tested. These parameters will be specific to the anchor design conditions.
- Minimum age of concrete at time of anchor installation. Per ACI 318 D.2.2, adhesive anchors must be installed in concrete having a minimum age of 21 days at time of anchor installation. Simpson Strong-Tie® has performed in-house testing of SET-XP®, AT-XP®, and ET-HP® adhesive anchors installed in 7-day- and 14-day-old concrete. The results of testing are published in an engineering letter (L-A-ADHGRNCON15.pdf), which can be viewed and downloaded at www.strongtie.com.
- Concrete temperature range. This is the in-service temperature of the concrete into which the adhesive anchor is installed. Temperature Ranges are categorized as 1, 2 or 3. Some manufacturers use A, B, or C as the category designations. Each Temperature Range category has a maximum short-term concrete temperature and a maximum long-term concrete temperature. Short-term concrete temperatures are those that occur over short intervals (diurnal cycling). Long-term concrete temperatures are constant temperatures over a significant time period.
- Moisture condition of concrete at time of installation. Moisture conditions, as designated by ACI 355.4, are “dry,” or “water-saturated.” Moisture condition impacts the characteristic bond stress of an adhesive.
- Type of lightweight concrete, if applicable.
- Requirements for hole drilling and preparation. These requirements are specific to the adhesive, and are described in the Manufacturer’s Printed Installation Instructions (MPII). Reference to the MPII in the contract documents is sufficient.
Adhesive anchors installed in a horizontal or upwardly inclined orientation that resist sustained tension loads require a “certified” installer.
A certification program has been established by ACI/CRSI. Installers can obtain certification by successful completion of this program. Contact your local ACI or CRSI chapter for more information. Other means of certification are permitted, and are the responsibility of the licensed design professional.
The installation of adhesive anchors in a horizontal or upwardly inclined orientation presents unique challenges to the installer. Simply put, the effects of gravity for these applications make it difficult to prevent air bubbles and voids, which can limit full adhesive coverage of the insert (threaded rod or reinforcing bar). Due to the increased installation difficulty of these anchors, they are required to be continuously inspected by a certified special inspector.
Suggested General Structural Notes or specifications for post-installed anchors can be viewed and downloaded at here, or contact a Simpson Strong-Tie® representative for help with your post-installed General Structural Notes or specifications.
Simpson Strong-Tie Suggested General Note for Anchor Products
Post-Installed Anchors into Concrete, Masonry and
Steel and Cast-in-Place Anchors into Concrete
The below products are the design basis for this project. Substitution requests for products other than those listed below may be submitted by the contractor to the Engineer-of-Record (EOR) for review. Substitutions will only be considered for products having a code Report recognizing the product for the appropriate application and project building code. Substitution requests shall include calculations that demonstrate the substituted product is capable of achieving the equivalent performance values of the
design basis product. Contractor shall contact manufacturer’s representative (800-999-5099) for product installation training and a letter shall be submitted to the EOR indicating training has taken place. Refer to the building code and/or evaluation report for special inspections and proof load requirements.
- For anchoring into cracked and uncracked concrete
a) Mechanical anchors shall have been tested in accordance with ACI 355.2 and/or ICC-ES AC193 for cracked concrete and seismic applications. Pre-approved products include:
i. Simpson Strong-Tie® Strong-Bolt® 2 (ICC-ES ESR-3037)
ii. Simpson Strong-Tie® Titen HD® (ICC-ES ESR-2713)
iii. Simpson Strong-Tie® Torq-Cut® (ICC-ES ESR-2705)
iv. Simpson Strong-Tie® Titen HD® Rod Hanger (ICC-ES ESR-2713)
v. Simpson Strong-Tie® Blue Banger Hanger® (ICC-ES ESR-3707, except roof deck insert)
b) Adhesive anchors shall have been tested in accordance with ACI 355.4 and/or ICC-ES
AC308 for cracked concrete and seismic applications. Adhesive anchors shall be installed
by a certified adhesive anchor installer where designated on the contract documents.
Pre-approved products include:
i. Simpson Strong-Tie® AT-XP® (IAPMO-UES ER-263)
ii. Simpson Strong-Tie® SET-XP® (ICC-ES ESR-2508)
III. Simpson Strong-Tie® ET-HP® (ICC-ES ESR-3372)
The intent of this technical bulletin is to clarify code language and outline the correct path for the design of concrete anchors under the International Building Code (IBC). The reader will be able to clearly distinguish between “code anchors” and anchors that are considered “alternative materials,” as well as understand the logical sequence of code language for designing each type. The distinction between “cracked” concrete and “uncracked” concrete anchor design will be made. This technical bulletin will lend clarity to the qualification of post-installed anchors for use in concrete. Excerpts from the IBC and its Referenced Standards will be provided to facilitate the description of the design requirements.
More than a decade after the introduction of the American Concrete Institute’s ACI 318, Appendix D design methodology for anchor design in 2002, many design professionals either do not fully understand or are unaware of the code requirements for the design of concrete anchors. Several factors contribute to the challenges associated with understanding the code mandates:
1. The incorrect notion that ACI 318, Appendix D is exclusively for anchors designed for “cracked concrete,” leading to regionally varying degrees of enforcement and implementation of the design requirements
2. Multiple Reference Standards for the design and qualification of different anchor types
3. The evolving scope of Reference Standards, which have reclassified some anchors as “Code Anchors” that were previously considered “Alternative Materials”
4. Confusing language in IBC sections that address concrete anchorage
5. Complexity of the anchor design methodology itself
6. Varying levels of special inspections enforcement
It is nevertheless incumbent upon the licensed design professional to design anchors in accordance with the minimum provisions of the code in order to protect public safety, reduce liability risk and fulfill professional responsibilities.
The International Building Code, beginning with the 2000 edition, describes the design methodology of concrete anchors by virtue of the language within the IBC itself, or through language in the Referenced Standard (ACI 318). In this technical bulletin, specific reference to the 2012 IBC and ACI 318-11 will be made, since this is currently the most widely adopted edition of the IBC.
“Code Anchors” and “Alternative Materials”
Anchors can be divided into two major categories: 1) “Code Anchors”, which are those that are specifically addressed in the IBC or its Referenced Standards, and 2) “Alternative Materials”, the design and qualification of which are not addressed in the IBC or its Referenced Standards.
The following “Code Anchors” recognized by the 2012 IBC:
- Headed studs
- Headed bolts
- Hooked (J- or L-) bolts
- Expansion anchors(such as Simpson Strong-Tie® Strong-Bolt® 2)
- Undercut anchors (such as Simpson Strong-Tie® Torq-Cut™)
- Adhesive anchors (such as Simpson Strong-Tie® SET-XP®, AT-XP®, and ET-HP®)
Anchor types not listed above are considered “Alternative Materials.”
The following are anchors qualified as such:
- Screw anchors (such as Simpson Strong-Tie® Titen HD®)
Alternative materials also apply to anchor types specifically excluded from ACI 318-11 calculation and analysis requirements.
- Specialty inserts (such as Simpson Strong-Tie® Blue Banger Hanger®)
- Multiple anchors connected to a single steel plate at the embedded end
- Grouted anchors
- Powder- or gas-actuated fasteners (such as Simpson Strong-Tie® PDPA)
Designing “Code Anchors”
The starting point for the design of all anchors is Section 1908 of the 2012 IBC.
Section 1908.1 states that only cast-in-place headed bolts and headed studs are permitted to be designed using “Allowable Stress Design,” provided that they are not used to resist earthquake loads or effects. For these anchors, Section 1908.2 references Table 1908.2 for the determination of the allowable service load. Section 1908.1 makes explicit reference to post-installed anchors (anchors installed into hardened concrete), stating that the provisions of “Allowable Stress Design” is not permitted. For the design professional, this means that determining anchor by means of “Allowable Load Tables” based on previous test criteria that used a safety factor of 4.0 to determine allowable loads, as in the example below, is not permitted under the IBC.
Section 1909 of the 2012 IBC, “Anchorage to Concrete – Strength Design” makes explicit reference to Appendix D of ACI 318 as the required design standard for the anchors listed in this section.
Cast-in-place headed bolts and headed studs used to resist earthquake loads or effects must be designed using “Strength Design” in accordance with ACI 318 Appendix D. Additionally, Section 1909 does not make reference to adhesive anchors, despite their status as “code anchors.” ACI 318-11 was the first edition to include adhesive anchors in its scope; however, the 2012 IBC was approved prior to the approval of ACI 318-11. This resulted in the omission of adhesive anchors from the language in Section 1909 of the 2012 IBC. Section 1901.3 of the 2015 IBC, entitled “Anchoring to Concrete” includes language for adhesive anchors and their applicability to the ACI 318-14 design and qualification requirements. The omission of adhesive anchors from Section 1909 of the 2012 IBC, however, does not exclude them from the design and qualification requirements of ACI 318-11 by virtue of their inclusion in ACI 318-11 Section D.2.2. The design professional must then reference Section D.2 of ACI 318-11, Appendix D to confirm that the anchors being designed fall within its scope.
Note that anchors used for temporary construction means, such as tilt wall panel bracing, are not addressed in the IBC. As a result, they are not required to be designed in accordance with the provisions of ACI 318, Appendix D. Section D.2.2 lists anchor types that fall within its scope, and those that are excluded (considered “Alternative Materials”).
Code Anchors are required to meet the ACI 318-11 Section D.2.3 qualification requirements described below.
ACI 355.2 (Qualification standard for expansion and undercut anchors) and ACI 355.4 (Qualification standard for adhesive anchors) are referenced here as the qualification criteria for specific types of postinstalled anchors. For the design professional it can be difficult to determine, without fully investigating these Referenced Standards, whether a specific proprietary anchor has been tested and is qualified for use in concrete. A simpler means by which to identify whether a proprietary anchor has been qualified to the Referenced Standard is a current Research Report (e.g., Evaluation or Code Report) which provides third-party review and verification that the product has been tested to and meets the qualification standard. There are two primary Research Report providers: IAPMO UES (International Association of Plumbing & Mechanical Officials Uniform Evaluation Service) and ICC-ES (International Code Council Evaluation Service).
These agencies are ANSI ISO 17065 accredited. They review independent laboratory test data, witnessed or conducted by an accredited third party, for a product and verify its conformance to publicly developed and majority-approved qualification criteria (or acceptance criteria) established for a given anchor type. Research Reports are an invaluable tool to the design professional and building official as evidence of conformance with the IBC.
There are two acceptance criteria that apply to post-installed “Code Anchors”:
- ICC-ES AC193 – Acceptance Criteria for Post-Installed Mechanical Anchors in Concrete Elements
- ICC-ES AC308 – Acceptance Criteria for Post-Installed Adhesive Anchors in Concrete Elements
These acceptance criteria reference ACI 355.2 and ACI 355.4, respectively, as the foundation for the test program by which the anchor is evaluated, and establish minimum performance standards for qualification. A Research Report is issued for an anchor that meets these minimum standards.