Showing papers in "Materials and Structures in 2009"

Fibre reinforced concrete: new design perspectives

Abstract: Although the use of Fibre Reinforced Concrete (FRC) for structural applications is continuously increasing, it is still limited with respect to its potentials, mainly due to the lack of International Building Codes for FRC structural elements. Within fib (Federation Internationale du Beton), the Special Activity Group 5 is preparing a New fib Model Code that aims to update the previous CEB-FIP Model Code 90, published in 1993, that can be considered as the reference document for Eurocode 2. The New Model Code includes several innovations and addresses among other topics, new materials for structural design. In this respect, FRC will be introduced. The Technical Groups fib TG 8.3 “Fibre reinforced concrete” and fib TG 8.6 “Ultra high performance FRC” are preparing some sections of the New Model Code, including regular and high performance FRC. This paper aims to briefly explain the main concepts behind the structural rules for FRC structural design.

Chemical changes and phase analysis of OPC pastes carbonated at different CO2 concentrations

Abstract: Chemical changes and phase analysis of OPC pastes exposed to accelerated carbonation using different concentrations of CO2 (3%, 10% and 100%) have been undertaken and compared with those of natural carbonation (≅0.03%). 29Si Magic Angle Spinning-Nuclear Magnetic Resonance (29Si M.A.S-N.M.R), Thermogravimetric analyses (TG) and X-Ray Diffraction (XRD) have been used for characterisation. The carbonation of the samples has resulted in a progressive polymerisation of CSH that leads to formation of a Ca-modified silica gel and calcium carbonate. The carbonation of CSH and portlandite occurs simultaneously and the polymerisation of the CSH after carbonation increases with the increase in concentration of CO2. When ≅0.03% and 3% CO2 are used, CSH gel with a lower Ca/Si than that of the uncarbonated sample, and quite similar for both samples remained. When carbonating at 10% and 100% of CO2, the CSH gel completely disappears. For every condition, a polymerised Ca-modified silica gel is formed, as a result of the decalcification of the CSH. From these results it can be deduced that among the different concentrations of CO2 tested, carbonation up to a 3% of CO2, (that is to say, by a factor of 100) results in a microstructure much more similar to those corresponding to natural carbonation at ≅0.03% CO2 than those at the 10% and 100% concentrations.

Structural concrete with incorporation of coarse recycled concrete and ceramic aggregates: durability performance

Abstract: The growing difficulty in obtaining natural coarse aggregates (NCA) for the production of concrete, associated to the environmental issues and social costs that the uncontrolled extraction of natural aggregates creates, led to a search for feasible alternatives. One of the possible paths is to reuse construction and demolition waste (CDW) as aggregates to incorporate into the production of new concrete. Therefore, a vast and detailed experimental campaign was implemented at Instituto Superior Tecnico (IST), which aimed at determining the viability of incorporating coarse aggregates from concrete and ceramic brick wall debris, in the production of a new concrete, with properties acceptable for its use in new reinforced and pre-stressed structures. In the experimental campaign different compositions were studied by incorporating pre-determined percentages of recycled coarse concrete aggregates and recycled coarse ceramic plus mortar particles, and the main mechanical, deformability and durability properties were quantified, by comparison with a conventional reference concrete (RC). In this article, these results are presented in terms of the durability performance of concrete, namely water absorption, carbonation and chlorides penetration resistance.

Application of ECC for bridge deck link slabs

Abstract: In this article, the application of ECC in a bridge deck link slab is described. The unique ultra high tensile ductility and tight crack width of self-consolidating ECC is exploited in this application to improve bridge deck constructability, durability, and sustainability. Design guidelines and material specifications were developed for implementation of this ECC link slab technology. A construction project implementing these guidelines and specifications was conducted in 2005 on an ECC-concrete bridge deck in southeast Michigan, USA. This article summarizes the experience of this project.

Dynamic modulus simulation of the asphalt concrete using the X-ray computed tomography images

Abstract: The objective of this study is to predict the dynamic modulus of asphalt mixture using both two-dimensional (2D) and three-dimensional (3D) Distinct Element Method (DEM) generated from the X-ray computed tomography (X-ray CT) images. The 3D internal microstructure of the asphalt mixtures (i.e., spatial distribution of aggregate, sand mastic and air voids) was obtained using the X-ray CT. The X-ray CT images provided exact locations of aggregate, sand mastic and air voids to develop 2D and 3D models. An experimental program was developed with a uniaxial compression test to measure the dynamic modulus of sand mastic and asphalt mixtures at different temperatures and loading frequencies. In the DEM simulation, the mastic dynamic modulus and aggregate elastic modulus were used as input parameters to predict the asphalt mixture dynamic modulus. Three replicates of a 3D DEM and six replicates of a 2D DEM were used in the simulation. The strain response of the asphalt concrete under a compressive load was monitored, and the dynamic modulus was computed. The moduli of the 3D DEM and 2D DEM were then compared with both the experimental measurements results. It was revealed that the 3D discrete element models successfully predicted the asphalt mixture dynamic modulus over a range of temperatures and loading frequencies. It was found that 2D discrete element models under predicted the asphalt mixture dynamic modulus.

Use of ultrafine rice husk ash with high-carbon content as pozzolan in high performance concrete

Abstract: Rice husk ash (RHA) has been generated in large quantities in rice producing countries. This by-product can contain non-crystalline silica and thus has a high potential to be used as cement replacement in mortar and concrete. However, as the RHA produced by uncontrolled burning conditions usually contains high-carbon content in its composition, the pozzolanic activity of the ash and the rheology of mortar or concrete can be adversely affected. In this paper the influence of different grinding times in a vibratory mill, operating in dry open-circuit, on the particle size distribution, BET specific surface area and pozzolanic activity of the RHA is studied, in order to improve RHA’s performance. In addition, four high-performance concretes were produced with 0%, 10%, 15%, and 20% of the cement (by mass) replaced by ultrafine RHA. For these mixtures, rheological, mechanical and durability tests were performed. For all levels of cement replacement, especially for the 20%, the ultra-fine RHA concretes achieved superior performance in the mechanical and durability tests compared with the reference mixture. The workability of the concrete, however, was reduced with the increase of cement replacement by RHA.

Compression behaviour of non-industrial materials in civil engineering by three scale experiments: the case of rammed earth

Abstract: In order to give an example of a scientific approach adapted to non-industrial materials, we chose to study a structural element: a load-bearing building wall made of rammed earth material. Rammed earth construction is an ancient technique which is attracting renewed interest throughout the world today. Although rammed earth is currently regarded as a promising material in the construction sector in the context of sustainable development, it is still difficult to quantify its durability, as well as its thermal and mechanical performances, which discourages people from using it. This paper is devoted to the study of the last problem. Three different scales were studied. The first is the scale of in-situ walls. Dynamic measurements were carried out on site to determine the Eigen frequencies of the walls. The elastic modulus was determined from the frequencies measured by using a finite element model. The second is the scale of a representative volume element (RVE). Rammed earth RVE samples with dimensions similar to those of the walls on site were manufactured and tested in the laboratory. Finally, at the last scale, called the micro-mechanical scale, tests were performed on equivalent compressed earth blocks (CEBs), which can replace the rammed earth RVE samples to facilitate laboratory tests.

High performance fiber reinforced concrete: Progress in knowledge and design codes

Abstract: High performance fiber reinforced concrete is developing quickly to a modern structural material with a high potential. As for instance testified by the recent symposium on HPFRC in Kassel, Germany (April 2008) the number of structural applications increases. At this moment studies are carried out with the aim to come to an international recommendation for the design of structures with HPFRC. Research projects are being carried out in order to supply missing information in relevant areas. Some examples of recent research at TU Delft are given. For the preparation of an internationally acceptable design recommendation for HPFRC a number of principles should be respected. The code should as much as possible be in harmony with the code for conventional fiber concrete. Moreover it should be consistent with existing design recommendations for structural concrete. Second thoughts on the introduction of such a new code are given.

Micromechanical fracture modeling of asphalt concrete using a single-edge notched beam test

Abstract: Cracks in asphalt pavements create irreversible structural and functional deficiencies that increase maintenance costs and decrease lifespan. Therefore, it is important to understand the fracture behavior of asphalt mixtures, which consist of irregularly shaped and randomly oriented aggregate particles and mastic. A two-dimensional clustered discrete element modeling (DEM) approach is implemented to simulate the complex crack behavior observed during asphalt concrete fracture tests. A cohesive softening model (CSM) is adapted as an intrinsic constitutive law governing material separation in asphalt concrete. A homogenous model is employed to investigate the mode I fracture behavior of asphalt concrete using a single-edge notched beam (SE(B)) test. Heterogeneous morphological features are added to numerical SE(B) specimens to investigate complex fracture mechanisms in the process zone. Energy decomposition analyses are performed to gain insight towards the forms of energy dissipation present in fracture testing of asphalt concrete. Finally, a heterogeneous model is used to simulate mixed-mode crack propagation.

Shear resistance of masonry walls and Eurocode 6: shear versus tensile strength of masonry

Abstract: In the case of masonry structures subjected to seismic loads, shear failure mechanism of walls, characterised by the formation of diagonal cracks, by far predominates the sliding shear failure mechanism. However, as assumed by Eurocode 6, the latter represents the critical mechanism for the assessment of the shear resistance of structural walls. The results of a series of laboratory tests are analysed to show that in the case of the diagonal tension shear failure the results of the Eurocode 6 based calculations are not in agreement with the actual resistance of masonry walls. The results of calculations, where the diagonal tension shear mechanism and tensile strength of masonry are considered as the critical parameters, are more realistic. Since the results of seismic resistance verification, based on the Eurocode 6 assumed sliding shear mechanism, are not in favour of structural safety, it is proposed that in addition to sliding shear, the diagonal tension shear mechanism be also considered. Besides, in order to avoid misleading distribution of seismic actions on the resisting shear walls, the deformability characteristics of masonry at shear should be determined on the basis of experiments and not by taking into account the Eurocode 6 recommended G/E ratio.

Applications and recommendations of high performance fiber reinforced cement composites with multiple fine cracking (HPFRCC) in Japan

Abstract: High performance fiber reinforced cement composites with multiple fine cracking (HPFRCC) show remarkably high ductility under uniaxial tensile stress and excellent performance distinguished from conventional cementitious materials by multiple cracking and strain hardening behaviors. Characteristics of HPFRCC include crack width controlling capability keeping crack width in a permissible range. HPFRCC has been applied to bridge decks and building dampers, making use of its excellent mechanical performance. It has also been used for surface repair of concrete dams, water channels, and retaining walls making use of its finely distributed cracking behavior. Appropriate use of the tensile performance can work out a structural component excellent in both durability and mechanical performance. The Japan Society of Civil Engineers (JSCE) has published the first design recommendations for HPFRCC. Examples of HPFRCC applications in Japan and the outlines of the JSCE recommendations for HPFRCC are introduced in this article.

Rate-dependent tensile behavior of high performance fiber reinforced cementitious composites

Abstract: High Performance Fiber Reinforced Cementitious Composites (HPFRCC) show strain hardening behavior accompanied with multiple micro-cracks under static tension. The high ductility and load carrying capacity resulting from their strain hardening behavior is expected to increase the resisting capacity of structures subjected to extreme loading situations, e.g., earthquake, impact or blast. However, the promise of HPFRCCs for dynamic loading applications stems from their observed good response under static loading. In fact, very little research has been conducted to investigate if their good static response translates into improved dynamic response and damage tolerance. This experimental study investigates the tensile behavior of HPFRCC using High strength steel fibers (High strength hooked fiber and twisted fiber) under various strain rates ranging from static to seismic rates. The test results indicate that the tensile behavior of HPFRCC using twisted fiber shows rate sensitivity while that using hooked fiber shows no rate sensitivity. The results also show that rate sensitivity in twisted fibers is dependent upon both fiber volume fraction and matrix strength, which influences the interface bond properties.

Influence of microcracking on water absorption and sorptivity of ECC

Abstract: This paper presents the results of an experimental investigation on the water absorption and sorptivity properties of mechanically loaded Engineered Cementitious Composites (ECC). ECC is a newly developed high performance fiber reinforced cementitious composite with substantial benefit in both high ductility and improved durability due to tight crack width. By employing micromechanics-based material design, ductility in excess of 3% under uniaxial tensile loading can be attained with only 2% fiber content by volume, and the typical single crack brittle fracture behavior commonly observed in normal concrete or mortar is converted to multiple microcracking ductile response in ECC. In this study, water absorption (ASTM C642) and sorptivity tests (ASTM C1585) were conducted to determine absorption capacity and sorptivity of microcracked ECC. The experimental program described in this paper indicated that microcracks induced by mechanical loading increases the sorptivity value of ECC without water repellent admixture. However, the use of water soluble silicone based water repellent admixture in the production of ECC could easily inhibit the sorptivity even for the mechanically loaded ECC specimens. Moreover, the incorporation of the water repellent admixture reduced the absorption capacity of the resulting ECC mixture. Based on this study, the risk of water transport by capillary suction in ECC, cracked or uncracked, is found to be low compared with that in normal sound concrete. The incorporation of water repellent admixture further lowers this risk.

Minimum cement content requirements: a must or a myth?

Abstract: The objective of the present work was to systematically investigate the issue of minimum cement content requirements, by studying the behavior of concretes with different water to cement ratios ( w/c) in the range of 0.45–0.70, in which the cement content was varied, by controlling the water content, using water reducing admixtures. The effect of cement content was noted to be different for various properties: strength was a function of w/c and independent of cement content; total water absorption was proportional to the paste content at a given w/c, while capillary absorption and chloride ingress reduced with a reduction in the cement content for a given w/c, to an extent which was much greater than the reduction in total porosity. Carbonation and shrinkage were largely independent of cement content for a given w/c. The trends observed were discussed in terms of the effect of paste content on concrete properties, and the influence of admixtures on the paste properties. These results suggest that requirements for minimum cement content in standards should be revisited.

Shrinkage and restrained shrinkage cracking of self-compacting concrete compared to conventionally vibrated concrete

Abstract: Self-compacting concrete (SCC) used in Switzerland contains about 80 l/m3 more volume of paste than conventionally vibrated concrete (CVC). Consequently, there are some systematic differences in the properties of the hardened concrete. Normally, shrinkage of SCC is higher than shrinkage of CVC. Therefore, risk of cracking in case of restrained deformations can be increased for SCC. In this study shrinkage of thirteen different SCC mixtures using volume of paste, water content, type of binder, grain size distribution or content of shrinkage reducing admixture (SRA) as variables was compared with shrinkage of three different CVC mixtures with constant volume of paste but variable w/b. Furthermore, the risk of cracking of the different SCC- and CVC-mixtures in restrained conditions was studied under constant and varying curing conditions. The results show that shrinkage is mainly depending on volume of paste. Due to the higher volume of paste, SCC displayed higher shrinkage than CVC. Adding an SRA was the only measure to reduce shrinkage of SCC to values of CVC. Restrained shrinkage cracking is depending on shrinkage rate, mechanical properties and drying velocity. For slow shrinkage stress development, cracking risk of SCC can be lower compared to CVC despite the higher shrinkage rate.

Double Punch Test to control the energy dissipation in tension of FRC (Barcelona test)

Abstract: Current testing methods used to measure tensile properties of Fiber Reinforced Concrete (FRC) are mainly based on bending test of beam specimens They normally show a considerable scatter that makes difficult the quality control, as in particular when such properties are intended to estimate the strength of structural members In order to improve the material assessment procedure, the Double Punch Test (DPT) has been recovered for the quality control of the tension behaviour of FRC Former experimental research showed the feasibility of the test and a reduction of the scatter in the values of the tensile strength and of the toughness This paper describes the results of an experimental program carried out using both DPT and bending test on FRC with different type of fibers, concretes and fiber contents In addition, a correlation between both tests is proposed Its application to steel and polyolefin FRC specimens shows very good results

Analysis of geopolymer concrete columns

Abstract: Ordinary portland cement (OPC) has been traditionally used as the binding agent in concrete. However, it is also necessary to search for alternative low-emission binding agents for concrete to reduce the environmental impact caused by manufacturing of cement. Geopolymer, also known as inorganic polymer, is one such material that uses by-product material such as fly ash instead of cement. Recent research has shown that fly ash-based geopolymer concrete has suitable properties for its use as a construction material. Since the strength development mechanism of geopolymer is different from that of OPC binder, it is necessary to obtain a suitable constitutive model for geopolymer concrete to predict the load–deflection behaviour and strength of geopolymer concrete structural members. This article has investigated the suitability of using an existing stress–strain model originally proposed by Popovics for OPC concrete. It is found that the equation of Popovics can be used for geopolymer concrete with minor modification to the expression for the curve fitting factor, to better fit with the post-peak parts of the experimental stress–strain curves. The slightly modified set of stress–strain equations was then used in a non-linear analysis for reinforced concrete columns. A good correlation is achieved between the predicted and measured ultimate loads, load–deflection curves and deflected shapes for 12 slender test columns.

The effects of supplementary cementing materials in modifying the heat of hydration of concrete

Abstract: This paper is intended to provide guidance on the form and extent to which supplementary cementing materials, in combination with Portland cement, modifies the rate of heat evolution during the early stages of hydration in concrete. In this investigation, concretes were prepared with fly ash, condensed silica fume and ground granulated blastfurnace slag, blended with Portland cement in proportions ranging from 5% to 80%. These concretes were subjected to heat of hydration tests under adiabatic conditions and the results were used to assess and quantify the effects of the supplementary cementing materials in altering the heat rate profiles of concrete. The paper also proposes a simplified mathematical form of the heat rate curve for blended cement binders in concrete to allow a design stage assessment of the likely early-age time–temperature profiles in large concrete structures. Such an assessment would be essential in the case of concrete structures where the potential for thermally induced cracking is of concern.

Alkali-silica reaction in concrete containing glass

Abstract: The use of surplus waste glass in concrete has been avoided on the grounds that it is known to undergo harmful alkali-silica reaction (ASR). As part of a research project to develop draft specifications for glass in concrete, a major ASR testing programme was undertaken to establish appropriate use of glass in concrete which avoided harmful ASR. The British Standard for assessing ASR reactivity of aggregate—BS 812-123—was used. Testing was conducted on concrete mixes containing glass as either fine aggregate, filler aggregate, or as a Type II addition. Glass used as fine aggregate was found to produce significant expansion for both green and amber glass. GGBS and metakaolin had the effect of reducing this expansion considerably. Concrete containing powdered glass displayed much less expansion. A simple schematic model for the alkali-silica reaction of glass, based on glass dissolution mechanisms is proposed, and related to the test results.

Hydration monitoring of cement-based materials with resistivity and ultrasonic methods

Abstract: Two test setups, the electrical resistivity and ultrasonic techniques, were used to monitor the hydration process of cement-based materials. In the electrical resistivity method, a non-contacting device was used. In the ultrasonic method, a wave was transmitted and measured by the embedded piezoelectric ultrasonic transducers, which had good coupling with the surrounding materials. The focus of the study was to detect the setting and hardening behaviors of cement paste during the first 7 days of hydration using the above techniques. Immediate after placing the cement paste into the mould, the measurement started and continued throughout the hydration process. The obtained resistivity and ultrasonic data were used to interpret the hydration process of the specimens. The correlation of two techniques was also studied. The results illustrated that both electrical resistivity and ultrasonic techniques were effective to accurately monitor the hydration of cement pastes. The resistivity method was able to study both the chemical reaction and physical change during hydration, while ultrasonic method was sensitive to physical change of cement only.

Geometrical and mechanical aspects of fabric bonding and pullout in cement composites

Abstract: Fabric reinforced cement composites are a new class of cementitious materials with enhanced tensile strength and ductility. The reinforcing mechanisms of 2-D fabric structures in cement matrix are studied using a fabric pullout model based on nonlinear finite difference method. Three main aspects of the composite are evaluated: nonlinear bond slip characteristic at interface; slack in longitudinal warp yarns, and mechanical anchorage provided by cross yarn junctions. Parametric studies of these key parameters indicate that an increase in the interfacial bond strength directly increases the pullout strength. Grid structures offering mechanical anchorage at cross yarn junctions can substantially increase the pullout resistance. Presence of slack in the yarn geometry causes an apparently weaker and more compliant pullout response. The model was calibrated using a variety of test data on the experimental pullout response of AR-Glass specimens, manufactured by different techniques to investigate the relative force contribution from bond at interface and from cross yarn junctions of alkaline resistant glass fabric reinforced cement composites.

Cracking in cement paste induced by autogenous shrinkage

Abstract: Detection and quantification of microcracks caused by restrained autogenous shrinkage in high-performance concrete is a difficult task. Available techniques either lack the required resolution or may produce additional cracks that are indistinguishable from the original ones. A recently developed technique allows identification of microcracks while avoiding artefacts induced by unwanted restraint, drying, or temperature variations during sample preparation. Small cylindrical samples of cement paste are cast with steel rods of different diameters in their centre. The rods restrain the autogenous shrinkage of the paste and may cause crack formation. The crack pattern is identified by impregnation with gallium and analyzed by optical and scanning electron microscopy. In this study, a non-linear numerical analysis of the samples was performed. Autogenous strain, elastic modulus, fracture energy, and creep as a function of hydration time were used as inputs in the analysis. The experimental results and the numerical analysis showed that samples with larger steel rods had the highest probability of developing microcracks. In addition, the pattern and the width of the observed microcracks showed good agreement with the simulation results.

Tensile behaviour of FRC under high strain-rate

Abstract: This paper presents experimental results on two types of concrete reinforced with steel and polyvinyl-alcohol (PVA) fibres subjected to dynamic tensile loading. The tests were carried out by using a Modified Hopkinson Bar apparatus on fibre reinforced concrete notched-specimens under three different strain-rates (50, 100, and 200 s−1). From the experiments it was found that there is a significant enhancement in tensile strength with increasing strain-rates. The dynamic tests on steel FRC with the smaller loading rate (50 s−1) showed a strength similar to the one measured from static tests; however, for increasing loading rates, a remarkable decrease of post-peak strength and ductility occurs. In specimens with PVA fibres, an enhancement of the tensile strength was also observed and a significant reduction of fracture energy and ultimate deformation occurred. Some experimental aspects are also discussed as the specimen shape, its dimension, the loading rate as well as the different post-peak behaviour from static and dynamic tests.

Durability of self-consolidating concrete to different exposure regimes of sodium sulfate attack

Abstract: This study investigates the durability of a wide scope of self-consolidating concrete (SCC) mixture designs to sodium sulfate attack. The mixture design variables included the type of binder (single, binary, ternary and quaternary), air-entrainment, sand-to-aggregate ratio and hybrid fibre reinforcement. Since current standard test methods (e.g. ASTM C 1012) generally do not address various sulfate attack exposure scenarios that may exist under field conditions, three different sulfate attack exposure regimes (full immersion, wetting-drying and partial immersion) were investigated in the current study. In the wetting-drying and partial immersion exposure regimes, results of the physico-mechanical properties revealed performance risks associated with some SCC mixture designs. Such risks were not captured by the full immersion exposure. Thermal, mineralogical and microscopy studies elucidated the complexity of degradation mechanisms, which in some cases varied at different locations of the same specimen. Findings from this study emphasize the need for performance standard tests that can better simulate various realistic field exposure regimes in order to achieve a more reliable and comprehensive evaluation of the resistance of concrete to sulfate attack.

Serviceability Limit State criteria based on steel–concrete bond loss for corroded reinforced concrete in chloride environment

Abstract: This paper will focus on the study of reinforced concrete beams stored in a chloride environment for a period of 14–23 years under service loading. According to the experimental results, a Serviceability Limit State (SLS) criteria is proposed based on an excessive steel–concrete bond reduction. Corrosion of reinforcement in chloride environment leads to a specific local steel cross-section loss as well as a steel–concrete bond loss. Experimental results have shown that, in the first stage of corrosion propagation period, the deflection is more sensitive to chloride-induced corrosion than the ultimate capacity due to the effect of the tension steel–concrete bond loss even if both are correlated. Given this high sensibility of the bending stiffness to corrosion pitting attacks, it appears that a Serviceability Limit State (SLS) criteria based on excessive deflection of structural members is an adequate factor for SLS assessment. Later in corrosion propagation period, when the bond is already significantly reduced, only the ultimate capacity is affected by the steel cross-section loss. This does not affect the serviceability, because pitting attacks are very localised with an insignificant influence on the global deflection. Then, once the steel–concrete bond is lost in critical parts of the beams (high bending moment areas), pitting corrosion propagation does not affect anymore serviceability (stiffness reduction, bending or corrosion cracks patterns) but still leads to an ultimate capacity reduction, which is not acceptable. As a result, excessive steel–concrete debonding can be considered as the SLS criteria.

Structural cast-in-place SFRC: technology, control criteria and recent applications in spain

Abstract: Steel fiber-reinforced concrete (SFRC) has been traditionally applied to pavements or tunnelling. However, classical structural applications have not been developed so fully. This paper examines different reasons which may justify the lack of this application. Several technologies on the fiber dosing system are presented and assessed. The SFRC reception control and acceptance criteria have been developed in accordance with the upcoming Spanish code. Adopting such criteria is justified on previous experiences and test viability under cast-in-place conditions. Some lines of action to promote the use of fibers with structural purposes are proposed. Two examples of recent applications are also set out, and a justification of the convenience of SFRC in terms of structural evaluation and durability under seawater conditions is provided.

Wet packing of crushed rock fine aggregate

Abstract: It is well known that the performance of concrete is dependent on the packing density of the aggregate. All existing methods of measuring the packing density of aggregate are carried out under dry condition. However, these dry packing methods are sensitive to the amount of compaction applied and do not account for the effect of water in the concrete mix. In this research, a new method, which measures the packing density of aggregate under wet condition, has been developed. It is called the wet packing method and has the advantages that it is less sensitive to the amount of compaction applied and that it includes the effect of water. The wet packing method was compared to the dry packing method by applying both methods to measure the packing density of crushed rock fine aggregate. It was found that the packing density of crushed rock fine aggregate can be 24% higher under wet condition than under dry condition and that the addition of superplasticizer can have a marginal contributing effect. Furthermore, it was demonstrated that the beneficial effect of blending different size aggregates together is better revealed by the wet packing method than by the dry packing method.

Evaluation of crack width in FRC structures and application to tunnel linings

Abstract: When steel bars are placed in a concrete structure, the evaluation of crack width and crack spacing is generally required in the serviceability stage. According to more or less aggressive conditions, crack width shall be limited in order to avoid, for instance, the corrosion of steel reinforcement. The presence of fibers in the concrete cast may help to achieve this goal, since fibers remarkably increase the bridging actions across a crack. However, new mechanical models are needed to evaluate these effects, which are generally neglected by classical approaches. Code requirements are based on semi-empirical formulae, in which the average structural performances are analyzed by referring to a single cross-section, instead of a wide portion of an R/FRC or RC element in bending. To evaluate crack patterns more accurately, a suitable block model is therefore introduced in this paper. With the new approach, the bridging effects of fibers, as well as the bond-slip mechanism between steel bars and FRC in tension, are taken into account. By means of such model, it is possible ble to predict at one time the values of crack width, crack spacing, and crack depth, and compare them to data obtained by bending tests on concrete beams. Moreover, to evaluate the possible crack patterns in R/FRC tunnel linings, the proposed block model has been extended to the serviceability stage of massive structures subjected to combined compressive and bending actions. This paper follows a previous work by the same authors (Chiaia et al. Mater Struct 40(6):593–694, 2007) and completes the design procedures for FRC cast-in-place tunnel linings.

Steel-concrete bond strength of lightweight self-consolidating concrete

Abstract: The bond behavior of lightweight self-consolidating concrete (LWSCC) must be understood in order to use this type of high performance concrete in structural members. The objective of this research program is to assess the bond behavior of reinforcing steel bars embedded in LWSCC members. Three different classes of LWSCC mixtures were developed with two different types of lightweight aggregates. In addition, one normal weight SCC (NWSCC) was developed and used as a control mixture. A total of twenty four pullout tests were conducted on deformed reinforcing bars with an embedded length of either 100 or 200 mm and the load-slip responses, failure modes and bond strengths of LWSCC and NWSCC were compared. Based on the results of this study, the bond strength of deformed bars for LWSCCs are found to be less (between 16 and 38%) as compared with NWSCC. Under the conditions of equivalent workability properties and compressive strength, bond slip properties were shown to be significantly influenced by the type of lightweight aggregate used. In this study, the use of expanded shale in the production of LWSCC significantly enhanced the pullout strength when compared with lightweight slag aggregate.

Formation and properties of C-S-H–PEG nano-structures

Abstract: Results of an investigation of the intercalation potential of polyethylene glycol (PEG) with synthetic and pre-treated C-S-H are reported. The partial intercalation of PEG molecules in the interlayer of C-S-H is discussed. The effective and strong interaction of PEG molecules with the C-S-H surface was shown using XRD, 13C CP and 29Si MAS NMR, and DTGA. The position and character of the 002 low angle XRD peak of C-S-H are affected by drying procedures and concomitant chemical treatment preceding intercalation and the reaction temperature. Recovery of the initial 002 position after severe drying and intercalation with distilled water or PEG is incomplete but is accompanied by an increase in intensity. It is inferred that the stability of C-S-H binders in concrete can be impacted by a variation in nanostructure dependent on curing temperature and use of chemical admixtures.