Standard Concrete Specimens Research Articles

Verification of Interaction Between Cl− Erosion and Carbonation in Marine Concrete

Marine concrete frequently experiences performance degradation due to the combined effects of chloride ion (Cl−) erosion and carbonation. While many studies have examined the separate effects of Cl− erosion and carbonation, their combined impact on concrete is still debated. Investigating the interaction mechanisms between Cl− erosion and carbonation is crucial for improving the durability of concrete structures. This study utilizes a method where concrete specimens are immersed in artificial seawater with NaCl concentrations of 5%, 10%, and 15% prior to carbonation, with carbonation depth serving as a key indicator for analyzing the impact of Cl− erosion on carbonation. Both carbonation-treated and standard concrete specimens are immersed in 5% artificial seawater to evaluate the impact of carbonation on chloride erosion, with the free chloride content in the concrete serving as the assessment criterion. Scanning electron microscopy (SEM) is employed to examine the microstructure of the concrete, elucidating the interplay between Cl− erosion and carbonation. This study reveals that (1) Cl− erosion hinders concrete carbonation as NaCl crystals and Friedel’s salt in the pores limit CO2 penetration, with this effect intensifying at higher artificial seawater concentrations; (2) carbonation has a dual impact on Cl− erosion: in fully carbonated areas, carbonation products block pores and restrict Cl− diffusion, while at the interface between carbonated and non-carbonated zones, carbonation depletes Ca(OH)2, reducing Cl− binding capacity, increasing free Cl− content, and promoting Cl− diffusion.

Harnessing Path Optimization to Enhance the Strength of Three-Dimensional (3D) Printed Concrete

The path-dependent strength of three-dimensional printed concrete (3DPC) hinders further engineering application. Printing path optimization is a feasible solution to improve the strength of 3DPC. Here, the mix ratio of 3DPC was studied to print standard concrete specimens with different printing paths using our customized concrete 3D printer, which features fully sealed extrusion and ultrathin nozzles. These paths include crosswise, vertical, arched, and diagonal patterns. Their flexural and compressive strengths were tested. In order to verify the tested results and expose the mechanism of strength enhancement, digital image correlation (DIC) was used to capture the dynamic gradual fracture in the flexural tests. Also, the meso- and microstructures of the 3D-printed concrete specimens were pictured. The results reported here show that arched-path concrete has 30% more flexural strength than others because it makes better use of filament-wise strength. The findings here provide a pathway to improve the strength of 3D-printed concrete by path optimization, boosting 3DPC’s extensive application in civil engineering.

Compressive Behavior of Concrete Subjected to High Rate of Loading Using SHPB

Concrete is the most widely used construction material for strategic structures which are subjected to high rate of loading (impact and blast loading). Hence, the study of the concrete material becomes essential day by day for the structural engineers for the analysis and design of the concrete structures for safety and security purposes. This paper focuses on investigating the dynamic compressive response of the standard concrete (M35) and high-strength concrete (M60) under impact loading by using the newly developed split Hopkinson pressure bar setup (SHPB) apparatus. First, the calibration of SHPB performed at a 4.90[Formula: see text]ms[Formula: see text] impact velocity generates almost similar incident and transmission trapezoidal profile waves with minor reflection showing accuracy. Second, the dynamic compression tests were performed on the standard concrete (SC) and high-strength concrete (HSC) specimens of 29.5[Formula: see text]mm in diameter and [Formula: see text][Formula: see text]mm in length with an aspect ratio of about 1([Formula: see text]) and a comparative study of strain rate effect and quasi-static compressive strength on the material properties has been investigated. The dynamic compression behavior of SC and HSC thus studied under varying strain rates has been presented in terms of the compressive strength, dynamic increase factor, stress–strain relationship, material failure, and fragmentation. Compressive strength increments of 114% and 41% were observed in SC and HSC as the loading condition changed from quasi-static to dynamic. In addition, the HSC described the [Formula: see text]% and [Formula: see text]% higher compressive strength corresponding to 290[Formula: see text]s[Formula: see text] and 131[Formula: see text]s[Formula: see text] strain rate as compared to SC describing that the HSC is more sensitive to the strain rate. The dynamic mechanical properties increased with an increase in the strain rate, and the DIFs were found to vary in the range of 1.42–2.14 and 1.07–1.41 for SC and HSC, respectively. The critical strain and ultimate strain were also found to be sensitive to strain rate and increased with increase in strain rate for both grades of concrete. Moreover, strain rates exert a noteworthy influence on fragmentation, leading to an increase in the percentage weight of fine fragments and a decrease in the percentage weight of large fragments as the strain rate increases. Additionally, there is an overall increase in the quantity of fragments for both SC and HSC specimens under these conditions. The flaky, elongated, angular, and irregular shape fragments were observed but their quantity and size varied significantly with strain rate.

A methodology for parameter identification and calibration of the cohesive element based meso-scale concrete model

Meso-scale analysis techniques have demonstrated superiority in analyzing structural cracking and crack propagation compared to conventional methods. The cohesive element based meso-scale analysis technique has gained widespread acceptance due to its clear analytical concepts and the ability to simulate complex fracture modes. However, the cohesive zone model (CZM) requires numerous parameters for different fractures patterns, and the impact of each cohesive element parameter on the mechanical properties of the meso-scale model remains uncertain. This limitation restricts the applicability of CZM models in concrete structure analysis. Even experienced researchers often rely on trial-and-error methodologies or empirical methods to determine cohesive element parameters. The present study aims to investigate the effects of cohesive element parameters in the CZM concrete model under static axial loading and propose a feasible method for determining these parameters through a parametric analysis. Initially, a Python script was developed to generate random aggregate models, modify polygonal aggregates, and insert cohesive elements. Subsequently, numerical simulations were conducted on two-dimensional standard concrete specimens under axial compression and tension. Based on the results of the parametric analysis, a novel prediction formula and an inverse method for cohesive element parameters were introduced. Additionally, the accuracy of the novel method was validated in accordance with the Chinese Code. The results indicate that the proposed method exhibits exceptional capabilities in predicting initial stiffness, uniaxial strength, and corresponding strain. However, the prediction of the descending segment of the stress–strain curve lacks precision. In summary, the methods presented in this study demonstrate high accuracy, which can provide a reference basis for researchers and promote the application of CZM meso analysis method.

Effect of high temperature on the compressive and flexural performance of fibrous concrete- an experimental investigation

Concrete is a versatile building material and is used for different structures including hazardous occupancy. The discharge of thermal load on reinforced structural elements is rare; but can occur as a result of fire breakout, post- earthquake effect or long-term temperature exposure. The thermal effects on concrete are of primary concern as they determine the integrity of structure after exposure to very high temperatures. From researches so far; the addition of fibres was found to improve the mechanical properties of concrete. So the aim of the present study is to evaluate the changes which the fibres can make in the residual properties of concrete after exposure to elevated temperatures. The study considers two types of synthetic fibres such as polypropylene fibres and glass fibres; added to concrete at various percentages by volume and subjected to different elevated temperatures. For the experimental study, the standard concrete specimens were prepared and exposed to varying temperatures from 100 to 800 °C. Then, the compressive strength and flexural strength of concrete samples were determined as per Indian standards. The parametric studies were carried out on the effect of types of fibres and fibre content for different elevated temperatures. The present study found that the fibrous concrete exhibit higher strength than ordinary concrete at temperatures up to 600 °C, despite the usual decrease in strength with increasing temperature. Moreover, a higher content of fibres in fibrous concrete enabled the concrete to withstand thermal effects even at high temperatures. The rate of decrease in residual strength was also observed to be lower in fibrous concrete compared to ordinary concrete. The study also found that incorporating higher content of glass fibre into concrete improved its residual mechanical properties. These results suggest that fibrous concrete could be a promising material for concrete elements under high-temperature applications.

Evaluation of Hybrid Fiber Multiscale Polymer Composites for Structural Confinement under Cyclic Axial Compressive Loading

Fiber reinforced polymer (FRP) confinement is recognized as the most promising technique for the strengthening and retrofitting of concrete structures. In order to enhance the performance of conventional epoxy-based FRP composites, nano filler modification of the epoxy matrix was implemented in the current study. In particular, the cyclic loading response of standard concrete specimens externally confined by epoxy-based natural and hybrid fiber reinforced polymer systems was investigated. The confinements were realized with sisal fiber reinforced polymer (SFRP) and hybrid sisal basalt fiber reinforced polymer (HSBFRP). Moreover, the effects of multiwalled carbon nanotubes (MWCNT) were also investigated. Three different specimen sets were considered for study: (i) unconfined specimens, (ii) epoxy-based FRP confined specimens and (iii) MWCNT incorporated epoxy-based FRP confined specimens. The specimens were tested in repeated compressive mode in loading-unloading cycles at increasing displacement levels. The test results revealed that FRP wrapping could enhance the mechanical behavior of unconfined columns in terms of strength and ductility. Moreover, it was evident that the mechanical properties of the epoxy matrix were enhanced by MWCNT incorporation. The developed epoxy-based FRP confinement containing MWCNT ensures improvement in axial strength by 71% when compared with unconfined specimens. The epoxy-based FRP confinement, with and without MWCNT, exhibited a high strain redistribution behavior around the concrete core. In comparison to the unconfined specimens, the confinement could increase the sustained axial strain from 0.6 to 1.4% using epoxy-based FRP confinement and to 1.6% with MWCNT incorporated epoxy-based FRP confinement. Further, an empirical model was developed to predict the ultimate axial stress of concrete columns confined externally with FRP jackets. The ultimate compressive strength obtained from the experimental study was compared with the proposed model, and the observed deviation was lower than 1%.

Study on Crack Development of Concrete Lining with Insufficient Lining Thickness Based on CZM Method.

The most common structural defect of a tunnel in the operation period is the cracking of concrete lining. The insufficient thickness of tunnel lining is one of the main reasons for its cracking. This study studied the cracking behavior of standard concrete specimens and the failure behavior of tunnel structures caused by insufficient lining thickness using Cohesive Zone Model (CZM). Firstly, zero-thickness cohesive elements were globally inserted between solid elements of the standard concrete specimen model, and the crack development process of different concrete grades was compared. On this basis, a three-dimensional numerical model of the tunnel in the operation period was established. The mechanism and characteristics of crack propagation under different lining thicknesses were discussed. In addition, the statistics of cracks were made to discuss the development rules of lining cracks quantitatively. The results show that the CZM can reasonably simulate the fracture behavior of concrete. With the increase in concrete strength grade, the number of cohesive damaged elements and crack area increases. The insufficient lining thickness changes the lining stress distribution characteristics, reduces the lining structure’s overall safety, and leads to the cracking of the diseased area more easily. When surrounding rock does not contact the insufficient lining thickness, its influence on the structure is more evident than when surrounding rock fills the entire lining thickness. The number of cohesive damaged elements and the size of the crack area increases significantly.

Experimental and analytical investigation on the age-dependent tensile strength of low-calcium fly ash-based concrete

In this study, the age-dependent splitting and flexural tensile strength have been investigated by incorporating the various percentages of fly ash in the plain concrete mixes. The partial replacement of cement by fly ash was varied from 0 to 60% on an equal weight basis. Standard concrete specimens were cast for measuring splitting and flexural tensile strength at different ages, i.e., 7, 28, 56, 90, 150, and 180 days, for all plain and fly ash concrete mixes. Experimental results show that the fly ash produced a significant effect on the tensile strength of concrete mixes. It has been observed that the fly ash concrete mixes gain considerable tensile strength with respect to age beyond 28 days. In the low-calcium fly ash concrete mixes, the rate of development of tensile strength from 28 to 180 was observed higher in comparison with the plain concrete mixes. The assessment of the existing models for the estimation of age-dependent tensile strength recommended by design codes and researchers with experiments has also been done on various mixes of plain and fly ash concrete. New models to predict the age-dependent splitting and flexural tensile strength of concrete having different percentages of fly ash are proposed. The present experimental and analytical study will be helpful for the designers and practicing engineers for fixing preliminary dimensions of reinforced and prestressed concrete members and mix proportioning of low-calcium fly ash concrete mixes.

考虑混凝土材料非均质特性的近场动力学模型

The classical continuum mechanics and some related numerical methods often fail in analysis of the damage evolution and progressive failure process of concrete materials, due to the difficulties in handling discontinuity and considering the intrinsic microstructure, which is of significance to the damage evolution. A peridynamic concrete model in view of the heterogeneity was established. The digital image processing (DIP) technology was firstly adopted to extract the information of aggregate shape and distribution in a concrete specimen, and then the bondbased peridynamic model reflecting 3 different bonds was applied to analyze the tension and compression failure of standard concrete specimens. The proposed analysis tool was further developed with the FORTRAN language in the framework of ABAQUS. The numerical results are in good agreement with reported experimental data, indicating that the proposed peridynamic model for heterogeneous concrete can simulate the complex failure process of concrete well, and paves a way to further exploration of the failure mechanism of concrete materials.

Modeling Adhesive Anchors in a Discrete Element Framework

In recent years, post-installed anchors are widely used to connect structural members and to fix appliances to load-bearing elements. A bonded anchor typically denotes a threaded bar placed into a borehole filled with adhesive mortar. The high complexity of the problem, owing to the multiple materials and failure mechanisms involved, requires a numerical support for the experimental investigation. A reliable model able to reproduce a system’s short-term behavior is needed before the development of a more complex framework for the subsequent investigation of the lifetime of fasteners subjected to various deterioration processes can commence. The focus of this contribution is the development and validation of such a model for bonded anchors under pure tension load. Compression, modulus, fracture and splitting tests are performed on standard concrete specimens. These serve for the calibration and validation of the concrete constitutive model. The behavior of the adhesive mortar layer is modeled with a stress-slip law, calibrated on a set of confined pull-out tests. The model validation is performed on tests with different configurations comparing load-displacement curves, crack patterns and concrete cone shapes. A model sensitivity analysis and the evaluation of the bond stress and slippage along the anchor complete the study.

Effect of Metakaolin and Slag blended Cement on Corrosion Behaviour of Concrete

The present paper is aimed to investigate the influence of Metakaolin (MK) and Portland slag Cement (PSC) on corrosion behaviour of concrete. For this purpose, Ordinary Portland Cement (OPC) was replaced by 15% MK by weight and readymade available PSC were used. The standard concrete specimens were prepared for both compressive strength and half- cell potential measurement. For the aforesaid experiments, the specimens were cast with varying water to binder ratios (w/b) such as 0.45, 0.5 and 0.55 and exposed to 0%, 3%, 5% and 7.5% of sodium chloride (NaCl) solution. The specimens were tested at wide range of curing ages namely 7, 28, 56, 90 and 180 days. The effects of MK, w/b ratio, age, and NaCl exposure upon concrete were demonstrated in this investigation along with the comparison of results of both MK and PSC concrete were done. It was also observed that concrete with MK shows improved performance as compared to concrete with PSC.

COMPARISON OF PROPAGATION CHARACTERISTICS OF EXCITED STRESS WAVES IN PILE AND ANCHORAGE WITH BOLT SYSTEMS

Dynamic testing methods for pile systems and anchorage-with-bolt systems are similar andoften confused with each other, possibly resulting in incorrect assessment of the quality of the pileand the anchorage systems. The stress wave velocity is one of the most important parameters forevaluating the quality of dynamic tests. In this paper, the stress wave velocities in standardconcrete specimens of the free rock bolt and the different ages were a reference for comparing thestress wave velocities in a pile-and-anchor system. The tests were conducted using the low strainreflected wave method. Results indicate that the wave velocity in a pile is larger than that in thestandard samples, and that the wave velocity in an anchorage-with-bolt system is smaller than thatin a free steel bar, but larger than that in a pile and in standard samples. Wave velocities in the pileand in the standard samples were found to raise as the ages of samples increased. The wavevelocity in the anchorage-with-bolt system initially decreased as specimen age increased, butincreased with increasing specimen age afterward.

Mesoscopic analysis of concrete under excessively high strain rate compression and implications on interpretation of test data

Abstract The strain rate effect on the behaviour of brittle materials like concrete has been a classical topic of interest in the shock and impact engineering community. For concrete under high strain rate compression, a dynamic increase factor (DIF) is commonly used to account for the nominal dynamic strength enhancement for engineering applications. The cause of the experimentally observed DIF on standard concrete specimens has been a subject of securitization in recent years. This paper presents an investigation on the dynamic behaviour of concrete specimens under high strain rate compression with the aid of mesoscale numerical simulation. Beyond a further observation on the so-called lateral inertia confinement effect, special attention is paid to the transient shock wave effect and the propagation of material failure when a specimen is loaded with a strain rate exceeding a theoretical limit for a given specimen size, i.e., in the “excessive” strain rate regime as referred to in this paper. Based on the simulation, it is argued that the validity of many existing test data on the nominal compression DIF for concrete, especially those in the very high strain regime, is rather questionable. The correlation between the externally measured (inferred) strength-strain data and the actual material dynamic response within the specimen is examined. The influence of the material heterogeneity on the DIF is also discussed with quantification.

Study on the Material Mechanical Performance of Large Scale Self-Compacting Rock-Filled Concrete

In order to study the influence of large size aggregate on self-compacting rock-filled concrete, the compactness between self-compacting concrete (SCC) and rocks, as well as the whole workability of self-compacting rock-filled concrete, the experimental test in the present paper was carried out on the 500-ton electro-hydraulic serve testing machine and the sizes of specimens are 450mm×450mm×450mm and 450mm×450mm×900mm. The results show that the exterior damage of self-compacting rock-filled concrete specimens are different from that of standard concrete specimens; the damage sections are flat and rocks are cut through in some areas, which shows mainly bond failure at interfaces between self-compacting concrete (SCC) and rocks. The compactness inside is well and the strength of self-compacting rock-filled concrete can almost reach that of SCC, which provides theoretical basis for the application of self-compacting rock-filled concrete.

Characterization of ASR damage in concrete using nonlinear impact resonance acoustic spectroscopy technique

This research reports on a successful application of the nonlinear impact resonance acoustic spectroscopy (NIRAS) technique for the characterization of progressive damage in standard concrete specimens. Damage in the specimens is introduced, following ASTM C 1293 testing procedures, through the deleterious alkali–silica reaction (ASR), which leads to the formation of a gel-like reaction product, microcracks, and interfacial debonding between cement and aggregate phases. The microcracks and debonded interfaces act to increase the nonlinearity of concrete. The response of the specimen to impact loading is analyzed to obtain a nonlinearity parameter, which is used as a measure of damage. Measurements are performed on concrete prisms undergoing the ASTM C 1293 expansion test; three aggregates with varying reactivity are examined. The results from the expansion test are compared with those from the NIRAS measurements. For potentially reactive aggregate, the NIRAS technique offers a more definitive assessment of the damage state of the specimen and can be used to distinguish marginally reactive aggregates. The NIRAS results not only demonstrate a clear distinction between nonreactive and reactive aggregates using the nonlinearity parameter, but also the capability to quantitatively track ASR-induced damage in concrete, potentially forming the foundation for field assessment and monitoring.

Test Method for Appraising Future Durability of New Concrete Bridge Decks

The nondestructive test procedure for quantifying the future durability of bridge deck concrete is based on the fundamental relationship between ultrasonic pulse velocity (UPV) and the permeability of an elastic medium. An experimental study using standard concrete cylindrical specimens (ASTM C192) documented adequate sensitivity between UPV and permeability. The test procedure uses a parameter directly proportional to increase in-field concrete permeability called paste quality loss (PQL). The PQL is computed from UPV measurements on standard concrete specimens made from field concrete mixture (ASTM C31) and measurements on field concrete. Deck replacement projects on three National Highway System bridges were used as demonstration sites to implement the test procedure. The 56-day PQLs were calculated as 15%, 28%, and 9%, demonstrating a significant variability in the permeability of the three bridge decks. Field permeability tests were also conducted with Figg’s apparatus for comparison purposes. The PQL evaluation from postconstruction measurements is an effective and reliable method for testing the future durability of bridge decks.

Experimental study on the acoustic wave velocity in steel-reinforced mortar under external pull-out load

One of the major safety issues concerning the bond stress of mortar and concrete elements under external loads has been investigated. The relationship between bond stress and acoustic wave velocity has been analyzed for different cement/sand ratios. This study pioneers the combination of pull-out tests on standard concrete specimens with acoustic through-transmission measurements, and lays the groundwork for future on-site inspection of steel-reinforced structures. Changes in acoustic wave velocity were observed as the bond stress exceeded a certain level. As the external pull-out load (transferred to induced bond stress) increases, the acoustic wave velocity decreases. This phenomenon becomes very significant as the pull-out load approaches the maximum bond strength of the specimen. For a one-to-one cement/sand ratio, a mere 5% decrease in velocity ratio is critical to structural safety, since a 1.4 safety-factor can assure the bond condition only up to 71% of the maximum load, compared to 70% of the maximum load in more than 60% of the measurements.

Accelerated strength and testing of concrete using blended cement

Abstract Accelerated strength testing using the boiling water procedure of ASTM C 684 was performed to evaluate this test method for use in the routine quality control of concrete made of local materials with particular emphasis on the use of blended cements, and in the prediction of potential quality and strength of concrete at later ages. Large number of groups of standard concrete specimens are sampled; in each group one cube represented the accelerated strength and is tested at 28.5 h while the other one is normally cured and tested at 28 days. Test results were recorded and statistically evaluated. A computer program was developed to carry out the numerical statistical computations and regression analysis. Correlation between the 28-day compressive strength and the corresponding accelerated strength was established considering the utilization of local materials and practices. The outcome of this study in the form of prediction models confirm that accelerated strength testing could be accepted in lieu of the standard 28-day testing. The conclusions derived may provide experimental evidence in favor of implementing the standard boiling water method for use in relation to quality control and prediction of concrete strength at later ages. Advanced Cement Based Materials 1997, 5, 49–56.

Accelerated strength and testing of concrete in Jordan

Accelerated strength testing using the boiling water procedure of ASTM C 684 was performed to evaluate this test method for use in the routine quality control of concrete made of local materials, and in the prediction of potential quality and strength of concrete at later ages. 190 groups of standard concrete specimens were sampled: in each group two cylinders represented the accelerated strength and were tested at 28.5 h, while the other two were normally cured and tested at 28 days. Test results were recorded and statistically evaluated. A computer program was developed to carry out the numerical statistical computations and regression analysis. Correlations between the 28-day compressive strength and the corresponding accelerated strength were established, taking account of the utilization of local materials and practices. The outcome of this study confirms that accelerated strength testing can do everything the 28-day test can do but 27 days sooner. The conclusions derived provide experimental evidence in favour of implementing the standard boiling water method for use in relation to quality control and prediction of concrete strength at later ages.