In general, the design of tunnel lining segments used in mechanized tunneling is dominated by the joints. In fact, the effects of partial area loading dominate segmental failure and, consequently, the concrete bearing capacity of the central region remains underutilized. This paper presents two types of new hybrid segments with enhanced utilization ratios throughout their bodies. The first type comes with strengthened longitudinal joints using high performance steel fiber reinforced concrete, in addition to conventional reinforcement. The second incorporates one-sided barrel-shaped recesses in the central region, which yield volume savings of up to 23.8%. The performance of the new designs is experimentally evaluated on a full-size testing rig made from two steel frames with a capacity of 5 MN each. It captures the on-site conditions in tunnels during service (final state). The first type of segments shows a 74.3% higher loading capacity than a conventional specimen for reference. The second performs even better and possesses 97% higher capacity. In both cases, the failure occurs in the central region. This increases structural safety and simplifies the design. Especially the design of the central region subjected to bending and axial forces gains more relevance against the controversially discussed design of the longitudinal joints. © 2023 Elsevier Ltd

The durability of a concrete structure ultimately depends on the quality of the cast concrete, whereby especially the edge zone of the manufactured concrete segments is crucial for environmental exposures. In order to ensure that the concrete provides sufficient resistance to these exposures, the corresponding material properties must be quantified according to the so-called performance-based durability design. However, the quantification of these properties is carried out using mainly standardized concrete test specimens, cast and cured under optimized laboratory conditions. On the other hand, the real structure is built under different in situ and curing conditions. This may lead to a deviation of the concrete properties achieved at the construction site in contrast to those determined in the laboratory. In this context, systematic investigations were conducted to assess this deviation. To accomplish the studies, separately manufactured test specimens, as well as dummy walls (4 m2), were cast at different construction sites. The separately manufactured test specimens were cured in the laboratory following current standards. The wall was cured in its formwork for a defined number of days (mostly 7 days). Subsequently, drill cores were taken and compared to the laboratory samples in terms of the corresponding material properties (compressive strength, chloride migration and carbonation rate of the concrete). As a result, the core drill samples underperformed the laboratory-cured samples, indicating that the performance achieved at the construction site tends to be lower than that of laboratory specimens. © 2023, The Author(s), under exclusive license to Springer Nature Switzerland AG.

Chloride ingress and carbonation pose a significant risk of steel rebar corrosion in concrete structures. Various models exist to simulate the initiation phase of rebar corrosion, addressing both carbonation and chloride ingress mechanisms separately. These models also consider the environmental loads and material resistances, typically determined through laboratory testing based on specific standards. However, recent findings show significant differences between material resistances obtained from standardized laboratory specimens and those extracted from real structures, with the latter exhibiting inferior performance on average. To address this issue, a comparative study was conducted between laboratory specimens and on-site test walls or slabs, all cast using the same concrete batch. This study encompassed five construction sites featuring different concrete compositions. While laboratory specimens adhered to European curing standards, the walls were subjected to formwork curing for a predetermined period (typically 7 days) to simulate practical conditions. In some instances, a portion of the test walls/slabs received only one day of surface curing to emulate inadequate curing conditions. Subsequent testing of compressive strength and resistance to chloride ingress revealed that field specimens exhibited lower material resistance compared to their laboratory counterparts. This trend was also observed in the modulus of elasticity and carbonation rate. Notably, shorter curing periods further compromised performance, particularly resistance to chloride ingress and carbonation. These findings highlight the importance of establishing acceptance criteria not only for concrete delivered to construction sites but also for ensuring the quality of the actual structure. © 2023 by the authors.

Tunnel linings installed in difficult geological conditions characterized by a high expansion potential, such as Opalinus Clay and rocks containing anhydrite, may exhibit a loss of integrity due to excessive ground deformations in case water penetrates the rocks. One of the potential remedies against lining deterioration and failure of the tunnel structure is the incorporation of a compressible cementitious layer around the tunnel. This compressible layer serves as a cushion that protects the tunnel structure by tolerating a certain amount of deformations prior to collapse. The deformation capacity of the compressible layer can be enhanced by introduction of various soft inclusions or air bubbles in the cementitious mix. However, such soft inclusions reduce the overall elasticity modulus and strength of the composite. Hence, it is crucial to determine the optimal mix that provides the required compaction potential without compromising on stiffness and strength. For this design purpose computational modeling can be adopted. The model should be able to account for the material properties of the individual components and their volume fraction, which strongly influence the overall behavior of the cementitious composite, and to capture the main features characterizing compressible composites, such as a yielding plateau after a certain threshold loading has been reached. The compaction process of such a composite is described by a voxel model with a discrete distribution of material properties, where the voxels are compacted by applying an Eigenforce when the local stress state reaches a threshold value. This threshold is determined within the framework of continuum micromechanics on the scale of individual inclusions. The model predictions are compared with data from experimental investigations on the compression behavior of different cementitious mixes. © 2023 Elsevier Ltd

High-performance concrete is often heat-treated to increase early strength. Standardized temperature conditions for at least 24 h and sulphate-resistant cement thereby preventing delayed ettringite formation (DEF). In contrast, this paper investigates the risk of DEF for rapidly heat-treated HPC with a minimum temperature duration of 1 h to achieve a rapid early strength for stripping. In addition, a binder is used that does not exhibit increased sulphate resistance. For this purpose, standard prisms (L x W x H = 16 × 4× 4 [cm]) are heat-treated at 80 °C between 1 and 6 h without a pre-storage time. Samples with a temperature duration of 24 h serve as reference. First, the pore structure is analyzed using Mercury porosimetry to identify damage to the concrete matrix due to the rapid heat treatment. The investigation of DEF is achieved by two common approaches. The sulphate resistance is determined by means of wet-dry cycles on specimens for 90 days, whereby the changes in mass and strain as well as the cracking pattern on the concrete surface serves as a qualitative evaluation criterion. In addition, the SVA method on specimens directly after heat treatment, which determines the change in mechanical properties and the swelling behaviour of the concrete, leads to a quantitative evaluation of sulfate resistance. The measured pore distribution of the samples shows a significant increase of the capillary pores of up to 109% for decreasing temperature durations. However, the porosity does not affect the sulphate resistance of the concrete. The wet-dry cycles cause no damage due to DEF, since the change in mass and strains decreases with increasing cycles from up to 62.7% to − 2.5%. Furthermore, no visual differences were detected on the concrete surface between specimens with and without sulphate attack. As a result of the SVA test, maximum absolute strains of about 0.1 mm/m occur that are below the limit value of 0.152 mm/m. Also, the mechanical properties do not show a reduction in strength, but even an increase in tensile strength. In conclusion, the high-performance concrete formulation used in combination with rapid heat treatment investigated here did not result in detectable damage due to DEF. © 2023

To achieve a performance-based durability design for concrete, the corresponding materials’ properties must first be quantified. These properties, together with the expected environmental loads, are used as model inputs to assess the durability of the concrete structure. The quantification of these properties is carried out using mainly standardised concrete test specimens, cast and cured under optimal laboratory conditions. However, the real structure is built under different conditions and using different methods. This naturally leads to a difference between the concrete properties achieved at the construction site and those determined in the laboratory. This difference is currently being assessed in the context of an ongoing German research project. For this, samples of fresh concrete used at different construction sites were taken and used to cast separate, standardised test specimens, as well as a 2 m × 2 m, 25 cm thick dummy wall, which was built by the construction workers with the available tools and methods on site. The separate standardised test specimens were cured in the laboratory in accordance with current German standards. The wall was cured in the form for a determined number of days; in most cases 7 days. Core-drill samples were later taken from it and used to prepare test specimens for comparison against the laboratory specimens. Tests were carried out to determine the compressive strength, the carbonation rate and the chloride migration coefficient of the used concrete. The core-drill samples performed worse than the laboratory-cured samples, suggesting that the performance achieved in the construction may be lower than that of laboratory specimens. © 2023, The Author(s), under exclusive license to Springer Nature Switzerland AG.

Prefabricated superstructures for bridges made of high-strength concrete – faster, slimmer and more sustainable. In order to speed up the urgent modernization of bridges in the German road and rail network, it is also necessary to modernize the construction technology for bridges. The new bridges should be able to be built as quickly as possible and be beautiful, sustainable and robust. It turns out that prefabricated construction methods can be particularly advantageous for this requirement profile. The opportunities offered by this construction method should therefore be used more intensively in the future. The new option of using prefabricated parts with high-strength concrete has proven to be a particularly powerful innovation. This means that the area of application for bridge superstructures made of prefabricated concrete parts can be significantly expanded and adapted to the requirements of bridge modernization. The existing long-term experience with bridges with high-strength concrete also suggests that these structures will be of excellent durability and sustainability. © 2023 Ernst & Sohn GmbH.

Deck caps of bridges—in Germany shortly known as “bridge caps”—are usually made of steel reinforced concrete to form the anchoring area for the guardrail of bridge deck edges on existing cantilever slabs. Combined with protective devices, the outside arranged deck caps provide safety against lateral breaking or crashes of vehicles. Due to their exposed position in the cross section of bridges and the particularly intensive stresses, the deck caps are considered as wear parts and have to be renewed several times during the service life of a bridge. The decisive factor here is the discrepancy between the frost resistance of the concrete and the crack width restriction. On the one hand, only very small cracks (<0.2 mm) can be accepted to prevent corrosion of the rebars under the strong exposure of these elements. To confine crack widths without applying an excessively high degree of reinforcement, a low concrete compressive strength is advantageous. On the other hand, a sufficient frost resistance requires a correspondingly high compressive strength. With carbon reinforcement, these contrary points could be defused and simultaneously the needed durability could be provided. Therefore, slightly modified deck cap concretes combined with a carbon reinforcement mesh were tested to examine the bond behavior with and without freeze–thaw attack. To prove the characteristics of this combined system, the crack formation and crack distribution were investigated experimentally. The test results were compared to calculated values from a mathematically tool to be able to develop different reinforcement concepts in future that can ensure an optimized crack formation and crack width for deck caps. © 2022 The Authors. Structural Concrete published by John Wiley & Sons Ltd on behalf of International Federation for Structural Concrete.

In the case of partial-area loading, compressive forces are transmitted into concrete members only over a limited area. For plain concretes and conventionally reinforced concretes, numerous investigations have already been carried out analyzing the load-bearing behavior under partial-area loading. Due to the tendency towards higher concrete strengths and the increasingly widespread use of steel fibers in recent years, it becomes also necessary to investigate the performance of high-strength steel fiber reinforced concrete (SFRC) under partial-area loading. This paper describes experimental tests on high-strength steel fiber reinforced concrete under partial-area loading with spatial and plane load distribution. Different area ratios and concretes with different fiber types and contents as well as fiber cocktails were considered. On the basis of the test results, a calculation approach is proposed for the determination of the bearable ultimate local stress. It is shown that by referring to the flexural tensile strength, instead of the compressive strength, as in the case of common calculation approaches, a more precise approximation of the ultimate local stresses for high-strength steel fiber reinforced concrete is possible. © 2021, RILEM.

In the present study, the capability of high-strength short steel fibers to control the degrada-tion in high-performance concrete was experimentally examined and numerically simulated. To this end, notched prismatic high-performance concrete specimens with (HPSFRC) and without (HPC) short steel fibers were subjected to static and cyclic tensile tests up to 100,000 cycles. The cyclic tests showed that the rate of strain increase was lower for HPSFRC specimens and that the strain stagnated after around 10,000 cycles, which was not the case with HPC specimens. The microscopic examinations showed that in HPSFRC, a larger number of microcracks developed, but they had a smaller total surface area than the microcracks in the HPC. To further investigate the influence of fibers on the behavior of HPSFRC in the cracked state, displacement-controlled crack opening tests, as well as numerical simulations thereof, were carried out. Experiments have shown, and the numerical simulations have confirmed, that the inclusion of short steel fibers did not significantly affect the ultimate strength; however, it notably increased the post-cracking ductility of the material. Finally, the unloading/reloading behavior was examined, and it was observed that the unloading stiffness was stable even for significant crack openings; however, the hysteresis loops due to unloading/reloading were very small. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

According to the current European standard EN 14487-1 (residual) flexural strengths of steel fiber-reinforced sprayed concrete are determined by performing a four-point bending test on beams according to EN 14488-3. Controversial to this test method, a three-point bending test on notched square panels is recommended by EFNARC. According to EFNARC, one substantial advantage of this test method is the minor scattering of the test results. Against this background, a round robin test was performed at European level. The aim was to investigate the comparability and correlation on the one hand, on the other hand to assess the precision of both test methods. The results shall provide initial pointers for the classification of the residual strengths for steel fiber-reinforced sprayed concrete with two different test methods. The results showed that the scattering of the residual strengths is marginally smaller with the test method on panels in comparison to the test method on beams. As a result, a slightly higher precision was achieved. Therefore, the EFNARC test shall be included as an alternative test procedure in EN 14487-1 in future. © 2021. The Authors. Structural Concrete published by John Wiley & Sons Ltd on behalf of International Federation for Structural Concrete

There is an increasing need for the development of novel technologies for tunnel construction in difficult geological conditions to protect segmental linings from unexpected large deformations. In the context of mechanized tunneling, one method to increase the damage tolerance of tunnel linings in such conditions is the integration of a compressible two-component grout for the annular gap between the segmental linings and the deformable ground. In this regard, expanded polystyrene (EPS) lightweight concrete/mortar has received increasing interest as a potential “candidate material” for the aforementioned application. In particular, the behavior of the EPS lightweight composites can be customized by modifying their pore structure to accommodate deformations due to specific geological conditions such as squeezing rocks. To this end, novel compressible cementitious EPS-based composite materials with high compaction potential have been developed. Specimens prepared from these composites have been subjected to compressive loads with and without lateral confinement. Based on these experimental data a computational model based on the Discrete Element Method (DEM) has been calibrated and validated. The proposed calibration procedure allows for modeling and prognosis of a wide variety of composite materials with a high compaction potential. The calibration procedure is characterized by the identification of physically quantifiable parameters and the use of phenomenological submodels. Model prognoses show excellent agreement with new experimental measurements that were not incorporated in the calibration procedure. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

Deformation behavior of concretes with high plastic compressibility. For special applications, for example compressible layers in tunnel constructions, there are already initial approaches for mortars and concretes, which are characterized by a high plastic compressibility. This paper describes experiments on the compressive behavior of highly porous concretes with low compressive strength (&lt; 5 N/mm2) under transverse strain constraint. Concretes with different compressible additives (expanded glass / EPS) and various water-cement ratios were considered in the tests. Furthermore, the deformation behavior under both partial- and full-area loading was investigated in order to cover different application-related boundary conditions. On the basis of the determined stress-deformation curves as well as investigations on the structure of specimens before and after the compression tests essential findings on the compressive behavior of the concretes could be gained and material- and test-related parameters could be identified. © 2020, Ernst und Sohn. All rights reserved.

In special cases, concrete members are exposed to high locally concentrated loadings. Such concentrated loadings lead to a multi-dimensional stress state beneath the loaded area. Due to the load diffusion, large splitting tensile stresses are generated in the upper regions of the concrete member (i.e. St. Venant disturbance zone) and spread along directions perpendicular to the load. In order to resist these splitting tensile stresses, the state of the art is to reinforce concrete members with transverse steel reinforcement. An alternative approach is to add steel fibers to the concrete matrix. However, regarding economic concerns it may not appropriate to reinforce the entire concrete member with an adequate high amount of steel fibers, rather only those zones where high splitting stresses are expected. The main objective of the presented experimental study was to investigate the load-bearing and fracture behavior of hybrid concrete elements with splitting fiber reinforcement under concentrated load. For this purpose, in a first step, hybrid specimens were produced containing both plain and fiber concretes. The reference specimens consisted exclusively of plain concrete, while the hybrid specimens were partially strengthened with various types of steel fibers only in the St. Venant disturbance zone, instead of a full range fiber reinforcement. The thickness of the reinforcement layer was varied in order to determine the optimal configuration of fiber reinforcement. Taking into account the influence of the casting direction on the fiber orientation and consequently on the bearing and fracture behavior, the hybrid specimens were cast either in standing or in lying molds by means of a "wet-on-wet" casting technique. These hybrid elements were then tested under concentrated load. The test results showed that under concentric loads the maximum bearing capacity of the hybrid specimens increased progressively with growing thickness of the fiber reinforced concrete layer. In contrast to the plain concrete specimens, the fiber reinforcement led to a remarkable improvement in the post-cracking ductility. Compared to the fully reinforced specimens, the hybrid specimens that were only reinforced in the St. Venant disturbance zone exhibited - besides an almost identical bearing capacity - a similar local behavior in the postcracking zone. Furthermore, a significant impact of the casting direction on the bearing as well as fracture behavior could be proved. © 2020 American Concrete Institute. All rights reserved.

Repair of concrete traffic areas with slim carbon reinforced concrete layers. Concrete pavements are exposed to high loads from existing traffic volumes and extreme weather conditions. In order to prevent wild cracking caused by the resulting constraining stresses, transverse joints are usually installed. However, these are also weak points in the concrete pavement, so that continuous maintenance and upkeeping is necessary. An ecologically and economically viable alternative to renewing the entire concrete slab is to rehabilitate the concrete runway with the aid of a jointless as well as thin top layer of carbon reinforced concrete. By separating the bond between the old concrete layer and the carbon concrete layer in the area of the transverse joint, a distribution of cracks is ensured and the crack width of the individual cracks in the carbon reinforced concrete pavement is reduced by multiple crack formation. Liquids can therefore penetrate less deeply into the concrete and damage neither the old concrete layer nor the corrosion-resistant carbon reinforcement. © 2020, Ernst und Sohn. All rights reserved.

According to the actual European standard EN 14487-1 the potential of steel fiber reinforced sprayed concrete is characterized by flexural strength tests (first peak, ultimate and residual). In most cases, this is performed by a four-point bending test on beam specimens, specified in EN 14488-3. As an alternative test method, a three-point bending test on square panels with notch is recommended by EFNARC. It is argued as main benefit of the latter test method, that the geometry and dimensions of the panels are equal to those of specimens used for measuring the energy absorption capacity according to EN 14488-5. Hence, the specification of the ductility of fiber reinforced concretes according to EN 14487-1 in terms of residual strength and energy absorption capacity can be achieved preparing only one type of specimen. Furthermore, the EFNARC guideline points out a smaller scatter of test results, compared to the beam tests according to EN 14488-3. Before the EFNARC method will be considered in EN 14487-1, the relevant CEN TC 104/WG10 requests for adequate proofs. These were performed within a round-robin test (RRT) on testing the flexural behavior of steel fiber reinforced sprayed concrete by the standardized EN 14488-3 method as well as by the proposed EFNARC method. The aim of this RRT was to investigate the comparability and correlation between the two test methods. Furthermore, the scatter of both methods was assessed. The RRT has been organized by Ruhr University Bochum, in whose labs the steel fiber reinforced sprayed concrete specimens (beams according to EN 14488-3 and square panels according to EFNARC guideline) were produced using a robotic spraying machine. The specimens were then tested in five independent laboratories Europe-wide. The results of this RRT are presented in detail. With regard to the residual strength, the relevant material parameter for steel fiber reinforced sprayed concrete, a tendency towards a slightly lower scatter was detected for the EFNARC test method. © 2020 American Concrete Institute. All rights reserved.

In mechanized shield tunneling, the annular gap between the tunnel structure and the surrounding soil needs to be filled with an adequate grouting mortar to ensure a rapid and safe bedding of the segment rings and to minimize settlements on the surface above the tunnel lining. After mounting of the segment rings and filling of the annular gap, a rapid solidification of the used grout must prevent possible displacements or a floating of the tunnel. In the case of nearly impermeable soils, two-component grouts are necessary, which develop an adequate strength and stiffness in a short period of time by the use of powerful activators like water glass (component B). In addition to the commonly activated cementitious materials, it is feasible to ensure an immediate and sufficient bedding by physical effects. Therefore, the use of superabsorbent polymers (SAP) as component B has been investigated. Experimental studies have been carried out in a systematic way in order to determine the type of the SAP, which leads to a sufficient absorption rate in the alkaline pore water of a cementitious grout. After identification of a suitable “alkali-stable” polymer, tests were carried out in order to examine the necessary amount of SAPs in a slightly modified one-component grout (component A) to cause a sufficient solidification of the whole system within a short period of time. Next to this, the short and long term strength development like shear strength or compressive strength of the combined system (component A and B) were determined. Considering the state of the art of the structural design of the grouting technology on a tunnel boring machine and the generally used liquid activators, a permanent pre-suspension of the SAP was tested and also the strength development of the activated system was examined. © RILEM 2020.

Bearable local stress of high-strength steel fiber reinforced concrete. In the case of partial-area loading, compressive forces are transmitted into concrete members only over a limited area. For plain concretes and conventionally reinforced concretes, numerous investigations have already been carried out analysing the load-bearing behavior under partial-area loading. Due to the tendency towards higher concrete strengths and the increasingly widespread use of steel fibers in recent years, it becomes also necessary to investigate the performance of high-strength steel fiber reinforced concrete under partial-area loading. The article describes experimental tests on high-strength steel fiber reinforced concrete under partial-area loading with spatial and plane load distribution. Different area ratios and concretes with different fiber types and contents as well as fiber cocktails were considered. On the basis of the test results, a calculation approach is proposed for the determination of the bearable ultimate local stresses. It is shown that by referring to the flexural tensile strength, instead of the compressive strength as in the case of common calculation approaches, a more precise approximation of the ultimate local stresses for high-strength steel fiber reinforced concrete is possible. © 2019, Ernst und Sohn. All rights reserved.

Two contradictory requirements are basically demanded on single-component grouting mortars. On the one hand, a sufficient workability lasting for several hours; on the other hand, a rapid development of shear strength immediately after grouting. The latter is normally achieved through dewatering of the mortar into the surrounding soil. During dewatering also particles are transported to the soil and lead to a clogging in the interface, which can have a significant influence on the further dewatering process and thus on the consolidation behavior of the grout. In this research study, the dewatering behavior and redistribution of the particles of single-component grouting mortars have been systematically investigated under variation of the relevant material-specific parameters, such as the granulometry of the fines and aggregates. For the determination of the filtrate water, a filter press test was developed, simulating the conditions within the annular gap. For the evaluation of the redistribution of the particles, the particle size distribution, density and water content of individual layers of the dewatered grout were determined over the specimen height. The investigations revealed a significant influence of the granulometry of the particles on the grout properties. At early mortar ages, the amounts of filtrate water were at the same level independent of the granulometry and specific surface of the binder. However, the shear strengths increased steadily with increasing specific surface of the binder and age. With regard to the granulometry broken particles with a rough surface provided an essential contribution to the development of shear strength. © 2018, Iran University of Science and Technology.

In mechanized tunneling, an early bedding of the recently installed segment ring is a constructive necessity and of considerable importance for the durability of the whole structure. Alongside the actual material technology of the grout, procedural aspects like advance rate of the tunnel boring machine, therefore the interacting parameters like grout volume per unit of time and gelation and hardening of the grout also play an important role for a successful injection process. Due to adjustments of the grout composition during the construction the flow properties before and especially after activation will change. In order to observe the flow behavior and the interactions between the grout and soil a large-scale test setup was developed to simulate the annular gap grouting under realistic boundary conditions. Herein, it is possible to observe the flow behavior and to examine the material for its homogeneity and strength characteristics after grouting. © 2019 Taylor & Francis Group, London.

Systematic investigations of hardened cement paste, high-performance concrete and mortar with and without microfibers, subjected to static and cyclic tensile loadings, were conducted. The material degradation was investigated by means of microscopic analyses of the microcrack development. Notched specimens were subjected to a predefined number of load cycles. A nonsteady increase of microcracking with increasing load cycles was observed in high-strength concrete, whereas the addition of steel fibers lead to a steady increase of microcracks. High-strength mortar often showed premature failure, while addition of steel micro fibers allowed completion of the cyclic tests. To obtain a deeper insight into physical mechanisms governing fatigue and structural failure, high-performance concrete (HPC) and fiber-reinforced concrete (FRC) under static and cyclic tensile loadings have been modeled using cohesive interface finite elements, micromechanics, and a fiber-bundle model. Analysis of model predictions shows the significance of strength disorder and fiber properties on the structural behavior. © 2019 The Authors. Structural Concrete published by John Wiley & Sons Ltd on behalf of International Federation for Structural Concrete

In order to utilize the benefits of steel fiber reinforcement and ensure an economic design at the same time, a new approach to produce hybrid lining segments was developed and is presented. The basic principle is to strengthen the vulnerable zones of segments with a high performance steel fiber reinforced concrete (HPSFRC), while for less critical zones an ordinary lining concrete is used. Focusing on the longitudinal joints, an experimental study on the bearing and fracture behavior of hybrid specimens, produced with a so called “wet-on-wet” production procedure, has been conducted under concentrated load. It was proven that the installation of a HPSFRC layer only in zones where critical splitting stresses occur leads to a remarkable increase in the bearable loads. The results showed that the use of vertical instead of horizontal formworks had a positive impact on the fiber orientation and thus consequently on the bearing behavior. © 2019 Taylor & Francis Group, London.

The BBQ-Guideline of DAfStb – Best Practice for the quality chain in concrete construction. The standards of concrete construction presently only partially illustrate the different complexity of the construction tasks. The structure of the rules also complicates interaction or communication. However, the concrete construction standards will only be sustainable if they provide for the various requirements, expectations and boundary conditions in planning and construction and building material production in the same way appropriate and therefore differentiated solutions and better coordinated. In order to support such a better coordination between planning, building materials and construction, the German Committee for Reinforced Concrete (DAfStb) has decided to establish the concept of ConcreteConstructionQuality-classes (BetonBauQualitätsklassen BBQ). In this BBQ-concept quality measures will be organised depending on the complexity of tasks in concrete construction. This report summarises the latest state-of-the-art of the development of the BBQ-concept and the work on the DAfStb-BBQ-Guideline. © 2019, Ernst und Sohn. All rights reserved.

This study investigates the evolution of the microstructure and mechanical properties of mortar. Mortar samples consisting of Portland cement CEM I42.5 R (~ 60 vol% of quartz sand 0/2 mm, w/c-ratio of 0.5) were prepared and stored according to EN 1015. After 1, 2, 7, 14 and 28 days, the samples were oven-dried until constant weight as well as vacuum-dried. The microstructure of the mortar samples was investigated using scanning electron microscopy. Phase analysis was performed using X-ray diffraction, allowing the description of the crystalline phase evolution during hardening. Mechanical properties were evaluated using nanoindentation. Based on the nanoindentation results, the effective Young’s modulus was calculated using the model by Hashin and Shtrikman. The moduli calculated based on the values of the nanoindentation experiments were compared to the Young’s modulus determined in compression experiments. The results show that the Young’s modulus determined by the nanoindentation and compression test describes a degressive curve progression. The studies show a correlation between the results from nanoindentation tests and the mechanical properties obtained from the compression tests. Therefore, the microstructural evolution of mortar, including the influence of pores on Young’s modulus, must be taken into account to estimate the macroproperties from the nanoindentation tests. © 2018, Iran University of Science and Technology.

In mechanized tunneling, the annular gap between the segmental lining and the surrounding soil must be filled with a suitable grouting material simultaneously. When using two-component grouts as grouting material with a composition adapted to the geological and structural conditions, the early bedding of the recently installed segment ring but also a permanent position stability of the tunnel can be reliably achieved. In the compilation of two-component grouts, by now exclusively on an empirical basis, primarily hydraulic binders are used. In order to increase the economics of such systems, it is obvious to replace the hydraulic fraction in the binder by suitable substituents without adverse consequences. As a result of systematic investigations, the use of latent-hydraulic blast furnace slag has proved to be suitable in the system of activated annular gap grouts. In combination with different activator configurations, exchange rates of up to 70% of the cement are possible. © 2019 Taylor & Francis Group, London.

Concrete pavements are exposed to a number of stresses during their service life, mostly resulting from traffic and climate conditions. In consideration of the continuously rising traffic volume, the durability requirements of concrete pavements become more and more significant. In this context, maintenance and repair become increasingly important. Small-scale repairs like spalling at edges up to the replacement of whole slabs are proven in several cases. In contrast, large-scale maintenance techniques for partial repairs of whole pavement sections are not available, yet. If the upper layer concrete is deteriorated, while the lower layer and the base course are still intact, the whole pavement needs to be replaced, due to a lack of alternatives. Therefore new maintenance techniques like the application of concrete overlays are needed for an economic rehabilitation and the prevention of an unnecessarily long traffic disruption by time-consuming maintenance of complete pavements. The relevant questions how a durable bond between old and new concrete can be ensured and which parameters affect this bond, were investigated in representative studies on large-scale concrete beams with a thin concrete overlay on existing concrete. © 2018 The Authors, published by EDP Sciences.

Impact of fatigue-induced microcracks in concrete on the fluid ingress. The durability of concrete structures is sustainably impaired by the ingression of deleterious substances (carbon dioxide, chloride, acids, sulfates, alkalis, etc.) into the concrete microstructure. In order to minimize these processes, it is important to create a dense structure of the mortar matrix through an accordingly low water-cement ratio. If concrete structures are exposed to cyclic loading during their operating life, a degradation takes place within the concrete in the form of microstructural damages. These do not show macroscopically, but rather in the form of microcracks with widths from 5–15 μm within the mortar matrix. These in turn promote the ingress of fluid media and thus deleterious substances. Typical examples are concrete traffic areas, that are exposed to cyclic loading through traffic on the one hand and alkaline de-icing agents during wintertime on the other hand. Such an external alkali supply promotes a deleterious alkali silica reaction (ASR) significantly. Similar conditions can be observed for offshore wind energy plants in alkaline seawater. The focus of the relevant investigations lay on the characterization of the degradation effects on a microstructural level as well as the transport behavior of fluid media in intact as well as pre-damaged concrete structures. © 2018, Ernst und Sohn. All rights reserved.

Impact of fatigue-induced microcracks in concrete on the fluid ingress. The durability of concrete structures is sustainably impaired by the ingression of deleterious substances (carbon dioxide, chloride, acids, sulfates, alkalis, etc.) into the concrete microstructure. In order to minimize these processes, it is important to create a dense structure of the mortar matrix through an accordingly low water-cement ratio. If concrete structures are exposed to cyclic loading during their operating life, a degradation takes place within the concrete in the form of microstructural damages. These do not show macroscopically, but rather in the form of microcracks with widths from 5–15 Μm within the mortar matrix. These in turn promote the ingress of fluid media and thus deleterious substances. Typical examples are concrete traffic areas, that are exposed to cyclic loading through traffic on the one hand and alkaline de-icing agents during wintertime on the other hand. Such an external alkali supply promotes a deleterious alkali silica reaction (ASR) significantly. Similar conditions can be observed for offshore wind energy plants in alkaline seawater. The focus of the relevant investigations lay on the characterization of the degradation effects on a microstructural level as well as the transport behavior of fluid media in intact as well as pre-damaged concrete structures. © 2018, Ernst und Sohn. All rights reserved.

Innovative test setup for the cyclic loading of multiple large-format concrete specimens. In case of concrete structures, cyclic loading can cause a microstructural degradation long before critical fatigue failure. In combination with other exposures, such as de-icing agents or sea water, this can result in an impairment of the durability. Therefore, load-dependent and load-independent cyclic loads must be distinguished. Load-independent but recurring loads are defined by thermal and hygroscopic deformations during the annual and daily changes for each concrete component. Load-dependent cyclic loads can be caused by i.e. traffic on concrete pavements or bridges or from the rotor blades of wind power plants. In order to be able to record the influence of the cyclic loads and their consequences on the durability (degradation of the concrete structure), also in combination with other exposures, a test setup was developed at the Institute of Building Materials at the Ruhr-University Bochum. This test setup allows a realistic simulation of cyclic loads on large-scale concrete specimens. The development, the functional principle and the successful use of the multiple test setup in several research projects are presented in this publication. Copyright © 2018 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin

Concrete for agricultural or industrial applications is often subject to intense acid attack. Most affected structures are sewage structures and biogas plants, natural draught cooling towers or silage silos. Widely independent from acid type, in most cases the acid attack on concrete runs the same way, starting with dissolution of easily soluble calcareous phases like calcium hydroxide. With ongoing attack, calcium-silicate-hydrate crystals (CSH) are also affected by acidic media. In contrast, siliceous phases like silicon-dioxide (SiO2) are widely unaffected by acid attack. While the dissolution of the matrix is increasing with ongoing attack, quarzitic aggregates remain unchanged. Beside the use of coarse SiO2-aggregates, the resistance against acid attack is mainly increased by a minimization of the porosity. For this purpose on one hand, a low water/cement-ratio has to be sought, on the other hand also the fines should be distributed with an optimized grading curve (e.g. Fuller-principle). In practice, this results in a combination of various fine and ultra-fine components, e.g. fly ash, GGBS, silica fume or metakaolin. Such binder compositions lead to a particularly dense microstructure, especially at pore sizes below 1 micron, and a higher chemical resistance due to a lower Ca(OH)2 content. This paper gives an overview on typical acid-resistant concretes, most common applications as well as the effects of the related acid attack and points out the potential of granulated blast furnace slag addition to such concretes. © 2018 The Authors, published by EDP Sciences.

Treatment of Fresh Concrete Surfaces – An Approach to the Evaluation of Appropriate Fresh Concretes. For the professional treatment of fresh concrete surfaces, i.e. smoothing or the like, fresh concretes, which do not exhibit an overly high adhesiveness, are appropriate. Therefore, the rheological properties of the binder paste or mortar in concrete are most important. For some time, problems concerning this matter occurred, especially when concretes were produced with high binder contents and plasticizers based on PCE. Hitherto, simple and practicable methods testing the fresh concretes on site with regard to their relevant properties are not yet provided. In orienting investigations attempts were made, to also evaluate the rheological behavior of the delivered concrete quickly and sufficiently accuratly on site, through the modification of common test methods (flow table test) and through the development of new test methods (ball-sink test). Copyright © 2018 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin

Durable concrete pavements using different binders in top and bottom lift. It is a matter of common knowledge that ground granulated blast furnace slag and siliceous fly ash can be used to mitigate alkali-silica-reaction (ASR) in concrete. Nonetheless such materials are seldom used for the construction of concrete pavements in Germany, as their application can impair the deicer-scaling resistance of the concrete. In two-lift concrete pavements a possible approach to avoid this problem is to use cements with a higher proportion of blast furnace slag only in the thick bottom lift, thus minimizing its susceptibility to ASR. The thin top lift, which is subjected to a high level of mechanical and freeze-thaw stress, is still produced with Ordinary Portland Cement, hence ensuring a sufficient deicer-scaling resistance. However, this approach is currently hindered by the German technical standards for concrete pavements, which require the use of the same cement in the top and bottom lift. Furthermore the standards do not permit the application of siliceous fly ash to partly substitute cement in concrete pavements. It was therefore the aim of a research project to obtain the necessary knowledge for assessing potential benefits and risks of a more flexible use of binders in concrete pavements. Copyright © 2017 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin

According to the objectives of the research group 1498, this paper deals with degradation effects in concrete structures that are caused by cyclic flexural loading. The goal is to determine their influence on the fluid transport processes within the material on the basis of experimental results and numerical simulations. The overall question was, to which extent the ingress of externally supplied alkalis and subsequently an alkali-silica reaction are affected by such modifications in the microstructure. Degradation in the concrete microstructure is characterized by ultrasonic wave measurements as well as by microscopic crack analysis. Furthermore, experiments on the penetration behavior of water into the investigated materials were performed. The penetration behavior into predamaged concrete microstructures was examined by the classical Karsten tube experiment, nuclear magnetic resonance method, and time domain reflectometry techniques. In order to create an appropriate model of the material's degradation on the water transport, the Darcy law was applied to describe the flow in partially saturated concrete. Material degradation is taken into account by an effective permeability that is dependent on the state of degradation. This effective permeability is obtained by the micromechanical homogenisation of the flow in an Representative Elementary Volume (REV) with distributed ellipsoidal microcracks embedded in a porous medium. The data gained in the microscopic crack analysis is used as input for the micromechanical model. Finite element simulations for unsaturated flow using the micromechanical model were compared with the experimental results showing good qualitative and quantitative agreement. © 2017 fib. International Federation for Structural Concrete

Durable concrete pavements using different binders in top and bottom lift. It is a matter of common knowledge that ground granulated blast furnace slag and siliceous fly ash can be used to mitigate alkali-silica-reaction (ASR) in concrete. Nonetheless such materials are seldom used for the construction of concrete pavements in Germany, as their application can impair the deicer-scaling resistance of the concrete. In two-lift concrete pavements a possible approach to avoid this problem is to use cements with a higher proportion of blast furnace slag only in the thick bottom lift, thus minimizing its susceptibility to ASR. The thin top lift, which is subjected to a high level of mechanical and freeze-thaw stress, is still produced with Ordinary Portland Cement, hence ensuring a sufficient deicer-scaling resistance. However, this approach is currently hindered by the German technical standards for concrete pavements, which require the use of the same cement in the top and bottom lift. Furthermore the standards do not permit the application of siliceous fly ash to partly substitute cement in concrete pavements. It was therefore the aim of a research project to obtain the necessary knowledge for assessing potential benefits and risks of a more flexible use of binders in concrete pavements. Copyright © 2017 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin

In practice, concrete structures are often subjected to concentrated loads. Under such high stress concentrations, large splitting stresses are generated due to the resulting stress diffusion. In order to resist these splitting stresses, the state of the art is to place transverse steel reinforcement in zones where the most critical splitting stresses occur. An alternative approach gaining increasingly importance, is to add steel fibers to the concrete mixture. Regarding economic concerns, however, it is recommended not to reinforce the entire concrete element with steel fibers, rather in zones where high splitting stresses are expected. Based on this fact, a new design concept to manufacture hybrid concrete elements has been developed. In this paper, an experimental study on the load-bearing capacity and failure mode of steel fiber reinforced concrete elements under centric strip-loading has been conducted. Forthis purpose, hybrid concrete specimens (50 × 25 × 50 cm3) containing both plain and fiber concretes were produced. The reference samples were produced only with plain concrete (PC), while the hybrid specimens were additionally strengthened with a layer of high performance steel fiber reinforced concrete (HPSFRC). The thickness and position of this HPSFRC layer has been varied in order to determine the most efficient and economic configuration of the splitting fiber reinforcement. The test results have shown that the load-bearing capacity could be enhanced up to 200% depending on the thickness and position of the incorporated layer of HPSFRC. As a fundamental finding of this experimental study it is shown that the ratio of fiber reinforcement (e.g. the thickness of the HPFSRC layer) was not solely decisive for the load bearing capacity, rather the position of the reinforcement layer according to the location of the crucial splitting stresses. © Springer International Publishing AG 2018.

According to the goals of the research group 1498, this paper deals with the effects of cyclic flexural loading in a four-point bending test on the fluid transport processes within a concrete structure. Therefore, the degradation of the microstructure is characterized through ultrasonic wave measurements as well as microscopic crack analysis. In order to numerically model these processes, experiments on the penetration behavior of water into the concrete were carried out. The penetration behavior over time as well as the influence of degradation on the water transport were investigated. To predict the influence of concrete degradation on alkali diffusivity, a multi-scale continuum micromechanics model is incorporated into the numerical model, which accounts for the topology and the three-dimensional distribution of microcracks. As expected, the numerical simulation predicts larger alkali-penetration in pre-damaged concrete. Regarding the micro-crack distribution, an anisotropic distribution of micro-cracks tangential to the direction of the alkali and water flux increases their penetration depth. © Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

Steel-fiber-reinforced concretes are used more and more also in structural concrete elements. Examples for these are among others tunnel lining segments. In this case there is a specific characteristic, that concretes for such elements usually have a compressive strength between 70 and 90 N/mm2. These concretes are very brittle with only low fracture toughness, so that damages in form of cracking and spalling often occur in the periphery of the segments. By the addition of steel fibers the ductility of the concrete can be considerably improved. The primary question is, to which extend the stresses can be transferred in concrete cracks by the fibers bridging these cracks. To investigate the bonding behavior between steel fiber and concrete matrix, single fiber pullout tests were performed in high-strength concrete matrix. Various parameters such as fiber shape, geometry, tensile strength, surface properties, inclination angle and embedment length were investigated. Based on the experimental results, the effect of those parameters on pullout force versus slip relationship, pullout work, fiber utilization and fiber/matrix failure mode were analyzed and discussed. Copyright © 2014 Ernst &amp; Sohn Verlag für Architektur und technische Wissenschaften GmbH &amp; Co. KG, Berlin.

The pullout behaviour of single steel fibres embedded in a concrete matrix is investigated for various configurations of fibre types and embedment lengths and angles by means of laboratory tests and analytical models. Laboratory tests for fibre pullout are performed to investigate the fibre-matrix bond mechanisms. Parameters influencing the fibre pullout response, such as fibre shape, fibre tensile strength, concrete strength and fibre inclination angle are systematically studied. The effects of these parameters on the pullout force versus displacement relationship, fibre efficiency and fibre/matrix failure response are analysed based on the experimental results. For the analytical modelling of the fibre pullout behaviour of straight fibres, an interface law is proposed for the frictional behaviour between fibre and matrix. In the case of inclined fibres, the plastic deformation of the fibre and the local damage to the concrete are also considered. For hooked-end fibres, the anchorage effect due to the hook is analysed. Combining these sub-models allows the pullout response of single fibres embedded in a concrete matrix to be predicted. In addition, numerical simulations of pullout tests are performed to obtain insights into the local fibre-concrete interactions and to provide supporting information for the analytical modelling. The models are successfully validated with the experimental results. Copyright © 2014 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

In the Guideline "Production and use of cementitious grouting concrete and grouting mortar" [1] by the German Committee for Structural Concrete, the casting thickness is limited to 25-times of the maximum aggregate size. Background are concerns that concrete with a larger thickness heats up so intensively that loss in strength, microcracking and impairment of durability as e.g. reduced frost resistance or potential secondary ettringite formation become relevant. In a research project, sponsored by the German Society for Concrete and Construction Technology, the Institute for Building Materials of the Ruhr-University Bochum examined the effects of higher temperatures during hydratation of the cementitious grouting concrete with higher installation thickness on its strength and durability. It was found that, with the currently limited thickness, more or less the same thermal conditions are existent as with larger heights. Thus no impairs on the properties of hardened concrete could be observed [2]. © Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

In mechanized tunneling with segment lining the annular gap between segment lining and soil, caused by tunnel driving, must be backfilled instantaneously with an adequate grouting mortar, to avoid bulking of the surrounding soil, to embed the tunnel lining and to minimize settlements of the ground surface. The decisive requirements for such annular gap grouts are on the one hand optimal flow properties lasting for several hours and sufficient stability against segregation at same time, on the other hand a fast development in shear strength immediately after grouting within a few minutes. Latter is usually achieved by dewatering of the mortar into the surrounding soil. Thus, two contradictory requirements are demanded on annular gap grouts. The main objective of this research study was to investigate the dewatering behavior of grouting mortars, which can be influenced by the constituents and the composition significantly. In systematic and extensive investigations the correlation of these properties has been studied fundamentally as well as the consistency. For this purpose, a test setup was developed, simulating the conditions within annular gaps up to 20. cm in width. The amount of filterable water, including temporal effects, has been determined. Additionally, the development of shear strength, influenced by the dewatering effect, has been examined. In comparison, shear strengths were also determined at non-dewatered specimens, reflecting the worst case of non-dewatering. The investigations were based on grout compositions, which were primarily used in major traffic tunnels. These mixes have been modified by parameters such as fineness of the binder (cement, fly ash, inert additions) and types of aggregates. From the experimental results, the main influencing parameters on the dewatering behavior and on the development of the required shear strength could be defined. © 2014 Elsevier Ltd.

The annular gap between segment lining and bedrock, caused by tunnel driving, must be filled instantaneously with an adequate grouting mortar. The decisive requirements for such grouts are, on the one hand, optimal flow properties lasting for several hours and sufficient stability and, on the other hand, a rapid development of shear strength immediately after grouting. The latter is normally achieved by dewatering under pressurization into the surrounding soil. Thus, two contradictory requirements are demanded for such grouts. The dewatering behavior of these grouts, which is significantly influenced by their constituents, is an important key factor. The correlation of these properties has been studied in systematic and extensive tests. The comprised grout mixtures are also performed in accordance with real design concepts to ensure an adequate application in practice. For this purpose, a test set-up was developed, simulating the conditions within annular gaps up to 18 cm in width. The amount of filterable water, including temporal effects, has been determined. Additionally, the development of shear strength by the dewatering effect has been examined at dewatered grouts. The investigations were carried out on typical grout mixtures with different variations of parameters (e.g. fly ash: additive ratio). From the results, the main influencing parameters on the dewatering behavior and development of shear strength could be defined. Consequently, well-proved mixtures can be recommended for further purpose. © 2014 American Society of Civil Engineers.

During the assembly stage of precast segmental tunnel linings, the segments are often subjected to impact and concentrated loads. Since plain concrete is a quasi-brittle material exhibiting low tensile strength and fracture toughness, concrete damages - in the form of cracking and spalling - are very likely to occur on the periphery of the segments. By the addition of steel fibers into the concrete matrix, the robustness and ductility of this quasi-brittle material can be significantly enhanced due to the crack-bridging effect of fibers. To simulate the segments subjected to concentrated loads in small-scale, partial-area loading tests were carried out on plain and fiber concrete prisms under laboratory conditions. The principal variables investigated in this paper were fiber reinforcement, area ratio and loading eccentricity. The effects of those variables on the load-bearing capacity, failure mode and crack pattern were analyzed and discussed. From the experimental results, it was found that the presence of steel fibers led to a remarkable increase of the load-bearing capacity of concrete and changed its failure mode from a brittle to a ductile one. Increasing the area ratio or loading eccentricity also had significant influence on the bearing strength and failure pattern. © 2014 American Society of Civil Engineers.

Two nearly contrary requirements are set to the concrete of edge beams on bridges. On the one hand a low water-cement-ratio is sought with respect to a sufficient freeze deicing salt resistance (XF 4). On the other hand the same characteristic value should be not too low as the concrete strength and in sequence the crack-limiting reinforcement should be kept below certain ranges. In the last few years at several times scaling at concrete surfaces of edge beams on bridges due to the attack by freeze and deicing salt has been observed. This casually is associated with the fact, that today also in concretes for these edge beams more and more CEM II- and CEM III-cements are used - contrary to the past, where traditionally CEM I-cement had been applied. To check the influence of the various parameters of concrete and execution on the freeze deicing salt resistance considering the specific random conditions of bridges appropriate lab tests (CDF-tests) had been performed. Amongst others it could be demonstrated that the w/c-ratio, with 0.50 set in ZTV-ING as maximum allowable value, seems to be marginal. Only a small exceeding of this value resulted in significant increased scaling. This was quite more evident in the concretes with CEM II-cements than in those with CEM I. Thus the specific adjustment between cement and water-cement-ratio is of very high importance in concretes for edge beams on bridges. Copyright © 2013 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

In the last few years cracking in concrete pavements was observed in several highway sections in some regions of Germany. Within the scope of extensive investigations no definite single cause of cracking could be observed in the majority of these cases. Besides incremental load-induced stresses by increasing heavy traffic, within this scope especially load-independent stresses due to hygral and thermal changes are of relevance. Several investigations additionally substantiate reaction products of an alkali silica reaction (ASR). Because of this multiplicity of potential causes for cracking it has not been definitely clarified up to now to which extent especially the ASR contributes to cracking in concrete pavements. Rather it seems that superposition and/or interactions of different mechanisms are responsible. © 2011 ASCE.