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Cecília Nemessányi Department of Mechanics, Materials and Structures, Faculty of Architecture, Budapest University of Technology and Economics, K. II. 61, Műegyetem rkp. 3, H-1111 Budapest, Hungary

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Anikó Pluzsik Department of Mechanics, Materials and Structures, Faculty of Architecture, Budapest University of Technology and Economics, K. II. 61, Műegyetem rkp. 3, H-1111 Budapest, Hungary

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High performance fibre reinforced concrete (HPFRC) materials with tensile hardening behaviour can effectively be used for strengthening reinforced concrete beams. A perfect bond between the original and the reinforcing layer cannot be formed, the load-bearing capacity and ductility of the strengthened beam can significantly be affected by the interfacial bond strength between the contacting surfaces. In this paper, beam retrofitting with cast in-situ strengthening type is examined. The purpose of this experimental study is to investigate the impact of the different bond types on the load-bearing capacity, ductility, and failure mode of the strengthened beams in the case of cast in-situ strengthening. Twenty-four beam tests were performed with untreated and rough surfaced beams, with or without connecting elements. The effect of the bond type proved to be significant regarding the failure mode in the case of compression side strengthening, stronger bond resulted in higher load bearing capacity and ductility, too. When tensile side reinforcement was investigated no average increment was experienced in the maximal force and ductility due to the stronger bond. Based on the results, it can be concluded that the generally applied analytical models that assume perfect connection may lead to exaggerated results in the case of a compressed side HPC-strengthened beam. Therefore, it is necessary to develop a model that considers the effect of the imperfect bond.

A húzásra felkeményedő anyagtulajdonságú, szálerősítésű, nagyszilárdságú betonhabarcs (HPFRC) hatékonyan használható vasbeton gerendák megerősítéséhez. Tökéletes kapcsolat az eredeti vasbeton gerenda és a megerősítő HPFRC réteg között nem tud kialakulni, a megerősített gerenda teherbírási képességét és duktilitását jelentősen befolyásolhatja a rétegek közötti adhéziós kapcsolat. Ezen kísérleti tanulmány során a helyszínen öntött megerősítéstípust vizsgáltuk. A tanulmány célja a különböző kapcsolati típusok hatásának vizsgálata a megerősített beton gerendák teherbírására, duktilitására, valamint tönkremeneteli módjára helyszínen öntött HPFRC megerősítés esetén. Huszonnégy gerendakísérlet készült kezeletlen, valamint érdesített felületű gerendákkal kapcsolóelemek használatával, valamint azok nélkül. Az eredmények azt mutatják, hogy a kapcsolat típusának hatása a nyomott oldali megerősítés esetén jelentős, míg a húzott oldali megerősítés esetében a tönkremeneteli módot nem befolyásolta a kapcsolat típusa. A nyomott oldali megerősítés eredményei alapján kijelenthető, hogy a tökéletes kapcsolatot feltételező analitikus modellek, amelyeket gyakran használnak a HPC-vel megerősített gerenda teherbíró képességének számításához, túlzó eredményekhez vezethetnek, ezért szükséges olyan modellt kidolgozni, amely figyelembe veszi a tökéletlen kapcsolat hatását.

Abstract

High performance fibre reinforced concrete (HPFRC) materials with tensile hardening behaviour can effectively be used for strengthening reinforced concrete beams. A perfect bond between the original and the reinforcing layer cannot be formed, the load-bearing capacity and ductility of the strengthened beam can significantly be affected by the interfacial bond strength between the contacting surfaces. In this paper, beam retrofitting with cast in-situ strengthening type is examined. The purpose of this experimental study is to investigate the impact of the different bond types on the load-bearing capacity, ductility, and failure mode of the strengthened beams in the case of cast in-situ strengthening. Twenty-four beam tests were performed with untreated and rough surfaced beams, with or without connecting elements. The effect of the bond type proved to be significant regarding the failure mode in the case of compression side strengthening, stronger bond resulted in higher load bearing capacity and ductility, too. When tensile side reinforcement was investigated no average increment was experienced in the maximal force and ductility due to the stronger bond. Based on the results, it can be concluded that the generally applied analytical models that assume perfect connection may lead to exaggerated results in the case of a compressed side HPC-strengthened beam. Therefore, it is necessary to develop a model that considers the effect of the imperfect bond.

1 INTRODUCTION

There are several materials and techniques for strengthening Reinforced Concrete (RC) beams. Traditional materials used for retrofitting are reinforced concrete, ferrocement and steel. The main disadvantage of the concrete retrofitting is the significantly increased cross section and additional weight, while externally bonded steel plates are exposed to corrosion (Askar– Hassan–Al-Kamaki 2022. 1). Ferrocement strengthening is a relatively new and cost-effective method with the use of thin steel mesh in a thin concrete layer. Its strength, weight, durability, and fire resistance are all beneficial, but it is a labour-intensive technology and there are no applicable codes (Kaish et al. 2018. 1–3).

Recently, a more widely researched strengthening material is fibre reinforced plastic (FRP). FRP materials are lightweight, high strength materials with very good durability and designable characteristics. They are made up of fibres and matrix, which connects the fibres together and protects them from mechanical impacts and environmental effects. Fibres can be short or long. The long fibres may have a random or ordered direction inside the matrix, or they can be woven into fibre mats (Kollár–Springer 2003). The most used fibre materials for strengthening are carbon and glass fibres. Carbon fibres have high strength and elastic modulus that is comparable to steel, while glass fibres have less strength and elastic modulus, but they are more affordable (Hollaway–Teng 2008). In all reinforcement methods, the key to the effectiveness of the strengthening lies in the bond between the original beam and the FRP material. The adhesive transfers the stresses between the FRP and the contacting surface of the beam. Debonding and partial connection have a negative impact on the load-bearing capacity, ductility, and the failure mode of the strengthened beam (Kalfat–Al-Mahaidi–Smith 2013). FRP strengthening techniques have been studied comprehensively in Askar–Hassan–Al-Kamaki (2022), Danraka–Mahmod–Oluwatisin (2003), Csuka–Kollár (2012), Szabó–Balázs (2007), Hollaway–Teng (2008). The main limitations of FRP retrofitting are low fire resistance, rigid failure mode, heat sensitivity, labour intensiveness, complicated installation process and its cost (Askar–Hassan–Al-Kamaki 2022. 8).

With improved material properties, concrete can also be used effectively in retrofitting. High-performance concrete materials (HPC) are characterised by high strength, ductility, toughness, durability, and stiffness (Neville–Aïtcin 1998. 112; Walraven 2009. 1247). Adding macro steel fibres (30–50 mm) to the material (high-performance fibre-reinforced concrete, HPFRC) prevents the propagation of cracks and further increases the tensile and flexural strength, and the ductility performance while reducing the shrinkage and creep deformations of concrete (Seyam–Balázs 2023. 2–3; Afroughsabet–Biolzi–Ozbakkaloglu 2016. 2). Drying shrinkage can also be affected by variation aggregate proportions (El-Mir et al. 2017. 8). A special HPC is called ultra-high performance concrete (UHPC or UHPFRC) when the characteristic compressive strength of the HPC is over 150 MPa and the cement matrix contains steel fibres (Seyam– Balázs 2023. 2–3).

Among strengthening materials, fire resistance properties of HPFRC are relatively good. The low permeability of HPC is favourable under normal conditions because it protects the steel reinforcement from corrosion. However, it can hinder the evaporation of water particles from the material, resulting in the cracking of the concrete layer due to the increased pressure caused by the trapped water vapor (Neville–Aïtcin 1998. 116).

Important material properties of FRC are already established for use in the development of design guidelines and codes (ACI 544.1R-96 2002; CNR-DT 204 2006; DAfStb Guideline 2015; Silfwerbrand 2008), however, there is no existing guideline for strengthening RC structures with FRC or HPFRC.

This paper discusses the strengthening of RC beams with HPFRC. Recently, such structural applications have become more popular (Abdal et al. 2023; Bertola et al. 2021; Tanaka et al. 2011). The most common technique of HPFRC strengthening is the cast in-situ technique due to its convenience and simplicity. Other strengthening methods include prefabricated elements attached to the existing structure with a bonding agent or with mechanical anchoring (Huang et al. 2022. 11). The lack of defined methods for use, legal design codes, and unified test for concrete-to-concrete interface behaviour are the biggest challenges for HPFRC strengthened beam applications (Huang et al. 2022. 2). Zhu et al. (2020) state that the roughened surface of the concrete substrate helps to increase the strength in the case of UHPFRC strengthening due to better adhesion. Most of the tested specimens were reinforced on the tension side.

Analytical models for flexural strengthening in the literature assume plain section and perfect bond between the UHPFRC strengthening and the RC element. Huang et al. (2022) and Zhu et al. (2020) summarize analytical models developed to study beam flexural strengthening. These models had two base assumptions that 1) the Bernoulli-Navier hypothesis is true; meaning that the untreated sections remain flat, and 2) the bond between the strengthening layer and the original cross-sectional part is perfect, so no slippage occurs at the interface. Finite Element studies were also examined and compared with the analytical and experimental results (Zhu et al. 2020). In the case of tensile side strengthening experimental studies show that the UHPFRC strengthened beams almost always behave monolithically without debonding and with limited slip at the interface (Huang et al. 2022).

2 PROBLEM STATEMENT

While flexural debonding of FRP sheets is an intensively researched area, and it is established that interfacial stresses play a significant role in the behaviour of the FRP strengthened beams (Kalfat–Al-Mahaidi–Smith 2013. 15–24), it is a general assumption in the case of a HPFRC strengthening that interface behaviour is not governing the behaviour of the strengthening system (Noshiravani–Brühwiler 2014. 2). In the available analytical or numerical calculation methods of the HPFRC strengthened beams the bond is assumed perfect between the contacting materials and the slip at the interface is neglected (Zhu et al. 2020. 15–16; Huang et al. 2022. 14). Although the interfacial bond strength is usually higher than the tensile strength of concrete, and debonding is not always detected at the interface, damage in the near interface concrete zone can occur due to the stiffness change in the materials and the stress concentrations near the interface in the concrete part (Noshiravani–Brühwiler 2014. 2). So, the strengthened structure does not always behave monolithically, interfacial stresses have to be taken into account.

In this study structural beam tests were performed to investigate the interfacial bond and the overall behaviour of the HPFRC strengthened beams. In practice, the surface of the concrete beam is roughened before casting the strengthening layer to improve the bond, but no additional connecting elements are used (Mapei Kft. 2023). In the case of untreated and rough surfaced beams, only the cohesion and the adhesion transfer the loads between the contacting surfaces. Due to slipping between the surfaces, the deformations of the original beam and the new layer are different. The stresses that arise from adhesion are the function of the material and the roughness of the contacting surfaces. Connecting elements, like bent rebars or studs, can improve the connection between the strengthening layer and the original beam. Strengthening a partial connection between the contacting materials by connecting elements ensures that the horizontal and vertical displacements of the two components are the same in the connected points. Thereby debonding can only occur between these connecting elements.

In this study, lab experiments were performed using untreated and rough surfaced specimens with or without contacting element to investigate the effect of the different types of bond types on the load-bearing capacity and ductility of the strengthened beams. It was assumed that a stronger bond would impact the failure mode of the strengthened beam, too.

In the literature mostly tensioned side experiments are examined, however, in several cases strengthening on the compressed side is beneficial (Lampropoulos et al. 2016. 2–3; Martinola et al. 2010. 734), for example in the case of over-reinforced beams. In the case of over-reinforced beams, the reinforcement on the tensioned side does not yield at the ultimate limit state so strengthening on the tensioned side cannot effectively increase the load-bearing capacity. The question arises whether the statements drawn from reinforcement on the tensioned side remain valid in the case of reinforcement on the compressed side.

3 EXPERIMENT

3.0 Test matrix

In this study beam retrofitting with cast in-situ strengthening type is examined. Preliminary analytical calculations (see section 4) were performed to choose the reinforcement of the original beams in order to find configurations where the effect of the connection of the strengthening layer predicted to be significant.

In the first step, two types of original beams were made with similar shear and compressed side steel reinforcement, but with different diameter steel reinforcement on the tensioned side. The slightly over-reinforced samples (two Φ10 at the tensioned side) were strengthened on their compressed side in the second step of the experiment. The slightly under-reinforced samples (two Φ6 at the tensioned side) were later strengthened on their tensioned side. The tensioned side rebars were bent up for better anchorage (Fig. 1).

Figure 1.
Figure 1.

Geometrical data of the performed specimens

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

In the second step HPFRC strengthening layer was casted to the original beams. The slightly over-reinforced and the slightly under-reinforced samples were strengthened on their compressed and on their tensioned side, respectively. Additional steel reinforcement (two Φ6) was placed in the tensioned side strengthening layer. To ensure appropriate concrete cover and embedding of the rebars, the strengthening layer for the tensioned side strengthening was chosen to be 50 mm. While on the compressed side, the thickness of the reinforcement layer was chosen to be 30 mm to avoid the compressed concrete zone falling into the thickness of the strengthening layer. Compressed and tensioned side strengthening were casted with four different types of surface connection: untreated surface, roughened surface, untreated surface with connecting element and roughened surface with connecting element. Schematic drawings of the sample types and reinforcements are shown in Figure. 2. The surfaces were roughened with an angle grinder, at a depth of 5–10 mm per cm. The connecting elements were made from 120 mm long Φ6 steel rebars bent in half. Figures 34 show the different surface types before and after the connecting elements have been inserted respectively. Untreated surfaced beams were slightly polished with angle grinder to remove ruggedness and the topmost layer of the concrete prior to the casting of the strengthening layer. Three test specimens were prepared for each surface connection type, therefore twenty-four 1100 mm long reinforced concrete beam test specimens with a cross section of 100×100 mm were made. Figure 1 and Table 1 summarise the geometrical data of the performed specimens. The codes of the beams are formed as follows: lower (A)/upper (B) reinforcement, surface type (1–4) and casting number (01–03) in the end. The surface types are numbered as follows: 1) untreated surface, 2) roughened surface, 3) untreated surface with connecting elements and 4) roughened surface with connecting elements. Overall, six 40-liter pourings were made; three for the upper and three for the lower reinforcement, and four beams were casted with each pouring. From each pouring, four different surface connections were made: one with each of the previously mentioned four surface types. The casting number (01–03) refers to the same pourings in the case of same side strengthening.

Figure 2.
Figure 2.

Different types of beams made for the experiment

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Figure 3.
Figure 3.

Untreated and roughened surfaced beams with predrilled holes

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Figure 4.
Figure 4.

Bended 6mm steel rebars glued in the predrilled holes

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Table 1.

Geometrical data of performed samples

Sample IDBase RC beam dimensions [mm]Compressed/ Tensioned steel reinforcementHPC layer thicknessSteel reinforcement in HPC
A100×100×11002×6 mm (B500A) /

2×6 mm (B500A)
50 mm2×6 mm (B500A)
B2×6 mm (B500A) /

2×10 mm (B500B)
30 mm

3.1 Materials

Tables 23 show the mix proportion of concrete and HPC elements. In Table 4 and hereafter the letters A and B indicate test specimens reinforced on the tensioned side and on the compressed side, respectively.

Table 2.

Base concrete mixture proportions [kg/m3]

Aggregate fractionsLimestoneCementWaterWater/cement ratio
0/4 mm4/8 mm8/16 mm
798371687502901740.60
Table 3.

HPFRC mixture proportions [kg/bag]

Mapei Planitop HPCMapei Fibres HPCWater
25.001.6253.00
Table 4.

Measured average compressive strength values of the preliminary test specimens and test specimens made during the casting of the beams. The bending test was performed 28 days after casting the HPC as a strengthening layer

Measuring dayType/code of test specimenCompressive cube strength [N/mm2]Density [kN/m3]
28 daysPreliminary test specimens37.3223.81
A, B44.1023.72
The day of the bending testA54.5423.23
B60.6823.42
HPCx128.6024.40

Concrete compressive strength affects the quality of bonding and the strength of the bond itself (Shunmuga Vembu–Ammasi 2023. 1). To measure the compressive strength of the materials cube test specimens were also made during the manufacturing of the beams. The specimens were stored under water for a week after casting. The compressive strength was determined based on the MSZ-EN-196-1:2016 standard. Table 4 shows the three measured compressive cube strength values of the concrete used for the beam specimens; the preliminary test specimens, the test specimens made during the casting of the beams measured at 28 days after the mixing and on the day of the test.

The strengthening material was fibre reinforced cementitious mortar namely Mapei Planitop HPC with Mapei Fibres HPC which had 128.6 N/mm2 compressive cube strength (Table 4). The manufacturer’s instructions were followed during mixing. The original concrete material had an average 57.61 N/mm2 compressive cube strength, which is approximately 30% of the compressive strength of the strengthening material.

The grade of steel reinforcement is B500A for the 6 mm diameter and B500B for the 10 mm diameter bars for both the original cross-section and the reinforcement. The grades of the reinforcements were chosen based on the available products on the market. The connecting elements and the stirrups were produced by bending 6 mm steel bars. The connecting elements (see section 3.0) were glued in place with Hilti HIT-HY 200-A injectable hybrid mortar.

3.2 Manufacturing

In real strengthening scenarios the new and the original concrete layers are of different age and have different material characteristics. Therefore, additional stresses arise in the contacting surface due to the restrained shrinkage of the new concrete layer. These stresses may trigger the debonding of the new and original layers. Furthermore, preloading the original beam leads to internal stresses, so the stress distribution and strain are not uniform in the original beam and the reinforcement. In HPFRC strengthening, the shrinkage cracks are smaller due to the bridging effect of the fibres. Therefore, these stresses do not result in debonding of the layers.

In this experiment, the strengthening HPFRC layer was added seven months after the original beams were casted. The original beams were stored during winter under unheated circumstances. The reinforcement layer was casted to the beams in early summer using self-made wooden formwork, when the beams were 208–213 days old. The reinforced beams were tested 29 days after the HPFRC reinforcement was added. Representative compressive test samples of the original concrete and of the reinforcement were made during casting to evaluate the compressive strengths after 28 days and on the day of the tests.

3.3 Testing

Simply supported beams were tested in three-point bending. The degrees of freedom of the supports were free when moving in the axis of the beam and when rotating in the width axis of the beam, otherwise they were fixed. The tests were conducted using ZWICK/ROELL Z150 universal material testing machine. The experimental setup can be seen in Fig. 5. The beams were loaded in a displacement-controlled way at a 0.8 mm/min loading rate. (One of the beams (B1_02) was loaded at 0.2 mm/min loading rate. However, due to the long test time, the loading rate was increased to 0.8 mm/min for the rest of the beams.) During the measurements, failure modes and force-displacement diagrams were examined. The force-displacement diagrams were produced by the test machine, where the displacements correspond to the movement of the crosshead. During the study, it was assumed that the movement of the crosshead corresponded approximately to the displacement of the beam. These results were sufficient for comparison, as there were still noticeable differences between the various cases. The maximum displacement of the head was set to 80 mm. Although some of the specimens reached this value, they were already in the descending branch and very damaged at this stage, therefore continuing the experiment was not necessary. Similarly, testing of the B3_02 beam was finished before it completely failed, so that the beam failure would not cause damage to the cameras and gauges located nearby.

Figure 5.
Figure 5.

Loading configurations

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

4 EVALUATION OF THE TEST RESULTS

To evaluate the test results, a simple analytical model based on internal cross-sectional stress balance within the middle cross section was used to determine the approximate load-bearing capacity of the samples. This method is similar to the one described by Zhu et al. (2020). Linear- elastic and perfectly plastic material models were used for the steel and the HPFRC, while the original concrete material was assumed to be linear-elastic for tension and linear-elastic and perfectly plastic for compression. The analytical model given in (Zhu et al. 2020) was improved by taking into consideration the partial connection in the bond at a cross-sectional level. In the improved model, the bond could be set to be perfect or partial by a coefficient. When partial connection is assumed, the cross-section does not remain flat, but a jump arises in the strain diagram proportionally to the coefficient at the point where the two surfaces meet. In the case of perfect bond (coefficient is equal to 1) the strain diagram remains continuous (Fig. 6). This improved model was used for the preliminary calculations to determine the scenario where the connection has the greatest effect to the cross-sectional moment resistance based on the concrete type and the reinforcement ratio. It is important to note that the value of the coefficient is continuously changing along the beam and the real value of this coefficient in the different cross sections cannot be determined without the examination of the entire beam. In order to gain the calculated force-displacement diagram of the beam, it is necessary to develop a whole beam model that considers the effect of the imperfect bond along the length of the beam. To compare the experimental results, it was assumed that perfect bond exists between the materials as a base case, so the value of the coefficient was chosen to be 1 for the whole beam. Failure occurs in the model when one of the components fail; that is rupture of the steel bars or crushing of the concrete based on their material models. Table 5 summarises the calculated load-bearing capacities and failure modes for the original and the strengthened beams. The calculations were made with the average strength values of the concrete elements that were measured on the day of the bending test (Table 3).

Figure 6.
Figure 6.

Base assumption of the analytical model. “xc” indicates the compressed zone, which can be obtained from a force balance calculated from stresses. Figures 1a) and 2a) show the case of perfect bond (coefficient=1), Figures 1c) and 2c) show the complete absence of bond (coefficient=0), and Figures 1b) and 2b) show the case of a partial bond (coefficient is between 0 and 1)

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

Table 5.

Calculated load-bearing capacities and failure modes of the original and strengthened beams

Beam ‘A’Beam ‘B’
OriginalStrengthenedOriginalStrengthened
Failure load [kN]9.8633.98 (increment 245%)26.249.11 (increment 87%)
Failure modeCompressed concrete crushing tensioned steel yieldingCompressed concrete crushing, steel is elastic in the original cross section, yields in the strengtheningCompressed concrete crushing tensioned steel yieldingCompressed concrete crushing tensioned steel yielding

5 EXPERIMENTAL RESULTS

The most important failure modes can be seen in Figure 7. Figures 89 show the average load- displacement diagrams of the strengthened beams. The detailed load-displacement diagrams produced by the testing machine can be seen in Figures 1019. A brief summary of the experimental results related to the failure modes and load-bearing capacities of the beams that were strengthened on the tensioned and compressed sides can be found in Tables 67, respectively. Ductility can be determined in several ways, among which the calculation was performed based on the method defined in Jen Hua Ling–Yong Tat Lim–Euniza Jusli (2023). Equation (1) was used to determine ductility, where Δy is the displacement corresponding to 75% of the measured maximum force before reaching the maximum force, and Δu is the displacement corresponding to the 85% of the measured maximum force after reaching the maximum force.

μΔ=ΔuΔy 1
Figure 7.
Figure 7.

Different failure modes of HPC strengthened beams

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Figure 8.
Figure 8.

Average force-displacement diagrams of test specimens strengthened on the tensioned side

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Figure 9.
Figure 9.

Average force-displacement diagrams of test specimens strengthened on the compressed side

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

Figure 10.
Figure 10.

Force-displacement diagrams of beams with untreated surface strengthened on the tensioned side

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

Figure 11.
Figure 11.

Force-displacement diagrams of beams with roughened surface strengthened on the tensioned side

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

Figure 12.
Figure 12.

Force-displacement diagrams of beams with connecting element strengthened on the tensioned side

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

Figure 13.
Figure 13.

Force-displacement diagrams of beams with roughened surface and connecting elements strengthened on the tensioned side

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Figure 14.
Figure 14.

Force-displacement diagrams of beams strengthened on the tensioned side

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Figure 15.
Figure 15.

Force-displacement diagrams of beams with untreated surface strengthened on the compressed side

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Figure 16.
Figure 16.

Force-displacement diagrams of beams with roughened surface strengthened on the compressed side

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

Figure 17.
Figure 17.

Force-displacement diagrams of beams with connecting elements strengthened on the compressed side

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Figure 18.
Figure 18.

Force-displacement diagrams of beams with connecting elements strengthened on the compressed side

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

Figure 19.
Figure 19.

Force-displacement diagrams of beams strengthened on the compressed side

Citation: Építés – Építészettudomány 52, 1-2; 10.1556/096.2024.00113

5.1 Failure modes of beams

5.1.1. Strengthening on the tensioned side

In the case of tensioned-side reinforcement, the failure modes of the beams were roughly the same despite the different roughening modes. Each beam failed due to one bending crack in the middle. Other cracks rarely occurred due to the closing effect of the fibres. A moderate ductility arose from the fibres in HPC material. During the crack opening, the fibres crossing the crack were slipping out from the HPC matrix, thereby preventing sudden failure. However, the ductility of the samples strengthened on the tensioned side was low. According to the analytical calculations, the steels in the original cross-section remained elastic during the failure. The originally slightly under-reinforced beams became over-reinforced due to the strengthening which might cause the experienced rigid behaviour. A more ductile result could probably have been achieved by the proper choice of the thickness of the strengthening layer or the quantity of the steel bars in it fitted to the original cross-section. This experimental result shows that more research is needed on what material properties, geometry and reinforcement are necessary to achieve better ductility.

There was no delamination at the end of the beams in either case. This may have been caused by the anchorage effect of the load on the supports, as the reinforcement layer continued beyond the supports along the entire length of the beam. As the reinforcement layer fell between the supports and the original concrete beam (Fig. 5), the support force increased the friction, thereby anchoring the reinforcement. Delamination starting from the middle opening crack was observed in some samples with an untreated surface (Fig. 7b). Further experiments could be carried out to investigate the outcomes in cases where the reinforcement layer ends before the support, and does not connect the reinforcement layer to the end of the beam.

Figures 7a7b show examples of the failure modes for tensioned-side reinforcement. Table 6 shows the results for each tensioned side strengthened sample.

Table 6.

Test results for specimens strengthened on the tensioned side. Calculated value: 33.98 kN. Coefficient of variation (CV) is given below the average value

Type of connectionSignMaximum Force [kN]/(compared to calc. value [%])Bending displacement at maximum force [mm]Ductility based on equation (1)Failure modeAverage maximum force [kN]/(compared to calc. value [%])Average bending displacement at maximum force [mm]Average ductility
Untreated surfaceA1_0137.58/ (+10)7.1652,860Bending failure with one localized crack in the middle started from the bottom36.84/ (+8) CV: 1.687.208 CV: 0.672,888 CV: 0,13
A1_0234.92/ (+2)6.5603,032
A1_0338.02/ (+11)7.9002,772
Roughened surfaceA2_0135.69/ (+5)7.4362,93234.71/ (+2) CV: 1.657.318 CV: 0.482,967 CV:0,04
A2_0235.64/ (+4)7.7263,015
A2_0332.80/ (-4)6.7912,954
With connecting elementA3_0138.50/ (+13)7.7072,67533.95/ (-0) CV: 4.087.079 CV: 0.632,930 CV:0,24
A3_0230.61/ (-10)6.4492,954
A3_0332.75/ (-4)7.0823,161
Roughened surface + connecting elementA4_0133.13/ (-3)6.9433,03133.67/ (-1) CV:1.537.613 CV: 0.583,002 CV:0,18
A4_0232.49/ (-5)7.9493,166
A4_0335.40/ (+4)7.9482,810

5.1.2. Strengthening on the compressed side

Compressed side reinforcement showed greater variance in failure mode, ductility, and load-bearing capacity caused by the different interfaces. Inner or end debonding of the strengthening layer occurred for the untreated interface as shown in Figure 7c. When connecting elements were used, the debonding stopped at the connecting elements and in these cases the crack continued as a shear crack from the connecting element towards the supports (Fig. 7e). For the roughened interface, debonding was not detected at the interface. Moreover, where coupling elements did not help the transmission of force, separation was experienced in the original concrete layer. In one case, failure occurred in the concrete substrate parallelly with the contacting interface at the end of the beam (Fig. 7d). For the roughened interface with or without connecting elements, the beams failed in bending or had a combined bending-shear failure (Fig. 7e). Due to the strong connection between the surfaces, these samples showed higher ductility during the failure. In Table 7 the failure mode of each sample is described for the compressed side strengthening.

5.2 Load-bearing capacity and ductility

5.2.1 Strengthening on the tensioned side

The force-displacement diagrams for all the beams which were strengthened on the tensioned side with HPFRC with steel reinforcement looked similar (Figs 8, 10–14). After reaching the peak force, a crack occurred, and samples failed in a quasi-rigid manner. Maximal forces and the deflections are summarised in Table 6.

Results measured at the maximum force showed high deviation when the tensioned side was reinforced. The reason for the high deviation is that the increment in the load-bearing capacity was also relied on the tensional stiffness of the HPFRC material. This material is strongly affected by the randomly distributed steel fibres in it. Experimental results proved that the bond type had no significant effect on the load-bearing capacity in the case of strengthening on the tensioned side which corresponds to the literature results (Huang et al. 2022. 11). The highest and lowest peak forces were both observed in specimens with untreated surfaces and connecting elements. Based on the average peak forces, the best performing specimens were the untreated surfaced ones. These beams could withstand 9.41% more than the least performing specimens with roughened surface and connecting elements. Compared with the calculated load-bearing capacity of the original beam, all strengthened beams improved the original load-bearing capacity more than three times (see Tables 56). The measured load-bearing capacities were consistent with the calculated values of the strengthened beams where a perfect bond was assumed (Table 6). In some cases, the measured values exceeded the calculated one. Considering the average deflections for the highest force in the different cases, the largest deflection values were achieved by the beams with connecting elements and a roughened surface (Table 6). The calculated ductility values were not significantly different from each other in the case of tensioned-side strengthening. However, the highest value was achieved by the beams with connecting elements and a roughened surface, and the lower value by the untreated surfaced beams. The ductility increment was 4.24% between their average ductility values.

5.2.2 Strengthening on the compressed side

Comparing the force-displacement diagram of the beams strengthened on the compressed side in Fig. 9, and Figs 1519, the bonding type showed a significant effect on the ductility of the strengthened beams. Noticeable increment in the maximum deflection was seen between the most rigid, untreated surfaced beams and the most ductile, roughened surfaced ones with connecting elements. The maximum deflection was affected by the failure mode. In the case of delamination (B2_03), the maximum deflection was reduced compared with the other roughened surfaced beams. The force-deflection diagrams can be considered stable after entering the plastic state, with just a few jumps until failure. When connecting elements were used, the early spikes in the force-displacement diagrams were usually followed by hardening. The calculated ductility values varied greatly in the case of compressed-side strengthening. The highest average value was achieved by the elements with untreated surface and connecting elements. The average calculated ductility value of beams with an untreated surface and connecting elements was more than three times the average value of the beams with an untreated surface. A stronger bond resulted in increment in the load-bearing capacity, too. However, the measured load-bearing capacities did never achieve the calculated values that assumed a perfect bond. Compared to the calculated load-bearing capacity of the original beam, they improved the load-bearing capacity more than 1.7 times (see Tables 5 and 7).

Table 7.

Test results for specimens strengthened on the compressed side. Calculated value: 49.11 kN. Coefficient of variation (CV) is given below the average value. The beam which was loaded at a different loading rate is signed by x

Type of connectionSignMaximum force [kN] / (compared to calc. value [%])Bending displacement at maximum force [mm]Ductility based on equation (1)Failure modeAverage maximum force [kN]/(compared to calc. value [%])Average bending displacement at maximum force [mm]Average ductility
Untreated surfaceB1_0146.10/ (–7)20.2814,367Bending failure44.90/ (–9)

CV: 1.26
20.333

CV: 0.18
3,492

CV:0,876
B1_02x43.58/ (–12)20.1832,615Delamination of reinforcement layer in the middle of the beam
B1_0345.03/ (–9)20.5343,493Complete delamination of the reinforcement layer at the end of the beam
Roughened surfaceB2_0146.73/ (–5)28.2239,657Bending-shear failure46.64/ (–6)

CV: 1.18
28.953

CV: 12.63
9,013

CV:2,488
B2_0247.77/ (–3)41.92911,116Bending failure
B2_0345.42/ (–8)16.7076,267Complete delamination of the reinforcing layer along with the original concrete layer at the end of the beam
With connecting elementB3_0144.62/ (–10)17.85017,552Bending-shear failure with delamination45.70/ (–7)

CV: 1.57
25.557

CV: 6.78
12,578

CV:4,401
B3_0244.97/ (–9)30.5729,187Bending-shear failure with delamination
B3_0347.49/ (–4)28.25010,995Bending failure
Roughened surface + connecting elementB4_0149.16/ (+0)39.7009,704Bending failure48.31/ (–2)

CV: 1.15
46.432

CV: 10.21
10,994

CV:1,924
B4_0247.00/ (–5)58,18613,205Bending failure
B4_0348.75/ (–1)41.41210,073Bending failure, failure of steel reinforcement

In terms of average load-bearing capacity and the average deflections of the peak force, the beams with both connecting element and surface roughening performed best, followed by the ones with only surface roughening, the beams with untreated surface and connecting element, while the beams with untreated surface and without connecting element performed the least well. Comparing the peak force averages of the beams with connecting element with the untreated surfaced ones without connecting elements the former was able to withstand 7.57% more force than the latter.

6 CONCLUSION

It has been proven by several practical applications that reinforced concrete structures with insufficient load-bearing capacity can be effectively retrofitted and strengthened by increasing the original cross-section with HPFRC. In this paper, beam retrofitting with cast in-situ strengthening type was studied. Beam specimens with differently bonded strengthening layers were evaluated to find the suitable connection mode between the original beam and the strengthening layer. Untreated and rough surfaced beams with or without connecting elements were examined. A simple analytical model was used to calculate the load-bearing capacities and predict the failure mode of the cross sections in the case of perfect bond between the different layers. The behaviour of the beams strengthened on the compressed side and on the tensioned side was completely different, so they were discussed separately.

Overall, comparing the experimental results presented in Tables 67 and the calculated load-bearing capacities and failure modes that assumed a perfect bond between the strengthening and the original cross-section showed that the experimental results best approached the calculated values when both roughened surface and connecting elements were used. This may be due to these cases being closest to the perfect relationship used for the calculations.

In the case of the presented tensioned side reinforcement, the following conclusions can be drawn:

  • All the specimens had the same rigid failure mode: bending failure with one localised crack in the middle that started from the bottom. Except the middle cross-section, the whole beam remained uncracked. Debonding was also observed in some samples with an untreated surface. The lack of formation of additional cracks may provide a basis for further research. Further experiments are recommended with different cross-sections and material quality ratios.

  • No average increment was experienced in the maximal force and ductility due to the stronger bond, which is consistent with previous studies (Huang et al. 2022).

For the presented compressed side reinforcement, the following conclusions can be drawn:

  • According to the experiments, the untreated surfaced beams can fail due to debonding, which occurs exactly at the contacting surface. It is important to note that the roughened surface could not stop the debonding either. However, applying connecting elements can change the failure mode and type.

  • The stronger the bond is between the contacting surfaces, the higher the load-bearing capacity of the strengthened beam can be. In these experiments, surface roughening together with connecting elements increased the maximum force by 7.57% compared with the untreated surfaced strengthened beam.

  • Significant difference was observed in the deflection and calculated ductility of the beams: the maximum deflection was measured to be more than two times higher in cases of all rough surfaced specimens with or without connecting elements compared with the untreated surfaced ones. Applying connecting elements to untreated surface also effectively improved the ductility.

FURTHER RESEARCH

Based on the experimental results, it can be concluded that the connection between the original beam and the strengthening can determine the strength and behaviour of the beams when the strengthening is applied on the compressed side. Since debonding can occur, the generally applied analytical models that assume a perfect connection can lead to exaggerated results. Therefore, it is necessary to develop a beam model that considers the imperfection of the relationship. The models used for FRP reinforcement (Rabinovitch 2004), which consider the possibility of delamination, can serve as a good basis.

On the tensioned side strengthening further experiments are needed where the strengthening layer is not anchored with supports, making delamination more probable. The reason for the relatively brittle failure observed in the case of the tension side reinforcement may also be the subject of further research.

ACKNOWLEDGEMENT

The authors would like to thank Mapei Kft. for providing the materials, the Material Testing Laboratory of the Department of Building Materials and Civil Engineering of the Budapest University of Technology and Economics for their help in preparing the test specimens, as well as the Czakó Adolf Laboratory of Materials and Structures of the Department of Mechanics, Materials and Structures at Budapest University of Technology and Economics for the assistance in performing the experiments.

References

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    • Search Google Scholar
    • Export Citation
  • Rabinovitch, O.: Fracture-Mechanics Failure Criteria for RC Beams Strengthened with FRP Strips–A Simplified Approach. Composite Structures 64 (2004) 3–4. 479492. doi: .

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    • Search Google Scholar
    • Export Citation
  • Seyam, A. M.Balázs, G. L.: A Review in Technologies, Definitions, Properties and Applications of Ultra High-Performance Concrete (UHPC). Vasbetonépítés 24 (2023) 105111. doi: .

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    • Search Google Scholar
    • Export Citation
  • Shunmuga Vembu, P.R.–Ammasi, A. K.: A Comprehensive Review on the Factors Affecting Bond Strength in Concrete. Buildings 13 (2023) 3. 577. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Silfwerbrand, J.: Codes for SFRC Structures – A Swedish Proposal. In Tailor Made Concrete Structures. New Solutions for our Society. International FIB Symposium, 2008. 553558. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Szabó, Z. K.Balázs, L. G.: Near Surface Mounted FRP Reinforcement for Strengthening of Concrete Structures. Periodica Polytechnica Civil Engineering 51 (2007) 1. 33. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Tanaka, Y.Maekawa, K.Yutaka KameyamaAkio OhtakeHiroyuki MushaWatanabe, N.: The Innovation and Application of UHPFRC Bridges in Japan. (2011) 149188. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Walraven, J. C.: High Performance Fiber Reinforced Concrete: Progress in Knowledge and Design Codes. Materials and Structures 42 (2009) 9. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhu, Y.Zhang, Y.Hussein, H. H.Chen, G.: Flexural Strengthening of Reinforced Concrete Beams or Slabs Using Ultra-High Performance Concrete (UHPC). A State of the Art Review. Engineering Structures (2020) 205. 110035. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Abdal, S.Mansour, W.Agwa, I.Nasr, M.Abadel, A.Onuralp Özkılıç, Y.Akeed, M. H.: Application of Ultra-High-Performance Concrete in Bridge Engineering. Current Status, Limitations, Challenges, and Future Prospects. Buildings 13 (2023) 1. 185. [online] doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • ACI 544.1R-96. Report on Fiber Reinforced Concrete. ACI (American Concrete Institute) 2002.

  • Afroughsabet, V.Biolzi, L.Ozbakkaloglu, T.: High-Performance Fiber-Reinforced Concrete: A Review. Journal of Materials Science 51 (2016) 14. 65176551. [online] doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Askar, M. K.Hassan, A. F.Al-Kamaki, Y. S. S.: Flexural and Shear Strengthening of Reinforced Concrete Beams Using FRP Composites. A State of the Art. Case Studies in Construction Materials (2022) e01189. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Bertola, N.Schiltz, P.Denarié, E.Brühwiler, E.: A Review of the Use of UHPFRC in Bridge Rehabilitation and New Construction in Switzerland. Frontiers in Built Environment (2021) 7. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • CNR-DT 204. Guidelines for Design, Construction and Production Control of Fiber Reinforced Concrete Structures. Italy: National Research Council of Italy, 2006.

    • Search Google Scholar
    • Export Citation
  • Commentary on the DAfStb Guideline ‘Steel Fibre Reinforced Concrete’. (2015). 1st ed.: DAfStb.

  • Csuka, B.Kollár, L. P.: Analysis of FRP Confined Columns Under Eccentric Loading. Composite Structures 94 (2012) 3. 11061116. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danraka, M. N.Mahmod, H. M.Oluwatisin, O. J.: Strengthening of Reinforced Concrete Beams Using FRP Technique: A Review. International Journal of Engineering Science and Computing 7 (2003) 6. 1319913213.

    • Search Google Scholar
    • Export Citation
  • El-Mir, A.Nehme, S. G.Sinka, Z.Vági, I.: Properties of Ultra-High Performance Concrete Made Utilizing Supplementary Cementitious Materials. In: Vági, I. (ed.): The Eleventh High Performance Concrete (11th HPC) and the Second Concrete Innovation Conference (2nd CIC). 2017.

    • Search Google Scholar
    • Export Citation
  • Hollaway, L. C.Teng, J. G.: Strengthening and Rehabilitation of Civil Infrastructures Using Fibre-Reinforced Polymer (FRP) Composites. Elsevier 2008.

    • Search Google Scholar
    • Export Citation
  • Huang, Y.Grünewald, S.Schlangen, E.Luković, M.: Strengthening of Concrete Structures with Ultra High Performance Fiber Reinforced Concrete (UHPFRC). A Critical Review. Construction and Building Materials (2022) 336. 127398. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Jen Hua LingYong Tat LimEuniza Jusli: Methods to Determine Ductility of Structural Members: A Review. Journal of the Civil Engineering Forum 9 (2023) 2. 181194. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kaish, A. B. M. A.Jamil, M.Raman, S. N.Zain, M. F. M.Nahar, L.: Ferrocement Composites for Strengthening of Concrete Columns: A Review. Construction and Building Materials (2018) 160. 326340. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kalfat, R.Al-Mahaidi, R.Smith, S. T.: Anchorage Devices Used to Improve the Performance of Reinforced Concrete Beams Retrofitted with FRP Composites: State-of-the-Art Review. Journal of Composites for Construction 17 (2013) 1. 1433. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kollár, L. P.Springer, G. S.: Mechanics of Composite Structures. Cambridge University Press, 2003.

  • Lampropoulos, A. P.Paschalis, S. A.–Tsioulou, O. T.–Dritsos, S. E.: Strengthening of Reinforced Concrete Beams Using Ultra High Performance Fibre Reinforced Concrete (UHPFRC). Engineering Structures (2016) 106. 370384. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Mapei Kft.: Planitop HPC Technical Data Sheet. 2023.

  • Martinola, G.Meda, A.Plizzari, G. A.Rinaldi, Z.: Strengthening and Repair Of RC Beams with Fiber Reinforced Concrete. Cement and Concrete Composites 32 (2010) 9. 731739. .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Neville, A.Aïtcin, P.-C.: High Performance Concrete. An Overview. Materials and Structures 31 (1998) 2. 111117. doi: .

  • Noshiravani, T.Brühwiler, E.: Analytical Model for Predicting Response and Flexure-Shear Resistance of Composite Beams Combining Reinforced Ultrahigh Performance Fiber-Reinforced Concrete and Reinforced Concrete. Journal of Structural Engineering 140 (2014) 6. 04014012. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Rabinovitch, O.: Fracture-Mechanics Failure Criteria for RC Beams Strengthened with FRP Strips–A Simplified Approach. Composite Structures 64 (2004) 3–4. 479492. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Seyam, A. M.Balázs, G. L.: A Review in Technologies, Definitions, Properties and Applications of Ultra High-Performance Concrete (UHPC). Vasbetonépítés 24 (2023) 105111. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Shunmuga Vembu, P.R.–Ammasi, A. K.: A Comprehensive Review on the Factors Affecting Bond Strength in Concrete. Buildings 13 (2023) 3. 577. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Silfwerbrand, J.: Codes for SFRC Structures – A Swedish Proposal. In Tailor Made Concrete Structures. New Solutions for our Society. International FIB Symposium, 2008. 553558. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Szabó, Z. K.Balázs, L. G.: Near Surface Mounted FRP Reinforcement for Strengthening of Concrete Structures. Periodica Polytechnica Civil Engineering 51 (2007) 1. 33. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Tanaka, Y.Maekawa, K.Yutaka KameyamaAkio OhtakeHiroyuki MushaWatanabe, N.: The Innovation and Application of UHPFRC Bridges in Japan. (2011) 149188. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Walraven, J. C.: High Performance Fiber Reinforced Concrete: Progress in Knowledge and Design Codes. Materials and Structures 42 (2009) 9. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhu, Y.Zhang, Y.Hussein, H. H.Chen, G.: Flexural Strengthening of Reinforced Concrete Beams or Slabs Using Ultra-High Performance Concrete (UHPC). A State of the Art Review. Engineering Structures (2020) 205. 110035. doi: .

    • Crossref
    • Search Google Scholar
    • Export Citation
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Építés - Építészettudomány
Language English Hungarian
Size B5
Year of
Foundation
1957
Volumes
per Year
1
Issues
per Year
4
Founder Magyar Tudományos Akadémia  
Founder's
Address
H-1051 Budapest, Hungary, Széchenyi István tér 9.
Publisher Akadémiai Kiadó
Publisher's
Address
H-1117 Budapest, Hungary 1516 Budapest, PO Box 245.
Responsible
Publisher
Chief Executive Officer, Akadémiai Kiadó
ISSN 0013-9661 (Print)
ISSN 1588-2764 (Online)

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