Fibre Reinforced Shotcrete Research Articles

Теплотехнические свойства бетона, торкрет-бетона и торкрет-фибробетона в условиях углеводородного пожара

Теплотехнические свойства бетона, торкрет-бетона и торкрет-фибробетона в условиях углеводородного пожара

Damage layer thickness and formation mechanism of shotcrete with and without steel fiber under sulfate corrosion of dry–wet cycles by ultrasound plane testing method

In recent years, nondestructive testing, especially ultrasound testing method, has been increasingly used to detect concrete strength, compactness, cracks, and other aspects. In this work, the damage layer, thickness measured by ultrasound plane testing method and the formation mechanism of the damage layer were studied. Simultaneously, the damage process and microstructure of shotcrete under multiple coupling effects of sulfate attack and cyclic drying and wetting were also investigated. The results indicated that ultrasound plane testing accurately characterizes the damage layer thickness of shotcrete with and without steel fiber. The variation in performance of the original specimen was significant with increased thickness. An exponential distribution existed between the damage layer thickness and the original performances expected weight loss ratio. In the corrosion process, sulfate ions were transported into micropores and microcracks layer by layer, and velocity was slowly diffused from a distance from the surface. For ordinary shotcrete, sulfate attack is divided into three stages: ettringite action, combined ettringite and gypsum action, and gypsum action. However, for steel fiber-reinforced shotcrete, sulfate attack only occurred at the combined ettringite and gypsum action.

Age-dependent changes in post-crack performance of fibre reinforced shotcrete linings

It is commonly assumed that when a mix achieves satisfactory performance in Quality Control tests at 28days this result will translate into satisfactory performance throughout the design life of the corresponding concrete structure. While this is generally true of the compressive strength of concrete it is not necessarily true for other parameters. The post-crack performance of fibre reinforced concrete (FRC) differs from that of conventionally reinforced concrete in that the post-crack performance of fibres is related in a complex manner to the characteristics of the concrete matrix. Age-dependent changes in the characteristics of the concrete matrix can effect changes in the post-crack behaviour of fibres. The present investigation has examined how the post-crack energy absorption of fibre reinforced shotcrete (FRS) changes with aging and has found that some types of fibre exhibit dramatically different performance characteristics at late age compared to that displayed at 28days. This change can have significant consequences for the design of ground support based on fibre reinforced shotcrete. Tunnel linings required to resist sustained ground stresses, or which may be subject to deformation associated with seismicity or ground movement at later ages, should be designed with consideration of a possible long-term loss of ductility exhibited by some types of fibre reinforced shotcrete.

Influence of Friction in Supporting Rollers on the Apparent Flexural Performance of Third-Point Loaded Fiber Reinforced Concrete Beams

Abstract Existing beam-based ASTM test methods for the assessment of post-crack performance of Fiber Reinforced Concrete (FRC) and Fiber Reinforced Shotcrete (FRS) contain detailed specifications for many components of the test apparatus including the yoke assembly for measuring specimen deflection and the control system required for the test machine. However, there is limited attention paid to the design of the supporting rollers with the result that laboratories have been left to develop their own designs for this component of the test apparatus. Simple free-body force calculations for a cracked FRC beam indicate that friction in the supporting rollers can significantly influence the apparent post-crack performance of a beam. The present investigation has confirmed experimentally that the apparent post-crack performance of a third-point loaded FRC beam will be biased as a result of friction in the supporting rollers. Rollers that fail to roll during a test will lead to over-estimated post-crack performance. Due to the potentially serious consequences of unconservative FRC performance estimation a “standard” design for a supporting roller has been proposed for use in all ASTM beam tests. Use of this supporting roller design will lead to a reduction in performance variability between laboratories and improve the confidence users can have in the post-crack properties of FRC.

Development of a 1200-mm-Diameter Round Panel Test for Post-Crack Assessment of Fiber-Reinforced Concrete

AbstractThe ASTM C1550 round panel test is now used widely to assess the post-crack performance of fiber-reinforced concrete (FRC) and fiber-reinforced shotcrete (FRS) and is firmly established as the most repeatable means of post-crack performance assessment available for these materials. However, the 75-mm standard specimen thickness for this test method has limited its application to relatively thin-walled structures such as FRS linings in mines and tunnels. The performance characteristics of FRC in bending depend on the thickness of a member, and thus a specimen with a thickness close to that in the intended application should be used for representative performance assessment. For this reason, tests such as ASTM C1609/C1609M, based on a 150-mm-deep beam, are typically used for thick-walled applications such as FRC tunnel linings and pre-cast elements. Unfortunately, the high single-operator variability characteristic of performance parameters obtained using beam tests has burdened the construction industry with unnecessarily expensive FRC designs, because the resulting margin between target performance and design requirements becomes excessively large. In response to this, demand has arisen for an enlarged version of the 75-mm-thick ASTM C1550 panel that can be used as a quality assurance tool for thick-walled FRC applications. The present paper describes the development of a version of the ASTM C1550 round panel 150 mm thick and 1200 mm in diameter with specific emphasis on developing thickness correction factors and characterizing the single-operator variability for these large test specimens. It also establishes a correlation between residual strengths measured using ASTM C1609/C1609M beams and using 1200-mm-diameter round panels at comparable crack widths.

Influence of Test Machine Control Method on Apparent Performance of ASTM C1550 Fiber Reinforced Concrete Panels

Abstract The ASTM C1550 round panel test is used to assess the post-crack performance of fiber reinforced concrete (FRC) and fiber reinforced shotcrete (FRS) and has become firmly established as the most repeatable means of post-crack performance assessment available for these materials. When the test method was first published, two methods of test machine control were permitted: displacement control, in which the displacement of the loading actuator is used as the control parameter in the test, and strain control, in which the central deflection of the specimen is used as the controlling parameter. Both these methods of control, however, require expensive testing machines, with the result that some operators have used cheaper open-loop machines to test ASTM C1550 round panels. Doubts have arisen among specifiers of FRC and FRS in tunneling and mining projects as to whether specimens tested using open-loop machines result in the same apparent performance as would be expected using displacement- or strain-controlled testing machines. This investigation has established that under certain circumstances, an open-loop testing machine can produce results for ASTM C1550 panels that are similar to results obtained using a strain- or displacement-controlled testing machine. It was also found that the measurement of specimen deflection on the underside of the panel is problematic at large deformations and can lead to substantial variations in apparent performance.

Use of Synthetic, Fiber-Reinforced, Initial Shotcrete Lining at Devil's Slide Tunnel Project in California

Fiber-reinforced shotcrete is widely used as a temporary excavation support in the construction of transportation tunnels and in other civil applications that are performed with the Sequential Excavation Method. Traditionally, steel fibers have been used in tunnel construction; however, the use of synthetic fibers is becoming more common. At the Devil's Slide Tunnel Project, on Highway CA-1 in Pacifica, California, a synthetic fiber mix was used to achieve the contract initial lining performance. The synthetic (polypropylene) fiber was used throughout the excavation phase of the twin highway tunnels and was tested with the round determinate panel test (ASTM C1550) for its capability to absorb energy after shotcrete section failure (flexural strength). The contract required the ASTM C1550 test to produce at least 320 J at 40 mm of deflection after 7 days. This paper discusses the testing program and summarizes its results. The overall performance of the fiber-reinforced shotcrete liner is discussed and evaluated on the basis of its reaction to deformation and its use in the tunnel.

Influence of geometric factors on the punching load resistance of early-age fibre reinforced shotcrete linings

Understanding the early-age properties of fibre reinforced shotcrete (FRS) is critical to the accurate estimation of the time to safe re-entry under freshly sprayed FRS linings. Previous research in the field has indicated that shear failures dominate early-age load resistance and thereby govern the performance of a lining during the first few hours after spraying. This investigation has examined the influence of several geometric variables on shear resistance in a punching mode of failure. In particular, the research has addressed the influence of the size, shape, eccentricity, and inclination of punching loads on the apparent shear strength of an early-age fibre reinforced shotcrete (FRS) lining.

Influence of Fiber Type on Creep Deformation of Cracked Fiber-Reinforced Shotcrete Panels

This article describes how experience in underground construction to date has indicated that creep deformations in conventionally reinforced shotcrete and steel fiber-reinforced shotcrete (SFRS) are small once hardening beyond the initial few weeks of hydration has occurred. At the same time, shotcrete reinforced with macrosynthetic fibers has become well established as a means of ground support in mines. This type of fiber-reinforced shotcrete (FRS) has demonstrated an ability to deform in response to highly plastic ground and thereby control stability. However, debate has arisen about the merits of using shotcrete reinforced with macrosynthetic fibers in civil tunnel linings. In particular, concerns have been expressed about the magnitude of post-crack creep-related deflections likely in the long term. The present investigation was instigated to determine the relative magnitude of time-dependent post-crack deflections typical of shotcrete reinforced with macrosynthetic fibers and to compare this with SFRS tested under nominally identical conditions.

An improved test method and numerical analysis for crack opening resistance of FRC round determinate panels

This paper presents a numerical analysis of a material test program investigating the crack opening properties of fibre reinforced concrete (FRC) round determinate panels (RDP). The objective of this research is to set up a modified RDP test method to improve the current ASTM C1550 test method for FRC and fibre reinforced shotcrete (FRS) composite. By this test method, a kind of light small-diameter panels are applied and more crack information can be obtained by a new rotation angle measuring technique. It is shown that this modified test method can be used to effectively evaluate the crack opening resistance of FRC. The finite element analysis was then performed to clarify the crack propagations and failure mode of FRC RDP panels. It helps establish a reasonable theoretical method to predict the structural response of RDP, combining with this modified testing technique.

Strength, ductility and stiffness of fibre-reinforced shotcrete

In mining flexible linings are preferred because large displacements of underground openings are often allowed. This often results in extensive cracking of the shotcrete. Therefore, ductility of shotcrete is very important. In an investigation comprising 45 beam tests and 33 panel tests, Young's modulus, residual flexural strength and absorbed energy were determined for shotcrete reinforced by polypropylene fibres, steel fibres and steel mesh. The residual flexural strength was determined as the average strength over the displacement range 0–4 mm. Beams reinforced with steel fibres had higher residual flexural strength than beams reinforced with polypropylene fibres or steel mesh. The shotcrete reinforced with steel fibres and the best polypropylene fibres had almost the same energy absorption in the panel tests (0–25 mm displacement range) as well as in the beam tests (0–20 mm displacement range). Young's modulus of reinforced shotcrete determined by beam tests was lower than that derived from compression tests of cast concrete

New Testing Procedure for Shrinkage Cracking of Fiber-Reinforced Shotcrete

Plain shotcrete and fiber-reinforced shotcrete (FRS) are commonly employed to produce layers or linings with large surface/volume ratios. Restrained shrinkage cracking is hence an important concern from both structural and durability aspects. In this investigation, a new testing procedure is adopted to study the shrinkage cracking behavior for plain shotcrete and four different types of FRS (polypropylene, steel, polyvinylalcohol, and hybrid). To introduce shrinkage restraint, specimens cut from a shotcrete panel are bonded to a steel fixture. By measuring the shrinkage strain of the restrained specimen at different times, the occurrence of cracking can be easily observed from a sudden “jump” in the strain values. To find the actual strain induced on the restrained specimen, unrestrained members were also monitored to obtain the free shrinkage. For various plain and reinforced shotcrete mixes, the induced strain when cracking occurs is found to follow an order that is generally consistent with physical in...

Construction of Paramithia tunnels, Egnatia motorway, Greece

The paper describes in detail the construction process followed for site set-up, portal formation, tunnel excavation and primary lining of two tunnels in the presence of heavily fragmented cataclastic material. Excavation techniques used to secure the crown of the tunnel and to avoid extensive collapses included, inter alia, grouted forepoles of various types and density, down the hole drilling, the use of self-drilling anchors and fibre-reinforced shotcrete. The optimum set of measures was selected according to block size, interlocking of blocks and cohesion of each unit at the tunnel crown and sides. Attention is paid to the observational method applied, the treatment of special cases with novel techniques such as horizontal cement piles and the treatment of localised ground collapses, and to the production rates achieved during construction. A procedure applicable when mixed conditions at the tunnel face are encountered during excavation in tunnelling projects is proposed.

Constitutive Modeling of Fiber Reinforced Shotcrete Panels

Beam and panel specimens are commonly used to assess the flexural performance of fiber reinforced shotcrete (FRS). Constitutive modeling of the flexural performance of ASTM C-1550 panels made with FRS is useful for explaining the relationship between the performance of specimens and the behavior of in situ FRS tunnel linings. A series of analyses have therefore been undertaken using yield line theory to derive the nonlinear load-deflection response of C-1550 panels based on the flexural capacity of beams of similar composition and thickness. A Monte Carlo simulation has been incorporated into the analysis to account for the stochastic nature of the material properties. This has demonstrated that the postcracking behavior of C-1550 panels can be successfully modeled using yield line theory and moment-crack rotation relationships developed from tests on beams made of the same material. Moreover, the variation in performance of C-1550 panels is primarily caused by variations in the flexural capacity of elements comprising the panel rather than variations in the location of the radial cracks that form during a test.

Construction of two road underpasses at Ma On Shan, Hong Kong SAR

Two road underpasses each some 200 m long, 16 m wide and 12 m high were constructed in tunnel during 2002-2003 to serve as slip roads as part of a new major trunk road (Road T7) in Ma On Shan, Hong Kong Special Administration Region. The tunnel alignments are curved horizontally with gentle vertical gradients and encounter weak, altered and metamorphosed sedimentary rock and strong granite rock. Power tools and drill and blast techniques were employed for the tunnel excavation. Due to the presence of heavily sheared stratified rocks and an inclined principal joint set system, the influence of groundwater seepage on tunnel stability was critical. The initial temporary support system adopted for the weak rock excavation comprised a combination of support canopy (forepoling), a lightweight steel set and fibre-reinforced shotcrete. Detailed rock mapping and monitoring of excavated rock faces were undertaken to optimise temporary tunnel supports. The initial temporary support design was re-assessed by numerical analysis (FLAC model) with a range of geological input parameters based on detailed face mapping carried out during construction. This led to the subsequent omission of the invert strut of the steel set. The effects of blasting vibration on the closely spaced large span tunnels was monitored using seismographs and the results fed back into the design and procedures associated with production blasting. The vibration as detected at ground surface was monitored to be all within the allowable maximum peak particle velocity of 25 mm/s. Permanent lining construction was facilitated with the use of movable lining shutters and the underpass construction was able to be completed one month ahead of programme. Prior to and during construction there was significant interaction between the contractor and a geotechnical specialist acting as an advisor to the tunneling contractor. The full-time input of the specialist advisor has proven to be worthwhile and cost effective leading to optimization in temporary support systems in poor quality rock and blasting patterns in more competent rock. The specialist geotechnical advisor was engaged in regular and detailed mapping of exposed rock faces, collection and interpretation of monitoring data and high level analyses of construction options. This led to an efficient and productive communication and problem solving relationship with the engineer's staff on site and in their design office. The regular input from the advisor made a significant contribution to the completion of the tunnel excavation works one month ahead of programme, despite a delayed start. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

The Bothnia Railway Link - improving transportation in northern Scandinavia

On the 27th of November 1997, an agreement for the construction of the Bothnia Line was entered into between the Swedish government and four municipalities (Kramfors, Oernskoeldsvik, Nordmaling and Umeaa) in the counties of Vaesternorrland and Vaesterbotten. The Bothnia Line, a 190 km, single-track, high-speed rail link running from Kramfors in the south to Umeaa in the north, is one of Scandinavia's largest railway projects in modern times. As part of the agreement, a company, Botniabanan AB, was set up to be in charge of designing and building the line. The project's goal is the improvement of communications and freight transport in the north of Scandinavia. The present main railway, which runs inland and was built early in the 20th century, is nearing its capacity ceiling. A fully developed Bothnia Line will vastly improve passenger and goods transport to and from northern Scandinavia. Amongst the features of the new, 190 km, single-track rail link are 22 passing places, 140 bridges and around 39 km of rock tunnels. The project is split into 130 major contracts. The largest of these is for the 32 km line between Offersjoen and Bjaellstaaan. Work started in August 1999 and is scheduled for completion in 2008. The total estimated cost is SEK 15 billion. The project's 39 km of rock tunnels includes 15 rail tunnels totalling 24.4 km in length. Service tunnels account for the remaining 14.6 km. Four of the service tunnels are being built parallel to the larger rail tunnels - Aaskott (3.3 km), Namntall (6.0 km), Bjoernboele (5.2 km) and Varvsberg (2.1 km). The service tunnels for Aaskott and Varvsberg are 2.3 and 1.1 km long, respectively. The total volume of the rock tunnels is around 2.2 million cubic metres. The tunnels are being excavated through Scandinavian gneiss/granite formations and meta-greywacke. The rock is generally of good quality with typical Q-values (Barton et al., 1974) of between 1 and 100. Grouted in rock bolts and fibre-reinforced shotcrete are being employed as the standard forms of rock reinforcement. Continuous pre-injection is being used in the construction of all tunnels. Two separate stretches (118 and 102 m) of the 1,450 m long Stranneberg tunnel had little or no rock burden. In these cases, reinforcement was provided by, respectively, jet grouting and soil freezing. Final sealing and rock reinforcement is provided by concrete cladding. Jet grouted sections have a 65 cm lining for the floor and a 55 cm lining for roof and walls. The figures for soil frozen sections are 70 and 65 cm, respectively. So far (late July 2003), four tunnels, with a combined length of 4.4 km, have been completed. (A) Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

Properties of wet-mixed fiber reinforced shotcrete and fiber reinforced concrete with similar composition

Fiber reinforced shotcrete (FRS) is commonly used in slope protection, tunnel linings as well as structural repair and rehabilitation. For the design of shotcrete mixes, it is of interest to see if data on fiber reinforced concrete (FRC) can be employed as an initial guideline. In this study, various properties of FRS, including its compressive strength, flexural behavior, permeability and shrinkage behavior, are compared with FRC of similar composition. The results, based on five different mixes, indicate that the fabrication process (i.e., shotcreting vs. casting) can significantly affect compressive strength and permeability, but has relatively little effect on shrinkage behavior. The flexural strength of FRS is slightly higher than that for FRC in most cases, but the residual load carrying capacity in the postcracking regime can be significantly lower. Based on the differences in the properties of FRC and shotcrete, implications to material design are discussed.

Crack Offset Corrections to Post-crack Performance Parameters Obtained from Third-point Loaded Fiber Reinforced Shotcrete Beams

Abstract Fiber Reinforced Shotcrete (FRS) is used as an economically competitive method of ground support in tunnel excavations throughout the world. Fibres are incorporated into this material to impart post-crack load capacity. Beam tests are commonly used to assess the post-crack performance of FRS but high within-set variability has hindered the usefulness of beam test data. A method of correcting performance data to account for the offset between the position of the crack and the center of the beam has been suggested as a means of improving within-batch variability for post-crack performance parameters obtained from third-point loaded beam tests. In this paper, an offset correction method has been applied to a large set of beam test data obtained as part of Quality Control testing for a tunnel-lining project to determine whether the method is effective in reducing within-batch variability.

Correlations in the behaviour of fibre reinforced shotcrete beam and panel specimens

Satisfactory structural performance of Fibre Reinforced Shotcrete (FRS) in applications such as tunnel linings is dependent on an ability to support load after cracking. At present, both beam and panel specimens are used to measure the post-crack flexural capacity of this material. Although these tests have been widely used to develop improved fibres and FRS mix designs, the relationship between performance data produced in beam and panel tests is unclear. This investigation was therefore undertaken to examine possible correlations in behaviour between beam and panel specimens and determine which was the most appropriate type of test for a given FRS application.

Design of fibre-reinforced shotcrete support for coal-mine galleries

Shotcrete is a surface support. The decisive factors in mobilizing a surface support to interact with the rock and to stabilize the surface are its bearing capacity, deformation resistance and toughness. Surface support in the form of strong arches of concrete, shotcrete or steel may be sufficient in itself to stabilize a gallery. In other cases it is possible to use surface supports of lower strength that interact with the bolt support to achieve the necessary stability.