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2 literature review

  1. 1. Chapter 2 LITERATURE REVIEW2.1 CONCRETE BOND Bond stress is the shear stress acting parallel to the bar on the interfacebetween the bar and the concrete. Bond stress may be considered as the rate of transfer offorce between concrete and steel. In other words, if there is bond stress there is change insteel stress and vice-versa. Bond is due to combined effect of adhesion, friction andbearing (for deformed bars). Concrete, on its own, is strong in compression but weak in tension. As amatter of fact, the compressive strength of concrete is about ten times greater than itstensile strength. This negative trait is remedied by placing steel reinforcing bars into theconcrete to form reinforced concrete (RC). This approach allows a material with muchhigher tensile strength, such as steel, to take on the tensile load that the concrete cannotsupport. In order for this relationship to work, however, the concrete and the reinforcingsteel must have a sufficient bond between them so the tensile load can be transferredeffectively to the steel. There are three different aspects that contribute to bond strength:chemical adhesion, friction, and mechanical interlock. The chemical adhesion is a bondbetween the concrete and the steel, the friction is caused by the bar deformations, or ribs,slipping along the concrete, and the mechanical interlock is a bearing force caused by theribs bearing against the concrete (Swenty, 2003). In order to insure an adequate bond, ACI 318 (2008) regulates how long a barmust be imbedded into the concrete based on factors such as concrete type, concretestrength, bar diameter, and bar type. This regulated factor is called the developmentlength of the bar, and prevents a bond failure from being the controlling failure mode of astructure. Bond failure usually occurs in two different ways. In structures, the mostcommon is known as a splitting failure. A splitting failure occurs when a small clearcover or small spacing between reinforcing bars exists. The small amount of concretearound the bars can crack or split, exposing the reinforcement and ultimately leading to 6
  2. 2. bond failure. Also contributing to a splitting failure are the mechanical properties of thesurrounding concrete such as concrete tensile strength, bar geometry, and the presence oftransverse reinforcement such as stirrups (ACI Committee 408, 2003). This result tendsto be the more catastrophic of the bond failure modes (Swenty, 2003). Another commonbond failure type is pull-out. This mode occurs when the reinforcing bar slips, and as aresult, the concrete between the bar deformations is crushed, leading to a simple pullingout of the bar. Usually pull-out controls when there is a larger concrete clear cover andspacing between the reinforcing bars making splitting less likely. A less common bondfailure is known as a conical failure. This occurs when the concrete cracks propagateoutward from the ribs on a reinforcing bar, and the bar ultimately pulls out along with a“cone” of concrete upon failure. Bond slip behavior of reinforcement bars in reinforced concrete members hasa pronounced influence on the design of anchorage of reinforcing bars and their splicelengths and on the structural ductility. Parameters that affect the concrete-steel bondproperties include concrete density, concrete cover, aggregate type, confinementconditions (e.g. transverse reinforcement), type, diameter, location and orientation of thereinforcing bar, and mix additives such as silica fume or fibers (Dancygier, 2010). Themain source of bond of deformed bars is the mechanical interlocking between theconcrete and the lugs of the rebar. Plain bars are more sensitive to the voids beneathhorizontal reinforcement because of the decrease of the contact area between concreteand steel and hence the adhesion. The bond behavior is significantly affected by theconcrete type in specimens with deformed bars (Soylev, 2011). The study of Harajli (2004) concentrated on the analytical evaluation of theaverage bond strength at failure, or development strength of reinforcing bars embedded inplain HSC in comparison with NSC. The analysis adopted a numerical solution schemeof the bond problem and incorporated an experimentally derived local bond stress-slipresponse, applicable for both NSC and HSC. The bond strength results predicted by theanalysis were in very good agreement with a collection of experimental data for bothNSC and HSC. The analytical results demonstrated that the average bond stressdistribution along the bar development length at bond failure is generally non-uniform, 7
  3. 3. and that the degree of non-uniformity in the corresponding bond stress distribution ismore pronounced for HSC as compared to NSC and increases with an increase in thedevelopment length. Increasing the development length leads to a concentration of thebond force in only a portion of the bar at the loaded end.2.2 BOND TESTING Testing for bond strength is carried out in a variety of ways. The mostcommon and traditional method is the standard pull-out test. One issue with the pull-outtest is that a compressive stress is induced on the bond that normally does not exist in anactual structure. To remedy this, ACI 408R-03 outlines several other methods such as thebeam anchorage, beam end, and splice tests that place the bond in situations that are moresimilar to those present in the field (ACI Committee 408, 2003). Note that the followingACI bond tests do not have specimen dimensions. This is because ACI does not specifyspecific dimensions. The pull-out test is popular due to its ease of construction and testing. ASTMC234 was developed to standardize the testing method, but was later disbanded due to thehigh level of inconsistency that the test yields. RILEM, however, has provided a set ofrecommendations for the test in order to provide some form of uniformity and minimizesome of the inconsistencies. The RILEM test recommends casting a single reinforcingbar into a concrete cube with only half of the bar inside the specimen actually bonded tothe concrete, as shown in Figure 2.1 (RILEM 7-II-28, 1994). This approach is to preventa conical bond failure at the bottom and is achieved using a bond breaker of some type.The bar is fed through a metal plate and a pulling force is applied to the bar while themetal plate pushes up on the concrete block until a bond failure occurs. Usually a deviceis installed on the unloaded end of the reinforcing bar in order to measure slip. While thistest has been modified by RILEM, it is still not accepted as an accurate way ofdetermining development lengths for reinforcement (ACI Committee 408, 2003).Therefore, this test is commonly used as a means of comparison between a controlspecimen of known development requirements and an experimental specimen. Data forthis test is often compiled into force vs. slip and stress vs. slip plots. 8
  4. 4. Figure 2.1: Typical pullout specimen (db=bar diameter)The IS: 2770 (PART IV) 1967 covers the method for the comparison of bond resistanceof different types of reinforcing bars with concrete by means of a pullout test. It states thewhole test procedure with method to calculate the bond stress. The moulds for bond test specimens shall be of size suitable for castingconcrete cubes of dimensions specified. According to that, 150mm size of cubes is usedfor 16mm diameter bars. Apparatus shall be provided for measuring the movement of thereinforcing bar with respect to the concrete at both loaded end and free ends of the bar.Dial micrometers shall be used at both locations. At the free end of the bar a dialmicrometer graduated to read 0.0025mm and having a range of not less than 2.5mm shallbe used. 9
  5. 5. Figure 2.2 Pullout test specimen with LVDTs The cube shall be reinforced with a helix of 6mm diameter plain mild steelreinforcing bar such that the outer diameter of the helix is equal to the size of cube. Tostudy the effect on bond stress in the situation of without transverse reinforcement, thishelix reinforcement is not used. Instead of that, various doses of steel fiber are used andits effects are studied. The slip at the loaded end of the bar shall be calculated as averageof the readings of the two dial gauges, corrected for the elongation of the reinforcing barin the distance between the bearing surface of the concrete block and point on thereinforcing bar at which the measuring device was attached.2.3 EFFECTS OF STEEL FIBERS ON BOND Fiber reinforced concrete (FRC) may be defined as a composite material madewith Portland cement, aggregate and incorporating discrete discontinuous fibers. Plain,unreinforced concrete is a brittle material, with a low tensile strength and a low straincapacity. The role of randomly distributes discontinuous fibers is to bridge across thecracks that develop provides some post-cracking ductility. If the fibers are sufficientlystrong, sufficiently bonded to material, and permit the FRC to carry significant stressesover a relatively large strain capacity in the post-cracking stage. The real contribution of 10
  6. 6. the fibers is to increase the toughness of the concrete, under any type of loading. Thefibers tend to increase the strain at peak load, and provide a great deal of energyabsorption in post-peak portion of the load vs. deflection curve. When the fiberreinforcement is in the form of short discrete fibers, they act effectively as rigidinclusions in the concrete matrix. The fiber reinforcement may be used in the form ofthree – dimensionally randomly distributed fibers throughout the structural member whenthe added advantages of the fiber to shear resistance and crack control can be furtherutilized. The fiber concrete may also be used as a tensile skin to cover the steelreinforcement when a more efficient two – dimensional orientation of the fibers could beobtained (Nguyen) The effect of fibers on the variation of bond between steel reinforcement andconcrete with casting position has not been sufficiently studied. Bond strength decreasesas concrete depth beneath horizontal reinforcement increases. This phenomenon is knownas top-bar effect and bleeding is considered to be the most important factor behind thisphenomenon. As the heavier materials settle in fresh concrete, bleed water movesupwards and it is trapped under large aggregates and horizontal reinforcement. The voidformation due to concrete settlement and water accumulation under the reinforcementcauses reduction in bond strength. The main source of bond of deformed bars is the mechanical interlockingbetween the concrete and the lugs of the rebar. Plain bars are more sensitive to the voidsbeneath horizontal reinforcement because of the decrease of the contact area betweenconcrete and steel and hence the adhesion. The bond behavior is significantly affected bythe concrete type in specimens with deformed bars. The results of compressive strengthand splitting tensile strength tests indicate increase in compressive strength but noincrease in splitting tensile strength by steel fiber addition with respect to the controlspecimen. Steel fiber reinforced concrete specimens remained integral after the pulloutfailure. The strength term does not change for steel fiber reinforced concrete as the tensilestrength did not change by steel fiber addition. Steel fiber has a confinement effect andthe development length can be reduced by confinement factor. However, as thecontribution of steel fibers to bond strength depends on crack length and width. The 11
  7. 7. confinement with fiber decreases as the concrete depth increases due to segregation ofsteel fibers. Therefore, there is need for an additional top-bar factor to define the decreaseof confinement for top-cast bars. The steel fiber reinforced concrete had higher bondstrengths. However, the decrease in bond strength from bottom cast to top cast bar washigher, mainly due to segregation of steel fibers. The superiority in the bond strength wasattributed to the improvement in the fracture behavior by the presence of steel fibers(Soylev 2011). Concrete is most widely used construction material in the world. However, ithas some deficiencies such as low tensile strength, low post cracking capacity, brittlenessand low ductility, limited fatigue life, not capable of accommodating large deformations,low impact strength. The weakness can be removed by inclusion of fibers in the mix. Thefibers can be imagined as an aggregate with an extreme deviation in shape from therounded smooth aggregate. The fibers interlock and entangle around aggregate particlesand considerably reduce the workability, while the mix becomes more cohesive and lessprone to segregation. Fibers help to improve the compressive strength, flexural strength,tensile strength, post peak ductility performance, pre-crack tensile strength, fatiguestrength, impact strength and eliminate temperature and shrinkage cracks. Fibers act ascrack arrester restricting the development of cracks and thus transforming an inherentlybrittle matrix into a strong composite with superior crack resistance (Shende, 2011). It is known that the addition of steel fibers leads to a reduction of crackwidth of bending elements in reinforced concrete. It is however, not established whetherthis effect is only due to prove transfer of tensile force by the fibers across cracks or alsobecause of an improvement of bond of the embedded deformed bar reinforcement byfibers. As the cover decreases, the bond strength decreases. The corner position of the barleads to a lower bond strength than the edge position. The results also show that the bondsplitting strength depends primarily on the relative cover irrespective of fiber additionwithin the investigated range of fiber contents. However the post-peak ductility afterreaching the bond splitting strength is markedly enhanced by fiber addition. The observedreduction of crack width and deformation of reinforced concrete bending members withsteel fiber addition is caused by the transfer of tensile force across primary cracks by the 12
  8. 8. fibers, acting as randomly oriented reinforcing bars. The post-peak ductility of specimensfailing by splitting is greatly improved by fiber addition (Rostasy, 1988). 13