Post Installed Reinforcement

7/25/2019 Post Installed Reinforcement

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design issues forstructural engineers

STRUCTURAL DESIGN

Richard T. Morgan, P.E., isthe Manager for Software andLiterature in the Technical

Marketing Department of HiltiNorth America. He is responsible

for PROFIS Anchor and PROFISRebar software. He can be reachedat [email protected].

By Richard T. Morgan, P.E.

Design Using Adhesive

Anchor Systems

Post-Installed Reinforcement

R einforced concrete is a constructionmethod that relies on widely understoodand historically validated concepts.

Traditionally, reinforcing bars areplaced in formwork prior to concrete placement.However, many applications require reinforce-ment to be added to existing structures by meansof reinforcing bars grouted into drilled holes,usually with injectable adhesives. is article pro-

vides an overview of how reinforcing bars can bedesigned in accordance with the development andsplice requirements of the American ConcreteInstitute ACI 318, Building Code Requirements for Structural Concrete , using a post-installedadhesive anchor system. Specific reference to the2012 International Building Code (2012 IBC)and ACI 318-11 will be made because the firstInternational Code Council-Evaluation Service(ICC-ES) Evaluation Service Reports (ESRs) con-taining provisions for designing post-installedreinforcing bars as “reinforcement” are recog-

nized within the 2012

IBC provisions.Post-installed rein-forcing bars, for thepurposes of this article,to refer to reinforcing

bars installed into hardened concrete using a qual-ified adhesive anchor system. e primary focus

will be to discuss post-installed reinforcing bardesign using the development length provisions

within ACI 318-11. Alternative design methodsfor post-installed reinforcement based on anchortheory and shear-friction will also be mentioned.

Anchor Design versusReinforcement Design

Before discussing the new provisions to designpost-installed reinforcing bars for development,it is a good idea to review the current provisionsfor post-installed anchor design. Post-installedadhesive anchor systems are commonly designed with the Appendix D, Anchoring to Concrete ,provisions of the ACI 318 code. Anchor ele-ments used with adhesive anchor systems includethreaded rod, internally threaded inserts, pro-prietary anchor elements and reinforcing bars.

ACI 318-11 Appendix D contains provisionsfor calculating design strengths corresponding toanchor failure modes. Bond strength provisionsare given in Part D.5.5, Bond strength of adhesiveanchor in tension. e key concept when using Appendix D to design with reinforcing bars is thatthe bars act as “anchors” (Figure 1). Essentially,

when using Appendix D provisions, the bars aredesigned in the same manner as anchor bolts.is concept assumes the “anchorage” is subjectto three possible failure modes in tension: steelfailure, concrete breakout and bond failure; andthree possible failure modes in shear: steel failure,

concrete breakout and concrete pryout. e barscan be treated as a group, such that the effects of

spacing and edge distance relative to a specificembedment and characteristic bond strengthare considered. Consideration is also given tosplitting via the modification factors ψ cp,N andψ cp,Na ; however, calculation of these parametersis concerned with the increased edge distancerequired to preclude splitting failure rather thanthe embedment required to develop the bar topreclude splitting failure.e predictive expressions for concrete break-

out, pryout, etc. in Appendix D do not explicitlyconsider the influence of reinforcement. However, Appendix D does permit consideration of “supple-

mentary reinforcement” or “anchor reinforcement”to enhance the capacity of an anchorage. e term“supplementary reinforcement” in Appendix Drefers to reinforcement capable of controlling split-ting, or providing an increased calculated concretebreakout capacity. Reinforcement designed for thestrength and serviceability of a concrete membershould not automatically be assumed to act assupplementary reinforcement for an anchorage.e term “anchor reinforcement” in AppendixD refers to additional reinforcement specificallydesigned to preclude concrete breakout failure bytransferring the loads applied to the anchorage into

bars that will be developed. Reference ACI 318-11D.5.2.9 (tension) and D.6.2.9 (shear) for moreinformation about Appendix D anchor reinforce-ment provisions.In contrast to development length provisions,

Appendix D anchorage provisions are not predi-cated solely on design controlled by the steelstrength of the anchor element. Rather, AppendixD provisions simply provide the means to calcu-late various strengths corresponding to possibleanchor failure modes. Furthermore, Appendix Dprovisions consider “steel strength” to be definedby the specified ultimate tensile strength ( f uta )

Figure 1. Reinforcing dowels designed usinganchor theory.


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of the steel element. is assumption dif-fers from what is assumed when designing areinforced concrete member (RCM), in whichsteel reinforcing bars are designed to yield.RCM design assumes the reinforcing bars

will provide the necessary strength, service-ability and ductility via yielding. RCM designfurther assumes the reinforcing bars will yieldif they are installed at a deep enough embed-

ment to preclude either a splitting failure(small cover) or a pullout failure (large cover).Bars installed at an embedment to obtainyielding are assumed to be “developed”, andthe embedment required to “develop” thebar is referred to as the “development length”(Figure 2 ). All of this is well understood byStructural Engineers. e reason it is notedhere is to draw a distinction between rein-forcing bars designed with the provisions of

Appendix D, and reinforcing bars designedspecifically for development. erefore, unlike Appendix D, design of reinforcing bars for

development assumes (a) the bars reach theirspecified minimum yield strength ( f y ), (b) thebar design is controlled by the yield strengthinstead of the ultimate bar strength and (c) theembedment required to yield the bar will bedeep enough to preclude splitting or pullout.

Post-Installed Reinforcing Bar

Testing and Assessment

e ICC-ES Acceptance Criteria for Post-Installed Adhesive Anchors in ConcreteElements (AC308), establishes requirements

for post-installed adhesive anchor systemsto be recognized for compliance with theInternational Building Code (IBC). Anchorsystems that satisfy these requirements receivean (ICC-ES) Evaluation Service Report. eESR will note the IBC versions for whichrecognition has been obtained, describe thematerials and components that comprise theanchor system, note design, application andinstallation parameters, provide tables withdesign data, and provide Manufacturer’sPrinted Installation Instructions (MPII).

Additional information is given in a specificESR. e ESR references the IBC, whichin turn references the ACI 318 code. Forexample, an adhesive anchor system having anESR that references compliance with the 2012IBC can be used to design an anchorage perthe provisions of ACI 318-11 Appendix D. AC308 (approved June 2013 for compliance

with January 2014 and January 2015) nowalso addresses the assessment and design ofpost-installed reinforcing bars for use withthe provisions of ACI 318-11 Chapter 12,Development and Splices of Reinforcement , andChapter 21, Earthquake-Resistant Structures .

e AC308 post-installed reinforcing barqualification test program includes consid-eration of the following:Service Condition Tests• bond resistance of the post-installed

adhesive system• bond/splitting behavior of the post-

installed adhesive system at deepembedment

Reliability Tests• sensitivity to hole cleaning • sensitivity to freeze/thaw conditions• sensitivity to sustained load at elevated

temperature• decreased installation temperature• sensitivity to installation direction

Installation Procedure Verification• installation at deep embedment• injection verification

Durability • chemical and corrosion resistance

Special Conditions• seismic qualification

Some of these tests are mandatory and someare optional. Reference Section 2.0, USES ,and Section 5.0, CONDITIONS OF USE ,in the ESR for information about the param-eters for which the adhesive system has beentested. Reference AC308 for specific detailson all test parameters. e intent of the testprogram is to demonstrate equivalence withcast-in place bars, which will permit a rein-forcing bar post-installed using an adhesivesystem to be designed in accordance withthe development and splice requirements of ACI 318. Adhesive systems that successfullycomplete this test program will receive recog-nition by way of an ESR, which then permitsthe adhesive to be used with reinforcing bars

designed as “reinforcement” per the provi-sions of ACI 318 Chapters 12 and 21. ESRsreferencing 2012 IBC compliance will bethe first such reports to recognize this typeof design. erefore, post-installed reinforc-ing bars can now be designed as either an“anchor” using the provisions of ACI 318-11 Appendix D, or as “reinforcement” using theprovisions of ACI 318-11 Chapters 12 or 21.

is means that post-installed reinforcingbars can now be designed for a developmentlength calculated using the provisions ofeither Chapter 12 or Chapter 21.Successfully completing the AC308 test pro-

gram for post-installed reinforcing bars permitsthe bars to be designed for development intension (l d ), or development in compression(l dc ), in the same manner as a cast-in-placebar. AC308 qualification testing for designper the provisions of Appendix D limits theembedment depth of an anchor element to amaximum of 20(d anchor ). AC308 qualification

testing for design per the provisions of ACI 318Chapter 12 or Chapter 21 limits the embed-ment depth of a post-installed reinforcing barto a maximum of 60(d anchor ). erefore, a keyparameter for using adhesive systems with theprovisions of Chapter 12 or Chapter 21 is toqualify for installation at deep embedmentvia the test program defined in AC308 Table3.8. Satisfying this parameter is one way in which the system demonstrates equivalence with cast-in-place reinforcing bars. Note thatpost-installed reinforcing bar installation isonly relevant to straight bars. e structural

analysis required to determine the area of rein-forcement ( As ) for post-installed reinforcingbars will be per the relevant provisions of the

ACI 318 code.

Designing Post-Installed

Reinforcing Bars as

Reinforcement

Consider an application for a slab extension in which reinforcing bars post-installed with anadhesive will be used. e post-installed bars will need to be spliced to the reinforcementin the existing member (Figure 3, page 16 ). Assume the slab is subjected to non-seismictension loads, and design is per the 2012 IBC. Assume the adhesive has been qualified perthe test program defined in AC308 Table 3.8.Reinforced concrete design principles wouldbe used to determine a post-installed bar sizeand grade. e development length for thepost-installed bars would be calculated using ACI 318-11 Eq. (12-1). Lap splices wouldfollow Section 12.15, Splices of deformed barsand deformed wires in tension. If the existing

Figure 2. Post-installed reinforcement designedas a lap splice.


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reinforcement bar size was the same as thepost-installed bar size, the lap splice length would be calculated using the provisions of

Section 12.15.1. If the criteria of 12.15.2 couldbe satisfied, a Class A splice could be used;otherwise, a Class B splice would be used.Likewise, if the existing reinforcement bar size

was different from the post-installed bar size,the lap splice length would be calculated usingthe provisions of Section 12.15.3. Once a barsize and splice length has been determined, thedetailing would be in accordance with ACI318-11 Section 7.6, Spacing limits for reinforce-ment , and Section 7.7, Concrete protection forreinforcement ; subject to any additional con-ditions per the required code parameters and

the product-specific ESR. e MPII would befollowed when installing the new bars.Now, consider reinforcing bars post-

installed to permit extension of an existingspecial moment frame (Figure 4 ). Reinforcedconcrete design principles would be used todetermine a post-installed bar size and grade. Assuming the adhesive has been qualified perthe test program defined in AC308 Table 3.8,the development length for the post-installedbars would be calculated using the provisionsof ACI 318-11 Section 21.7.5, Developmentlength of bars in tension. Specifically, Eq.(21-6) in Section 21.7.5.1 and the provisionsof Sections 21.7.5.2 and 21.7.5.3 would beused to calculate the development length.Once a bar size and development lengthhas been determined, detailing would be inaccordance with ACI 318-11 Section 7.6 andSection 7.7, subject to any additional condi-tions per the required code parameters andthe product-specific ESR. e MPII wouldbe followed when installing the new bars. Adhesive systems qualified per AC308 permitdesign and detailing of post-installed rein-forcing bars with the same criteria as that for

cast-in-place bars. In order to complete thepost-installed reinforcing bar design, the instal-lation requirements given in the MPII must be

followed. Determining the method for drillingthe hole in the existing concrete, making surethat the hole is drilled to the required depthand properly cleaned, installing the adhesivein a manner that eliminates voids and permitsproper bar insertion, and allowing the adhesiveto cure without any disturbance are parametersthat must be considered when post-installingthe reinforcing bars. Additional installation parameters are given inthe ACI 318 building code and in the product-specific ESR. e ACI 318-11 code addressesadhesive anchor installation in Appendix D

via D.9, Installation and inspection of anchors . Adhesive anchor ESRs address installa-tion requirements in Section 4.0, DESIGN AND INSTALLATION , and Section 5.0,CONDITIONS OF USE . e MPII, ACI318-11 D.9 provisions, and product-specificESR provisions must be followed when design-ing and installing post-installed reinforcingbars with an adhesive anchor system.

Alternative Design Methods

Design of post-installed reinforcing barsis an ongoing area of research and testing.Charney et al have proposed an approachthat utilizes anchor theory to calculate apost-installed reinforcing bar developmentlength. is approach is premised on calcu-lating a development length that takes intoconsideration concrete breakout failure andbond failure when splitting failure no longercontrols. e calculations utilize the concretebreakout and bond strength equations of

ACI 318-11 Appendix D.Palieraki et al have proposed an approach

that does not require bar development for

applications being designed for shear transfer.e resulting bar embedment could be lessthan that required per ACI 318-11 Section

11.6.4, Shear-friction design method .Developing a code-approved approach todesign anchor bolts for development is anotherarea where research would be beneficial.

Summary

e focus of this article was post-installedreinforcing bar design using the developmentand splice requirements of ACI 318-11.Only adhesive systems that have beenqualified per the post-installed reinforcingbar provisions of the ICC-ES Acceptance

Criteria AC308 are relevant for this design.Recognition for this design will be given in aproduct-specific ICC-ES Evaluation ServiceReport. e importance of understandingthe requirements and limitations given in theESR, following the Manufacturer’s PrintedInstallation Instructions, and following allcode-related provisions for design and instal-lation, are emphasized.▪

References

Charney, A., Pal, K., and Silva, J.(2013).

Recommended Procedures forDevelopment and Splicing of Post-Installed Bonded Reinforcing Bars inConcrete Structures , ACI Structural

Journal, Vol. 110, No. 3, pp. 437-447,Palieraki, V., Vintzileou, E., Trezos, K.

(2014). Shear Transfer Along Interfaces:Constitutive Laws , Proceedingsof the 2nd European Conferenceon Earthquake Engineering andSeismology, Istanbul, Turkey (elec-tronic source).

Figure 4. Extension of an existing special moment frame.Figure 3. New slab extension to an existing slab.

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Post Installed Reinforcement