International RILEM Conference on Early-Age and Long-Term Cracking in RC Structures

One of the key parameters governing the stresses due to restrained deformation, and thereby the associated risk of cracking, is the elastic modulus of concrete. The early-age evolution of the elastic modulus is intrinsically a function of the development of hydration degree. For concrete subjected to sustained loads, the elastic modulus evolves with creep and relaxation properties. Many design methods use the concept of effective elastic modulus or age-adjusted effective elastic modulus to account for such effects. The challenge with the existing approaches is the lack of in-depth knowledge and reliable test data to accurately determine creep and aging effects for predicting elastic modulus evolution under sustained loading conditions. This paper first provides a brief review of the current approaches to determine the elastic modulus evolution under sustained loading at early age. Second, a method is proposed to directly obtain the age-adjusted effective elastic modulus (Eaaef(t, t0)) evolution experimentally by utilizing an advanced Temperature Stress Testing Machine. Such evolution of Eaaef(t, t0) in a high-performance concrete subjected to a sustained tensile stress of 30% of the 3-day tensile strength is experimentally obtained. The test data are compared with the pure elastic modulus evolution. The creep and aging coefficients are derived based on the newly-measured test data. Differences between obtained early-age creep coefficients and that predicted by existing approaches are comparatively investigated. In addition, on the basis of newly-measured test data and analytical investigations, a convenient approach to determine Eaaef(t, t0) using time-dependent profiles of reduction factors is proposed.

Low calcium alkali-activated cement composite known as geopolymer has been around for more than 40 years. The main benefit of geopolymer based composites is the environmental aspect—it is partially made by utilizing waste products, such as fly-ash, slags, and others. It has been estimated that geopolymer binder production makes up to 6 times less CO2 than the production of Portland cement. Due to the polymerization or in other words nature of the geopolymer binding process, there are some differences in creep and shrinkage development. Because of this microstructure of the specimen could be dissimilar to ordinary Portland cement. There has been an absence of investigations regarding the geopolymer composite long-term properties and micro-analysis. Also, the conditions affecting the long-term properties of the geopolymer composites have been little studied. The subject of the research is geopolymer concrete that has been tested for creep and shrinkage in compression and tension. The specimens for microstructure analysis were acquired from the cylindrical shape (compression) and compact tension (tension) specimens. Polished sections were used for SEM microanalysis. Acquired polished section image cross-sections were analyzed by determining the amount of geopolymer binder, filler, and air void in the analyzed cross-section. The results were cross-referenced with creep and shrinkage test results. After creep and shrinkage tests in compression and tension specimen cross-section zones that have been subjected to the highest stresses were chosen and analyzed. The article’s main aim is to determine the geopolymer composite microstructure and applied load influence on long-term properties.

The estimation of crack widths in steel fibre reinforced concrete (SFRC) with and without conventional reinforcement is still relatively uncharted territory. Engineers are faced with many challenges and uncertainties when designing SFRC members, such as precast SFRC tunnel segments, steel fibre sprayed concrete tunnel linings and other underground structures, owing to the peculiarities associated with SFRC behaviour, particularly when it is used without conventional reinforcement. At present, Eurocode 2 focuses on the design conventional reinforced concrete, as such, its recommended methodology for crack control is not applicable to SFRC. However, other guidance documents, such as fib MC2010, Concrete Society TR63, RILEM TC 162-TDF σε method, DafStb Guideline and research works by Johnson et al., are available and provide some recommendations with respect to crack width estimation in SFRC with and without conventional reinforcement. This paper reviews the aforementioned methods and evaluates their peculiarities, assumptions and differences with regards to the calculation of crack widths in SFRC.

A mathematical model for the cracking analysis of a concrete tie reinforced by ordinary and prestressing steels, subjected to a statically applied axial force, is proposed. The two types of steels do not have only different diameters, but also different bond properties, since in general the prestressing steel presents a lower bond quality with respect to the ordinary steel. The model is based on the assumptions of linear elastic σ-ε laws for both types of steels, as well as for the concrete, and on the bond law proposed in fib Model Code 2010. Both the crack formation stage and the stabilized cracking stage are analyzed. In particular, in the stabilized cracking stage, under the assumption that the crack spacing is maximum and equal to twice the transmission length, the effect of primary and secondary cracks is taken into account. This leads to distinguish, in the stabilized cracking stage, an initial transitional phase where the transfer length of ordinary steel is smaller than the transfer length of prestressing steel, followed by a fully developed phase, where the two transfer lengths are equal to each other. Finally, the theoretical results of the crack spacing and the crack width, obtained with this refined model are compared to the experimental data available in the literature.

The development of concrete mixtures with low potential for cracking is necessary for durable, sustainable, and efficient concrete construction. Due to the wide range of crack-inducing parameters, efforts on the development, evaluation, and specification of concrete mixtures with low potential for cracking are paramount. On the design phase, criteria related to the selection and performance of concrete mixture constituents, mixture proportions, and laboratory testing are necessary for crack mitigation. During concrete production, efforts to maintain control of batch water and water-to-cementitious ratio are paramount. Results presented in this paper will compare the performance of concrete mixtures with low and high potential for cracking in both the laboratory and field.

Reinforced cement-based rendering mortars are used as the protection layer in External Thermal Insulation Composite Systems (ETICS). Thermal insulations, when used in renovation, have a big impact on the reduction of CO2 emissions. The interactions with the environment changing temperature and relative humidity lead to thermal and hygral strains, which when restrained, may lead to stresses that can attain the tensile strength of the material causing then the mortar cracking. The eventual penetration of water inside the cracks may cause the insulator to lose partially its efficiency and durability. Here, we focus on the cracking development in the reinforced mortar layer using experimental techniques. To understand the crack initiation and propagation in the reinforced mortar layer, and the role of the fiber-glass mesh as reinforcement inside the mortar, a new mechanical setup is developed. This setup is designed to perform 3-point bending tests using in-situ X-ray tomography. The latter allows observing the cracks inside the mortar sample shedding lights on the reinforcement mechanisms of the fiber-glass mesh and its impact on the initiation and the propagation of the cracks. The role of the mortar heterogeneities is also analyzed and information about cracks characteristics such as openings and lengths may be extracted.

Distributed fibre optic sensors (DFOS) provide new possibilities in structural technical condition assessment in comparison with traditional spot measurements. It is possible to analyze strains and temperature changes continuously over structural member length with spatial resolution starting from as fine as 5 mm. Thanks to the appropriate sensor construction and its installation before concreting it is possible to analyze material behaviour starting from its early stage, when thermal-shrinkage strains appear. This phenomenon depends on many factors, such as the type of concrete mix, dimensions of structural member, the way of concrete care, external conditions (temperature, humidity), formwork and constraints related to reinforcing bars or external friction and resulting with crack appearance. Stage of early-age concrete (hydration process) is thus very important for its final durability and performance. The article presents the very new measuring tools which allow for comprehensive analysis of concrete temperatures and strain state including all local nonlinearities (cracks). The attention was paid to hydration process, but sensors installed inside the structural members can be also effectively used during other phases such us: activation of prestressing tendons, construction stages or operation. Selected examples of laboratory tests as well as the unique in situ installations realized in Poland during last few years are presented and discussed hereafter. Except of traditional concrete, also other materials were examined, such as concrete made on lightweight sintered aggregate and fibre-reinforced concrete mixed with the ground.

The study carried out at Vilnius Gediminas Technical University (Vilnius Tech) reports test results on cracking of seven RC beams of the rectangular section. The beams were subjected to a short-term loading under a four-point bending configuration. The experimental program was aiming at evaluating the effect of section height, reinforcement ratio, and bar diameter on cracking of RC beams. The beam cross-sections were designed to have two different cases of section heights (200 and 600 mm), reinforcement ratios (0.36% and around 1.9%), and bar diameters (Ø10 and Ø20 mm). The paper reports test results on mean and maximum crack spacings, as well as mean and maximum crack widths recorded using digital cameras and an electronic microscope. These characteristics were compared to the predictions by Eurocode 2 and Model Code 2010. For the beams with high reinforcement ratio, the mean and maximum crack width predictions by the Eurocode 2 and Model Code 2010 were close to the test results. However, the calculated values for the beams with low reinforcement ratio at most load levels notably exceeded the test results.

Early-age cracks in the RC walls of the tanks very often cause leaks. In turn, limiting the value of imposed strains reduces the risk of cracking or the width of cracks if cracking is not to be excluded in general. In the paper the analysis of the impact of a cooling pipe system (CPS) on the changes of temperature and strains in a RC semimassive wall was presented. In the numerical calculations a model covering non-linear and non-stationary temperature field variations resulting from cement hydration, internal cooling and heat exchange with the surroundings was used. It was demonstrated that during concrete maturing CPS contributes not only to mean temperature changes reduction but also to the reduction of temperature gradients, which helps effectively limit or eliminate concrete cracking. The analysis of the so-called “self-equilibrated stress” inducing temperature indicated that the application of CPS contributes to the reduction to zero the positive temperature difference in the immediate surroundings of the cooling pipes and to the reduction of temperature extreme changes, i.e. positive changes in the wall interior and negative ones in its corners. As a result, the restrained part of the imposed strains, to a greater extent, may remain below the tensile strain capacity of concrete. This solution can protect the tank wall from cracking or significantly reduce it.

In the current regulations there is no clear indication about what is the limit of crack opening that can produce a decrease in the service life of reinforced concrete structures, in terms of the arrival of chlorides to the reinforcement and the initiation of corrosion. The crack becomes a preferential way of entry of aggressive agents, especially chlorides in solution. In this work, the transport of chlorides in cracked concrete is studied, obtaining the coefficients of stationary and non-stationary diffusion, the period of initiation of the corrosion, as well as the decrease of this according to the width of the crack. Concrete samples have been manufactured with different cement compositions that, after their conditioning, have been cracked by indirect traction test. The method includes natural and accelerated chloride transport tests, the latter by applying an electric current, according to current Spanish regulations.

Development of thermal cracks is inevitable in concrete structures under extreme fire conditions. Intense heat flux on the exposed surface and the thermal incompatibility between the mortar and aggregate phase results in micro-cracks. The geometrical properties of cracks such as width, length, density and pattern are to be quantified accurately to adopt suitable control methods. This study aims to analyse the crack pattern and crack control measures on Self-Compacting Concrete (SCC) exposed to standard fire temperature. SCC mixes were developed to achieve a strength of 30MPa using Fly Ash (FA), Ground Granulated Blast Furnace Slag (GGBFS) and Expanded Perlite Aggregate (EPA). The rheological behaviour of the developed mixes was confirmed as per European Federation of National Associations Representing for Concrete (EFNARC 2005) guidelines. After the curing process, specimens were exposed to elevated temperatures for different durations (30, 60, 90, and 120 min) following the International Organization for Standardization ISO 834 standard fire curve. A mortar combination of Cement-Perlite Plaster (CPP) was considered as the sacrificial layer (protection layer) over the concrete surface to understand the efficiency of perlite in controlling the crack growth. GGBFS-SCC specimens exhibited severe surface cracking than FA-SCC specimens. The protected (CPP) specimens showed better resistance on mitigating the surface cracks even at 120 min of heating. Attempts have been made to quantify the cracks on the concrete surface with the help of Image-Processing Tool (IPT). Scanning Electron Microscopy (SEM) analyses have been carried out to analyse the micro-cracks in the fire-damaged concrete.

Numerical and analytical simulation of cracking in reinforced concrete (RC) structures requires laboratory testing for validation of assumptions and calibration of modelling strategies. The scale, complexity and innovation of these experiments can present a challenge, even for experienced professionals, and a significant amount of time, effort and money might be inadequately invested on unsuitable test setups, or poorly planned experimental campaigns. This paper deals with the lessons learned from a complex long-term experimental campaign on RC slabs subjected to flexural cracking and restrained shrinkage. It intends to serve as a steppingstone for future works of the same nature. The final version of the restraining device is described and decisions on tested specimens, test conditions, sensing and material characterization are explained based on the available resources. The most relevant tasks during preparation of the long-term experimental campaign are described in detail. Finally, the hard-ships and problems faced during implementation of the experimental program, and the way they were overcome, are addressed. The application of concepts in line with Building Information Modelling (BIM) methodologies, such as object-oriented modelling and process maps, for design of the test setup and task scheduling, was of crucial importance for the success of the experimental campaign.

Cement-based materials usually undergo drying which influences their mechanical behaviour. This study investigates the evolution of mechanical properties of a high-performance self-compacting concrete subjected to drying at an early age and after long-term maturation. Uniaxial compression and bending tests are performed at different drying times. The evolution of the mechanical properties obtained is controlled by a competitive effect between material strengthening (due to capillary depression, disjoining pressure, hygral gradients, and hydration if it still occurs) and drying-induced micro-cracking (due to material heterogeneity and differential shrinkage). The competitive effect shows a great dependence on maturation level. Such couplings have to be taken into account for a reliable durability analysis.

This research study aims to better understand the mechanical behavior of a Na-geopolymer used to immobilize the nuclear waste (MgZr alloys). Based on a large experimental campaign that has been carried out at LGCGM, the mechanical properties of Na-geopolymer are investigated as well as its cracking sensitivity at early age. This last depends on several parameters studied in the present project: tensile strength, Young’s modulus, and delayed deformations such as autogenous shrinkage and basic creep. All of these parameters are characterized experimentally under autogenous condition. These results obtained for a Na-Geopolymer are compared to those of a Portland cement-based mortar. From restrained ring tests, it appears that the cracking sensitivity is lower for Na-Geopolymer. This behavior is mainly due to a low Young’s modulus, a fast stabilization of autogenous shrinkage and a high creep capacity.

The positive influence of longer duration of wet curing in improving the mechanical and durability properties of concrete is fairly well established. However, it is increasingly becoming apparent that longer duration of wet curing, unfavorably affects the drying shrinkage characteristics of concrete, resulting in both increased amount of shrinkage and also increased cracking potential under restrained conditions. Various types of fibers have been developed and used to enhance the durability of concrete structures by controlling the restrained shrinkage cracking. However, the influence of longer duration of wet curing on the age at cracking and crack characteristics of concrete containing macro polymeric fibers has not been previously determined. In this research, the effects of three different curing durations on strength characteristics and free and restrained drying shrinkage performance of mixes with and without macro polymeric fibers were studied. The results show that drying shrinkage and cracking potential of the control mixes increased with longer duration of wet curing, confirming the trend reported by recent literature. Interestingly the mixes containing fibers also showed the same trend and both the amount of free shrinkage and the restrained shrinkage potential increased with increased duration of wet curing. With regards to crack widths, the results show that with longer wet curing, the width of the single crack for the control mixes increased significantly while the cumulative width of the multiple cracks of the mixes with polymeric fibers increased only modestly.

Concrete crack mitigation in hydro-engineering projects belongs to important tasks during design and operational stages. The paper focuses on a design of reinforced upstream concrete face of a polder. The face is made of cast concrete blocks, 15 m long, 2.5 m tall and up to 2.5 m thick. First, the paper summarizes design of two concretes with low hydration heat and their strength evolution, basic creep, calorimetry data, and freeze-thaw resistance. Heat release was further validated on a small insulated concrete cube. Second, a thermo-mechanical model has been assembled, taking into account heat of hydration, ageing basic creep, autogenous shrinkage, evolution of tensile strength. A multi-directional fixed crack model captures crack initiation, crack opening is controlled by exponential softening and dissipation of correct amount of energy is guaranteed by crack-band approach. Summer casting scenario is presented, showing crack evolution due to different reinforcement ratios and concrete mix design. The results show that minimizing cement amount in concrete is a must and crack width can remain under required 0.20 mm with the reinforcement ratio of 0.4–0.6%, depending on mix design.

Precise prediction of the elastic response is crucial to model cracking at early and late ages of cement-based materials and structures. Here, we use Machine Learning (ML) techniques to predict the elastic properties of Ordinary Portland Cement (OPC) pastes. A database with 365 observations is built on experimental studies from in the literature. We show that micromechanics-based estimations may provide missing data in databases to be interrogated by ML, increasing the accuracy of predictions. Finally, we explore the formulation space of OPC pastes using Monte Carlo computations, which enables guessing which are the compositions that can be associated with a given elastic response. Applications of our results include the development of tailored formulations for a target elastic response and also in the forensics of existing cement pastes.

This study aims to develop an analytical tool to predict the expansion of Hauyne and free-lime based expansive additives in RC structures. Hydration of species was based on the multi-component heat of hydration model which incorporates referential heat rate at a certain temperature and thermal activities following Arrhenius' law of chemical reaction. All other time-dependent properties of concrete such as elastic modulus, temperature, pore pressure, creep, moisture status, total porosity of interlayer, gel and capillary pores are computed internally based on the micromodels of materials inside the system. Coupling this material information with poromechanics, volumetric change generated by cement hydration and shrinkage is systematically included in the modelling of concrete mechanics, which deals with macroscopic structural responses based on the space-averaged constitutive laws on the fixed four-way cracked concrete model. Finally, the models are then verified with experimental results that could prove the models’ capability in predicting the amount of expansion under various replacement ratios.

EDF operates a fleet of 56 nuclear reactors. For 24 of these reactors, the concrete containment building is a double wall structure. The inner wall is prestressed and has no metallic liner. Every ten years, the leak-tightness of the inner wall is verified by performing a pressure test. The leakage rate has to remain below a given threshold. As time passes, the leakage rate is getting closer to this threshold, so important coating programs are underway to keep the leakage rate within the regulatory limits. Therefore, it is important for EDF to be able to forecast the evolution of leakage, which depends on the concrete saturation and the level of prestress. The prestress decrease is related to creep and shrinkage of concrete and tendons relaxation (this last factor is considered negligible at ambient temperature). To tackle this issue, EDF has launched an important research program around the VERCORS mock-up, which is a containment building at scale 1/3 and has used this research program to improve its ability to predict the evolution of leak-tightness with time. In this paper, the different tools of the digital twin are presented, as well as an example of the use of the models originally developed on the VERCORS mock-up to help the choice of coating strategy for the third pressure test of an industrial containment building.

The mesoscale fracture modeling of concrete requires explicit descriptions of the mesostructure and the fracture behavior of its phases. To identify the fracture properties, an in situ three-point bending test on a notched beam was carried out in an X-ray scanner. The kinematic fields at different loadings and crack propagation steps were measured via Digital Volume Correlation. A realistic mesh was created for mechanical simulations to be run. The experimental displacement fields were used as kinematic boundary conditions of a region of interest around the crack. Fracture of the cement paste was modeled with a phase field method. The numerical cracking pattern is compared to the experimental observation.

The control of random cracking in reinforced concrete members is of crucial interest to assess their serviceability and durability. From a regulatory point of view, such control within the design phase is ensured using a set of guidelines (definition of the reinforcement ratio, limitation of the crack width, limitation of the deflection, etc.). One can find corresponding formulae in the Eurocode2 (Design of concrete structures—Part 1-1: General rules and rules for buildings, [1]), Code Models (fib Model Code for concrete structures, [2]) or others. On the one hand, those regulatory formulations do not provide accurate results for all concrete types as the used experimental results for fitting show a large discrepancy. On the other hand, and mostly, they do not account for size effects observed on the tensile strength of concrete. This contribution aims at analyzing size effects on the cracks’ distribution in axially reinforced concrete members subjected to pure tension. For the most part, the paper presents an alternative approach (to existing regulatory formulae) based on the use of spatially correlated Weibull random field coupled to an energetic-statistical size effect law. The main interest is geared towards the estimation of the mean and maximal spacing values once the stabilized cracking stage is reached. The method is validated based on the set of tests performed by Farra and Jacoud (Rapport des essais de tirants sous déformation imposée de courte durée. Projet: Influence du béton et de l’armature sur la fissuration des structures en béton. Publication N°140. IBAP. Ecole Polytechnique Fédérale de Lausanne, [3]). The obtained results show a logarithmic decrease of the mean and maximal spacing values with the effective size of the ties which should be considered for future regulatory formulations.

Cement based material structures are designed to work in pair with steel reinforcement. The cracking of reinforced concrete is a normal phenomenon given the working principle of this material. But, for aesthetic or durability reasons, the openings of these cracks must be controlled. In the modelling, there is often an ambiguity between damage (micro-cracking at the scale of a REV) and crack opening at the scale of the material. The goal of this study is to estimate the crack opening in concrete structures under various multi-physic loadings. To do so, a method based on continuum mechanics is developed. This method post-processes the mechanical damage induced by the diverse loadings. To illustrate the procedure, a numerical application is run on an experimental reinforced shear wall made of ordinary concrete submitted to drying prior to loading. A hygro-mechanical model previously developed by the authors is firstly calibrated on experimental data and secondly used to compute the shear wall's damage induced by drying shrinkage. The resulting computed crack openings are compared to the experimental results. Interestingly, the post-processing technique managed to locate the areas where cracks are of major importance. However, the assumption of perfect adhesion between concrete and steel leads to a slight underestimation of the amplitude of the crack opening. In a first attempt to capture cracks, the proposed method based on continuum mechanics seems to be a good alternative to sophisticated methods.

Internal curing using fine lightweight aggregate is an effective method to address early-age cracking. This study focuses on the sustainability of using fine lightweight aggregates for internally curing concrete. Application of fine lightweight aggregates has a positive influence on transport properties of concrete and enhances the durability and service life of concrete infrastructure. These properties contribute to the performance of those infrastructures in various environments and climate zones. This paper highlights the sustainability of internal curing application through cost, energy, and emission analyses based on existing service life prediction models and using transport properties of internally-cured concrete obtained through experimental investigations. Presented methodology covers data acquisition and refinement procedures to perform such analyses. Analysis includes a range of concrete applications to simulate current trends in concrete market. Results highlight quantitative sustainability performance measures, such as energy and emissions in addition to cost and time for selected applications in parking structures and bridge decks.

Crack width control is an important design task to achieve the required durability and serviceability of reinforced concrete (RC) structures. The thickness of the member is an important contributing factor for the mode of cracking—and thus for the required reinforcement for crack control. While in thin RC elements, e.g. industrial floors or tank walls, visible cracks usually penetrate through the entire element thickness, in thick elements with common reinforcement configuration (reinforcement is located near the surface) the cracking process is much more complex: initially, a primary crack occurs and separates the element over its entire thickness; and in addition, secondary cracks may occur due to the transfer of the steel force in the primary crack back into the concrete by bond action. These secondary cracks do not penetrate through the entire thickness, but end somewhat behind the surface zone. This paper presents the validation of a thermo-hygro-mechanical framework for simulation of the cracking process in thick restrained RC members. The study focusses on the short-term experimental campaign on restrained reinforced concrete panels made at TU Graz. The experimental measurements are compared with the results of the finite element simulation. The comparison is made for the crack patterns, the crack width development for primary and secondary cracks and the development of the restraint force in function of the imposed elongation. In addition, a parametric study is performed to investigate the drying shrinkage effect on the long-term crack width evolution of primary and secondary cracks.

The article presents the results of bleeding tests for cement pastes modified with the addition of granite powder. Bleeding is the process of dispensing the water from the inside of the mixture towards the top. Bleeding in cementitious mixes is an important process related to early-age properties of composites (like cracking of concrete slabs). There are known examples of damage to horizontally formed concrete structures due to the uncontrolled bleeding process. The research was performed for four series of modified cement slurries of the addition of granite powder. The test was carried out with the use of a method allowing to obtain high accuracy of measurement (ASTM C243-95 Standard Test Method for Bleeding of Cement Pastes and Mortars (Withdrawn 2001), [1]) of the released water over time, with particular emphasis on the first 30 min of the process. Three phases of the bleeding process were observed during the investigation (initial bleeding, accelerated bleeding, stable bleeding) into cement pastes. An additional division of the first phase on three phases (start of the process, components ordering, appropriate bleeding) was also proposed. The results of the research also present that thanks to the addition of granite powder, it is possible to reduce the volume of dispensed water in the bleeding process. The main issues were recognized and directions of future research area are indicated.

Expansive additives are being widely used as shrinkage reducing/compensating admixture to reduce the early-age cracking and chemical prestressing. Such expansive additives influence the composition of cement hydration products and mechanical properties (stiffness and strength) of the cement and concrete, which ultimately affects the early age deformation. In the current study, the EMM-ARM (Elastic Modulus Measurement by Ambient Response Method), is used because of its ability to continuously monitor cement's real-time stiffness development paste with expansive additives. Stiffness development of the cement paste with expansive additives is captured in the unrestrained condition by allowing the free expansion during the stiffness measurement. An expansive additive, based on free lime, with a 10% (by weight) cement replacement ratio is studied in the current work. Results from the EMM-ARM are compared to the elastic modulus measurement from the compression testing of cylindrical specimens, and the mechanisms of expansive action on the microstructure in early-age are discussed.

As shrinkage can affect the concrete durability, several solutions were designed to limit this deformation. One of them consists to add expansive agents like MgO-based products. The aim of the present research work is to better understand the effect of MgO-based expansive agent on the self-healing potential of cementitious materials. At 28 days old, Portland cement mortars with different contents of expansive agent (0%, 5% and 10%) are pre-cracked by means of splitting test and stored under water. Self-healing is monitored using 2D and 3D tests: optical microscope observations and water permeability measurements respectively. Mechanical resistances and deformations of the studied mortars are also monitored and their microstructures are characterized by means of thermogravimetric analysis. The results show that mortars containing MgO-based expansive agent present a better self-healing capacity. It can be due to its swelling capacity decreasing/deleting shrinkage deformations and the formation of supplementary products like brucite.

Carbon nanotubes (CNT) modify not only the mechanical but also the electrical properties of the cement matrix, allowing the development of self-sensing cement composites (SSCC). These composites can sense their own deformation through changes in their electrical resistivity, acting as strain sensors, increasing the operational safety of concrete structures by indicating preventive or corrective maintenance. The efficiency of SSCC depends on the electrical properties of the conductive filler and its dispersion degree throughout the matrix. The most commonly employed method of ensuring adequate dispersion of CNT is using chemical dispersing agents. This work aims to compare the effectiveness of two types of plasticizers as dispersing agents for CNT, identifying how they affect the self-sensing capacity of SSCC. CNT contents of 0.10 and 0.20%, by mass of cement, were blended in a cement matrix using two types of commercial superplasticizers as dispersant agents, one naphthalene based and one polycarboxylate ether based. SSCC cement pastes were cast in 50 mm cubic molds with four copper embedded electrodes and cured for 30 days in a humid environment. After curing, an electrical potential was applied through the electrodes and the electrical resistivity of the SSCC was acquired during successive compressive load applications. A 10 V DC power supply was used for this purpose. The changes in electrical resistivity of each cube were correlated with deformations measured by LVDT. Results showed that both the type of superplasticizer used and the CNT content affect the piezoresistive response of SSCC. Naphthalene-based superplasticizer presented better performance as dispersing agent and led to SSCC with better self-sensing response. Results indicated that the chemical basis of the dispersing agent for CNT has great influence on the fabrication of efficient SSCC.

Concrete incorporates a natural self-healing capability that is primarily created by hydration and carbonation to repair small cracks. This ability is limited and is only activated when direct contact with water takes place. The study mainly dealt with crack repair and sustainability. For the repair of cracks, autogenous healing mechanism is adopted. Autogenous healing can be accelerated by the addition of mineral additives. The mineral additive used for autogenous healing is metakaolin. The use of metakaolin in high performance concrete can improve both the strength and durability properties of concrete. The ability of autogenous self-healing will increase with higher cement content. The experiment was conducted on high performance concrete of M70 grade. 10% replacement of cement with metakaolin is considered for the study. Specimens with addition of metakaolin and without metakaolin were subjected to 0 day, 3 days and 7 days curing. This study mainly focused on the strength retained, type of crack formed, propagation of cracks and amount of crack healed. The compressive strength and flexural strength at 7 day, 28 day and 180 days were carried out. Flexural strength test carried on beam specimens of size 700 × 150 × 150 mm was subjected to two-point loading. It has been observed that the specimens with metakaolin gives increased compressive and flexural strength than those specimens without adding metakaolin. The early age cracks in concrete structure will get reduced due to the addition of metakaolin. Also the addition of metakaolin promoted autogenous self-healing.

Monitoring and rehabilitation of cracks in concrete structures can be rather expensive, labour intensive and often presents technical difficulties. Nonetheless, it is vital for prolonging their lifespan. As an alternative to traditional repair techniques self-healing in cementitious materials has been proven a promising technology for crack remediation. The use of bacteria in self-healing relies on their ability to metabolically facilitate the precipitation of calcite, which can act as a crack sealant. Since cracking is likely to occur throughout the life of concrete it is important to investigate the long-term effectiveness of the method. In this study bacterial spores of the species B. cohnii were added in cement mortar mix, along with nutrients for the bacteria. Reference samples of plain mortar and control samples containing only nutrients were also prepared for comparison. Two sets of samples were prepared: one with samples that were cracked after 28 days of curing and one with samples cracked after 9 months. A single crack of 0.4–0.5 mm crack width was introduced to all samples. After cracking, samples were left to heal semi-submerged in water. Healing was examined at two temperatures, at 7.5 and 20 °C, for both sets of samples. Optical microscopy and water-flow tests were used for evaluating the healing. Results showed that substantial healing can be achieved in 9 months old samples containing bacteria, both at 7.5 and 20 °C, comparable to that of 28 days old samples. The long-term competence of bacteria-based self-healing technology for crack remediation in concrete is, therefore, demonstrated.

This paper reports experimental findings on the effect of Bacillus subtilis on crack remediation in thermally degraded Limestone Calcined Clay Cement (LC3) mortars. Mortar prisms measuring 160 mm × 40 mm × 40 mm were cast using LC3 at water/cement (w/c) ratio of 0.5 and cured for 28 days. Half of the 28-day cured mortar prisms were thermally degraded by heating them at 1000 °C in a furnace to induce the cracks while the other half was used as a control. Both cracked and un-cracked mortar prisms were subjected to compressive strength, porosity and accelerated chloride ingress tests. Moreover, half of the cracked and un-cracked mortars were immersed in bacterial solution containing Bacillus subtilis while the other half was separately immersed in curing water until the 90th day. Compressive strength, porosity and chloride ingress tests were also repeated on the 90th day. Microstructural changes in cracked LC3 mortars were carried out using Scanning Electron Microscope (SEM) before and after immersion in bacterial solution. Results showed that at 28 days of curing, un-cracked mortars exhibited higher compressive strength, lower porosity and lower apparent chloride diffusion coefficients than cracked mortars. However, compressive strength, porosity and apparent chloride diffusion coefficients of both cracked and un-cracked mortars were equivalent after 90 days of curing in bacterial solution. SEM images showed visible micro-cracks after thermal treatment and healed cracks with calcite deposition after curing in bacterial solution. In conclusion, Bacillus subtilis was found to improve the crack healing capacity of thermally cracked mortars.

This paper comprises the enhanced self-healing properties of fiber-reinforced cementitious composites by taking advantage of synergistic effect of using different mineral admixtures. To do this, single and binary use of metakaolin (MK) and zeolite (Z) were utilized in cement-based mixtures, and early age self-healing behavior of specimens was assessed based on sorptivity tests. The autogenous self-healing performance of each mixture was also evaluated via crack width assessments by using a video microscope. Results were compared with the sound and preloaded specimens having different crack width levels. In addition, mechanical properties of four different mixtures were evaluated by conducting experiments on 7 and 28 days. The multiple crack width closures and sorptivity coefficients of preloaded specimens indicated that an effective combination of MK and Z can trigger a higher level of self-healing in comparison with reference mixture. However, single-use of Z in mixtures was also promising for achieving autogenous self-healing attributes compared to mixtures containing only MK and to reference mixtures. Having said that, this behavior should also be addressed for different pre-cracking and prolonged curing age of each mixture.