BOND STRENGTH AND RIB GEOMETRY: A COMPARATIVE STUDY OF THE INFLUENCE OF DEFORMATION PATTERNS ON ANCHORAGE BOND STRENGTH

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1 BOND STRENGTH AND RIB GEOMETRY: A COMPARATIVE STUDY OF THE INFLUENCE OF DEFORMATION PATTERNS ON ANCHORAGE BOND STRENGTH Michel S. Lorrain, PhD, Dept. of Civil Engineering, INSA/LaSAGec², Toulouse, France Luciane F. Caetano, MSc, LEME/PPGEC, UFRGS, Porto Alegre-RS, Brazil Bruno V. Silva, MSc, Dept. of Civil Engineering, UNESP, Ilha Solteira-SP, Brazil Lara E.S. Gomes, Phis, Dept. of Design, LdSM/UFRGS, Porto Alegre, Brazil Mônica P. Barbosa, Dr, Dept. of Civil Engineering, UNESP, Ilha Solteira-SP, Brazil Luiz Carlos P. Silva Filho, PhD, LEME/PPGEC, UFRGS, Porto Alegre, Brazil Bond between steel and concrete has been constantly investigated due to its importance in the performance of concrete and also due to its constant modifications as a result of technological advances in the production of steel and concrete. Bond strength is also known to be influenced by several different factors, such as: concrete strength, concrete cover, geometry, size and surface condition of rebars, position of the bars during casting, among others. This study aims to evaluate the influence of geometric properties of the bar (height, rib face inclination, rib spacing and rib inclination in relation to the bar axle) on bond strength through pull-out test. For that, bars coming from different countries, such as Brazil and France, and from different manufacturers were analyzed. The nominal diameter of the bar was fixed in 12.5 mm and the concrete strength was ranging from 20 to 25 MPa. The geometric properties (height, spacing, inclination of ribs) were measured considering a new technique based on a the three-dimensional laser scanner of the Laboratory of Design and Selection of Materials (LDSM) of the Federal University of Rio Grande do Sul (UFRGS). Relative rib areas were calculated according to ACI Committee 408 and CEB-FIP recommendations. Keywords: bond strength, bar geometry, 3-D laser scanner profiling, pull-out test.

2 INTRODUCTION The mechanical contact between the concrete matrix and the rib area of reinforcing steel bars is considered as the utmost factor responsible for the steel-concrete bond strength. The insertion of a portion of concrete between consecutive ribs (as seen in figure 1) creates a mechanical interlock between both materials that end up anchoring the ribs, restraining the relative displacement between the steel and concrete until this concrete console is sheared-off or the whole concrete cover splits. concrete consoles Fig. 1 Detail of the whole cover concrete and bar spacing. Naturally, the geometric properties of the rib, which are dependent on the fabrication process of deformed rebars, will therefore influence the development of bond strength. Due to this fact, several standards, such as the Brazilian NBR¹ 7780/07 and the ASTM² A706/09, establish specific requirements for some geometrical properties such as rib area, rib height, rib inclination and rib spacing³. The problem was that these properties are not easily measured or controlled. This work proposes to overcome this difficulty by using a novel technique for rebar geometric assessment, by means of 3-D laser profiling. This allows the generation of a precise model of the rebar, that can be manipulated in a CAD environment, as discussed in more detail ahead. The first results obtained with this technique were very promising and a major collaboration study was therefore initiated to characterise and assess the impact of rib geometry on bond strength, taking into account deformed bars produced in several countries, starting with Brazil, France and Tunísia. The proposal is to measure the specific geometric properties of rebars from various manufacturers, with several diameters, and check the resulting effects of these variations in bond strength, using concretes with a wide range of compressive strength. The main aim is to determine more precisely how rib geometry affects bond strength and check the variability of the more relevant rib properties between different countries and manufacturers. This paper presents some of the preliminary data obtained. It was gathered from 12.5 mm diameter bars coming from five different manufacturers (four Brazilian and one French). Each one was subject to laser profiling in order to determine the precise geometric parameters that could influence bond strength. Later, the bars were inserted on concrete cubes with a mean compressive strength value of 20 MPa (minimum strength requirement for RC structures in Brazil) and subjected to the traditional pull-out test. The paper presents some graphs showing the initial results of the analysis that are being conducted to check how changes in rib geometry and spacing affect bond behaviour. 2

3 CONSIDERATIONS ON THE INFLUENCE OF RIB GEOMETRY ON BONDING Due to the relevance of rib geometry in the bond mechanism, several previous studies can be found in the literature dealing with this issue. Among them, it is interesting to highlight the work undertaken by Clark (1946 4, ), who analysed 17 bar configurations and suggested that the average spacing between two consecutive ribs and the rib height should be equal to 70% and 4% of the diameter, respectively, for bars with nominal diameters smaller than 13 mm. These recommendations are in accordance with the requirements incorporated on the ASTM A 706/09 and on the Brazilian Standard NBR 7780/07. A recent study undertaken by Hamad 6 (1995) investigated the role of several geometric variables, such as: rib spacing (ranging from 35 to 60% of bar diameter); rib height (5 and 12.5% of the diameter) and rib inclination (from 30 to 90 ). Analysing the results of this study it can be verified that the configuration that resulted in the better bond performance was: rib inclination equal to 60, rib spacing equal to 50% and rib height equal to 10% of the bar diameter. Regarding the influence of rib inclination, a thorough work by Lutz and Gergeley 7 (1967) observed that inclinations ranging from 40 to 105 are sufficient to avoid relative displacements between the steel bar and the concrete, leading to a well-defined slipping behaviour after the shear failure of the concrete console. Relative movement is not restrained when the angle drops lower than 30, affecting the failure mode and changing the strengthslip curve. However, it is useful to be careful when considering such data nowadays, given that modern concrete tends to have higher strength and may present a more fragile behaviour. Other authors, such as Leonhardt and Monnig 8 (1977), have highlighted the importance of considering the relative surface of the ribs (f R ) as a parameter for bonding efficiency. Basically, this parameter is defined as a relation between the rib surface (fr), which is equal to the contact area of the concrete consoles between the ribs with the ribs, and the lateral surface (F M ) of the concrete cylinder to be cut in order to allow the bar to slip. Even though the basic idea is the same, there are small but important variations on the equations proposed by the codes and some authors to consider this parameter. Soretz e Holzenbein 9 (1979) suggest the use of equation (1), while the CEB 1 10 (1999), the CEB 2 11 (July e Sept, 1999) and the EUROCODE 2 12 (1993) propose a simplified formula (equation 2), which follows the general principle of equation (1). The ACI 408.3R³, nonetheless, uses a different way to define this relationship, as shown in equation (3). k An senβ fr = π φ Sn hs fr = γ Bearing area hs fr = ( 0,8 a 0,9) Sn Shearing area Sn Equation 1 Equation 2 Equation 3 3

4 Where: fr = rib relative area; k = number of transversal ribs around the bar perimeter; An = area of the longitudinal section of the rib; β = inclination angle of the rib; φ = bar diameter Sn = rib spacing γ = constant that depends on the bar geometry (usually = 0,5) and hs = maximum height of the transversal rib. Caetano 13 (2008) has concluded in her work that bond strength varies with the cross sectional rib area. However, it was shown that this relationship is not linear. For a 70% increase on the rib cross area, the bond strength rose just around 31%. These and various other studies have confirmed that parameters linked to the rib geometry are vital to determine bond behaviour. There is still a need, however, to improve the knowledge about how the changes in rebar geometry, derived from differences in the fabrication processes, will impact bond performance. A new research tool was used in this work, as described below, opening up the opportunity to enhance the way we characterise bar geometry, and may constitute an important contribution for the investigations in this field. A NOVEL APPROACH FOR BAR GEOMETRY MEASUREMENT BY LASER SCANNER PROFILING An important advancement proposed in this study was the use of an innovative technology, already well known and used in the design area but yet novel in the civil engineering field, which consists on the generation of a 3D image of an object using laser profiling. The technique allows a detailed reconstitution of bar geometry and a precise determination of rib shape by means of a continuous scanning using a laser beam, as observed in figure 3. Figure 3 Detail of bar laser scanning procedure. 4

5 The laser profiling was carried out in the Laboratory of Design and Selection of Materials (LdSM/UFRGS) using a Digimill 3D laser machine. For this study measurement points were spaced 0.05 mm in both directions. After the scanning procedure, a precise 3D geometric model of each bar was generated (as seen in figure 4), which could then be exported and manipulated in a CAD environment. This made possible a precise determination of the geometric parameters, which was done with a much better accuracy than it was possible with other methods used previously for rebar measurement. Fig. 4-3D bar model generated with the laser profiling technique. REBAR TYPES USED IN THE STUDY The direct variables considered for this work were the country of origin and the manufacturer of the bars whilst the indirect variables were the geometric characteristics of the bars (rib height, rib spacing and rib face inclination). Five types of bars were analysed in this preliminary study: four Brazilians bars, labelled A to D, from two different manufacturers. Bars A, B and D are all from the same manufacturer, but bar A is from one plant while bars B and D are from another one. Bar D has a different configuration, which is being tried by the manufacturer to ease rebar bending. Label E represents a French rebar. All presented nominal yield strengths of 500 MPa and nominal diameters of 12.5 mm. Figure 5 shows the detailed rib configuration of each bar, obtained with the laser profiling technique. To obtain the profiles, small segments 100 mm long with at least 8 ribs in each side were subjected to laser scanning. The geometric characteristics of each bar were determined by an average of at least 10 measurements made in the resulting CAD model. 5

6 Lorrain, et al 3rd fib International Congress A B C D Fig. 5 BOND TEST PROCEDURE E E-Face I E-Face II Laser profiles of the bars tested in this study. The specimenss used for the pull-out test were prepared and tested following the requirements of RILEM (1973). It is important to mention thatt prior to casting, the bars were subjected to a mechanical cleaning procedure to remove the superficial rust and any other residue thatt might be attached to the bar surface. Afterwards, according to the RILEM recommendation, the section of the bar which was not to be bonded to the concrete was marked and isolated by the placement of a plastic tube. Figure 6 shows the experimental set-up used for the pull-out tests. Concrete specimen LVDT Magnetic base Leather layer Mettalic plate Fig. 6 Experimental test setup. 6

7 As seen in figure 6, the concrete cubes were placed against the bottom movable arm of the testing machine, while the bar was fixed on the upper arm. The free end of the bar was instrumented with a displacement transducer (an LVDT) fixed on the concrete block to avoid any contamination of the slip measurements. CONCRETE MIX PROPORTION AND CURING PROCEDURES The pull-out tests were conducted on cubes 125mm (10 dia.) wide, made with 20 MPa concrete. The mix proportion used was 1 (binder): 3,42 (fine aggregate) : 4,08 (coarse aggregate), with a water/cement ratio (w/c) of A Type III Portland cement; a local siliceous river sand with a fineness modulus of 2.69 and maximum particle size of 4.8 mm; and a basaltic coarse aggregate with a fineness modulus of 6.83 and maximum particle size of 19.0 mm were used in the mix. All the specimens were demoulded 24h after casting and placed in a curing room with an average temperature of 23 C and a 99% relative humidity for 21 days. Specimens were then transferred to the laboratory environment for 6 days (being stored at a temperature around 20 C and relative humidity of 78%) to avoid using wet specimens during tests. EXPERIMENTAL PROGRAMME Prior to presenting the results of this study, it is important to clarify the terminology used to define each geometric characteristic of the bars, as seen in the schematic drawing of figure 7. Rib Spacing 11,08 mm 150,20 Rib face angle 1,14 Rib Heigth mm Rib Inclination (a) (b) Fig. 7 - Schematic representation of the geometric characteristics of the bars; typical section on XY plane (a), typical section on the XZ plane (b). The data collected for each bar, considering the geometric parameters defined above, is presented in table 1. It is important to highlight that the geometric data of bar D was taken considering face I. On face II, the rib height is similar to face I, however the spacing varies from 4.41 to mm, due to variations on rib inclinations. 7

8 Bars Tab. 1 Bond strength and geometric characteristics for each bar type. Bond Strength [MPa] Rib Heigth [mm] Rib Spacing [mm] Rib Face Angle [ ] Rib Inclination [ ] A 14,59 0,97 9, B 14,28 0,88 9, C 11,47 0,79 8, D 9,90 0,64 8, E 8,99 0,94 7, The data was used to create Figure 8, which shows the individual variations, in terms of ratios of rib height and rib spacing to bar diameter. It is easy to verify in this figure that all bars tested conformed to the minimum requirements specified in the NBR 7780/07 and ASTM A706/09 standards. On graph 8(a) it can be noticed that the bar labelled as D presented a notably smaller height/diameter ratio than the others, but the value was still 19% higher than the minimum suggested by both standards. The other bars presented values 58, 76, 88 and 94% higher than the minimum requirement. Relative Heigth/Diameter [%] (a) Steel Type A B C D E Fig. 8 Results; height/diameter ratio (a), spacing/diameter (b). Relative Spacing/Diameter [%] (b) Steel Type A B C D E NBR ASTM NBR Regarding the ratio of rib spacing to bar diameterthe requirement vary according to the standard considered. While the NBR 7780/07 indicates that the rib spacing should range between 50 and 80% of the diameter, the ASTM A706/09 adopts 70% as the maximum value allowable. Looking at graph 8(b) it can be verified that all bars comply with the Brazilian standard. However, when considering the ASTM criterion, bars A and B would have slightly higher than admissible rib spacing values (3.2 and 4.2% higher, respectively). 8

9 When the results were analysed in terms of both rib inclination and rib face angle, it was not possible to find any direct relationship with the results of bond strength. This leads to the conclusion that inclinations inside a certain range do not significantly influence the bond behaviour, as suggested by Lutz and Gergeley (1967). However, it cannot be dismissed the hypothesis that this behaviour may vary with the increase of concrete compressive strength. Further analyses are being carried out to investigate this possibility and the results will be published in the near future. A stronger relation with the bond strength is noticed when rib height and rib spacing are taken into account: the larger are these geometric parameters, the better is the bond performance, with the exception of the bar labelled as E. The authors believe that this distinction is caused by the variation of rib spacing that exists on face II of this bar. As explained before, due to the change in the inclinations of two consecutive ribs, the shorter distance between the ribs is close to 4.41 mm at a certain point, a value 30% smaller than the minimum suggested by the Brazilian standard. The impression that this geometry affected the bond performance was supported by visual observations that pointed out that the bond failure occurred in the face II of the bar. The conclusion is that the short space between ribs reduced the concrete console area, thus producing a concentration of stresses. This behaviour might be mitigated when higher strength concrete is used. Figure 9 shows a plot of the relative bond strength against the relative rib height. It can be seen that a straight regression line fits quite well the experimental data, resulting on a R 2 value equal to 0.94, which demonstrates that there is a direct relationship between rib height and bond performance. 120 Relative Bond Strength [%] y = 1,1174x - 9,1301 R² = 0, Relative Rib Heigth [%] Fig. 9 - Plot and adjusted line of relative bond strength against relative rib height. When analyzing the bond performance in relation to rib spacing, as done in figure 10, it is possible to conclude that there is also a relationship between the variables, although not as 9

10 well defined as in the case of rib height. An exponential curve provided the best adjust to the data, resulting in a R² value of 73.2%. It is clear from figure 10 that bar D does not fit well with the adjusted curve, presenting a different behavior from the other ones, probably due to the very distinct rib configuration used by the manufacturer to enable easier bending. 120 Relative Bond Strength [%] y = 9,9e 0,022x R² = 0, Relative Rib Spacing [%] Fig Plot and adjusted curve of relative bond strength against relative rib height. Figure 11 was therefore plotted without the data from this bar, which enabled the exponential adjusted curve to reach a considerably higher R² value, close to 90%, which indicates a very good fit. This result indicates that, to obtain a good fit between bond performance and rib spacing it is important to consider bars with basically similar rib geometry. 120 Relative Bond Strength [%] y = 10,23e 0,022x R² = 0, Relative Rib Spacing [%] Fig Plot and adjusted curve of relative bond strength against relative rib height without data from bar D. FINAL CONSIDERATIONS From the preliminary results obtained it is possible to reach some considerations: 10

11 The laser profiling technique, has shown great potential, allowing the generation of 3d models that could be used on a CAD environment for a quick and precise determination of geometric bar and rib properties; There are important differences in rib geometry between manufacturers and plants. In general, the geometric characteristics of the bars tested complied with the specifications of the Brazilian NBR 7780/07 and the ASTM A706/09 standards; The bond strength results for all bars were compatible with the ones expected for the concrete strength adopted in the tests; It was confirmed that rib geometry has a critical influence on bond strength, proving that a good bar geometry is vital for good bonding behavior; Rib inclination and rib face angle did not influence bonding performance, for the range of values tested; Rib height and rib spacing were the most influential parameters for bond performance. A linear relationship with very strong adjustment was observed between rib height and bond strength. An exponential adjustment with a slightly worse fit was obtained for the rib spacing versus bond strength relationship. Furthermore, the results from bar E have shown that, for the test conditions used, the variation in bar spacing caused by variation of rib inclination alongside one of the faces of the bar reduced the bond strength; Further studies are being carried out to confirm and expand these initial trends, considering more manufacturers, other bar diameters and different concrete strengths. Final conclusions must wait until the whole range of tests is performed. REFERENCES 1. Associação Brasileira de Normas Técnicas. Aço destinado a armaduras para estruturas de concreto armado especificação. ABNT/NBR 7480, Rio de janeiro, ASTM A706/09 Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement, ACI 408 3R: Bond and Development of straight reinforcing bars in tension, do Comitê 408 do American Concrete Institute, Clark, A.P., Comparative Bond Efficiency of Deformed Concrete Reinforcing Bars. Journal of the American Concrete Institute, V.18, N.4, Dez. 1946, pp Clark, A.P. Bond of Concrete Reinforcing Bars. Journal of the American Concrete Institute, V.46, N.11, Nov. 1949, pp

12 6. Hamad, B.S., Bond Strength Improvement of Reinforcing Bars With Specially Designed Rib Geometries. ACI structural journal, V. 92, No. 1, Jan. 1995, pp Lutz, L.A.; Gergely, P. Mechanics of Bond and Slip of Deformed Bars in Concrete. ACI Journal Proceedings, V.64, N.11, Nov. 1967, pp Leonhardt, F; Mommig, E. Construções de Concreto: Princípios Básicos do Dimensionamento de Estruturas de Concreto Armado. 1ª ed. Rrio de Janeiro: Interciência, V. 1 e 3, Soretz, S.; Holzenbein, H. Influence of Rib Dimensions of Reinforced Bars on Bond and Bendability. ACI Structural Journal, V.76, N.1, Jan. 1979, pp Comité Euro-International Du Béton: Structural Concrete. Paris, Bulletin N.1, Jul, Comité Euro-International Du Béton: Structural Concrete. Paris, Bulletin N.2, Jul, Eurocode 2. Stahlbeton Und Spannbeton. Springer Verlag, Caetano, L. F., Estudo do Comportamento da Aderência de Elementos de Concreto Armado Sujeitos a Corrosão e às Altas Temperaturas, dissertation (Master), School of Engeneering at UFRGS,