Volume and deviator creep of calcium-leached cement-based materials

This paper provides new experimental evidence of the specific volume creep and deviator creep of cement-based materials. Creep tests conducted on calcium-leached cement pastes and mortars under various triaxial loading conditions provide conclusive evidence of a specific short-term creep and a specific long-term creep. It is found that at least two competing dissipative mechanisms are at work in short-term creep of cement-based materials: (1) a creep relaxation mechanism in the calcium-silicate-hydrate (C-S-H) solid phase activated by microstress concentrations in the heterogeneous microstructure; and (2) stress relaxation by microcracking beyond a relatively small stress threshold. If the first dominates over the second, the short-term volume creep is contracting, and in the inverse case, a short-term dilating behavior is found. Furthermore, provided that microcracking stabilizes during the short-term creep, the long-term creep occurs at constant volume. Based on these results, we argue that the concept of a creep Poisson's ratio, which is commonly employed in engineering practice to extrapolate uniaxial creep tests to multiaxial stress conditions, should be abandoned when triaxial stress states affect the durability performance of concrete structures.     

Correlation of hollow and solid cylinder dynamic pressurization tests for measuring permeability

An experimental apparatus and analytical derivation are developed for quantifying the permeability of cementitious materials using dynamic pressurization of a hollow cylinder. Experimental results from the newly developed hollow dynamic pressurization technique for measuring permeability are then compared to results obtained using the solid dynamic pressurization test. The measured permeabilities obtained from testing Vycor^® glass and hardened cement paste indicate close agreement between the two test methods, which supports the validation of the hollow dynamic pressurization test as an accurate and repeatable method for measuring the permeability of cementitious materials. Three different pore fluids with widely varying viscosities were tested, each yielding equivalent intrinsic permeabilities. Additionally, the permeability values from this study agree reasonably well with relevant values presented in the recent literature.     

Simulated microstructure and transport properties of ultra-high performance cement-based materials

Ultra-high performance cement-based materials expected to be used in nuclear waste containers were submitted to a leaching test in order to evaluate their long-term durability. Reactive powder concretes (RPC) were attacked by de-ionized water. Previous studies revealed a superficial degradation after leaching with a sound zone underneath an altered porous zone in which anhydrous silicates C₃S and C₂S were dissolved. To predict the long-term durability of RPC, the hydration rate of cement minerals, pozzolanic reactivity of silica fume, pore structure, and mechanisms of chemical reactions were needed. So first, the microstructure of RPC matrix was simulated using the NIST microstructural model. Then the transfer of Ca ions through percolating water was estimated using DIFFU-Ca, a model based on the local chemical equilibrium. This double modeling validates the damage process related to an instantaneous dissolution of anhydrous cement silicates at the degradation front which results in a higher connected pore space, and is in good agreement with experimental results. The long-term behavior is expressed as the depth of the altered zone, the leaching kinetics and the evolution of Ca concentration in the material.     

Experimental study on properties of pervious concrete pavement materials

In this paper, a pervious concrete pavement material used for roadway is introduced. Using the common material and method, the strength of the pervious concrete is low. Using smaller sized aggregate, silica fume (SF), and superplasticizer (SP) in the pervious concrete can enhance the strength of pervious concrete greatly. The pervious pavement materials that composed of a surface layer and a base layer were made. The compressive strength of the composite can reach 50 MPa and the flexural strength 6 MPa. The water penetration, abrasion resistance, and freezing and thawing durability of the materials are also very good. It can be applied to both the footpath and the vehicle road. It is an environment-friendly pavement material.     

Degradation of recycled PET fibers in Portland cement-based materials

In order to investigate the durability of recycled PET fibers embedded in cement-based materials, fiber-reinforced mortar specimens were tested until 164 days after mixing. Compressive, tensile, and flexural strengths, elasticity modulus, and toughness of the specimens were determined. The mortars were also analyzed by SEM. The results have shown that PET fibers have no significant influence on mortars strengths and elasticity modulus. However, the toughness indexes I₅, I₁₀, and I₂₀ decreased with time due to the degradation of PET fibers by alkaline hydrolysis when embedded in the cement matrix. Fourier transform infrared spectroscopy (FT-IR) and SEM analysis of PET fibers immersed and kept for 150 days in alkaline solutions supported the conclusions.     

Degradation of cement-based materials by various organic acids in agro-industrial waste-waters

The paper analyses the mechanisms of the degradation of cementitious materials by three organic acids: oxalic, citric and tartaric acids, and evaluates their relative aggressiveness compared to that of acetic acid. These acids have different chemical characteristics in terms of poly-acidity and chemical and physical properties of their calcium salts. The aim was to highlight the links between these properties and the aggressiveness of each acid toward concrete. Degradation mechanisms and kinetics were investigated (using EPMA, XRD and SEM) on cement paste specimens immersed in acid solutions for one year.The results showed that oxalic acid was not aggressive to the matrix. Citric acid caused the highest kinetics and severe degradations in the paste. Tartaric acid attack developed in two stages: no visible degradation was observed in the first weeks and then damage occurred, the kinetics remaining moderate. Acetic acid had intermediate aggressiveness.It was confirmed that the solubility of the calcium salts was a major parameter of aggressiveness. It also appeared that, even if salts precipitated, they were not necessarily protective, the protection being correlated with the salt's molar volume. Moreover, poly-acidity seemed, in some cases, to increase the aggressiveness of the acid.     

Toughening cement-based materials through the control of interfacial bonding

Mortars are made from inherently brittle components: sand grains and hardened cement paste. Under normal circumstances, cracks will propagate rapidly through the cement matrix, bypassing the strong sand grains but fracturing some of the weakest. The approach of the work described in this paper was to modify the mortar in order to alter this process. These modifications produced tensile residual stresses between the matrix and the aggregate, which when released by an additional applied tensile stress produced microcracking, debonding of matrix from aggregate, a small expansion and increased toughness. This work demonstrates toughening in sand/Portland cement mortars modified with different expansive admixtures: sodium sulphate or dead-burnt lime. Additionally, mortars of sand/ASTM Type K cement were tested. In order to give additional insight into the toughening mechanism, spherical and angular aggregate have been used to ascertain the consequences of microcracking and aggregate-bridging. The role of aggregate-bridging, especially when related to fracture paths, is also discussed and suggests that the bond between the aggregate and the matrix has been found in some cases to control not only the crack path but consequently the apparent toughness.     

An experimental study on the retrofitting effects of reinforced concrete columns damaged by rebar corrosion strengthened with carbon fiber sheets

This paper presents the results of an experiment study on the structural behavior of reinforced concrete (RC) columns damaged by rebar corrosion and the retrofitting effects of damaged RC columns strengthened with carbon fiber sheets (CFS). In the experiment, a cyclic horizontal loading test was carried out using RC columns damaged by different degrees of rebar corrosion and strengthened with CFS. As a result, it was revealed that the deterioration of their structural behavior was mainly caused by the decline in the confining effect due to the falling off of concrete cover and the reduction of mechanical properties of corrosion rebar. In addition, the test proved that shear strengthening using CFS is an very effective retrofit technique that prevents bond splitting cracks and shear cracks from growing and improves the ductility of RC columns with corroded rebars due to the confining effect of CFS.     

Modelling and simulations of the chemo-mechanical behaviour of leached cement-based materials: Interactions between damage and leaching

The assessment of the durability of cement-based materials, which could be employed in underground structures for nuclear waste disposal, requires accounting for deterioration factors, such as chemical attacks and damage, and for the interactions between these phenomena. The objective of the present paper consists in investigating the long-term behaviour of cementitious materials by simulating their response to chemical and mechanical solicitations. In a companion paper (Stora et al., submitted to Cem. Concr. Res. 2008), the implementation of a multi-scale homogenization model into an integration platform has allowed for evaluating the evolution of the mineral composition, diffusive and elastic properties inside a concrete material subjected to leaching. To complete this previous work, an orthotropic micromechanical damage model is presently developed and incorporated in this numerical platform to estimate the mechanical and diffusive properties of damaged cement-based materials. Simulations of the chemo-mechanical behaviour of leached cementitious materials are performed with the tool thus obtained and compared with available experiments. The numerical results are insightful about the interactions between damage and chemical deteriorations.     

高强耐磨水泥基材料的设计与性能

运用最紧密堆积理论对高强耐磨材料的集料进行配比设计,通过提高高强耐磨水泥基材料的集料堆叠密实度,提高其强度和耐磨性能。并参照RPC材料设计原理,通过外加剂的优选降低水固比、矿物掺合料掺量的复配以及增强材料的使用改善材料内部的多尺度结构,从而优化材料的施工工作性、抗裂性能、耐磨性,达到提高耐磨材料的服役性能与年限。     

Patch microstructure in cement-based materials: Fact or artefact?

The appearance of patch microstructure, i.e. broad dense and porous regions separated by sharp and distinct boundaries and occurring randomly in bulk and interfacial transition zones, has been reported previously in various site- and laboratory-mixed concretes and mortars. In this paper, evidence is presented to show that patch microstructure is an artefact of sample preparation and does not reflect the true nature of the hydrated cement paste. The appearance of dense patches comes from paste areas that have been ground and polished beyond the epoxy resin intrusion depth. In a backscattered electron image, pores not filled with epoxy are not visible because the signal is generated from the base or side walls of the pores. A modified method for epoxy impregnation, which can achieve a much deeper epoxy penetration than conventional vacuum impregnation, is presented.     

Transport properties of concrete pavements with excellent long-term in-service performance

Excellent long-term performance in concrete pavements is associated with both concrete strength and durability properties like permeability and chloride ion penetration resistance. Water and air permeability are investigated, as is the rapid chloride permeability test (RCPT). Strong relations between compressive strength and permeability and chloride ion penetration resistance of typical ordinary Portland cement (OPC) concrete pavements are observed. Effects of environmental exposure on permeability and RCPT results are explained. Through-depth variation of permeability and RCPT results within a pavement slab are shown and discussed.     

Influence of the porosity gradient in cement paste matrix on the mechanical behavior of mortar

This paper is devoted to the study of the influence of the microstructure of mortar on its mechanical behavior. For this purpose, mechanical tests have been carried out on mortars and a mathematical model, the (n + 1)-phase model, has been used to take into account three variable parameters of the microstructure of mortar (the thickness of the interfacial transition zone, the porosity gradient in the cement paste matrix and the nature of the constituents of the Interfacial Transition Zone) for some given parameters (volume fraction of aggregates, porosity of the mortar and mechanical behavior of the aggregates and the cement paste). By fitting some measured moduli to the model predictions, we can estimate in a non-destructive manner, the possible distribution of porosity within the Interfacial Transition Zone. Our results provide information on the data such micromechanical models can deal with in order to predict the elastic behavior of mortars.     

Influence of metakaolin on the properties of mortar and concrete: A review

Supplementary cementing materials (SCM) have become an integral part of high strength and high performance concrete mix design. These may be naturally occurring materials, industrial wastes, or byproducts or the ones requiring less energy to manufacture. Some of the commonly used supplementary cementing materials are fly ash, silica fume (SF), granulated blast furnace slag (GGBS), rice husk ash (RHA) and metakaolin (MK), etc. Metakaolin is obtained by the calcination of kaolinite. It is being used very commonly as pozzolanic material in mortar and concrete, and has exhibited considerable influence in enhancing the mechanical and durability properties of mortar and concrete. This paper presents an overview of the work carried out on the use of MK as partial replacement of cement in mortar and concrete. Properties reported in this paper are the fresh mortar/concrete properties, mechanical and durability properties.     

Pyroelectric behavior of cement-based materials

The pyroelectric effect, which is useful for temperature sensing, was observed in cement-based materials. The use of short steel fibers (8 μm diameter), together with polyvinyl alcohol (PVA), as admixtures greatly enhances the effect, thereby attaining pyroelectric coefficient 6×10⁻⁸ C/m² K (10 kHz). However, due to the high value (2500) of the relative dielectric constant, the pyroelectric voltage is lower than those of plain cement paste or carbon fiber (15 μm diameter) cement paste. Carbon fiber cement paste and plain cement paste are comparable in the pyroelectric voltage, though the pyroelectric coefficient is higher for carbon fiber cement paste than plain cement paste. The pyroelectric effect in cement-based materials is attributed to the increase in mobility of the ions as the temperature increases.     

A review of cement-based materials as electroceramics

The dielectric behavior of unpoled cured cement-based materials enables these materials to serve as electroceramics. The behavior entails the DC polarization (apparent DC electrical resistivity increase), permittivity (AC polarization, capacitance measurement) and DC electret (permanent electric dipole, voltage measurement). The dielectric behavior is not derived from functional admixtures such as the perovskite ceramics. The polarization involves charge-carrier polarization, with the carriers being primarily the ions in the pore solution. Dipolar polarization associated with the polar water molecules plays a minor role. Silica fume, if present, decreases the permittivity, partly due to the pore refinement. A polymer admixture, if present, increases the permittivity, with significant polarization resulting from the cement-polymer interface. Carbon fiber, if present, affects the electronic and ionic conduction, with the fiber’s ozone treatment promoting the ionic conduction and enhancing the permittivity. As the water/cement ratio increases, the permittivity increases, but the DC polarization decreases. The DC polarization occurs faster and more significantly than the subsequent depolarization. This reflects the electret, which discharges upon short circuiting (as in capacitor discharge) and subsequently charges back upon open circuiting. The temperature increases the permittivity or the electret’s electric field, whereas tension decreases the same, enabling capacitance-based/voltage-based self-sensing of temperature and stress/strain.     

Microstructural and chemical effects of wet/dry cycling on pulp fiber-cement composites

The microstructural and chemical mechanisms responsible for pulp fiber-cement composite degradation during wet/dry cycling are being investigated through environmental scanning electron microscopy (ESEM), energy dispersive spectroscopy (EDS), and mechanical testing. Based on these results, a three-part progressive degradation mechanism for cast-in-place kraft pulp fiber-cement composites is proposed, which involves: (1) initial fiber-cement or fiber interlayer debonding, (2) reprecipitation of needle-like or sheath-like ettringite within the void space at the former fiber-cement interface or between the S1 and S2 fiber layers, and (3) fiber mineralization due to reprecipitation of calcium hydroxide filling the spaces within the fiber cell wall structure. This investigation also revealed that kraft pulp fibers exhibit poor resistance to degradation due to their inferior dimensional stability, as compared to thermomechanical pulp (TMP) fibers. TMP fibers contain significant amounts of lignin, which is alkali sensitive. Despite this, TMP fiber-cement composite exhibit improved resistance to degradation during wet/dry cycling. It is proposed that this improvement in durability may be attributed to the presence of lignin in the cell wall restricting fiber dimensional changes during wetting and drying, and hence, minimizing fiber-cement debonding. Additionally, it is proposed that lignin acts as physical barrier to calcium hydroxide formation within the fiber cell wall, minimizing fiber mineralization of TMP fibers.     

Influence of limestone addition on calcium leaching mechanisms in cement-based materials

In order to investigate the mechanisms of calcium leaching and their implications on the long-term durability of cement-based materials, four different systems (C₃S, C₃S+C₃A+CaSO₄·2H₂O, C₃S+C₄AF+CaSO₄·2H₂O, and Portland cement) were prepared, mixed with water, and cured for several weeks. Four percentages of calcium carbonate addition (0%, 5%, 10%, and 20%) were used in the preparation of these systems. In all cases, the water/solid ratio was fixed at 0.5. At the end of the curing period, 1-mm thick samples were cut and immersed in distilled water. The kinetics of degradation was assessed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) analysis after 10, 20, 40, and 90 days of immersion. The volumetric stability was also followed by length-change measurements. Test results clearly indicate that the mechanisms of leaching are directly affected by the mineralogical composition of the system, in particular by the calcium carbonate content.     

Mechanical and fracture properties of cement-based bi-materials after thermal cycling

This study investigates the effect of thermal cycles on the fracture properties of the cement-based bi-materials. Sixty eight cubes were exposed to a varied number of 24-hour thermal cycles ranging from 0 to 90 and subsequently were tested in a wedge splitting configuration. The mechanical and fracture properties of normal strength and high strength concretes are substantially improved after 30 thermal cycles, but less so after 90 thermal cycles both in isolation and when bonded to an ultra high-performance fibre-reinforced cement-based composite.     

Enhanced dielectric and mechanical properties of CaCu3Ti4O12/Ti3C2Tx MXene/silicone rubber ternary composites

Dielectric polymer composites with conducting fillers would have great potential for diverse applications if their severe leakage loss could be addressed. In this regard, ternary composites using both ceramic and conducting materials as fillers might be an enabler for high dielectric constant and low dielectric loss. Herein, ternary composites with both Ti₃C₂T_x MXene conducting nanosheets and CaCu₃Ti₄O₁₂ (CCTO) dielectric particles embedded in silicone rubber were studied. It was found that a ternary composite with 1.2 wt% (0.40 vol%) Ti₃C₂T_x MXene and 12 wt% (2.58 vol%) CCTO could provide an overall superior performance that include a high dielectric constant of 8.8, low dielectric loss of less than 0.0015, good thermal stability up to 450 °C, and excellent mechanical properties with tensile strength of 569 kPa, elastic module of 523 kPa and elongation at break of 333%. The outstanding performance is attributed to the improved uniform dispersion and good interfacial compatibility of mixed fillers in the polymer matrix, suggesting ternary composites might be a better option over their binary counterparts in preparing high performance dielectric composites.