Numeral modeling of fracture failure of recycled aggregate concrete beams under high loading rates

Fracture tests of recycled aggregate concrete (RAC) beams of different sizes were conducted under high loading rates. In order to characterize the effect of high loading rate on the behavior of RAC beams, two new material models were used together with the commercial finite element software \(\hbox {ABAQUS}^{\mathrm{R}}\). One model is a viscoelastic model that can predict the increase of stiffness (modulus of elasticity) of RAC with increasing loading rate, and the other model is a multiphase composite model that can determine the effective stiffness of RAC taking into account the special internal structure of recycled aggregate. Two different cases were considered in the numerical simulation. Case 1 is for fixed beam size under different loading rates, and Case 2 is for fixed loading rate with different beam sizes. For Case 1, the simulation results of the maximum loads under three different strain rates agreed with test data quite well. The Force-CMOD curves of the numerical simulation and test data showed similar trends. The higher the strain rates, the wider the high stresses spread in the crack propagation zone. The good agreements with the test data indicated that the two new material models can characterize the effect of high loading rate on RAC beams very well. For Case 2, three beam sizes and one loading rate was studied. The post-peak Force versus CMOD curves from the simulation follow the same trend of the test data. The stress distributions in the beams of different sizes are similar. On the other hand, the maximum loads predicted by the numerical model did not agree very well with test data. This is due to the fact that the maximum forces of RAC notched beams exhibited size effect, which was not considered in the fracture criteria adopted in \(\hbox {ABAQUS}^{\mathrm{R}}\) and not in the two new material models. This will be a topic for future research.     

Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete

The use of recycled aggregate from construction and demolition waste (CDW) as replacement of fine and coarse natural aggregate has increased in recent years in order to reduce the high consumption of natural resources by the civil construction sector. In this work, an experimental investigation was carried out to investigate the influence of steel fiber reinforcement on the stress–strain behavior of concrete made with CDW aggregates. In addition, the flexural strength and splitting tensile strength of the mixtures were also determined. Natural coarse and fine aggregates were replaced by recycled coarse aggregate (RCA) and recycled fine aggregate (RFA) at two levels, 0% and 25%, by volume. Hooked end steel fibers with 35 mm of length and aspect ratio of 65 were used as reinforcement in a volume fraction of 0.75%. The research results show that the addition of steel fiber and recycled aggregate increased the mechanical strength and modified the fracture process relative to that of the reference concrete. The stress–strain behavior of recycled aggregate concrete was affected by the recycled aggregate and presented a more brittle behavior than the reference one. With the addition of steel fiber the toughness, measured by the slope of the descending branch of the stress–strain curve, of the recycled concretes was increased and their behavior under compression becomes similar to that of the fiber-reinforced natural aggregate concrete.     

The effect of meta-halloysite on alkali–aggregate reaction in concrete

Damage to concrete structures may occur as a result of internal effects. Alkali silica reaction (ASR) is a long term reaction between alkalis and reactive aggregate present in the concrete. The reaction product is sodium–potasium–calcium silica gel, able to absorb water, resulting in the expansion and cracking of concrete. The key problem is to find the right method for mitigating the internal damage. This paper presents the results of an investigation into the effectiveness of calcined halloysite (meta-halloysite) in improving the resistance to alkali-silica reaction (ASR). The pozzolanic reactivity of meta-halloysite was also evaluated using Thermo-Gravimetric Analysis. Microstructures of mortar bars were observed by Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (EDS) to investigate the location and chemical composition of ASR gel. The results from this study showed satisfactory level of pozzolanic reactivity when cement was partially replaced by meta-halloysite. It was demonstrated that a 20% addition of meta-halloysite are able to mitigate ASR and lower expansion of mortar bars with reactive aggregate to a safe level of not more than 0.1% at 14 days. Microstructural observations of the specimens containing meta-halloysite indicated the presence of a calcium–alkali–silicate–hydrate gel. But fewer reaction products and with different composition than those forming in the pastes without mineral additives are present.     

Steel–concrete bond strength of lightweight self-consolidating concrete

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.     

The effect of shale in quartzite aggregate on the creep and shrinkage of concrete—A comparison with RILEM model B3

This paper presents the results of an investigation into the effects of increasing shale content in nominally “quartzite” aggregates on the creep and shrinkage behaviour of concretes made with such aggregate. The investigation was prompted by concern because of the increasing presence of the weaker and poorly shaped shale particles appearing in the quartzite aggregates being used in the Gauteng region of South Africa. In this investigation, two strength grades of concrete were prepared using varying proportions of shale in the stone and sand fractions of the quartzite aggregates. A secondary outcome of this investigation was the opportunity to assess the effect of aggregate type on the prediction of concrete creep and shrinkage using Model B3. The results indicate that increasing shale content in concrete has a significant effect in increasing both creep and shrinkage strains. Shrinkage increases almost linearly with increasing shale content of the total aggregate. On the other hand, shale in the sand fraction appears to have a larger effect in increasing creep than shale in the stone fraction. An assessment of the need for an aggregate-based adjustment to Model B3 is also presented.

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Résumé Cet article présente les résultats d’une recherche relative aux effects que peut avoir l’augmentation de la teneur en schiste dans des granulats nominalement “quartzites” sur le comportement des bétons faits avec de tels granulats. La présente recherche est née du souci de comprendre la teneur croissante des particules de schiste plus faibles et mal formées apparaissant dans les granulats quartzites utilisés dans la région de Gauteng en Afrique du sud. Au cours de cette recherche, deux gammes de résistance de béton ont été préparées en utilisant des proportions variables de schiste dans les fractions de pierre et de sable des granulats quartzites. La recherche a aussi été l’occasion d’évaluer les effets du type granulat sur la prévision du fluage et du retrait du béton en utilisant le modèle B3. Les résultats indiquent qu’une augmentation de la teneur de schiste dans du béton a un effet significatif sur l’augmentation des contraintes du fluage et du retrait. Le retrait augmente presque linéairement avec l’augmentation de la teneur en schiste du granulat total. D’autre part, le schiste dans la fraction de sable semble avoir un plus grand effet sur l’augmentation du fluage que le schiste dans la fraction en pierre. Une évaluation du besoin d’un réglage basé sur le granulat par rapport au modèle B3 est également présentée.

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Editorial Note Dr. Yunus Ballim is a RILEM Senior Member.     

“Collapsible” lightweight aggregate concrete. Part II: characterization under static and dynamic loadings

In this work a granular cementitious composite has been developed, tailoring its performance to a low compressive strength and high deformation and energy dissipation capacity, which can be required to the material when employed in post-installed screeds for protection of structures and infrastructures against accidental actions such as impact and blast. The required level of performance can be achieved by uniform grain size distribution, paste content as low as minimum theoretical void ratio and low paste strength: it is believed that the synergy between the aforementioned three requirements can allow for energy dissipation capacity after paste cracking due to both rearrangement of grain meso-structure and, in case, grain crushing. After the mix design concept and optimization of the material composition, illustrated in the first part of this companion paper study, the mechanical performance of the composite under static and impact compressive loadings has been thoroughly characterized, as affected by mix-design variables, such as paste volume fraction, water to cement ratio and aggregate size. The reliability will thus be thoroughly checked, of the employed material concept, and the influence will also be investigated, if any, of specimen shape, size and boundary conditions.     

“Collapsible” lightweight aggregate concrete. Part I: material concept and preliminary characterization under static loadings

In this work a granular cementitious composite has been developed, tailoring its performance to low compressive strength as well as to high deformation and energy dissipation capacity. This peculiar performance can be required to the material when employed in post-installed screeds for protection of structures and infrastructures against accidental actions such as impact and blast. The required level of performance can be achieved through uniform grain size distribution, paste content as low as minimum theoretical void ratio and low paste strength. It is believed that the synergy between the aforementioned three requirements can allow for energy dissipation capacity after paste cracking due to both rearrangement of grain meso-structure and, in case, grain crushing. This part I of a companion paper study first of all details the optimization of the material composition, in terms of mix-design variables such as w/c ratio, content of air entraining agent, mixing protocol, paste volume fraction, grain size distribution of the employed lightweight expanded clay aggregate. The mechanical performance of a trial collapsible concrete mix will be then checked. In part II extensive mechanical characterization under static and impact loadings will be performed as pertinent to the intended aforementioned application.     

Round-Robin analysis of over-reinforced concrete beams—Comparison of results

Following the Round-Robin test on compressive softening, RILEM TC 148-SSC (“Strain Softening of Concrete”, see [1]) has proposed a second Round-Robin on the application of strain-softening data from simple uniaxial compression tests for the analysis of over-reinforced beams. The beams were tested at Aalborg University in Denmark in 1996. The Round-Robin is carried out in collaboration with ACI-ASCE committee 447 (“Finite Element Analysis of Reinforced Concrete Structures”). The results of the beam tests have been kept confidential, and researchers were invited to analyze the beams given the compressive and tensile properties of the concrete and the reinforcement. In total six contributions were submitted, which were compared to the experimental results in a workshop at the 3rd International Conference on Fracture Mechanics of Concrete and Concrete Structures, in Gifu, Japan in October 1998. In this report the outcome of the different analyses are summarized and the results are compared to the experimentally obtained peak loads, ductility and size effect.     

Predicting the service-life of concrete structures – Limitations of simplified models

North American civil infrastructure systems are deteriorating. Roads, bridges, overpasses, marine and airport facilities are all impacted. The primary causes of premature concrete deterioration are harsh climatic conditions and chemical attacks, particularly exposure to winter de-icing salts and seawater. Given the growing concern for concrete degradation, numerous computer-assisted tools have been developed to assist engineers in the prediction of the service-life of structures. Many of these models are based on simplified equations that significantly restrict the scope of their application. The limitations of these approaches for the design of new construction and rehabilitation of existing structures are discussed. The theoretical assumptions at the basis of these models are first reviewed. Special attention is paid to the consequences of these simplifying assumptions on the reliability of the models. The difficulties of using these simplified models for the treatment of actual structures exposed to natural conditions are then discussed.     

Effect of confinement level,aspect ratio and concrete strength on the cyclic stress–strain behavior of FRP-confined concrete prisms

Behavior of rectangular concrete columns confined with FRP composites depends on several parameters, including unconfined concrete strength, confinement level, aspect ratio of cross-section (defined as the depth/width of the cross-section), and the sharpness of the section corners. For modeling the cyclic stress–strain behavior of FRP-confined rectangular concrete columns, effect of column parameters on the cyclic behavior of these columns should be examined properly. In this paper, effects of unconfined concrete strength, confinement level and the aspect ratio of cross-section are studied. The test database includes 10 prisms from recent study of authors and 18 prisms from a new experiment. Results of tests show that some aspects of cyclic behavior of FRP-confined concrete prisms such as envelope curve and stress deterioration are unaffected by the considered parameters. Results also indicate that the plastic strain decreases as the unconfined concrete strength increases, but it is independent of the aspect ratio and the confinement level. While the reloading path in all specimens was almost linear, the unloading path was highly nonlinear and was affected by unconfined concrete strength.     

Evaluation of asphalt–aggregate interaction based on the rheological properties

The silicon dioxide (SiO₂) and calcium oxide (CaO) analytical reagents are selected to prepare asphalt mastics and the effects of aggregate chemical composition on asphalt–aggregate interactions (AAI) are evaluated based on the complex modulus and phase angle. It is found that the oxide analytical reagents significantly affect the rheological properties such as complex shear modulus and phase angle, and the effects of CaO are greater than SiO₂ due to the stronger interaction between asphalt binder and CaO analytical reagents. Both the modulus stiffening ratio and the phase angle-based K. Ziegel-B coefficient could be used to evaluate the AAI, and the latter is the better index. Results show that the indexes increase with the test temperature, but decrease with the loading frequency, and tend to be constant. The higher adhesive strength between asphalt binder and limestone than basalt is likely attributed to the higher content of CaO in limestone aggregate and the stronger asphalt–CaO interaction.     

Assessment of alkali–silica reaction damage through quantification of concrete nonlinearity

A newly developed nondestructive evaluation technique, Nonlinear Impact Resonance Acoustic Spectroscopy (NIRAS), is applied to concrete specimens in an ongoing assessment of aggregate alkali reactivity during standard concrete prism testing. NIRAS measures the nonlinearity in a specimen caused by the inception and growth of microcracks throughout the sample and debonding at the aggregate/cement interface. NIRAS is used to exploit the nonlinear effect of excitation amplitude dependent resonance frequency changes, which are related to nonlinearity measurements of concrete samples cast with aggregates of varying reactivity. To relate microstructural changes to changes in nonlinearity and expansion, sample characterization is performed with uranyl-acetate staining. The results demonstrate the utility of NIRAS for not only assessing the potential for ASR under standardized test conditions, but for more general damage characterization in concrete and assessment of “job mixtures.” NIRAS can distinguish reactive from nonreactive aggregates without ambiguity, as supported by sample characterization results.     

Pavement performance evaluation of recycled styrene–butadiene–styrene-modified asphalt mixture

More and more styrene–butadiene–styrene (SBS)-modified asphalt waste materials are being discarded with the increase in road service life. The recycling of these waste pavement materials can reduce environmental pollution and help save resources. However, the low-temperature performance and the fatigue resistance of recycled asphalt mixture are significantly affected by the addition of reclaimed asphalt pavement (RAP). In order to evaluate the low-temperature performance and the fatigue resistance of recycled SBS-modified asphalt mixture, three points bending test, Fénix test and Ensayo de BArrido de DEformaciones test were conducted. Additionally, the differences of recycling between SBS-modified RAP with different ageing conditions and ordinary unmodified RAP were compared. The results showed that fatigue resistance of modified recycling of asphalt mixture with different RAPs did not vary much under low temperature (?5 °C) while displaying an obvious difference under higher temperature. SBS-modified RAP under light ageing condition was suitable for modified recycling. However, the SBS-modified asphalt from RAP under serious ageing condition would lose modification effect resulting in a great reduction of the low-temperature crack resistance and the fatigue resistance. Therefore, it is necessary to evaluate the ageing degree of RAP before recycling SBS-modified asphalt mixture. The SBS-modified RAP under serious ageing condition (SM-RAP) is not recommended for directly modified recycling. But considering for further utilisation, the SM-RAP used for unmodified recycling as ordinary unmodified RAP can be regarded as a good choice and the RAP content should be restricted to less than 30%.     

Analysis of FRP–concrete debonding via boundary integral equations

The problem of debonding of FRP plates glued over a concrete element is studied making use of boundary integral equations. Mode II cohesive crack model is adopted for the interface, whereas linear elasticity is used for the two materials outside the process zone. Symmetric Galerkin boundary element method is used, adopting the arc-length technique to follow the equilibrium path beyond its critical point. It is shown that, due to the presence of a softening branch in shear stress-slip law, the behavior of a specimen undergoing debonding may be strongly non-linear, and is associated with a very brittle failure mechanism. For bond lengths longer than minimum anchorage length, a snap-back branch typically occurs after the attainment of the maximum force. Two different test setups have been numerically simulated and results in good agreement with experimental tests are found.     

Performance of connections for prefabricated timber–concrete composite floors

Timber–concrete composite beams and slabs require interlayer connectors, which provide composite action in the cross-section. A range of mechanical connectors is available on the market with an extensive variety of stiffness and strength properties, which are fundamental design parameters for the composite structure. Another crucial parameter is the cost of the connector, including the labour cost, that if too high may prevent the use of the composite system. In order to reduce the construction cost and make timber–concrete structures more widespread on the market, it is believed that a high degree of prefabrication should be achieved. For a simple and cost effective construction process, the use of “dry” connections, which do not require the pouring and curing of concrete on site, may represent a possible solution. This paper reports the outcomes of an experimental programme aimed to investigate a number of different mechanical “dry–dry” connectors previously embedded into a prefabricated concrete slab. Direct shear tests on small blocks made of a glulam segment connected with a prefabricated concrete slab were performed. The shear force-relative slip relationships were measured and all the relevant mechanical properties such as slip moduli and shear strengths were calculated. It was found that some of the new developed connection systems for prefabricated concrete slab can perform as satisfactorily as those for cast-in-situ slabs, with the additional benefit of being relatively inexpensive.     

Processing conditions—fracture toughness relationships of asphalt concrete mixtures

The effect of processing conditions (dynamic compaction) on the fatigue crack propagation behaviour of AC-20 asphalt concrete mixture was studied. Beams were prepared from AC-20 asphalt binder containing 8% asphalt by weight with and without dynamic compaction. The gradation used was Ohio Department of Transportation item 403, and was kept the same. Flexural static tests were conducted to determine the effect of dynamic compaction on both the ultimate strength and flexural modulus. Flexural fatigue tests were conducted on three identical notched specimens prepared using each of the two compaction techniques. Parameters controlling the crack propagation process were evaluated; namely, the energy release rate and the change in work, W _i expended on damage formation and history dependent viscous dissipation processes within the active zone (region ahead of the crack tip). The modified crack layer (MCL) model was employed to extract the specific energy of damage, , a material parameter characteristic of the asphalt concrete mixture's resistance to crack propagation, and the dissipative coefficient, . It has been found that the dynamically and statically compacted AC-20 mixture displayed superior fracture resistance, as reflected in and . Also, the ultimate strength and modulus increased by about two-fold. Scanning electron microscopic examination revealed an obvious change in the morphology of the fracture surface. This is manifested in the appearance of a finer more dense texture in the case of the dynamically and statically compacted mixture. In addition, smaller more frequent dimples in binder rich areas are indicative of better adhesion between the binder and the aggregate. This in turn contributes to the increased fracture resistance of the dynamically and statically compacted AC-20 asphalt concrete mixture.     

Evaluation of non-linear behavior of timber–concrete composite structures using FE model

This article describes a numerical model that was developed for the analysis of composite timber–concrete beams. This model presents a simplified methodology for determining the effective bending stiffness of the timber–concrete composite structure. It is based on previous work done usually referred to in some non-normative literature by γ-method. The implemented methodology assumes some simplifications, as for instance, linear elastic behavior of all components, constant stiffness of the connection and sinusoidal loading. For comparison purposes, the work benefits from an experimental program in which full-scale beams were tested in bending and timber–concrete connections were tested in shear. The FE model has shown the ability to overcome the simplifications of the Eurocode, namely the variation of shear force along the beam axis. The numerical model is capable of detecting and quantifying the influence of the non-linear behavior of the connections on the composite structure. Different parameters are analyzed and, for instance, the ductility behavior of the timber–concrete connection could be more important than the maximum strength, which is an interesting result. By comparing theoretical predictions with test results, it is clear that the numerical model used in this work is a very interesting method when compared with the usual design models, such as that of Annex B of Eurocode 5 (EN 1995-1-1). The influence of the connections behavior on the ultimate load of the composite structure is very important and the described approach proved to give good predictions.     

Fracture properties of concrete reinforced with steel–polypropylene hybrid fibres

This research discusses polypropylene fibres and three sizes of steel fibres reinforced concrete. The total fibre content ranges from 0% to 0.95% by volume of concrete. A four-point bending test is adopted on the notched prisms with the size of 100×100×500 mm³ to investigate the effect of hybrid fibres on crack arresting. The research results show that there is a positive synergy effect between large steel fibres and polypropylene fibres on the load-bearing capacity and fracture toughness in the small displacement range. But this synergy effect disappears in the large displacement range. The large and strong steel fibre is better than soft polypropylene fibre and small steel fibre in the aspect of energy absorption capacity in the large displacement range. The static service limitation for the hybrid fibres concrete, with “a wide peak” or “multi-peaks” load–CMOD patterns, should be carefully selected. The ultimate load bearing capacity and the crack width or CMOD at this load level should be jointly considered in this case. The K_IC and fracture toughness of proper hybrid fibre system can be higher than that of mono-fibre system.     

Air voids reduction phenomena of asphalt concrete – A continuum approach

Asphalt concrete used in flexible highway pavement construction has 5–8 percent air voids immediately after laying of the roadway. Constitutive laws for asphalt concrete developed till now have modelled the mix as a linear elastic or viscoelastic material and have not taken into account the effect of void concentration on the mechanical behaviour of the material. In this paper, the theory of linear elastic material with voids is used to model asphalt concrete under isothermal conditions. Two cases of void reduction behaviour are studied, one in which the void volume reduces asymptotically under a constant load and the other in which it reaches the refusal air void content. The model is used to predict the creep behaviour under constant compressive stress as well as to obtain the hysteretic stress-strain behaviour. Solutions for the case of uniaxial deformation are derived and the strains are simulated for a constant compressive stress. Use of the air voids reduction measure as a possible damage parameter is also examined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development and mechanical properties of metal–particulate glass matrix composites from recycled glasses

The great number of glasses available from recycling activity and vitrification treatment of industrial wastes leads to the need for new applications, with the development of new materials, such as low-cost composite materials from a powder technology route. In the present work a variety of recycled glasses is investigated, in order to obtain aluminium reinforced glass matrix composites via cold-pressing and viscous flow sintering. A good compatibility between lead silicate glasses from cathode ray tubes dismantling and aluminium reinforcement is found to be effective. Composites exhibiting good mechanical properties were developed from these materials. A particular attention was due to fracture toughness (K_IC) determination. The absolute K_IC of glass matrix composites value remains low, but a notable increment in relation to unreinforced matrix is observed.