ACI 408 2R:1992 (R2005) download.Bond under Cyclic Loads.
SCOPE
The purpo of ACI 408 2R is to reiieiv the current state-of-the-art on bond, with particular eniphs on bond under cydicloaiing. Two genera typof cydic loais are aidresd: high-cydic(fatigue) aid lcivcydic (earthquce and 9mila) loals The behai’ior of straight anchores, hooks and icsunder both ba] rimes is diausJ. The report is intended to ‘ve both degners and reErches and is organized into eight chapters. Chapters 1, 2, and 3 prent background information on bond under cydic baling and should be of i nte’est to aU riers Chapters 4, 5, aid 6 deal with results of rech and dedopmeit of anaiytic bond mode and should beof u primarily to reseach-s Chter 7 presents a reiiev c current de sgn guiddines, from both the U.S. aid roaJ, ding with bond under cyclic loals, and should be cf particular intere to designers Chter 8 provides a summary of the rearch results and resch needs The document is meait to veas an introduction for degnersto the bc mechaiisiisinvdved in bond, the vaies that fect them, aid the diffeences between behavior under cydic aid non-cydicloais An extensve rerence list, induding milar reports,7 is provided for reaias dering aiditional daIs Bond behavior of prestres9ng tendons and behavior under shock or impat baling are not aldred in this report.
forced concrete members arise from two distinct tualions The first is anchore or devopmeit where bas are terminated. The second is flexural bond or the cha-ge of force along a bar due to a change in bending moment along the member.
The fidency of bond can be conveniently quantified by looking at bond stressversus bar ippe curves that represent the change in local stress in the bar Ub versus the total movement of the bar rdative to the surrounding concrete s(Fig. 1). Theip srqxesents the rigid body motion of the bar with reect to some fixed point in the surrounding concrete. Bond stress, as used in this report, rers to an avere bond stress computed along alength of bar at l 15 dianeters long, and not to the local stress at an individual bar dormation or at a point along the interfae between bar and concrete. The limit of 15 bar diameters is some what arbitary and constitutes a lower bound to tycal aichore lengths, but it isthe values of av6-e bond stressover typical anchore lengths that areof importance in degn. For monotonically incrng dips, values for maximum bond (a strength measured over short distances) are reported in the literature to vary from about 1500 to 3000 psi (10.3 to 20.7 M Pa). Avere values of bond stress for use in degn range from 560to 8OOp (3.8to 5.5 MPa). Thevaluesof ipat maximum bond stress show considerable scatter, de pending primarily on the deformation pattern,2 but typcally maximum bond stress will be rhed at Vdues of slip of 0.01 to 0.1 in. (0.25 to 2.5 mm).
LOADS
Loads on structural members can be subdivided into monotonic and cydic loads Monotonic loading imies that some parameter, in this case slip, is always incr ng. Cyclic loads imply that the same parameter reverses in direction many times during the load history. Cydic loadings are divided into two general categories The first category is the so-called “low-cyde” loading, or a load history conta fling few cydes (less than 100) but having large ranges of bond stress(ub,> 6apsi). Thebond stressrarlgeafristhedifferencebetween the avere bond stresses at the maximum and minimum load, taking into acountthedirection of the loading. Low-cycle loadings commonly arise in s m C and high wind loadings The loading is also referred to as “low-cycle, high-stress’ loading. The second category isthe so-called “high-cyde” or fatigue loading, which is a load history containing many cycles (typically thousands or millions), but at a cm bond stress range(o, < 300 psl). Bridge members, offshore structures, and members supporting vibrating mahinery are often subjected to “high-cycle” or fatigue loading. High-cyde loadings are consdered a problem at service load les while low-cycle loading produce problems at the ultimate limit state.
The bond behavior under cydic loa:iing can further be subdivided axording to the type of stress plied. The first is repeated or unidirectional loading, which irTplies that the bar stress does not change se (tenon to compreson) during a loading cyde, as is the usual tuation for fatigue loang. The second is stress reversal, where the bar is subjected alternativdy to tenon and compreson. Stress reversals are the typcal cfor smicloaJing.
FAILURE MODES
Under monotonic loading, two types of bond falures are typical. The first is a direct pullout ci the bar, which occurs when ample confinement is provided to the bar. The second type of fallure is a splitting of the concrete cover when the cover or confinement is inaifficient to obtain a pullout falure. Failure loads under low-cydeloading are very milar to those under monotonic loading, but cracking occurs in both directions with cyding (Fig. 2) and fatigue failures of both ranforcing bar and concrete need to becondered.
COMPONENTS OF BOND RESISTANCE
Although the concept of average bond stress is convenient, the force transfer is a combination of rsta,ce due to aiheson V,, mechanical anchorage due to bearing ci the lugs Vb, and frictional restanceV (Fig. 3). Adheson is rated to the shear strength of the steelconcrete interface, and is primarily the result of chemical bonding. Mechanical anchorage arises from bearing forces perpendicular to the lug face as the bar is loaded and triesto ith These bing forces, in turn, give rise to frictional forces along the bar-concrete intart ace. The latter forces are an important component when failure is governed by splitting.
Under monotonic loading, typical values for adheon range from 7Oto 150 ps (0.48 to 1.03 M Pa), while those for friction range from 60 to 1450 ps (0.41 to 10.0 Mpa). It has generally been assumed that under monotonic loads, atesaon can be broken due to service loads or to shrinkage of the concrete, and that bearing against the lugs is the primary load-transfer mechanism at loads near ultimate. However, recent data comparing the performance of plan and epoxy- coated rant orcing ba-sunder monotonic loads indicate that adhesion may play a much greater role in anchorage failures governed by i tti ngof the concrete cover.
Under cyclic loads, most of the bond stresses are transferred mechaically by bearing of the bar del ormations against the surrounding concrete. The tensle and compressve strength of the concrete, the geometry and spacing of the deformations, cover and spacing, and amount ci transverse renforcement play a dominant role in controlling the bond behavior for this loading c.
Thebond stress-sip response of aba- loaded by lowcyde loads is shown in Fig. 42 The initial part of the curve follows the monotonic envelope. If the load is reversed, large ipoccursboretheba lug bsanst the concre and bond stresses incre. The man differences batween monotonic and cydic beds are that in the latter, aiheon is sumed to be lost after the first cyde, and the friction component (flat portion of the curve) decrs with cyding. In framed structures, the loss of bond in beam-column joints cai teed to large drifts if the joints hate been subjected to indic boal reiersais becaise of th horizontai portions (near zero stiffness) of the bond stress-sip curves
Under hgh-cyde loais, the behiior is very dependent on the stressandlor strain amplitude and the number of cydesof toed apied.’ Fig. 5 shows a typicai cuve for this ca Four separate regimes can be identified. First, lageipsoccur with constant loaling(A), the ip then decrees (B), and stlizes (C), and fin ly it incres rapicly with cyding until falure (D).
FACTORS AFFECTING BOND STRENGTH
UNDER CYCLIC LOADS
The man fa.tors fecting bond behauior under cydic beds are:
1. Concrete compressve strength
2. Cover and bar aing
3. Ba sze
4. Anchorage length
5. Rib geometry
6. Sted yidd strength
7. Amount and potion cl transverse sted
8. Cting potion, vibration, and ribration
9. Strain(or stress) range
10. Type and rate of baling
11. Temperature
12. Surf ae condition (coatings)
The influence of these ftors on bond strength and faluremechanism isunderstood only quaiitatively in many cs Chapters 3, 4, and 5 of the report cied with &meof the rech behind the abvationsjust listed, and an øten9ve list of rerences (more than 160 citations) is attathed to suplement the ci aus on.
DESIGN APPROACHES
Chapter 7 contains a deription of code-proposed equations to design anchorages subjected to cydic loaJs, intended to suplement those for monotonic
1. The higher the loaJ amplitude, the larger the 3J- ditional sip, especially after the first cycle. Some premaient damage ses to occur if 6Db 70 percent of the static bond caity is reahed. For degn conederations, a damage threshold can be suggested at 50 percent of the bond strength (400 ps).
2. When loaiing a bar to an arbitrary bond stress or ip value bdow the damage threshold (afout 64) percent of ultimate) and unloaiing to zero, the monotonic stress ip rdationship for all pratic purposes can be attaned agan during roating. This behavior al occurs for a age number of loalings, provided that no bond failureoccurs during cydic loalings
3. Loaiing a bar to a bond stress higher than 80 percent of its ultimate bond strength will result in 9gnif icant permanent sip. Loa:iing beyond the ip corresponding to the ultimate bond stress results in large losses of stiffness and bond strength.
4. Bond deterioration under large stress ranges (greater than 50 percent of ultimate bond strenght) cannot be preiented, cqt by the use of very long anchorage lengths (at le a fator of 1.5 on the devdopment lengths currently used) and substantial transverse r ntorcernent (two to three 11 rnes that requi red by the current codes). Even in this c, bond damage n the most highly stressed ar cannot be totally &minated.