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Frontiers of Structural and Civil Engineering

Front. Struct. Civ. Eng.    2016, Vol. 10 Issue (2) : 214-223     https://doi.org/10.1007/s11709-016-0332-3
RESEARCH ARTICLE |
The effect of carbon nanotubes and polypropylene fibers on bond of reinforcing bars in strain resilient cementitious composites
Souzana P. TASTANI(),Maria S. KONSTA-GDOUTOS,Stavroula J. PANTAZOPOULOU,Victor BALOPOULOS
Department of Civil Engineering, Democritus University of Thrace, Vas. Sofias Street, #12 Xanthi 67100, Greece
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Abstract

Stress transfer between reinforcing bars and concrete is engaged through rib translation relative to concrete, and comprises longitudinal bond stresses and radial pressure. The radial pressure is equilibrated by hoop tension undertaken by the concrete cover. Owing to concrete’s poor tensile properties in terms of strength and deformability, the equilibrium is instantly released upon radial cracking of the cover along the anchorage with commensurate abrupt loss of the bond strength. Any improvement of the matrix tensile properties is expected to favorably affect bond in terms of strength, resilience to pullout slip, residual resistance and controlled slippage.The aim of this paper is to investigate the local bond of steel bars developed in adverse tensile stress conditions in the concrete cover. In the tests, the matrix comprises a novel, strain resilient cementitious composite (SRCC) reinforced with polypropylene fibers (PP) with the synergistic action of carbon nano-tubes (CNT). Local bond is developed over a short anchorage length occurring in the constant moment region of a four-point bending short beam. Parameters of investigation were the material structure (comprising a basic control mix, reinforced with CNTs and/or PP fibers) and the age of testing. Accompanying tests used to characterize the cementitious material were also conducted. The test results illustrate that all the benefits gained due to the synergy between PP fibers and CNTs in the matrix, namely the maintenance of the multi-cracking effect with time, the increased strength and deformability as well as the highly increased material toughness, were imparted in the recorded bond response. The local bond response curves thus obtained were marked by a resilient appearance exhibiting sustained strength up to large levels of controlled bar-slip; the elasto-plastic bond response envelope was a result of the confining synergistic effect of CNTs and the PP fibers, and it occurred even without bar yielding.

Keywords carbon nanotubes      strain resilient cementitious composite      polypropylene fibers      tensile bending      bond     
Corresponding Authors: Souzana P. TASTANI   
Online First Date: 05 April 2016    Issue Date: 11 May 2016
 Cite this article:   
Souzana P. TASTANI,Maria S. KONSTA-GDOUTOS,Stavroula J. PANTAZOPOULOU, et al. The effect of carbon nanotubes and polypropylene fibers on bond of reinforcing bars in strain resilient cementitious composites[J]. Front. Struct. Civ. Eng., 2016, 10(2): 214-223.
 URL:  
http://journal.hep.com.cn/fsce/EN/10.1007/s11709-016-0332-3
http://journal.hep.com.cn/fsce/EN/Y2016/V10/I2/214
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Souzana P. TASTANI
Maria S. KONSTA-GDOUTOS
Stavroula J. PANTAZOPOULOU
Victor BALOPOULOS
Fig.1  Bond shear stresses fb along an elementary segment and radial stresses sn sustained by hoop media stresses shoop.
mix design portl. cem., 42.5 fine fly ash fine sand (dagg <0.5 mm) water HRWRa) PPb) (vol.)
M1 1 2.8 1.12 1.4 (0.368)c) 0.09 1.5%
M2 1 2 1.1 1.1 (0.366)c) 0.1 2%
Tab.1  Mix designation
Fig.2  Comparative representation of the tested mixtures M1 and M2 regarding (a) the fresh state and (b) the cracking pattern of the associated 3-point bending samples
Fig.3  Average load- deflection histories of the 3-point bending prims of M1 and M2 mixtures
Fig.4  Control and control with CNTs matrices: (a) average load – mid deflection envelopes of the 3-point bending tests on prims and (b) associated fractured surfaces
Fig.5  SRCC and SRCC-CNT matrices: (a-b) average load—mid deflection responses of the 3-point bending tests on prisms, (c) comparative representation of the average envelopes of both matrices at both ages and (d) the associated failure patterns at both ages. (Within brackets is the testing age in days)
Matrix ID – (age in days) Experimental results Tensile indices
Dy (mm) Du(mm) Du. 85% (mm) Py (kN) Pu (kN) Gf(Nt?m?1) ft,fl(MPa)
Control(70 d) 0.16 0.16 0.16 1.45 1.45 73 4.75
Control –CNT(30 d) 0.23 0.23 0.23 2.39 2.39 172 7.83
SRCC(30 d) 0.28 1.80 2.62 1.55 2.53 3275 5.10
SRCC(70 d) 0.28 0.93 1.47 2.70 2.36 1997 8.86
SRCC-CNT(30 d) 0.18 1.56 2.25 1.66 3.01 3307 5.43
SRCC-CNT(70 d) 0.27 2.21 3.01 1.66 2.84 4193 5.46
Tab.2  Experimental average values for applied force and mid deflection at milestones (apparent first cracking-subscript y, peak-subscript u and ultimate-subscript u.85%) and analytical estimations of tensile indices (apparent flexural toughness Gf and modulus of Rupture ft,fl as per [13])
Fig.6  (a,b) Geometry and (c) detailing of the 4-point bending short beam for investigation of local bond in the constant moment region. (d) Setup and instrumentation of the specimens
Fig.7  (a) Average load—mid deflection responses of the bond tests. (b) Cracking pattern near failure on the lateral face of all bond tests. (c) Splitting cracking of the bottom cover along the bonded length occurred in every case
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