ACI - 355.4 IN-LB
Qualification of Post-Installed Adhesive Anchors in Concrete (ACI 355.4-20) and Commentary
|Publication Date:||1 January 2020|
This standard applies only to post-installed adhesive anchors as defined herein.
R1.2 Adhesive anchors resist tension loads with a combination of adhesion and mechanical bond (micro-interlock). Different anchor designs and adhesive types may exhibit a range of performance characteristics. In particular, the sensitivity of adhesive anchors to variations in installation and service-condition parameters (such as hole cleaning, installation orientation, and cracked concrete characteristics) may vary widely from each system. ACI 318 addresses this situation by matching capacity reduction factors to anchor performance categories that are, in turn, established through a series of reliability tests.
This standard applies to anchors with a diameter da of 1/4 in. or larger. The drilled hole shall be approximately cylindrical with a diameter do ≤ 1.5da. This standard also applies to anchors with an anchor embedment depth hef not less than four diameters (4da), or 1-5/8 in., and an embedment depth not exceeding 20da.
R1.2.1 The minimum diameter of 1/4 in. is based on practical considerations regarding the limit of structural anchor applications. The upper limit on the ratio of hole diameter to anchor element diameter provides a demarcation between conditions where a single bond strength can be used to evaluate anchor strength and conditions where bond strengths at both the anchor interface and concrete interface must be determined to evaluate anchor strength. In addition, the value of 1.5da is based on consideration of typical practice whereby most organic adhesives are used with thin bond lines to limit both adhesive shrinkage and creep of the anchor when under load. The design method deemed to satisfy the anchor design requirements of ACI 318, Appendix D, is based on an analysis of an anchor database with a maximum diameter of 2 in. While ACI 355.4 gives no limitations on maximum anchor diameter, for anchors beyond this dimension, the testing authority should decide if the tests described in this standard are applicable or if alternative tests and analyses are more appropriate. It may also be desirable to reconsider those tests where only small, medium, and large diameters are tested when the upper diameter is much larger than 1-1/2 in.
A limitation on the minimum embedment length of adhesive anchors is necessary to ensure conformance with the design method deemed to satisfy the anchor design requirements of ACI 318, Appendix D.
The minimum member thickness shall not be less than the value given by Eq. (10-21). Values of Δh in Eq. (10-21) shall be permitted if they are verified by tests according to Table 3.1, Test no. 14, and Table 3.2, Test no. 20, or Table 3.3, Test no. 15.
This standard does not address the following systems and use conditions:
1. Bulk adhesives mixed in open containers without automatically controlled metering and mixing of adhesive components.
2. Adhesives to adhere structural elements to concrete surfaces outside of a drilled hole.
3. Adhesive anchors in aggressive environments not specifically considered in this standard.
4. Adhesive anchors to resist fatigue or shock loading.
5. Injection-type adhesive anchor systems for horizontal and upwardly inclined installations that do not employ a piston plug or similar device to provide back pressure during the adhesive injection process.
R1.2.3 Correct proportioning (metering) and mixing of adhesive components is critical to their performance. Bulk mixing and delivery of adhesives (for example, those with paddle mixers in buckets), while appropriate for some applications, may not provide anchor performance consistent with the assumptions of this standard. These systems are not considered to provide controlled metering of adhesive components. Bulk dispensing equipment that provides automatic metering and mixing of the adhesive components is included; however, ongoing monitoring is required to check that the equipment is operating within tolerances in accordance with the Manufacturer's Printed Installation Instructions (MPII), particularly with respect to mixture ratios, leak tightness, and dwell time.
This standard is not appropriate for assessing the use of adhesives to adhere structural elements to the concrete surface. Examples include bonded steel plates or external carbon fiber reinforcement. Other standards exist for these purposes. This standard includes tests to assess the sensitivity of adhesive anchor systems to a limited range of aggressive environments, including moisture, highly alkaline fluids, and sulfur dioxide. While it is believed that these exposure environments envelop a range of possible exposures, specific environments (for example, radiation exposure and chemical production environments) may require unique assessment.
Due to the variety of possible loading conditions associated with fatigue and shock loading, this standard does not include tests for these loading variants. Fatigue and shock loading may result in reductions in bond strength, steel strength, and concrete strength, and these effects are not addressed by this standard. Caution should be exercised in the determination of whether cyclic loading should be explicitly considered. These conditions may be evaluated separately for specific systems using generally accepted principles. Fatigue is generally less of a problem for the adhesive than for the anchor element; provisions of preload in the anchor to reduce the level of stress fluctuation in the anchor element is only effective if sufficient unbonded length is provided to ensure a reasonable degree of elastic stretch.
Void-free injection of adhesive is critical for the performance of adhesive anchors, particularly for cases involving sustained tension load. This standard includes several criteria for assessing the effectiveness of the adhesive anchor injection system. Nevertheless, the injection of adhesive into horizontal and upwardly inclined holes presents special challenges. The collapse of a tunnel ceiling in Boston, Massachusetts in 2006 highlights this issue. NTSB (2006) documented improper installation of the adhesive based on observation of failed anchors and anchors adjacent to the collapsed section. Subsequent laboratory investigations confirmed these findings, see Ocel and Hartmann (2007). The piston plug was developed to minimize injected air voids (see Fig. 2.2). Laboratory investigations (Silva 2016) indicate that injection of adhesive with only an extension tube, i.e., without the use of a device such as a piston plug to provide back pressure during the injection process, does not result in a sufficient degree of reliability in the installation process. The use of a piston plug during the injection process consistently results in good installation. For small hole diameters (1/4-in. to 3/8-in.), the same effect is accomplished when the extension tube diameter equals the hole diameter.
Consequently, the injection of adhesive in the horizontal or upwardly inclined direction without the use of a piston plug or similar back-pressure device to avoid air voids is no longer included in the scope of this Standard. It is also important to note that the use of the piston plug for proper injection is not limited to embedments of large diameter or depth.
Adhesive anchors shall be evaluated for sustained loading with the provisions of this standard. Qualification of adhesive anchors exclusively for short-term loads is not permitted by this standard.
R1.2.4 While it is permissible to use adhesive anchors to resist short-term loads such as those from wind or earthquake, the sustained load tests and corresponding assessment described herein are not optional. All anchors qualified in accordance with ACI 355.4 are suitable for sustained loads within the use parameters established in the assessment (7.17 and 10.4.7).
Adhesive anchor systems shall exhibit characteristic bond strengths as determined in accordance with Eq. (10-12) equal to or exceeding the minimum permissible bond strength τk,min. Adhesive anchor systems that do not exhibit characteristic bond strengths equal to or exceeding the minimum permissible bond strength in accordance with 10.2, shall not be qualified according to this standard.
R1.2.5 ACI 318, Appendix D, provides default bond stress values for specific constellations of use parameters that may be used in place of values from an evaluation report in accordance with this standard. Because the default values are independent of the adhesive anchor system selected, they represent minimum values for the assessment of any adhesive anchor system under this standard. The minimum values and corresponding use parameters are given in Table 10.2.
In general, ACI 355.4 is intended to address the assessment of adhesive anchors for cases where anchor design theory applies. It is not intended to address the assessment or design of post-installed reinforcing bars proportioned according to the concepts of development and splicing of reinforcement.
R1.2.6 This standard is intended to provide parameters for the design of adhesive anchors in conjunction with the provisions of ACI 318, Appendix D. Those provisions are derived from the principles of anchor theory, whereby anchor forces are transferred to the concrete in a manner that generally precludes splitting of the concrete and where spacing, edge distance, and member thickness are explicitly considered in the evaluation of the concrete breakout capacity (Fig. R1.1(a)). It is not intended to address the assessment or design of post-installed reinforcing bars proportioned according to the concepts of development and splicing of reinforcement (Fig. R1.1(b)). While the provisions of Chapter 12 of ACI 318 may be used to establish embedment lengths for post-installed reinforcing bars in such cases, the ability of an adhesive anchor system to transfer loads to adjacent embedded bars, particularly where longer splice lengths are required, should be verified by appropriate testing. Testing for the splice length is outside the scope of this standard.