ANCHORING WITH BONDED FASTENERS

Transcription

1 ANCHORING WITH BONDED FASTENERS Ronald A. Cook and Robert C. Konz Department of Civil Engineering, University of Florida, USA Abstract Bonded fasteners have been used extensively during the past twenty years. In most cases, the estimated strength of these anchors has been determined from information provided in manufacturers literature and has not been based on rational design models. During the past several years, research in the US, Europe, and Japan has led to a better understanding of the behavior of bonded fasteners. The results of this research has led to the development of rational design models for determining strength and to proposed product approval test procedures that can be used to ensure that bonded anchor products will perform as intended by the designer. This paper presents an overview of the stateof-the-art in bonded fasteners. Basic bonded fastener behavior, design models, and factors influencing bond strength are discussed. 1. Introduction Bonded fasteners can be divided into two distinct areas: adhesive bonded fasteners and grouted fasteners. An adhesive fastener is a reinforcing bar or threaded rod inserted into a drilled hole in hardened concrete with a structural adhesive acting as a bonding agent between the concrete and the steel. Typically, the hole diameter is only about 10 to 25% larger than the diameter of the reinforcing bar or threaded rod. Structural adhesives for this type of anchor are available prepackaged in glass capsules or foil packets, in dualcartridge injection systems, or as two-component systems requiring user proportioning. A grouted fastener may be a headed bolt, threaded rod with or without a nut at the embedded end, or deformed reinforcing bar with or without end anchorage installed in a pre-formed or drilled hole with a cementitious or filled polymer based grout. Grouted fasteners are typically installed in holes at least one and one-half times the diameter of the fastener. Figure 1 shows typical adhesive and grouted fasteners. 361

2 h ef Adhesive Un-headed Grouted Headed Grouted Figure 1. Schematic diagram of adhesive and grouted fasteners 2. Bonded Fastener Systems Figure 2 shows the general types of bonded fastener systems available. Adhesive and grouted fastener systems are typically composed of organic polymers or inorganic cementitious materials. In some cases, hybrid systems utilizing both organic and inorganic materials are available. The primary difference between the adhesive and grouted systems is the introduction of a filler material (e.g., fine sand) into the bond mixture. Bonded Fasteners Adhesive Fasteners Grouted Fasteners Capsule & Foil Type Injection Type Manually Mixed Organic Compounds Organic Compounds Inorganic Compounds Organic Compounds Inorganic Compounds Epoxy Epoxy Cementitious Epoxy Cementitious Polyester Polyester others Polyester others Vinylester Vinylester others Figure 2. Types of bonded fastener systems 362

3 3. Behavior of Bonded Fasteners Tests of adhesive fasteners have shown failure modes as indicated in Figure 3. For shallow embedments, the failure mode appears to be the same as that of headed cast-inplace and mechanical fasteners. For deeper installations (the type typically used in practice), embedment failure results in a shallow concrete cone with a bond failure below the shallow cone. Given the thin bond line between the fastener and the concrete, it is very difficult to determine which of the three center failure modes shown in Figure 3 actually occurred. For very deep embedments, steel failure will occur as shown on the far right of Figure 3. concrete cone adh./conc. interface steel/adh. interface adh./conc. and steel/adh. interface steel Figure 3. Failure modes of adhesive bonded fasteners Un-headed grouted fasteners typically fail at the grout/steel interface. The left diagram in Figure 4 shows the typical failure mode of un-headed grouted fasteners (i.e., a shallow cone with a bond failure at the grout/steel interface). Headed grouted fasteners eliminate the possibility of bond failure at the grout/steel interface due to the anchor head and force the bond failure to the grout/concrete bond line (with a shallow cone) for low bond strength grouts or result in a full concrete cone failure for high bond strength grouts (as shown in the right diagram of Figure 4. Test reports on the behavior of adhesive fasteners have been collected in Europe, in the USA and in Japan. From 38 reports, a database containing the results of 2929 tests has been established. The database contains tensile and shear load testing in uncracked and cracked concrete with single fasteners, groups of two fasteners and groups of four fasteners. The database contains tests carried out with threaded rods, insert sleeves and rebars. Finally, the database contains tests with epoxies, vinyl esters, unsaturated polyesters, hybrid adhesives and inorganic adhesives. A database for grouted fasteners is also being developed. Currently there are over 400 single grouted fasteners tests available using both polymer and cementitious grouts. Both the adhesive anchor 363

4 database and grouted fastener database are being maintained for ACI 355 by Ronald A. Cook, Department of Civil Engineering, University of Florida, Gainesville, Florida Un-headed Bond Failure Headed Bond Failure Headed Cone Failure Figure 4. Failure modes of unheaded and headed grouted fasteners (excluding steel failure) 4. Design of Bonded Fasteners Several design models have been presented for adhesive fasteners over the last several years These are summarized in Cook et al (1998) 1 and Kunz et al (1998) 2. A wide variation of possible models for single fastener strength were evaluated in Cook et al (1998) 1. These models included: Concrete cone models Bond models Bond models neglecting the shallow concrete cone Combined concrete cone models and bond models Bond models considering bond failure at two interfaces The results of the Cook et al (1998) 1 paper indicate that a simple model based on a uniform bond stress fits the test data from the international database best. The expression for determining the mean strength of single fasteners in tension is given by Eqn. 1. N = τ π d (1) bond h e Terms in Eqn. 1 and other equations are given in the Notation section at the end of the paper. 364

5 Although each individual product has a unique mean bond stress (τ), it is possible to normalize all products to a unique bond stress value. Figure 5 shows a comparison of Eqn. 1 with 888 single anchor tests of products in the international data base normalized to 10 MPa. Figure 5 also shows a comparison of a nominal strength of 0.67 of the mean strength compared to the test data. Note that the final design strength will also include an appropriate capacity reduction factor, φ Uniform Bond Model (mean) Load (kn) % fractile, V=0.20 (0.67 mean) Bond Area (mm 2 ) Figure 5. Comparison of measured loads with the uniform bond model As an additional verification of the uniform bond stress model, non-linear computer analyses were performed (McVay, et al 1996) 3. A typical result is shown in Figure 6. As shown by Figure 6, as the load increases (curves moving from left to right) the bond stress distribution changes from what might be expected in elastic analysis to a nearly uniform bond distribution at failure. Both the database and the non-linear finite element analysis indicate that a uniform bond stress model is appropriate for adhesive fasteners. 365

6 Bond Stress (MPa) Figure 6. Bond stress distribution versus normalized depth with increasing load The strength of grouted fasteners depends on whether or not the fastener is headed. Detailed test results and design recommendations for grouted fasteners are presented in another paper in these symposium proceedings. The information provided here is only intended to provide a brief summary of the results presented in that paper. For unheaded fasteners, bond failure typically occurs at the grout/steel interface and Eqn. 1 provides the basis for determining the mean strength of the fastening. For headed grouted fasteners, two failure modes are possible. For low bond strength grouts, bond failure at the grout/concrete interface may occur. Tests have shown that this failure mode can best be represented by a uniform bond stress model calculated using the grout/concrete bond strength of the product (τ 0 ) applied to the bonded area at the grout/concrete interface. This is given by Eqn. 2: Nbond, d τ 0 π d 0 0 = h (2) For higher bond strength grouts, a full concrete breakout failure occurs and the mean concrete breakout strength developed by Fuchs et al (1995) 4 is appropriate. This is given by Eqn. 3: e 1.5 N cone = 16.7 fc h (3) e The predicted mean strength of a headed bonded fastener is determined by the lower value of Eqn. 2 and Eqn. 3. Although grout/concrete bond failure is typically not observed in tests of unheaded fasteners using engineered grouts, it may be prudent to base the strength of these 366

7 fasteners on the smaller of the bond strength determined at the grout/steel interface (Eqn. 1) and the bond strength determined at the grout/concrete interface (Eqn. 2). Eqns 1-3 provide predictions for the mean strength of bonded fasteners. For design purposes, these strengths must be reduced. For Load and Resistance Factor Design, the determination of design strength from behavioral models which represent mean strengths is typically based on establishing a nominal strength (some lower bound fractile of the mean strength) and then applying a capacity reduction factor (φ) to limit the probability of failure. In current US and European design standards, the nominal strength is commonly taken as the lower 5% fractile of the test data. The 5% fractile represents the value where it would be expected that 95% of the tests performed would exceed the specified nominal strength. The determination of the 5% fractile depends on the number of tests available and the scatter of the test results. The scatter of the test results is typically expressed as the coefficient of variation (V) which is defined as the standard deviation of the test results divided by the mean. This leads to the following for nominal bond strengths: (4) τ ' = τ (1-α V) ' τ = τ (1- V) (5) 0 0 α The selection of the α factor depends on the number of tests available. The selection of an appropriate capacity reduction factor (φ) for bond can be based on detailed studies of probability of failure and/or on what φ factors are used for similar failure modes in existing building codes. Bond failure can be compared to shear-friction since it involves slip along an interface. In ACI 318, the φ factor for shear-friction and shear is A capacity reduction factor (φ) for bond of 0.85 is recommended for designs controlled by bond failure. Various behavioral models for both edge effects and group effects for bonded fasteners are being studied in both the US and Europe. Proposals for modification factors for edge effects and group effects for bonded fasteners are presented in other papers in these symposium proceedings. 5. Factors Influencing the Strength of Bonded Fasteners The evaluation of both the mean bond stress (τ and τ 0 ) and design bond stress (τ and τ 0 ) must be based on product approval tests that include the effects of installation and in-service conditions. As noted in Cook et al (2001) 5, there are significant differences 367

8 between adhesive products. Basic tests for mean bond stress in clean, dry holes at room temperature indicate that the mean bond stress can range from 2 MPa to 20 MPa for adhesives and 7 MPa to 21 MPa for grouts as shown for 20 adhesives and 9 grouts in Figure 7. The coefficient of variation for these tests can vary between 0.05 and In many cases, products that exhibit high bond stress in clean, dry holes at room temperature are inadequate under typical installation and in-service conditions such as damp holes and elevated temperatures. It is mandatory that designers require product testing for expected in-service and installation conditions prior to the final design. Average Uniform Bond Stress, [MPa] Adhesive τ mean = 12.7 MPa Grouted τ mean = 17.9 MPa A B C D E F G H I J K L M N O P Q R S T Product Figure 7. Bond stress variation for adhesive and grout products The following provides examples of the factors influencing bond strength that need to be considered for product approval tests of bonded fastener products: Concrete mix (equal concrete strength does not ensure equal results) Temperature effects Damp hole Improperly cleaned hole Curing time Freeze-thaw effects Installation direction (vertical down, horizontal, overhead) Creep (normal and elevated temperatures) Mix proportioning (primarily manually-mixed products) Fire resistance Wet (submerged) hole Maximum torque Repeated load 368

9 Seismic load Environmental effects (chemicals) Cracked concrete (static cracks and moving cracks) Other possible tests: Age of concrete Oil presence (compressed air cleanout of holes) Capsules driven rather than drilled Hammer installed capsules installed upside-down Hole size Hole drilling Radiation As can be observed from the above list, a product approval standard for bonded fasteners must be quite comprehensive to ensure reliable performance of products. 6. Status of Design and Product Approval Standards Design models for bonded fasteners are currently being finalized in the United States and Europe. Many of these design recommendations are presented in the symposium proceedings. Product approval standards are also underway with the European Organization for Technical Approvals leading the way with Part Five of the ETAG N o 001 standard. In the United States, the American Society for Testing and Materials Committee E06.13 is currently developing draft product approval standards. Notation: d = outside diameter of fastener [mm] d 0 = drilled hole diameter of fastener [mm] f c = concrete strength, measured on 150 by 300 mm cylinders [MPa] h e = embedment depth [mm] N bond = mean strength of the fastener as controlled by bond strength N bond,d0 = mean strength of the fastener as controlled by grout/concrete bond strength V = coefficient of variation (standard deviation divided by the mean) α = a statistically determined coefficient based on the tolerance limit and confidence to be used for design φ = capacity reduction factor (0.85 is recommended for bond failure) τ = mean uniform bond stress for adhesive fasteners (MPa) τ 0 = mean uniform bond stress for headed bonded fasteners (MPa) τ = nominal uniform bond stress (MPa) at the fastener/adhesive interface τ 0 = nominal uniform bond stress (MPa) at the grout/concrete interface 369

10 References: 1. Cook, R. A., Kunz, J., Fuchs, W., and Konz, R., Behavior and Design of Single Adhesive Anchors Under Tensile Load in Uncracked Concrete, ACI Structural Journal, ACI, V. 95, No. 1, January-February 1998, pp Kunz, J., Cook, R. A., Fuchs, W. and Spieth, H, Tragverhalten und Bemessung von chemischen Befestigungen (Load Bearing Behavior and Design of Adhesive Anchors), Beton- und Stahlbetonbau 93 (1998), H.1, S , H. 2, S McVay, M., Cook, R. A., and Krishnamurthy, K., "Behavior of Chemically Bonded Anchors," Journal of Structural Engineering, American Society of Civil Engineers, V. 119, No. 9, September, 1993, pp Fuchs, W., Eligehausen, R., Breen, J. E., "Concrete Capacity Design (CCD) Approach for Fastening to Concrete," ACI Structural Journal, American Concrete Institute, V. 92, No. 1, January-February 1995, pp Cook, R. A., and Konz, R., Factors Influencing the Bond Strength of Adhesive Anchors, ACI Structural Journal, American Concrete Institute, V. 98, No. 1, January-February 2001, pp