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Strengthening of RC slabs with reinforced concrete overlay on the tensile face
Engineering Structures 132 (2017) 540–550
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Engineering Structures journal homepage: www.elsevier.com/locate/engstruct
Strengthening of RC slabs with reinforced concrete overlay on the tensile face Hugo Fernandes a,⇑, Válter Lúcio b, António Ramos b a b
FCT – Universidade NOVA de Lisboa, Portugal CERIS - ICIST, FCT – Universidade NOVA de Lisboa, Portugal
a r t i c l e
i n f o
Article history: Received 21 October 2015 Revised 19 August 2016 Accepted 10 October 2016
Keywords: Slab strengthening Concrete layer Reinforced concrete Steel connectors Interfacial bond
a b s t r a c t Strengthening of concrete structures with a new concrete layer has been commonly used for columns, beams and slabs. This technique is economic and efficient for structural strengthening since it uses the same base materials, steel and concrete. It is usually applied on the compressed face of the concrete element due to concrete’s recognized behaviour under compression, posing several challenges to control cracking and resistance when applied on the tensile face. For assessing the performance of the strengthening method, twelve slab specimens were designed and tested monotonically. The main parameters to assess in this work were the debonding behaviour and load, and the relationship between the latter and the relative displacements at the interface of the two concrete layers. The performance of the strengthened structures strongly relies on the interaction of the two concrete layers, with this being the main subject of the research about overlaid concrete. The load transfer capacity of the interface depends on the interface shear strength, which in turn is highly dependable on substrate roughness, cleanliness and curing conditions of the newly added layer. Interface performance may be improved by using steel connectors crossing the interface, properly anchored on both layers. The importance for these elements grows as the existing concrete is more deteriorated, since adhesion strength will decrease with lower quality concrete. This paper presents the experimental research for the application of bonded concrete overlays on the tensile face of reinforced concrete slabs, mainly aimed at office buildings and parking facilities, where spatial clearances or inaccessibility to the lower side of the slabs are recurrent. A ductile behaviour upon debonding was achieved for the specimens with reinforcement crossing the interface, and a debonding load up to three times that of the reference specimens without reinforcement crossing the interface. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Strengthening of concrete structures by adding a new concrete layer is well known when applied to the compressed face of concrete elements. Examples of that are beams and columns strengthened with concrete jacketing. Depending on the intended reason for strengthening/retrofitting the existing structure should be relieved of existing loads until strengthening is applied. This is more important with damaged or deteriorated structures since loading will further aggravate the existing structural condition. Bonded concrete overlay becomes a relevant strengthening and repairing technique since the base material has to be replaced with new concrete.
⇑ Corresponding author. E-mail address: [email protected] (H. Fernandes). http://dx.doi.org/10.1016/j.engstruct.2016.10.011 0141-0296/Ó 2016 Elsevier Ltd. All rights reserved.
When applied on the tensile face of concrete elements, bonding between two concrete layers is affected by the normal stresses that appear due to the difference in stiffness of the concrete layers. This technique relies on the quality of bond between the two concrete layers, therefore varying with surface preparation, and if steel connectors are installed crossing the interface. If no connectors are used, adhesion is the only component of the resisting mechanism acting on the interface, and brittle failure shall occur. This relies strongly on roughness, which allows for interlocking of the two layers and consequently bonding stresses to develop along the interface. With steel connectors crossing the interface between the two concrete layers, three components of the resisting mechanism illustrated in Fig. 1 shall develop [1]: 1. Adhesion, due to chemical bond between the two layers, and mechanical interlocking, if macroscopic surface roughness is present, shall be considered for slips up to 0.5 mm [2].
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Adhesion/Interlocking + Friction
541
Dowel Action
Fig. 1. Resisting mechanism for concrete-to-concrete interfaces with steel connectors adapted from [4].
2. Friction is a direct consequence of external loads perpendicular to the interface, or due to steel connectors crossing the interface that are mobilized in tension for higher relative displacements, usually larger than 0.5 mm [2], with equilibrium guaranteed by compressive forces at the interface. 3. Larger relative slips mobilize dowel action inducing bending, shear and tension in the steel connectors crossing the interface. The dowel action maximum value depends on the resistance of the steel connector (bending + tension and shear) and the crushing of the surrounding concrete [2,3]. Bond performance of the interface will depend on the appropriate conditions for these resisting mechanisms. Some authors provide recommendations for surface preparation and curing conditions of the added concrete layer. Contamination of the concrete surface before casting the new layer, methods used for surface preparation, and surface microcracking, are according to [2] the main quality parameters for the application of this technique. A special attention to edge zones is also referred to in the document since the discontinuity of the cross section allows for the development of significant tensile and shear stresses. Interface bond performance varies from null to full transfer of horizontal stresses between the two layers, allowing monolithic behaviour to be achieved, according to Fig. 2. Usually interaction between layers in the cross section is characterized as partial, and relative displacement between the two layers is accounted for. Several behaviour models have been developed over the years [5–7], where relative displacement, both horizontal and vertical, is comprised.
Stress transfer between the two concrete layers is based on the shear friction theory [8–12]. The behaviour models developed using the basis of this theory account for relative slip and interface crack opening or dilation, which are increased in an unconfined state. Normal and tangential relative displacements are of the utmost importance when tensile stresses govern the interface stress field. Models for relative displacement analysis and some models for stress analysis do not specifically account for steel connectors crossing the interface. This is a behaviour changing aspect for detailing the interface since it limits interface crack opening, providing for greater stresses to develop and reaching of larger slips. Shear connectors crossing the interface, properly anchored to both layers to improve strength, can reduce uncertainty about interface performance, and should be accounted for in the design of these interfaces [13]. Due to the orthogonal directions of interface deformations, behaviour of reinforcement crossing the interface will be ruled by pullout of the embedded length and dowel action of the steel bar. The relationship between the two phenomena has been identified and studied, since pullout can be affected by transverse shear, and dowel action in turn is affected by the pullout of the steel bar [14]. Another ruling aspect for these mechanisms to develop is aggregate interlock, which causes the interface crack to open as the two layers slip relatively to another [4,15]. The latter mechanism is highly dependable on the quality of the surface preparation chosen for the existing structure substratum. Surface preparation also plays a major role on interface behaviour due to the interlocking mechanism, which causes the interface crack to open when relative slip occurs [16]. There are several methods for surface preparation, as jackhammering,
Fig. 2. Interface performance regarding shear stress transfer between layers.
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recognized as being one of the most aggressive surface preparation methods, and hydro-demolition, one of the least aggressive methods, allowing also for selective concrete removal [17–20]. These methods are more or less intrusive in terms of microcracking of the existing concrete layer and can cause premature debonding of concrete chunks and reduced interface performance, reaching a depth of 3 mm [21] to 10 mm [22] on the existing concrete layer. Since a new concrete layer is to be cast against the existing one, a greater roughness is required, with exposed aggregate particles for improved interlocking. This is quantified in terms of an average roughness parameter, which can be assessed through several techniques. This value defines the interface in terms of roughness, according to [23], from smooth to very rough, with a specific profile geometry. For improved performance of the interface, a very rough profile is recommended, with an average roughness parameter equal or greater than 3.0 mm, since it directly improves all other components of the resisting mechanism. Chipping with an electric or pneumatic hammer and steel moil point, despite the disadvantages of microcracking, and hydro demolition with high-pressure water jet, are the most common surface preparation methods. The latter technique is the most efficient in terms of roughening and reduced microcracking of the surface, but also characterized by difficult logistics and higher costs. For this reason, chipping with an electric hammer and steel moil point is the easiest and least specialized method for surface roughening in a strengthening/retrofitting situation. Alongside the surface preparation techniques, it is also referred to in [24] the importance of moisture on the existing surface. An optimum combination of factors is presented for saturated substrate and apparently dry surface, for better bonding of the overlaid concrete. Values for adhesive tensile stress are provided in [18] for the steel moil point and the high-pressure water jet of 1.10 MPa and 1.46 MPa, respectively. Besides geometrical characteristics of the interface, performance of the strengthened structure also depends on materials of both substrate and the new layer to be overlaid. They directly influence both local and global behaviour, through adhesive capacity and crushing resistance of concrete, and through bending and shear resistance of the composite layers. According to [25], the newly added layer should be of greater resistance than the substrate and low shrinkage, and should be fluid enough to penetrate the grooves on the existing surface.
2. Experimental programme 2.1. Definition of the strengthened specimens For characterizing the behavioural aspects of concrete layers in a strengthening situation, three different detailings of the interface and one for reference, were tested. The reference specimens were tested with only surface roughening and no rebar crossing the interface. Relevant data regarding specimens geometry is provided in Table 1, where As,o and As,s are the longitudinal reinforcement area for the overlay and substratum, respectively, and As,i the area of the reinforcement crossing the interface. The experimental tests were comprised by the following specimens: Surface roughening only (S-REF). Steel connectors distributed along the interface, with 50 mm of anchorage length (S-STC, 1–3), and 70 mm of anchorage length (S-STC-4). Anchoring of the longitudinal reinforcement bars of the new layer at the ends, since the deformation of the specimens in bending suggests the lifting of the overlaid concrete in this zone [25], with 50 mm of anchorage length (S-ANC, 1–3) and 70 mm of anchorage length (S-ANC-4). All of the aforementioned techniques combined for improving bond at the interface (S-STANC). Also in the MC 2010 [23] the lifting phenomenon at the overlaid concrete edge is considered relevant since deformation due to shrinkage is greatest in this zone and debonding shall occur. Roughening of the substrate surface was accomplished for all specimens with an electric hammer and steel moil point. The slenderness of the slabs was always a limiting factor for the intended stress at the interface. Therefore, the solution that guaranteed structural integrity until debonding consisted on bundled longitudinal reinforcement and reduced contact area between the two layers. The slab substrates were then reinforced with 10 mm bundled longitudinal bars and reinforcement for the overlaid concrete layer consisted of six double 12 mm bars (Fig. 3). The contact area between the two layers was reduced transversally to increase bond stress. This layer also did not reach the supports for all specimens, since confinement of the rebar does not necessarily happen in a real strengthening situation. Surface
Table 1 Relevant specimen geometrical parameters. S-REF
Overlay
Thickness [m] Length [m] Width [m] As,o [–]
Interface
As,i [mm2]
Substrate
As,s [–] Width [m] Length [m] Thickness [m]
S-STC
S-ANC
S-STANC
0.07 1.00 0.60 2Ø12//0.10 m (22.62 cm2/m) –
336
678 2
2Ø10//0.10 m (15.7 cm /m) 1.00 2.30 0.12
Fig. 3. Rebar detailing and strain gauge placing for reference specimens (S-REF).
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preparation for casting the new layer was performed with an electric chipping hammer and steel moil point eighteen days after casting of the substrate concrete. The final surface can be observed in Fig. 4. Despite the disadvantages concerning microcracking of the existing surface, this method was chosen due to its practical and economical characteristics. Surface roughness was evaluated for each specimen, with concern that the moil tip should not go deeper than 10 mm. This protects the longitudinal rebar and the minimum concrete cover. Several methods for surface roughness assessment can be used [17], with the sand patch method (SPM) being the most widely referred in the literature and recommended in the MC 2010 [23]. Due to the availability of other methods, and with knowledge from [26] that this method is limited for very rough surfaces, another method by point measurement was used to evaluate the roughness profile, shown in Fig. 4. Since the SPM can only determine the average ‘‘peak-to-mean” roughness Rp [23], this parameter was calculated according to Eq. (1), where n is the number of peak pi to mean z profile measurements [17].
Rp ¼
n 1X jp zj n i¼1 i
ð1Þ
The resulting parameters for each of the specimens are presented in Table 2, where six assessment lengths (n = 6) were considered. Characterization of the surface roughness was performed according to [27], where a lower limit roughness is set at 3.0 mm for using the SPM (Rt,SPM), or 2.2 mm when considering the Rp parameter. Since the actual SPM was not performed the latter
parameter was considered, allowing for a surface characterization of ‘‘very rough” according to [27] and the classification provided on the MC 2010 [23]. The overlaid concrete was cast seventy-five days after casting the substrate concrete, with the rebars instrumented with strain gages, as well as the substrate rebars, at midspan and 0.25 m from this point. Interface detailing, with longitudinal rebars crossing the interface or steel connectors with anchorage lengths of 50 mm and 70 mm, can be observed in Figs. 5–7. Shear connectors particularly, which consisted on right angle rebars, were tied to the longitudinal reinforcement to prevent slipping of the reinforcement at the overlay anchorage portion. Bonding of the anchorages in the substrate layer was performed with SikaÒ Grout, rather than an epoxy solution, due to the economy and ease of application in a practical situation. 2.2. Materials characterization Material characterization was performed for the different concrete layers, rebar sizes, and grout used for anchoring the rebar crossing the interface. The mechanical properties for the grout were tested for the compressive and tensile strength, and with pullout tests for assessing bond strength. The parameters for this material were a flexural tensile strength of 9.7 MPa, a compressive strength of 78.8 MPa, and an average bond strength of 16.2 MPa. Pullout testing of unconfined rebar embedded in concrete holes with a diameter at least double that of the rebar allowed for determining the latter strength. Anchorage failure resulted in the pull out of the rebar sliding through the steel-grout interface. The test
Fig. 4. Substrate surface roughness assessment setup and detail.
Table 2 Average peak-to-mean roughness parameter. Rp [mm]
Reference (REF)
Steel connectors (STC)
Anchorage (ANC)
Steel connectors + Anchorage (STANC)
S-1 S-2 S-3 S-4
2.2 2.2 2.2 –
2.2 2.2 2.3 2.6
2.2 2.9 2.3 2.9
2.6
Fig. 5. Rebar detailing and strain gauge placing for S-ANC specimens.
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Fig. 6. Rebar detailing and strain gauge placing for S-STC specimens.
Fig. 7. Rebar detailing and strain gauge placing for S-STANC specimens.
results for the other materials can be observed below in Tables 3 and 4.
Table 4 Strength characteristics for the steel bars used.
2.3. Test setup The specimens were subjected to monotonic loading in a three point bending test, as shown in Fig. 8. Loading was imposed at midspan with hydraulic jacks, and reactions at two symmetrical supports, according to the figures below. Forces at the supports were measured with four TMLÒ CNC-200KNA load cells and the deformation at the coordinates identified in the left picture below, with TMLÒ CDP-100 displacement transducers. These tests were performed with prestressing strands at the supports. For assessing relative displacements between the two layers, TMLÒ CDP-50 displacement transducers were fixed on the substrate layer, as shown in Fig. 9. For data acquisition, four HBMÒ Spider8 datalogger units were used along with one HBMÒ UPM100 datalogger unit, all monitored by HBMÒ Catman V6.0 software. Loading was controlled by force with a WALTER+BAIÒ PKNS19D electronically controlled hydraulic pump, at a speed of 0.10 kN/s for all tests.
Diameter
Ø6
Ø 10
Ø 12
fy [MPa] fu [MPa]
541.0 692.7
530.6 627.5
532.1 627.4
fy – mean yield stress of steel. fu – mean tensile strength of steel.
the interface level for the reference specimens started from the ends of the overlaid concrete layer and evolved along the interface to midspan, according to Fig. 10. The phenomenon occurs when the tensile strength of the interface is reached, and subsequent failure of the substrate layer (Fig. 10, right, flexural failure). For specimens with longitudinal rebar anchored on the substrate layer, beginning of interface cracking was similar to the reference specimens, occurring again at the overlaid concrete ends. The debonding load was very close to the failure load on these specimens. One specimen failed in shear (S-ANC-1), as shown in Fig. 11, with the other two specimens resulting in flexural failure of the substrate layer rebar. Some specimens with smaller anchorage length (50 mm), had failure controlled by pullout of the rebars anchored in the substrate, as shown in Fig. 12. For specimens with steel connectors distributed along the interface, cracking was similar to anchored rebar ones, with two specimens failing in shear (see Fig. 13). There was also no visible full debonding of the overlaid concrete, with failure occurring for the two layers resisting the load.
3. Experimental results 3.1. Failure modes Behaviour was consistent for all specimens in terms of cracking, debonding of the overlaid concrete, and failure load. Cracking at
Table 3 Concrete strength characteristics for all specimens. Reference (S-REF, 1–3)
Steel connectors (S-STC, 1–3)
Anchorage (S-ANC, 1–3)
Steel connectors (S-STC, 4)
Anchorage (S-ANC, 4)
Steel connectors + Anchorage (S-STANC)
fc,cube [MPa]
Substrate Overlay
45.3 47.8
40.6 47.8
37.4 47.8
55.5 41.1
54.8 39.4
56.4 41.5
fct [MPa]
Substrate Overlay
2.8 2.9
2.5 2.9
2.3 2.9
3.9 3.1
3.9 3.0
4.0 3.1
fc,cube – mean value for the compressive strength of concrete in cubic specimens. fct – mean value for the tensile splitting strength of concrete.
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Fig. 8. General test setup.
Fig. 9. Relative displacement measurement setup.
Fig. 10. Interface failure for reference specimens (S-REF-1).
Fig. 11. Shear failure for S-ANC-1 specimen.
Anchorage failure occurred for some specimens with pullout at the steel-grout interface visible through the interface crack right after testing. Through a longitudinal sectioning of the specimens, the evolution of the shear crack for the specimen that failed in bending can be observed. The insufficient anchorage length caused the shear crack to fail intercepting the steel connectors (see Fig. 14). All three specimens with a larger anchorage length of rebars crossing the interface (70 mm) failed in shear, as visible in Fig. 15. This attests the increase in performance when reinforcement crosses the interface, properly anchored, achieving an almost monolithic behaviour until failure.
load and deflection at midspan, comparing specimens with reinforcement crossing the interface and reference ones. The marker on the load-deflection curves illustrates the moment when the highest strain was registered for the overlaid concrete rebars. The load-deflection curves above show an approximately linear behaviour for all tests up to a load of 40 kN at midspan, followed by a reduction in stiffness. This was consistent with the cracking load for the geometry of the cross section. For the reference specimens, debonding of the overlaid concrete occurred for a load of 80 kN at midspan. Stiffness was reduced to approximately zero, observed graphically by an horizontal plateau. This was followed by reloading of the substrate layer, until flexural failure occurs for a load of 160 kN at midspan. For the specimens with longitudinal rebar anchored 50 mm in the substrate (Fig. 16, left), the failure load was about the same as for the reference specimens. The debonding load of 142 kN represents an increase of 79% when compared to reference specimens.
3.2. Debonding and failure loads The relationship between load and deflection at midspan was analysed to characterize the behaviour of the strengthened specimens. Figs. 16 and 17 present the relationship between vertical
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Fig. 12. Failure for specimens with small anchorage length of the longitudinal rebars.
Fig. 13. Shear failure for specimens with steel connectors (S-STC-1).
For specimens with steel connectors anchored 50 mm in the substrate (Fig. 16, right), debonding load increased about the same as the latter (76%), with the resulting failure load 10% higher than the reference tests. Debonding phenomenon was not visible during testing, and only identified through the strain measurement at the longitudinal rebars. The three specimens with 70 mm anchorage length differed from the first set of tests mainly due to greater debonding and ultimate loads. The relationships between vertical load and deflection at midspan for these specimens are presented in Fig. 17. From the load-deflection curves, one can observe that the increase in anchorage length also increased debonding and failure loads. The latter showed an increase of 39% for the anchored longitudinal rebar specimens, 27% for the slabs with steel connectors, and 43% for all techniques combined. Also noticeable is the small increase for the failure load of the specimen that combined all techniques (S-STANC), when compared to anchored longitudinal rebar tests (S-ANC). This attests the impact of the latter in the overall behaviour of the interface, due to longitudinal rebars with higher diameter and anchorage positioning at the overlaid concrete
ends. The limitation on the evolution of both interface crack opening and relative slip results in smaller relative displacements that reach the steel connectors, thus reducing its contribution to the resisting strength of the interface. However, these are activated later in the loading history, with smaller stresses. Table 5 lists the failure loads for all strengthened specimens, and the load for which maximum strain was reached in the overlaid concrete rebars, which is also the load for maximum stress at the interface. Failure modes are identified as shear of the substrate layer (Ss) or flexural failure of the substrate rebar (Fs). The vertical and horizontal relative displacements at the overlay end are summarized in Table 6 for each detailing of the interface. Regarding the specimens where the overlaid concrete fully debonded, the displacement values were considered for maximum steel strain at the overlay rebars. For the specimens without strain gauge information, this value was estimated based on the irrespective load-deflection curve, comparing with instrumented specimens with the same detailing of the interface. Highlighted values refer to the highest values of relative displacement registered for specimens where failure occurred by shear of the substrate layer. The results also show higher crack dilation and interface slip for the higher failure loads of specimens S-STC-4, S-ANC-4, and S-STANC, denoting the mobilization of the dowel action of the reinforcement crossing the interface with proper anchoring in the substrate (see Table 6).
4. Discussion The provisions on concrete-to-concrete interface resistance presented in the Model Code 2010 [23] were considered for behaviour characterization and quantification of the components of the resisting mechanism, aggregate interlock, friction, and dowel action. Shear stresses at the interface are a consequence of the variation of force DFso along the longitudinal rebar of the overlaid concrete, that are transferred to the substrate layer. These stresses are responsible for the integrity of the composite section, since failure
Fig. 14. Steel connectors anchorage detail and section for the S-STC specimens.
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Fig. 15. Shear failure for specimens with larger anchorage length of rebars crossing the interface (left, S-ANC-4; centre, S-STC-4; right, S-STANC).
Deflection at midspan [mm]
Deflection at midspan [mm]
Fig. 16. Load-deflection curves for specimens w/reinforcement crossing the interface anchored 50 mm.
highest strain registered for the overlay concrete rebars.
forces leads to an average shear stress mi at the interface level. This is proportional to the variation of forces in length of the overlaid concrete rebars, according to Eq. (2), where b is the width of the interface.
vi ¼
Fig. 17. Load-deflection curves for specimens w/reinforcement crossing the interface anchored 70 mm. highest strain registered for the overlay concrete rebars.
of the interface can cause failure of the strengthened element. Shear stress can be evaluated in terms of the force transferred between the two concrete layers over its interface. In a concrete element of length Dl between cracked sections, equilibrium of
DF so Dl b
ð2Þ
The strain gages installed in the longitudinal rebars provided a linear distribution of steel strains, which is consistent with a uniform distribution of shear stresses across the interface. The values for the steel strains at midspan (es) and corresponding values for shear stress at the interface (mi) can be observed in Table 7. These values are the maximum strains registered for the longitudinal rebars at the overlaid concrete. Only two specimens of each detailing were instrumented with strain gages at the rebars. Evaluation of the shear resistance at the interface was carried out according to Randl [2] and the MC 2010 [23]. The resisting mechanisms at the interface are divided in three main components: aggregate interlock, friction, and dowel action of the reinforcement crossing the interface. The low stiffness of slabs results in large vertical deflections and large horizontal displacements. In [2] a distinction is made between stiff and more brittle behaviour of the interface in terms of the relative slip s. This limit is set at 0.05 mm, governed by
Table 5 Maximum strain at the overlaid concrete rebars and failure load at midspan. Specimen
Values for maximum strain at the overlay rebars Failure load at midspan [kN] Failure mode
S-REF
Midspan load [kN] Overlay rebars Fso [kN] Substrate rebars Fs [kN]
S-STC
S-ANC
S-STANC
1
2
3
1
2
3
4
1
2
3
4
82.4 138.9 90.8
– – –
80.1 141.3 88.6
140.3 218.7 179.7
– – –
144.3 226.5 175.9
200.3 375.9 408.1
– – –
150.0 305.5 153.6
140.0 273.6 137.3
190.3 408.2 272.7
214.7 483.4 235.2
163.1
155.3
160.8
177.2
171.2
180.1
201.8
151.1
153.6
157.3
221.2
227.7
Fs
Fs
Fs
Ss
Fs
Ss
Ss
Ss
Fs
Fs
Ss
Ss
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Table 6 Relative displacements at the interface end. S-REF
Load at midspan [kN] Crack dilation [mm] Interface slip [mm]
S-STC
S-ANC
S-STANC
1
2
3
1
2
3
4
1
2
3
4
82.4 1.52 0.58
79.6 0.87 0.39
80.1 1.42 0.57
177.2 4.11 1.45
148.1 2.89 1.02
180.1 4.33 1.43
201.8 4.98 2.38
151.1 2.33 0.35
150.0 2.06 0.51
140.0 2.51 0.49
221.2 5.46 2.49
227.7 4.18 2.21
Table 7 Overlay steel strains and shear stress at the interface. Specimen
es [106] mi [MPa]
S-REF
S-STC
S-ANC
S-STANC
1
3
1
3
4
2
3
4
–
511.7 0.46
520.6 0.47
805.7 0.73
834.5 0.76
1384.9 1.25
1125.5 1.02
1008.0 0.91
1503.9 1.36
1780.9 1.61
aggregate interlock and friction from external forces. For slips over this limit, the behaviour is considered more ductile, with adhesion replaced by friction/interlocking and dowel action. The following equations taken from [2] quantify both these scenarios, without accounting for external actions that are favourable to the resisting mechanism:
s 6 0:05 mm;
v R;ad ¼ ca f ctd 6 0:5 m f cd
s P 0:05 mm;
v R;ilþfrþdow ¼ cr f 1=3 ck þ l qi k1 f yd þ k2 qi
qffiffiffiffiffiffiffiffiffiffiffiffiffi f yd f cd 6 bc
Fig. 18. Vertical tensile stresses at the end of the interface.
ð3Þ
m f cd
ð4Þ
where
vR,ad is the design value for the adhesive shear stress for the interface; ca ; cr are the coefficients for surface condition; f ctd is the design value for concrete tensile strength; m is the reduction coefficient for compressive forces
(m ¼ 0:55 ð30=f ck Þ 6 0.55); f cd is the design value for concrete compressive strength; vR,il+fr+dow is the design value for shear stress of the interface for interlocking, friction, and dowel action; f ck is the characteristic compressive strength for concrete; l is a friction coefficient; qi is the ratio of reinforcement crossing the interface (qi ¼ As;i =Ai ); k1 is the latter reinforcement performance reduction factor (k1 ¼ rs;i =f y 6 1.0) where rs,i is the actual tensile stress in the steel crossing the concrete interface; f yd is the design value for steel yielding stress; k2 is the interaction coefficient for flexural resistance of the rebar (61.6 for circular cross-sections and C20/25 - C50/60); bc is the coefficient that accounts for the strut inclination of concrete in compression. 1=3
An adjustment is proposed for some coefficients in an analysis situation that fits better with test results. Coefficient cr is proposed as 0.2 for the very rough surfaces of the present test specimens which is lower than recommended in [2,23] when considering mean values. One should notice that for the present case the interface comprises perpendicular tensile stresses, whereas the cr values according to [2,23] refer to non-tensioned interfaces only. The related shear strength due to aggregate interlock for the reference specimens thus resulted in 0.66 MPa, calculated considering the lower concrete compressive strength of the two concretes. This value overestimates the test results of about 0.19 MPa for the two reference slabs in Table 7. The microcracking due to the surface
preparation and the tensile stresses that result from equilibrium at the end of the interface (Fig. 18) can justify this behaviour, debonding prematurely for a brittle resisting mechanism. The coefficient of friction l was calculated as proposed in [23] and also in [28] for very rough interfaces. Considering the roughness classification presented in Section 2.1, the friction coefficient takes the value of 1.4. For the coefficient k1 a new method is proposed, that contemplates the amount of horizontal force at the interface not resisted by dowel action of the rebar. This shear force at the interface (kFDFso) is accounted for in the quantification of the steel stresses for the rebars crossing the interface, by means of a coefficient ‘kF’ as follows:
rsi ¼
F s;c ðkF DF so Þ tan h kF ti tan h ¼ ¼ Asi Asi qsi
ð5Þ
where Fs,c is the tensile force in the steel connectors or longitudinal rebar anchorage and qsi is the reinforcement ratio crossing the interface (Asi/Ai). h is the angle between the interface plane and the concrete strut that results from nodal equilibrium, as illustrated in Fig. 19. The values for the coefficient kF were determined according to the amount of stresses resisted by dowel action of the rebar crossing the interface. Around 70% of the total horizontal load at the interface for the steel connectors was determined for the aggregate interlock and friction resisting mechanisms (kF = 0.7). The remaining stresses were then resisted by dowel action of the rebars. For the specimens with longitudinal rebar anchored, the value kF was reduced in half (kF = 0.35) empirically in good approximation to the test results, since the edge lifting phenomenon is present, thus resulting in tension of the anchored rebars. An angle h of 21.8° was considered, with good correlation to test results, which is also the lower bound for the angle of concrete struts according to [29]. The considered values for these parameters are a tentative approach accounting for the rather complex situation at the considered overlay edges subject to strong delamination and shall be confirmed with further test results. Dowel action resistance alone can be calculated according to [2] with Eq. (6), where maximum allowable dowel action of the
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Fig. 19. Resisting mechanism of the overlay rebars anchorage (left) and shear connectors (right). Table 8 Dowel action resistance. Specimen
S-STC 1
Asi [mm2] s [mm] k1 [–] VF [kN] mF [MPa] mF/mi [–]
S-ANC
3
336 0.48 0.34 62.5 0.21 0.29
0.53 0.35 62.2 0.21 0.27
4
2
2.38 0.59 60.6 0.20 0.16
678 0.51 0.24 91.1 0.30 0.30
S-STANC
3 0.49 0.21 91.7 0.31 0.34
4
(Ø6)
(Ø12) 678
1.32 0.32 140.1 0.47 0.34
336 1.69 0.25 72.7 0.73 0.45
146.7
Table 9 Shear stress for friction and interlocking of protruding aggregates. Specimen
S-STC 1
mil [MPa] mfr [MPa] mfr+il/mi [–] mfr+il+F [MPa] mfr+il+F/mi [–]
0.64 0.29 1.27 1.13 1.55
S-ANC
3
4
2
0.30 1.24 1.14 1.51
0.64 0.49 0.90 1.33 1.06
0.62 0.20 0.81 1.12 1.10
Shear stress at the interface [MPa]
1.6 1.2 0.8 0.4 0 S-STC-1 S-STC-3 S-STC-4 S-ANC-2 S-ANC-3 S-ANC-4 S-STANC Friction
Interlocking
Experimental
Fig. 20. Estimated shear stresses in the interface and experimental shear stresses.
reinforcement is scaled down due to the interaction between bendqffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ing and tensile stresses in the rebars ( 1 k1 ), and due to the relpffiffiffiffiffiffiffiffiffiffiffiffiffi ative slip of the interface crack ( s=smax 6 1:0). This phenomenon is particularly important for surfaces with a higher roughness, where horizontal relative displacement leads to vertical displacement, causing the interface crack to open.
V F ðsÞ ¼ V F;max
pffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi2ffi Asi f y s=smax 1 k1 6 pffiffiffi 3
where V F;max ¼ k2 Asi
3
4
0.18 0.88 1.11 1.21
0.63 0.27 0.66 1.37 1.00
0.64 0.31 0.59 1.69 1.05
The coefficient k2 is taken as 1.5, fitting within the values prescribed in [2], without safety factor. The value for the resisting stress that results from dowel action, considering smax = 0.10Ø, are presented in Table 8. Contribution of the rebar crossing the interface is significant, accounting in the S-STANC solution for almost one-half of the horizontal load at the interface by dowel action of the rebar. For most of the other specimens, this mechanism accounted for around one third of the horizontal load at the interface. The remaining resisting mechanisms at the interface can then be estimated according to [2] with Eq. (7), and are presented in Table 9. along with the total value for the resisting strength of the interface.
2
Dowel action
S-STANC
ð6Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi f c;cube f y is the maximum allowable force for
dowel action; Asi is the area of the reinforcement crossing the interface; f c;cube is the concrete compressive strength in cubic specimens; f y is the steel yield stress; s is the relative slip of the layers at the interface; smax is the relative slip for V F;max , limited to 0.10Ø–0.20Ø.
v ilþfr ¼ cr f 1=3 cm þ l k1 qi f y
ð7Þ
An overestimation for the shear stress at the interface seems characteristic for the specimens with smaller anchoring of the reinforcement crossing the interface (S-STC-1, S-STC-3, S-ANC-2 and SANC-3, see Fig. 20). A good correlation with the model is found for the specimens with proper anchoring of the reinforcement crossing the interface (S-STC-4, S-ANC-4, and S-STANC, see Fig. 20). The anchorage length of the reinforcement crossing the interface shall be determined considering the resistance of the grout used in the hole, the total roughness of the surface, and the failure mechanisms of grouted anchors under shear and tension, according to [30]. 5. Conclusions Twelve flexural tests were performed on slab specimens strengthened with a new concrete overlay. Failure mode for each detailing of the interface was identified, along with several constraints to the application of this strengthening technique. Full brittle debonding of the new layer occurred for the reference specimens, attesting the importance of reinforcement crossing the
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interface. The smaller dispersion of results attests the constant behaviour of concrete elements strengthened with this technique, thus crediting its application in actual strengthening and retrofitting situations. Zones where the interface surface could be observed were identified with adhesive or cohesive failure. The latter could be observed generally in the substrate layer, where concrete was weakest. One constraint identified after testing was the insufficient anchorage length of the reinforcement crossing the interface in a first series of tests, which lead to a complementary experimental campaign. Insufficient bonding length in the existing layer penalized the resisting strength of the interface, leading to lower resistance and premature debonding. The embedment length of the steel connectors or anchoring of longitudinal rebar must account for the anchorage failure mechanisms and the maximum roughness of the interface. This parameter should also be controlled on site. When compared to the reference specimens, detailing with rebars crossing the interface reached a performance gain for each solution in terms of maximum shear stress at the interface. Rebar crossing the interface with greater anchorage length resulted in a performance gain of more than double the shear stress of the reference specimens. Even with insufficient anchorage length, the reinforcement crossing the interface resulted in a performance gain of 60–110% for the shear stress at the interface. For the solution with both steel connectors and longitudinal rebar anchored, the shear stress at the interface was three times that of the reference specimens. Provisions on the Model Code 2010 [23] for concrete-toconcrete interfaces, considering the proposed coefficients, fit well with the experimental results for specimens with proper anchoring of the reinforcement crossing the interface. This work, by simplifying the horizontal shear stress problem to one direction, allowed for a better understanding of the concreteto-concrete interaction phenomenon on a tensile face, and will allow for the development of two direction shear specimens, characteristic for punching strength on flat slabs. Acknowledgements This work was supported by Fundação para a Ciência e Tecnologia – Ministério da Ciência, Tecnologia e Ensino Superior through project EXPL/ECM–EST/1371/2013 and grant SFRH/ BD/89505/2012. Contributions of CONCREMAT SA and SIKA AG are also acknowledged for the production of concrete specimens and providing the bonding agent for anchoring the reinforcement crossing the interface. References [1] Münger F, Wicke M, Randl N. Design of shear transfer in concrete-concrete composite structures. IABSE reports; 1997. p. 163–8.
[2] Randl N. Design recommendations for interface shear transfer in fib Model Code 2010. Struct Concr 2013;14:230–41. [3] Koseki K, Breen J. Exploratory study of shear strength of joints for precast segmental bridges. Research report 248-1; 1983. [4] Millard S, Johnson R, Gambarova P, Karakog C. Shear transfer across cracks in reinforced concrete due to aggregate interlock and to dowel action. Mag Concr Res 1985;36:47–51. [5] Walraven J, Reinhardt H. Theory and experiments on the mechanical behavior of cracks in plain and reinforced concrete subjected to shear loading. Concr Mech 1981;26:65. [6] Okamura H, Maekawa K. Nonlinear analysis and constitutive models of reinforced concrete. Tokyo: Gihodo-Shuppan; 1991. [7] Vecchio F, Lai D. Crack shear-slip in reinforced concrete elements. J Adv Concr Technol 2004;2:289–300. [8] Birkeland P, Birkeland H. Connections in precast concrete construction. J Am Concr Inst 1966;63:345–67. [9] Mattock A, Hawkins N. Shear transfer in reinforced concreterecent research. J Prestress Concr Inst 1972;17:55–75. [10] Loov R, Patnaik A. Horizontal shear strength of composite concrete beams with rough interface. PCI J 1994;39:48–69. [11] Walraven J. Aggregate interlock: a theoretical and experimental analysis [Ph.D. thesis]. TU Delft; 1980. [12] Randl N. Investigations on load transfer between old and new concrete at different surface roughnesses [Ph.D. thesis]. University of Innsbruck; 1997. [13] Silfwerbrand J, Beushausen H, Courard L. Bonded cement based material overlays for the repair, the lining or the strengthening of slabs or pavements. Netherlands: Springer; 2011. p. 51–79. [14] Maekawa K, Qureshi J. Embedded bar behavior in concrete under combined axial pullout and transverse displacement. J JSCE, Divis E: Mater, Concr Struct Pavem 1996;30:183–95. [15] Bazˇant Z, Gambarova P. Crack shear in concrete: crack band microplane model. J Struct Eng 1985;110:2015–35. [16] Santos P, Júlio E. Interface shear transfer on composite concrete members. ACI Struct J 2014;111:113–21. [17] Santos P, Júlio E. A state-of-the-art review on roughness quantification methods for concrete surfaces. Constr Build Mater 2013;38:912–23. [18] Silfwerbrand J. Improving concrete bond in repaired bridge decks. Concr Int 1990;12:61–6. [19] Saucier F, Pigeon M. Durability of new-to-old concrete bonding. In: Proceedings of the ACI international conference on evaluation and rehabilitation of concrete structures and innovations in design; 1991. p. 689–707. [20] Abu-Tair A, Rigden S, Burley E. Testing the bond between repair materials and concrete substrate. ACI Mater J 1996;93:553–8. [21] Bissonnette B, Courard L, Beushausen H, Fowler D, Trevino M, Vaysburd A. Recommendations for the repair, the lining or the strengthening of concrete slabs or pavements with bonded cement-based material overlays. Mater Struct 2012;46:481–94. [22] Transportation Research Board. Removing concrete from bridges. NCHRP synthesis. Report 169, Washington, D.C.; 1991. p. 42. [23] fib Model Code for Concrete Structures 2010. Ernst & Sohn; 2013. 434p. [24] Emmons P. Concrete repair and maintenance. R. S. Means Company; 1994. 314p. [25] Bissonnette B, Courard L, Fowler D, Granju J. Bonded cement based material overlays for the repair, the lining or the strengthening of slabs or pavements. State-of-the-art report of the RILEM Technical Committee 193-RLS; 2011. [26] Bissonnette B, Vaysburd A, von Fay K. Best practices for preparing concrete surfaces prior to repairs and overlays. Report number MERL 12-17; 2012. [27] Lenz P. Beton-Beton-Verbund, Potenziale für Schubfugen [Ph.D. thesis]. Technische Universität München; 2012. [28] Santos P, Júlio E. A state-of-the-art review on shear-friction. Eng Struct 2012;45:435–48. [29] European Committee for Standardization. EN 1992-1-1: eurocode 2-design of concrete structures - part I: general rules and rules for buildings, Brussels; 2004. [30] Miltenberger P. Capacity design of grouted anchors. In: 16th international conference on structural mechanics in reactor technology, Washington, DC; 2001.
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Engineering Structures journal homepage: www.elsevier.com/locate/engstruct
Strengthening of RC slabs with reinforced concrete overlay on the tensile face Hugo Fernandes a,⇑, Válter Lúcio b, António Ramos b a b
FCT – Universidade NOVA de Lisboa, Portugal CERIS - ICIST, FCT – Universidade NOVA de Lisboa, Portugal
a r t i c l e
i n f o
Article history: Received 21 October 2015 Revised 19 August 2016 Accepted 10 October 2016
Keywords: Slab strengthening Concrete layer Reinforced concrete Steel connectors Interfacial bond
a b s t r a c t Strengthening of concrete structures with a new concrete layer has been commonly used for columns, beams and slabs. This technique is economic and efficient for structural strengthening since it uses the same base materials, steel and concrete. It is usually applied on the compressed face of the concrete element due to concrete’s recognized behaviour under compression, posing several challenges to control cracking and resistance when applied on the tensile face. For assessing the performance of the strengthening method, twelve slab specimens were designed and tested monotonically. The main parameters to assess in this work were the debonding behaviour and load, and the relationship between the latter and the relative displacements at the interface of the two concrete layers. The performance of the strengthened structures strongly relies on the interaction of the two concrete layers, with this being the main subject of the research about overlaid concrete. The load transfer capacity of the interface depends on the interface shear strength, which in turn is highly dependable on substrate roughness, cleanliness and curing conditions of the newly added layer. Interface performance may be improved by using steel connectors crossing the interface, properly anchored on both layers. The importance for these elements grows as the existing concrete is more deteriorated, since adhesion strength will decrease with lower quality concrete. This paper presents the experimental research for the application of bonded concrete overlays on the tensile face of reinforced concrete slabs, mainly aimed at office buildings and parking facilities, where spatial clearances or inaccessibility to the lower side of the slabs are recurrent. A ductile behaviour upon debonding was achieved for the specimens with reinforcement crossing the interface, and a debonding load up to three times that of the reference specimens without reinforcement crossing the interface. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Strengthening of concrete structures by adding a new concrete layer is well known when applied to the compressed face of concrete elements. Examples of that are beams and columns strengthened with concrete jacketing. Depending on the intended reason for strengthening/retrofitting the existing structure should be relieved of existing loads until strengthening is applied. This is more important with damaged or deteriorated structures since loading will further aggravate the existing structural condition. Bonded concrete overlay becomes a relevant strengthening and repairing technique since the base material has to be replaced with new concrete.
⇑ Corresponding author. E-mail address: [email protected] (H. Fernandes). http://dx.doi.org/10.1016/j.engstruct.2016.10.011 0141-0296/Ó 2016 Elsevier Ltd. All rights reserved.
When applied on the tensile face of concrete elements, bonding between two concrete layers is affected by the normal stresses that appear due to the difference in stiffness of the concrete layers. This technique relies on the quality of bond between the two concrete layers, therefore varying with surface preparation, and if steel connectors are installed crossing the interface. If no connectors are used, adhesion is the only component of the resisting mechanism acting on the interface, and brittle failure shall occur. This relies strongly on roughness, which allows for interlocking of the two layers and consequently bonding stresses to develop along the interface. With steel connectors crossing the interface between the two concrete layers, three components of the resisting mechanism illustrated in Fig. 1 shall develop [1]: 1. Adhesion, due to chemical bond between the two layers, and mechanical interlocking, if macroscopic surface roughness is present, shall be considered for slips up to 0.5 mm [2].
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Adhesion/Interlocking + Friction
541
Dowel Action
Fig. 1. Resisting mechanism for concrete-to-concrete interfaces with steel connectors adapted from [4].
2. Friction is a direct consequence of external loads perpendicular to the interface, or due to steel connectors crossing the interface that are mobilized in tension for higher relative displacements, usually larger than 0.5 mm [2], with equilibrium guaranteed by compressive forces at the interface. 3. Larger relative slips mobilize dowel action inducing bending, shear and tension in the steel connectors crossing the interface. The dowel action maximum value depends on the resistance of the steel connector (bending + tension and shear) and the crushing of the surrounding concrete [2,3]. Bond performance of the interface will depend on the appropriate conditions for these resisting mechanisms. Some authors provide recommendations for surface preparation and curing conditions of the added concrete layer. Contamination of the concrete surface before casting the new layer, methods used for surface preparation, and surface microcracking, are according to [2] the main quality parameters for the application of this technique. A special attention to edge zones is also referred to in the document since the discontinuity of the cross section allows for the development of significant tensile and shear stresses. Interface bond performance varies from null to full transfer of horizontal stresses between the two layers, allowing monolithic behaviour to be achieved, according to Fig. 2. Usually interaction between layers in the cross section is characterized as partial, and relative displacement between the two layers is accounted for. Several behaviour models have been developed over the years [5–7], where relative displacement, both horizontal and vertical, is comprised.
Stress transfer between the two concrete layers is based on the shear friction theory [8–12]. The behaviour models developed using the basis of this theory account for relative slip and interface crack opening or dilation, which are increased in an unconfined state. Normal and tangential relative displacements are of the utmost importance when tensile stresses govern the interface stress field. Models for relative displacement analysis and some models for stress analysis do not specifically account for steel connectors crossing the interface. This is a behaviour changing aspect for detailing the interface since it limits interface crack opening, providing for greater stresses to develop and reaching of larger slips. Shear connectors crossing the interface, properly anchored to both layers to improve strength, can reduce uncertainty about interface performance, and should be accounted for in the design of these interfaces [13]. Due to the orthogonal directions of interface deformations, behaviour of reinforcement crossing the interface will be ruled by pullout of the embedded length and dowel action of the steel bar. The relationship between the two phenomena has been identified and studied, since pullout can be affected by transverse shear, and dowel action in turn is affected by the pullout of the steel bar [14]. Another ruling aspect for these mechanisms to develop is aggregate interlock, which causes the interface crack to open as the two layers slip relatively to another [4,15]. The latter mechanism is highly dependable on the quality of the surface preparation chosen for the existing structure substratum. Surface preparation also plays a major role on interface behaviour due to the interlocking mechanism, which causes the interface crack to open when relative slip occurs [16]. There are several methods for surface preparation, as jackhammering,
Fig. 2. Interface performance regarding shear stress transfer between layers.
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recognized as being one of the most aggressive surface preparation methods, and hydro-demolition, one of the least aggressive methods, allowing also for selective concrete removal [17–20]. These methods are more or less intrusive in terms of microcracking of the existing concrete layer and can cause premature debonding of concrete chunks and reduced interface performance, reaching a depth of 3 mm [21] to 10 mm [22] on the existing concrete layer. Since a new concrete layer is to be cast against the existing one, a greater roughness is required, with exposed aggregate particles for improved interlocking. This is quantified in terms of an average roughness parameter, which can be assessed through several techniques. This value defines the interface in terms of roughness, according to [23], from smooth to very rough, with a specific profile geometry. For improved performance of the interface, a very rough profile is recommended, with an average roughness parameter equal or greater than 3.0 mm, since it directly improves all other components of the resisting mechanism. Chipping with an electric or pneumatic hammer and steel moil point, despite the disadvantages of microcracking, and hydro demolition with high-pressure water jet, are the most common surface preparation methods. The latter technique is the most efficient in terms of roughening and reduced microcracking of the surface, but also characterized by difficult logistics and higher costs. For this reason, chipping with an electric hammer and steel moil point is the easiest and least specialized method for surface roughening in a strengthening/retrofitting situation. Alongside the surface preparation techniques, it is also referred to in [24] the importance of moisture on the existing surface. An optimum combination of factors is presented for saturated substrate and apparently dry surface, for better bonding of the overlaid concrete. Values for adhesive tensile stress are provided in [18] for the steel moil point and the high-pressure water jet of 1.10 MPa and 1.46 MPa, respectively. Besides geometrical characteristics of the interface, performance of the strengthened structure also depends on materials of both substrate and the new layer to be overlaid. They directly influence both local and global behaviour, through adhesive capacity and crushing resistance of concrete, and through bending and shear resistance of the composite layers. According to [25], the newly added layer should be of greater resistance than the substrate and low shrinkage, and should be fluid enough to penetrate the grooves on the existing surface.
2. Experimental programme 2.1. Definition of the strengthened specimens For characterizing the behavioural aspects of concrete layers in a strengthening situation, three different detailings of the interface and one for reference, were tested. The reference specimens were tested with only surface roughening and no rebar crossing the interface. Relevant data regarding specimens geometry is provided in Table 1, where As,o and As,s are the longitudinal reinforcement area for the overlay and substratum, respectively, and As,i the area of the reinforcement crossing the interface. The experimental tests were comprised by the following specimens: Surface roughening only (S-REF). Steel connectors distributed along the interface, with 50 mm of anchorage length (S-STC, 1–3), and 70 mm of anchorage length (S-STC-4). Anchoring of the longitudinal reinforcement bars of the new layer at the ends, since the deformation of the specimens in bending suggests the lifting of the overlaid concrete in this zone [25], with 50 mm of anchorage length (S-ANC, 1–3) and 70 mm of anchorage length (S-ANC-4). All of the aforementioned techniques combined for improving bond at the interface (S-STANC). Also in the MC 2010 [23] the lifting phenomenon at the overlaid concrete edge is considered relevant since deformation due to shrinkage is greatest in this zone and debonding shall occur. Roughening of the substrate surface was accomplished for all specimens with an electric hammer and steel moil point. The slenderness of the slabs was always a limiting factor for the intended stress at the interface. Therefore, the solution that guaranteed structural integrity until debonding consisted on bundled longitudinal reinforcement and reduced contact area between the two layers. The slab substrates were then reinforced with 10 mm bundled longitudinal bars and reinforcement for the overlaid concrete layer consisted of six double 12 mm bars (Fig. 3). The contact area between the two layers was reduced transversally to increase bond stress. This layer also did not reach the supports for all specimens, since confinement of the rebar does not necessarily happen in a real strengthening situation. Surface
Table 1 Relevant specimen geometrical parameters. S-REF
Overlay
Thickness [m] Length [m] Width [m] As,o [–]
Interface
As,i [mm2]
Substrate
As,s [–] Width [m] Length [m] Thickness [m]
S-STC
S-ANC
S-STANC
0.07 1.00 0.60 2Ø12//0.10 m (22.62 cm2/m) –
336
678 2
2Ø10//0.10 m (15.7 cm /m) 1.00 2.30 0.12
Fig. 3. Rebar detailing and strain gauge placing for reference specimens (S-REF).
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preparation for casting the new layer was performed with an electric chipping hammer and steel moil point eighteen days after casting of the substrate concrete. The final surface can be observed in Fig. 4. Despite the disadvantages concerning microcracking of the existing surface, this method was chosen due to its practical and economical characteristics. Surface roughness was evaluated for each specimen, with concern that the moil tip should not go deeper than 10 mm. This protects the longitudinal rebar and the minimum concrete cover. Several methods for surface roughness assessment can be used [17], with the sand patch method (SPM) being the most widely referred in the literature and recommended in the MC 2010 [23]. Due to the availability of other methods, and with knowledge from [26] that this method is limited for very rough surfaces, another method by point measurement was used to evaluate the roughness profile, shown in Fig. 4. Since the SPM can only determine the average ‘‘peak-to-mean” roughness Rp [23], this parameter was calculated according to Eq. (1), where n is the number of peak pi to mean z profile measurements [17].
Rp ¼
n 1X jp zj n i¼1 i
ð1Þ
The resulting parameters for each of the specimens are presented in Table 2, where six assessment lengths (n = 6) were considered. Characterization of the surface roughness was performed according to [27], where a lower limit roughness is set at 3.0 mm for using the SPM (Rt,SPM), or 2.2 mm when considering the Rp parameter. Since the actual SPM was not performed the latter
parameter was considered, allowing for a surface characterization of ‘‘very rough” according to [27] and the classification provided on the MC 2010 [23]. The overlaid concrete was cast seventy-five days after casting the substrate concrete, with the rebars instrumented with strain gages, as well as the substrate rebars, at midspan and 0.25 m from this point. Interface detailing, with longitudinal rebars crossing the interface or steel connectors with anchorage lengths of 50 mm and 70 mm, can be observed in Figs. 5–7. Shear connectors particularly, which consisted on right angle rebars, were tied to the longitudinal reinforcement to prevent slipping of the reinforcement at the overlay anchorage portion. Bonding of the anchorages in the substrate layer was performed with SikaÒ Grout, rather than an epoxy solution, due to the economy and ease of application in a practical situation. 2.2. Materials characterization Material characterization was performed for the different concrete layers, rebar sizes, and grout used for anchoring the rebar crossing the interface. The mechanical properties for the grout were tested for the compressive and tensile strength, and with pullout tests for assessing bond strength. The parameters for this material were a flexural tensile strength of 9.7 MPa, a compressive strength of 78.8 MPa, and an average bond strength of 16.2 MPa. Pullout testing of unconfined rebar embedded in concrete holes with a diameter at least double that of the rebar allowed for determining the latter strength. Anchorage failure resulted in the pull out of the rebar sliding through the steel-grout interface. The test
Fig. 4. Substrate surface roughness assessment setup and detail.
Table 2 Average peak-to-mean roughness parameter. Rp [mm]
Reference (REF)
Steel connectors (STC)
Anchorage (ANC)
Steel connectors + Anchorage (STANC)
S-1 S-2 S-3 S-4
2.2 2.2 2.2 –
2.2 2.2 2.3 2.6
2.2 2.9 2.3 2.9
2.6
Fig. 5. Rebar detailing and strain gauge placing for S-ANC specimens.
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Fig. 6. Rebar detailing and strain gauge placing for S-STC specimens.
Fig. 7. Rebar detailing and strain gauge placing for S-STANC specimens.
results for the other materials can be observed below in Tables 3 and 4.
Table 4 Strength characteristics for the steel bars used.
2.3. Test setup The specimens were subjected to monotonic loading in a three point bending test, as shown in Fig. 8. Loading was imposed at midspan with hydraulic jacks, and reactions at two symmetrical supports, according to the figures below. Forces at the supports were measured with four TMLÒ CNC-200KNA load cells and the deformation at the coordinates identified in the left picture below, with TMLÒ CDP-100 displacement transducers. These tests were performed with prestressing strands at the supports. For assessing relative displacements between the two layers, TMLÒ CDP-50 displacement transducers were fixed on the substrate layer, as shown in Fig. 9. For data acquisition, four HBMÒ Spider8 datalogger units were used along with one HBMÒ UPM100 datalogger unit, all monitored by HBMÒ Catman V6.0 software. Loading was controlled by force with a WALTER+BAIÒ PKNS19D electronically controlled hydraulic pump, at a speed of 0.10 kN/s for all tests.
Diameter
Ø6
Ø 10
Ø 12
fy [MPa] fu [MPa]
541.0 692.7
530.6 627.5
532.1 627.4
fy – mean yield stress of steel. fu – mean tensile strength of steel.
the interface level for the reference specimens started from the ends of the overlaid concrete layer and evolved along the interface to midspan, according to Fig. 10. The phenomenon occurs when the tensile strength of the interface is reached, and subsequent failure of the substrate layer (Fig. 10, right, flexural failure). For specimens with longitudinal rebar anchored on the substrate layer, beginning of interface cracking was similar to the reference specimens, occurring again at the overlaid concrete ends. The debonding load was very close to the failure load on these specimens. One specimen failed in shear (S-ANC-1), as shown in Fig. 11, with the other two specimens resulting in flexural failure of the substrate layer rebar. Some specimens with smaller anchorage length (50 mm), had failure controlled by pullout of the rebars anchored in the substrate, as shown in Fig. 12. For specimens with steel connectors distributed along the interface, cracking was similar to anchored rebar ones, with two specimens failing in shear (see Fig. 13). There was also no visible full debonding of the overlaid concrete, with failure occurring for the two layers resisting the load.
3. Experimental results 3.1. Failure modes Behaviour was consistent for all specimens in terms of cracking, debonding of the overlaid concrete, and failure load. Cracking at
Table 3 Concrete strength characteristics for all specimens. Reference (S-REF, 1–3)
Steel connectors (S-STC, 1–3)
Anchorage (S-ANC, 1–3)
Steel connectors (S-STC, 4)
Anchorage (S-ANC, 4)
Steel connectors + Anchorage (S-STANC)
fc,cube [MPa]
Substrate Overlay
45.3 47.8
40.6 47.8
37.4 47.8
55.5 41.1
54.8 39.4
56.4 41.5
fct [MPa]
Substrate Overlay
2.8 2.9
2.5 2.9
2.3 2.9
3.9 3.1
3.9 3.0
4.0 3.1
fc,cube – mean value for the compressive strength of concrete in cubic specimens. fct – mean value for the tensile splitting strength of concrete.
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Fig. 8. General test setup.
Fig. 9. Relative displacement measurement setup.
Fig. 10. Interface failure for reference specimens (S-REF-1).
Fig. 11. Shear failure for S-ANC-1 specimen.
Anchorage failure occurred for some specimens with pullout at the steel-grout interface visible through the interface crack right after testing. Through a longitudinal sectioning of the specimens, the evolution of the shear crack for the specimen that failed in bending can be observed. The insufficient anchorage length caused the shear crack to fail intercepting the steel connectors (see Fig. 14). All three specimens with a larger anchorage length of rebars crossing the interface (70 mm) failed in shear, as visible in Fig. 15. This attests the increase in performance when reinforcement crosses the interface, properly anchored, achieving an almost monolithic behaviour until failure.
load and deflection at midspan, comparing specimens with reinforcement crossing the interface and reference ones. The marker on the load-deflection curves illustrates the moment when the highest strain was registered for the overlaid concrete rebars. The load-deflection curves above show an approximately linear behaviour for all tests up to a load of 40 kN at midspan, followed by a reduction in stiffness. This was consistent with the cracking load for the geometry of the cross section. For the reference specimens, debonding of the overlaid concrete occurred for a load of 80 kN at midspan. Stiffness was reduced to approximately zero, observed graphically by an horizontal plateau. This was followed by reloading of the substrate layer, until flexural failure occurs for a load of 160 kN at midspan. For the specimens with longitudinal rebar anchored 50 mm in the substrate (Fig. 16, left), the failure load was about the same as for the reference specimens. The debonding load of 142 kN represents an increase of 79% when compared to reference specimens.
3.2. Debonding and failure loads The relationship between load and deflection at midspan was analysed to characterize the behaviour of the strengthened specimens. Figs. 16 and 17 present the relationship between vertical
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Fig. 12. Failure for specimens with small anchorage length of the longitudinal rebars.
Fig. 13. Shear failure for specimens with steel connectors (S-STC-1).
For specimens with steel connectors anchored 50 mm in the substrate (Fig. 16, right), debonding load increased about the same as the latter (76%), with the resulting failure load 10% higher than the reference tests. Debonding phenomenon was not visible during testing, and only identified through the strain measurement at the longitudinal rebars. The three specimens with 70 mm anchorage length differed from the first set of tests mainly due to greater debonding and ultimate loads. The relationships between vertical load and deflection at midspan for these specimens are presented in Fig. 17. From the load-deflection curves, one can observe that the increase in anchorage length also increased debonding and failure loads. The latter showed an increase of 39% for the anchored longitudinal rebar specimens, 27% for the slabs with steel connectors, and 43% for all techniques combined. Also noticeable is the small increase for the failure load of the specimen that combined all techniques (S-STANC), when compared to anchored longitudinal rebar tests (S-ANC). This attests the impact of the latter in the overall behaviour of the interface, due to longitudinal rebars with higher diameter and anchorage positioning at the overlaid concrete
ends. The limitation on the evolution of both interface crack opening and relative slip results in smaller relative displacements that reach the steel connectors, thus reducing its contribution to the resisting strength of the interface. However, these are activated later in the loading history, with smaller stresses. Table 5 lists the failure loads for all strengthened specimens, and the load for which maximum strain was reached in the overlaid concrete rebars, which is also the load for maximum stress at the interface. Failure modes are identified as shear of the substrate layer (Ss) or flexural failure of the substrate rebar (Fs). The vertical and horizontal relative displacements at the overlay end are summarized in Table 6 for each detailing of the interface. Regarding the specimens where the overlaid concrete fully debonded, the displacement values were considered for maximum steel strain at the overlay rebars. For the specimens without strain gauge information, this value was estimated based on the irrespective load-deflection curve, comparing with instrumented specimens with the same detailing of the interface. Highlighted values refer to the highest values of relative displacement registered for specimens where failure occurred by shear of the substrate layer. The results also show higher crack dilation and interface slip for the higher failure loads of specimens S-STC-4, S-ANC-4, and S-STANC, denoting the mobilization of the dowel action of the reinforcement crossing the interface with proper anchoring in the substrate (see Table 6).
4. Discussion The provisions on concrete-to-concrete interface resistance presented in the Model Code 2010 [23] were considered for behaviour characterization and quantification of the components of the resisting mechanism, aggregate interlock, friction, and dowel action. Shear stresses at the interface are a consequence of the variation of force DFso along the longitudinal rebar of the overlaid concrete, that are transferred to the substrate layer. These stresses are responsible for the integrity of the composite section, since failure
Fig. 14. Steel connectors anchorage detail and section for the S-STC specimens.
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Fig. 15. Shear failure for specimens with larger anchorage length of rebars crossing the interface (left, S-ANC-4; centre, S-STC-4; right, S-STANC).
Deflection at midspan [mm]
Deflection at midspan [mm]
Fig. 16. Load-deflection curves for specimens w/reinforcement crossing the interface anchored 50 mm.
highest strain registered for the overlay concrete rebars.
forces leads to an average shear stress mi at the interface level. This is proportional to the variation of forces in length of the overlaid concrete rebars, according to Eq. (2), where b is the width of the interface.
vi ¼
Fig. 17. Load-deflection curves for specimens w/reinforcement crossing the interface anchored 70 mm. highest strain registered for the overlay concrete rebars.
of the interface can cause failure of the strengthened element. Shear stress can be evaluated in terms of the force transferred between the two concrete layers over its interface. In a concrete element of length Dl between cracked sections, equilibrium of
DF so Dl b
ð2Þ
The strain gages installed in the longitudinal rebars provided a linear distribution of steel strains, which is consistent with a uniform distribution of shear stresses across the interface. The values for the steel strains at midspan (es) and corresponding values for shear stress at the interface (mi) can be observed in Table 7. These values are the maximum strains registered for the longitudinal rebars at the overlaid concrete. Only two specimens of each detailing were instrumented with strain gages at the rebars. Evaluation of the shear resistance at the interface was carried out according to Randl [2] and the MC 2010 [23]. The resisting mechanisms at the interface are divided in three main components: aggregate interlock, friction, and dowel action of the reinforcement crossing the interface. The low stiffness of slabs results in large vertical deflections and large horizontal displacements. In [2] a distinction is made between stiff and more brittle behaviour of the interface in terms of the relative slip s. This limit is set at 0.05 mm, governed by
Table 5 Maximum strain at the overlaid concrete rebars and failure load at midspan. Specimen
Values for maximum strain at the overlay rebars Failure load at midspan [kN] Failure mode
S-REF
Midspan load [kN] Overlay rebars Fso [kN] Substrate rebars Fs [kN]
S-STC
S-ANC
S-STANC
1
2
3
1
2
3
4
1
2
3
4
82.4 138.9 90.8
– – –
80.1 141.3 88.6
140.3 218.7 179.7
– – –
144.3 226.5 175.9
200.3 375.9 408.1
– – –
150.0 305.5 153.6
140.0 273.6 137.3
190.3 408.2 272.7
214.7 483.4 235.2
163.1
155.3
160.8
177.2
171.2
180.1
201.8
151.1
153.6
157.3
221.2
227.7
Fs
Fs
Fs
Ss
Fs
Ss
Ss
Ss
Fs
Fs
Ss
Ss
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Table 6 Relative displacements at the interface end. S-REF
Load at midspan [kN] Crack dilation [mm] Interface slip [mm]
S-STC
S-ANC
S-STANC
1
2
3
1
2
3
4
1
2
3
4
82.4 1.52 0.58
79.6 0.87 0.39
80.1 1.42 0.57
177.2 4.11 1.45
148.1 2.89 1.02
180.1 4.33 1.43
201.8 4.98 2.38
151.1 2.33 0.35
150.0 2.06 0.51
140.0 2.51 0.49
221.2 5.46 2.49
227.7 4.18 2.21
Table 7 Overlay steel strains and shear stress at the interface. Specimen
es [106] mi [MPa]
S-REF
S-STC
S-ANC
S-STANC
1
3
1
3
4
2
3
4
–
511.7 0.46
520.6 0.47
805.7 0.73
834.5 0.76
1384.9 1.25
1125.5 1.02
1008.0 0.91
1503.9 1.36
1780.9 1.61
aggregate interlock and friction from external forces. For slips over this limit, the behaviour is considered more ductile, with adhesion replaced by friction/interlocking and dowel action. The following equations taken from [2] quantify both these scenarios, without accounting for external actions that are favourable to the resisting mechanism:
s 6 0:05 mm;
v R;ad ¼ ca f ctd 6 0:5 m f cd
s P 0:05 mm;
v R;ilþfrþdow ¼ cr f 1=3 ck þ l qi k1 f yd þ k2 qi
qffiffiffiffiffiffiffiffiffiffiffiffiffi f yd f cd 6 bc
Fig. 18. Vertical tensile stresses at the end of the interface.
ð3Þ
m f cd
ð4Þ
where
vR,ad is the design value for the adhesive shear stress for the interface; ca ; cr are the coefficients for surface condition; f ctd is the design value for concrete tensile strength; m is the reduction coefficient for compressive forces
(m ¼ 0:55 ð30=f ck Þ 6 0.55); f cd is the design value for concrete compressive strength; vR,il+fr+dow is the design value for shear stress of the interface for interlocking, friction, and dowel action; f ck is the characteristic compressive strength for concrete; l is a friction coefficient; qi is the ratio of reinforcement crossing the interface (qi ¼ As;i =Ai ); k1 is the latter reinforcement performance reduction factor (k1 ¼ rs;i =f y 6 1.0) where rs,i is the actual tensile stress in the steel crossing the concrete interface; f yd is the design value for steel yielding stress; k2 is the interaction coefficient for flexural resistance of the rebar (61.6 for circular cross-sections and C20/25 - C50/60); bc is the coefficient that accounts for the strut inclination of concrete in compression. 1=3
An adjustment is proposed for some coefficients in an analysis situation that fits better with test results. Coefficient cr is proposed as 0.2 for the very rough surfaces of the present test specimens which is lower than recommended in [2,23] when considering mean values. One should notice that for the present case the interface comprises perpendicular tensile stresses, whereas the cr values according to [2,23] refer to non-tensioned interfaces only. The related shear strength due to aggregate interlock for the reference specimens thus resulted in 0.66 MPa, calculated considering the lower concrete compressive strength of the two concretes. This value overestimates the test results of about 0.19 MPa for the two reference slabs in Table 7. The microcracking due to the surface
preparation and the tensile stresses that result from equilibrium at the end of the interface (Fig. 18) can justify this behaviour, debonding prematurely for a brittle resisting mechanism. The coefficient of friction l was calculated as proposed in [23] and also in [28] for very rough interfaces. Considering the roughness classification presented in Section 2.1, the friction coefficient takes the value of 1.4. For the coefficient k1 a new method is proposed, that contemplates the amount of horizontal force at the interface not resisted by dowel action of the rebar. This shear force at the interface (kFDFso) is accounted for in the quantification of the steel stresses for the rebars crossing the interface, by means of a coefficient ‘kF’ as follows:
rsi ¼
F s;c ðkF DF so Þ tan h kF ti tan h ¼ ¼ Asi Asi qsi
ð5Þ
where Fs,c is the tensile force in the steel connectors or longitudinal rebar anchorage and qsi is the reinforcement ratio crossing the interface (Asi/Ai). h is the angle between the interface plane and the concrete strut that results from nodal equilibrium, as illustrated in Fig. 19. The values for the coefficient kF were determined according to the amount of stresses resisted by dowel action of the rebar crossing the interface. Around 70% of the total horizontal load at the interface for the steel connectors was determined for the aggregate interlock and friction resisting mechanisms (kF = 0.7). The remaining stresses were then resisted by dowel action of the rebars. For the specimens with longitudinal rebar anchored, the value kF was reduced in half (kF = 0.35) empirically in good approximation to the test results, since the edge lifting phenomenon is present, thus resulting in tension of the anchored rebars. An angle h of 21.8° was considered, with good correlation to test results, which is also the lower bound for the angle of concrete struts according to [29]. The considered values for these parameters are a tentative approach accounting for the rather complex situation at the considered overlay edges subject to strong delamination and shall be confirmed with further test results. Dowel action resistance alone can be calculated according to [2] with Eq. (6), where maximum allowable dowel action of the
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Fig. 19. Resisting mechanism of the overlay rebars anchorage (left) and shear connectors (right). Table 8 Dowel action resistance. Specimen
S-STC 1
Asi [mm2] s [mm] k1 [–] VF [kN] mF [MPa] mF/mi [–]
S-ANC
3
336 0.48 0.34 62.5 0.21 0.29
0.53 0.35 62.2 0.21 0.27
4
2
2.38 0.59 60.6 0.20 0.16
678 0.51 0.24 91.1 0.30 0.30
S-STANC
3 0.49 0.21 91.7 0.31 0.34
4
(Ø6)
(Ø12) 678
1.32 0.32 140.1 0.47 0.34
336 1.69 0.25 72.7 0.73 0.45
146.7
Table 9 Shear stress for friction and interlocking of protruding aggregates. Specimen
S-STC 1
mil [MPa] mfr [MPa] mfr+il/mi [–] mfr+il+F [MPa] mfr+il+F/mi [–]
0.64 0.29 1.27 1.13 1.55
S-ANC
3
4
2
0.30 1.24 1.14 1.51
0.64 0.49 0.90 1.33 1.06
0.62 0.20 0.81 1.12 1.10
Shear stress at the interface [MPa]
1.6 1.2 0.8 0.4 0 S-STC-1 S-STC-3 S-STC-4 S-ANC-2 S-ANC-3 S-ANC-4 S-STANC Friction
Interlocking
Experimental
Fig. 20. Estimated shear stresses in the interface and experimental shear stresses.
reinforcement is scaled down due to the interaction between bendqffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ing and tensile stresses in the rebars ( 1 k1 ), and due to the relpffiffiffiffiffiffiffiffiffiffiffiffiffi ative slip of the interface crack ( s=smax 6 1:0). This phenomenon is particularly important for surfaces with a higher roughness, where horizontal relative displacement leads to vertical displacement, causing the interface crack to open.
V F ðsÞ ¼ V F;max
pffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi2ffi Asi f y s=smax 1 k1 6 pffiffiffi 3
where V F;max ¼ k2 Asi
3
4
0.18 0.88 1.11 1.21
0.63 0.27 0.66 1.37 1.00
0.64 0.31 0.59 1.69 1.05
The coefficient k2 is taken as 1.5, fitting within the values prescribed in [2], without safety factor. The value for the resisting stress that results from dowel action, considering smax = 0.10Ø, are presented in Table 8. Contribution of the rebar crossing the interface is significant, accounting in the S-STANC solution for almost one-half of the horizontal load at the interface by dowel action of the rebar. For most of the other specimens, this mechanism accounted for around one third of the horizontal load at the interface. The remaining resisting mechanisms at the interface can then be estimated according to [2] with Eq. (7), and are presented in Table 9. along with the total value for the resisting strength of the interface.
2
Dowel action
S-STANC
ð6Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi f c;cube f y is the maximum allowable force for
dowel action; Asi is the area of the reinforcement crossing the interface; f c;cube is the concrete compressive strength in cubic specimens; f y is the steel yield stress; s is the relative slip of the layers at the interface; smax is the relative slip for V F;max , limited to 0.10Ø–0.20Ø.
v ilþfr ¼ cr f 1=3 cm þ l k1 qi f y
ð7Þ
An overestimation for the shear stress at the interface seems characteristic for the specimens with smaller anchoring of the reinforcement crossing the interface (S-STC-1, S-STC-3, S-ANC-2 and SANC-3, see Fig. 20). A good correlation with the model is found for the specimens with proper anchoring of the reinforcement crossing the interface (S-STC-4, S-ANC-4, and S-STANC, see Fig. 20). The anchorage length of the reinforcement crossing the interface shall be determined considering the resistance of the grout used in the hole, the total roughness of the surface, and the failure mechanisms of grouted anchors under shear and tension, according to [30]. 5. Conclusions Twelve flexural tests were performed on slab specimens strengthened with a new concrete overlay. Failure mode for each detailing of the interface was identified, along with several constraints to the application of this strengthening technique. Full brittle debonding of the new layer occurred for the reference specimens, attesting the importance of reinforcement crossing the
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interface. The smaller dispersion of results attests the constant behaviour of concrete elements strengthened with this technique, thus crediting its application in actual strengthening and retrofitting situations. Zones where the interface surface could be observed were identified with adhesive or cohesive failure. The latter could be observed generally in the substrate layer, where concrete was weakest. One constraint identified after testing was the insufficient anchorage length of the reinforcement crossing the interface in a first series of tests, which lead to a complementary experimental campaign. Insufficient bonding length in the existing layer penalized the resisting strength of the interface, leading to lower resistance and premature debonding. The embedment length of the steel connectors or anchoring of longitudinal rebar must account for the anchorage failure mechanisms and the maximum roughness of the interface. This parameter should also be controlled on site. When compared to the reference specimens, detailing with rebars crossing the interface reached a performance gain for each solution in terms of maximum shear stress at the interface. Rebar crossing the interface with greater anchorage length resulted in a performance gain of more than double the shear stress of the reference specimens. Even with insufficient anchorage length, the reinforcement crossing the interface resulted in a performance gain of 60–110% for the shear stress at the interface. For the solution with both steel connectors and longitudinal rebar anchored, the shear stress at the interface was three times that of the reference specimens. Provisions on the Model Code 2010 [23] for concrete-toconcrete interfaces, considering the proposed coefficients, fit well with the experimental results for specimens with proper anchoring of the reinforcement crossing the interface. This work, by simplifying the horizontal shear stress problem to one direction, allowed for a better understanding of the concreteto-concrete interaction phenomenon on a tensile face, and will allow for the development of two direction shear specimens, characteristic for punching strength on flat slabs. Acknowledgements This work was supported by Fundação para a Ciência e Tecnologia – Ministério da Ciência, Tecnologia e Ensino Superior through project EXPL/ECM–EST/1371/2013 and grant SFRH/ BD/89505/2012. Contributions of CONCREMAT SA and SIKA AG are also acknowledged for the production of concrete specimens and providing the bonding agent for anchoring the reinforcement crossing the interface. References [1] Münger F, Wicke M, Randl N. Design of shear transfer in concrete-concrete composite structures. IABSE reports; 1997. p. 163–8.
[2] Randl N. Design recommendations for interface shear transfer in fib Model Code 2010. Struct Concr 2013;14:230–41. [3] Koseki K, Breen J. Exploratory study of shear strength of joints for precast segmental bridges. Research report 248-1; 1983. [4] Millard S, Johnson R, Gambarova P, Karakog C. Shear transfer across cracks in reinforced concrete due to aggregate interlock and to dowel action. Mag Concr Res 1985;36:47–51. [5] Walraven J, Reinhardt H. Theory and experiments on the mechanical behavior of cracks in plain and reinforced concrete subjected to shear loading. Concr Mech 1981;26:65. [6] Okamura H, Maekawa K. Nonlinear analysis and constitutive models of reinforced concrete. Tokyo: Gihodo-Shuppan; 1991. [7] Vecchio F, Lai D. Crack shear-slip in reinforced concrete elements. J Adv Concr Technol 2004;2:289–300. [8] Birkeland P, Birkeland H. Connections in precast concrete construction. J Am Concr Inst 1966;63:345–67. [9] Mattock A, Hawkins N. Shear transfer in reinforced concreterecent research. J Prestress Concr Inst 1972;17:55–75. [10] Loov R, Patnaik A. Horizontal shear strength of composite concrete beams with rough interface. PCI J 1994;39:48–69. [11] Walraven J. Aggregate interlock: a theoretical and experimental analysis [Ph.D. thesis]. TU Delft; 1980. [12] Randl N. Investigations on load transfer between old and new concrete at different surface roughnesses [Ph.D. thesis]. University of Innsbruck; 1997. [13] Silfwerbrand J, Beushausen H, Courard L. Bonded cement based material overlays for the repair, the lining or the strengthening of slabs or pavements. Netherlands: Springer; 2011. p. 51–79. [14] Maekawa K, Qureshi J. Embedded bar behavior in concrete under combined axial pullout and transverse displacement. J JSCE, Divis E: Mater, Concr Struct Pavem 1996;30:183–95. [15] Bazˇant Z, Gambarova P. Crack shear in concrete: crack band microplane model. J Struct Eng 1985;110:2015–35. [16] Santos P, Júlio E. Interface shear transfer on composite concrete members. ACI Struct J 2014;111:113–21. [17] Santos P, Júlio E. A state-of-the-art review on roughness quantification methods for concrete surfaces. Constr Build Mater 2013;38:912–23. [18] Silfwerbrand J. Improving concrete bond in repaired bridge decks. Concr Int 1990;12:61–6. [19] Saucier F, Pigeon M. Durability of new-to-old concrete bonding. In: Proceedings of the ACI international conference on evaluation and rehabilitation of concrete structures and innovations in design; 1991. p. 689–707. [20] Abu-Tair A, Rigden S, Burley E. Testing the bond between repair materials and concrete substrate. ACI Mater J 1996;93:553–8. [21] Bissonnette B, Courard L, Beushausen H, Fowler D, Trevino M, Vaysburd A. Recommendations for the repair, the lining or the strengthening of concrete slabs or pavements with bonded cement-based material overlays. Mater Struct 2012;46:481–94. [22] Transportation Research Board. Removing concrete from bridges. NCHRP synthesis. Report 169, Washington, D.C.; 1991. p. 42. [23] fib Model Code for Concrete Structures 2010. Ernst & Sohn; 2013. 434p. [24] Emmons P. Concrete repair and maintenance. R. S. Means Company; 1994. 314p. [25] Bissonnette B, Courard L, Fowler D, Granju J. Bonded cement based material overlays for the repair, the lining or the strengthening of slabs or pavements. State-of-the-art report of the RILEM Technical Committee 193-RLS; 2011. [26] Bissonnette B, Vaysburd A, von Fay K. Best practices for preparing concrete surfaces prior to repairs and overlays. Report number MERL 12-17; 2012. [27] Lenz P. Beton-Beton-Verbund, Potenziale für Schubfugen [Ph.D. thesis]. Technische Universität München; 2012. [28] Santos P, Júlio E. A state-of-the-art review on shear-friction. Eng Struct 2012;45:435–48. [29] European Committee for Standardization. EN 1992-1-1: eurocode 2-design of concrete structures - part I: general rules and rules for buildings, Brussels; 2004. [30] Miltenberger P. Capacity design of grouted anchors. In: 16th international conference on structural mechanics in reactor technology, Washington, DC; 2001.