Precast Concrete Segments Research Articles

Research on signal processing of segment-grout defect in tunnel based on impact-echo method

In shield tunneling construction, grout of precast concrete segments is widely applied to various geological conditions. However, the connection performance between the grout and segments after the grout cannot be determined by destructive means, such as coring. The impact-echo method is used extensively in nondestructive testing of civil engineering structures. Nevertheless, there are inadequate researches on determining defects in segment-grout structure. In this paper, the segment-grout structure was simulated experimentally and artificial defects were introduced in the grout. The Short-time Fourier Transform (STFT) and Wavelet Transform (WT) of signals obtained by the impact-echo method were carried out to analyze the time-frequency domain features of the impact-echo signals with and without defects in the grout. On this basis, defects in the grout layer were determined. Finally, energies of wavelet packet were calculated and characteristic relative energy indexes of wavelet packet at different positions were detained. It’s found that the values at positions without defects are relatively low. It’s demonstrated that the signal analysis of impact-echo method has to combine time-frequency domain information and energy distribution for detection of defects in the grout layer. Wavelet transform and energy analysis methods are effective means to detect defects in the grout layer.

Interaction diagrams for design of hybrid fiber-reinforced tunnel segments

Fiber reinforcement has emerged as an alternative to traditional reinforcing bars and welded wire mesh reinforcement for precast concrete tunnel segments. This is mainly due to improved postcracking behavior and crack control characteristics of fiber-reinforced concrete (FRC) segments. A hybrid solution of fibers and reinforcing bars is adopted when FRC is not adequate as the sole reinforcing system. Often times, this is the case in large-diameter tunnels with large curved length segments in order to achieve required strength for embedment loads in shallow cover, TBM thrust jack forces, and loading from imperfect construction and irregularities. P–M interaction diagrams are used as one of the main design tools since segment cross section, under most of governing load cases, is subjected to a combined axial force and bending moment. Standard FRC constitutive laws recently allows for a significant residual strength in tension zone below the neutral axis. However, design capacity of hybrid fiber-reinforced concrete (HRC) segment is significantly underestimated using conventional Whitney’s rectangular stress block method. Methods that currently incorporate contribution of fibers on P–M diagrams are based on numerical and finite-element analyses. However, closed-form solutions offer important advantages. This paper presents material models, derivations and for the first time closed-form solutions to construct P–M interaction diagram of HRC segments. Parametric studies are conducted and validity of the model is verified by simulating experimental results of HRC columns and model-predicted results of precast and cast-in-place concrete linings. Results show that using appropriate material models for fiber and reinforcing bar, engineers can use the proposed methodology to obtain P–M interaction diagrams for HRC tunnel segments.

Curvilinear GFRP bars for tunnel segments applications

Abstract The possibility of adopting glass-fiber reinforced polymer (GFRP) composite bars in tunnel segments as a substitute of the traditional steel reinforcement has been demonstrated. Nevertheless, this application requires a curvilinear shape for the reinforcement, and thus a different production process and rebar geometry. The aim of the paper is the experimental evaluation of the physical and mechanical properties of non-conventional GFRP rebars. The test procedures and obtained results are highlighted and compared with the acceptance criteria available for straight pultruded rebars. Finally, the presented outcomes show the suitability of GFRP non-conventional bars as reinforcement for precast concrete segments in tunnel applications.

Chemical and mineralogical alterations of concrete subjected to chemical attacks in complex underground tunnel environments during 20–36 years

This paper investigates the chemical and mineralogical alterations of concrete in underground tunnel structures built from 1980 to 1996, located on the China's east seashore. The underground water around the tunnels had once been treated to be not or little aggressive. However, the complex environments in the tunnels had increased the aggressiveness of certain ingredients, thus causing chemical attacks. The chlorides in the leakage underground water cause chloride attack, and the NaCl crystallizations on the surface of the C50 pre-cast concrete segments induce a deeper chloride contamination. When the concrete suffers flowing leakage water, calcium leaching also occurs, leading to decalcification and magnesium incorporation. Under this circumstance, C50 pre-cast concrete shows a higher resistance compared with C30 cast-in-place concrete, mainly due to the low water to binder ratio. Within the C30 cast-in-place concrete facing concentrated leakage water and NaCl crystallizations, a strong magnesium chloride attack is observed. Besides the materials factors, the environmental factors, including the high concentrations of both chloride and magnesium ions, the removal timing of calcium ion, and the water saturation, are believed to take responsibility for the magnesium chloride attack.

Classification of Precast Concrete Segments Damages during Production and Transportation in Mechanized Shield Tunnels of Iran

Precast concrete segments used in shield tunnel linings are prone to damage in many situations. These damages can occur at different stages such as fabrication in segment factory, transportation to tunneling site, during tunneling process, and at serviceability stage. The aim of the present article is to study the damages inflicted on concrete segments during production and transportation, and to present a new classification of these damages throughout the two stages. The developed classification is based on field observations and examinations of major subway and water conveyance mechanized shield tunnels of Iran, located in Tehran, Tabriz, Mashhad, Kermanshah (Nosood) and Isfahan (Golab). The quality of tunnel lining suffers from what, as a direct consequence of any damage to concrete segments, during production and transportation, which will be also discussed in this article. For further investigation, more than 250 concrete segments from Tehran subway line 3 and 350 segments of concrete segments from Tehran subway line 7 were selected and studied for a statistical analysis of chipping and crack, consecutively. Absence of preventive measures to limit segment damages in precast segment factories is one of the main reasons for increased number of damaged concrete segments, and as a result, increased costs of tunnel construction at later stages. In this paper, production phase damages and factors contributing to these damages are studied. According to the findings of the study, the human (operator) error was the most important cause for chipping, and, time-dependent behavior of concrete was the essential reason in crack of precast segments. Eventually, final section of the article presents practical solutions for reduction of damages during fabrication and transportation of concrete segments.

Development of a Skewed Pipe Shear Connector for Precast Concrete Structures.

Joint connection methods, such as shear key and loop bar, improve the structural performance of precast concrete structures; consequently, there is usually decreased workability or constructional efficiency. This paper proposes a high-efficiency skewed pipe shear connector. To resist shear and pull-out forces, the proposed connectors are placed diagonally between precast concrete segments and a cast-in-place concrete joint part on a girder. Design variables (such as the pipe diameter, length, and insertion angle) have been examined to investigate the connection performance of the proposed connector. The results of our testing indicate that the skewed pipe shear connectors have 50% higher ductility and a 15% higher ratio of maximum load to yield strength as compared to the corresponding parameters of the loop bar. Finite element analysis was used for validation. The resulting validation indicates that, compared to the loop bar, the skewed pipe shear connector has a higher ultimate shear and pull-out resistance. These results indicate that the skewed pipe shear connector demonstrates more idealized behavior than the loop bar in precast concrete structures.

Special segments at the cross passages in the Filder Tunnel – Interpretation of the data from monitoring segments

AbstractThe 9,468 m long twin‐bored Fildertunnel is the longest railway tunnel of the Stuttgart 21 project. The maximum overburden is approximately 220 m, the tunnel tubes lie up to 60 m below the water table and the distance between the cross‐passages is approximately 500 m. The precast concrete segments for the mechanised excavation of the Fildertunnel are equipped with measuring devices in proximity of the cross‐passages to assess the lining strains prior to the construction of the cross‐passages and to serve as a basis for the further design. The lining stresses are derived by applying an appropriate calculation method, which transforms the measured strains into material stresses, hence achieving the active load configuration. The strains measured on the outside and inside of the lining with embedded vibrating wire strain gauges are converted into stresses and internal forces, enabling a direct assessment of the internal forces along the entire circumference of the segmental lining. In this article the measurement results in the area of the cross passages 6a and 7a of the Fildertunnel are described and interpreted. The significant internal forces in the segments after their installation and their time development are described, their dependence on influence factors such as self‐weight, thrust forces, grout pressure, rock mass and water pressure is discussed and the results are compared to the design calculation.

Performance of in situ stitches in precast segmental bridges

In situ concrete stitching with subsequent prestressing is often used in construction to join together precast concrete segments. An example of an application is the stitching of the approaching decks of precast segmental bridges constructed using the balanced cantilever method. The relatively narrow stitches used in segmental bridges are often provided with nominal reinforcement only. Therefore, the in situ stitch is a location of potential weakness, but relatively little work has been done in this area. In light of this, extensive experimental and numerical studies on the behaviour of precast concrete members with in situ concrete stitches have been conducted. The effects of stitch width, prestressing force, concrete strength, bonding of tendon and provision of shear keys on the performance of such members are presented in this paper. Unlike members that are continuously cast, cracking initiates along the construction joint between the precast segment and the stitch at low load levels for members with in situ stitches, while no cracking is observed elsewhere. The cracks widen extensively as the load level increases and can become detrimental to the serviceability of the members. Such members should be designed so that the joints are subject to no tension under service load.

Detection of Rockfall on a Tunnel Concrete Lining with Ground-Penetrating Radar (GPR)

Experiments were conducted using Ground-Penetrating Radar (GPR). The performance of six GPR systems was assessed in terms of: (1) remotely mapping cavities behind concrete linings, (2) detecting rockfall from the tunnel roof onto an inner lining comprising, for example, precast concrete segments. Studies conducted in Norway and the United States demonstrate that the GPR technique is a simple and reliable method that can assist stability inspection in existing Norwegian tunnels. The ground-coupled GPR systems represent a step forward in the remote detection of rockfall on tunnel concrete linings, and are particularly suited to self-standing inner linings. The analysis of the data is relatively straightforward and reasonably accurate.

“Sandwich-cladding” – Protection Measure against Contact Detonations

As part of the German national research project “Protection of critical bridges and tunnels – SKRIBTPlus” HOCHTIEF Engineering has conducted the development and testing of a protection measure against contact detonations in tunnels. The measure consists of a two layer cladding – therefore called “Sandwich-cladding” – which is installed at the inner side of the tunnel lining. It minimizes the effects of contact detonations to the tunnel lining significantly. An unprotected tunnel lining might be seriously damaged by a contact detonation, especially if the lining thickness is relatively small. If in addition the hydrological situation comprises flowable sands and high ground water pressure, the result might be a tunnel collapse or at least a water and soil swallet.The two layered “Sandwich-cladding” is build up as a precast concrete segment. The outer layer consists of micro-reinforced high-strength concrete which is known as DUCON (DUctile CONcrete). Between the outer layer and the proper tunnel lining the inner layer is placed, consisting of extremely compressible concrete. Both layers are poured as precast concrete segments, connected by reinforcement bars and then fixed to the tunnel lining using dowels and screwsThe effectiveness of the measure has been assessed in a series of blasting tests. Different materials as well as various combinations of layer thickness were tested. In order to get quantifiable results, the damaged specimens also have been subject to a load bearing test. Based on the results of these tests, in particular the remaining load bearing capacity after damage, the effectiveness of the “Sandwich-cladding” could be demonstrated and verified. In addition this was underlined by the results of numerical simulations of the test procedure.The “Sandwich-cladding” also functions as a fire protection measure, as it protects the tunnel lining from overheating and spalling. This has been assessed and verified in real scale fire-tests.

Project Spotlight: Precast concrete Z-shaped superstructure segments provide cantilever for seawall sidewalk

Project Spotlight: Precast concrete Z-shaped superstructure segments provide cantilever for seawall sidewalk

Fatal Compressed Air Jet Injury to the Chest while Working on Tunnel Boring Machine.

Compressed air is used to power various tools and machinery. It is a concentrated jet of air with high pressure that can cause various fatal and non-fatal injuries. But people don’t understand the lurking danger and use it improperly and negligibly. Tunnel boring machine (TBM) uses compressed air to balance earth pressure and to push off the precast concrete tunnel lining segment. Compressed air is stored in the thrust cylinders, capable of producing a pressure of about 100 psi. This is a case of a person, who was working on a TBM, as a mechanic. On the day of the incident, he was detaching the pipeline from the nozzle of the thrust cylinder. Somehow, he came in front of the air jet from the cylinder accidentally and injured himself in the chest and died at once. At postmortem examination, a lacerated wound has been present over the right side of the chest. Underlying rib was fractured. Right lung was lacerated through middle and lower lobes. Adjacent diaphragm and liver were lacerated too. The deceased died due to multiple visceral injuries. The injury pattern was suggestive of blast effect of gunshot wound with blank cartilage, without any gunshot residue effect.

Design procedure and experimental study on fibre reinforced concrete segmental rings for vertical shafts

Structural fibres are used to replace partially or totally the passive reinforcement in precast concrete segments for tunnel linings constructed with TBM, showing several advantages. Fibre reinforced concrete (FRC) could also be applied with similar benefits to vertical shafts. However, to the author's knowledge, this material has not been used in such application yet. The Model Code 2010 gathers an approach for the design of FRC structural elements. This approach should be adapted according to the structural needs of precast segment, for which the transient load stages are often the most critical and specific ductility requirements should be established. The objective of this paper is twofold: propose a general analytical formulation to assess the minimum mechanical requirements that FRC must fulfil in case of partial or complete substitution of the steel rebars and confirm that it is possible to replace the rebars by using fibres in vertical shaft linings. First, the general analytical formulation is proposed. Then, the segments of the Montcada vertical shaft (Barcelona) are redesigned considering the total substitution of the traditional reinforcement by fibres. Finally, two full-scale tests of the FRC precast segments were performed to verify the suitability of the analytical formulation proposed.

Experiments on flexural strength on composite modular bridge pier cap for CFT columns

Modular bridge substructures consisting with Concrete-Filled steel Tube (CFT) columns, composite precast segments have been suggested to accelerate bridge construction. The precast segmental pier cap consists of a composite pier table and prestressed precast concrete segments on the table. The pier table has embedded steel sections to mitigate stress concentration at the connection between concrete pier cap and small steel tubes. The shear strength of the pier cap was obtained by extending vertical reinforcing bars from the table to the precast segment. Transverse prestressing was introduced to control tensile stresses of the segments with service loadings. Structural performance of the proposed modular system was evaluated by static tests. Test results showed that the horizontal shear force from the vertical load should be resisted by bond, friction and vertical reinforcements crossing the interface. When the anchorage of vertical reinforcements and preventing of premature fracture of prestressing steels were provided, the flexural strength of the pier cap module at the joint location showed reasonable prediction.

Assessment the influence of ground stratification, tunnel and surface buildings specifications on shield tunnel lining loads (by FEM)

Urban development and rapid extension of cities have been accompanied by a considerable growth in mechanized shield tunnelling. Commonly precast concrete segments are used as tunnel lining which comprises relatively considerable part of tunnelling cost. The optimum design of lining needs to an accurate evaluation of loads acting on the lining.In this paper, the effects of ground stratification, surface buildings specifications and tunnel depth on lining loads were studied. For this purpose a 3D finite element model was used employing the ABAQUS software (Ver. 6.11). The geometry of tunnel, lining segments, injection grout and the surrounding soil properties were adopted from the under construction Tabriz urban railway line 2 project.The results show that the studied parameters have considerable effects on lining loads. For mentioned case study, surface buildings with 5 and more story have a considerable effect on lining loads, especially for shallow tunnels. The geometry of surface buildings influences the internal forces of the tunnel lining and increase of buildings width and length increases the lining loads. The building width is the most important parameter and with increase of that the influence of other parameters increases. Also by comparison of obtained results in this study with results of 2D analysis, reliability of 2D models was investigated. The comparisons show that 3D analysis is superior to 2D analysis, particularly in the cases of surface buildings presence. The difference between results increases with decrease of building length and increase of tunnel depth and building weight.

Structural Performance of Precast Concrete Arch with Reinforced Joint

내구성과 미관성이 우수한 석조 아치교량은 단경간 교량으로서 가장 오래되고 경험적으로 시공되었던 구조물로서 유럽에서는 지금까지도 현장에 시공되고 있으며, 이에 대한 활발한 연구가 진행되고 있다. 그러나 국내에서는 시공법의 비효율성과 비경제성으로 인해 교량 목적으로 시공되는 경우는 거의 없는 상태이다. 따라서 본 연구에서는 기존 연구자들이 제안한 시스템에서 구조성능의 향상이 가능한 보강된 이음부의 접합방법을 제안하였다. 제안된 이음부는 횡방향 루프이음과 종방향 보강근을 적용한 형식으로 프리캐스트 콘크리트 블록으로 이루어진 아치에 적용하여 실험을 통해서 구조적 성능을 평가하였다. 실험결과, 보강된 이음부는 소량의 보강근으로도 블록 상호 간의 상대변위와 힌지거동을 억제시켜 기존의 형식보다 충분한 내력을 향상시킬 수 있을 것으로 판단된다. The masonry stone arch bridge, due to its superior durability and aesthetics, has been one of the oldest and popular types of short span bridges. In Europe, this type of bridges have been continuously constructed, and numerous related researches have been conducted until recently. However, there are few construction cases in Korea since the economic efficiency and the construction effectiveness is not contentable. Therefore, this study proposed the reinforced joint to improve structural performance of the conventional arch systems which is proposed by previous researchers. The structural performance of the proposed reinforced joint, which consists of the transverse loop joint and the longitudinal reinforcement, is validated by experimental test of an arch bridge which is constructed using precast concrete segments. Based on this results of the experimental test, it is concluded that the strength of arch bridges can be enhanced by applying the proposed reinforced joints since the reinforced joint restrains hinge behavior and relative displacement between segments with a little reinforcement.

Cover Picture: Structural Concrete 4/2013

AbstractThe new Europe Bridge (inaugurated in June 2013) is 1791 m long, with 1391 m of that over the river, making it the longest bridge over the Danube. Seven 80 m long and one 52 m long concrete box girder spans make up the crossing to the Bulgarian abutment near Vidin, where the highway runs onto a long embankment. There are also five spans over the navigable part of the river: 124 m + 180 m + 180 m + 180 m + 115 m. The bridge foundations – 2000 mm diameter bored cast in situ concrete piles – extend 80 m into the subsoil.FCC erected the main bridge using glued segmental match‐cast deck girder sections, then cast more than 450 deck units – each weighing up to 250 t – near Vidin. These precast concrete segments were lifted using special equipment manufactured specifically for this project (photo: FCC)

Numerical Evaluation of Symmetric Prism Method to Analyze the Splitting Forces Due to TBM Jack Loads

Final lining in mechanized excavation includes the precast concrete segments. These segments are designed for applied loads during construction, moving, depot, assembling in the tunnel and service loads that are affected by earth conditions. One of these loads that are applied to the segments after assembling in the ring, are TBM jack loads especially when the TBM should excavate squeeze zones with single mode. As the jack pad section is smaller than the segment section, it causes splitting loads in the segment. Symmetric prism method is an approximate solution to evaluate these forces. In this paper, calculated results by this method are compared to that of numerical solution by ANSYS software. It shows 10 - 20 percent difference between numerical and analytical results.

Concrete segment tunnel lining sealant performance under earthquake loading

This work was conducted to provide an understanding of the leakage behavior of gasketed segmental tunnel linings subjected to static ground loads and earthquake shaking of the recommended design of the Los Angeles (LA) Metro. The proposed lining was made of bolted and double gasketed precast concrete segment lining with convex to convex longitudinal joint surfaces. Lining evaluation included the sealant performance of different gasket materials under water pressure less than 90psi (Neoprene and EPDM gaskets with open base. Testing program was designed to evaluate the longitudinal joint and T-joint sealant behavior under static and dynamic loading using large scale concrete segments. The results showed that concrete around the gasket groove, cracked zones, gasket–gasket groove interface, and bolting pockets are the places of leakage in concrete segments. Besides that, damage to the side of the gasket groove reduced the gasket confinement and led to leakage. Also, the results showed that longitudinal joint sealant capacity was improved by cycling as a result of the increase in bonding between the gasket and gasket groove.

3-D Numerical Analysis on Performance of Prestressed and Precast Concrete Segment Joint

The rotational angle of joint is the key factor for segment lining design. Using the nonlinear finite element program of MARC (MSC2003), the 3D numerical analysis on segment joint of prestressed concrete lining has been carried out. During the analysis, Coulomb and Stick-slip friction models were used as the friction type between the concrete and the prestressing steel strands. The selection of friction model used in the numerical analysis is very important. The study showed that the Stick-slip friction model is more suitable than Coulomb friction model for analysis of the P&PCSL joint. The value of friction coefficient should be thought over carefully.