Fresh Cement-based Materials Research Articles

Physical Characterization of Cementitious Materials on Casting and Placing Process.

Coagulation of cement particles is an inevitable phenomenon of fresh cement-based materials undergoing solidification. Coagulation can be classified into two types, reversible flocculation and irreversible coagulation, wherein microstructural change affects the rheological properties, including shear thinning and thixotropy, and the hydration process. This paper attempts to measure the mechanical property and the coagulation of cement particles according to the mix proportions of cement paste. Experimental setups were proposed for two different types of coagulations using a laser backscattering instrument. Volume fraction and size distribution of coagulated particles were obtained, and their variations were discussed. From the obtained results the microstructural buildup of freshly mixed cement pastes can be divided into three categories: permanent coagulation and strong and weak flocculation.

Effect of coarse particle volume fraction on the hydraulic conductivity of fresh cement based material

Hydraulic conductivity or permeability is a key parameter that affects the stability of fresh cement-based materials. For example, bleeding and water retention are among the most important issues governed by concrete permeability. Whereas this parameter is relatively easy to measure at the cement paste scale, it is difficult to evaluate with the addition of sand or gravel due to the sample size needed to obtain a representative sample. For ordinary permeability measurement, the test duration is linearly linked to the sample height. The aim of this study is to relate the concrete permeability to the cement paste permeability for various aggregate volume fractions in mortar or concrete. The proposed model uses an adaptation of Darcy’s law to compute concrete permeability from measured cement paste permeability and mix-design parameters. This model has been successfully tested with kaolin and cement pastes with spherical aggregates (glass beads) and sand.

Determination of the consolidation coefficient of low compressibility materials: application to fresh cement-based materials

This paper focuses on the determination of the consolidation characteristics of mineral materials. The main objectives were to improve measurement accuracy and to reduce test duration. Consolidation characteristics govern fluid migration and the evolution of the properties; compressibility, permeability and consolidation coefficient are required to describe hydro-mechanical behaviour. In this paper, a method of consolidation coefficient determination is proposed. After the measurement of compressibility and permeability, the evolution of the consolidation coefficient as a function of void ratio was computed. The method was applied to two different mineral materials: a clay material (kaolin paste) and a fresh cement paste. The results obtained were compared with commonly used methods described in national standards; comparison showed that the developed procedure is quicker and provides more reliable results.

Prediction of extrusion load and liquid phase filtration during ram extrusion of high solid volume fraction pastes

For perfect and homogeneous plastic materials, the extrusion stress and flow typology have previously been well described. However for firm pastes, during extrusion, liquid phase migration may appear leading to unsuccessful processing. Reviewed literature explains that there is a competition between the extrusion velocity and the liquid filtration velocity (or time).The aim of this work is to develop a new model to predict the ram extrusion force of frictional plastic materials such as firm fresh cement-based materials or clay pastes. A quantitative one-dimensional model based on consolidation theory has been developed to predict the extrusion force and quantify liquid phase migration.Model results are compared with experimental data from tests carried out on kaolin paste undergoing ram extrusion. Influences of extrusion rate, rheological parameters and filtration characteristics on extrusion load are investigated.

Cement-based mixes: Shearing properties and pore pressure

This study presents original results on the rheological measurement of concrete mixes. It focuses on how to determine their mechanical and physical behavior under shearing stress. More specifically, the influence of aggregate content on shearing properties is studied. A vane rheometer was developed to characterize fresh cement-based materials. In addition to the conventional concrete rheometer, a special hydraulic pressure transducer was fitted to the container to monitor the pore water pressure variation while shearing the material. Experiments on cement paste, mortar, and concrete bring a new approach to help us understand the behavior of fresh-state mixes. The results show 1) a correlation between water pore pressure and torque applied on the vane; 2) a critical sand volume fraction, ϕ c, as a limit between colloidal interaction behavior and frictional behavior in mortars; beyond this critical fraction, a leap in yield stress and a drop in pore pressure due to granular dilatancy are noticed; 3) the granular content clearly influences the increase in yield stress of the cement mixes: above ϕ c, this increase becomes negligible.

Assessment of Air Entrainment in Fresh Cement Paste Using Ultrasonic Nondestructive Testing

Abstract It is understood that the frost protection afforded by entrained air voids in cement-based materials is dependent on their size and distribution or spacing factor. The common practice of adding air-entraining admixtures (AEAs) to concretes and mortars demands economical quality control measures of the air-entrained voids. However, conventional methods for qualifying air content in fresh cement-based materials, such as the pressure, volume, and gravimetric methods, measure only total air volume and cannot assess size (i.e., allow discrimination between entrained and entrapped air voids) or spacing. Ultrasonic monitoring may present an alternative in situ approach for these measurements. In this investigation, using matched pairs of transducers, ultrasonic pulses were transmitted through fresh cement paste specimens (containing 0.0 % up to 0.6 % AEA by weight of cement). The received signals were recorded every 5 min during the first 6 h and then every 15 min thereafter. Analysis of the signals shows strong distinctions between specimens with and those without the AEA. In general, the addition of AEA suppresses the peak-to-peak signal strength, pulse velocity, and peak frequency of the signal transmissions through the specimens. The data also suggest correlations between Vicat setting times, heat of hydration, and autogenous strain and ultrasonic metrics. The findings of this research should be most appropriate as a foundation for an inversion process and improved air-entrainment detection methods.

Simulation of capillary shrinkage cracking in cement-like materials

Abstract In drying suspensions, water loss leads to a capillary pressure build-up in the liquid phase. This effect may also be observed in fresh cement-based materials subjected to evaporation at an open surface. If under decreasing water content the near-surface solid particles are no longer covered by a plane water film, menisci develop along with an associated build-up of negative capillary pressure, resulting in shrinkage and possibly in cracking. A 2D model for simulating the described physical process is presented. For arranging the particles in the 2D specimen a stochastic–heuristic algorithm is used. Subsequently, the course of the water front between the particles is calculated by assuming a constant curvature of the water surface. Particle mobility is taken into account by adopting interparticle forces and performing equilibrium iterations. The model allows one to study the influences of the particle size distribution as well as of the properties of the liquid phase on the capillary pressure build-up and on the cracking risk.

Capillary pressure in fresh cement-based materials and identification of the air entry value

Plastic shrinkage in fresh cement-based materials is caused by capillary pressure in the pore system. Within the first few hours after casting, evaporation results in the loss of water and possibly in the formation of menisci in the water surface between the superficial solid particles. With decreasing radius of the menisci the capillary water pressure rises. At a certain pressure, the system becomes unstable and air penetrates into the system. This event is called air entry and assumed to be the origin of strain localization and, consequently, of early age cracking. Similar phenomena may be observed in inert materials. If the capillary pressure is kept below the air entry value by a closed-loop controlled concrete rewetting, the risk of early age cracking is reduced significantly. Experimental results of capillary pressure, deformation and electrical conductivity measurements are presented and a technique for the identification of the air entry value is proposed.

A simple, robust, and on-site microwave technique for determining water-to-cement ratio (w/c) of fresh Portland cement-based materials

Inspection and evaluation of cement-based materials such as concrete is of great interest to the construction industry. In particular, real-time and on-site evaluation of water-to-cement ratio (w/c) is an important practical issue, since the compressive strength of a concrete structure is significantly influenced by its w/c. Currently, there is no single real-time, on-site, relatively inexpensive, easy-to-implement, and operator friendly technique for evaluating this parameter. Microwave nondestructive testing and evaluation techniques have shown great promise when used for inspection and evaluation of the properties of cement-based materials. In this paper, the optimal design of a monopole antenna probe used to evaluate w/c of fresh cement-based materials in real-time and in-situ is presented. This probe, operating at 3 GHz, is used along with a reflectometer whose DC output voltage is shown to be linearly correlated to w/c of fresh cement paste and fresh concrete specimens. This paper presents the optimal probe design procedure, the experimental verification of the results, and the results of using the custom-made reflectometer for quick and robust w/c measurement of fresh cement paste and concrete.