Critical Crack Research Articles

Crack Detection of Asphalt Concrete Using Combined Fracture Mechanics and Digital Image Correlation

Cracking is a common failure mode in asphalt concrete (AC) pavements. Many tests have been developed to characterize the fracture behavior of AC. Accurate crack detection during testing is crucial to describe AC fracture behavior. This paper proposes a framework to detect surface cracks in AC specimens using two-dimensional digital image correlation (DIC). Two significant drawbacks in previous research in this field were addressed. First, a multiseed incremental reliability-guided DIC was proposed to solve the decorrelation issue due to large deformation and discontinuities. The method was validated using synthetic deformed images. A correctly implemented analysis could accurately measure strains up to 450%, even with significant discontinuities (cracks) present in the deformed image. Second, a robust method was developed to detect cracks based on displacement fields. The proposed method uses critical crack tip opening displacement (δc) to define the onset of cleavage fracture. The proposed method relies on well-developed fracture mechanics theory. The proposed threshold δc has a physical meaning and can be easily determined from DIC measurement. The method was validated using an extended finite-element model. The framework was implemented to measure the crack-propagation rate while conducting the Illinois-flexibility index test on two AC mixes. The calculated rates could distinguish mixes based on their cracking potential. The proposed framework could be applied to characterize AC cracking phenomenon, evaluate its fracture properties, assess asphalt mixture testing protocols, and develop theoretical models.

Comparative Analysis of Reinforced Asphalt Concrete Overlays: Effects of Thickness and Temperature.

Reflection cracking in asphalt concrete (AC) overlays is a common form of pavement deterioration that occurs when underlying cracks and joints in the pavement structure propagate through an overlay due to thermal and traffic-induced movement, ultimately degrading the pavement's lifespan and performance. This study aims to determine how alterations in overlay thickness and temperature conditions, the incorporation of chopped fibers, and the use of geotextiles influence the overlay's capacity to postpone the occurrence of reflection cracking. To achieve the above objective, a total of 36 prism specimens were prepared and tested using an overlay testing machine (OTM). The variables considered in this study were the thickness of the overlay (40, 50, and 60 mm), temperature (20, 30, and 40 °C), mix type (reference mix and mix modified with 10% chopped fibers by weight of asphalt cement), and the inclusion of geotextile fabric at two positions (one-third of the depth from the base and at the bottom). The research outcomes revealed that a decreased temperature and thicker overlay led to a higher resistance to crack initiation and full propagation, as indicated by the values of critical fracture energy (Gc) and crack progression rate (CPR). Furthermore, the study observed the enhanced crack resistance of overlays in the presence of geotextiles, whether at the bottom or one-third of the depth from the bottom, with superior performance of the former. Despite a slight enhancement in certain properties, the incorporation of chopped fibers in the overlays did not substantially improve the overall performance compared to the reference specimens. Overall, the study provides valuable insights into the variables that influence the ability of AC overlays to mitigate reflection cracking. These findings will aid engineers and designers in making informed decisions regarding overlay design and construction.

Study on the dynamic fracturing characteristics of aggregate-mortar interface under various loading rates using DIC

The dynamics of fracture characteristics at the aggregate-mortar interface under varying loading rates present a significant area of investigation, pivotal for comprehending the response of concrete to dynamic load conditions. In concrete, the interface between the aggregate and the mortar forms a significant region, fundamentally dictating concrete structures' comprehensive strength and durability. This study aims to investigate the interface crack propagation process, including the crack propagation velocity and the propagation direction using Digital Image Correlation (DIC). Three-point bending beams with a single-size aggregate were fabricated. Beams with different initial crack tip-to-aggregate distances, d0 (aggregate position) of 20 mm, 40 mm, and 60 mm, were tested at five different displacement-loading rates (0.012 mm/min, 0.12 mm/min, 1.2 mm/min, 12 mm/min, and 120 mm/min) using an electro-hydraulic servo testing machine. The crack propagation process was documented utilizing a high-speed video camera. The study reveals a direct correlation between loading rates and concrete's mechanical attributes. The results indicate that as loading rates rise, the compressive strength, tensile strength, and modulus of elasticity of concrete notably increase, while the Poisson's ratio decreases. The initial fracture toughness (KIcini) also escalates with increasing the rates, its sensitivity notably influenced by d0. Concurrently, fracture energy increases with the increase of loading rate and the decrease of d0, a trend more evident at high loading rates. The crack initiation load (Pini) for all single-size aggregate concrete beams was higher than that of plain concrete, and Pini decreased as d0 increased. Observations were made on the trajectory of crack formation, further scrutinizing the effects of d0 and the variable loading rate. Notably, the presence of the interface was found to slow the crack propagation velocity, and this ability to inhibit crack propagation decreased as d0 increased. As the loading rate increased, the crack propagation velocity at the mortar matrix and interface also increased. This study also reports additional material properties, such as critical crack length, and interfacial fracture mode.

Strain-rate sensitivity analysis of the fracture behaviour of polyoxymethylene-fibre-reinforced airport-pavement concrete

Polyoxymethylene (POM), a novel type of fibre, can be incorporated into airport-pavement concrete as a reinforcing fibre. Airport pavements are thin-plate concrete structures continuously subjected to various complex random loads. Thus, understanding the fracture behaviour of POM-fibre-reinforced concrete under various strain rates is essential. In this study, ordinary airport-pavement concrete and POM-fibre-reinforced airport-pavement concrete were subjected to three-point bending fracture tests at five strain rates (10−6, 10−5, 10−4, 10−3 and 10−2 s−1), and the effects of fibre content and strain rate on the fracture performance of these concrete were systematically investigated. The results showed that POM fibres could enhance the crack resistance of concrete used in airport pavements; for a POM fibre concentration of > 1.2 vol%, the fracture toughness increased by ∼ 50 %. The analysed critical crack propagation rates demonstrated the ability of POM fibres to inhibit fracture propagation. Although PFAPC showed distinct strain-rate-dependent fracture characteristics, this dependency weakened as the fibre concentration increased. Finally, statistical analyses of the fractured surface were performed to assess the fracture behaviour of PFAPC under various stresses and quantify the contribution of POM. These results provide a critical foundation for the practical application of PFAPC in airport pavements.

Experimental study on local crack propagation of concrete under three-point bending

To investigate the propagation process of local cracks in concrete, three-point bending tests were performed on the beams with a local rectangular pre-crack. Pre-cracks with different sizes were set in the specimens with the widths ranging from 30 mm to 100 mm and the heights ranging from 30 mm to 60 mm. The three-point bending tests were carried out with a controlled displacement rate of 0.024 mm/min. The compressive strength, elastic modulus and tensile strength of the concrete were 40.89 MPa, 32.57 GPa and 3.40 MPa, respectively. By monitoring the initiation and propagation of local cracks on the specimen surfaces, the fracture toughness of the concrete specimen with a local crack was determined and the crack propagation process along the thickness and height directions was investigated. The experimental results indicated that, under three-point bending loading, the local cracks always propagated firstly on the bottom surface of the specimen and formed a through crack in the thickness direction. After that, the crack in the specimen began to propagate in the height direction until full failure. In addition, the initial fracture toughness obtained from the specimens with bottom-through cracks was almost the same as the one from the specimens with local cracks. However, the unstable fracture toughness obtained from the specimens with bottom-through cracks was not consistent with the one from the specimens with local cracks, where the average unstable fracture toughness calculated using the critical crack lengths at different thicknesses was taken as the unstable fracture toughness of the local crack specimens. This indicated that the initial fracture toughness could be considered as a material characteristic, while the unstable fracture toughness was dependent on the crack geometry.

DeepCrackAT: An effective crack segmentation framework based on learning multi-scale crack features

The detection of cracks is essential for assessing and maintaining building and road safety. However, the large appearance variations and the complex topological structures of cracks bring challenges to automatic crack detection. To alleviate the above challenges, we propose a deep multi-scale crack feature learning model called DeepCrackAT for crack segmentation, which is based on an encoder–decoder network with feature tokenization mechanism and attention mechanism. Specifically, we use hybrid dilated convolutions in the first three layers of the encoder–decoder to increase the network’s receptive field and capture more crack information. Then, we introduce a tokenized multilayer perceptron (Tok-MLP) in the last two layers of the encoder–decoder to tokenize and project high-dimensional crack features into low-dimensional space. This helps to reduce parameters and enhance the network’s ability of noise resistance. Next, we concatenate the features corresponding to the encoder–decoder layers and introduce the convolutional block attention module (CBAM) to enhance the network’s perception of the critical crack region. Finally, the five-layer features are fused to generate a binary segmentation map of the crack image. We conducted extensive experiments and ablation studies on two real-world crack datasets, and DeepCrackAT achieved 97.41% and 97.25% accuracy on these datasets, respectively. The experimental results show that the proposed method outperforms the current state-of-the-art methods.

Application of Bilinear Softening Laws and Fracture Toughness of Foamed Concrete

This study examined the fracture and failed performance of foamed concrete materials by testing normalized notched beams under three-point bending via three methods: inverse analysis, digital image correlation (DIC), and finite element modeling (FEM). It also discussed both experimental and FEM characteristics. However, inverse analysis is only applicable for specimens with a notch height of 30 mm. Bilinear softening of the tested beams was estimated to identify the fracture energy (GF), critical crack length (ac), and elastic modulus (E). Additionally, the fracture toughness was calculated by adopting the double-K method (initiation fracture, unstable fracture, and cohesive fracture). Two-dimensional FEA modeling of the fracture was conducted using the traction-separation law (TSL), incorporating the extended finite element method (XFEM) and cohesive zone (CZM) techniques. A finite element sensitivity for the XFEM and CZM was performed, with the global mesh size of 2 and the damage stabilization cohesion of 1 × 10−5 showed good convergence and were used in other models. Further comparison of the DIC experiment findings with those from the FEM demonstrated good agreement in terms of crack propagation simulation.

Analysis of the shear behavior of reinforced recycled aggregate concrete beams based on shear transfer mechanisms

This study aims to contribute to a more rational understanding of the shear behavior of reinforced recycled (concrete) aggregate concrete beams. Twelve reinforced concrete beams - six with natural aggregate concrete (NAC) and six with 100% coarse recycled concrete aggregates (RAC) - with same concrete compressive strength (38 MPa) and proportions of cement and aggregates per volume, varying the longitudinal (1.15%, 1.75%, and 2.50%) and transverse (0%, 0.086%, and 0.114%) reinforcement ratios, were tested under a 4-point bending setup with a full-field displacement monitoring. A thorough examination of the failure span along the tests, associating critical crack shape and kinematics with an investigation of the shear transfer mechanisms provided by mechanical models from the literature was conducted. Direct shear tests on push-off specimens followed by analyses of their failure surface evaluated the characteristics of shear crack surfaces and a proper modeling of the aggregate interlock contribution. The results showed a 28% average reduction in the shear strength of RAC beams with no stirrups in comparison with their reference beams (NAC), whereas beams with stirrups of same characteristics showed a similar shear strength, regardless of the type of coarse aggregate. The evaluation of the shear transfer mechanisms enabled an understanding of the reasons for those behaviors. Experimental results were also compared with those predicted in design codes. They were conservative for beams with stirrups, and, in general, less accurate than those obtained by the shear transfer mechanisms.

A Study on the Distribution of Shear Forces Between Resisting Mechanisms in an RC Element Without Stirrups

This paper explores how the shear force distributes itself among the three main shear resistance mechanisms: shear resistance of the uncracked compressed chord, aggregate interlock, and dowel effect. Today’s dominating shear models, the critical crack theory (Muttoni et al. [1]) and the compression field theory (Collins et al. [2]) maintain that the main shear-resisting mechanism is aggregate interlock, while more recent studies (Marí et al. [3]), maintain that the main resistance mechanism is the shear resistance on the uncracked compression chord.
 In this paper FEM modelling is used to study a test carried out at the Universidad Politécnica de Madrid (UPM) to try to assign the shear force to the different shear mechanisms for different loading steps and elucidate what finally causes the failure of the structure. The results show that as load is increased the relative part of the shear force taken by the uncracked compressed chord increases until finally shear failure is reached when the principal tensile stress in the area located close to the load but towards the support reaches the tensile resistance of concrete, generating a crack that precipitates the failure of the beam.

Methodology to assess damage mechanisms of rail steels based on small-scale experiments

A new methodology to distinguish between the damage mechanisms wear only and RCF crack initiation in combination with wear in rolling contacts is proposed in this work. The new methodology distinguishes between the damage mechanisms by comparing the experimental crack depth against a critical crack depth which is governed by the surface roughness and by the plastic shear deformation depth in the near-surface layer. The new methodology is used to assess the results of twin disc experiments that have been conducted with three pearlitic rail steels at different normal contact pressures and different longitudinal creep values. The observed experimental results show agreement with the criterion and the threshold of the new methodology.

Effects of pressurized carbonation with presoaking in calcium hydroxide solution on the fracture behaviours of recycled coarse aggregate concrete

Carbonation is effective in enhancing recycled coarse aggregate, but the shortage of carbonizable Ca2+ in recycled coarse aggregate and the low permeability efficiency of CO2 at ambient pressure limit the carbonation efficiency and industrial application. To solve these issues, the Ca2+ content in recycled coarse aggregate and the carbonation effectiveness were increased by presoaking recycled coarse aggregate in calcium hydroxide and enhancing the carbonation pressure, respectively. The optimal carbonation condition was obtained based on the physical properties of the recycled coarse aggregate and the strength of recycled coarse aggregate concrete: the carbonation pressure was 4 bar, and the concentration of the calcium hydroxide solution was 0.04 mol/kg. The fracture behaviour and microstructure of the recycled coarse aggregate concrete under different carbonation conditions were evaluated. The recycled coarse aggregate concrete after pressurized carbonation with presoaking in calcium hydroxide solution (RCAC-C0.04-P4) has higher toughness and a more stable crack propagation process than untreated recycled coarse aggregate concrete. Pressurized carbonation with presoaking in calcium hydroxide solution significantly increased the initial cracking load, peak load, Young's modulus, initial toughness, unstable toughness, and fracture energy of the recycled coarse aggregate concrete by 67.35%, 2.83%, 12.19%, 60.77%, 2.31%, and 5.38%, respectively, and reduced the critical crack mouth (CMODc) and tip opening displacement (CTODc) by 9.13% and 10.96%. Simple pressurized carbonation made the toughness of the specimens worse than that of untreated recycled coarse aggregate concrete. Compared with untreated recycled coarse aggregate concrete, although the initial toughness, unstable toughness, and fracture energy of recycled coarse aggregate concrete after simple pressurized carbonation (RCAC-P4) were increased by 16.02%, 1.86%, and 0.28%, respectively, the Young's modulus was slightly reduced by 1.35%, and the CMODc and CTODc were increased by 5.37%. In addition, a microstructural analysis showed that the contents of Ca(OH)2 and CaCO3 of RCAC-C0.04-P4 were increased by 50.56% and 37.62% compared to untreated recycled coarse aggregate concrete, respectively, and the average indentation modulus of the old aggregate and old mortar interface of RCAC-C0.04-P4 was increased by 24.37% compared to untreated recycled coarse aggregate concrete, which confirmed that pressurized carbonation with presoaking in a calcium hydroxide solution made CaCO3 fill the cracks in the interface and improved the macroperformance. These results can promote the application of recycled coarse aggregate in engineering.

Modified Maximum Likelihood Estimation Metal Magnetic Memory Quantitative Identifying Model of Weld Defect Levels Based on Dempster–Shafer Theory

Metal magnetic memory (MMM) is a nondestructive testing technology based on the magnetomechanical effect, which is widely used in the qualitative detection of stress concentration zones for welded joints. However, there is inevitable residual stress after welding, which brings the bottleneck of quantitative identification between the weld residual stress concentration and the early hidden damage. In order to overcome the bottleneck of quantitative identification of weld defect levels with MMM technology, a modified maximum likelihood estimation (MLE) MMM quantitative identifying model is first proposed. The experimental materials are Q235B welded plate specimens. Fatigue tension experiments were operated to find the MMM feature laws of critical hidden crack by comparing with synchronous X-ray detection results. Six MMM characteristic parameters, which are, ΔHp(x), Gxmax, Zxmax, ΔHp(y), Gymax and Zymax, are extracted corresponding to the normal state, the hidden crack state and the macroscopic crack, respectively. The MLE values of the six parameters are obtained by the kernel density functions with optimized bandwidth from the view of mathematical statistics. Furthermore, the modified MLE MMM quantitative identifying model is established based on D–S theory to overcome the partial overlap of MLE values among different defect levels, of which the uncertainty is as low as 0.3%. The verification result from scanning electron microscopy (SEM) is consistent with the prediction of the modified MLE MMM model, which provides a new method for quantitative identification of weld defect levels.

Contrasting the major characteristics of pinewood and Amazon hardwoods to provide high-quality cement-bonded particleboards

The combined effect of wood chemical composition, anatomical traits, and density on cement curing, matrix-reinforcement interface, and final performance of cement-bonded particleboards (CBPBs) needs deeper investigation. Besides, many uncharacterized Amazon hardwoods challenge softwoods' often-reported superiority for producing CBPBs. Certified Cordia goeldiana, Brosimum parinarioides, Parkia gigantocarpa, and Pinus oocarpa woods were characterized by histology, cell biometry, basic density, and chemical composition. The hydration kinetics of the Portland cement pastes was evaluated by isothermal heat conduction. The CBPBs production parameters were wood:cement ratio of 1:2.75, water:cement ratio of 1:2.5, 4% of CaCl2 additive (based on cement mass), and target density of 1.25 g cm−3. They were submitted to physical and mechanical tests, and the polished surfaces were analyzed by scanning electron microscopy. The cement kinetics showed that wood's higher contents of extractives and hemicelluloses harmed the initial dissolution stage of Portland cement curing but not the essential stages of acceleration and deacceleration. C. goeldiana wood showed the highest compatibility indexes with cement, and all hardwoods surpassed P. oocarpa. Thin-walled fibers, big-diameter fibers, and big-diameter pores favored cement impregnation. Debonding at wood-cement interfaces of CBPBs reinforced with higher-density woods was critical for crack propagation throughout the matrix. Less critical inter-particle cracks occurred in CBPBs reinforced with lower-density woods. The better interface and the highest volume of wood particles of the lowest-density P. gigantocarpa wood provided the CBPBs with the best physical and mechanical performance overcoming the chemical incompatibility relevant only in the initial dissolution stage of cement curing.

Crack extension at the concrete–rock interface damaged by high-pressure water environment: DIC research

As a weak link in hydraulic structures such as arch dams, the fracture property of the concrete–rock interface is impacted by various factors, including the material properties of the individual constituents and the fracture process zone (FPZ) at the crack tip. Because the heel area of an extra-high dam has been exposed to a high-pressure water environment, this paper aims to investigate the whole fracture process of the concrete–rock interface after the water pressure effect. In this test, the water pressure was set at 0, 1, 2, and 4 MPa, and the crack deformation of the concrete–rock composite beam under three-point bending (TPB) was monitored in real-time by the digital image correlation (DIC) technique. DIC has been discovered to aid in understanding the damage caused by a high-pressure water environment to material properties. The stable crack extension of TPB composite beams after the high-pressure water exposure was greatly limited, and the corresponding crack length was decreased by up to 35%. The FPZ characteristic length was reduced by 19% when exposed to a high-pressure water environment, resulting in a drop in cohesive toughness on the fictitious crack. KR resistance curves for the concrete–rock interface fracture were constructed to consider the damage to material properties and FPZ evolution by high-pressure water exposure. For a reference for fracture analysis in the dam heel of extra-high dams, the calculation model of critical crack size in elastic fracture and the prediction equation of FPZ characteristic length in nonlinear fracture were constructed for a TPB composite beam.

Methodology for determination of critical crack depth in a class-I component

In this study, a methodology for the determination of critical crack depth in a class-I component using the failure assessment diagram has been proposed and implemented through the finite element method. Overviewing the literature in the field of fracture mechanics, the need for the development of such methodology was felt, as such straightforward methodologies are difficult to find in contemporary literature. As a case study, the proposed methodology was then successfully applied to a pressurized water reactor’s pressure vessel closure head (class-I component). Subsequently, the critical crack depth was determined through a failure assessment diagram under pressurized thermals shock induced during the small break loss of coolant accidental and Rancho-Seco transient accidental conditions of the reactor.

Blending hydrogen in existing natural gas pipelines: Integrity consequences from a fitness for service perspective

Blending hydrogen in existing natural gas pipelines compromises steel integrity because it increases fatigue crack growth, promotes subcritical cracking and decreases fracture toughness. In this regard, several laboratories reported that the fracture toughness measured in a hydrogen containing gaseous atmosphere, KIH, can be 50% or less than KIC, the fracture toughness measured in air. From a pipeline integrity perspective, fracture mechanics predicts that injecting hydrogen in a natural gas pipeline decreases the failure pressure and the size of the critical flaw at a given pressure level. For a pipeline with a given flaw size, as shown in this work, the effect of Hydrogen Embrittlement (HE) in the predicted failure pressure is largest when a failure occurs by a brittle fracture. The HE effect on failure pressure diminishes with a decreasing crack size or increasing fracture toughness. The safety margin after a successful hydrostatic test is reduced and therefore the time between hydrotests should be decreased. In this work, all those effects were quantified using a crack assessment methodology (level 2, API 579-ASME FFS) considering literature values for KIH and KIC reported for an API 5L X52 pipeline steel. To characterize different scenarios, various crack sizes were assumed, including a small crack with a size close to the detection limit of current in-line inspection techniques and a larger crack that represents the largest crack size that could survive a hydrotest to 100% of the steel Specified Minimum Yield Strength (SMYS). The implications of a smaller failure pressure and smaller critical crack size on pipeline integrity are discussed in this paper.

Effect of double crack on fatigue crack growth life of 3D printing compressor impeller

A compressor impeller with pressure ratio less than 3 is taken as the research object. The 3D printing residual stress was introduced into damage tolerance assessment of blades, and the effect of double crack on Fatigue Crack Growth Life (FCGL) was investigated. The impeller was printed by selective laser melting technology and then heat treated. Finite element method was applied to numerically simulate 3D printing process, heat treatment process and actual working process of impeller, and mechanical quantities such as stress and deformation were completely analyzed. Numerical simulation results agreed well with the experimentally observed crack location in the middle of the blade. Forman–Newman–de Koning model and linear elastic fracture mechanics theory were applied to give the growth law of impeller under single and double crack conditions. The result shows that crack initiation locations of impeller during printing and operating are respectively middle part and root part of blade. The stress intensity factor at the initial crack tips of single and double cracks exhibits a quadratic relationship with rotational speed when only external load is considered. The crack growth paths of single crack in middle and root are radial and axial respectively, and the FCGL of single crack in root is lower than that of single crack in middle. With the increase of rotational speed, the critical crack length decreases linearly and the cycles decreases nonlinearly in both single and double crack conditions. The FCGL of double crack is between two single cracks at low speed and lower than two single cracks at high speed. The critical crack length of double crack is lower than that of corresponding single crack. At 60000rpm, the dominant crack changes from middle crack to root crack.

A simplified approach to hydraulic fracturing of rocks based on Finite Fracture Mechanics

AbstractThis work presents a coupled approach, based on Finite Fracture Mechanics (FFM), to preliminarily investigate hydraulic fracturing of rocks. The novelty of the criterion relies on the assumption of a finite crack extension and on the simultaneous fulfillment of a stress requirement and the energy balance. Two material constants are involved, namely, the tensile strength and the fracture toughness. The FFM unknowns are represented by the critical crack advance, which results a structural parameter and not a simple material one, and the breakdown pressure. The study investigates the longitudinal failure behavior of both impermeable and permeable rocks by supposing that growing cracks are loaded by a pressure proportional to that acting on the borehole wall. The stability growth of hydraulic fractures is discussed case by case. The approach is validated against experimental data available in the literature by considering the effect of rock permeability, fluid viscosity and flow rate.

Shear behaviour of seawater sea-sand coral aggregate concrete beams reinforced with FRP strip stirrups

The preparation of concrete for remote islands or reefs using locally available seawater, sea sand, and coral aggregate can greatly shorten the construction period and mitigate the shortage of river sand and natural gravel resources. This study is the first to experimentally investigate the shear behaviour of seawater sea-sand coral aggregate concrete (SSCAC) beams reinforced with non-corrosive carbon fibre-reinforced polymer (CFRP) strip stirrups. The test parameters included concrete strength, concrete type, and stirrup configuration. The experimental results suggested that CFRP-reinforced SSCAC beams exhibited a brittle shear failure mode because of the sudden CFRP rupture at the bend portions. The ratio of the measured maximum strain to the ultimate tensile strain of the CFRP strip stirrups varied from 36.6% to 47.8%. The critical diagonal shear cracks of SSCAC beams directly penetrated through the coral aggregates and split the aggregates, whereas those in the natural aggregate concrete (NAC) beams bypassed the natural gravel. In this regard, the SSCAC beams exhibited approximately 10% lower shear strength than their NAC counterparts owing to the loss of the aggregate interlocking effect after the occurrence of smooth diagonal shear cracks. The higher the strength grade of SSCAC, the greater the shear strength and service load.

The Effect of Polypropylene Fibers on the Fracture Characteristics of Lightweight Aggregate Crumb Rubber Concrete Composites

The increasing use of rubber tires and their low recycling ratio have made them a serious environmental problem. This work aims to develop and investigate enhanced lightweight aggregate crumb rubber concrete (PFLWACRC) composites regarding the fracture properties of concrete. Polypropylene (PP) fibers are commonly familiar with increased crack growth endurance of concrete. On the other hand, the reuse of waste rubber in concrete plays a major role in the mitigation of the effects of climate change. Various concrete mixtures were designed with conventional Portland cement and common lightweight coarse aggregates. The variables considered in this study are PP fibers in different percentages (0.2%, 0.4%, and 0.6% volume fraction), and crumb rubber with various substitution proportions (0%, 5%, 10%, 15%, 20%, and 25% of fine aggregates). Cement with 450kg/m3 density with 10% silica fume was used. The fracture characteristics, which involve fracture toughness (KIC) and fracture energy generation (GF), of all concrete mixtures were evaluated by testing two types of samples, i.e. 54 notched concrete beams with dimensions of 10×10×52cm and 54 cylinders with diameter×height equal to 15×30cm. The results showed that the fracture toughness generation addresses the energy scattering limit of concrete mixtures. The findings showed that the existence of PP fibers increased the fracture energy and critical Crack Mouth Opening Displacement (CMOD)c. The PP fibers had a limited effect on the compressive strength and may even reduce it, but a remarkable enhancement of the energy absorption was observed.