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Highlights • Concrete slabs prestressed with CFRP are loaded in bending for 1 and 7 years. • Residual strength after 1 and 7 years is compared with short-term test results. • FE simulations use material model based on Microprestress-solidification theory. • Certain deficiencies of the material model related to cyclic humidity are pointed out. • The viscoelastic behavior of CFRP is not found to be significant. Abstract The present study investigates behavior of concrete slabs prestressed with carbon-fiber-reinforced polymers (CFRP) subjected to four-point bending. The main experimental set comprised 10 identical 4.5 m long specimens from which 8 were subjected to quasi-static loading until failure. The remaining two specimens were exposed to sustained loading with duration of 1 and 7 years, and subsequently were loaded until failure. The main objective of this study is to analyze the long-term behavior in order to assess the assumption of several authors that no significant creep or stress relaxation occurs in CFRP prestressed rebars. To achieve that, the experimental data were compared to the results of a complex finite element model in which the CFRP is modeled by a linear elastic material (without creep) which allowed us to confirm the initial hypothesis that no considerable creep or stress relaxation occurs in the prestressed CFRP rebars. [ABSTRACT FROM AUTHOR]

This paper lays the foundation for the development of a high-load bearing energy absorbing system with controlled deformation. Brittle ceramic blocks made of bricks with thin-walled cellular structures are presented and tested with impact loading. The authors demonstrate that such blocks are able to absorb impact energy because of the gradual brittle fracture process which occurs in the cellular structures. Full-scale specimens were subjected to laboratory impact tests: two non-deformable flat-nosed cart tests as well as two full-scale field crash tests involving passenger vehicles with crumple zones. The experiments were designed specifically to prove that block specimens are able to gradually absorb different levels of impact energy and to examine the applicability of using such blocks in the design of cross-drainage culverts. Based on a comparison of the two collisions with test culverts, the authors show that consequences of the collision can be significantly reduced by using brittle blocks with cellular structures in culverts. In fact, the crumple zone of the passenger vehicle which collided with a brittle block culvert was not crumpled; vehicle bounce off was eliminated and gradual deceleration of the vehicle was recorded. [ABSTRACT FROM AUTHOR]

The New Austrian Tunnelling Method (NATM) is widely used for the design and construction of tunnels and one of its defining characteristics is double shell lining. When evaluating the long-term stability of a tunnel, the primary lining of sprayed concrete is not considered due to concerns about long-term alternations in properties; only the secondary lining is used when assessing long-term stability for permanent loads. However, several studies have now shown that primary sprayed concrete linings (SCLs) have reasonable mechanical properties even after years of operation and can therefore be used in evaluating long-term stability in low aggressive environments. Because of this, considerable savings might be achieved in future stability evaluations by taking into account the shear capacity of spray-applied waterproofing membranes applied between both linings. Such double shell linings can be considered to be compound structures able to withstand permanent loads. [ABSTRACT FROM AUTHOR]

Penetration resistance of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) was investigated within the framework of this study. Fiber volume fraction was set as the main test variable and two types of projectiles have been used. Deformable and non-deformable projectile had a full metal jacket with soft lead core and full metal jacket with mild steel core, respectively. Both projectiles were 7.92 mm in diameter and weighed 8.04 grams. Semi-infinite targets made of UHPFRC have been constructed with five different fiber volume fractions with gradual increments ranging from 0.125% to 2%. For the means of comparison, additional plain mixture specimens were casted. Semi-infinite targets have been introduced by using UHPFRC cubes of 200 mm in size. Projectile impact was simulated by using semi-automatic rifle calibre 7.62 × 39 mm at the shooting distance of 20 m. Muzzle velocities for both type of projectiles were around 710 m/s and only front crater with the tunnelling section occurred at the proximal face of the specimen. Depth of penetration, crater diameter and crater volume have been investigated based on various fiber volume fractions and various projectiles. The results from UHPFRC targets were compared with theirs plain mixture counterparts and among different fiber volume fractions. It has been experimentally verified that using steel fibers in the ultra-high performance concrete matrix results in lower damage in terms of depth of penetration, crater area and crater volume, while the latter two are far more affected by the increase of the fiber volume content than the former. Using short steel fibres as disperse micro-reinforcement provides enhanced performance of semi-infinite UHPFRC targets and is therefore essential for construction of protective structures in order to reduce the damage of the impacted elements and to increase the safety of the personnel. [ABSTRACT FROM AUTHOR]

Ultra-high-performance fibre-reinforced concrete (UHPFRC) is an advanced cementitious composite with enhanced mechanical and durability properties that outperforms conventionally used concretes in many ways. Such a material with certain properties and specifications is well suited for energy absorption facade panels and key elements of building structures that may be exposed to impacts or blast loads. It can be stated that the resistance of civil infrastructure is strongly related to the energy absorption capacity of concrete, which is the main material property that benefits from fibre reinforcement. The aim of this study is to investigate the fracture energy of the UHPFRC with various fibre aspect ratios. Different behaviour of UHPFRC in terms of fracture energy can be expected for various aspect ratios of fibres, as the fibres are the key component of the UHPFRC that result in enhanced energy absorption and dissipation capacity. The aspect ratios (length-to-diameter) of fibres used in this study ranged from 50:1 to 108:1. It was verified experimentally that the fracture energy increases as the aspect ratio increases. In addition, it was found that the dependence of the fracture energy on the aspect ratio of the fibres tends to follow a linear trend. The results provided in the present study can serve as valuable information for verifying material models, and also for design purposes. [ABSTRACT FROM AUTHOR]

The main purpose of this study is to analyse the projectile impact resistance on slabs constructed of ultra-high-performance fibre-reinforced concrete (UHPFRC). The fibre volume fraction of the UHPFRC was set as the main test variable in the framework of this study. The fibre volume fraction was ranging from 0.125% to 2% and accordingly the compressive strength of the UHPFRC was ranging from 118 MPa to 144 MPa. The primary importance was given to the shape of the crater that was produced on the UHPFRC slabs as a result of the projectile impact. The resistance of the slab was therefore evaluated on the basis of the crater depth, crater area and crater volume at the front and rear side of the slab. The resistance to impact loading was tested on rectangular slabs with the measurements of 300 mm × 400 mm × 50 mm (length × width × thickness). It was verified experimentally that fibres are important inhibitor of the damage when dealing with projectile impact. The damage of the slab was reduced significantly when the fibre volume fraction was increased. Also it was shown that the best performance was achieved with the highest fibre volume fraction. It is very important to note that using 2% of steel fibres in the thin UHPFRC targets resulted in no scabbing from the back face of the slab with only a front crater forming. This is very important for facade panels and walls that are used for the purpose of protecting people where no perforation and scabbing of the concrete are allowed to occur. [ABSTRACT FROM AUTHOR]

This paper is devoted to the experimental development of a high load bearing energy absorbing system (EAS) made of high performance concrete (HPC). The HPC investigated in this study was a self-consolidating concrete with fast strength development which did not require heat curing or special mixing techniques. It was also an inherently very dense material which lead to the excellent environmental resistance and resulted in the long term durability. The proposed EAS was subjected to the thorough testing and its functionality was verified under impact and quasi-static loading. Numerical simulations were compared with the experimental data and good agreement was obtained. It was demonstrated that it is possible to develop an energy absorbing system made of high performance concrete with controlled impact force and deceleration. This demonstration together with load bearing capability and long term durability, predetermined the developed EAS for wide range of applications including those where currently manufactured energy absorbers cannot be applied. [ABSTRACT FROM AUTHOR]

Significant buildings, transportation hubs, protective or defence structures are at exposure risk to extreme load event such as blast loading or direct armed attack. This kind of loading is typical by its rapid increase in release of energy in a very short time. It is believed that emerging cementitious materials such as Engineered Cementitious Composite (ECC) or Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) provide better passive protection to significant buildings compared to High-Strength Concrete (HSC) or conventional Fibre Reinforced Concrete (FRC). In this study, response of several UHPFRC slabs to projectile impact was examined in order to simulate the effects of possible damage by small fire arms or by fragments generated by an explosion. Real ammunition was used with impact velocity in the range of 691–720 m/s and fibre content of the UHPFRC was selected as the main test variable. The optimal fibre content in the UHPFRC mixture was determined using damage parameters such as crater diameter, penetration depth, debris fragment mass and residual penetration potential of the bullet. It was verified experimentally that the optimal fibre content in the UHPFRC mixture is 2% by volume. Using less than 2% of fibre volume fraction might be unsafe in the slim UHPFRC targets due to increased volume of secondary fragments generated from the back side of the slab and also due to the higher residual penetration potential of the bullet exiting the back side of the slab. Using more that 2% of fibre volume fraction could be inefficient. No improvement in all damage parameters was observed when the fibre volume fraction was changed from 2% to 2.5% or 3%. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper is to describe mix design of Ultra High Performance Fiber Reinforced Concrete (UHPFRC) and its response to deformable and non-deformable projectile impact. UHPFRC represents a class of cementitious composite in which stress–strain response in tension undergoes strain hardening behaviour accompanied by multiple cracking, leading to a high strain prior to failure. The compressive strength of the resulting UHPFRC mixtures exceeded 130 MPa and direct tensile strength was in the range of 10 MPa. Several UHPFRC mixtures with different content of fibers were subjected to deformable projectile impact. It was found that specimens containing 2% of fibers by volume have optimal resistance against deformable projectile impact. Slabs containing 2% of fibers were further subjected to a non-deformable projectile impact. In addition, response of slabs made of traditional fiber reinforced concrete (FRC) is discussed. The magnitude of the damage was assessed based on the penetration depth, crater diameter and loss of mass. [Copyright &y& Elsevier]

Abstract: Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) is currently one of the biggest challenging issues in modern concrete technology. The UHPFRC represents a concrete with improved durability and strength, which is significantly higher than in conventional normal strength concrete. This may result in reduced cross-sections and as a consequence smaller dead-load of the structure. The uniaxial compressive cylinder strength of UHPFRC used in this study exceeded 150MPa and the uniaxial direct tensile strength exceeded 10MPa. Several UHPFRC mixtures with different content of fibers were subjected to deformable projectile impact. The ogive-nose projectile weighed 8.04 grams and its average velocity was determined to be 710 m/s. It was shown that plain UHPC specimen failed in the brittle manner which caused that the slab split into several pieces. Further, it was demonstrated that addition of steel micro fibers enhanced the resistance to deformable projectile impact and it was specified that specimens containing 2% of fibers by volume have optimal resistance against impact loading. In addition, response of slabs made of conventional fiber reinforced concrete (FRC) and normal strength concrete (NSC) was studied for comparison. The magnitude of the damage was assessed based on the penetration depth, crater diameter and mass loss. Experimental results clearly showed that implementation of high strength steel micro fibers significantly increased the resistance to projectile impact. It was stated, that UHPFRC has much better resistance to projectile impact in comparison to conventional FRC. [Copyright &y& Elsevier]

Abstract: This paper describes experimental investigations of various mechanical properties of Ultra High Performance Fiber Reinforced Concrete (UHPFRC). The UHPFRC is a modern cementitious composite containing a large amount of cement, both reactive and nonreactive very fine particles, chemical admixtures and short high strength steel fibers. This type of concrete has superior durability and excellent mechanical properties. In this paper an UHPFRC containing 2% of fibers by volume with a compressive strength exceeding 150MPa is presented. In this study detailed descriptions of the measurements of compressive strength, bending strength, direct tensile strength, fracture energy and modulus of elasticity are given. The study provides a comparison of the above mentioned UHPFRC properties with high performance concrete, fiber reinforced concrete and conventional structural concrete. The results indicate excellent performance of UHPFRC in all categories. The biggest difference to ordinary concretes can be seen in tensile behavior, where its response in tension exhibits strain hardening accompanied with multiple cracking followed by tensile softening. The direct tensile strength of the studied material is 10MPa and its fracture energy is 5 times higher than that of conventional fiber reinforced concrete. It is believed, that such type of material is the future of concrete industry, it will allow executing higher, more effective structures with excellent durability. [Copyright &y& Elsevier]

• The effect of the multi-layer composition on the impact response is examined. • A resistance of partially damaged laminate to successive impacts is investigated. • A properly tuned numerical model predicts the contact force and crack pattern well. Modeling of gradual fracture of individual layers of laminated glass produced by successive low-velocity impacts is presented. This subject is first examined experimentally focusing on fracture sequence of the laminate composed of three or four glass layers bonded by polyvinyl butyral (PVB) interlayers. While both laminate geometries, limited to four test samples, show a significant scatter in impact energies and breakage forces causing fracture as well as differences in fracture sequence with increasing impact height, they consistently support ability of partially damaged samples to withstand contact forces exceeding those leading to fracture of the previous glass layer. Also, the four-glass layer laminates seem to provide the post-fracture response consistent with the concept of a sacrificial-glass-ply design. However, many questions remain concerning stochastic behavior of glass laminates further promoted by the present experimental results. To open the door to probabilistic modeling, the present topic is addressed also computationally adopting the LS-DYNA software. It is illustrated that properly tuned computational model may provide results, which are comparable to experimental observations both in terms of contact forces and the resulting fracture patterns. [ABSTRACT FROM AUTHOR]

We aim at determining the value of minimal magnetic induction for magnetic orientation of fibres in cementitious composite in a given mixture and establish the effect of the magnetic field shape on the final mechanical properties. The magnetic orientation is a suitable method to ensure the required direction of steel ferromagnetic fibres in the composite. The strength of the magnetic field (magnetic induction) and rheology of the fresh mixture play a crucial role in the magnetic orientation. A special device generating alternating and direct magnetic fields with different magnetic inductions was built to determine the minimal magnetic induction for the given mixture. Moreover, the effects of using alternating magnetic field are explored. The used magnetic inductions are 60 mT, 80 mT, 100 mT and 120 mT. The nondestructive inductive measuring method is applied to confirm the orientation of fibres in the hardened composite. The effect of the shape of the magnetic field and magnetic induction on flexural strength is presented. Moreover, the effects of using the alternating or direct magnetic field on the microstructure are explored using a scanning electron microscope. • Magnetic orientation of fibres was examined. • Alternating magnetic field proves to be suitable for fibre orientation. • Scanning electron microscopy (SEM) confirmed that microstructure is not affected. • Magnetic orientation increased flexural strength by up to 50%. • Results of the non-destructive inductive method correlate well with the strength. [ABSTRACT FROM AUTHOR]

The material models for creep and shrinkage operating on the material point level in FEM are usually intended for challenging complex applications and structures, where the average cross-sectional approach does not suffice. The identification of the growing number of material parameters induced by increasing model capabilities relies on very specific and narrow-oriented yet interconnected experiments which are scarce. The presented comprehensive experiments aim to provide a clearer image of the complicated interaction among the basic phenomena: drying, shrinkage, creep, and microcracking. The cornerstone of this ongoing research is a unique set of 30 partially-sealed unreinforced concrete beams with span 1.75–3.0 m subjected to drying. To minimize material variation, all specimens in this study were cast from a single batch of ordinary strength structural concrete with slag-blended binder. The resulting experimental database will be suitable both for validation and development of the constitutive models. • Extensive experimental study on drying shrinkage, creep and microcracking of concrete • All specimens prepared from a single batch of ordinary-strength structural concrete • 1st year of measurements of (not only) non-uniformly drying beams with span up to 3-m • The database is downloadable from free-to-use research data repository • Modified MPS model for concrete creep used in blind prediction of all experiments [ABSTRACT FROM AUTHOR]

• Response of glass plate to a low velocity impact is analyzed. • Three types of simplified numerical models are introduced. • Models are verified against industry standard finite element software. • Models are validated against experimental data of two different glass plates. • Accuracy depends on the plate dimensions and the impactor stiffness. This article concerns numerical simulations of glass plates loaded by a force impulse caused by a low velocity impact. Three types of numerical models of different complexity – the spring-mass system, the modal decomposition based model, and the conventional finite element method – all exploiting the Hertz contact law are compared and their advantages and limitations are discussed. Their performance is verified against the results provided by the commercial software LS-DYNA and validated by two full scale experiments. It is observed that the accuracy of the spring-based model decreases with increasing dimension of the plate and increasing stiffness of the impactor. Nevertheless, the models based on modal decomposition and finite element methods deliver results reasonably close to experimental measurements. [ABSTRACT FROM AUTHOR]

Even though the number of deadly traffic incidents is decreasing, the proportion of motor vehicle crash deaths involving a collision with fixed objects is not. To address this issue, a novel load-bearing energy-absorbing system comprised of brittle blocks with thin-walled cellular structures is presented here, laying the foundation for the assessment of the system's crashworthiness. Six brittle hexagonal cellular structures were subjected to quasi-static and impact tests. The crushing process, force-displacement curves, and a wide range of indicators of crashworthiness were examined. Samples with different levels of crashworthiness were identified and several observations that could guide future crashworthiness design are given. Our findings show that relatively small changes in the design can lead to production of blocks with distinguishable levels of crashworthiness. • The foundation for the assessment of the crashworthiness of brittle blocks is laid. • Quasi-static and impact testing of brittle cellular structures is presented. • The gradual brittle fracture process occurring in a cellular structure is examined. • Several observations that could guide future crashworthiness design are given. • The manufactured blocks with different levels of crashworthiness are identified. [ABSTRACT FROM AUTHOR]