Cracking Of Cement-based Materials Research Articles

DEM simulation of crack evolution in cement-based materials under inclined shear test

The study aimed to investigate the initiation and propagation of cracks in cement-based materials subjected to shear stresses via discrete element method (DEM) simulations. It carried out numerical simulations and calibration of confinement stress-dependent bond parameters in inclined shear tests (IST), coupled with acoustic emission (AE) technique, on cement-based samples. Without modification, the default bonded models (DBM) in existing DEM software cannot consider the effect of confinement, which is critical for the strength and crack behavior of cemented materials. The study put forth a set of linear equations for determining confinement-dependent bond parameters, which are related to shear angle and confinement stress. The DEM simulation showed that the peak shear stresses were 6.26, 17.78 and 24.43 MPa at shear angles of 80°, 70° and, 65° respectively and corresponds to those results obtained from the tests. The broken bonds initiation (crack initiation) position was located around the center of the sample and subsequent crack propagation behavior was affected by the shear angle which agrees with the experimental results of AE evolution. Broken bonds proved to be a useful tool in studying micro-crack behavior. The proposed model is simple and yields results close to the test.

Evolution of temperature, humidity and deformation in early-age cement-based materials based on a multi-field model

Early-age cement-based materials undergo significant physical and chemical changes and have a complex mutual relationship among their temperature, humidity and mechanical behavior. As the second part of this study, this paper builds a multi-field coupling equation model taking into account the staged characteristics of the deformation for early-age cement-based materials based on the Hybrid Mixture Theory, and the accuracy of calculation results are verified with the test results obtained in the first part of this study. Moreover, the multi-field coupling model has been adopted to simulate the evolution of the different types of deformation for early-age cement-based materials. The results show that the multi-field coupling model established in this paper and the staged deformation calculation model established for early-age cement-based materials in the first part of this study can be used to better describe the development of their early-age temperature, humidity and deformation. Moreover, thermal strain is the earliest deformation that may cause cracking of cement-based materials, and chemical shrinkage has a certain compensation effect on thermal strain. In addition, the coefficient of thermal expansion (CTE) and the coefficient of stiffness influence (CSI) as well as their effects on thermal and chemical deformations have been analyzed. The results of this study can provide an important reference for analyzing the performance evolution of early-age cement-based materials.

A chemo-damage-transport model for chloride ions diffusion in cement-based materials: Combined effects of sulfate attack and temperature

Corrosion of reinforcement caused by chloride penetration has become the primary factor of concrete durability in the saline soil environment. A chemo-damage-transport model for chloride diffusion is studied, in which the effects of sulfate attack and temperature on chloride diffusivity in concrete are taken into consideration. Specifically, the volume changes of Friedel’s salt and ettringite can cause the expansion and cracking of cement-based materials. Meanwhile, temperature has a significant influence on the reactions rates, mass transfer and ions diffusivity. The model is validated by published experimental results. The main conclusion is that there exists a peak value of the bound chloride ions. Near the exposed surface of the samples, the bound chloride content decreases with the increase of temperature. For the peak corresponding position and inward part, with the temperature rises, the bound chloride content in cement-based materials increases obviously.

Calcium carbonate labeling for the characterization of self-healing cracks in cement-based materials

Biomineralization has been widely used to develop self-healing cement-based materials for infrastructure maintenance. To characterize the crack repair depth of cement-based materials after microbial self-healing, we used two methods to label calcium carbonate, the main product of microbial mineralization. One method involved the adsorption of Cu2+ by microbial calcium carbonate to form Cu2(OH)2CO3, which surrounded the repair products. The other method involved labeling the mineralized products by doping them with Eu3+ to form CaCO3: Eu3+. X-ray diffraction was used to analyze bicarbonate before and after labeling in solution and in the cement-based materials. Fluorescence excitation and emission spectroscopy was used to analyze CaCO3: Eu3+. X-ray computed tomography was used to characterize the depth of the repair products in cement-based materials labeled using the two methods. Both labeling methods indicated a similar repair depth of 3 mm, demonstrating that these methods can be used to evaluate the efficacy of microbial self-healing in cement-based materials.

Influences of different calcium sources on the early age cracks of self-healing cementitious mortar

Self-healing schematic diagram of mortar cracks. • Studied the changes of Ca 2+ concentration and calcification rate in alkali solution. • Early age cracks of mortar could be well repaired by microorganisms. • The maximum average healing depth of early age cracks could reach 4.0 mm. • The self-healing mechanism of mortar cracks was discussed. • Calcium lactate was more suitable as an external calcium source than calcium nitrate. Microbial induced calcium carbonate precipitation (MICP) has become an efficient and environmentally friendly technology for repair of cracks in cement-based materials. In this research, the alkali-tolerant microbial spores was prepared by spray drying, and two kinds of microbial self-healing agents were used to study the mineralization reaction in alkaline environment, then they were used in the self-healing of the early age cracks with a width of 0.2−0.3 mm. The self-healing efficiency of cracks was characterized by the area repair ratio, permeability coefficient and healing depth. Meanwhile, the morphologies and polymorphs of precipitates were analyzed by SEM equipped with an EDS and XRD. The results indicated that the calcification rate using calcium lactate as calcium source was higher than that of calcium nitrate, and had good self-healing efficiency on mortar cracks. However, compared with the specimens adding N-Ca and spores, the specimens adding L-Ca and spores had lower permeability coefficient and deeper healing depth, their average healing depth was 1.9 mm and 4.0 mm, respectively. In addition, the precipitates generated were most at the crack surface, with the increase of the crack depth, the precipitates gradually decreased. XRD analysis showed that the precipitates at the crack mouth were calcite. Finally, the reasons for the different healing depth of the microbial self-healing agents were analyzed and the self-healing mechanism was discussed.

Self-Healing of Later-Age Cracks in Cement-Based Materials by Encapsulation-Based Bacteria

AbstractBacteria-induced calcium carbonate precipitation has been considered as an intelligent and environment friendly technology to repair cracks in cement-based materials. However, because of th...

Investigation of self-healing capability on surface and internal cracks of cement mortar with ion chelator

Ion chelator is a novel crystalline admixture which can promote the self-healing of cracks in cement-based materials. In this paper, the self-healing capability of the surface and internal cracks of mortars with ion chelator of different age were investigated by the change of crack width, water permeability, recovery of compressive strength and X-ray computer tomography test. The results indicated that the surface crack up to 0.4 mm on the mortars with 0.5 wt% ion chelator pre-cracked at different age can be repaired after standard curing for 30 d. According to the water permeability test, the mortars with ion chelator pre-cracked at 3 d and 28 d have excellent self-healing ability for internal cracks, and the relative permeability coefficient of the mortars with 0.5 wt% ion chelator decreases most significantly. The compressive strength recovery test of mortars shows that ion chelator can promote the self-healing of internal cracks within the mortars. After curing for 56 d, the compressive strength recovery ratio of the mortars with 0.5 wt% ion chelator pre-loaded at 28 d reached 99.5%. And the X-ray computer tomography displayed that the self-healing process of cracks in mortars was from outside to inside.

Influence of concrete-related environmental stressors on biomineralizing bacteria used in self-healing concrete

Biogenic calcium carbonate precipitation has been identified as a way to improve durability and remediate cracks in cement-based materials. However, conditions that occur in cement-based materials (e.g., elevated temperature, nutrient depletion, and high pH) might impede microbial-induced calcium carbonate precipitation (MICCP) in these systems. Thus, the effects of heat, nutrient depletion, and pH treatments, all designed to mimic conditions in fresh cement paste, on the ureolytic bacterium Sporosarcina pasteurii were examined; specifically, impacts to bacterial viability, ability to hydrolyze urea, and surface charge were assessed. Viability and urea hydrolysis were most impacted by exposure to extreme temperature (55°C) and extreme pH (13.6), but the impact was greatly reduced with exposure to milder temperature (45°C) and pH (12.9) conditions. The zeta potential of S. pasteurii cells, which was used to approximate surface charge, was minimally affected by all tested conditions; the negative surface charge of S. pasteurii cells suggested that the cells might serve as heterogeneous nucleation sites for calcium carbonate precipitation. This was supported by the observation of substantially higher calcite concentrations in bacterial pastes containing viable cells or cells inactivated by autoclaving as compared to cell-free pastes. These new findings provide important insight as to how the conditions within cement paste could influence the viability of microorganisms and MICCP within cement-based materials. Overall, this study shows the importance of bacteria, whether viable or not, as potential heterogeneous nucleation sites for MICCP in cement-based materials.

Self-healing of drying shrinkage cracks in cement-based materials incorporating reactive MgO

Excessive drying shrinkage is one of the major issues of concern for longevity and reduced strength performance of concrete structures. It can cause the formation of cracks in the concrete. This research aims to improve the autogenous self-healing capacity of traditional Portland cement (PC) systems, adding expansive minerals such as reactive magnesium oxide (MgO) in terms of drying shrinkage crack healing. Two different reactive grades (high ‘N50’and moderately high ‘92–200’) of MgO were added with PC. Cracks were induced in the samples with restraining end prisms through natural drying shrinkage over 28 days after casting. Samples were then cured under water for 28 and 56 days, and self-healing capacity was investigated in terms of mechanical strength recovery, crack sealing efficiency and improvement in durability. Finally, microstructures of the healing materials were investigated using FT-IR, XRD, and SEM-EDX. Overall N50 mixes show higher expansion and drying shrinkage compared to 92–200 mixes. Autogenous self-healing performance of the MgO containing samples were much higher compared to control (only PC) mixes. Cracks up to 500 μm were sealed in most MgO containing samples after 28 days. In the microstructural investigations, highly expansive Mg-rich hydro-carbonate bridges were found along with traditional calcium-based, self-healing compounds (calcite, portlandite, calcium silicate hydrates and ettringite).

Evaluation of self-healing of internal cracks in biomimetic mortar using coda wave interferometry

Calcium carbonate biomineralization is a bio-chemical process in which calcium carbonate precipitation is obtained by leveraging the metabolic activity of microorganisms. Studies have shown that biomineralization can be used to repair surface cracks in cement-based materials. One of the challenges in determining whether biomineralization is a feasible option for internal crack repair pertains to how to monitor and quantify self-healing of internal microcracks. In this study, mortar samples with and without microcracks and microorganisms were cured in different environments until 50days. Coda wave interferometry measurements, a nondestructive method that is very sensitive to small changes in material, were conducted on these samples to evaluate the extent of self-healing during the entire curing period. Compressive strength tests were performed after 7 and 28days of curing. The results indicated that the cracked mortar samples with microorganisms showed significantly higher strength development and higher relative velocity change than samples without microorganisms.

Self-healing of early age cracks in cement-based materials by mineralization of carbonic anhydrase microorganism.

This research investigated the self-healing potential of early age cracks in cement-based materials incorporating the bacteria which can produce carbonic anhydrase. Cement-based materials specimens were pre-cracked at the age of 7, 14, 28, 60 days to study the repair ability influenced by cracking time, the width of cracks were between 0.1 and 1.0 mm to study the healing rate influenced by width of cracks. The experimental results indicated that the bacteria showed excellent repairing ability to small cracks formed at early age of 7 days, cracks below 0.4 mm was almost completely closed. The repair effect reduced with the increasing of cracking age. Cracks width influenced self-healing effectiveness significantly. The transportation of CO2and Ca2+ controlled the self-healing process. The computer simulation analyses revealed the self-healing process and mechanism of microbiologically precipitation induced by bacteria and the depth of precipitated CaCO3 could be predicted base on valid Ca2+.

Nondestructive evaluation of notched cracks in mortars by nonlinear ultrasonic technique

In this paper, a nonlinear ultrasonic technique is used to nondestructively characterise concentrated defects in cement-based materials. Cracks are artificially notched in mortar samples and five different crack widths are used to simulate increased damage of samples. The relative ratio of second harmonic amplitude to the square of fundamental ultrasonic signal amplitude is defined as the damage indicator of the nonlinear ultrasonic technique, which is measured for mortar samples in conjunction with a typical linear nondestructive evaluation parameter – ultrasonic pulse velocity. It is found that both linear and nonlinear damage parameters have a good correlation with the change of crack width, while the nonlinearity parameter shows a better sensitivity to the width increase. In addition, the nonlinearity parameter presents an exponential increase with the crack growth, indicating an accelerating nonlinear ultrasonic response of materials to increased internal damage in the late phase. The results demonstrate that the nonlinear ultrasonic technique based on the second harmonic principle keeps the high sensitivity to the isolated cracks in cement-based materials, similarly to the case of distributed cracks in previous studies. The developed technique could thus be a useful experimental tool for the assessment of concentrated damage of concrete structures.

A review on testing methods for autogenous shrinkage measurement of cement-based materials

Autogenous shrinkage is an important factor for cracking of cement-based materials at early ages. This review summarizes different definitions and testing methods for autogenous shrinkage in literature. Autogenous shrinkage should be defined as the sum of chemical shrinkage and self-desiccation shrinkage. All testing methods in the literature can be divided into two categories: direct and indirect testing methods. Direct testing methods usually measure volumetric or linear dimensional changes. Indirect methods usually measure porosity or change in relative humidity of cement-based materials, which can reflect the change in autogenous shrinkage. Indirect methods need to establish correlations between autogenous shrinkage and porosity or relative humidity.

Effect of CCWC on the Self-Healing Action and the Microstructure of Cement-Based Materials

The performance of Capillary Crystalline Waterproofing Coatings(CCWC) was measured by its impermeability and self-healing ability. At the meantime, the microstructure of mortar treated by CCWC was researched with SEM and XRD. The results showed that Ionic transmission could cause a chemical reaction, generate a great quantity hydrate products to block up the pores and cracks in cement-based materials, endow it with self- healing ability, and increase its durability. With increment of the coverage rate, pores and cracks of cement-based materials can be better healed with ionic transmission.

Semiempirical Electromagnetic Modeling of Crack Detection and Sizing in Cement-Based Materials Using Near-Field Microwave Methods

Detection and characterization of cracks in cement-based materials is an integral part of damage evaluation for health monitoring of civil structures. Microwave signals are able to penetrate inside of dielectric materials (e.g., cement-based materials) and are sensitive to local, physical, geometrical, and dielectric variations in a structure. This makes microwave nondestructive testing and evaluation (NDT&E) techniques suitable for inspection and health monitoring of civil structures. Near-field microwave NDT&E techniques offer the added advantage of providing high spatial resolution, requiring simple hardware that may be portable, low power, fast, real time, and robust. Additionally, these techniques are noncontact and one-sided. Besides the need for robust detection, electromagnetic modeling of a microwave probe response to a crack is also an important issue. Such a model can be used to obtain optimal measurement parameters and serve as the foundation for extracting important crack information such as its width and depth. In this paper, the utility of open-ended rectangular waveguide probes for detecting surface-breaking cracks in cement-based materials is discussed. Subsequently, the development of a semiempirical model capable of simulating the crack response is presented. The model described here translates the magnitude and phase of the reflection coefficient as a function of scanning distance into the complex reflection plane and takes advantage of the common shape of these signals for predicting a similar signal from an unknown crack. Finally, this empirical model is used to estimate crack dimensions from a set of measurements.

Self-healing action of permeable crystalline coating on pores and cracks in cement-based materials

The self-healing action of a permeable crystalline coating on the porous mortar was investigated by two times impermeability test. Moreover, the self-healing mechanism of cement-based materials with the permeable crystalline coating was studied by SEM. The results indicate that the permeable crystalline coating not only seals the pores and cracks in mortar during its curing process, but also heals the permeable pathway caused by first impermeability test or cracks produced by freeze-thaw cycles. Therefore, cement-based materials can be improved by the permeable crystalline coating for the self-healing function. SEM images prove that the self-healing function is realized by generating a great quantity of non-soluble dendritic crystalline within the pores and cracks, which prevents the penetration of water and other liquids.

New Experimental Method for Studying Early-Age Cracking of Cement-Based Materials

New Experimental Method for Studying Early-Age Cracking of Cement-Based Materials