Experimental Investigation of the Effect of Compressive Interface Stress on Interfaces in Reinforced Concrete Elements under Cyclic Action

Palieraki, Vasiliki; Zeris, Christos; Vintzileou, Elizabeth

doi:10.3390/app14114350

Open AccessArticle

Experimental Investigation of the Effect of Compressive Interface Stress on Interfaces in Reinforced Concrete Elements under Cyclic Action

by

Vasiliki Palieraki

^*,

Christos Zeris

and

Elizabeth Vintzileou

Laboratory of Reinforced Concrete, Faculty of Civil Engineering, National Technical University of Athens, Iroon Polytechniou 5, Zografou, 157 73 Athens, Greece

^*

Author to whom correspondence should be addressed.

Appl. Sci. 2024, 14(11), 4350; https://0-doi-org.brum.beds.ac.uk/10.3390/app14114350

Submission received: 15 April 2024 / Revised: 10 May 2024 / Accepted: 17 May 2024 / Published: 21 May 2024

(This article belongs to the Special Issue Advances in Building Materials and Concrete, 2nd Edition)

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Abstract

:

Reinforced concrete interfaces, either cracks within monolithic elements or joints between concretes cast at different times may become critical under cyclic actions, due to stiffness and interface resistance degradation. Among the numerous parameters affecting the behavior of interfaces, this paper focuses on the effect of externally applied compressive stress. In conjunction with this parameter, the diameter of the reinforcing bars crossing the interface, their embedment length, and the anchorage of the interface reinforcement, by bond or using epoxy resin, are investigated. Roughened concrete interfaces crossed by reinforcing bars were subjected to cyclic shear slips, with or without compressive stress normal to the interface. The presented experimental results prove the beneficial effect of the external compressive stress on the ultimate shear resistance of interfaces, accompanied by the reduction of the effect of small embedment length of the interface reinforcement, due to its reduced contribution: the externally imposed compression leads to smaller crack openings at the interface, in most cases smaller than 0.40 mm, and to reduction of the reinforcement clamping effect. The shear resistance is activated at reduced shear slip values (0.20 mm–0.40 mm compared to 0.20–0.80 mm for interfaces under zero external compression), while the interface resistance degradation is also reduced (e.g., during the second load cycle, to 15% on average, compared to 30% for interfaces under zero external compression). Finally, an equation previously proposed by the authors is applied for the prediction of the shear resistance of interfaces under normal force, leading to satisfying accuracy.

Keywords:

interface; shear friction; dowel action; experimental investigation; normal compressive stress

1. Introduction

Interfaces between concretes cast at different times are common in various repair and/or strengthening techniques, where a new, usually reinforced, concrete layer, is cast in contact with the existing structure member (e.g., the addition of a layer in the tensioned zone of a beam for flexural strengthening, the construction of an RC jacket to a column, etc.) or a new reinforced concrete element (e.g., an RC wall within an existing RC frame, etc.). The connection of the existing structure with the added elements, i.e., the behavior and the design of interfaces, is crucial for the effectiveness of the intervention technique and to reach the target of monolithic behavior of the resulting composite element. The interface overall behavior and resistance depend on the reinforcement ratio and type (reinforcing bars or industrial anchors), on the roughness of the surface of the old element (as-cast or intentionally roughened to various degrees), on the use of chemical products for the enhancement of the cohesion between the two concretes, and on the restrain of the interface, due to external compression. The design of the interfaces is decisive for the global behavior of the structures when they are subjected to cyclic actions, resulting in the degradation of the interface resistance.

One of the specific characteristics of interfaces between concretes cast at different times is the frequently insufficient embedment length of the interface reinforcement due to geometrical constraints of the connected elements. This may also be the case in cracks within monolithic RC elements (natural or load-induced cracks), especially in sub-standard structures or in cracks occurring close to supports.

In recognition of the significance of interfaces, numerous researchers have undertaken relevant experimental campaigns. The behavior of various types of interfaces, namely monolithic elements cracked before being subjected to shear, cold joints formed during construction, and connections between old and new concrete or between precast elements, has been experimentally studied. Available experimental results published in fifty-eight (58) papers were collected and compiled in a comprehensive database. A review of this database is presented elsewhere [1]. However, with the emphasis of this work being on the influence of external compressive stress on interfaces between concretes cast at different times and subjected to cyclic shear slip, a brief overview of the published experimental results investigating this parameter is initially provided herein.

The literature is rich in tests that experimentally investigate the behavior of unreinforced interfaces between concretes cast at different times, where an external compressive stress of varying amplitude is applied [2,3,4,5,6,7,8,9]. In that case, once a crack is formed along the interface, only the friction mechanism (due to the external compression) is activated. The overall behavior of the unreinforced interfaces differs significantly from that of reinforced interfaces. Indeed, in the latter, friction is due to the gradual mobilization of the interface reinforcement, while dowel action also contributes to the interface resistance. Thus, publications referring to the shear behavior of unreinforced interfaces are not reviewed herein.

External compressive stress may be due to the self-weight of the structure (in horizontal interfaces), the confinement provided, e.g., by the hoops of an RC jacket to a column (along the vertical interfaces between the original column and the jacket), etc. The effect of normal compressive stress on the behavior of reinforced interfaces between concretes cast at different times was investigated in several experimental campaigns; basic information on the published tests is summarized in Table 1.

Table 1. Basic information on tests from the literature involving external normal stress.

Paper	Number of Tests	Interface Roughness ⁽¹⁾	Dimen. of the Interface A_c (mm²)	Number of Bars, n	Bar Diameter d_b (mm)	Reinf. Ratio, ρ ⁽²⁾ (%)	Yield Strength of Steel Reinforcement (MPa)	Type of Anchor. ⁽³⁾/ Embedment Depth ⁽⁴⁾	σ_N,ext (MPa)
[5]	36 Mon.	Smooth, disk sander (12) Rough, special hammer (24)	100 × 170 (24) 100 × 240 (12)	0 (11) 1 (14) 2 (11)	No bars (11) 8 (25)	0.00 (11) 0.21 (4) 0.42 (4) 0.30 (10) 0.59 (7)	568.0	By bond/12.5 d_b (25)	0.5 (3) 1.0 (30) 2.0 (3)
[10]	12 Mon.	Smooth, as-cast	300 × 300	18	6	0.57	250.0	Closed hoops/Not reported	0.0 (3) 0.5 (3) 1.0 (3) 1.5 (3)
[11]	10 Mon.	Smooth (1) Shear keys (2) Saw teeth (2) Single Shear Key (5)	230 × 914.4	4 (8) 8 (2)	12.7 (9) 15.9 (1)	0.12 (8) 0.24 (1) 0.37 (1)	453.0 (5) 425.4 (4) 496.4 (1)	By bond/ 18.0 d_b (1), 22.5 d_b (9)	1.7 (5) 0.12 (2) 0.70 (2) 2.1 (1)
[12]	3 Mon. 8 Cyclic	Smooth, as-cast (10) Shear keys (1)	152 × 1200	2	12.7 (2) 15.8 (3) 25.4 (6)	0.14 (2) 0.21 (3) 0.56 (6)	1861.7 (2) 1082.5 (3) 413.4 (6)	By bond/47.2 d_b (2), 38.0 d_b (3), 23.6 d_b (6)	2.0
[13]	1 Mon. 4 Cyclic	Smooth, as-cast	150 × 1200	2	12.7 (1) 15.8 (2) 25.4 (1)	0.14 (1) 0.21 (2) 0.56 (1)	1861.7 (1) 1082.5 (2) 413.4 (1)	By bond/70.9 d_b (1), 57.0 d_b (2), 35.4 d_b (1)	2.0
[4]	2 Mon. 14 Cyclic	Rough, Sandblasting	101.6 × 812.8	0 (4) 3 (8) 6 (4)	No bars (4) 19 (12)	0.00 (4) 1.03 (8) 2.06 (4)	413.4	Adhesive mortar/ 8 d_b (12)	0.0 (6) 2.4 (1) 6.9 (8) 10.3 (1)
[14]	8 Cyclic	Very smooth, grease (3) Shear keys (5)	225 × 860	2	22	0.39	380.0	By bond/29.6 d_b	0.0 (3) 7.8 (3) −1.1 (2)
[15]	6 Mon. 2 Cyclic	Steel element, smooth	254 × 711	4	19	0.31	413.4	Welded- Headed Studs/6.7 d_b	0.0 (1) −0.35 (1)
[16]	8 Cyclic	Smooth	200 × 1700	5	16	0.30	401.0	Adhesive mortar/ 10.0 d_b	0.0 (4) 0.62 (2) 1.12 (2)
[17]	2 Mon. 11 Cyclic	Rough, exposed aggregates	90 × 400 (2) 120 × 400 (11)	8	6 (2) 8 (11)	0.47 (2) 0.84 (9) 1.12 (2)	521.0 (2) 546.0 (11)	By bond/ 33.3 d_b (2), 25.0 d_b (11)	4.30 ÷ 10.0

⁽¹⁾ Columns 3–10: The numbers in parenthesis indicate the number of test specimens per parameter. ⁽²⁾ Reinforcing ratio: ρ = (n*π*d_b²/4)/A_c, where n = number of bars crossing the interface, d_b = diameter of bars crossing the interface, and A_c = interface cross-section (for explanation, see Figure 1). ⁽³⁾ Anchorage type refers to the method of anchoring, namely (a) plain concrete bond, (b) use of adhesive resin mortar, (c) use of headed stud, and (d) closed hoops. ⁽⁴⁾ Embedment length of the reinforcement is reported in d_b; the shorter of the two embedment lengths of interface reinforcement is reported, as this governs the behavior of the interface.

Figure 1. (a) Geometry of the specimens with three cast-in bars crossing the interface. (b) Geometry of the specimens with three post-installed bars crossing the interface (dimensions in meters). Photo of the artificially roughened interface area A_c after drilling the holes for the installation of the interface reinforcement (n bars).

More specifically, in the studies by Papanicolaou and Triantafillou [5], Mazizah and Izni [10], and Williams et al. [11], interfaces were subjected to monotonic loading, whereas in those of Soudki et al. [12,13], Valluvan et al. [4], Nakano and Matsuzaki [14], Saari et al. [15], Shirai et al. [16], and Trost [17], the interfaces were subjected to cyclic loading. The test setups used by several researchers can be categorized as follows: push-off setup [5]; modified push-off, for the application of cyclic loading [4,12,14,16], push-through [11,15]; topping on a base [10,13]; and wall specimens subjected to diagonal compression [17]. Irrespective of other parameters adopted in the experimental campaigns, such as roughness, reinforcing steel ratio, etc., all researchers have reached the conclusions that (i) the external compressive stress results in higher interface resistance, and (ii) the external stress should be added to the clamping stress provided by the interface reinforcement for the calculation of the resistance. It is reminded that the shear slip imposed on the interface after the occurrence of a crack along the interface leads to the opening of this crack. Thus, the interface reinforcement develops tensile stresses. The tensile force of the reinforcement is equilibrated by clamping forces, resulting in compressive stresses across the interface. The conclusions of previous experimental campaigns are depicted in the provisions of currently enforced normative regulations (e.g., [18,19]), which, however, do not address the case of interfaces subjected to cyclic loading. In the research of Soudki et al. [12,13], the specimens were subjected to either monotonic or cyclic shear, with an external compressive stress applied on the interface in all specimens, equal to 2.0 MPa, throughout the test. Thus, the effect of varying this parameter on the interface behavior cannot be assessed. Valluvan et al. [4] tested interfaces under cyclic loading under external compressive stress of relatively high magnitudes, namely 29% or 57% of the concrete compressive strength. Thus, the reinforcement crossing the interface—also of relatively high percentage yet of limited embedment length (eight times the bar diameter)—did not contribute to the interface resistance. As a result, the failure mode changed from the pullout of the short embeds under zero external stress to an extensive interface failure under high external compression, described by the authors as “aggregate interlock failure”. The investigation of different types of interfaces, namely smooth, rough, or shear-keyed interfaces [14,16] between concretes, or steel to concrete interfaces [15], shows an increase in the interface resistance in the presence of external compressive stress. Nonetheless, in the respective publications, no further comments were offered on the failure mode or on the overall cyclic behavior of the tested interfaces. Finally, in the tests performed by Trost [17], due to the test setup, the stress perpendicular to the interface increased simultaneously with the stress parallel to the interface, and the effect of the external stress was not commented upon.

In conclusion, even though the effect of external compressive stresses on interfaces was investigated by several researchers, crucial information is still missing on the effect of normal stress on the failure mode, the shear slip at which the interface resistance is reached, and the resistance degradation due to cycling. To further understand interface responses, a systematic investigation of RC interfaces subjected to cyclically imposed shear slips was undertaken at the Laboratory of RC, National technical university of Athens (NTUA), active since the early 1980s [9,20]. The series of research programs that have been carried out comprise several test series, aiming to investigate the effect of the principal governing parameters, namely concrete strength, the percentage of reinforcement crossing the interface, the anchorage length of the reinforcing bars on both sides of the interface, the magnitude of the imposed cyclic slip, the level of normal load on the interface, etc.

In the present paper, the results of part of these tests are presented and commented upon. In this portion of the test series, the effect of several parameters on the cyclic behavior of the interfaces is investigated, with the main focus being on the effect of external compressive stress acting normal to the interface. Furthermore, the model developed at NTUA [21] to calculate the interface resistance is also applied to the experimental results in order to establish its suitability in this case. In this model, both mechanisms are activated at the interface—that is, the friction and the dowel action are taken into account—while the main parameters affecting the development of the two mechanisms are considered, namely the interface roughness, the properties of concrete and steel (and the percentage of the latter), the embedment length of the reinforcement, the value of the externally applied stress, if applicable, etc.

2. The Experimental Campaign

2.1. Specimen Geometry and Test Setup

The overall morphology of the specimens (Figure 1) was chosen to allow cyclic shear to be imposed exactly at the interface (i.e., with no eccentricity). The interfaces were 500 mm long and 100 mm wide. The overall dimensions of the specimens were, however, significantly larger to allow for the positioning of the testing equipment within the testing frame and to ensure that the specimen was fixed in the testing position at a distance from the interface, to avoid the positive effect of restraining forces on the interface shear resistance. The two parts of the specimens were reinforced and designed to avoid the formation of cracks, not related to the interface behavior [22]. The interfaces were crossed by three (3) reinforcing bars 8 mm, 12 mm, or 16 mm in diameter. To avoid parasitic rotation of the specimens during testing, the decision was taken to use three reinforcing bars throughout the experimental campaign. Thus, to reach different reinforcing ratios, the diameter of the bars varied. The reinforcing bars were positioned at mid-width of the interface. The axial distance between consecutive bars was equal to 21.25, 14.17, or 10.63 times the bar diameter for the specimens with 8 mm, 12 mm, or 16 mm diameter bars, respectively (Figure 1). The distances between consecutive bars are within the prescriptions of the EOTA TR066 [23]. B500C-grade (EN 10080 [24]) deformed steel bars were used, with a mean yield strength equal to 560 N/mm². Class C steel (according to [24,25]) is hot rolled and weldable. The bars have two longitudinal ribs, as well as transverse ribs of alternating direction on both sides.

The specimens were constructed in two phases. Initially, block “A” corresponding to old concrete was constructed. When the concrete was mature enough, the surface of block “A” corresponding to the interface was roughened (chipped) using a pickaxe (Figure 1b). Approximately 28 days after the construction of block “A”, the second block, “B”, simulating new concrete, was cast against the old concrete. The reinforcing bars were (a) cast-in in both concrete blocks. The anchorage length was a parameter of the test (6.25 d_b, 12.5 d_b, or 20.0 d_b, Table 2) and it was the same for the two concrete blocks (Figure 1a), each side of the roughened interface. Alternatively, and depending on the test objective, (b) the reinforcing bars were post-installed initially in the first block, “A”, using epoxy resin mortar to a depth (6.25 d_b or 12.5 d_b) that observed the manufacturer’s specification. The epoxy resin mortar is a high-performance thixotropic adhesive of two components, solvent-free, conforming to the requirements of EN 1504-6:2006 [26]. The bars protruded to a length sufficient for full anchorage by bond in the second concrete block, “B” (Figure 1b). The purpose of this option was to study the behavior of interfaces governed by the post-installed portion of the reinforcing bars.

After casting each part, the specimens were covered with hessian cloth and were kept wet for 2 to 3 days. Testing was performed one to two months after the construction of block “B”. Until then, the specimens were stored in the interior of the laboratory, in controlled environmental conditions. For the measurement of the compressive strength of concrete, conventional concrete cylinders (diameter/height = 2, diameter = 150 mm) were cast together with each concrete block, and they were tested in compression the day of testing the corresponding specimens (mean concrete compressive strength values are given in Table 2). As a matter of fact, the compressive strength and the overall properties of the concrete affect the behavior of the interface. In cases where the new concrete is governing the behavior of the interface (e.g., when the interface reinforcement is fully anchored into the old concrete, while the added concrete layer is thin, allowing for reduced activation of the interface reinforcement), the use of innovative types of concrete (see e.g., [27]) may contribute to improved interface behavior. However, this case is not examined within the current investigation and, thus, conventional concrete types and compressive strengths are used for the construction of the specimens. Furthermore, the case of deteriorated (e.g., due to environmental actions) existing concrete is not examined either.

Besides the presence of a normal compressive stress on the interface, the embedment length of the bars in the specimens was among the parameters investigated (equal to 6.25, 12.5, or 20 times the bar diameter). In case of cast-in reinforcing bars, the embedment length was smaller than the anchorage length required for full development of the yield strength of the bars (according to Section 8 of Eurocode 2 [18]).

The experimental results are presented for pairs of specimens differing only in the value of normal compressive stress σ_N,ext on the interface (either zero or 0.30–3.00 MPa, according to Table 2).

Figure 2 shows the test setup [22,28]. The MTS^® actuator (“M”) (maximum capacity = ±300 kN) was placed vertically on a steel frame (“F”). The specimen (“S”) was hanging from the actuator by means of four steel rods (“R”). The position of the actuator axis allows shear slips to be applied to the interface without eccentricity. A steel column (“C”) together with two metal plates (“P”) and four steel rods (“R1”) were used to keep the block simulating the new concrete in position. Shear slips were imposed on the interface by the actuator at low speed (approx. 0.05–2.00 mm/min). Where applicable, normal compressive stress, σ_N,ext, was applied prior to shearing on the interface by means of a horizontally placed actuator, “a” (max. capacity = 100 kN, Figure 2a).

2.2. Instrumentation and Loading History

In order to check the proper function of the experimental setup (e.g., whether parasitic rotation of the specimens was occurring), and to document the behavior of the interfaces as exhaustively as possible, measuring devices were provided on both faces of the specimens at as many locations as allowed by the dimensions of the tested interfaces. Furthermore, strain gauges were used to measure the development of tensile strains in the interface reinforcement. The measurements during testing were (a) the shear slip along the interface, (b) the interface resistance corresponding to the imposed shear slip, and (c) the crack opening, perpendicular to the interface. To measure the shear slip and the crack width, in total, 8 LVDTs (Linear Variable Displacement Transducers) were used, positioned on both faces of the specimen and both ends of the interface. The measuring devices are shown in Figure 3; LVDTs 5 to 8, parallel to the interface, were used for the measurement of the shear slip, while LVDTs 1 to 4, perpendicular to the interface, measured the crack opening. In addition, the strains that developed in the two end reinforcing bars were measured, using electrical strain gauges. The strain gauges were positioned on the bars at a distance approximately equal to 10 mm to 20 mm from the interface on both blocks, “A” and “B”.

Each specimen was subjected to stepwise increasing cyclic shear slips under displacement control, after the application of the external normal stress σ_N,ext, where applicable, and the corresponding shear resistance of the specimen was obtained using the load cell of the vertical actuator. For each relative displacement value, the recorded force constitutes the resistance of the interface to the imposed slip, and this term is used in the relevant plots. Three full reversals were imposed at each slip step, starting with a shear slip value equal to ±0.10 mm, with the exception of specimen R-16/B/12/0.2 (Table 2), for which the first cycle slip was ±0.20 mm. The test was terminated when the interface resistance degradation exceeded 50% of the maximum or when extensive damage of the interface hindered the continuation of testing.

3. Experimental Results and Discussion

3.1. Failure Mode

The overall performance of the specimens was satisfactory. Indeed, the reinforcement of the blocks simulating the old and new concrete was sufficient to prevent cracks not related to the interface behavior. As intended, the first recorded crack, in all specimens, formed along the interface. It is important to note that the opening of that crack was measurable and visible for small values of the imposed shear slip, in some cases as small as 0.10 mm (Figure 4a). However, as shown in detail in the following Sections, the width of the crack along the interface is affected by the parameters investigated within the experimental program.

In several specimens (Table 2), an inclined crack or multiple inclined cracks occurred in block “B” (Figure 4b,c) following cycles at increased slip values. This crack, visible for an imposed shear slip approximately equal to 0.50 mm, was initiated at the interface (at the location of one of the interface bars) and propagated within block “B” with an angle of approximately 45°. In some cases, especially where the embedment length of the bars was small, the occurrence of this crack did not allow the test to continue. The formation of such oblique cracks was observed by other researchers too (Valluvan et al. [4], Tassios and Vintzeleou [14], Hofbeck et al. [29]) and their occurrence was attributed to the effect of high normal stresses on the interface, either subjected to an external compressive stress [14] or provided with a high percentage of reinforcement [4,29].

In one case (Specimen Re-26/B/6/0.1), provided with post-installed bars anchored at 6d in the old concrete, a crack parallel to the interface formed in block “A”, simulating the existing concrete. This crack occurred at a distance almost equal to the anchorage length of the post-installed bars (Figure 4c). Similar cracks parallel and eccentric to the interface were also observed in other tests, in specimens which had been reinforced with post-installed bars of limited embedment length (Valluvan et al. [4], Vintzileou et al. [30], Randl [31]).

Figure 4. Cracks on the specimens: (a) Specimen R-31/B/20/0.1, crack along the interface; (b) specimen NR-25/Ε/6/0.1, oblique cracks; (c) specimen R-25/Ε/6/0.1, block “B”, parallel, multiple oblique cracks; (d) specimen Re-26/B/6/0.1, crack parallel to the interface, at a distance almost equal to the embedment length of the post-installed bars in block “A”.

3.2. Hysteretic Behavior and Maximum Shear Resistance

Figure 5 shows typical hysteresis loops for the tested interfaces. The behavior of the interfaces is characterized by the pinching effect and the limited area of the hysteresis loops, as well as by a pronounced reduction in the interface resistance during the second and the third loading cycles. Hysteresis loop envelopes for all tested specimens are presented in Figure 6. It is noted that the maximum interface resistance is not mobilized for the same shear slip value in the two loading directions. Thus, the τ_u,exp in Table 2 is calculated as the average of two values, namely the absolute maximum recorded resistance (usually during the first loading direction) and the resistance mobilized for the same shear slip value, s_u, in the other loading direction.

The hysteresis loop envelopes, together with the test results listed in Table 2, clearly show the effect of the presence of an external normal stress, σ_N,ext, of different magnitude, on the behavior of interfaces. Indeed, the interface resistance increases, together with the stiffness of interfaces (Figure 7). Thus, the ultimate shear resistance is mobilized for significantly smaller imposed shear slips than for interfaces without external compression, this being the case for both cast-in and post-installed interface reinforcement. It is also observed that interfaces subjected to external compression σ_N,ext exhibit a practically linear behavior up to their maximum resistance. It should be noted that this behavior does not seem to depend on the value of the imposed external compression. However, such a conclusion has to be considered as tentative, in view of the fact that the overall number of tested specimens is rather limited, while the inherent scatter of the experimental results must also be taken into account

The ratio V/s for the first cycle, corresponding to the shear stiffness of the specimens, is shown in Figure 7 to clearly demonstrate the significantly higher stiffnesses of the interfaces under external compression compared to those without normal stress. Furthermore, it is also observed that while, in the absence of external compression, the stiffness seems to depend on the reinforcement parameters (ratio, diameter of bars, and embedment length), in the presence of σ_N,ext, those parameters do not affect the interface stiffness.

The effect of the external normal stress on the maximum interface resistance τ_u,exp is illustrated in Figure 8, where it is interesting to observe the interaction between the presence of σ_N,ext and the embedment length of the interface reinforcement. Figure 8a shows the well-known (Hofbeck et al. [29], Vintzileou et al. [30], Randl [31]) dependence of the interface peak resistance on the embedment length for σ_N,ext equal to zero. Indeed, an increase in the normalized embedment length from 6.25 to 20.0 leads to an increase in the interface resistance. The increase in the interface resistance depends on the reinforcing ratio as well. On the contrary, as shown in Figure 8b, the presence of external compressive stress makes the interface resistance rather insensitive to the embedment length, as well as to the interface reinforcing ratio. This observation is in accordance with Valluvan et al. [4]. This is further confirmed by the reinforcement strain measurements shown in Figure 9. In the presence of external compressive stress, σ_N,ext, the tensile strains in the reinforcing bars are reduced, compared to those measured on reinforcing bars crossing an interface under zero normal stress, implying that the contribution of the interface reinforcement to the overall resistance is reduced. It should be noted that the large scatter of the strain values for interfaces under σ_N,ext = 0 is attributed to the fact that the respective points in Figure 9 correspond to specimens with various reinforcing ratios and embedment lengths.

Figure 10 shows the energy absorbed for all specimens during the first loading cycle at various imposed shear slip steps, estimated by the area of the respective hysteresis loops. Due to the fact that, for the same imposed shear slip at the interface, the specimens without external compression exhibit smaller shear resistance, the area of the hysteresis loops is smaller than for specimens subjected to external compression. This difference is depicted in the two graphs of Figure 10.

3.3. Crack Openings and Tensile Strains of the Bars Crossing the Interface

In Figure 11, the opening, w, of the crack at the interface is plotted against the imposed shear slip, s. It is observed that the crack opening increases when the applied shear slip increases. However, the number of applied shear slip cycles does not seem to significantly affect the magnitude of crack opening. Similar observations can be made in relation to the residual crack opening, i.e., the value recorded on the unloading branch, when the shear slip value is equal to zero. At that point, there is always a residual crack opening, the value of which depends on several parameters and mobilized mechanisms, such as (a) the cut-off of cement paste particles and aggregates forming the roughness of the interface as the new and the old concrete blocks slip relative to one another (where the roughness of the interface is gradually reduced, whereas the material produced by the cut-off of asperities remains along the interfaces, thereby preventing the crack from closing) and (b) the contribution of the interface reinforcement, which depends on its bonded length. Thus, when the interface reinforcement is short, due to the tension induced by the clamping mechanism, the bars exhibit significant and irreversible pullout displacements that contribute to the residual crack opening. If, on the contrary, the interface reinforcement is provided with anchorage length sufficient for its yield strength to be developed, there may be not fully reversible strains in the interface reinforcement post-yield. Those strains also contribute to the value of residual crack opening.

Further observations can be made in Figure 12 and Figure 13. In the absence of external compressive stress, larger embedment lengths allow larger shear slip values to be imposed (Figure 12a), associated with larger crack widths (Figure 13a) and, as a result, higher steel strains due to clamping effects. Thus, as shown in Figure 8a, the interface resistance increases for larger embedment lengths. The opposite is observed in cases where an external compressive stress, σ_N,ext, is acting in the interface: in this case, both the shear slip s_u and the crack opening at the moment of the maximum interface resistance is activated (w_u at s_u), which decrease with increasing embedment length of the reinforcement. As an example, consider the case of the post-installed interface reinforcement embedded at 6.25 d_b. In the absence of external compression, the (limited) interface resistance is mobilized at a relatively small imposed shear slip, causing concrete breakout failure (Figure 4d). In the presence of external compression, however, the interface can sustain larger shear slip values, associated with larger crack opening, and therefore exhibits higher resistance. As the embedment length of the reinforcement increases, the interface resistance increases significantly (Figure 8a). In this case, the positive effect of external compression is less pronounced (compare Figure 8a and Figure 8b). With the increase in interface resistance being smaller under external compressive stress, the maximum resistance is mobilized for smaller “s_u” and “w_u” values (Figure 12b and Figure 13b).

The smaller shear slip values at which interfaces subjected to external compression attain their maximum resistance (Figure 12, Table 2) are associated with smaller crack widths and, hence, reduced mobilization of the interface reinforcing bars, both as clamps and as dowels (Figure 9). As already mentioned, the tensile strains recorded during testing also depended on the presence of compressive stress normal to the interface. The ratio of strain increase with the increase in the imposed shear slip for the specimens with normal compressive stress was quite small compared with that of the specimens without compressive stress. In the case of an acting compressive stress, σ_N,ext, the bars were not under the limit state of being subjected to pullout. It is noted, however, that the relative contribution of the axial strain and the dowel action (dowel deformation because of kinking) of the bars was not clear.

The degradation of the interface resistance with cycling for the two groups of specimens, those without and those with external compression, is shown in Figure 14. A general observation is that, in the presence of external compression, the degradation of the interface resistance with cycling is smaller than for interfaces without σ_Ν,ext. Furthermore, in Figure 14a, one can distinguish more clearly the influence of the normalized embedment length on the resistance degradation, while, in the presence of external compression, this effect is more limited. This reduced degradation with cycling that is observed in the interfaces under external compression may be associated with the smaller shear slip values and, as also corroborated by the crack width increase with the number of cycles in Figure 11, increased mobilization of the clamping action of the dowels, hence leading to reduced deterioration of the concrete surfaces.

4. Calculation of the Interface Resistance

Palieraki et al. [32] developed a model able to numerically reproduce the behavior of RC interfaces under monotonic and cyclic shear slips. This model was used for the development of an equation allowing the interface resistance to be calculated. The validity of the equation was checked against numerous experimental results from the literature, compiled in a database (Palieraki et al. [21]). Following this work, the interface resistance is calculated according to Equation (1) as the sum of the contributions of friction τ_f due to the clamping effect and the externally acting normal stress, if applicable, and the dowel action, τ_d, of the transverse reinforcement crossing the interface:

τ_{u} = β_{d} \cdot τ_{d} + β_{f} \cdot τ_{f} = β_{d} \cdot \frac{(1.3 \cdot n \cdot d_{b}^{2} \cdot \sqrt{f_{c} \cdot f_{y})}}{A_{c}} + β_{f} \cdot 0.33 \cdot {(f_{c}^{2} \cdot σ_{c})}^{\begin{matrix} \frac{1}{3} \end{matrix}} (N, m m o r p s i)

(1)

β_f and β_d are contribution factors accounting for (a) the fact that the maxima of the two mechanisms do not occur for the same shear slip value and (b) the interaction of the two mechanisms. A set of values for the contribution factors was proposed by Palieraki et al. [21], depending on the interface roughness, the embedment length of the reinforcement, etc.

In Equation (1), the interface resistance due to friction is calculated for normal compressive stress on the interface, σ_c, due both to clamping effects, ρσ_s, and external (compressive or tensile) stress, σ_N,ext:

σ_{c} = ρ \cdot σ_{s} + σ_{N, e x t}

(2)

where ρ denotes the reinforcing percentage (area of steel to the interface area), σ_s is the tensile stress in the reinforcement due to clamping effects, and σ_N,ext denotes the external normal stress (positive for compression).

With the purpose of validating the above model for the case of interfaces under external compression, Equation (1) was applied to the experimental results obtained within this work, as well as to the relevant results of the literature, shown in Table 1. Figure 15a shows the comparison between experimental and calculated interface resistances for 119 specimens in total. The mean value of the ratio τ_u,calc/τ_u,exp is equal to 0.834, while the coefficient of variation is equal to 41.7%. As shown in Figure 15a, the experimental values of interface resistance measured by Valluvan et al. [4] and Papanicolaou and Triantafillou [5] are significantly larger than the predicted ones. Papanicolaou and Triantafillou [5] tested interfaces between two concretes significantly different in compressive strength (42.5 MPa vs. lightweight concrete of 10.0–13.7 Mpa). Thus, as shown in the photographs included in their publication, and as commented by the authors, in several cases, when the interface was smooth, there was no clear crack formation along the interface; the crack deviated into the weaker concrete to a path longer than a clear plane interface crack. This might explain the higher interface resistances obtained. In the tests by Valluvan et al. [4], the external compression was of very high value (~29% to 57% of the compressive strength of the weaker concrete), and the authors commented that, in this case, the failure was “aggregate interlock failure”. The photos provided in the relevant paper indicated a compressive failure, with pronounced crushing of concrete.

By excluding the specimens that did not fail due to interface failure, as well as the specimens with interfaces of very limited length, the revised plot of Figure 15b presents a better comparison between experimental and calculated interface resistances. In this case, the mean value of the ratio τ_u,calc/τ_u,exp is equal to 0.895, with a coefficient of variation equal to 37.8%. Thus, it seems that Equation (1) is able to quite accurately predict the experimental resistance of interfaces subjected to external compression.

5. Conclusions

The results of a set of cyclic shear tests of reinforced interfaces in the presence of an externally acting normal stress were presented and discussed in terms of the failure mode and the influence of the test parameters on the interface capacity, the shear slip corresponding to the interface capacity, and the width of the crack across the interface. Based on the experimental results presented in this paper, the following conclusions can be drawn:

The shear resistance of rough interfaces simulating cold joints (the new concrete is cast against the old concrete) depends on the value of the applied shear slip, as well as on the embedment length of the reinforcement, the increase in which has a positive effect on the mobilized shear resistance. Additionally, the shear resistance is affected by the anchoring mechanism of the reinforcement. Thus, for embedment lengths smaller than the development length, the cast-in bars mobilize smaller resistance than post-installed bars of the same embedment length. Nonetheless, in the presence of an external compressive stress, the effect of the embedment length of the interface reinforcement is reduced. This is due to the reduced activation of the clamping effect of the interface reinforcement. Indeed, in the presence of external compressive stress, the crack width across the interface is limited, and thus, the strains measured on the reinforcing bars are much smaller than for identical specimens tested without any external compression on the interface.
The application of cyclic shear slips has confirmed the negative effect of this type of loading in terms of ultimate shear resistance and stiffness degradation. Nonetheless, in this case, the presence of external compressive stress reduces the effect of cycling. Furthermore, the maximum resistance of interfaces subjected to external compression is mobilized for smaller shear slip values than the capacity of their counterparts with zero external stress. This is a feature to be accounted for in cases of displacement-based design.
The equation proposed by the authors to calculate the maximum resistance of interfaces was applied to the experimental results presented in this paper, as well as to relevant results from the literature, obtained from testing interfaces subjected to external compression. It is noted that the equation is based on a global model following the step-by-step development of the shear resisting mechanisms, as well as their interaction. The application of this equation was quite satisfying, as it was able to predict the ultimate shear resistance of interfaces quite accurately.
Taking into account that the equation proposed by the authors is already checked against a large number of experimental results from the literature, compiled in a database and covering a wide range of values for all influencing parameters, one may consider this equation as a sound basis for the design of interfaces.

Author Contributions

Conceptualization, V.P., C.Z. and E.V.; methodology, V.P., C.Z. and E.V.; validation, E.V.; investigation, V.P. and C.Z.; data curation V.P.; writing—original draft preparation, V.P. and E.V.; writing—review and editing, V.P., C.Z. and E.V.; supervision, V.P., C.Z. and E.V.; project administration, V.P. and C.Z.; funding acquisition, C.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research program was partly funded under the Basic Research Funding Program of the Research Accounts Department at NTUA. Financial support provided by the Hellenic State Scholarships Foundation, for the development of the doctoral research of the first author at NTUA, is gratefully acknowledged.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 2. Test setup of the specimens with external compressive stress: (a) sketch of the test setup; (b) photo of a specimen in testing position.

Figure 3. Position of LVDTs to measure shear slip and crack opening.

Figure 5. Typical hysteresis loops of interface resistance (V) versus slip (s).

Figure 6. Hysteresis loop envelopes for the first cycle for specimens (a) without external compressive stress and (b) with external compressive stress; (c) close up of the graphs (a,b), s ≤ 2.00 mm.

Figure 7. The ratio V/s for the first cycle, corresponding to the shear stiffness of the specimens (a) without external compressive stress and (b) with external compressive stress.

Figure 8. Maximum interface resistance, plotted against the embedment length. (a) Specimens without external compressive stress. (b) Specimens with external compressive stress, σ_N,ext. Empty symbols are used for specimens with cast-in bars, with full symbols for specimens with post-installed bars.

Figure 9. Strain of the reinforcement, measured for the maximum interface resistance, plotted against the applied external stress, σ_N,ext. Empty symbols are used for specimens with cast-in bars, with full symbols for specimens with post-installed bars.

Figure 10. Energy absorbed during the first cycle, estimated by the area included in the hysteresis loops. (a) Specimens without external compressive stress. (b) Specimens with external compressive stress, σ_N,ext.

Figure 11. Typical crack width (w) vs. shear slip(s) curves.

Figure 12. Slip at the maximum interface resistance, plotted against the embedment length. (a) Specimens without external compressive stress. (b) Specimens with external compressive stress. Empty symbols are used for specimens with cast-in bars, with full symbols for specimens with post-installed bars.

Figure 13. Crack opening at the maximum interface resistance, plotted against the embedment length. (a) Specimens without external compressive stress. (b) Specimens with external compressive stress. Empty symbols are used for specimens with cast-in bars, with full symbols for specimens with post-installed bars.

Figure 14. Effect of cycling on the shear resistance of interfaces (a) under zero normal stress and (b) subjected to external compression.

Figure 15. Plotted vs. experimental interface resistance calculated using Equation (1): (a) 119 specimens are included; (b) 102 specimens are included. The data experimental interface resistance is included in the current paper and in the literature [4,5,10,11,12,13,14,15,16,17].

Table 2. Main characteristics of specimens, experimental and calculated values of maximum shear.

Specimen Designation ⁽¹⁾	ρ (%), l_emb/d_b	σ_Ν,ext (N/mm²)	Mean Concrete Compressive Strength (MPa)		s_u (mm)	w_u at s_u (mm)	τ_u,exp (N/mm²)	τ_u,calc (N/mm²)	Failure Mode
Specimen Designation ⁽¹⁾	ρ (%), l_emb/d_b	σ_Ν,ext (N/mm²)	Block A	Block B
Specimens without external compressive stress
R-16/B/12/0.2	0.302, 12.5	0.00	36.00	15.94	(1.50) *	(0.97) *	0.85	1.02	I, OC
R-31/B/20/0.1	0.302, 20.0	0.00	31.10	34.76	0.40	0.36	2.36	2.65	I
Re-26/B/6/0.1	0.302, 6.25	0.00	49.14	26.04	0.10	0.19	1.28	1.58	I, P
Re-21/B/12/0.1	0.302, 12.5	0.00	25.57	21.38	0.40	0.25	2.86	2.18	I, OC
R-25/E/6/0.1	0.679, 6.25	0.00	25.06	31.10	0.35	0.35	2.27	1.76	I, MOCs
R-31/E/20/0.1	0.679, 20.0	0.00	31.10	34.76	0.80	0.44	3.53	3.91	I, MOCs
Re-21/E/12/0.1	0.679, 12.5	0.00	25.57	21.38	0.60	0.21	4.49	3.22	I, MOCs
R-16/C/6/0.1	1.206, 6.25	0.00	36.00	15.94	0.20	0.22	1.21	1.42	I, MOCs
R-21/C/12/0.1	1.206, 12.5	0.00	39.74	20.98	0.55	0.37	1.81	2.90	I, OC
Specimens with external compressive stress
NR-25/B/12/0.1	0.302, 12.5	1.60	31.63	25.06	0.11	0.01	4.50	3.42	I, OC
NR-25/Β/20/0.1	0.302, 20.0	1.60	25.06	31.10	0.34	0.29	5.31	3.60	I, OC
NRe-27/B/6/0.1	0.302, 6.25	3.00	36.21	27.03	0.80	0.54	4.18	4.18	I, OC
NRe-21/B/12/0.1	0.302, 12.5	1.60	25.57	21.38	0.20	0.06	4.78	3.39	I, OC
NR-25/E/6/0.1	0.679, 6.25	1.60	25.06	31.10	0.40	(1.29) *	5.08	3.97	I, OC
NR-25/E/20/0.1	0.679, 20.0	1.60	25.06	31.10	0.14	0.08	4.31	4.61	I, MOCs
NRe-21/E/12/0.1	0.679, 12.5	0.40	25.57	21.38	0.40	0.28	5.14	4.12	I, MOCs
NR-36/C/6/0.1	1.206, 6.25	3.00	49.14	36.21	0.20	0.002	4.46	6.40	I, MOCs
NR-38/C/12/0.1	1.206, 12.5	0.30	49.14	37.92	0.40	0.46	4.30	6.87	I, OC

⁽¹⁾ Description of terms in the designation of the specimens NR-c/D_b/λ/s: N: specimens with external compressive stress, equal to σ_N,ext. R: roughened interface. Re: roughened interface; anchorage of reinforcement by means of adhesive mortar (epoxy resins). c: The compressive strength of the concrete block in MPa (the minimum of blocks A and B, Figure 1). D_b: specimens with three bars; diameter of the bars: B: 8 mm, C: 16 mm, E: 12 mm. λ: ratio of the bar embedment length l_emb normalized to the bar diameter d_b (the minimum between embedment in blocks A and B, Figure 1, is used). s: shear slip amplitude during the first set of cycles (in mm), in all specimens but one, equal to 0.10 mm. Example of the designation: NRe-21/E/12/0.1: specimen with external compressive stress, roughened interface, anchorage by means of adhesive mortar; minimum compressive strength of blocks A and B equal to 21 MPa/three bars 12 mm in diameter/ratio l_emb/d_b = 12/shear slip amplitude during the first set of cycles = 0.10 mm. s_u: shear slip amplitude at the moment when the maximum interface resistance is activated. w_u: crack width corresponding to s_u. τ_u,exp: the average peak shear resistance obtained from the two loading directions, calculated as follows: (i) the absolute maximum recorded resistance (at a shear slip value defined as s_u) and (ii) the shear resistance recorded in the opposite loading direction at a shear slip value equal to s_u. τ_u,calc: the interface resistance calculated using the equation proposed by the authors, Section 4. Failure Mode: I: interface (Figure 4a), OC: oblique crack (Figure 4b), MOCs: multiple oblique cracks (Figure 4c), P: crack parallel to the interface (Figure 4d). * Unreliable measurement.

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Palieraki, V.; Zeris, C.; Vintzileou, E. Experimental Investigation of the Effect of Compressive Interface Stress on Interfaces in Reinforced Concrete Elements under Cyclic Action. Appl. Sci. 2024, 14, 4350. https://0-doi-org.brum.beds.ac.uk/10.3390/app14114350

AMA Style

Palieraki V, Zeris C, Vintzileou E. Experimental Investigation of the Effect of Compressive Interface Stress on Interfaces in Reinforced Concrete Elements under Cyclic Action. Applied Sciences. 2024; 14(11):4350. https://0-doi-org.brum.beds.ac.uk/10.3390/app14114350

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Palieraki, Vasiliki, Christos Zeris, and Elizabeth Vintzileou. 2024. "Experimental Investigation of the Effect of Compressive Interface Stress on Interfaces in Reinforced Concrete Elements under Cyclic Action" Applied Sciences 14, no. 11: 4350. https://0-doi-org.brum.beds.ac.uk/10.3390/app14114350

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Experimental Investigation of the Effect of Compressive Interface Stress on Interfaces in Reinforced Concrete Elements under Cyclic Action

Abstract

1. Introduction

2. The Experimental Campaign

2.1. Specimen Geometry and Test Setup

2.2. Instrumentation and Loading History

3. Experimental Results and Discussion

3.1. Failure Mode

3.2. Hysteretic Behavior and Maximum Shear Resistance

3.3. Crack Openings and Tensile Strains of the Bars Crossing the Interface

4. Calculation of the Interface Resistance

5. Conclusions

Author Contributions

Funding

Institutional Review Board Statement

Informed Consent Statement

Data Availability Statement

Conflicts of Interest

References

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