In this paper, mechanisms of self-healing in cementitious materials, i.e. autogenous self-healing, self-healing based on mineral admixtures, self-healing based on bacteria and self-healing based on ...adhesive agents, are reviewed. Literature shows that all mechanisms of self-healing are effective, to some extent, under some particular conditions. It reveals that not any particular method of self-healing is the best, but one can be the most suitable for a particular situation. For better application of self-healing concept in engineering practice, favorable situations for self-healing in cementitious materials are summarized. The required environmental conditions for each self-healing mechanism are analyzed. Additional costs for realizing self-healing in concrete structures are also discussed. Based on the aforementioned aspects of self-healing in cementitious materials, perspectives for further research on application of self-healing in engineering practice are proposed.
Display omitted
•For application of self-healing concept, favorable situations for self-healing in cementitious materials are summarized.•The required environmental conditions for each self-healing mechanism are analyzed.•Additional costs for realizing self-healing in concrete structures are discussed.•The studies of self-healing in cementitious materials in-situ are much far from satisfactory.•The repeatability of self-healing of a crack under repeated loads are still unknown.
•The stochastic micromechanical properties of cement paste were predicted.•Results show good agreement with data in the literature.•Specimen with lower w/c ratio has higher and less variable strength ...and elasticity.•A strong size effect exists in the modulus/stress ratio of cement paste.
This work presents a study of stochastic fracture properties of cement paste at the micro length scale based on a combination of X-ray computed tomography (XCT) technique and discrete lattice type fracture model. Thirty virtual specimens consisting of pore, outer hydration products, inner hydration products and anhydrous cement particles were extracted from 3D images obtained through XCT from real cement paste samples. These virtual specimens were subjected to a computational uniaxial tension test to calculate their tensile strengths and elastic moduli. The predicted stochastic strengths were analysed using Weibull statistics, showing that specimens with lower w/c ratio yield higher strength and less variability. The strength-porosity and modulus-porosity relations were investigated based on existing empirical models. It was shown that existing models can predict the properties in the studied porosity range quite accurately, with the exponential model having the highest determination coefficient among all the models for both relations. Finally, by comparing the existing data in the literate, it is found that the smaller cement paste specimens have higher modulus/tensile strength ratio, which indicates that they are able to have more strain at the peak load.
Display omitted
•Crystallisation inhibitor (sodium ferrocyanide) was encapsulated in chitosan complexed calcium alginate hydrogels.•Chitosan-calcium alginate capsules slow leaching of sodium ...ferrocyanide within mortars.•Presence of chitosan is necessary in achieving controlled release of sodium ferrocyanide.•Slow leaching of the inhibitor can prolong its long term effectiveness against salt weathering.•Chitosan-calcium alginate hydrogels can have several applications in the construction industry.
Crystallisation inhibitors, such as sodium ferrocyanide (NaFeCN), are highly effective in mitigating NaCl-induced weathering in lime-based mortars; however, direct addition of NaFeCN in lime-mortars increases its susceptibility to leaching and rapid depletion, thus compromising long-term performance. Here, we present hydrogel-capsules for the controlled-release of NaFeCN within hydraulic mortars for the prolonged prevention of salt weathering. Capsules were prepared by complexing chitosan and calcium-alginate in different ratios containing different concentrations of NaFeCN. The release of NaFeCN from these capsules was measured in (1) simulated lime-mortar solution (2) from mortar specimens incorporated with calcium alginate (CA) and chitosan-calcium-alginate (Cs-CA) capsules using ultraviolet–visible light spectrophotometry and Inductive Coupled Plasma-Optical Emission Spectroscopy. Mortars containing Cs-CA capsules exhibited controlled-release of NaFeCN with four times lower effective diffusion coefficient, compared to incorporating NaFeCN directly in mortar. Conversely, mortar containing CA capsules (without chitosan) released NaFeCN rapidly. Thus, chitosan’s presence in CA is necessary for tuning NaFeCN release and the reason may be attributed to chitosan’s role in reducing CA’s permeability and chitosan’s electrostatic-attraction to ferrocyanide anions, slowing diffusion of the latter. In conclusion, using Cs-CA capsules can control the release of NaFeCN within mortar, providing a steady NaFeCN supply to prolong mortar’s resistance against salt damage.
The application of concrete is rapidly increasing worldwide and therefore the development of sustainable concrete is urgently needed for environmental reasons. As presently about 7% of the total ...anthropogenic atmospheric CO
2 emission is due to cement production, mechanisms that would contribute to a longer service life of concrete structures would make the material not only more durable but also more sustainable. One such mechanism that receives increasing attention in recent years is the ability for self-repair, i.e. the autonomous healing of cracks in concrete. In this study we investigated the potential of bacteria to act as self-healing agent in concrete, i.e. their ability to repair occurring cracks. A specific group of alkali-resistant spore-forming bacteria related to the genus
Bacillus was selected for this purpose. Bacterial spores directly added to the cement paste mixture remained viable for a period up to 4 months. A continuous decrease in pore size diameter during cement stone setting probably limited life span of spores as pore widths decreased below 1
μm, the typical size of
Bacillus spores. However, as bacterial cement stone specimens appeared to produce substantially more crack-plugging minerals than control specimens, the potential application of bacterial spores as self-healing agent appears promising.
Reinforced concrete structures are frequently exposed to aggressive environmental conditions. Most notably, chloride ions from sea water or de-icing salts are potentially harmful since they promote ...corrosion of steel reinforcement. Concrete cover of sufficient quality and depth can ensure protection of the steel reinforcement. However, it is necessary to study the effects of material heterogeneity and cracking on chloride ingress in concrete. This is done herein by proposing a three-dimensional lattice model capable of simulating chloride transport in saturated sound and cracked concrete. Means of computationally determining transport properties of individual phases in heterogeneous concrete (aggregate, mortar, and interface), knowing the concrete composition and its averaged transport properties, are presented and discussed. Based on numerical experimentation and available literature, a relation between the effective diffusion coefficient of cracked lattice elements and the crack width was adopted. The proposed model is coupled with a lattice fracture model to enable simulation of chloride ingress in cracked concrete. The model was validated on data from the literature, showing good agreement with experimental results.
Self-healing cementitious materials containing a microencapsulated healing agent are appealing due to their great application potential in improving the serviceability and durability of concrete ...structures. In this study, poly(phenol-formaldehyde) (PF) microcapsules that aim to provide a self-healing function for cementitious materials were prepared by an in situ polymerization reaction. Size gradation of the synthesized microcapsules was achieved through a series of sieving processes. The shell thickness and the diameter of single microcapsules was accurately measured under environmental scanning electron microscopy (ESEM). The relationship between the physical properties of the synthesized microcapsules and their micromechanical properties were investigated using nanoindentation. The results of the mechanical tests show that, with the increase of the mean size of microcapsules and the decrease of shell thickness, the mechanical force required to trigger the self-healing function of microcapsules increased correspondingly from 68.5 ± 41.6 mN to 198.5 ± 31.6 mN, featuring a multi-sensitive trigger function. Finally, the rupture behavior and crack surface of cement paste with embedded microcapsules were observed and analyzed using X-ray computed tomography (XCT). The synthesized PF microcapsules may find potential application in self-healing cementitious materials.
New additive manufacturing methods for cementitious materials hold a high potential to increase automation in the construction industry. However, these methods require new materials to be developed ...that meet performance requirements related to specific characteristics of the manufacturing process. The appropriate characterization methods of these materials are still a matter of debate. This study proposes a rheology investigation to systematically develop a printable strain hardening cementitious composite mix design. Two known mixtures were employed and the influence of several parameters, such as the water-to-solid ratio, fibre volume percentage and employment of chemical admixtures, were investigated using a ram extruder and Benbow-Bridgwater equation. Through printing trials, rheology parameters as the initial bulk and shear yield stress were correlated with variables commonly employed to assess printing quality of cementitious materials. The rheology properties measured were used to predict the number of layers a developed mixture could support. Selected mixtures had their mechanical performance assessed through four-point bending, uni-axial tensile and compressive strength tests, to confirm that strain hardening behaviour was obtained. It was concluded that the presented experimental and theoretical framework are promising tools, as the bulk yield stress seems to predict buildability, while shear yield stress may indicate a threshold for pumpability.
Display omitted
•A quantitative methodology based on rheological parameters to develop printable cementitious composites is presented•The methodology was successfully applied to the development of printable strain hardening cementitious composites•A correlation between shape stability and buildability with the initial bulk yield stress was found•The use of VMA and the liquid/solid ratio are key factors to control mixture stability and fibre dispersion•Rheological properties measured with the ram extruder and Benbow-Bridgwater model can suggest the build height of an object.
A method is presented to model deformation and fracture behavior of 3D printed disordered lattice materials under uniaxial tensile load. A lattice model was used to predict crack pattern and ...load-displacement response of the printed lattice materials. To include the influence of typical layered structures of 3D printed materials in the simulation, two types of printed elements were considered: horizontally and vertically printed elements. Strengths of these elements were measured: 3 mm cubic units consist of lattice elements with two printing directions were printed and their strengths were tested in uniaxial tension. Afterwards, the measured element strengths and bulk material strength, respectively, were used as model input. Uniaxial tensile tests were also performed on the printed lattice materials to obtain their crack pattern and load-displacement curves. Simulations and experimental results were comparatively analyzed. For both levels of disorder considered, only when measured strengths were assigned to the elements with identical printing direction, are the predicted crack patterns and load-displacement curves in agreement with experimental results. The results emphasize the importance of considering printing direction when simulating mechanical performance of 3D printed structures. The influence of disorder on lattice material mechanical properties was discussed based on the experiments and simulations.
Display omitted
•Three-dimensional disordered lattice materials have been designed and printed using fused deposition modeling.•Fracture performance of the printed lattice material is simulated by an experiment informed discrete model.•The influence of printing directions on the lattice material can be captured by the discrete lattice model.•The influence of disorder degree on the printed materials is experimentally and numerically clarified.
This study aims to provide an efficient alternative for predicting creep modulus of cement paste based on Deep Convolutional Neural Network (DCNN). First, a microscale lattice model for short-term ...creep is adopted to build a database that contains 18,920 samples. Then, 3 DCNNs with different consecutive convolutional layers are built to learn from the database. Finally, the performance of DCNNs is tested on unseen testing samples. The results show that the DCNNs can achieve high accuracy in the testing set, with the R2 all higher than 0.96. The distribution of creep modulus predicted by the DCNNs coincides with that of the original data. Furthermore, through analyzing the feature maps, it is found that the DCNNs can correctly capture the local importance of different microstructural phases. The DCNN allows therefore prediction of the creep modulus based on microstructural input, which saves computational resources of segmentation procedure and multiple incremental FEM calculations.
In circumstances with wastewater and seawater, the behavior of multi-ions including calcium, chloride and others in concrete attracts attention. The present study investigated the multiple mechanisms ...that could happen under the special field situation above, including calcium leaching, chloride transport and multi-ion coupling. To realize the interactive ingress of multi ions, the simulation method for the processing of the individual mechanisms and the mutual influences is adopted. The distributions of the diversified ions are analyzed with the influence of the interfacial transition zone. The time-spatial distribution of porosity and its evolution mechanisms are investigated by considering the interaction with calcium ions in both pore solution and solid phase. The results indicate that calcium leaching would dominantly speed up chloride transport due to the coarsened pore structure, while the multi ions electrochemical coupling effect would facilitate calcium leaching in the early stage but subtly delay calcium leaching in the later stage.
PDF
