Bond between self-compacting concrete and reinforcement

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Universidad de la Costa Traian Onet, Technical University of Cluj-Napoca

1 Universidad de la Costa From the SelectedWorks of Marian Sabau November, 2012 Bond between self-compacting concrete and reinforcement Marian Sabau, Universidad de la Costa Traian Onet, Technical University of Cluj-Napoca Ioan Pop, Ghent University Available at:

2 Bulletin of the Transilvania University of Braşov Vol. 5 (54) Series 1: Special Issue No. 1 BOND BETWEEN SELF-COMPACTING CONCRETE AND REINFORCEMENT M. SABĂU 1 T. ONEȚ 1 I. POP 1 Abstract: This paper study the bond behaviour of self-compacting concrete (SCC) in comparison to normal vibrated concrete (NVC). In this paper are presented the following parameters: the influence of bar diameter, the influence of concrete quality, the top-bar effect, the influence of active and passive confinement. In literature, different test results are found for the bond strength in SCC, which deliver contradictory results, but internationally it seems to be agreed that bond strength in SCC is slightly higher than NVC. Key words: self-compacting concrete, bond, reinforcement, top-bar effect. 1. Introduction One of the more recent developments is self-compacting concrete (SCC). This concrete type has in contrast to normal vibrated concrete (NVC), no need for external vibration energy to be compacted. To obtain these properties a few modifications in the composition of the concrete are necessary. One of the methods to achieve self-compaction is the reduction of the coarse aggregate and an increase in the amount of powder. Self-compacting concrete is defined according to De Schutter et al. [1] as: a kind of concrete which needs to possess sufficient fluidity in order to be able to fill a formwork completely (filling ability) without the aid of other forces than gravity, even when having to flow through narrow gaps (passing ability), but also showing a sufficient resistance to segregation, during flow and in stationary conditions (stability). The definition given by EFNARC [2] is quite similar: A concrete that is able to flow and consolidate under its own weight, completely fill the formwork even in the presence of dense reinforcement, whilst maintaining homogeneity and without the need for any additional compaction. In both definitions three important requirements of fresh concrete are mentioned: the filling ability (the ability to fill the formwork), the passing ability (the resistance against blocking) and the stability (the resistance against segregation). The bond between steel and concrete has an important influence on the behaviour of reinforced elements in the cracked stage. Crack widths and deflections are influenced by the distribution of bond stresses along the reinforcement bars and by the slip between the bar and the surrounding concrete. Bond has been the subject of different studies on SCC, but the conclusions are very contradictory: some indicate that bond strengths of reinforcing bars in SCC are higher than those measured for NVC, other researchers see no differences between or even lower strengths. Most studies agree 1 Dept. of Structures, Faculty of Civil Engineering, Technical University of Cluj-Napoca

3 282 Bulletin of the Transilvania University of Braşov Vol. 5 (54) Series 1 that the bond strength of reebars in SCC is larger than that in NVC. 2. The influence of bar diameter Generally we can say that when the bar diameter increases, the medium and ultimate bond strength decrease. This decrease of the bond is not a linear variation in comparison with the bar area, being more accentuated for small diameters and smaller for big diameters. The SCC presents the same behaviour but in most of the studies [3], [4] SCC show a better behaviour than NVC. As we can see in Figure 1, the differences in the normalized bond for NVC and SCC are largest for bar diameters of 12 mm and the difference become smaller for higher diameters. Especially for self-compacting concrete the differences in the ultimate bond strength between these two concretes is largest for small diameters and became smaller for higher bar diameters. Fig. 1. Influence of bar diameter. In his study, Desnerck [4] observed that by increasing the bar diameter, the slip at maximum bond stress is increasing in all cases and no significant difference can be noticed between the results for SCC and the results for NVC. 3. The influence of concrete quality Bond action results from the localized pressure underneath the ribs and is directly related to the shear component of the interface forces. Bond performances depend on both concrete multi-axial behaviour in compression and on concrete tensile strength f ct. f c and f ct play a major role in pull-out and splitting failures respectively. The dependence, however, is less then linear and the position of bar during concreting is even more important than concrete strength as is shown in Figure 2.

4 M. SABĂU et al.: Bond between self-compacting concrete and reinforcement 283 Fig. 2. Bond stress versus concrete strength for different slip values and casting directions [5] The bars positioned at the bottom of the formwork and vertical bars loaded in opposite direction of the casting direction have higher bond strengths. For vertical reinforcing bars parallel to the casting direction, the combined effect of bleeding in and settlement of the fresh concrete leads to void formation underneath the ribs of the bar and the bond behaviour of the rebar is affected by the direction in which it is loaded during the experiments [6]. 4. Top-bar effect Top-cast bars have lower bond strengths than bars cast lower in a member. This behaviour is recognized in ACI318 [7] and EC2 [8]. Top reinforcement, horizontal reinforcement with more than 300 mm in ACI318 and 250 mm in EC2 of fresh concrete cast in the member below the development length or splice, requires a 30% increase in development length. Most research, however, indicates that while an increased depth of concrete below a bar reduces bond strength, the effect of shallow top cover is of greater significance. The impact of shallow top cover on the top-cast bar effect is emphasized by the fact that the strength reduction becomes progressively greater as cover is decreased. The lower bond strength of top-cast bars may be explained as follows: rising bleed water can be trapped under the bars, and any settling of the concrete can leave air voids under the bars which will compound the effect. The amount of bleeding increases with concrete depth below the bar, resulting in lower bond strength in the upper parts of a deep section. All studies performed to determine the top-bar effect in SCC are using pull-out test method. Domone [9] presents in his paper results of two programs. Figure 3 shows the results of tests on a set of five wall elements, each 1.5 m high, with deformed bars at four levels. Four of the elements were cast with SCC of different compositions and one with NVC. The in situ strength for the NVC was approximately 50 MPa, and the in situ SCC strengths varied from 35 to 43 MPa. All mixes showed a reduction in bond strength with increasing height in the wall. Three of the SCC mixes behaved similarly to the NVC mix and one somehow better at all heights. The NVC and two of the SCC mixes also showed a reduction greater than the EC2 [8] top-bar factor at the top of the section.

5 284 Bulletin of the Transilvania University of Braşov Vol. 5 (54) Series 1 Fig. 3. Variation of bond strength in wall slab and column elements [9] More extreme behaviour was obtained from tests on round bars in 2 m high columns, as can be seen in Figure 3. Each data point is the average from bars at three closely spaced levels. Columns with three NVC and two SCC mixes with varying strength levels were tested, and considerable reductions in bond strength (of up to 80%) were obtained with the two lower strength NVC mixes. The two SCC mixes and the highest strength NVC mix showed broadly similar behaviour, with reductions in bond strength similar to that recommended in EC2 [8]. In Figures 4 and 5 are presented results from a recent study made by Chan et al. [10] dealing with pull-out tests. They have reported that, as compared to NVC, SCC exhibits higher bond to reinforcing bars and lower reduction in bond strength due to topbar effect at all ages. Khayat [11] studied the bond strength of SCC with special focus on the effect of VMA to reduce the top-bar effect of anchored bars. Accumulation of bleed water under the reinforcement and separation of fresh paste from the reinforcement due to segregation and settlement can significantly reduce the bond. A total of 25 specimens were prepared by Khayat [11] to evaluate the effect of specimen height (500, 700 and 1100 mm) and bar anchored length (2.5 and 5 times bar diameter) on external bleeding, surface settlement, segregation and relative bond strength (from pull-out tests) of horizontally embedded bars. The findings indicated that the use of VMA reduced surface settlement (that is related to bleeding and segregation) and significantly reduced the top-bar factor. Sonebi et al. [12] performed bond tests (pull-out tests) with 12 and 20 mm deformed bars placed in concrete specimens of 100x100x150 mm to study the performance of SCC compared to NVC. The test results showed 10 40% higher normalized bond strength in SCC compared to NVC.

6 M. SABĂU et al.: Bond between self-compacting concrete and reinforcement 285 Fig. 4. Bond strength in NVC [10] Fig. 5. Bond strength in SCC [10] 5. The influence of passive and active confinement The stress state of the surrounding concrete has a significant effect on the bond strength of the steel bar. If the transverse stresses are compressive, the bond behaviour is favoured. Confinement can be active or passive as shown in Figure 6: (a) bar anchorage (partly active partly passive confinement); (b) lapped splice (passive confinement by stirrups) and (c) bar anchorage in an indirect support (active confinement). Fig. 6. Examples of bond-confinement interaction [5] Active confinement is resulting from a direct support or a column-beam joint and is more efficient than passive confinement, since its effects do not depend on the mobilized bond stress. Passive confinement is developed by the concrete cover and the stirrups and is less efficient, since it originates from concrete dilatancy, which accompanies crack formation and is strictly related to the actual bond stress. The major problem for passive confinement is how much transverse reinforcement is needed to be able to prevent splitting failure. The topical subject for active confinement is the transition from a pull-out failure to a splitting failure. The cover thickness and the transverse pressure help as long as bond failure is

7 286 Bulletin of the Transilvania University of Braşov Vol. 5 (54) Series 1 controlled by concrete splitting. 6. Conclusions For larger bar diameters, the difference between the values measured for NVC and those for SCC are small. For small diameters the bond strength for SCC is significantly higher than for NVC. Generally it can be concluded that the top-bar effect is less pronounced in SCC members. Acknowledgements This paper was supported by the project "Improvement of the doctoral studies quality in engineering science for development of the knowledge based society-qdoc contract no. POSDRU/107/1.5/S/78534, project cofunded by the European Social Fund through the Sectorial Operational Program Human Resources References 1. Poppe, A.-M., De Schutter, G., Audenaert, K., Boel, V.: Kennismaking met zelfverdichtend beton (1) Samenstelling en reologie. (Introducing self-compacting concrete (1) Composition and rheology).bouwkroniek 2002, EFNARC. The European Guidelines for Self-Compacting Concrete: Specification, Production and Use; ERMCO, 2005; p Fernando Menezes de Almeida et al.: Bond-slip behaviour of selfcompacting concrete and vibrated concrete using pull-out and beam tests. In: Material and Structures (2008) 41: Pieter Desnerck, Geert De Schutter et al.: Bond behaviour of reinforcing bars in self-compacting concrete: experimental determination by using beam tests. In: Material and Structures (2010) DOI //s fib bulletin 10. Bond of reinforcement in concrete, International Federation for Structural Concrete, august Castel, A., Vidal, T., Viriyametanont, K., François, R.: Effect of reinforcing bar orientation and location on bond with self-consolidating concrete. In: ACI Structural Journal 2006, 103(4), American Concrete Institute. ACI Building Code Requirements for Structural Concrete; ACI, 2011 p Eurocode 2: Design of concrete structures - Part 1-1: General rules and rules for buildings. 9. Domone P.L.: A review of the hardened mechanical properties of self-compacting concrete. In: Cement & Concrete Composites 29, Chan, Y., Chen, Y., Liu, Y.: Development of bond strength of reinforcement steel in self-consolidating concrete. In: ACI Structural Journal 2003, 100(4): Khayat, K.H., Manai, K. and Trudel, A.: In Situ Mechanical Properties of Wall Elements Cast Using Self-Consolidating Concrete. In: ACI Materials Journal, Vol. 94, No. 6, 1997, pp Sonebi, M., Bartos, PJM., Zhu, W., Gibbs, J., Tamimi, A.: Properties of hardened concrete. Final report. In: Advanced Concrete Masonry Centre, University of Paisley, Scotland, UK, 2000.

Hardened properties of selfcompacting

Universidad de la Costa From the SelectedWorks of Marian Sabau November, 2012 Hardened properties of selfcompacting concrete Marian Sabau, Universidad de la Costa Traian Onet, Technical University of Cluj-Napoca