Zhihui Sun

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.RésuméCet article présente les résultats d’une étude effectuée dans différents centres de recherche destinée à comparer deux méthodes non-destructives utilisant les ondes ultrasoniques: la première méthode, dite à transmission d’ondes utilise les les ondes «P» (pression) alors que la deuxième, dite à réflexion d’ondes, utilise les ondes «S» (S de l’anglais Shear, ou effort tranchant). Ces expérimentations font partie des activités du Comité RILEM TC ATC-185 qui a pour objectif d’évaluer la capacité de ces méthodes de surveiller en continu les processus de prise et de durcissement des matériaux cimentaires. Au total, 8 mortiers et bétons différents ont été soumis aux ultrasons. Des mesures de pénétration (ASTM C 403), d’accroissement de température, de chaleur adiabatique et rétrécissement chimique ont aussi été faites sur ces mêmes mélanges. Les résultats montrent que chacune des méthodes ultrasoniques rend compte de la prise du mortier et du béton de manière très différente. En dépit de ces différences, la comparaison entre les mesures aux ultrasons et les mesures de chaleur adiabatique et de rétrécissement chimique montre une corrélation directe entre les paramètres obtenus avec les ultrasons (vitesse des ondes «P» et perte de réflexion avec les ondes «S») et le processus d’hydratation du ciment.

Microstructure and Early-Age Properties of Portland Cement Paste-Effects of Connectivity of Solid Phases

Reliable information about the early-age properties of cementitious materials is vital in order to assure high quality and avoid problems in performance throughout the life of a concrete structure. This article describes the effect of the connectivity of the solid phase in the microstructure on the mechanical behavior of cement pastes. The authors report on an experimental study of the development of the compressive strength and elastic moduli of the Portland cement pastes with three different water-cement ratios (0.35, 0.5, and 0.6) cured under an isothermal condition (25 deg C.). In addition, the cement hydration model HYMOSTRUC3D was used to simulate the formation of the microstructure. The authors conclude that the percolation threshold of the solid phase in the microstructure of a cement paste is closely related to the w/c (water to cement ratio) that is used. The solid phase percolated earlier, and at a lower degree of hydration, in a cement paste with a lower w/c than in a paste with a higher w/c. The specific effective contact area is a microstructure parameter that describes the degree of interparticle bonding among the hydrating particles. The specific effective contact area appears to be directly related to both the measured compressive strength and the elastic moduli.

Rheological method to evaluate structural buildup in self-consolidating concrete cement pastes

Much research has focused on determining the rheological properties of cementitious materials, but limited information can be found on the influence of thixotropy in affecting the flow properties of these materials. A protocol consisting of hysteresis loops and energy methods, was developed to quantify the degree of structural rebuilding in cement pastes. This protocol takes into account the stiffening due to thixotropic rebuilding and irreversible structural changes by focusing on the rate in which the cement paste is able to regain its internal structure after shearing. The protocol was used to evaluate the rebuilding potential of 23 cement paste mixtures of various binder type and fluidity level. Results show that structural build up can be controlled through selection of proper powder type, high-range water-reducing admixture, and water-to-binder ratio (w/b).

Experimental simulation of self-consolidating concrete formwork pressure

Developing a laboratory device to describe self-consolidating concretes (SCCs) formwork pressure behavior was this investigations aim. The cost and time needed for the same research to be conducted on real structures was reduced by the development of such an apparatus. Pressurizing a volume of material inside a cylinder and recording lateral pressure evolution allowed casting rate and mixture composition effects to be studied. There was simulation of two different casting rates and columns measuring 14 m (46 ft) in height. Four different water-binder (w/b) ratios and different binder compositions were used in mixture design. That less than hydrostatic formwork pressures were achievable was shown through results. Higher w/b and casting rates were associated with higher pressures. A casting rate of 7 m/hour (23 ft/hour) and a w/b of 0.32 was recorded for a formwork pressure reduction up to 50% of the hydrostatic value. That fly ash incorporation reduces SCC formwork pressure was also shown by data.

Temperature effects on strength evaluation of cement-based materials with ultrasonic wave reflection technique

Among the various factors that influence the strength gain of cementitious materials at early age, curing temperature can be regarded as an important parameter due to its great effect on the hydration kinetics of portlandcement. The temperature effects on the relationship between the compressive strength of paste, mortar, or concrete and the reflection loss obtained with ultrasonic shear-wave reflection measurements were observed. In the study, three different constant curing temperatures (15, 25, and 35 °C) were used for a given paste or mortar. The temperature effects were observed by comparing the established relationship between reflection loss and compressive strength for portland cement mortar under different isothermal curing conditions. Additionally, the established correlations between the two studied parameters were verified by applying alternating curing temperatures to the mortar. As a third step, the temperature dependence of the relationship between the two studied parameters was further verified by observing concrete cured under various conditions. The study shows that for a given material, the relationship between the reflection loss and the compressive strength is independent of curing temperature.

Artificial Neural Network Modeling of Early-Age Dynamic Young’s Modulus of Normal Concrete

A comprehensive model for dynamic Young’s modulus of early-age normal concrete is proposed based on its original mixture design. Based on experimental work and the artificial neural network (ANN) technique, various cement paste, mortar, and concrete mixtures’ elastic moduli were modeled on a single platform. The role of hydration age, water-cement ratio (w/c), curing temperature, aggregate volume percentages, aggregate absorption capacities, and aggregate sizes on the development of elastic moduli were considered. Using the generalization capabilities of ANN, effective mixture design parameters affecting the concrete elastic modulus evolution at early ages were identified. From this study, it was possible to quantify the effect of age, w/c, coarse aggregate volume percentage, and curing temperature as main parameters in the evolution of concrete elastic modulus. It was concluded that with no need of complex modeling procedures, early-age concrete elastic modulus can be properly modeled by using an ANN technique based on original mixture proportions.

A Comparison of Test Methods for Early-Age Behavior of Cementitious Materials

A wide range of tests has been reported that can be used for probing different aspects of the chemical and structural properties of cement paste and concrete at early ages. In principle, these tests should be complementary and their results could be integrated to develop a broader understanding of early-age behaviour. Under the auspices of the Center for Advanced Cement Based Materials (ACBM), an inter-laboratory study was undertaken to compare the results of several of these different test methods on a single cement. A large quantity of cement was homogenized by the Cement and Concrete Reference Laboratory (CCRL), as part of their proficiency sample program, and distributed to several ACBM researchers for independent testing. Aggregates for making mortar were supplied from a single source (graded Ottawa sand). Tests on pastes and mortars included calorimetry, compressive strength, Vicat needle penetration, chemical shrinkage, autogenous shrinkage, AC-impedance spectroscopy, time domain reflectometry, and ultrasonic shear wave reflectometry. Numerical simulations of the same materials were undertaken for comparison. Results from these tests will be compared with respect to the way in which they capture solidification and strength development.

Prediction of Early Age Normal Concrete Compressive Strength Based on Dynamic Shear Modulus Measurements

In this research, the relationship between the compressive strength, fc, and the dynamic shear modulus, Gd, of normal concrete at an early age was studied. To investigate the correlation between fc and Gd at an early age, different types of mixtures, including mortar and concrete, were prepared, and the corresponding fc and Gd values were measured every 12 h after initial mixing up to 72 h after casting. The influences of hydration age, water-to-cement (w=c) ratio, curing temperature, aggregate volume content, and maximum aggregate size on the Gdfc relationship of concrete were studied. The Gdfc relationship was then mathematically modeled by using multivariable power laws. The developed model is reasonably accurate to predict the early age compressive strength of concrete with variations in hydration age, aggregate content, and sizes. This type of model can be used directly in the field for the estimation of concrete strength when nondestructive testing (NDT) techniques are employed. DOI: 10.1061/(ASCE)MT.1943-5533.0000528.

Observation of Cement Paste Microstructure Evolution

During early hydration in fresh concrete, the cement paste hardens and bonds the aggregates together to make a solid concrete mass. During this process, along with chemical reactions, physical changes in the cement paste at the microlevel and the nanolevel also play major roles in the development of the properties of the concrete. An attempt was made to study the microstructure of cement paste and the evolution of its corresponding elastic modulus during setting. For this purpose, two different in situ testing methods, scanning electron microscopy (SEM) with a Quantomix capsuling system and a nondestructive one-sided ultrasonic technique, ultrasonic wave reflection method, were used. The features in the SEM images were quantified by the use of image analysis techniques, and quantitative analysis was used to examine the relationship between the microstructure of the cement paste and the corresponding dynamic shear modulus during the initial and final setting times. Cement particle growth, motions, and rotations were observed during the evolution of the microstructure of the cement paste. A good correlation between solid-phase and elastic modulus developments was observed after 6 h of mixing for all three pastes tested. Finally, it has been concluded that in situ studies of the microstructures of fresh cement paste can greatly enhance knowledge of the development properties of concrete at an early age.

Hydration of Concrete Containing Hybrid Recycled Demolition Powders

AbstractIf not properly managed, the hybrid powders produced during recycling mixed construction and demolition (C&D) materials will present a serious hazard to the environment and human health. Re...