Wedge Splitting Specimens Research Articles

Prediction of fracture failure and fracture parameters for dam gallery concrete specimens of arbitrary size and age via boundary effect model and fracture extreme theory

Cracking-associated problems may occur in the construction and operation periods of dam galleries, necessitating attention to the fracture characteristics of gallery concrete. The fracture parameters of concrete exhibit size effects and also change with age. A correct evaluation of the cracking risk or crack stability of gallery structures at any given time requires a comprehensive understanding of both the size effect and time-variant characteristics of the gallery concrete fracture parameters. In the present study, 75 wedge-splitting specimens, with effective heights ranging from 200 mm to 600 mm, were cast using gallery concrete produced by the concrete mixing system at the Baihetan Dam construction site. Subsequently, fracture tests were conducted on these specimens at curing ages spanning from 7 days to 90 days. According to the test results, the fracture parameters of concrete with a specific size exhibited an increasing trend with age within 90 days, while the fracture parameters of concrete at specific ages show a significant size effect, tending to stabilize as the effective depth of specimens gradually increases. The boundary effect model was utilized for analyzing the fracture test results of concrete specimens at different ages, yielding the size-independent double-K fracture parameters and scale parameters for concrete at each age, along with their time-varying patterns. Subsequently, equations for predicting the failure loads of concrete specimens of any age and size were established. Based on the prediction results of failure loads, the double-K fracture parameters of concrete specimens of any age and size were solved by the fracture extreme theory. The predicted failure loads and fracture parameters were in good agreement with the experimental results. The findings of this study can be applied to the full lifecycle crack risk analysis or crack stability assessment of Baihetan Dam gallery structures.

Study on critical crack length of wet-sieved and fully graded concrete under mode I loading

The critical crack lengths of wet-sieved concrete and fully graded concrete obtained from experimental measurements, numerical simulation and an analytical solution based on linear elastic fracture mechanics were quantitatively compared. Experiments were conducted on wedge-splitting specimens with depths of 200–1500 mm and a maximum aggregate size of 150 mm. Strain gauges and clip gauges were pasted and mounted along the direction of crack growth to measure the critical crack length. Numerical and analytical methods were used to calculate the critical crack length. The average relative error between the analytical calculations and measurements was still 14%, even for a specimen depth of 1500 mm. The effective fracture toughness was calculated by combining the peak load and critical crack length, resulting in an average relative error of 17%. These results indicate that the existing analytical method for determining the critical crack length is inadequate. Consequently, a modified analytical method was adopted, using 95% of the peak load in the post-peak region and the corresponding crack mouth opening displacement, which provided results that agreed well with the experimental data. Furthermore, the effective fracture toughness could be reasonably derived using the critical crack length from the modified analytical method.

Modeling and fracture behavior of mesoscale concrete considering actual aggregate shapes and placement domain shapes

The establishment of meso-concrete models under complex boundary conditions still faces great challenges at present. In this study, a numerical framework for constructing concrete meso-structures in irregularly placement domains is developed based on the dot matrix idea. In addition, a solution to quickly obtain numerous 3D aggregates with realistic morphology as well as 2D cross-section aggregates is proposed. On this basis, wedge splitting (WST) tests with different coarse aggregate contents are conducted and the numerical results agree well with experimental results. The fracture mechanism of the wedge splitting specimen was then investigated by analyzing the contact force chain and crack evolution. Finally, parametric analysis was used to investigate the impact of aggregate and mortar strength on the fracture characteristics of WST specimens. The results show that the numerical framework has good robustness. Tensile stress is the driving force for crack extension in WST specimens, the area of the pure tensile stress zone and the distance between the front end of the pure tensile stress zone and the crack tip decreases as the crack extension length increases. With the increase of coarse aggregate content, the deflection effect of the crack is intensified, the crack path becomes more tortuous, and the fracture zone range also increases. The mechanical properties of concrete and the crack extension path are significantly influenced by the strength of the mortar and the aggregate, but the aggregate does not contribute as much to the overall strength of the concrete structure as the mortar matrix does.

An inverse analysis method for determining tensile softening relationship of concrete considering local response

This study presents an inverse analysis algorithm for deriving the softening relationship of concrete. The algorithm can not only reproduce the influence of local response on the σ-w curve by introducing additional load criteria but also reduce the dependence of optimization results on the initial guesses by combining global best-fitting with the automatic parameter updating technique, improving the prediction accuracy and lowing the possibility of obtaining pseudo-solutions. Then, the fracture tests were carried out on the wedge splitting specimens. The validity and versatility of the algorithm were verified using experimental data. The findings show that the simulation responses agree well with the test ones in all regions of the load-CMOD curve, and the values of fracture energy determined by the load-CMOD and σ-w curves are consistent. The derived σ-w curve is independent of the initial guesses of parameters. The widely used bi-linear σ-w curve tends to overestimate the load in the post-peak of load-CMOD, and the tri-linear model is recommended after considering the accuracy, computation time, and convergence. The tensile strength evaluated by the inverse analysis method is 75% of the splitting tension test and 91% of the fracture theory method. Further, the proposed algorithm exhibits outstanding versatility, applying to various concrete materials and specimen configurations.

Interfacial fracture performance of engineered cementitious composites and fire-damaged concrete

This study investigated the interfacial fracture properties of engineered cementitious composites (ECC) and fire-damaged concrete using bi-material wedge splitting specimens. The full load versus crack mouth opening displacement (CMOD) curves of the bi-material specimens were obtained using wedge splitting tests, and the fracture toughness and fracture energy were calculated. The effects of the maximum temperature experienced by the concrete (20–700 °C) and the interfacial roughness (0–5 mm) on the interfacial damage mode, fracture toughness, and fracture energy were studied. The results indicated that when the maximum temperature experienced by the concrete was lower than 400 ℃ or the interface was smooth, the damage mode of the specimens was primarily adhesive failure. However, when the maximum temperature experienced by the concrete was equal to or greater than 400 °C and the interface was rough, cohesive failure often occurred in the specimens. Owing to the change in the damage mode, the bi-material specimens had the highest values of the fracture toughness and fracture energy at 400 °C. When the maximum temperature experienced by the concrete exceeded 400 °C, both the fracture toughness and fracture energy decreased with the increasing temperature. In this study, the fracture toughness and fracture energy of the bi-material specimens first increased and subsequently decreased with the increaseing roughness, and the maximum fracture toughness and fracture energy values were obtained when the interface roughness was 3 mm. The study results would provide a reference for the surface treatment when ECC was used in the strenthening of fire damaged concrete.

Quasi-brittle fracture analysis of large and small wedge splitting concrete specimens with size from 150 mm to 2 m and aggregates from 10 to 100 mm

The wedge splitting (WS) test geometry, suitable for testing large concrete specimens up to a few meters in size because of the self-weight supports, will find more applications if a closed-form model is available. This study presented a linearized boundary effect model (BEM) on quasi-brittle fracture of large WS mass concrete specimens containing aggregates up to 100 mm. Two separate small WS specimens containing aggregates around 10 mm and 20 mm were also analyzed to show the versatility of the model and the necessity to include the aggregate size in modelling. Modelling concrete as a large particle composite led to a simple closed-form solution, and both the fracture toughness KIC and tensile strength ft for the formation of the crack/notch-tip fracture process zone (FPZ) were determined from the maximum splitting force Pmax-h of WS specimens. The material properties KIC and ft from the current WS model were also confirmed by three-point-bending (3-p-b) tests of the same concrete. Comparison between the linear BEM and the well-known size effect law (SEL) was provided for purposes of experimental data analysis and mathematical fitting principles.

Influence of different stirring speed on fracture properties of wedge splitting specimens

Because the stirring efficiency and the quality of finished products are impacted by the mixing speed of the concrete mixer, the influence of the mixing speed at the same process condition on the mechanical properties of concrete properties should be studied. The test adopts the wedge splitting process, and it indicates that the compressive strength of the concrete reaches its maximum at the stirring speed of 1.4 m/s, and the peak value exists around 1.4 m/s. Furthermore, there is a continuous reduction of the concrete strength at the speed extending. In accordance with the fracture test data, the ratio of crack load/limit load decreases continuously with the increase of mixing speed. The concrete is not liable to crack at a relatively low mixing speed and is unstable while with a significantly high mixing speed. The stirring model is established and the numerical simulation is carried out. Three pressure fields at different rotating speeds are simulated, which directly reflect the pressure distribution and value at different rotating speeds.

Initial cracking strength and initial fracture toughness from three‐point bending and wedge splitting concrete specimens

AbstractIn this study, a concept of initial cracking strength of concrete was elaborated. A fracture model and associated methods for determining independent initial cracking strength and initial fracture toughness by the three‐point bending (3‐p‐b) and wedge splitting (WS) concrete specimens are presented. The initial cracking strength and initial fracture toughness can be simultaneously determined using a curve fitting method from the proposed fracture model. All of the initial fracture curves were obtained by the determined concrete materials. The initial loads of the 3‐p‐b and WS specimens were predicted on the basis of the curves with ±15% ranges. Furthermore, the initial cracking strength and the initial fracture toughness of concretes were directly determined by analytical functions. The determined values within ±15% ranges cover most of the initial loads of the 3‐p‐b and WS specimens.

Fracture Properties of Concrete in Dry Environments with Different Curing Temperatures

This paper investigated the fracture properties of concrete in dry environments with different curing temperatures (5, 20, 40, and 60 °C). For each curing condition, the key fracture parameters of concrete were tested using wedge splitting specimens at five different ages (3, 7, 14, 28, and 60 d). The results show that in dry environments, the effective fracture toughness and fracture energy of concrete exposed to elevated temperatures increased at a relatively high growth rate at an early age. Nevertheless, the growth speed of effective fracture toughness and fracture energy decreased more quickly at elevated temperatures in the later stages. As a result, the concrete cured at higher temperature exhibited lower ultimate values of fracture parameters, and vice-versa. Namely, a temperature crossover effect was found in the effective fracture toughness and fracture energy of concrete under dry environments. Considering the early growth rate and ultimate values of fracture parameters, the optimum temperature suitable for concrete fracture properties development under dry condition was around 40 °C.

Determining the tensile strength of concrete for wedge splitting specimens based on the fracture extreme theory

Two theoretical methods were developed to determine the tensile strength ft of concrete using wedge splitting specimens based on nil-stress intensity factor (nil-SIF) criterion and force equilibrium theory, respectively. With the fracture extreme theory (FET), only experimental peak load of one specimen was needed and did not require the measurement of critical crack mouth opening displacement CMODc for calculation. The existing experimental data were utilized to verify the two methods. Results show that the values of ft were insensitive to the initial notched ratios a0/D and agreed well with the experiment data. The influence of the size of the specimen was analyzed, ft was affected by the specimen size when the nil-SIF criterion was adopted. Nevertheless, ft calculated by the force equilibrium theory appeared to be kept a constant when the height of the specimen varied from 200 mm to 600 mm. Finally, the critical effective crack length ac and critical crack tip opening displacement CTODc were obtained and compared using the two methods.

Determination of fracture parameter and prediction of structural fracture using various concrete specimen types

Concrete tensile strength and fracture toughness are two important material constants that should be independent of specimen size, geometry or type. In this study, on the basis of the improved boundary effect model, independent tensile strength and fracture toughness of concrete, which are unaffected by the specimen type, were determined by using various specimens, e.g., three-point-bending and wedge-splitting specimens. The concrete structural failure was fully predicted using independent material constants, and the constructed safe design diagrams with upper and lower limits (±15%) can cover all experimental results. The theoretical minimum size of the concrete meeting the linear elastic fracture mechanics were quantified, and the peak loads of concrete specimens were predicted using various average fictitious crack growth length.

Maturity model for fracture properties of concrete considering coupling effect of curing temperature and humidity

In this study, a new maturity model that considers the coupling effect of curing temperature and humidity is proposed. An innovative humidity rate function was introduced into the model to evaluate the influence of humidity on the fracture properties of concrete, which makes the maturity method more robust. Thereafter, fracture tests were performed on wedge-splitting specimens exposed to different constant and varying temperature and humidity to calibrate the parameters and validate the established model. The results indicated that the new model can accurately predict the fracture properties of concrete exposed to any curing condition with an error of less than 6%. In addition, although the temperature inversion phenomenon in the fracture properties of concrete was observed at all the humidity levels investigated, at low humidity, the phenomenon was not as pronounced as that at high humidity. Furthermore, the threshold age for the temperature crossover effect decreased as the curing humidity decreased.

Determining tensile strength of concrete based on experimental loads in fracture test

A theoretical method for determining the tensile strength ft of concrete was proposed based on the experimental loads in fracture tests. Using the fracture extreme method, only the initial cracking load Pini and the peak load Pmax were required to be measured. The experimental data from three-point bending notched beams and wedge splitting specimens were used to calculate ft with different initial crack length versus specimen depth ratios α0 and specimen dimensions. The results show that the ft obtained by the proposed method agree with the test values, and the method is insensitive to α0 and specimen dimensions.

Elevated temperature inversion phenomenon in fracture properties of concrete and its application to maturity model

The effect of elevated curing temperature on the fracture properties of concrete was investigated. Fracture experiments were carried out on wedge splitting specimens exposed to temperatures ranging from 5 to 60 °C. A new maturity model was then proposed for describing the combined influence of the temperature and aging on the fracture properties of concrete. The results show that the fracture energy of concrete subjected to high temperature is high at early ages; however, the effect of curing temperature on the fracture energy was reversed after 14 days, with greater later-age fracture energy corresponding to lower temperature and vice-versa. The effective fracture toughness of concrete also exhibited the same temperature inversion phenomenon. The optimum curing temperature was found to be approximately 40 °C for development of concrete fracture properties. The value of activation energy changed with fracture properties, temperature ranges and the development stage of a given fracture property. The proposed model accurately predicts the fracture properties of concrete and significantly simplifies the calculation process of the maturity index relative to the existing maturity method.

Effect of curing humidity on the fracture properties of concrete

This study investigates the influence of curing humidity on concrete fracture properties. Concrete fracture energy, effective fracture toughness, and characteristic length were tested at five different ages using wedge splitting specimens exposed to four different relative humidity values (30%, 50%, 70% and 98%). Four environmental chambers capable of automatically controlling temperature and relative humidity were constructed. A data averaging method for companion specimens was introduced. The test results show that at the early age, fracture energy, effective fracture toughness, and characteristic length remained almost constant during drying. As time elapsed, however, the influence of curing humidity on the concrete fracture properties became increasingly pronounced; larger values of fracture energy, effective fracture toughness, and characteristic length were measured by concrete specimens kept at high humidity. The concrete fracture mode also gradually changed from ductile to brittle with curing humidity decreasing and hydration time elapsing.

Wedge-splitting tests for tensile strength and fracture toughness of concrete

This study presents a new approach for determination of tensile strength ft and fracture toughness KIC of concrete using wedge-splitting (WS) specimens as neither property can be easily measured because of the heterogeneous material structures. The Boundary Effect Model (BEM) was extended to WS geometry and relevant equations were derived so that more concrete properties can be determined by the load and displacement measurements. The new model considers the influence of coarse aggregate structures on concrete fracture by linking the fictitious crack growth Δafic at peak load Pmax to the maximum aggregate size dmax. The random aggregate distribution and inevitable variations in Δafic and Pmax are considered by a discrete number β. WS results of concrete specimens with dmax from 10mm to 25mm and specimen size W from 150mm to 1200mm reported in literature are analyzed for a range of β from around 0.2 to 2.0. It is found that on average β≈1 is a good approximation for common WS specimens with W/dmax<20. Then reliable ft and KIC, which is the asymptotic limit for a large structure with a long crack, can be obtained, regardless whether WS specimens are geometrically similar, or have the same size but different notch lengths. Finally, it is concluded that quasi-brittle fracture of concrete is fully predicted if both KIC and ft are known.

An experimental study of the fracture behavior of particulate reinforced composites using digital image correlation

AbstractThis study aims to investigate the fracture behavior of particulate reinforced composites to determine resistance properties. It is important to evaluate the crack propagation characteristics of the materials under various temperatures and strain rates. The tests using a wedge splitting specimen are conducted under temperatures ranging from 60 °C to −60 °C and strain rates of 5 and 50 mm/min. KI is calculated using an equation suggested by ASTM (American society for testing and materials) E399. The splitting loads and crack resistance for crack extension at various temperatures are estimated. Also, the crack tip opening angle is calculated from the crack tip opening displacement and crack extension measured via the optical method. The crack tip opening angle consists of an unstable and stable region by which the critical crack tip opening angle can be determined. In addition, digital image correlation is used to calculate the strain fields around the crack tip as well as the whole specimen.

Size and boundary effects on notch tensile strength and fracture properties of PMMA bone cement

Poly (methyl methacrylate) bone cement is widely used for anchoring total joint replacements. In such an application, tensile and fracture properties of the cement are particularly important as they have strong implications for the stability of the implant in the bone bed and, hence, its life. Being a quasi-brittle material, it is to be expected that any mechanical property of bone cement would be size-dependent. This issue has not been addressed in the literature. In the present work, tests using modified wedge-splitting specimens were carried out to determine the size and boundary effects on the notch tensile strength (σNT) and on two fracture parameters, namely, specific fracture energy (Gf), and characteristic length based on the notch tensile strength (lchspl) of a widely used approved brand, Palacos® R. Size and boundary effects, of varying magnitudes, on each of these properties were found. For example, 1) for geometrically-similar specimens (specimen height (W) was varied from 15 mm to 36 mm but ratio of length of the starter crack (a) to W was fixed), the general trend was marked decrease in σNT with increase in W for a given a/W; and 2) for identical-sized specimens (W was fixed but a/W was varied from 0.06 to 0.81), the general trend was moderate decrease in Gf with increase in a/W for a given W. A validated boundary effect model presented in the literature and results from some of the specimens used in the present experimental work were used to compute the size-independent specific fracture energy and transition ligament length of the cement brand to be 2096 N m−1 and 4 mm, respectively.

Equivalent maturity for ambient temperature effect on fracture parameters of site-casting dam concrete

This paper presents the influence of ambient temperature on the fracture properties of site-casting (or cast in-situ in other words) dam concrete. By virtue of the mixing tower system in the site of a super-high arch dam, the wedge splitting specimens with the maximum aggregate size of 150mm were poured in summer and in winter seasons, respectively. The variations of fracture parameters obtained by experiment with different age were revealed, and the results show that the dam concretes poured in summer and in winter seasons show much difference in fracture characteristics at the age of 28days, however, with the increase of age, the difference in fracture behavior decreased, and was almost zero at the age of 180days. On top of that, by introducing the concept of equivalent maturity, the relationships between fracture parameters of site-casting dam concrete and equivalent maturity were obtained.

Experimental validation and fracture properties analysis on wedge splitting concrete specimens with different initial seam-height ratios

In order to study the fracture properties of non-standard concrete wedge splitting tensile specimens with different initial seam-height ratios and the impact on concrete fracture toughness based on aggregates cohesive force, five groups non-standard concrete wedge splitting specimens with different initial seam height ratios as 0.2, 0.25, 0.3, 0.35 and 0.4 were designed and constructed for the experiment. Based on double-K fracture model and fictitious crack model, a new fracture toughness calculation model for non-standard concrete wedge splitting tensile specimen with different initial seam height ratios was built. The results showed that: the value of initial cracking load Pini and unstable fracture load Pun decreased gradually with the increase of specimens’ initial seam height ratio. With the increase of seam height ratios, the value of the critical effective crack length ac showed a linearly increasing trend, but the subcritical crack propagation length Δac showed a decrease trend. The unstable fracture toughness KunIC and the cohesive toughness KcIC were affected by seam height ratios, decreased with initial seam height ratios increasing, and the initial fracture toughness KiniIC was independent of it that could be treated as material constant. When calculating the initial fracture toughness KiniIC, the model in this paper could eliminate the errors caused at the moment of determining initial cracking load Pini, which proved its advantage. For the specimens with smaller initial seam height ratio, its fracture features was influenced by maximum tension stress and showed some shearing failure.