Microcracks In Concrete Research Articles

Acoustic Emission During Flexural Deformation of Reinforced Concrete Under Incremental Cyclic Loading

Abstract Acoustic emissions (AE) released during the flexure tests of reinforced concrete (RC) beams under incremental cyclic loading conditions in the laboratory were subjected to improved b-value (Ib-value) analysis. Ib-value has been reported to be useful to detect, track, and investigate the various stages of micro-cracking and macro-crack damage in concrete. Further, Ib-value has been found to be free from the influence of monitoring conditions. We observed in the present study that the Ib-value began to decrease with the beginning of the formation of new micro-cracks, and then stabilized (∼1.0) and finally dropped down sharply to a very low value marking the onset of macro-crack damage and its growth in concrete. Additionally, when the Ib-value decreased, AE with steep rise angle (RA) values were noticed with the formation and growth of micro-cracks in concrete. On the contrary, the average frequency (AF) of AE events was found to decrease during the micro- and macro-cracking stages of the test beams. These experimental observations of AE-based Ib-value combined with RA and AF were useful for the detailed study of flexural deformation and the failure of concrete at various stages in the laboratory and, perhaps, in situ conditions as well.

Use of glass and nylon fibers in concrete for controlling early age micro cracking in bridge decks

One of the major defects in concrete bridge decks are the formation of early age micro cracking which ultimately reduces its durability. The inclusion of fibers in concrete slabs has showed satisfactory performance with improved mechanical properties and reduced early age micro cracking (EAMC). The relative high compressive strength (CS), achieved by using high cement content, is the main cause of early age micro cracking in concrete. The low tensile strength (TS) also results in less resistance to early age cracking. Other factors causing EAMC include temperature rise, water content and shrinkage strain. The EAMC can be reduced by increasing CS and TS with use of economical materials other than cement. In this work, mechanical properties of glass fiber reinforced concrete (GFRC) and nylon fiber reinforced concrete (NFRC) are determined for the application of concrete bridge decks. The effectiveness of glass fibers (GF) and nylon fibers (NF) in concrete will be checked by comparing mechanical properties of GFRC and NFRC with that of plain concrete (PC). The mix design ratio of PC is 1:3.33:1.67 (cement:sand:aggregate) with a water cement ratio of 0.71. The GF and NF having a length of 50mm and a fiber content of 5%, by mass of cement, are added in the concrete mixture for the production of GFRC and NFRC. Standard specimens are tested for determining compressive, splitting-tensile and flexural strengths of PC, GFRC and NFRC. Pre-crack energy absorption and toughness indices are also studied for different types of loadings i.e. compressive, splitting-tensile and flexural loadings. There is a decrease in compressive strength and an increase in splitting-tensile and flexural strengths of GFRC and NFRC compared to that of PC. Even though pre-crack energy absorption of GFRC and NFRC is less but their toughness indices are more than that of PC. Thus, GFRC and NFRC are suitable for reducing EAMC in bridge decks.

Influence of technological and environmental factors on the behaviour of the reinforcement anchorage zone of prestressed concrete sleepers

The article analyses the influence of technological factors on variations in the stress and strain state during the production of prestressed concrete sleepers. The paper focuses on technological damage to the anchorage zone and its influence on concrete and the pretensioned reinforcement of the last prestressed concrete sleepers located by abutments. The article presents the calculation principles of stresses caused by thermal and humidity deformations, compares theoretical and experimental results and specifies the nature of damages. The paper deals with concrete structure and an interface between reinforcement and concrete of new and three years old prestressed concrete sleepers, explores the results of laboratory tests on deteriorating the ends of new prestressed concrete sleepers under freezing and thawing cycles, looks at changes in concrete structure and the interface between reinforcement and concrete at the ends and in the middle of prestressed concrete sleepers applying a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). Secondary ettringite formation (DEF) in concrete micro-cracks and at the interface between reinforcement and concrete and elemental composition of the formed crystals have been determined. Significant difference in concrete structure and new solid phases at the ends of used and new sleepers has been defined. Concrete structure in the middle of the three years old railway sleeper has remained almost unchanged. The obtained results show that technological factors are affecting the deterioration nature of the ends of prestressed concrete sleepers.

Effects of alkali–aggregate reaction on concrete and structures

The effects of alkali–aggregate reaction (AAR) on concrete and structures start at the microstructural level, that is, at molecular and microscopic levels, such as the formation of reaction products in concrete and associated microcracking, which ultimately leads to macro-level effects such as visual cracking, differential movements, displacement and so on. The consequences of these effects are also wide ranging, and depend on the severity of the reaction, the configuration and functions of the affected element and the exposure conditions. The effects of AAR on concrete can range from minor cracking in non-load-bearing elements in benign environments, which may not have any significant consequence, to severe cracking in sensitive structural elements, particularly those exposed to aggressive environments, which could have serious detrimental effects on the mechanical and durability properties of concrete. This paper provides an overview of the changes that occur in the mechanical and durability properties of AAR-affected concrete elements.

Detection and localization of microcracking in polymer concrete using coda wave interferometry (CWI) at resonance

Nonlinear resonance and Coda Wave Interferometry (CWI) proved their potential to detect the evolution of a structure, namely, in the case of damage. Nevertheless, these techniques do not allow localizing defects in a structure. CWI may allow this localization through heavy statistical algorithms. In this case, the main problem remains the real time monitoring on one hand. On the other hand, in the case of highly scattering media, the amplitude of the acoustic pulse may cause the Coda signal to be affected by conditioning/relaxation effects. Moreover, Coda signal is very sensitive to experimental conditions, namely, to temperature. In order to overcome these inconveniences, we propose a method based on a comparative study of Coda signal contents in rest state and under a weak (linear) vibration. In this communication, we report the results obtained on a polymer concrete specimen excited under different resonance modes and for different dispositions of the sample which defect is not isotropic on which we apply simultaneously through transmission experiments in order to monitor changes in the coda signal.

A meso-scale approach to modeling thermal cracking of concrete induced by water-cooling pipes

Cooling by the flow of water through an embedded cooling pipe has become a common and effective artificial thermal control measure for massive concrete structures. However, an extreme thermal gradient induces significant thermal stress, resulting in thermal cracking. Using a mesoscopic finite-element (FE) mesh, three-phase composites of concrete namely aggregate, mortar matrix and interfacial transition zone (ITZ) are modeled. An equivalent probabilistic model is presented for failure study of concrete by assuming that the material properties conform to the Weibull distribution law. Meanwhile, the correlation coefficient introduced by the statistical method is incorporated into the Weibull distribution formula. Subsequently, a series of numerical analyses are used for investigating the influence of the correlation coefficient on tensile strength and the failure process of concrete based on the equivalent probabilistic model. Finally, as an engineering application, damage and failure behavior of concrete cracks induced by a water-cooling pipe are analyzed in-depth by the presented model. Results show that the random distribution of concrete mechanical parameters and the temperature gradient near water-cooling pipe have a significant influence on the pattern and failure progress of temperature-induced micro-cracking in concrete.

Effect of materials and age on the coefficient of thermal expansion of concrete paving mixture

This study examined the effect of the materials and age on the coefficient of thermal expansion (CTE) of concrete paving mixtures at 28 and 120 days. Concrete specimens were prepared in the laboratory by varying the mix design variables with different types of aggregate to produce different mixes using Portland cement. The microstructures of the concrete mixtures were also observed by scanning electron microscopy to determine the relationship between the volume change in concrete and the formation of microcracks. Statistical analyses of the experimental data suggested that the CTEs measured at 120 days are significantly lower than those measured at 28 days. An analysis of variance indicated that the mixture with granite results in significantly higher CTE reduction with time than the mixture with dolomite. The larger volume change with a higher CTE may be due to the formation of microcracks in concrete.

Thermo-mechanical assessment of concrete microcracking damage due to early-age temperature rise

The pursuit of high early-age strength concrete has led to mixtures with higher heat of hydration rates at early ages which produces higher temperatures and an overall increased risk of cracking. This study uses a two-phase micromechanical model to compute thermal stresses based on both coefficient of thermal expansion (CTE) and elastic Young’s modulus (E) mismatches between aggregates and the cementitious matrix. Concrete specimens were prepared using four types of coarse aggregates (different CTE and E), and subjected to temperature cycles to generate thermal cracking. Fluorescence microscopy, compressive strength, dynamic elastic Young’s modulus, and electrical resistivity were used to characterize the effect of this induced thermal cracking. Experimental results were in agreement with the two-phase model and it was concluded that the interaction pressure (P) between phases could be used to estimate the impact on the mechanical and transportation properties of a temperature gradient at early age.

Influence of flexural loading on chloride ingress in concrete subjected to cyclic drying-wetting condition

Chloride ingress implies a complex interaction between physical and chemical process, in which heat, moisture and chloride ions transport through concrete cover. Meanwhile, reinforced concrete structure itself undergoes evolution due to variation in temperature, relative humidity and creep effects, which can potentially change the deformation and trigger some micro-cracks in concrete. In addition, all of these process show time-dependent performance with complex interaction between structures and environments. In the present work, a time-dependent behavior of chloride transport in reinforced concrete beam subjected to flexural load is proposed based on the well-known section fiber model. The strain state varies because of stress redistribution caused by the interaction between environment and structure, mainly dominated by thermal stresses and shrinkage stress and creep. Finally, in order to clear the influence of strain state on the chloride diffusivity, experiment test were carried out and a power function used to describe this influence is proposed.

Numerical modelling of the concrete/rebar bond

The simulation of the behaviour of the rebar–concrete bond is of primary importance for an accurate description of cracking processes in reinforced concrete structures and an improved prediction of their durability. In this paper, the methodology used to identify the mechanical behaviour of a rebar-concrete bond in the case of a particular steel reinforcement is first mentioned. The methodology consists in simulating the probabilistic mechanical behaviour of RC tie-beams (170×10×10cm), subjected to tension, using a probabilistic approach for the mechanical behaviour of the concrete and a deterministic model for the concrete/rebar bond. The tie-beams are reinforced by a flat steel rebar with a rectangular cross section (25×3.5mm).This approach at macro-scale level is compared to another one at micro-scale level in which notches/indentations of the flat steel rebar are explicitly taken into account in the simulation, the bond behaviour being considered through the local concrete cracking around the rebar.The following conclusions can be made from this study:•The modelling of the concrete/rebar bond by an interface element having damage behaviour is relevant if the modelling of concrete cracking is physically and quantitatively relevant.•The concrete/rebar bond behaviour can mainly be considered as the consequence of concrete micro-cracking around the ribbed rebar (it means due to the presence of notches/indentations).•This micro-cracking concerns a zone with a thickness similar to the rebar one.

Field Verification of Low-pressure Silane Injection for the Inspection of Concrete Microcracking

Field Verification of Low-pressure Silane Injection for the Inspection of Concrete Microcracking

Acoustic emission monitoring of early corrosion in prestressed concrete piles

SUMMARY Prestressed concrete (PC) piles in marine environments are vulnerable to corrosion damage especially at their tidal zone because of the concurrent exposure of steel reinforcement to moisture, chlorides, and oxygen. Early detection of corrosion in prestressed strands is desirable to enable the effective planning and prioritizing of maintenance operations. This paper reports on experiments aimed at testing the hypothesis that acoustic emission (AE) monitoring is suitable to recognize and classify early corrosion damage in PC piles exposed to saltwater. In fact, AE is sensitive to the formation and growth of microcracks in both steel and concrete, which may develop upon depassivation of reinforcement and buildup of corrosion by-products. In addition, the influence of AE signals produced through flexural cracks in the concrete is of less concern for PC piles as such cracks are seldom produced under service loads. Five specimens representative of full-scale portions of PC piles were designed to accelerate corrosion and subjected for 1 year to wet/dry cycles in saltwater simulating tidal action. The specimens were monitored continuously with AE sensors. Benchmark electrochemical measurements were routinely performed to understand when depassivation of the steel strands occurred. Visual evidence of early corrosion was collected from strands removed from two decommissioned specimens. It is shown that AE intensity analysis is more effective than AE signal strength and cumulative signal strength analysis in recognizing corrosion damage. Intensity analysis-based criteria for the assessment of early corrosion damage are proposed, complementing previously developed criteria for more severe corrosion damage in steel strands. Copyright © 2014 John Wiley & Sons, Ltd. Received 22 May 2014; Revised 15 September 2014; Accepted 2 November 2014

Application of Digital Speckle Correlation Method in Observation of Conrete Micro-Crack Damage

The splitting test for concrete specimens in four different dimensions were carried out in order to build the relative even micro-crack damage fields. The real-time whole-field displacements of concrete surface were measured omni-directionally by DSCM(Digital Speckle Correlation Method). Through analyzing the obtained experimental data, two kinds of micro-crack damage parameter are established :one is damage area based on the MCOD (Micro-Crack Opening Displacement) and another is damage variable based on the weighting-damping elastic modulus, which provides a new index and method for characterization of concrete micro-crack damage.

Mesoscopic characterization and modeling of microcracking in cementitious materials by the extended finite element method

This study develops a mesoscopic framework and methodology for the modeling of microcracks in concrete. A new algorithm is first proposed for the generation of random concrete meso-structure including microcracks and then coupled with the extended finite element method to simulate the heterogeneities and discontinuities present in the meso-structure of concrete. The proposed procedure is verified and exemplified by a series of numerical simulations. The simulation results show that microcracks can exert considerable impact on the fracture performance of concrete. More broadly, this work provides valuable insight into the initiation and propagation mechanism of microcracks in concrete and helps to foster a better understanding of the micro-mechanical behavior of cementitious materials.

FE analysis of RC structures using DSC model with yield surfaces for tension and compression

The nonlinear finite element method with eight noded isoparametric quadrilateral element for concrete and two noded element for reinforcement is used for the prediction of the behavior of reinforcement concrete structures. The disturbed state concept (DSC) including the hierarchical single surface (HISS) plasticity model with associated flow rule with modifications is used to characterize the constitutive behavior of concrete both in compression and in tension which is named DSC/HISS-CT. The HISS model is applied to shows the plastic behavior of concrete, and DSC for microcracking, fracture and softening simulations of concrete. It should be noted that the DSC expresses the behavior of a material element as a mixture of two interacting components and can include both softening and stiffening, while the classical damage approach assumes that cracks (damage) induced in a material treated acts as a void, with no strength. The DSC/HISS-CT is a unified model with different mechanism, which expresses the observed behavior in terms of interacting behavior of components; thus the mechanism in the DSC is much different than that of the damage model, which is based on physical cracks which has no strength and interaction with the undamaged part. This is the first time the DSC/HISS-CT model, with the capacity to account for both compression and tension yields, is applied for concrete materials. The DSC model allows also for the characterization of non-associative behavior through the use of disturbance. Elastic perfectly plastic behavior is assumed for modeling of steel reinforcement. The DSC model is validated at two levels: (1) specimen and (2) practical boundary value problem. For the specimen level, the predictions are obtained by the integration of the incremental constitutive relations. The FE procedure with DSC/HISS-CT model is used to obtain predictions for practical boundary value problems. Based on the comparisons between DSC/HISS-CT predictions, test data and ANSYS software predictions, it is found that the model provides highly satisfactory predictions. The model allows computation of microcracking during deformation leading to the fracture and failure; in the model, the critical disturbance, Dc, identifies fracture and failure.

Microcracking and Fracture Process in Cement Mortar and Concrete: A Comparative Study Using Acoustic Emission Technique

This article reports the acoustic emission (AE) study of precursory micro-cracking activity and fracture behaviour of quasi-brittle materials such as concrete and cement mortar. In the present study, notched three-point bend specimens (TPB) were tested under crack mouth opening displacement (CMOD) control at a rate of 0.0004 mm/sec and the accompanying AE were recorded using a 8 channel AE monitoring system. The various AE statistical parameters including AE event rate $$ \left( {\frac{dn }{dt }} \right) $$ , AE energy release rate $$ \left( {\frac{dE }{dt }} \right) $$ , amplitude distribution for computing the AE based b-value, cumulative energy (ΣE) and ring down count (RDC) were used for the analysis. The results show that the micro-cracks initiated and grew at an early stage in mortar in the pre peak regime. While in the case of concrete, the micro-crack growth occurred during the peak load regime. However, both concrete and mortar showed three distinct stages of micro-cracking activity, namely initiation, stable growth and nucleation prior to the final failure. The AE statistical behavior of each individual stage is dependent on the number and size distribution of micro-cracks. The results obtained in the laboratory are useful to understand the various stages of micro-cracking activity during the fracture process in quasi-brittle materials such as concrete & mortar and extend them for field applications.

Evaluation on micro-cracks development and damage of concrete under axial compressive load

Laws of changing of microstructure and development of micro-cracks in concrete are revealed at macroscopical level by determination of longitudinal residual strains and calculation of specific crack area. Two opposite mechanisms of consolidation and cracking to the microstructure changing of concrete under axial compressive load are also revealed. Results show that the development of micro-cracks inside the concrete is consistent with the development of deformation. The residual strain and residual specific crack area are larger when the load level is higher after unloaded. There is obvious difference between the development process of micro-crack in concrete with different mix proportion. To the concrete with higher water to binder ratio and lower strength, the intial stress for micro cracks begining to expand and the critical stress of unstable crack propagation are both lower. The influence of uniaxial compressive load on the durability of concrete depends on the relative relations between these two opposite mechanisms.

Effect of various mixing properties of concrete on chloride penetration through cracks

Chloride penetration through cracks can threaten the durability of concrete substantially. Crack depth corresponded with critical crack width from the surface is a crucial factor in view of durability design of concrete structures. It is necessary to deal with chloride penetration through micro-cracks characterised with the features of concrete mixing. The purpose of this study is to identify and quantify the effect of the mixing features of concrete such as size of coarse aggregate, high strength and steel fibre reinforcement on chloride penetration through micro-cracks. In examining the effect of size of coarse aggregate on chloride penetration through cracks, the cracks should not impact on chloride penetration directly, although small size of coarse aggregate can lead to complicated micro-cracks in concrete. On the contrary, chloride should penetrate inwards through micro-cracks concentrated at the surface of coarse aggregates. In examining the effect of high strength concrete on chloride penetration through cracks, high strength concrete has an excellent performance to resist chloride penetration. If cracks are generated in high strength concrete, however, its performance is reduced up to the level of ordinary concrete. Finally, steel fibre reinforcement is effective to decrease chloride penetration through micro-cracks because of significant reduction of crack depth.

Chloride Penetration through Cracks in Concrete with Various Mixing Properties

The purpose of this study is to identify and quantify the effect of the mixing features of concrete such as size of coarse aggregate, high strength and steel fiber reinforcement on chloride penetration through micro-cracks. In examining the effect of size of coarse aggregate on chloride penetration through cracks, the cracks should not impact on chloride penetration directly, although small size of coarse aggregate can lead to complicated micro-cracks in concrete. On the contrary, chloride should easily penetrate through micro-cracks concentrated at the surface of coarse aggregates. In examining the effect of high strength concrete on chloride penetration through cracks, high strength concrete has an excellent performance to resist with chloride penetration. If cracks are generated in high strength concrete, however, its performance is reduced up to the level of ordinary concrete.

Microcrack Evolution and Stress-Strain Relationship of Saturated Concrete

The strength and stress-strain relationship of saturate concrete is investigated in this paper. The influence of moisture to concrete strength is assumed to be related to cement surface energy reduction by water. The initial elastic modulus of concrete is obtained by considering the deformation of both pore and microcracks in concrete. The stress-strain relationship is achieved with damage mechanics by comparing the damage evolution rules between dry and saturated concrete under external loading. The comparison between experiments and the results by the model proposed in this paper indicates a favorable agreement.