Flexural Properties of ECC-Concrete Composite Beam

Rebar corrosion-induced durability issue is a major concern for bridges. )e ECC cover was employed to prevent the intrusion of the corrosive agent. )is paper studied the flexural behavior of ECC-concrete composite beam. )e effects of bonding at the interface and fiber mesh reinforcement on the flexural properties and cracking pattern were investigated. )e strain distribution and midspan deflection were evaluated. Test results show that the bonded composite beam had a higher loading capacity. But the unbonded composite beam showed better postcrack energy absorption capacity with higher midspan deflection. )e fiber mesh reinforcement could further improve the flexural properties regardless of the bonding condition. )e strain at the bottom of the unbonded beam was much smaller than that of the bonded beam. )e penetrated cracks were observed at the ECC layer of the bonded composited beam.


Introduction
e long-term durability of bridges has become a major concern, especially for bridges exposed to aggressive environmental conditions.In 2015, about 79.6 thousand bridges, which is 10.2% of the total bridges, were classified as dangerous in China [1].e main cause of bridge deterioration is the corrosion of reinforcing steel, which results in reduced service life [2] or even collapse.To prevent steel corrosion, a certain thickness of concrete cover is designed for reinforced concrete structures.However, in practice, concrete cover will definitely crack due to mechanical and environmental loading, low tensile strength of concrete, and so on, thereby creating a fast entry path for corrosive agents and causing corrosion [3].erefore, one of the key points of prolonging bridge service life is to prevent cracking and reduce the permeability of the concrete cover.
Engineered cementations composite (ECC) developed based on micromechanics has a high ductility and crack control capacity (crack widths less than 100 μm) even at large deformation [4,5].Besides the excellent mechanical properties, a large amount of researches show that ECC has much higher durability than that of normal concrete [6][7][8].Researches indicate that even strained in tension up to 3%, the permeability and chloride ion diffusivity of ECC were similar to that of uncracked concrete [9,10].ese unique properties make ECC very suitable as concrete cover for bridges under aggressive environments [11].Researches have been carried out to investigate the mechanical properties of the reinforced ECC-concrete composite beam [12][13][14][15][16][17][18][19].Compared to normal reinforced concrete, properties, including load-carrying capacity, deformability, crack controlling ability, and fatigue, of beam with ECC layer were improved significantly.e ultimate strength and deflection improvement in composite beams are mainly dependent on the tensile and compressive ductility of the matrix [20].
In most of the current studies, the ECC layer was fully bonded with the normal concrete.
erefore, these two layers could deform together and increase the load-carrying capacity.However, large cracks developed at the concrete could cause localized cracking in the ECC layer [15,19].Some cracks could still penetrate through the ECC layer, resulting in an entry path for corrosive agents.In this situation, the high ductility and durability of the ECC could not be fully utilized.In order to prevent the penetrated crack in the ECC cover, the unbound composite beam is proposed in this paper.A plastic sheet was placed at the interface of normal concrete and ECC to break the bond.Large strain caused by the cracking in the normal concrete will be distributed across the entire ECC layer, avoiding the strain concentration.A ber mesh reinforcement was also placed in the middle of the ECC layer to further increase its strength and ductility.In this paper, the e ects of bonding and reinforcement of ECC on the exural properties of the composite beam were investigated.

Materials.
e cement used was ordinary 42.5# Portland cement with a 28-day compressive strength of 42.5 N/mm 2 .
e main chemical components of cement are provided in Table 1.e class F y ash containing 3.88% CaO from Jinan, Shandong province, was adopted.e characteristics of used PVA ber are listed in Table 2. e high-range water reducer (HRWRA) and viscosity-modifying agent (VMA) were employed simultaneously to obtain the proper workability.Based on the previous study, the water to cement ratio of 0.32, y ash to cement ratio of 1.2, silica sand to cement ratio of 0.8, and 2% (by volume) of ber were adopted for ECC mix.e concrete with the compressive strength of 40 MPa (C40) was used for the composite beam.

Experiment Design.
e 100 × 100 × 400 mm composite beam with a 20 mm ECC layer at the bottom was casted for the study.Two types of the ECC layer were considered.One is the pure ECC layer and the other is the reinforced ECC layer with ber mesh reinforcement in the middle.Two interface bonding conditions, fully bonded and unbonded, between normal concrete and ECC layer were designed.erefore, four di erent types of ECC-concrete composite beams were studied.During the four-point bending testing, the strain at di erent locations and midspan de ection was monitored.
e con gurations of the strain gauges are shown in Figure 1.

Specimen Preparation.
e C40 concrete was rst mixed according to GB/T 50081-2002 [21] and casted in the mold.
e reinforcing steel was then embedded into the concrete.After cured for 24 hours, the ECC was mixed and casted on top of the normal concrete.e beam was then cured in the standard curing room with 20 ± 2 °C temperature and 95% humidity for 28 days.For the unbonded composite beam, a plastic sheet was place on top of the normal concrete before placing ECC to prevent the bonding between ECC and normal concrete.e ECC layer was anchored at both ends.For the reinforced ECC layer, the ber mesh was placed at the middle of the ECC layer before casting ECC.

Testing Methods.
e uniaxial tensile and four-point bending tests were conducted to evaluate the properties of ECC. e 15 mm × 50 mm × 350 mm specimen was used for both tests.e LVDT displacement sensors were employed to measure displacement.
e universal testing machine (WDW-100E) was used for loading.e loading rates were 0.1 mm/min and 0.5 mm/min for direct tensile and four-point bending tests, respectively.e testing setups are shown in Figure 2.For the composite beam, the four-point bending test was carried out by using the microcomputer-controlled electronic universal testing machine under displacement control at the rate of 0.5 mm/min until its failure.e strain at di erent locations was collected by DH3818-4 strain acquisition box.e midspan de ection was measured by LVDT.Advances in Civil Engineering

Results and Discussion
3.1.Properties of ECC Material.Figures 3 and 4 show that, under both uniaxial tensile and flexural loading, ECC exhibits strain hardening behavior.e first cracking strength was 2.9 MPa.After the first cracking, the load continued to increase without fracture localization.Sequentially, more cracks developed, resulting in the inelastic strain at increasing stress.e ultimate tensile strength and tensile strain capacity were 4.4 MPa and 4.5%, respectively.e flexural behavior was similar to that under uniaxial tensile loading (Figure 4).e four-point bending test could be used as an indirect evaluation method for the strainhardening properties of ECC [22].e midspan deflection reached 20.5 mm at failure.e first cracking strength and flexural strength were 7.7 and 14.7 MPa, respectively, which were much higher than those of normal concrete.
e typical microcracking patterns of specimen under uniaxial tensile and flexural loading are shown in Figure 5.As observed in the figures, microcracks with very tight crack width were uniformly distributed with an average spacing less than 1 mm.e cracking pattern also indicated that the ECC had a very good strain-hardening property.

Flexural Behavior of Composite Beam.
As shown in Figure 6, both composite beams show elastic and plastic behavior under the flexural loading.At the beginning, the midspan deflection increased linearly with the flexural loading.At the end of the linear portion there was a force drop, it could be caused by the cracking of ECC and yielding of reinforcement steel.After that, more deflection occurs.Figure 6 also indicates that, regardless of whether the fiber mesh was embedded in the ECC layer, the type of bonding between the concrete and ECC layer had significant effect on both the flexural loading capacity and midspan deflection.Since the ECC had high tensile strength and ductility, it will carry the tensile strength together with the steel reinforcement after concrete cracks, resulting in higher strength.is trend is consistent with other research works [15,18,19].Differently, the unbonded composite beam showed a great postcrack energy absorption capacity due to the deformation of the ECC layer.e unbonded ECC cover could be treated as an external strengthening reinforcement.Since slip at the interface was allowed, the longitudinal strain was distributed across the ECC cover, thereby, allowing higher deflection. is finding is similar to Kamada and Li's research.ey also found that the interface property could affect the flexural behavior of the composite beam.
e smooth surface specimen was able to redistribute the load and utilize more materials than rough surface specimens.erefore, the deflection of the smooth beam was larger than that of the rough beam [23].
Placing a fiber mesh in the middle of the ECC layer could further increase its tensile strength and ductility, which in turn increased the loading capacity and midspan deflection at failure (Figure 7). is e ect is more prominent for the bonded composite beam.e increments were 50% and 70% for the loading capacity and midspan de ection at failure, respectively.e midspan de ections were 2.1 and 3.0 mm for the unbonded beam with and without ber mesh reinforcement, respectively.

Load-Strain Pattern of Composite Beams.
e load versus strain at di erent locations of the bottom is plotted in Figure 8. ese two types of beam possessed totally di erent patterns.For the unbonded composite beam, the tensile strains at points 1 and 2 were negligible due to very small stress and bending moment.e major strain occurred at the   middle of the span.Di erently, the strain at point 1 of the unbonded composite beam was close to that of point 3.
Because of the slip at the interface, there was limited shear resistance.e load carried by the ECC layer would pass to the anchors at both ends.In this case, all ECC layers were under tension and had similar tensile strain at the longitudinal direction.is means that under the same de ection, the ECC layer of the unbonded composite beam had smaller strain and lower risk of cracking than that of the bonded Figure 9 shows the strain distribution at the midspan section of the composite beam.Figure 9(a) indicates that the bonding between the ECC and normal concrete was strong and no slip occurred.e trend is similar to the current research work [16].But for the unbonded composite beam, the strain of the ECC layer increased slowly with the load.When loaded at 15 kN, the strain was 1328 με at point 7.But the bottom strain at point 3 was only 227 με. e high-strain value at point 7 was caused by the cracking of normal concrete.Figure 9 also indicates that under the same loading the strain at the bottom of the unbonded beam was much smaller than that of the bonded beam. is further proves that the ECC layer of the unbonded composite beam had lower risks of cracking, making the ECC suitable as concrete cover for corrosion resistance.Advances in Civil Engineering ECC cover.Although the ECC cover improved the beam's mechanical performance, there were still risks of corrosion of the rebar inside the beam and reduction of service life of the bonded composite beam.Differently, there were no cracks penetrating through the ECC cover for the unbonded composite beam.Also, the crack's width at the ECC cover was limited due to the high crack control capacity.e average crack widths at failure were 115 and 98 μm for the unbonded composite beam without and with fiber reinforcements, respectively.erefore, even though there were cracks at the ECC cover, the permeability would be limited due to very small crack width, resulting in high durability.Figure 11 also shows cracking at the left end of the ECC layer.Since the ECC layer could be treated as the external reinforcement of the unbonded composite beam, the force provided by the ECC layer is transferred to the normal concrete in the compression mode through end anchors, resulting in tensile stresses in the end anchors.If the stress is large enough, it will lead to cracking and failure of the anchor.

Conclusions
is paper studied the effects of bonding and fiber mesh reinforcement on the flexural properties and cracking pattern of the composite beams.e following findings and conclusions can be drawn: (1) ECC exhibits strain-hardening behavior.
e first cracking strength, ultimate tensile strength, and tensile strain capacity were 2.9 MPa, 4.4 MPa, and 4.5%, respectively (2) e bonded composite beam had a higher loading capacity.But the unbonded composite beam showed a better postcrack energy absorption capacity.e fiber mesh reinforcement could further improve the flexural properties, regardless of the bonding condition (3) e unbonded ECC layer had the ability to distribute the strain across the beam.Under the same loading, the strain at the bottom of the unbonded beam was much smaller that of the bonded beam (4) cracking could penetrate through the ECC cover of the bonded composite beam.e average cracking widths were controlled at 115 and 98 μm for unbonded composite beam without and with fiber reinforcements.
e results of this study indicate that the unbonded ECC cover is more effective in terms of controlling the cracking and preventing the corrosion-induced damage.erefore, the unbonded composite beam could be used for bridges under aggressive environments to enhancing its service life.However, further study is needed to quantify the effect of the bonding on the beam behavior and to explore the durability of the composite beam.Advances in Civil Engineering

Figure 2 :Figure 3 :Figure 4 :
Figure 2: Test setups for direct tensile and four-point bending tests of ECC.(a) Uniaxial tensile test.(b) Four-point bending test.

Figure 11 :
Figure 11: Cracking pattern of the unbonded composite beam.

Table 1 :
e main chemical components of cement.

Table 2 :
Characteristics of PVA ber.Con gurations of strain-testing points of the composite beam.