- Email: [email protected]
Rehabilitation of reinforced concrete axially loaded elements with polymer-modified cementicious mortar
Construction and Building Materials 23 (2009) 3129–3137
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Rehabilitation of reinforced concrete axially loaded elements with polymer-modified cementicious mortar Carlo Pellegrino *, Francesca da Porto, Claudio Modena Department of Structural and Transportation Engineering, University of Padova, Via Marzolo 9, 35131 Padova, Italy
a r t i c l e
i n f o
Article history: Received 21 November 2008 Received in revised form 16 May 2009 Accepted 18 June 2009
Keywords: Reinforced concrete Repair mortars Interface Cracking Columns
a b s t r a c t The aim of the paper is to investigate the compatibility and the efficiency of the rehabilitation intervention on reinforced concrete columns with polymer-modified cementicious mortar. This paper presents the results of experimental tests on axial behaviour of reinforced concrete columns, with square crosssection, repaired by polymer-modified cementicious mortar. Tests were repeated varying repair thickness, which included or did not include the steel reinforcement on one face of the square column. Despite this type of intervention is quite common in practice, the effect of repair thickness on the intervention efficiency, in relation to the existing steel reinforcement configuration, had not been previously studied in detail for axially loaded elements. Results were discussed and compared with those from control columns, which were tested in nondamaged, non-repaired conditions. The main findings of this work can be summarized as follows. The repair cannot restore the load-bearing capacity of non-damaged control columns, although they give acceptable results. Repairs that include the longitudinal reinforcement show good properties, with stable behaviour, sharing of loads, and plasticization of the material before failure, whereas thin repairs that do not include the reinforcement do not have adequate performance due to premature debonding. Non-linear numerical models also confirmed the different behaviour of the two types of repair. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The field of rehabilitation and strengthening of reinforced concrete structural elements shows an increasing interest for existing constructions and various projects have been carried out around the world over the past two decades. Structural strengthening and repairing is aimed at increasing or restoring the load-bearing capacity of the element, due to changes in conditions of use (e.g. increased loading) or deterioration and damage of the concrete structure (for example due to environmental conditions or seismic events). Historically, steel has been the primary material used to strengthen concrete structures. Bonded steel plates or stirrups have been applied externally to successfully repair reinforced concrete elements. However, using steel as a strengthening element adds additional dead load to the structure and normally requires corrosion protection. Externally bonded fiber-reinforced polymers (FRP) sheets/plates exhibit several attractive properties, such as low weight-to-strength ratios, non-corrosiveness, and ease of application. A number of experimental programs and analytical studies have been developed in the last few years at the University of Padova on flexural [19,28], shear [16,17,18] and bond behaviour [21,20] of FRP strengthened elements. In this context, adding or * Corresponding author. Tel./fax: +39 049 8275618. E-mail address: [email protected] (C. Pellegrino). 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.06.025
applying mortar, spraying concrete or mortar with the aim of rehabilitating and/or strengthening of existing reinforced concrete structures is also a possible way of intervention with a more traditional and common material [5]. Emberson and Mays [1] carried out one of the first extensive experimental studies on the influence of mechanical and physical properties of repair mortars, applied on axially loaded (in tension) reinforced concrete elements. They numerically modelled the axial load transfer through repair and substrate in the linear elastic range. They also worked on flexural elements, and studied the effect of repairs applied either in the compression or tension regions of reinforced concrete beams [2]. Following, most research focused on flexural elements. For example, Hassan et al. [9] tested the compatibility of cementicious, polymer, and polymer-modified mortar repairs to concrete. Río et al. [24] tested beams designed to fail in flexure, after localized artificial corrosion at midspan and localized patch-repair with three types of mortar (cement based, epoxy resin binder, and polymer-modified mortar). Park and Yang [15] tested eight beams repaired in the tension region with ordinary Portland and polymer-modified cement mortar. They varied reinforcement ratio and repair length. Shannag and Al-Ateek [26] tested 30 under-reinforced concrete beams, repaired in the tension region with five materials: ordinary Portland cement and four types of fiber-reinforced cementicious materials. Once repaired, the beams were tested as they were or after accelerated corrosion. Nounu and
3130
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
Chaudhary [14] compared ordinary Portland cement with free flowing micro-concretes, obtaining better results with the latter. Kim et al. [12] applied fiber-reinforced cementicious materials at the intrados of reinforced concrete beams with and without stirrups. Recently, Jumaat et al. [11] made a review of various repair materials and techniques for reinforced concrete beams. Experimental review of ten different repair methods for axially loaded columns, which took into account not only the structural performance and failure modes, but also the applicability and cost-efficiency of the repairs, was carried out by Ramirez [23], who found good results for methods based on application of cementicious materials. Among the wide literature on repair of reinforced concrete columns oriented to the seismic field (a comprehensive literature review can be found in [6], few authors considered the case of jacketing with mortar or concrete without adding additional reinforcement. Fukuyama et al. [7] tested eight damaged columns, repaired with different techniques, and found that replacement of cover with concrete can restore the original column shear strength and deflection capacity, without any increase of cross-section or steel percentage. The use of shrinkage compensating mortar can even improve strength, although faster strength degradation may occur. In general, the main aim of cementicious repair of concrete columns is to assist the repaired columns to carry the axial load, particularly when a significant amount of material in compression is lost due to the action of corrosion. Rahman et al. [22] numerically studied the problem of drying shrinkage and creep in cementicious repair of reinforced concrete columns. Shambira and Nounu [25] experimentally studied the long-term behaviour of this type of intervention. They applied two types of localized repairs, polymer and polymer-modified mortars, on one side of axially loaded columns. Despite the short-term behaviour was acceptable, the long-term behaviour worsened due to high shrinkage, and shrinkage forces induced parasite bending in the columns. Mangat and O’Flaherty [13] applied seven different ordinary and polymer-modified cementicious materials, on the unpropped compression members of two existing bridges. The case studies showed that repairs displayed structural interaction with the structure, and those made with stiff materials were more efficient than others, which is in disagreement with other results. For example, Sharif et al. [27] assessed the effectiveness of localized repairs on two sides of axially loaded columns. They applied two cementicious materials with low and high elastic modulus, under loaded and unloaded conditions. They demonstrated that repairs are structurally effective only if applied on unloaded columns, or become effective only if further loads are added to the columns. The load distribution between the repair layer and the concrete is even, only if the elastic modules of the two materials are similar. Some issues related to the rehabilitation of reinforced concrete columns, such as the choice of the repair material properties and its geometric configuration with the aim of improving the compatibility of the intervention, are thus still objects of research. In this framework, the effect of thickness of repair material on the efficiency of rehabilitation interventions, in relation to the existing steel reinforcement configuration, was not studied in detail for axially loaded elements. The objective of this study is to give some new insights on validating the effectiveness of repair intervention with polymer-modified cementicious mortar repairs applied to square columns, under axial loads, to recover the original properties of columns. In particular the aim is to verify the effect of repair thickness (including steel reinforcement or not), on cracking pattern and, in general, structural behaviour of axially loaded element. For this reason, experimental and numerical study on square columns, subjected to axial loads, repaired with polymer-modified cementicious mortar is carried out. The repair material had similar mechanical prop-
erties, slightly higher tensile strength than the concrete substrate and two different thicknesses (including steel reinforcement or not) and was applied over the entire length of one face of the columns. Experimental results were compared in terms of cracking pattern, ultimate capacity, axial and transversal strains with those from control columns, which were tested in non-damaged, non-repaired conditions. Simplified three-dimensional numerical models, implementing non-linear constitutive laws for materials, were developed to simulate the behaviour of control and repaired columns. 2. Experimental program The main objective of the experimental program was to assess the static behaviour under axial loading of six reinforced concrete columns repaired with polymermodified cementicious material. The following subsections describe the specimens used for experimental testing, the materials adopted for their construction and repair, and the testing procedure. 2.1. Design and preparation of specimens Six columns were made with square section of 300 300 mm area, 20 mm concrete cover, 0.8 m total height. Longitudinal reinforcement was constituted by four 12 mm diameter reinforcing bars. Stirrups having 8 mm diameter were placed at 140 mm spacing. The six columns were divided into three test series. Two specimens were used as control columns (P00_a;b) and were tested in non-damaged/ non-repaired conditions. Other four columns were repaired on one face. They were cast leaving the reinforcement non-covered with a curing process of 28 days. After this period, the non-covered surface was prepared. The preparation of the surface included roughening, cleaning of dust, powders and any impurities to improve the adhesion between concrete core and mortar, and wetting. The polymer-modified cementicious mortar for repair was applied after eventual evaporation of water in excess. Repair mortar was 50 mm thick and included the longitudinal reinforcement on two columns (P50_a;b). 15 mm of repair mortar were sufficient to obtain the original 300 300 mm section on other two columns (P15_a;b). In the latter case, repair was intended to restore only the concrete cover. Fig. 1 shows the details and Table 1 lists the data of the tested columns. 2.2. Materials The main mechanical properties of concrete were experimentally evaluated after 28 days curing. Average compressive strength on 150 150 150 mm cubic specimens, determined from results of four samples casted during the column
P0_a;b
P15_a;b
P
P50_a;b
P
800
P
800
300
800
300
300 15
2 φ 12
300
50 2 φ 12
300
300
2 φ 12
300
2 φ 12
2 φ 12
300
2 φ 12
300
Stirrup spacing 140mm Fig. 1. Dimensions, rebars and repairs arrangement of columns.
3131
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137 Table 1 Details of specimens. Type of element/test
Section (mm2)
Column Axial
300 300
Longitudinal reinforcement Tension
ql (%)
Transversal reinforcement
qw (%)
Condition
Designation
0.50
1U8/140 mm
0.24
Control column Repair 15 mm Repair 50 mm
P00_a; P00_b P15_a; P15_b P50_a; P50_b
Compression
4U12
construction was 34.8 N/mm2. Mean tensile strength, measured by splitting tests on three cylindrical samples having diameter 150 mm and height 300 mm, was 3.19 N/ mm2. Elastic modulus was not measured, but according to the measured cubic compressive strength and Eurocode 2 formulations [3], it was assumed to be around 32,500 N/mm2. Ribbed bars used for longitudinal reinforcement and for transversal reinforcement were both tested in tension. Mechanical properties were similar for the two types of bars; mean yield stress was 532 N/mm2 and mean tensile strength was 628 N/mm2. Strain at failure was 25%. Finally, the cementicious material, used for repairing all columns, was premixed, tixotropic, polymer-modified mortar with high-strength hydraulic binders and aggregates having maximum thickness of 4 mm. This product has high bond properties, low CO2 and vapour permeability, limited shrinkage. It is generally used for cover repair in reinforced concrete structures. Mechanical properties of the repair mortar were measured on samples having dimensions of 40 40 160 mm, cast during the repair interventions on columns. These samples were tested after 28 days curing. Density of hardened mortar was 2170 kg/m3. Mean tensile strength deducted from six flexural tests was equal to 3.48 N/mm2. Mean cubic compressive strength (six samples) was 39.6 N/mm2, mean elastic modulus (three samples) was 26,200 N/mm2. Table 2 compares the mechanical properties of the concrete support and the repair material. It can be seen that the measured compressive strengths differ for 4.8 MPa, the measured tensile strengths differ for about 0.3 MPa and concrete and mortar elastic modules differ for less than 10 kN/mm2. 2.3. Testing procedures Axial tests on columns were carried out monotonically, under a 10,000 kN loading machine, with loads increased between 0.5 and 2.5 kN/s. Pressure transducer mounted on the loading machine was used to measure the applied loads. The control columns were instrumented with six strain transducers (DD1; 100 mm measuring base), placed at mid-height along the columns. Four DD1 were placed on two adjacent orthogonal faces, two in the horizontal and two in the vertical direction, to measure transverse and axial strains. On the other two faces, two horizontal DD1 were placed close to the column corners, in order to gather information on possible instability of the reinforcement. Two linear variable differential transducers (LVDT) with 600 mm measuring base were also placed vertically on two faces, to measure overall axial strains of the columns. For the repaired columns, other two strain transducers (DD1) were placed across the repair layer-concrete column interface, to gather information on the behaviour of the interface. The other instruments were placed on two adjacent orthogonal faces, one of which was repaired and the other left in the original conditions, in order to gather information on the behaviour of the repair material. Fig. 2 shows the test setup and instrumentation on the four sides of a repaired column and some details of displacement and strain transducers on the repair layer and across the interface between concrete support and mortar layer.
3. Test results In the following the main results of the experimental program are shown in terms of failure modes, cracking patterns, stress vs. (axial and transversal) strain curves of undamaged and repaired columns. 3.1. Failure modes and ultimate loads All columns showed compressive failure with crushing of concrete. Vertical and sub-vertical cracks generally developed close Table 2 Mechanical properties of concrete and repair mortar. Property
Concrete
Mortar
Density of hardened material (kg/m3) Mean cubic compressive strength (N/mm2) Mean elastic modulus (kN/mm2) Mean tensile strength (N/mm2)
2380 34.8 32.5a 3.19
2168 39.6 26.2 3.48
a
Evaluated on the basis of EN 1992-1-1.
to one column end, where damage was concentrated. Cracks also connected each other close to the column corners, with spalling of the reinforcement cover at corners. Differences in failure modes were determined by presence and thickness of the repair. Fig. 3 shows the crack patterns of the tested columns. For the control columns P00_a and P00_b, failure occurred by crushing of concrete, respectively, at the lower and at the upper end of the column. Cracks had vertical or sub-vertical patterns and were distributed on the four column sides. Larger cracks were located close to the column corners and were connected to those on the adjacent column face. Ultimate loads reached by the control columns, 2929 and 2869 kN, corresponded to average stresses on the cross-sections of 31.0 and 31.9 N/mm2 (see Table 3). Columns P15_a and P15_b were repaired on one side only, with 15 mm thick mortar layer. At failure, cracks had vertical or subvertical patterns and were distributed on the three non-repaired column sides. They were localized at the upper end of column (P15_a) and on the overall specimen height (P15_b). Larger cracks were located close to the column corners and were connected to those on the adjacent column faces. However, in both specimens, the repair did not crack, but clearly debonded from the concrete substrate (Fig. 4) at 1901 and 1575 kN, corresponding to average stresses on the cross-sections of 21.1 and 17.5 N/mm2 (76% and 58% of ultimate load). Ultimate loads were 2507 and 2709 kN, respectively, corresponding to average stresses on the cross-sections of 27.9 and 30.1 N/mm2, on average 92% of the ultimate capacity reached by the control columns (see Table 3). Columns P50_a and P50_b were repaired on one side only with 50 mm thick mortar layer, and showed different behaviour. Cracks had vertical or sub-vertical patterns at failure and were distributed on the four column sides, including the repaired one. The repair only partially debonded from the concrete substrate at the column corners (Fig. 5), at 2430 and 2160 kN, corresponding to average stresses on the cross-sections of 27.0 and 24.0 N/mm2 (93% and 86% of ultimate load). Ultimate loads were 2606 and 2501 kN, respectively, and corresponded to average stresses on the crosssections of 29.0 and 27.8 N/mm2, on average 90% of the ultimate capacity reached by the control columns (see Table 3). 3.2. Stresses and strains Fig. 6 shows the stress–strain curves of three columns: without repair (P00_a), with 15 mm thick repair (P15_a) and with 50 mm thick repair (P50_a). In these diagrams, axial strains (compression) are plotted positive and transversal strains (tension) negative. Fig. 7 compares all stress–axial strain curves, with axial strains measured on the non-repaired sides of the columns (above) and on the repairs (below; in this diagram, axial strains of control columns are left as reference values). Fig. 8 compares all stress–transversal strain curves, with transversal strains measured on the non-repaired sides of columns (above) and on repairs (below; in this diagram, transversal strains of control columns are left as reference values). In the diagrams of Fig. 7 and 8, axial and transversal strains are both plotted positive. In control columns, the strains measured on orthogonal faces had the same trend and the stress–axial strain relationship was almost parabolic (see P00_a in Fig. 6). The ratio of transversal to axial
3132
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
STRAIN TRASDUCER LVDT TRASDUCER LVDT TRASDUCER
STRAIN TRASDUCERS
Fig. 2. Test setup and instrumentation for axial tests on columns.
strains in the elastic phase, on average, was about 0.2. Axial strains at ultimate load, on average, were 2.45 10 3, i.e. failure occurred after concrete plasticization (see Table 4). The columns repaired with thick mortar layer (see P50_a in Fig. 6) showed similar behaviour. The strains measured on orthogonal faces had the same trend but, in this case, they were gauged on the repaired and the non-repaired sides of the column, which were thus working together, although the repair always gave higher values of strain. The ratio of transversal to axial strains measured on the non-repaired sides of the columns in the elastic
phase, on average, was around 0.22. Despite the higher strains measured, also the ratio of transversal to axial strains measured on the repairs was, on average, 0.20 (see Table 4). The repairs partially debonded at the column corners at average stress level of 90% of the ultimate capacity. When this circumstance occurred, repair mortar had already started plasticizing on both columns (average axial strains on repair of 2.33 10 3; Table 4). At that point, the stiffness of the repaired columns, which initially was higher than that of the control columns, decreased and became lower (Fig. 7). Debonding, however, was not complete since the non-re-
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
3133
Fig. 3. Crack pattern of columns at failure.
paired and repaired sides of the columns kept working together (see P50_a in Fig. 6) until reaching the ultimate load. Average axial strains on the non-repaired and repaired sides of P50_a and P50_b at ultimate load were 2.25 10 3 and 3.01 10 3, i.e. failure occurred with plasticization of both concrete on the non-repaired column sides, and mortar repairs (see Table 4). The columns repaired with thin mortar layer (see P15_a in Fig. 6) presented different behaviour. The strains measured on orthogonal faces, i.e. on the repaired and the non-repaired sides of the columns, had the same trend only during the first elastic phase. During this phase, the ratio of transversal to axial strains measured on the non-repaired sides of the columns, on average, was again around 0.21, and it was slightly lower on the repaired sides, where the axial strains were higher (in average 0.17; Table 4). At stress level of 76% and 58% of the ultimate capacity (in P15_a and P15_b, respectively), the repairs debonded from the concrete. When this circumstance occurred, the repairs could not carry load any more, as demonstrated by the strain release (see P15_a in Fig. 6; axial strains of repair for P15_a and P15_b in Fig. 7 and transversal strain across the interface between concrete support and mortar layer for P15_a and P15_b in Fig. 8). In this case, plasticization had not yet started (average axial strain on repaired and non-repaired sides: 0.76 10 3 and 1.19 10 3; Table 4). At that point, the stiffness of the repaired columns, which was initially higher than that of the control columns, decreased and became lower (Fig. 7). At ultimate load, average axial strains on the non-repaired sides of P15_a and P15_b were 2.18 10 3, while the values on the repairs were not significant (Table 4). Failure thus occurred immediately after plasticization of concrete on the original, non-repaired portion of the columns, without any contribution of the mortar repairs. The stress–strain curves of undamaged (P00_b), and repaired (P15_b, P50_b) columns were not shown in Fig. 6 as they are similar to the corresponding (P00_a), and (P15_a, P50_a) columns. Stress–strain curves for P00_b, P15_b and P50_b columns are compared with the others in Figs. 7 and 8. 4. Finite element modeling of tested columns Three-dimensional non-linear finite element models were used to simulate the experimentally observed behaviour. The Straus7
code (G+D Computing [8]) was used for the numerical analyses. Eight-node solid elements were used to model concrete substrate, while beam elements were used for the steel reinforcement. In repaired columns type P15 and P50, the mortar repair was also modelled with eight-node solid elements, having properties different from those adopted for concrete. The interface between concrete substrate and mortar repair was modelled by means of link elements with tension cut-off. Translation of nodes at the upper and lower bases was restrained in the two orthogonal horizontal directions to reproduce friction between upper and lower faces of the columns and the loading machine plate. Simplified parabolic stress–strain relationships derived from Hognestad [10] were adopted for the concrete substrate and the mortar repair, while elasto-plastic bilinear relationship with hardening was used for steel. The properties of materials were derived from the experimental tests and are listed in Table 5. The Poisson ratio of the concrete and steel were assumed according to Eurocode 2 [3] and Eurocode 3 [4], respectively. The Poisson ratio of the mortar was assumed equal to that of the concrete. The described models were used to carry out non-linear static analyses, under constantly increasing loads. Fig. 9 compares experimental and numerical stress–strain curves. For the non-damaged, non-repaired columns (type P00), the model reproduces very well the mean stress–strain curves of the two tested specimens until peak value. Initial elastic stiffness and value of ultimate load are well reproduced, as can be seen by the stress–transversal strain curve and stress–axial strain curve. Only the latter is slightly stiffer in the model. In the case of columns type P15, repaired with 15 mm thick mortar layer the model reproduces quite well the values of initial elastic stiffness and ultimate load, as can be seen by the stress–transversal strain curve and stress–axial strain curve obtained on the non-repaired side of the column. The axial strains on the repair had trend similar to that measured on the non-repaired side of the column only during the first elastic phase, and the model still gives good results. During experimental tests, at stress level between 76% and 58% of the ultimate capacity, the repairs debonded from the concrete. Although the model cannot reproduce the subsequent strain release, the numerical curve presents a sudden discontinuity at the upper bound of this range of stresses. The model is thus able to show the debonding of repair. This
3134
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
Table 3 Results of axial tests. Column
Debonding load (kN) (a)
Ultimate load (kN) (b)
(a)/(b)
P00_a P00_b P15_a P15_b P50_a P50_b
– – 1901 1575 2430 (partial) 2160 (partial)
2929 2869 2507 2709 2606 2501
– – 0.76 0.58 0.93 0.86
phenomenon is also described by the model displacement in horizontal direction (see Fig. 10). In the case of columns type
P50, repaired with 50 mm thick mortar layer, the model is not able to get the difference in the initial elastic stiffness of the original column and the repair, but can average the two trends and does not present any discontinuity (Fig. 9), nor evidence marked debonding, according to experimental evidence.
5. Discussion The experimental results obtained on axially loaded columns showed that repairs on one side of the columns could not re-establish completely the load-bearing capacity of the non-damaged control columns. Ultimate load of the repaired columns was on
Fig. 4. Failure with debonding of mortar layer (P15_b).
Fig. 5. Cracking of mortar and along the interface (P50_a).
3135
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
35
30
30
25
25
20 15 transv. strain column transv. strain column axial strain column axial strain column
10 5
Stress [N/mm 2]
Stress [N/mm 2]
Average stress-strain on P00_a 35
Average stress-axial strain on column
20 15
P00_a (column) P00_b (column) P15_a (column) P15_b (column) P50_a (column) P50_b (column)
10 5 0
0
0.0
-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
Strain [10-3]
Average stress-strain on P15_a 35
20 15
transv. strain column transv. strain interface transv. strain repair axial strain column axial strain repair
10 5
-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Strain [10-3]
Average stress-strain on P50_a 35
Stress [N/mm 2]
Stress [N/mm 2]
3.5
4.0
30
25
0
3.0
Average stress-axial strain on repair
35
30
1.5 2.0 2.5 Strain [10-3]
25 20 15
P00_a (column) P00_b (column) P15_a (repair) P15_b (repair) P50_a (repair) P50_b (repair)
10 5 0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-3
Strain [10 ] Fig. 7. Stress–axial strain curves, on original column and on repair (all specimens).
Stress [N/mm 2]
30 25 20 15
transv. strain column transv. strain interface transv. strain repair axial strain column axial strain repair
10 5 0
-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -3
Strain [10 ] Fig. 6. Stress–strain curves of undamaged (P00_a), and repaired (P15_a, P50_a) columns.
average 91% that of the control columns. Thin repairs (15 mm), substituting only the reinforcement cover, showed premature debonding from the concrete substrate, on average at 67% of the ultimate load. After debonding, the repairs were not effective, as revealed by strain analysis, and the entire load was transferred to the original, non-repaired portion of the columns. When the repair layer included the reinforcement (50 mm thick), the global behaviour of the repaired columns was improved. Debonding of repair from the concrete substrate was limited and occurred only at the column corners, on average at 90% of ultimate load. After partial debonding, the repairs kept on collaborating with the concrete support, as revealed by strain analysis. Plasticization of both concrete on the non-repaired column sides and mortar repairs revealed that both portions of the repaired columns were
contributing to the column capacity, even in the non-linear phase until failure. Simplified three-dimensional numerical models, implementing non-linear constitutive laws for materials, could simulate fairly well not only the behaviour of control columns, but also that of repaired columns. In the case of thin repairs, the models evidenced the premature debonding of the repairs. Similar models can be thus effectively used for assessment and design of interventions. 6. Conclusions In this work an experimental investigation to control the effectiveness of polymer-modified cementicious mortar repairs applied to square columns under axial loads is carried out. The repair material had similar mechanical properties and slightly higher tensile strength than the concrete substrate and was applied over the entire length of one face of columns. The aim is to give some new insights on validating the effectiveness of such materials in recovering the properties of non-damaged, non-repaired columns, verifying the effect of repair thickness (including the steel reinforcement or not), on cracking pattern, strength and deformability of axially loaded element. The main conclusions arising from the experimental tests show that polymer-modified cementicious mortars, with limited shrinkage and mechanical properties similar to those of the concrete substrate, can be effective for the repair of reinforced concrete columns. The effectiveness of the intervention depends also on position and thickness of the repair layer. For columns repaired on one side only, the repairs cannot restore the load-bearing capacity of non-damaged control columns,
3136
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
Table 4 Stresses and strains of axial tests. 1/3 Ultimate load Transv. strain (a) (10 3) P00_a P00_b P15_a
Column Repair Column Repair Column Repair Column Repair
P15_b P50_a P50_b
10.3 10.6 9.3
0.08 0.08 0.06 0.09 0.06 0.06 0.05 0.12 0.05 0.24
10.0 9.7 9.3
(a)/(b) Axial strain (b) (10 3) 0.41 0.39 0.31 0.51 0.30 0.38 0.23 0.74 0.24 1.00
0.19 0.20 0.21 0.18 0.21 0.15 0.20 0.16 0.23 0.24
21.1
27.0 24.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
15
P00_a transv. column P00_b transv. column P00 transv. col. model P00_a axial column P00_b axial column P00 axial col. model
10
-2 0 -1.5 -1.0 -0.5 0.0
Stress [N/mm 2]
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Strain [10-3]
Average stress-strain on P15
P15_a transv. column P15_b transv. column P15 transv. col. model P15_a axial column P15_b axial column P15 axial col. model P15_a axial repair P15_b axial repair P15 axial rep. model
20 15 10 5
0 -2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
4.0
Strain [10-3]
Average stress-strain on P50
Average stress-transv. strain on repair
35 30 Stress [N/mm 2]
30 25 20 15
P00_a (column) P00_b (column) P15_a (repair) P15_b (repair) P50_a (repair) P50_b (repair)
10 5 0
27.8
0
Strain [10-3]
35
29.0
5
0
0.0
30.1
20
25
10
0.97 1.30 0.55 1.08 1.48 2.25 1.06 2.40
2.41 2.52 2.25 0.79 2.11 0.54 2.28 3.30 2.21 2.71
25
25
P00_a (column) P00_b (column) P15_a (column) P15_b (column) P50_a (column) P50_b (column)
31.0 31.9 27.9
Axial Strain (10 3)
30
30
15
Stress (N/mm2)
Axial strain
Average stress-strain on P00
35
20
Ultimate Load
35
30
5
Stress [N/mm 2]
0.20 0.23 0.13 0.06 0.25 0.43 0.29 0.54
17.5
Average stress-transv. strain on column
35
Debonding Transv. strain (10 3)
although they give acceptable results (91% of the ultimate capacity of the control columns). In any case, repairs that include the longitudinal reinforcement show good properties, with stable behaviour, sharing of loads, and plasticization of the material before failure, whereas thin repairs that do not include the reinforcement do not have adequate performance due to premature debonding. Hence, from the practical point of view, a rehabilitation intervention on axially loaded elements that include the longitudinal steel reinforcement may be generally recommended since it is more effective than that involving only the concrete cover without including steel bars. In fact, the action of the transverse steel allows sharing of loads between the mortar and the concrete core until failure, avoiding premature debonding of the mortar layer, when the steel bars are included in the repair layer.
Stress [N/mm 2]
Stress (N/mm2)
Stress [N/mm 2]
Stress (N/mm2)
Column
25 20
P50_a transv. column P50_b transv. column P50 transv. col. model P50_a axial column P50_b axial column P50 axial col. model P50_a axial repair P50_b axial repair P50 axial rep. model
15 10 5 0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Strain [10-3] Fig. 8. Stress–transversal strain curves, on original column and on repair (all specimens).
-2 0 -1.5 -1.0 -0.5 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Strain [10-3] Fig. 9. Experimental and numerical stress–strain curves of undamaged (P00), and repaired (P15; P50) columns.
3137
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137 Table 5 Model material properties. Material
fc (N/mm2)
ecu (%)
fct (N/mm2)
ectu (%)
fy (N/mm2)
ey (%)
ft (N/mm2)
et (%)
E (N/mm2)
m (–)
Concrete Mortar Steel
28.90 32.87 –
0.18 0.25 –
3.19 3.48 –
0.01 0.01 –
– – 532
– – 0.27
– – 628
– – 25
32,500 26,200 200,000
0.20 0.20 0.30
Fig. 10. Model displacement in horizontal direction for P15 at debonding (left) and P50 at same vertical load level (right).
The behaviour of axially loaded columns repaired with thin or thick repairs can be also reproduced by simplified three-dimensional non-linear finite element models. Acknowledgements The authors gratefully acknowledge Tassullo S.p.A. that provided materials and specimens for experimental testing, and Eng. Diego Testolin for his contribution to the experimental investigation developed during his MSc thesis. The experimental tests were carried out at the Laboratory of Structural Materials Testing of the University of Padova, Italy. References [1] Emberson NK, Mays GC. Significance of property mismatch in the patch repair of structural concrete. Part 2: Axially loaded reinforced concrete members. Mag Concrete Res 1990;42(152):161–70. [2] Emberson NK, Mays GC. Significance of property mismatch in the patch repair of structural concrete. Part 3: Reinforced concrete members in flexure. Mag Concrete Res 1996;48(174):45–57. [3] European Committee for Standardization. Eurocode 2 – Design of concrete structures. Part 1-1: General rules and rules for buildings. EN 1992-1-1, Brussels, Belgium; 2004. [4] European Committee for Standardization. Eurocode 3 – Design of steel structures. Part 1-1: General rules and rules for buildings. EN 1993-1-1, Brussels, Belgium; 2005.
[5] European Committee for Standardization. Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 3: Structural and nonstructural repair. EN 1504-3, Brussels, Belgium; 2005. [6] Fib. Seismic assessment and retrofit of reinforced concrete buildings. State of Art Report, Bulletin 24. Federation International du Beton: Lausanne (Switzerland); 2003. [7] Fukuyama K, Higashibata Y, Miyauchi Y. Studies on repair and strengthening methods of damaged reinforced concrete columns. Cement Concrete Compos 2000;22:81–8. [8] G+D Computing. Straus7 user’s manual. Sydney; 2005. [9] Hassan KE, Brooks JJ, Al-Alawi L. Compatibility of repair mortars with concrete in a hot-dry environment. Cement Concrete Compos 2001;23:93–101. [10] Hognestad E. A study of combined bending and axial load in reinforced concrete members, Bulletin No. 399. University of Illinois, Urbana IL USA: Engineering Experimental Station; 1951. [11] Jumaat MZ, Kabir MH, Obaydullah M. A review of the repair of reinforced concrete beams. J Appl Sci Res 2006;2(6):317–26. [12] Kim JHJ, Lim YM, Won JP, Park HG, Lee KM. Shear capacity and failure behaviour of DFRCC repaired RC beams at tensile region. Eng Struct 2007;29:121–31. doi:10.1016/j.engstruct.2006.04.023. [13] Mangat PS, O’Flaherty FJ. Influence of elastic modulus on stress redistribution and cracking in repair patches. Cement Concrete Res 2000;30:125–36. [14] Nounu G, Chaudhary Z-UL-H. Reinforced concrete repair in beams. Constr Build Mater 1999;13:195–212. [15] Park SK, Yang DS. Flexural behaviour of reinforced concrete beams with cementitious repair materials. Mater Struct 2005;38:329–34. doi:10.1617/ 14051. [16] Pellegrino C, Modena C. FRP shear strengthening of RC beams with transverse steel reinforcement. J Compos Constr 2002;6(2):104–11. [17] Pellegrino C, Modena C. FRP shear strengthening of RC beams: experimental study and analytical modelling. ACI Struct J 2006;103(5):720–8. [18] Pellegrino C, Modena C. An experimentally based analytical model for shear capacity of FRP strengthened reinforced concrete beams. Mech Compos Mater 2008;44(3):231–44. [19] Pellegrino C, Modena C. Flexural strengthening of real-scale RC and PRC beams with end-anchored pre-tensioned FRP laminates. ACI Struct J 2009;106(3):319–28. [20] Pellegrino C, Modena C. Influence of FRP axial rigidity on FRP-concrete bond behaviour: an analytical study. Adv Struct Eng, in press. [21] Pellegrino C, Tinazzi D, Modena C. An experimental study on bond behavior between concrete and FRP reinforcement. J Compos Constr 2008;12(2):180–9. [22] Rahman K, Baluch MH, Al-Gadhib A. Modeling of shrinkage and creep stresses in concrete repair. ACI Mater J 1999;96(5):542–50. [23] Ramirez JL. Ten concrete column repair methods. Constr Build Mater 1996;10:195–202. [24] Río O, Andrade C, Izquierdo D, Alonso C. Behaviour of patch-repaired concrete structural elements under increasing static loads to flexural failure. J Mater civil Eng 2005;17(2):168–77. doi:10.1061/(ASCE)0899-1561(2005)17:2. 168. [25] Shambira MV, Nounu G. On the effect of time-dependent deformations on the behaviour of patch-repaired reinforced concrete short columns. Constr Build Mater 2000;14:425–32. [26] Shannag MJ, Al-Ateek SA. Flexural behaviour of strengthened concrete beams with corroding reinforcement. Constr Build Mater 2006;20:834–40. [27] Sharif A, Rahman MK, Al-Gahtani AS, Hameeduddin M. Behaviour of patch repair of axially loaded reinforced concrete beams. Cement Concrete Compos 2006;28:734–41. doi:10.1016/j.cemconcomp.2006.05.013. [28] Valluzzi MR, Grinzato E, Pellegrino C, Modena C. IR thermography for interface analysis of FRP laminates externally bonded to RC beams. Mater Struct 2009;42(1):25–34.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Rehabilitation of reinforced concrete axially loaded elements with polymer-modified cementicious mortar Carlo Pellegrino *, Francesca da Porto, Claudio Modena Department of Structural and Transportation Engineering, University of Padova, Via Marzolo 9, 35131 Padova, Italy
a r t i c l e
i n f o
Article history: Received 21 November 2008 Received in revised form 16 May 2009 Accepted 18 June 2009
Keywords: Reinforced concrete Repair mortars Interface Cracking Columns
a b s t r a c t The aim of the paper is to investigate the compatibility and the efficiency of the rehabilitation intervention on reinforced concrete columns with polymer-modified cementicious mortar. This paper presents the results of experimental tests on axial behaviour of reinforced concrete columns, with square crosssection, repaired by polymer-modified cementicious mortar. Tests were repeated varying repair thickness, which included or did not include the steel reinforcement on one face of the square column. Despite this type of intervention is quite common in practice, the effect of repair thickness on the intervention efficiency, in relation to the existing steel reinforcement configuration, had not been previously studied in detail for axially loaded elements. Results were discussed and compared with those from control columns, which were tested in nondamaged, non-repaired conditions. The main findings of this work can be summarized as follows. The repair cannot restore the load-bearing capacity of non-damaged control columns, although they give acceptable results. Repairs that include the longitudinal reinforcement show good properties, with stable behaviour, sharing of loads, and plasticization of the material before failure, whereas thin repairs that do not include the reinforcement do not have adequate performance due to premature debonding. Non-linear numerical models also confirmed the different behaviour of the two types of repair. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The field of rehabilitation and strengthening of reinforced concrete structural elements shows an increasing interest for existing constructions and various projects have been carried out around the world over the past two decades. Structural strengthening and repairing is aimed at increasing or restoring the load-bearing capacity of the element, due to changes in conditions of use (e.g. increased loading) or deterioration and damage of the concrete structure (for example due to environmental conditions or seismic events). Historically, steel has been the primary material used to strengthen concrete structures. Bonded steel plates or stirrups have been applied externally to successfully repair reinforced concrete elements. However, using steel as a strengthening element adds additional dead load to the structure and normally requires corrosion protection. Externally bonded fiber-reinforced polymers (FRP) sheets/plates exhibit several attractive properties, such as low weight-to-strength ratios, non-corrosiveness, and ease of application. A number of experimental programs and analytical studies have been developed in the last few years at the University of Padova on flexural [19,28], shear [16,17,18] and bond behaviour [21,20] of FRP strengthened elements. In this context, adding or * Corresponding author. Tel./fax: +39 049 8275618. E-mail address: [email protected] (C. Pellegrino). 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.06.025
applying mortar, spraying concrete or mortar with the aim of rehabilitating and/or strengthening of existing reinforced concrete structures is also a possible way of intervention with a more traditional and common material [5]. Emberson and Mays [1] carried out one of the first extensive experimental studies on the influence of mechanical and physical properties of repair mortars, applied on axially loaded (in tension) reinforced concrete elements. They numerically modelled the axial load transfer through repair and substrate in the linear elastic range. They also worked on flexural elements, and studied the effect of repairs applied either in the compression or tension regions of reinforced concrete beams [2]. Following, most research focused on flexural elements. For example, Hassan et al. [9] tested the compatibility of cementicious, polymer, and polymer-modified mortar repairs to concrete. Río et al. [24] tested beams designed to fail in flexure, after localized artificial corrosion at midspan and localized patch-repair with three types of mortar (cement based, epoxy resin binder, and polymer-modified mortar). Park and Yang [15] tested eight beams repaired in the tension region with ordinary Portland and polymer-modified cement mortar. They varied reinforcement ratio and repair length. Shannag and Al-Ateek [26] tested 30 under-reinforced concrete beams, repaired in the tension region with five materials: ordinary Portland cement and four types of fiber-reinforced cementicious materials. Once repaired, the beams were tested as they were or after accelerated corrosion. Nounu and
3130
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
Chaudhary [14] compared ordinary Portland cement with free flowing micro-concretes, obtaining better results with the latter. Kim et al. [12] applied fiber-reinforced cementicious materials at the intrados of reinforced concrete beams with and without stirrups. Recently, Jumaat et al. [11] made a review of various repair materials and techniques for reinforced concrete beams. Experimental review of ten different repair methods for axially loaded columns, which took into account not only the structural performance and failure modes, but also the applicability and cost-efficiency of the repairs, was carried out by Ramirez [23], who found good results for methods based on application of cementicious materials. Among the wide literature on repair of reinforced concrete columns oriented to the seismic field (a comprehensive literature review can be found in [6], few authors considered the case of jacketing with mortar or concrete without adding additional reinforcement. Fukuyama et al. [7] tested eight damaged columns, repaired with different techniques, and found that replacement of cover with concrete can restore the original column shear strength and deflection capacity, without any increase of cross-section or steel percentage. The use of shrinkage compensating mortar can even improve strength, although faster strength degradation may occur. In general, the main aim of cementicious repair of concrete columns is to assist the repaired columns to carry the axial load, particularly when a significant amount of material in compression is lost due to the action of corrosion. Rahman et al. [22] numerically studied the problem of drying shrinkage and creep in cementicious repair of reinforced concrete columns. Shambira and Nounu [25] experimentally studied the long-term behaviour of this type of intervention. They applied two types of localized repairs, polymer and polymer-modified mortars, on one side of axially loaded columns. Despite the short-term behaviour was acceptable, the long-term behaviour worsened due to high shrinkage, and shrinkage forces induced parasite bending in the columns. Mangat and O’Flaherty [13] applied seven different ordinary and polymer-modified cementicious materials, on the unpropped compression members of two existing bridges. The case studies showed that repairs displayed structural interaction with the structure, and those made with stiff materials were more efficient than others, which is in disagreement with other results. For example, Sharif et al. [27] assessed the effectiveness of localized repairs on two sides of axially loaded columns. They applied two cementicious materials with low and high elastic modulus, under loaded and unloaded conditions. They demonstrated that repairs are structurally effective only if applied on unloaded columns, or become effective only if further loads are added to the columns. The load distribution between the repair layer and the concrete is even, only if the elastic modules of the two materials are similar. Some issues related to the rehabilitation of reinforced concrete columns, such as the choice of the repair material properties and its geometric configuration with the aim of improving the compatibility of the intervention, are thus still objects of research. In this framework, the effect of thickness of repair material on the efficiency of rehabilitation interventions, in relation to the existing steel reinforcement configuration, was not studied in detail for axially loaded elements. The objective of this study is to give some new insights on validating the effectiveness of repair intervention with polymer-modified cementicious mortar repairs applied to square columns, under axial loads, to recover the original properties of columns. In particular the aim is to verify the effect of repair thickness (including steel reinforcement or not), on cracking pattern and, in general, structural behaviour of axially loaded element. For this reason, experimental and numerical study on square columns, subjected to axial loads, repaired with polymer-modified cementicious mortar is carried out. The repair material had similar mechanical prop-
erties, slightly higher tensile strength than the concrete substrate and two different thicknesses (including steel reinforcement or not) and was applied over the entire length of one face of the columns. Experimental results were compared in terms of cracking pattern, ultimate capacity, axial and transversal strains with those from control columns, which were tested in non-damaged, non-repaired conditions. Simplified three-dimensional numerical models, implementing non-linear constitutive laws for materials, were developed to simulate the behaviour of control and repaired columns. 2. Experimental program The main objective of the experimental program was to assess the static behaviour under axial loading of six reinforced concrete columns repaired with polymermodified cementicious material. The following subsections describe the specimens used for experimental testing, the materials adopted for their construction and repair, and the testing procedure. 2.1. Design and preparation of specimens Six columns were made with square section of 300 300 mm area, 20 mm concrete cover, 0.8 m total height. Longitudinal reinforcement was constituted by four 12 mm diameter reinforcing bars. Stirrups having 8 mm diameter were placed at 140 mm spacing. The six columns were divided into three test series. Two specimens were used as control columns (P00_a;b) and were tested in non-damaged/ non-repaired conditions. Other four columns were repaired on one face. They were cast leaving the reinforcement non-covered with a curing process of 28 days. After this period, the non-covered surface was prepared. The preparation of the surface included roughening, cleaning of dust, powders and any impurities to improve the adhesion between concrete core and mortar, and wetting. The polymer-modified cementicious mortar for repair was applied after eventual evaporation of water in excess. Repair mortar was 50 mm thick and included the longitudinal reinforcement on two columns (P50_a;b). 15 mm of repair mortar were sufficient to obtain the original 300 300 mm section on other two columns (P15_a;b). In the latter case, repair was intended to restore only the concrete cover. Fig. 1 shows the details and Table 1 lists the data of the tested columns. 2.2. Materials The main mechanical properties of concrete were experimentally evaluated after 28 days curing. Average compressive strength on 150 150 150 mm cubic specimens, determined from results of four samples casted during the column
P0_a;b
P15_a;b
P
P50_a;b
P
800
P
800
300
800
300
300 15
2 φ 12
300
50 2 φ 12
300
300
2 φ 12
300
2 φ 12
2 φ 12
300
2 φ 12
300
Stirrup spacing 140mm Fig. 1. Dimensions, rebars and repairs arrangement of columns.
3131
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137 Table 1 Details of specimens. Type of element/test
Section (mm2)
Column Axial
300 300
Longitudinal reinforcement Tension
ql (%)
Transversal reinforcement
qw (%)
Condition
Designation
0.50
1U8/140 mm
0.24
Control column Repair 15 mm Repair 50 mm
P00_a; P00_b P15_a; P15_b P50_a; P50_b
Compression
4U12
construction was 34.8 N/mm2. Mean tensile strength, measured by splitting tests on three cylindrical samples having diameter 150 mm and height 300 mm, was 3.19 N/ mm2. Elastic modulus was not measured, but according to the measured cubic compressive strength and Eurocode 2 formulations [3], it was assumed to be around 32,500 N/mm2. Ribbed bars used for longitudinal reinforcement and for transversal reinforcement were both tested in tension. Mechanical properties were similar for the two types of bars; mean yield stress was 532 N/mm2 and mean tensile strength was 628 N/mm2. Strain at failure was 25%. Finally, the cementicious material, used for repairing all columns, was premixed, tixotropic, polymer-modified mortar with high-strength hydraulic binders and aggregates having maximum thickness of 4 mm. This product has high bond properties, low CO2 and vapour permeability, limited shrinkage. It is generally used for cover repair in reinforced concrete structures. Mechanical properties of the repair mortar were measured on samples having dimensions of 40 40 160 mm, cast during the repair interventions on columns. These samples were tested after 28 days curing. Density of hardened mortar was 2170 kg/m3. Mean tensile strength deducted from six flexural tests was equal to 3.48 N/mm2. Mean cubic compressive strength (six samples) was 39.6 N/mm2, mean elastic modulus (three samples) was 26,200 N/mm2. Table 2 compares the mechanical properties of the concrete support and the repair material. It can be seen that the measured compressive strengths differ for 4.8 MPa, the measured tensile strengths differ for about 0.3 MPa and concrete and mortar elastic modules differ for less than 10 kN/mm2. 2.3. Testing procedures Axial tests on columns were carried out monotonically, under a 10,000 kN loading machine, with loads increased between 0.5 and 2.5 kN/s. Pressure transducer mounted on the loading machine was used to measure the applied loads. The control columns were instrumented with six strain transducers (DD1; 100 mm measuring base), placed at mid-height along the columns. Four DD1 were placed on two adjacent orthogonal faces, two in the horizontal and two in the vertical direction, to measure transverse and axial strains. On the other two faces, two horizontal DD1 were placed close to the column corners, in order to gather information on possible instability of the reinforcement. Two linear variable differential transducers (LVDT) with 600 mm measuring base were also placed vertically on two faces, to measure overall axial strains of the columns. For the repaired columns, other two strain transducers (DD1) were placed across the repair layer-concrete column interface, to gather information on the behaviour of the interface. The other instruments were placed on two adjacent orthogonal faces, one of which was repaired and the other left in the original conditions, in order to gather information on the behaviour of the repair material. Fig. 2 shows the test setup and instrumentation on the four sides of a repaired column and some details of displacement and strain transducers on the repair layer and across the interface between concrete support and mortar layer.
3. Test results In the following the main results of the experimental program are shown in terms of failure modes, cracking patterns, stress vs. (axial and transversal) strain curves of undamaged and repaired columns. 3.1. Failure modes and ultimate loads All columns showed compressive failure with crushing of concrete. Vertical and sub-vertical cracks generally developed close Table 2 Mechanical properties of concrete and repair mortar. Property
Concrete
Mortar
Density of hardened material (kg/m3) Mean cubic compressive strength (N/mm2) Mean elastic modulus (kN/mm2) Mean tensile strength (N/mm2)
2380 34.8 32.5a 3.19
2168 39.6 26.2 3.48
a
Evaluated on the basis of EN 1992-1-1.
to one column end, where damage was concentrated. Cracks also connected each other close to the column corners, with spalling of the reinforcement cover at corners. Differences in failure modes were determined by presence and thickness of the repair. Fig. 3 shows the crack patterns of the tested columns. For the control columns P00_a and P00_b, failure occurred by crushing of concrete, respectively, at the lower and at the upper end of the column. Cracks had vertical or sub-vertical patterns and were distributed on the four column sides. Larger cracks were located close to the column corners and were connected to those on the adjacent column face. Ultimate loads reached by the control columns, 2929 and 2869 kN, corresponded to average stresses on the cross-sections of 31.0 and 31.9 N/mm2 (see Table 3). Columns P15_a and P15_b were repaired on one side only, with 15 mm thick mortar layer. At failure, cracks had vertical or subvertical patterns and were distributed on the three non-repaired column sides. They were localized at the upper end of column (P15_a) and on the overall specimen height (P15_b). Larger cracks were located close to the column corners and were connected to those on the adjacent column faces. However, in both specimens, the repair did not crack, but clearly debonded from the concrete substrate (Fig. 4) at 1901 and 1575 kN, corresponding to average stresses on the cross-sections of 21.1 and 17.5 N/mm2 (76% and 58% of ultimate load). Ultimate loads were 2507 and 2709 kN, respectively, corresponding to average stresses on the cross-sections of 27.9 and 30.1 N/mm2, on average 92% of the ultimate capacity reached by the control columns (see Table 3). Columns P50_a and P50_b were repaired on one side only with 50 mm thick mortar layer, and showed different behaviour. Cracks had vertical or sub-vertical patterns at failure and were distributed on the four column sides, including the repaired one. The repair only partially debonded from the concrete substrate at the column corners (Fig. 5), at 2430 and 2160 kN, corresponding to average stresses on the cross-sections of 27.0 and 24.0 N/mm2 (93% and 86% of ultimate load). Ultimate loads were 2606 and 2501 kN, respectively, and corresponded to average stresses on the crosssections of 29.0 and 27.8 N/mm2, on average 90% of the ultimate capacity reached by the control columns (see Table 3). 3.2. Stresses and strains Fig. 6 shows the stress–strain curves of three columns: without repair (P00_a), with 15 mm thick repair (P15_a) and with 50 mm thick repair (P50_a). In these diagrams, axial strains (compression) are plotted positive and transversal strains (tension) negative. Fig. 7 compares all stress–axial strain curves, with axial strains measured on the non-repaired sides of the columns (above) and on the repairs (below; in this diagram, axial strains of control columns are left as reference values). Fig. 8 compares all stress–transversal strain curves, with transversal strains measured on the non-repaired sides of columns (above) and on repairs (below; in this diagram, transversal strains of control columns are left as reference values). In the diagrams of Fig. 7 and 8, axial and transversal strains are both plotted positive. In control columns, the strains measured on orthogonal faces had the same trend and the stress–axial strain relationship was almost parabolic (see P00_a in Fig. 6). The ratio of transversal to axial
3132
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
STRAIN TRASDUCER LVDT TRASDUCER LVDT TRASDUCER
STRAIN TRASDUCERS
Fig. 2. Test setup and instrumentation for axial tests on columns.
strains in the elastic phase, on average, was about 0.2. Axial strains at ultimate load, on average, were 2.45 10 3, i.e. failure occurred after concrete plasticization (see Table 4). The columns repaired with thick mortar layer (see P50_a in Fig. 6) showed similar behaviour. The strains measured on orthogonal faces had the same trend but, in this case, they were gauged on the repaired and the non-repaired sides of the column, which were thus working together, although the repair always gave higher values of strain. The ratio of transversal to axial strains measured on the non-repaired sides of the columns in the elastic
phase, on average, was around 0.22. Despite the higher strains measured, also the ratio of transversal to axial strains measured on the repairs was, on average, 0.20 (see Table 4). The repairs partially debonded at the column corners at average stress level of 90% of the ultimate capacity. When this circumstance occurred, repair mortar had already started plasticizing on both columns (average axial strains on repair of 2.33 10 3; Table 4). At that point, the stiffness of the repaired columns, which initially was higher than that of the control columns, decreased and became lower (Fig. 7). Debonding, however, was not complete since the non-re-
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
3133
Fig. 3. Crack pattern of columns at failure.
paired and repaired sides of the columns kept working together (see P50_a in Fig. 6) until reaching the ultimate load. Average axial strains on the non-repaired and repaired sides of P50_a and P50_b at ultimate load were 2.25 10 3 and 3.01 10 3, i.e. failure occurred with plasticization of both concrete on the non-repaired column sides, and mortar repairs (see Table 4). The columns repaired with thin mortar layer (see P15_a in Fig. 6) presented different behaviour. The strains measured on orthogonal faces, i.e. on the repaired and the non-repaired sides of the columns, had the same trend only during the first elastic phase. During this phase, the ratio of transversal to axial strains measured on the non-repaired sides of the columns, on average, was again around 0.21, and it was slightly lower on the repaired sides, where the axial strains were higher (in average 0.17; Table 4). At stress level of 76% and 58% of the ultimate capacity (in P15_a and P15_b, respectively), the repairs debonded from the concrete. When this circumstance occurred, the repairs could not carry load any more, as demonstrated by the strain release (see P15_a in Fig. 6; axial strains of repair for P15_a and P15_b in Fig. 7 and transversal strain across the interface between concrete support and mortar layer for P15_a and P15_b in Fig. 8). In this case, plasticization had not yet started (average axial strain on repaired and non-repaired sides: 0.76 10 3 and 1.19 10 3; Table 4). At that point, the stiffness of the repaired columns, which was initially higher than that of the control columns, decreased and became lower (Fig. 7). At ultimate load, average axial strains on the non-repaired sides of P15_a and P15_b were 2.18 10 3, while the values on the repairs were not significant (Table 4). Failure thus occurred immediately after plasticization of concrete on the original, non-repaired portion of the columns, without any contribution of the mortar repairs. The stress–strain curves of undamaged (P00_b), and repaired (P15_b, P50_b) columns were not shown in Fig. 6 as they are similar to the corresponding (P00_a), and (P15_a, P50_a) columns. Stress–strain curves for P00_b, P15_b and P50_b columns are compared with the others in Figs. 7 and 8. 4. Finite element modeling of tested columns Three-dimensional non-linear finite element models were used to simulate the experimentally observed behaviour. The Straus7
code (G+D Computing [8]) was used for the numerical analyses. Eight-node solid elements were used to model concrete substrate, while beam elements were used for the steel reinforcement. In repaired columns type P15 and P50, the mortar repair was also modelled with eight-node solid elements, having properties different from those adopted for concrete. The interface between concrete substrate and mortar repair was modelled by means of link elements with tension cut-off. Translation of nodes at the upper and lower bases was restrained in the two orthogonal horizontal directions to reproduce friction between upper and lower faces of the columns and the loading machine plate. Simplified parabolic stress–strain relationships derived from Hognestad [10] were adopted for the concrete substrate and the mortar repair, while elasto-plastic bilinear relationship with hardening was used for steel. The properties of materials were derived from the experimental tests and are listed in Table 5. The Poisson ratio of the concrete and steel were assumed according to Eurocode 2 [3] and Eurocode 3 [4], respectively. The Poisson ratio of the mortar was assumed equal to that of the concrete. The described models were used to carry out non-linear static analyses, under constantly increasing loads. Fig. 9 compares experimental and numerical stress–strain curves. For the non-damaged, non-repaired columns (type P00), the model reproduces very well the mean stress–strain curves of the two tested specimens until peak value. Initial elastic stiffness and value of ultimate load are well reproduced, as can be seen by the stress–transversal strain curve and stress–axial strain curve. Only the latter is slightly stiffer in the model. In the case of columns type P15, repaired with 15 mm thick mortar layer the model reproduces quite well the values of initial elastic stiffness and ultimate load, as can be seen by the stress–transversal strain curve and stress–axial strain curve obtained on the non-repaired side of the column. The axial strains on the repair had trend similar to that measured on the non-repaired side of the column only during the first elastic phase, and the model still gives good results. During experimental tests, at stress level between 76% and 58% of the ultimate capacity, the repairs debonded from the concrete. Although the model cannot reproduce the subsequent strain release, the numerical curve presents a sudden discontinuity at the upper bound of this range of stresses. The model is thus able to show the debonding of repair. This
3134
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
Table 3 Results of axial tests. Column
Debonding load (kN) (a)
Ultimate load (kN) (b)
(a)/(b)
P00_a P00_b P15_a P15_b P50_a P50_b
– – 1901 1575 2430 (partial) 2160 (partial)
2929 2869 2507 2709 2606 2501
– – 0.76 0.58 0.93 0.86
phenomenon is also described by the model displacement in horizontal direction (see Fig. 10). In the case of columns type
P50, repaired with 50 mm thick mortar layer, the model is not able to get the difference in the initial elastic stiffness of the original column and the repair, but can average the two trends and does not present any discontinuity (Fig. 9), nor evidence marked debonding, according to experimental evidence.
5. Discussion The experimental results obtained on axially loaded columns showed that repairs on one side of the columns could not re-establish completely the load-bearing capacity of the non-damaged control columns. Ultimate load of the repaired columns was on
Fig. 4. Failure with debonding of mortar layer (P15_b).
Fig. 5. Cracking of mortar and along the interface (P50_a).
3135
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
35
30
30
25
25
20 15 transv. strain column transv. strain column axial strain column axial strain column
10 5
Stress [N/mm 2]
Stress [N/mm 2]
Average stress-strain on P00_a 35
Average stress-axial strain on column
20 15
P00_a (column) P00_b (column) P15_a (column) P15_b (column) P50_a (column) P50_b (column)
10 5 0
0
0.0
-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
Strain [10-3]
Average stress-strain on P15_a 35
20 15
transv. strain column transv. strain interface transv. strain repair axial strain column axial strain repair
10 5
-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Strain [10-3]
Average stress-strain on P50_a 35
Stress [N/mm 2]
Stress [N/mm 2]
3.5
4.0
30
25
0
3.0
Average stress-axial strain on repair
35
30
1.5 2.0 2.5 Strain [10-3]
25 20 15
P00_a (column) P00_b (column) P15_a (repair) P15_b (repair) P50_a (repair) P50_b (repair)
10 5 0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-3
Strain [10 ] Fig. 7. Stress–axial strain curves, on original column and on repair (all specimens).
Stress [N/mm 2]
30 25 20 15
transv. strain column transv. strain interface transv. strain repair axial strain column axial strain repair
10 5 0
-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -3
Strain [10 ] Fig. 6. Stress–strain curves of undamaged (P00_a), and repaired (P15_a, P50_a) columns.
average 91% that of the control columns. Thin repairs (15 mm), substituting only the reinforcement cover, showed premature debonding from the concrete substrate, on average at 67% of the ultimate load. After debonding, the repairs were not effective, as revealed by strain analysis, and the entire load was transferred to the original, non-repaired portion of the columns. When the repair layer included the reinforcement (50 mm thick), the global behaviour of the repaired columns was improved. Debonding of repair from the concrete substrate was limited and occurred only at the column corners, on average at 90% of ultimate load. After partial debonding, the repairs kept on collaborating with the concrete support, as revealed by strain analysis. Plasticization of both concrete on the non-repaired column sides and mortar repairs revealed that both portions of the repaired columns were
contributing to the column capacity, even in the non-linear phase until failure. Simplified three-dimensional numerical models, implementing non-linear constitutive laws for materials, could simulate fairly well not only the behaviour of control columns, but also that of repaired columns. In the case of thin repairs, the models evidenced the premature debonding of the repairs. Similar models can be thus effectively used for assessment and design of interventions. 6. Conclusions In this work an experimental investigation to control the effectiveness of polymer-modified cementicious mortar repairs applied to square columns under axial loads is carried out. The repair material had similar mechanical properties and slightly higher tensile strength than the concrete substrate and was applied over the entire length of one face of columns. The aim is to give some new insights on validating the effectiveness of such materials in recovering the properties of non-damaged, non-repaired columns, verifying the effect of repair thickness (including the steel reinforcement or not), on cracking pattern, strength and deformability of axially loaded element. The main conclusions arising from the experimental tests show that polymer-modified cementicious mortars, with limited shrinkage and mechanical properties similar to those of the concrete substrate, can be effective for the repair of reinforced concrete columns. The effectiveness of the intervention depends also on position and thickness of the repair layer. For columns repaired on one side only, the repairs cannot restore the load-bearing capacity of non-damaged control columns,
3136
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137
Table 4 Stresses and strains of axial tests. 1/3 Ultimate load Transv. strain (a) (10 3) P00_a P00_b P15_a
Column Repair Column Repair Column Repair Column Repair
P15_b P50_a P50_b
10.3 10.6 9.3
0.08 0.08 0.06 0.09 0.06 0.06 0.05 0.12 0.05 0.24
10.0 9.7 9.3
(a)/(b) Axial strain (b) (10 3) 0.41 0.39 0.31 0.51 0.30 0.38 0.23 0.74 0.24 1.00
0.19 0.20 0.21 0.18 0.21 0.15 0.20 0.16 0.23 0.24
21.1
27.0 24.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
15
P00_a transv. column P00_b transv. column P00 transv. col. model P00_a axial column P00_b axial column P00 axial col. model
10
-2 0 -1.5 -1.0 -0.5 0.0
Stress [N/mm 2]
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Strain [10-3]
Average stress-strain on P15
P15_a transv. column P15_b transv. column P15 transv. col. model P15_a axial column P15_b axial column P15 axial col. model P15_a axial repair P15_b axial repair P15 axial rep. model
20 15 10 5
0 -2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
4.0
Strain [10-3]
Average stress-strain on P50
Average stress-transv. strain on repair
35 30 Stress [N/mm 2]
30 25 20 15
P00_a (column) P00_b (column) P15_a (repair) P15_b (repair) P50_a (repair) P50_b (repair)
10 5 0
27.8
0
Strain [10-3]
35
29.0
5
0
0.0
30.1
20
25
10
0.97 1.30 0.55 1.08 1.48 2.25 1.06 2.40
2.41 2.52 2.25 0.79 2.11 0.54 2.28 3.30 2.21 2.71
25
25
P00_a (column) P00_b (column) P15_a (column) P15_b (column) P50_a (column) P50_b (column)
31.0 31.9 27.9
Axial Strain (10 3)
30
30
15
Stress (N/mm2)
Axial strain
Average stress-strain on P00
35
20
Ultimate Load
35
30
5
Stress [N/mm 2]
0.20 0.23 0.13 0.06 0.25 0.43 0.29 0.54
17.5
Average stress-transv. strain on column
35
Debonding Transv. strain (10 3)
although they give acceptable results (91% of the ultimate capacity of the control columns). In any case, repairs that include the longitudinal reinforcement show good properties, with stable behaviour, sharing of loads, and plasticization of the material before failure, whereas thin repairs that do not include the reinforcement do not have adequate performance due to premature debonding. Hence, from the practical point of view, a rehabilitation intervention on axially loaded elements that include the longitudinal steel reinforcement may be generally recommended since it is more effective than that involving only the concrete cover without including steel bars. In fact, the action of the transverse steel allows sharing of loads between the mortar and the concrete core until failure, avoiding premature debonding of the mortar layer, when the steel bars are included in the repair layer.
Stress [N/mm 2]
Stress (N/mm2)
Stress [N/mm 2]
Stress (N/mm2)
Column
25 20
P50_a transv. column P50_b transv. column P50 transv. col. model P50_a axial column P50_b axial column P50 axial col. model P50_a axial repair P50_b axial repair P50 axial rep. model
15 10 5 0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Strain [10-3] Fig. 8. Stress–transversal strain curves, on original column and on repair (all specimens).
-2 0 -1.5 -1.0 -0.5 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Strain [10-3] Fig. 9. Experimental and numerical stress–strain curves of undamaged (P00), and repaired (P15; P50) columns.
3137
C. Pellegrino et al. / Construction and Building Materials 23 (2009) 3129–3137 Table 5 Model material properties. Material
fc (N/mm2)
ecu (%)
fct (N/mm2)
ectu (%)
fy (N/mm2)
ey (%)
ft (N/mm2)
et (%)
E (N/mm2)
m (–)
Concrete Mortar Steel
28.90 32.87 –
0.18 0.25 –
3.19 3.48 –
0.01 0.01 –
– – 532
– – 0.27
– – 628
– – 25
32,500 26,200 200,000
0.20 0.20 0.30
Fig. 10. Model displacement in horizontal direction for P15 at debonding (left) and P50 at same vertical load level (right).
The behaviour of axially loaded columns repaired with thin or thick repairs can be also reproduced by simplified three-dimensional non-linear finite element models. Acknowledgements The authors gratefully acknowledge Tassullo S.p.A. that provided materials and specimens for experimental testing, and Eng. Diego Testolin for his contribution to the experimental investigation developed during his MSc thesis. The experimental tests were carried out at the Laboratory of Structural Materials Testing of the University of Padova, Italy. References [1] Emberson NK, Mays GC. Significance of property mismatch in the patch repair of structural concrete. Part 2: Axially loaded reinforced concrete members. Mag Concrete Res 1990;42(152):161–70. [2] Emberson NK, Mays GC. Significance of property mismatch in the patch repair of structural concrete. Part 3: Reinforced concrete members in flexure. Mag Concrete Res 1996;48(174):45–57. [3] European Committee for Standardization. Eurocode 2 – Design of concrete structures. Part 1-1: General rules and rules for buildings. EN 1992-1-1, Brussels, Belgium; 2004. [4] European Committee for Standardization. Eurocode 3 – Design of steel structures. Part 1-1: General rules and rules for buildings. EN 1993-1-1, Brussels, Belgium; 2005.
[5] European Committee for Standardization. Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 3: Structural and nonstructural repair. EN 1504-3, Brussels, Belgium; 2005. [6] Fib. Seismic assessment and retrofit of reinforced concrete buildings. State of Art Report, Bulletin 24. Federation International du Beton: Lausanne (Switzerland); 2003. [7] Fukuyama K, Higashibata Y, Miyauchi Y. Studies on repair and strengthening methods of damaged reinforced concrete columns. Cement Concrete Compos 2000;22:81–8. [8] G+D Computing. Straus7 user’s manual. Sydney; 2005. [9] Hassan KE, Brooks JJ, Al-Alawi L. Compatibility of repair mortars with concrete in a hot-dry environment. Cement Concrete Compos 2001;23:93–101. [10] Hognestad E. A study of combined bending and axial load in reinforced concrete members, Bulletin No. 399. University of Illinois, Urbana IL USA: Engineering Experimental Station; 1951. [11] Jumaat MZ, Kabir MH, Obaydullah M. A review of the repair of reinforced concrete beams. J Appl Sci Res 2006;2(6):317–26. [12] Kim JHJ, Lim YM, Won JP, Park HG, Lee KM. Shear capacity and failure behaviour of DFRCC repaired RC beams at tensile region. Eng Struct 2007;29:121–31. doi:10.1016/j.engstruct.2006.04.023. [13] Mangat PS, O’Flaherty FJ. Influence of elastic modulus on stress redistribution and cracking in repair patches. Cement Concrete Res 2000;30:125–36. [14] Nounu G, Chaudhary Z-UL-H. Reinforced concrete repair in beams. Constr Build Mater 1999;13:195–212. [15] Park SK, Yang DS. Flexural behaviour of reinforced concrete beams with cementitious repair materials. Mater Struct 2005;38:329–34. doi:10.1617/ 14051. [16] Pellegrino C, Modena C. FRP shear strengthening of RC beams with transverse steel reinforcement. J Compos Constr 2002;6(2):104–11. [17] Pellegrino C, Modena C. FRP shear strengthening of RC beams: experimental study and analytical modelling. ACI Struct J 2006;103(5):720–8. [18] Pellegrino C, Modena C. An experimentally based analytical model for shear capacity of FRP strengthened reinforced concrete beams. Mech Compos Mater 2008;44(3):231–44. [19] Pellegrino C, Modena C. Flexural strengthening of real-scale RC and PRC beams with end-anchored pre-tensioned FRP laminates. ACI Struct J 2009;106(3):319–28. [20] Pellegrino C, Modena C. Influence of FRP axial rigidity on FRP-concrete bond behaviour: an analytical study. Adv Struct Eng, in press. [21] Pellegrino C, Tinazzi D, Modena C. An experimental study on bond behavior between concrete and FRP reinforcement. J Compos Constr 2008;12(2):180–9. [22] Rahman K, Baluch MH, Al-Gadhib A. Modeling of shrinkage and creep stresses in concrete repair. ACI Mater J 1999;96(5):542–50. [23] Ramirez JL. Ten concrete column repair methods. Constr Build Mater 1996;10:195–202. [24] Río O, Andrade C, Izquierdo D, Alonso C. Behaviour of patch-repaired concrete structural elements under increasing static loads to flexural failure. J Mater civil Eng 2005;17(2):168–77. doi:10.1061/(ASCE)0899-1561(2005)17:2. 168. [25] Shambira MV, Nounu G. On the effect of time-dependent deformations on the behaviour of patch-repaired reinforced concrete short columns. Constr Build Mater 2000;14:425–32. [26] Shannag MJ, Al-Ateek SA. Flexural behaviour of strengthened concrete beams with corroding reinforcement. Constr Build Mater 2006;20:834–40. [27] Sharif A, Rahman MK, Al-Gahtani AS, Hameeduddin M. Behaviour of patch repair of axially loaded reinforced concrete beams. Cement Concrete Compos 2006;28:734–41. doi:10.1016/j.cemconcomp.2006.05.013. [28] Valluzzi MR, Grinzato E, Pellegrino C, Modena C. IR thermography for interface analysis of FRP laminates externally bonded to RC beams. Mater Struct 2009;42(1):25–34.