Journal of Structural Engineering Vol.65A (March 2019) JSCE

Development of effective precast concrete barrier wall thickness for existing bridge

Basit Sarfaraz*, Takeshi Maki†, Hiroshi Mutsuyoshi**, Yosuke Ishihara***, Hitoshi Tajima****

*Ph.D Student, Dept. of Civil and Environmental Engg, Saitama University, Saitama-Shi, Saitama 338-8570 †Dr. of Eng., Professor, Dept. of Civil and Environmental Engg, Saitama University, Saitama-Shi, Saitama 338-8570 **Dr. of Eng., Professor, Dept. of Civil and Environmental Engg, Saitama University, Saitama-Shi, Saitama 338-8570

***Deputy Manager, Tokyo West Bureau, Metropolitan Expressway Company Limited, Chiyoda-ku, Tokyo 102-0093 ****Dr. of Eng., Deputy Director General, Highway Technology Research Center of Metropolitan Expressway, Minato-ku, Tokyo

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The existing barrier on bridges, having a wall thickness of 250mm is based on the initial standard of construction which follow the allowable stress design method. For the mass construction, reduction of onsite construction time and minimum traffic obstruction can be achieved by using of precast concrete construction. This research explores the structural behavior of precast concrete barrier by varying the wall thickness and verify its structural performance up to failure. A series of static loading tests were carried out to evaluate the validity of the proposed barrier. It is being observed from the test results, the designed strength and required structural response have been achieved by using the 150mm thick wall barrier. A finite element model technique in numerical modeling has been adapted to reproduce the designed structural response of the concrete barrier. Keywords: bridge, precast concrete barrier, nonlinear finite element, numerical modeling

1. INTRODUCTION

Concrete bridge barriers are installed for road safety facilities

to prevent vehicles or pedestrians on the road get out of the course or opposing the lane. Their primary functions are to contain vehicles and redirect them back onto the roadway or stop them within an acceptable distance1). In the earlier era of Japan, Metropolitan Expressway was used to design concrete bridge barrier of a standard wall thickness of 150mm which constructed until May 1965, and then designed has been changed to a standard wall thickness of 200mm which utilized until May 1992. From this point onwards, a high column with a wall thickness of 250mm is designed and constructed to secure the fog thickness and Fall Protection requirements2).

The existing concrete barrier under severe environmental conditions cause deterioration of concrete barrier because of internal steel reinforcement corrosion3). Cast-in-place concrete barriers have proven through crash-testing and in-field evaluations that they meet these structural requirements4). However, they are

prone to early-age cracking owing to a combination of shrinkage and thermal dilations5). The early-age cracks immediately expose barriers to water ingress increasing the risk of corrosion from chlorides and damage from freeze–thaw cycles and have also been observed to accelerate the deterioration processes leading to premature and expensive bridge repairs6). The above-mentioned severe problems, the bridge traffic barrier retrofit policy1) reason and for speeding up bridge construction are the reasons for the use of precast concrete barrier modules in several jurisdictions7).

For this purpose, a high strength precast concrete barrier system is being developed. Due to the improved durability and higher standard of fabrication, the precast concrete barriers provide a solid reason incentive for their use. It also provides the markable reduction in construction time associated with its installation provides another. The faster installation (or repair) time of precast concrete barriers is a significant financial benefit, particularly in urban areas where construction costs associated with traffic delays and hazards can reach up to 30% of the total project costs4),8). To achieve mass construction and renovation works, repeated use of formwork, reduction of onsite construction time, minimum traffic obstruction and significant economic benefits9) can be achieved by

† Corresponding author E-mail: maki@mail.saitama-u.ac.jp

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using of high-performance high strength precast concrete construction is adopted for this research.

Where we talk about the structural response of precast barrier depends upon the barrier wall thickness, barrier continuity, i.e., the barrier segment length and barrier-to barrier connections. The purpose of this research is to optimize the behavior of largescale precast concrete barrier with existing bridge deck overhangs subjected to transverse loads11) and the thinner section is better for avoiding an increase of dead load. This objective is achieved using nonlinear finite element (NLFE) models validated on full-size bridge barrier-deck units tested in the laboratory. The experimental tests used as a reference to justify structural behavior of precast concrete barrier to bridge deck during static loading. Once the numerical models are validated with these tests, NLFE models are used to gather more information about the micromechanical behavior of precast barrier with an existing deck slab15), 16). Then these findings are to be used for optimizing the design of the precast concrete barriers and propose design recommendations/ guidelines.

2. EXPERIMENTAL PROGRAM

The precast concrete barrier was used to install at the actual

bridge site. For this purpose, these precast barrier wall is going to attach to the existing deck slab by removing the existing cast in situ barriers. By demolishing the existing barrier, these barriers are joined with an existing deck slab by providing the loop reinforcement connection. This loop reinforcement joint is then filled with highly durable interlocked mortar to ensure its durability and ductile behavior at the ultimate stage.

This precast concrete barrier is designed for Japan Highway specification for bridges, Vehicle Class SB. Minimum vehicle speed for class SB about 60 km/h with 25Ton truck weight and collision angle is 15o. According to AASHTO, it is equal to Test Level-4 specification/requirement1). It is underlined that the precast concrete barrier considered in this research is tested under static design load test. Whereas, the static design load is calculated by estimating the crash energy of the designated vehicle10). According to Japan Specification for Highway Bridges, this crash energy (impact energy) is converted into a design collision load (static load) by using following equation2):

F = Kf 2 1+ev

Lw sin θ

W

Wr

2

Is∙ a (1)

Where, F = collision load (kN), Kf = correction proportional coefficient (=0.1), ev = vehicle rebound coefficient (=0.2), Lw =

axle spacing (m), = collision angle (degree), W = vehicle weight (kN), Wr = rear wheel axel weight (kN), Is = collision energy (kJ), a = slope reduction coefficient (=1.0).

The laboratory experiments are comprising on five (05) specimens. Out of these five specimens, four were small-scale

specimens and one was a full-scale specimen. Full-scale specimen was Five-metre-long. All specimens were having the 1.5 m wide base slab. The base slab was anchored to the laboratory’s floor

Fig. 1 Experimental setup and loading condition

Fig.2 Schematic scheme for laboratory loading

Fig.3 Test specimen details and variables

Loading Axis

Top Hz. Disp. Transducers

Loading Head

Actuator

Vert. Disp. Transducers PI Gauge

Base Hz. Disp.

Transducers Base Slab

Wal

l Bar

rier

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with post-tensioned high strength steel bar, tensioned to 300kN as shown in Fig. 1. These precast concrete barriers were loaded in the transverse direction at the top of the barrier wall and at the mid-length in a quasi-static controlled displacement as laboratory setup shown Fig. 2.

2.1 Specimen Details and Laboratory Setup

For this research, the precast concrete barrier, a precast barrier wall, and base slab were joined by a loop reinforcing bar. This portion was filled with highly durable interlocked mortar, as shown in Fig.3. Thus, the loop length of the bars determines the height of the joining portion and the volume of filling mortar which highly influences on the construction period.

The geometrical details of specimens are illustrated in Table 1. Specimen No. 1 and Specimen No. 2 were having the same geometry but different loop joint detailing to minimize the joining mortar and observed influence on the behavior to precast wall barrier. Where Specimen No. 3 having the same geometry with Specimen No. 1 except precast wall barrier thickness which 200mm instead of 250mm. Similarly, specimen No. 4 was going to further reduction of wall thickness up to 150mm.

Based on the above-said specimens results in a full segment length 5m specimen were tested under static loading having the wall thickness 150mm. The reinforcement bar assembly of the first four specimens is illustrated in fig. 4 & 5 respectively. Where Fig. 6 illustrates the reinforcement details and response measuring levels of Specimen No. 5.

The precast concrete barrier wall is attached with the base slab by using the cementitious grout injected as shown in Fig.3. By using this methodology of construction causes the two-possible boundaries in form of interface, 1st interface develop between the precast wall and injected mortar and 2nd interface is developed between mortar and base slab. In between these two interfaces, the loop reinforcement height is also varied from 260mm to 170mm to check the adequacy of class SB type safety performance of these kinds of the precast concrete barrier.

The non-deformed able rigid base slab is used to connect the precast barrier instead of an actual thin slab thickness due to the limitation of laboratory conditions. Because this research mainly focusses on the influence of wall thickness on the mechanical response of the wall barrier itself as a structural component. For this research, our target is to minimize the disturbance of the existing deck slab. In Fig. 4, 5 and 6 specimens are designed for

the new construction of an additional lane in the existing bridge. In these specimens, hanger bars and loop are constructed in the non-deformed able rigid slab and they are provided enough rigidity with main reinforcement of handrail bars and H4 in all cases. But for the construction with the actual thin slab of the bridge, these rebars are connected by using some other techniques, which cause minimum disturbance of existing deck slab.

Rigid base slab in connected with the laboratory floor with the

Table 1 Test specimen variables detail

Specimen No.

A C F H* (mm) (mm) (mm) (mm)

1 250 665 730 260 2 250 665 730 170 3 200 665 730 260 4 150 665 730 170 5 150 5000 5500 170

Fig.4 Reinforcement details and measuring level for No.1 & 2

Fig.5 Reinforcement details and measuring level for No.3 & 4

Fig.6 Reinforcement details and measuring level for No.5

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help of high strength steel bar. Lateral loading was applied with the actuator on the top of specimens as shown in Fig. 2. Displacement transducers were attached at the axis’s of loading to measure the lateral displacement of the test specimen. Horizontal displacement transducer was attached at the top and bottom of the base slab to measure the horizontal movement of the specimen. Some vertical displacement transducer was attached at the toe of the barrier wall to measure the opening of the 2nd interface. Whereas, the other vertical displacement transducer was attached at the toe of the specimen to measuring the uplift of the specimen. 2.2 Static Loading Test

In this research, the static loading test has been performed equivalent to dynamic impact load for Class SB vehicles. In experiments the structural response and mechanical behavior of reinforcement bars and concrete has been observed on different levels as shown in Fig. 4 & 5 for Specimen No. 1&2 and Specimen No. 3&4 respectively and Fig. 6 for Specimen No. 5.

Each specimen was composed with inside and outside main-bars, haunch bars, and loop bars. Each specimen was pushed from inside of the wall to outside at the height of 1000mm from an upper surface of footing as shown in Fig. 4, 5 and 6. The static displacement control loading is applied at a right angle on the longitudinal direction. Strain gauges were attached at reinforcement bars to measuring strain at different mention levels and concrete gauges were also attached at different measuring levels. PI gauges were attached to measure the opening of interface 1 between precast wall barrier and mortar. Where displacement gauges were attached to measure the opening of interface 2 between mortar and base slab as shown in Fig. 2. Material properties of first four specimens are shown in Table 2.

Table 2 Material properties of specimens No. 1 to 4

3. NUMERICAL MODELLING 3.1 Introduction and 3D Modelling

The optimization of the precast concrete barrier design was done with nonlinear finite-element calculations using the software program DIANA 10.2. The three-dimensional numerical model was developed for this research work to reproduce a Specimen No.5 (5-m-long) barrier with the associated base slab. Reset four (04) specimen were also modeled but, in this paper, only Specimen No. 5 was demonstrated.

Specimen No. 5 has full-scale length and its structural response was elaborated in this paper. The appropriate longitudinal length of the whole specimen is crucial because the resistance of the barrier against a crash can be fully exerted by biaxial bending, that is, flexures about its longitudinal and vertical axes. Here, the longitudinal length is directly related to the flexural behavior with respect to the vertical axis. As expected, the longer the longitudinal length of the specimen is kept 5m to consider the full precast segment only for specimen No. 5.

Interfaces between the precast wall and cementitious grout mortar structure were modeled by using interface element. Similarly, the connection between the mortar and base slab was also model by using the interface element as shown in Fig7. Thus, the loss in strength and rigidity caused by crack propagation, crack opening and reinforcement yielding were reproduced in the model. The numerical model used for the barrier design was slightly modified to reproduce the experimental setup for the tests carried out in the laboratory. Maekawa-Fukuura concrete model was used to analyze the precast reinforced concrete barrier. Reinforcement was modeled according to Von Mises and Tresca Plasticity model. Where the loading plate and post-tensioning bars were modeled as reinforcement.

The force-displacement relation of an individual element can be derived from the constitutive modeling of materials in the

Fig.7 Location of interface in 3D modelling

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element concerned, and the overall structural behaviors were computed by solving equilibrium and deformational compatibility among elements. Then, the accuracy of structural analysis mainly depends on the constitutive modeling defined in each finite element domain13). 3.2 3D Modelling Information

The Maekawa-Fukuura concrete model in DIANA is a combination of the Total Strain crack model combined with the Maekawa Cracked Concrete curves, and the Elasto-Plastic Fracture model. In contrary to the Total Strain crack model the Maekawa-Fukuura concrete model makes use of a non-orthogonal crack definition. The constitutive model based on total strain is developed along the lines of the Modified Compression Field Theory, like the multi-directional fixed crack model the total strain-based crack models follow a smeared approach for the fracture energy.

For this research, the finite element modeling following material properties of specimens are listed in Table 3. The compressive strength of the wall barrier was 78.42MPa. Due to high strength concrete property of wall barrier the tensile cracking strength is taken as 1MPa for analysis purpose. High strength concrete has the brittle failure plane that’s why the tensile strength was reduced up to 50%. A controlled displacement step was used to load the precast concrete barrier wall and constant stiffness method was used to solve the structural response. Hexa-quad mesh type was used having linear mesh order to mesh the specimen. For this purpose, the Iterative solution algorithm was used. To take care of the shear problem, constant stiffness was used. In Fig.8, 3D modeling configuration illustrated. 4. RESULTS AND DISCUSSION 4.1 Experimental and Numerical Response of Precast

Concrete Barrier A full-scale experiment was conducted on the specimen No. 5

having segment length of 5m. Static loading was applied in accordance with Class SB vehicle according to Japan highway specification for bridges. In the paper, the precast barrier wall was connected to the non-deformable concrete slab. The amount of vertical reinforcement at the front face behaves as the main reinforcement.

Fig.9 shows different views of the crack pattern of the tested barrier specimen. In this specimen, the first visible crack was observed in the front top side of the barrier wall at 70kN near the axis of loading head. Whereas, the 1st flexural crack appeared at the design section at level M as shown in Fig.6. Although flexural cracks at the barrier penetrated further into the barrier thickness at a higher load, at the load of 35kN & 63kN the interface 1 (precast wall barrier with mortar) & interface 2 (mortar with base slab) opening occur respectively. The crack pattern has drowned after the experimental test finished. Fig. 9 clearly shows the punching

shear failure in the precast wall barrier. The contour near the loading plate area shows the very large value of load 200kN and 225kN where the upper and lower contour shows the smaller values 170kN and 120kN respectively.

Table 3 Material properties used in NLFE modelling (Sp. No.5)

Fig.8 3D Modelling configuration (Sp. No. 5)

Fig.9 Experimental crack pattern after test (front & top view)

Fig.10 Numerical crack pattern at ultimate

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Force-displacement relationship and cracking behavior from the experimental and numerical results are compared in Figs. 9, 10 and 11. The schematic scheme for loading in fig. 2 shows the lateral displacement was recorded in line with the loading axis which is the centerline of an actuator.

The close agreement between experimental and numerical stiffness, ultimate load (peak load), cracking and failure mode indicate that nonlinear finite element analysis by using Diana and as explained earlier modeling assumptions accurately simulate the precast concrete barrier with the base slab.

Whereas, ultimate load is referring to peak load of the specimen. Ultimate stage is the failure of specimens due to concrete crushing in small specimens but in large specimen the ultimate stage refers to punching shear of concrete. Numerical results show some overestimate ultimate strength is due to the failure plane and shear retention factor which control the post-peak behavior of the structure. Some of the following micromechanical behavior of structural components has been checked in test results and numerical analysis results for a better understanding of precast concrete barrier and to evaluate some modifications in the design of the precast barrier.

Fig.11 Load-displacement curve (experimental &

numerical results)

Fig.12 Experimental & numerical results of strain in H2 rebar

Under the provision of guardrail installation standards, it is necessary to design the deformation capability of the rigid protective barrier in anticipation of deformation within the elastic limit of the main member constituting the guard fence at the time of loading with collision equivalent load10),14). Fig. 12 and 13 show the strain of reinforcements at different levels of measurements as shown in fig. 6. The deformation of reinforcement (H2) of precast barrier wall was started at load 38kN but yielding of a bar was started at the load of 220kN & 250kN in experimental and numerical results respectively.

Where the design load is 72kN at the topmost of the wall barrier. This load was converted to the load at the loading point in the test that was 81kN. The precast concrete barrier successfully resisted a 243kN which is 3 times higher than the calculated design load for Class SB vehicles under dynamic loading.

Similarly, rebar B8 at the connection between the mortar and base slab is started deforming at the load of 60kN and 75kN in experimental and numerical results respectively but after that load, a sudden jump in strain is due to the opening of the interface as shown in Fig.14.

Fig.13 Experimental & numerical results of strain in B8 rebar

Fig.14 Experimental & numerical results of interface 2 (mortar & base slab)

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The results indicate the validity of the numerical analysis applies to the experimental test. The slightly higher in ultimate load in numerical analysis is related to the failure that takes place in the precast barrier is due to punching shear failure at experiment as clearly shown in cracking diagram Fig. 9 while in the analysis its take in flexure and shear failure. Whereas, Fig. 13 and 14 are related to each other in such a way that there is a major involvement of interface 2 (connection between mortar and base slab). The interface behavior in this analysis has some critics regarding slippage bond between reinforcement and mortar. In the sequence of construction, the mortar is injected at the last and some bond slippage will occur in this area. This will cause in some higher in results values of numerical analysis as compared to experimental one. 4.2 Experimental Results of 1st Four Specimens (1) Structural response of specimens

The specimen No. 1 and 2, as shown in Table 1, with the barrier wall thickness of 250mm and variation was made in the joining portion which was from 260mm to 170mm. It was necessary to reduce this, but it was confirmed that there was no difference in the load bearing capacity, and itself structural behavior remain the same under static loading as shown in Fig.15.

Fig.15 Load-displacement curve for Sp. No. 1 & 2

Fig.16 Load-displacement curve for Sp. No. 1 & 3

Similarly, the specimen No. 1 & 3 have all similar configuration except the barrier wall thickness which is 200mm to 150mm. Fig.16 shows the slight reduction of ultimate capacity but still higher the design capacity. The third comparison is made between specimen 2 & 4, having the same geometrical configuration except for the barrier wall thickness which is 250mm and 150mm respectively. As Fig. 17 shows the large reduction of capacity occur but it is still 4.7 times higher the static design load capacity.

Table 4 illustrates the experimental results of above said four specimens. Fig.15, 16 & 17 shows the load-displacement relationship in which the vertical axis shows the load (kN) and the horizontal axis shows the displacement (mm) for four model experimental testing results. Specimen No. 1 and 2 shows the linear behavior up to 18kN and initial cracking has appeared.

After slightly decreasing the rigidity it goes at approximate 120kN start yielding of structure and approach to the final failure. For specimen No. 3, where we decrease the wall thickness and slightly increase the main handrail reinforcement proceed linearly up to 20kN, initial cracking appears and due to slightly degradation of its rigidity it goes to 113.25kN for the final failure of the specimen. Where the specimen No.3 is unique in its dimensions. The location of interface 1 (wall barrier to mortar) is very near to the design section of precast wall barrier. For this reason, the test result shows some increase in load after yielding is started. This research emphasis to minimize the mortar quantity so, onward all specimens were designed having the mortar height is about 170mm.

Fig.17 Load-displacement curve for Sp. No. 2 & 4

Table 4 Initial cracking load and ultimate load

Specimen No.

Static design load (kN)

Ultimate load (kN)

Factor of safety

1 18.76 120.25 6.40

2 18.5 120.0 6.48

3 20.0 113.25 5.66

4 15.0 70.5 4.70

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For specimen No. 4 the further decrease in wall thickness is adopted by keeping as low the mortar height, specimen behave linearly up to 15kN and final failure of the structure is at 70.5kN. Based on these four model experiments the specimen No. 5 was tested under static loading condition by keeping the same structural configuration as specimen No. 4. (2) Reinforcement response in experimental results of 1st four

specimens From the test results, it was found that the difference in section

rigidity due to the difference in wall thickness is dominant in the load carrying performance in the static loading. For preventing destruction in the deck, the cross-section rigidity was high at the embankment section, but it can be illustrated from the results as shown in Fig.18, 19, 20 & 21 that the reinforcing bar yielded at maximum yield strength at the base of the precast concrete barrier wall.

By comparing the strain results of reinforcing bars at the different level, the wall having the thickness of 250mm shows yielding of handrail bar is started at design load (initial cracking load) but on other hands, the yielding of hanger bar is started at the load of 33kN.

Similarly, by reducing the joining portion, loop reinforcement height, in specimen No.2 shows the same pattern of yielding of handrail bar but the change in hanger yielding position which is at 24kN. By reducing the rigidity of the main wall thickness in

specimen No. 3 from 250mm to 200mm but keeping the same loop reinforcement height, shows the yielding of handrail bar is started at a lower load but hanger bar is simultaneously started at the same load.

For specimen No. 4 the loop reinforcement height is 170mm, it shows the handrail bar start yielding at about 15kN and simultaneously the hanger bar start yielding at the same loading which shows the effective utilization for forces in the whole section.

Changes in the reinforcement strain at the level L which is the connection between the precast concrete barrier and joining mortar. Specimen No. 1, handrail reinforcement strain start yielding at 40kN whereas the hanger bar starts at a load of around 20kN. On the bases of specimen No. 2, the handrail rebar of about 35kN but it’s strain rate as compared to specimen No. 1 is higher as clearly shows in Fig.18 and Fig.19. For specimen No. 3 & 4 handrail bar was started to yield at 18kN and 10kN respectively. Where the hanger bar started yielding at the load of 5kN. 5. FUTURE PROSPECTS OF CURRENT RESEARCH

In this research, the precast concrete barrier was attached with

a non-deformable slab due to the connectivity problem with laboratory floor slab. The overhang of barrier on the bridges is not consider which may cause the influence of existing deck slab

Fig.18 Strain of handrail bar H1F at level H

Fig.19 Strain of handrail bar H4 at level H

Fig.20 Strain of handrail bar H1F at level L

Fig.21 Strain of handrail bar H4 at level L

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design which need some extra consideration for the redesign. The dynamic behavior of the precast barrier and bridge deck

overhang, especially in respect to the large vehicle impact load to be expected for Class SB vehicles loading, has yet to be evaluated. Addition research should be studied the barrier and deck behavior under dynamic loads representative of Japan highway specification under Class SB loading, notably 25ton truck at 65km/h velocity with 150 impact angle2). Based on this study the precast concrete barrier having thickness 150mm can be installed on the bridge deck and will be adequate. However, prior to industrial use of the barrier, crash tests should be performed on the bridge barrier system to satisfaction of the local authority14).

6. CONCLUSIONS

In this study, nonlinear finite element analysis models created

by using DIANA 10.2 accurately reproduced the results in term of ultimate load, cracking pattern and failure mode. The basic purpose of the numerical simulation is that the full-scale collision test cannot perform on the bridge and it also save the experimental time and cost. By using the commercial software, the numerical modeling help to investigate the micromechanical behavior of different structural components and that can make some modification/suggestion in design such a handrail having similar properties as mentioned before. Experimental results show that all precast concrete barrier models exceed the static design criteria as specified in Japan highways specification for Class SB vehicles. These validated numerical models were used to detail the mechanical behavior of structural components. Based on the results of this study, the following conclusions are:

(1) The ultimate failure load differs 1.7 times to the specimens

having the wall thicknesses of 250mm to 150mm. But the specimen having 150mm thickness also collides with load Collision equivalent load and main component behaves within the elastic limit.

(2) This study shows the full-scale test (Specimen No.5) shows the ductile behavior up to failure load and it resists the 3 times higher the calculated design load for SB class vehicle loading. Thus, this effective section of the precast barrier can be adequately used in a field after performing the crash load tests.

(3) The numerical modeling technique can be used to reproduce the experimental results for precast concrete barrier having a similar configuration as presented in this study.

ACKNOWLEDGEMENT:

The authors gratefully acknowledge the support of The Metropolitan Expressway. Co. I would also like to thank my colleague Yusuke Suzuki for enabling me to conduct this valuable research.

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(Received September 21, 2018)

(Accepted February 1, 2019)

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