Comparison of ultrasonic wave transmission and reflection measurements with P- and S-waves on early age mortar and concrete

This paper reports on results of round robin tests comparing two nondestructive, ultrasonic techniques: the wave transmission method using P-waves and the wave reflection method using S-waves. The experiments were conducted within the activities of the RILEM Technical Committee TC ATC-185 with the objective to evaluate the ability of these methods to continuously monitor the setting and hardening process of cementitious materials. In total, eight different mortar and concrete mixtures were subjected to the ultrasonic tests. Additionally, experiments were conducted to determine the penetration resistance (ASTM C 403), the in-situ temperature rise, the adiabatic heat release, and the chemical shrinkage (of the cement paste phase) of the investigated materials. The results revealed that, originating from the different wave types, the two ultrasonic methods monitor the setting process of mortar and concrete in significantly different ways. Despite these differences, the comparison of the ultrasonic test results with the development of the adiabatic heat and the chemical shrinkage has proven that P-wave velocity and reflection loss, as the parameters measured by the two methods, have a consistent and direct relationship to the cement hydration process.     

Application of ultrasonic P-wave reflection to measure development of early-age cement-paste properties

Ultrasonic wave reflection (UWR), well established for the study of stiffening and strength development of cement-based materials, normally uses shear waves (S-waves) because they are sensitive to microstructural development. This study demonstrates possible application of UWR with longitudinal waves (P-waves) using a low impedance buffer for investigating stiffening in hydrating cement paste. The P-wave reflection coefficient was seen to increase modestly as hydration progressed. Also, the P-wave reflection coefficient showed higher values for pastes with lower water to cement ratios, which are primarily attributed to the higher density of low w/c pastes, and similar effects with addition of fly ash and entrained air. Partial debonding between paste and buffer was observed in most pastes at a time that coincided with final set as measured using S-waves; and the debonding appears to be associated with the development of pore water under-pressure that occurs after solidification (due to chemical shrinkage).     

Hydration monitoring of cement-based materials with resistivity and ultrasonic methods

Two test setups, the electrical resistivity and ultrasonic techniques, were used to monitor the hydration process of cement-based materials. In the electrical resistivity method, a non-contacting device was used. In the ultrasonic method, a wave was transmitted and measured by the embedded piezoelectric ultrasonic transducers, which had good coupling with the surrounding materials. The focus of the study was to detect the setting and hardening behaviors of cement paste during the first 7 days of hydration using the above techniques. Immediate after placing the cement paste into the mould, the measurement started and continued throughout the hydration process. The obtained resistivity and ultrasonic data were used to interpret the hydration process of the specimens. The correlation of two techniques was also studied. The results illustrated that both electrical resistivity and ultrasonic techniques were effective to accurately monitor the hydration of cement pastes. The resistivity method was able to study both the chemical reaction and physical change during hydration, while ultrasonic method was sensitive to physical change of cement only.     

Electrode configurations and impedance spectra of cement pastes

Electrode effects on impedance spectra of cement pastes were investigated by two-, three-, and four-point measurements without a potentiostat over the frequency range 0.01 Hz–10 MHz. Electrode immittance effects arising from highly resistive/capacitive contacts cannot be fully corrected by nulling procedures. Two-point measurements are much more susceptible to such effects than three- or four-point measurements. The three- and four-point results on pastes suggest that there is negligible high-frequency offset resistance, and that bulk paste arcs are not significantly depressed below the real axis in Nyquist plots. The important impedance-derived equivalent circuit parameters are bulk resistance and capacitance; offset resistance and arc depression angle may not be physically meaningful parameters. Whereas all electrode configurations give reliable values of bulk paste resistance, only the three-point configuration provides the total paste/electrode dual arc spectrum involving a single electrode. Multielectrode (three- or four-point) measurements may be necessary to establish the true bulk paste dielectric constant.     

Penetration resistance and electrical resistivity of cement paste with superplasticizer

The purpose of this paper is to investigate the setting process and evolution of electrical resistivity of Portland cement pastes with constant water to cement ratio (w/c) of 0.3 and with different dosages of naphthalene superplasticizer (SP) from 0 to 1.2 %. The setting process of cement paste was monitored by the Vicat needle test. The depth of penetration was recorded and used to calculate the shear resistance generated by the cement paste. Electrical resistivity was measured by a non-contacting electrical resistivity apparatus. The hyperbolic curve of electrical resistivity versus time was plotted to determine the ultimate electrical resistivity. The results show that the addition of SP to the pastes with a fixed w/c can cause longer setting time and delay the evolution of electrical resistivity. The final setting time (t _f) and the occurring time of maximum rate of electrical resistivity (t _r) were both delayed when the dosage of SP was increased. This may indicates that the electrical resistivity measurement can be used to monitor the setting process of cement. The compressive strength at 28 days and the ultimate electrical resistivity show a same tendency for the cement pastes with different dosages of SP. Thus, it would be possible to predict the compressive strength of hardened cement paste by its ultimate electrical resistivity.     

硫酸盐环境下灌浆材料长龄期力学性能及微观结构分析

本文将抗硫酸盐水泥和中热水泥掺粉煤灰制备的水泥浆体浸泡在5%Na2SO4溶液至1110d,研究长龄期硫酸盐侵蚀下各试件的力学性能和微观结构。结果表明:限制空间中形成的细小钙矾石是引起基体开裂的主要原因,石膏的形成会引起水泥基材料剥落,抗硫酸盐水泥不能有效防止石膏型硫酸盐侵蚀;大掺量粉煤灰的二次水化反应能够消耗大量氢氧化钙,从而降低侵蚀过程中石膏相的形成且能有效改善浆体微结构;水电工程中采用中热水泥+50%粉煤灰制备的水泥基材料能够有效抑制钙矾石和石膏的形成,其抗硫酸盐侵蚀性能和经济性明显优于抗硫酸盐水泥制备的水泥基材料。     

The early-age short-term creep of hardening cement paste: AC impedance modeling

Impedance spectra were monitored at early ages on hydrating Portland cement pastes subjected to a sustained load. The pastes were prepared with two different water-cement ratios (0.35 and 0.50). The experiments were conducted in a controlled chamber maintained at (96±2)% relative humidity. The three ages at loading investigated were 18, 24 and 30 hrs. Real-time changes in paste microstructure due to sustained load were followed through the coupling of an AC impedance frequency analyzer with a miniature loading system. Cement paste specimens were in the form of T-shaped columns with a minimum thickness value (for the web and flanges) less than 1.25 mm. The impedance analysis included an assessment of the relevance of the high frequency arc depression angle to an understanding of the creep and shrinkage behavior of cement paste. Electrical models were developed in order to predict the creep coefficient of normal (w/c = 0.50) and high strength (w/c = 0.35) cement pastes from early age data.     

Optimization of calcium chloride content on bioactivity and mechanical properties of white Portland cement

This research investigates the optimization of calcium chloride content on the bioactivity and mechanical properties of white Portland cement. Calcium chloride was used as an addition of White Portland cement at 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10% by weight. Calcium chloride was dissolved in sterile distilled water and blended with White Portland cement using a water to cement ratio of 0.5. Analysis of the bioactivity and pH of white Portland cement pastes with calcium chloride added at various amounts was carried out in simulated body fluid. Setting time, density, compressive strength and volume of permeable voids were also investigated. The characteristics of cement pastes were examined by X-ray diffractometer and scanning electron microscope linked to an energy-dispersive X-ray analyzer. The result indicated that the addition of calcium chloride could accelerate the hydration of white Portland cement, resulting in a decrease in setting time and an increase in early strength of the pastes. The compressive strength of all cement pastes with added calcium chloride was higher than that of the pure cement paste, and the addition of calcium chloride at 8 wt.% led to achieving the highest strength. Furthermore, white Portland cement pastes both with and without calcium chloride showed well-established bioactivity with respect to the formation of a hydroxyapatite layer on the material within 7 days following immersion in simulated body fluid; white Portland cement paste with added 3%CaCl₂ exhibited the best bioactivity.     

Properties of polyvinyl alcohol cement pastes

Properties of cement pastes containing varying amounts of each of polyvinyl alcohol (PVAL), mixtures of polyvinyl alcohol and phenol formaldehyde (PF) and mixtures of poly vinyl alcohol and borax were studied in this paper. Though the strength parameters of the PVAL-cement pastes are comparable to virgin cement paste their resistance to acid is far superior. Soxhlet extraction with water, done to determine leachability of the polymer from the polymer cement paste, revealed that the percentage of polyvinyl alcohol leached out was less when borax or PF resin was added to the PVAL cement paste. The compressive strength of the poly vinyl alcohol–phenol formaldehyde cement paste was found to be inferior to the other two cases but the retention of compressive strength after immersing in each of acid, base and kerosene was much better. In general, polyvinyl alcohol when added to cement pastes improves the chemical resistance properties in terms of retention of compressive strength after exposure to chemicals.     

Monitoring Early Age Properties of Cementitious Material Using Ultrasonic Guided Waves in Embedded Rebar

The evaluation of early age properties of concrete is critical for ensuring construction quality. This paper presents a sensing method to use ultrasonic guided waves in a rebar for monitoring the early age properties of cementitious material. An EMAT sensor was used to excite the longitudinal mode L(0,1) wave in a rebar embedded in cement/mortar, and an ultrasonic transducer was used for receiving the echo signals. Guided wave dispersion curves were developed to select appropriate frequency range. The leakage attenuations of the L(0,1) mode wave from the rebar to the surrounding cement materials were continuously monitored for the first 10 h. The evolution of the shear wave velocity was also monitored simultaneously. The leakage attenuation from experimental measurements was compared with the theory-predicted attenuation in both time and frequency domains, and showed good agreement. Experiments were performed on three cement paste samples and three mortar samples. The results indicated that attenuation is nearly linearly related to the shear wave velocity, and shear wave velocity is linearly related to the penetration resistance (ASTM C403) in logarithmic scale. These results suggest that mechanical properties and hardening process of cement materials can be monitored by using the ultrasonic guided waves in a rebar.     

Penetration of cement pastes into sand packings during 3D printing: analytical and experimental study

3D printing processes of concrete and cement based materials could bring architectural and structural innovation in construction industry. Additive manufacturing and digital fabrication methods in civil engineering have recently been developed at laboratory scale. Among the 3D printing processes that could bring new perspectives in innovative and designed architectural elements, one of the most interesting is called the selective paste intrusion method. The component is built layer by layer by selectively applying cement paste on an aggregate packing using a 3D printer nozzle and a subsequent penetration of the paste into the aggregate layer. The implementability of the selective paste intrusion method requires the prediction of the flow of a yield stress fluid through a porous media. The rheological behaviour of the cement paste must be adapted for its penetration through the porous network of the aggregate particle packing. An adequate penetration depth of the cement paste produces homogeneous materials that are capable of sustaining a high mechanical stress. We show in this paper that the compressive strength of component made by such a technique is directly linked to the penetration depth of the cement paste into the aggregate layer; consequently, this paper aims at predicting the penetration depth of cement pastes into sand layers. A theoretical framework has been developed to propose an evaluation of penetration depth as a function of the average sand grain diameter and the yield stress of the cement paste, which is experimentally validated with specific penetration measurements. Finally, we stress that the prediction of penetration with an analytical model is an effective technique to ensure building homogeneous cement based materials with the 3D printing selective binding method.     

Susceptibility of Portland cement and blended cement concretes to plastic shrinkage cracking

The market share of different types of blended cements is increasing year by year. Generally, blended cements are ground to higher fineness and exhibit a slower development of mechanical properties compared to Ordinary Portland Cement (OPC), which might affect the concrete performance in terms of shrinkage cracking at early ages.In this paper, the performance of concretes made with different cement types is compared according to the ASTM C1579-13 standard for plastic shrinkage cracking. The cracking behavior was further correlated to the deformations of both unrestrained and restrained specimens measured by a 3D image correlation system. The main factors influencing the cracking behavior were discussed based on poromechanics. It is concluded that the bulk modulus evolution has a dominant effect on controlling the plastic shrinkage cracking. Concretes made of more reactive cements, in particular with higher clinker content, are less susceptible to plastic shrinkage cracking. For cements with the same clinker content, increasing the cement fineness reduces the risk of plastic shrinkage cracking.     

Computer simulation of the diffusivity of cement-based materials

A digital image-based model of the microstructure of cement paste, coupled with exact transport algorithms, is used to study the diffusivity of Portland cement paste. The principal variables considered are watercement ratio, degree of cement hydration and capillary porosity. Computational methods are described and diffusivity results are presented, which are found to agree with the available experimental measurements within experimental error. Model cement pastes prepared with different watercement ratios, and having different degrees of hydration, are found to have diffusivities that lie on a single master curve when plotted as a function of capillary porosity. Concepts from percolation theory are used to explain quantitatively the dependence of diffusivity on capillary porosity. The effect of silica fume addition on diffusivity is also examined.     

Microstructural Development of Hydrating Portland Cement Paste at Early Ages Investigated with Non-destructive Methods and Numerical Simulation

Microstructure development of hydrating cement paste at early ages is not only an indicator of the reactivity of cement, but also a factor on the workability of fresh concrete. In this study, the microstructure development of hydrating cement paste at early ages is investigated with non-destructive methods including ultrasound P-wave propagation velocity measurement and non-contact electric resistivity tests, together with conventional needle penetration depth and calorimetry tests. The hydration process and microstructural development of the cement paste is modeled with the three-dimensional computer model CEMHYD3D. Evolution of microstructural parameters including the volumetric fraction of phases and their percolation status are analyzed by using the results of the numerical simulation. Microstructural mechanisms of the two non-destructive techniques (ultrasound pulse propagation and electric resistivity measurements) are discussed. The main findings of this study are that the velocity of ultrasound P-wave propagation in hydrating cement paste is a function of the propagation routes in the material and inter-particle forces. The electric resistivity is controlled by the ionic concentrations in the pore solution during the early hours and later by the connectivity of pores. A model for the development of ultrasound P-wave propagation velocity is also proposed.     

An assesment of porosity and pore sizes in hardened cement pastes

The porosity of hardened cement paste was analysed using fluid displacement methods, optical, scanning and transmission electron microscopy and mercury intrusion porosimetry. Attention has been drawn to the problems of mercury porosimetry and in particular to the extent of the pore volume which is missed by mercury. Previous workers have assigned the lost porosity to pore sizes too fine to be seen by mercury but this paper considers the contribution of closed pores. Nitrogen and water adsorption studies have been carried out on the pastes to determine qualitatively the magnitude of the pore volume beyond the range of mercury porosimetry. This volume was found to be very small. Emphasis has been placed upon the importance of large spherical pores, which may be missed by mercury porosimetry and adsorption studies, in determining the strength of cement pastes.     

Anti-washout carboxymethyl chitosan modified tricalcium silicate bone cement: preparation,mechanical properties and in vitro bioactivity

Anti-washout CaF₂ stabilized C₃S (F-C₃S) bone cement was prepared by adding water-soluble carboxymethyl chitosan (CMCS) to the hydration liquid. The setting time, compressive strength and in vitro bioactivity of the CMCS modified F-C₃S (CMCS–C₃S) pastes were evaluated. The results indicate that CMCS–C₃S pastes could be stable in the shaking simulated body fluid (SBF) after immediately mixed. The addition of CMCS significantly enhances the cohesion of particles, at the same time restrains the penetration of liquid, and thus endows the anti-washout ability. The setting times of the pastes increase with the increase of CMCS concentrations in the hydration liquid. Besides, the compressive strengths of CMCS–C₃S pastes after setting for 1–28 days are lower than that of the pure F-C₃S paste, but the sufficient strengths would be suitable for the clinical applications. The crystalline apatite deposited on the paste surface is retarded from 1 to 2 days for the addition of CMCS, but the quantities of deposited apatite are same after soaking in SBF for 3 days. As the result that pure C₃S paste has shorter setting times than pure F-C₃S paste, CMCS modified pure C₃S pastes would have better anti-washout ability. Our study provides a convenient way to use C₃S bone cement with excellent anti-washout ability when the pastes are exposed to biological fluids. The novel anti-washout CMCS–C₃S bone cement with suitable setting times, sufficient strengths and in vitro bioactivity would have good prospects for medical application.     

Nanosilica effects on composition and silicate polymerization in hardened cement paste cured under high temperature and pressure

Cement pastes of water to cement ratio (w/c) of 0.45 with and without nanosilica are hydrated under two conditions, room condition (20 °C with 0.1 MPa pressure) and an oil well condition (80 °C with 10 MPa pressure) for 7 days. For the cement pastes with nanosilica, 1% and 3% of cements weights were replaced by nanosilica. The composition of the hardened cement pastes is investigated using X-ray diffraction (XRD). Nuclear magnetic resonance (NMR) experiments are used to quantify the silicate polymerization in hydrated cement paste. Microstructural phases are identified according to the corresponding mechanical property using nanoindentation. The results showed that under room curing conditions, hardened cement paste with 1% nanosilica has the highest level of calcium silicate hydrate (C–S–H) polymerization. However, under high temperature and pressure curing conditions, hardened cement paste with 3% nanosilica has the highest level of C–S–H polymerization. A new relatively stiff microstructural phase is observed in cement pastes incorporating nanosilica and cured under elevated pressure and temperature conditions. The significance of curing conditions and nanosilica content on the polymerization and stiffness of hydrated cement pastes are discussed.     

Flexural strength reduction of cement pastes exposed to CaCl2 solutions

Calcium chloride (CaCl₂) can react with calcium hydroxide (Ca(OH)₂) to form calcium oxychloride which can reduce flexural strength and damage concrete. This paper aims to characterize the reduction in flexural strength of cement pastes exposed to CaCl₂ solutions using the ball-on-three-balls test. The amounts of Ca(OH)₂ and calcium oxychloride in the cement paste are measured using thermogravimetric analysis and low-temperature differential scanning calorimetry, respectively. The volume change that occurs as a result of the reactions between the cement paste and CaCl₂ is also measured. The reduction in flexural strength increases as the concentration of the CaCl₂ solution increases and the exposure temperature decreases. The flexural strength reduction can be mitigated by increasing the amount of supplementary cementitious materials (fly ash) in the cement pastes. Lowering the water-cementitious materials ratio also reduces the flexural strength reduction. The flexural strength reduction is correlated with the amount of calcium oxychloride and the volume change in the cement pastes exposed to the CaCl₂ solution. While the flexural strength reduction is believed to be primarily due to the formation of calcium oxychloride, the formation of Friedel's salt and Kuzel's salt also contributes to the flexural strength reduction.     

Time-dependent mechanical properties of high-strength cements

A high-strength cement paste based on aluminous cement with the addition of water-soluble polymer was found to have a flexural strength which increased at about 12 MPa per decade increase in strain rate over the range 10⁻⁶ to 10⁻² sec⁻¹. The effect of soaking in water was to markedly reduce the strength. Swelling occurred on soaking in water for both Portland and aluminous cement-based pastes but was more gradual for the latter. Wetting swelling and drying shrinkage increased with polymer content for Portland-based pastes. Creep and stress relaxation in one type of Portland cement-based paste and in aluminous cement-based paste were markedly increased by soaking, but in a second type of Portland-based paste there was little effect. These effects are attributed to the polymer content of the pastes and its distribution in the pastes.     

Calcium aluminate cements in fly ash/calcium aluminate blend phosphate cement systems: Their role in inhibiting carbonation and acid corrosion at a low hydrothermal temperature of 90°C

Study was focused upon formulating sodium polyphosphate-modified fly ash/calcium aluminate blend (SFCB) geothermal well cements with advanced anti-carbonation and anti-acid corrosive properties. At a low hydrothermal temperature of 90°C, to improve these properties, we investigated the effectiveness of various calcium aluminate cement (CAC) reactants in minimizing the rate of carbonation and in abating the attack of H₂SO₄ (pH 1.6). We found that the most effective CAC had two major phases, monocalcium aluminate (CA) and calcium bialuminate (CA₂), and a moderate CaO/Al₂O₃ ratio of 0.4. The reaction between sodium polyphosphate (NaP) and CA or CA₂ at room temperature led to the formation of amorphous dibasic calcium phosphate hydrate and anionic aluminum hydroxide caused by the decalcification of CA and CA₂. When SFCB cement made with this CAC was exposed to 4% NaHCO₃-laden water at 90°C, some carbonation of the cement occurred, forming calcite that was susceptible to the reaction with H₂SO₄. This reaction resulted in the deposition of gypsum gel scales as the acid corrosion product on the cement surfaces. The scale layer clinging to the cement protected it from further corrosion. Under such protection, the amorphous dibasic calcium phosphate hydrate crystal hydroxyapatite and anionic aluminum hydroxide crystal boehmite phase transitions were completed in acid solution. Meanwhile, the further chemical and hydration reactions of NaP with fly ash led to the formation of additional crystalline Na-P type zeolite phases. Thus, we propose that passivation of the surface of the cement by deposition of gypsum, following the formation of these reaction products, which are relatively inert to acid, are the acid corrosion-inhibiting mechanisms of the SFCB cements.