Automated Determination of Entrained Air-Void Parameters in Hardened Concrete

Development of an automated linear traverse system for determination of air-void parameters in hardened concrete is described. The technique is comprised of an image analysis system for acquisition of surface images through a microscope, methodology for detection and identification of entrained air voids, and a computercontrolled two-dimensional motorized actuator for bringing the sampling points in line with the microscope. In addition to the sample preparation procedures involved in ASTM C 457, the automated technique requires an additional step that includes pigmenting of the sections to create color contrast between the voids and the concrete matrix. Following the sample preparation steps, the process requires approximately 15 min for the complete determination of air-void parameters along a 2642 mm (104 in.) traverse length. Data are automatically analyzed, and air-void parameters in terms of air content, specific surface, spacing factor, void frequency, and other parameters of importance are displayed and stored in a worksheet for reporting. Evaluation of the system included experimentation with a range of concretes with varying degrees of air content using both automated and manual measurements for the determination of air-void parameters.

Keywords: air content; air-entraining admixture; air void; concrete; lineartraverse method.

(ProQuest Information and Learning: ... denotes formulae omitted.)

INTRODUCTION

Concrete deteriorates under repeated exposure to cycles of freezing and thawing. For this reason, air-void agents are used to improve freezing-and-thawing resistance. Depending on the size of aggregates used in the mixture, the volume of air required for optimum resistance against frost damage generally varies from approximately 4.5 to 7% by volume of concrete. The air content percentage by volume is the easiest quantity measured during routine field tests. There are several techniques available to determine the air content percentage by volume including the gravimetric (ASTM C 138),1 volumetric (ASTM C 173),2 and pressure (ASTM C 231)3 methods. The total amount of air, however, is not the primary parameter that determines the adequacy of protection against frost damage. The parameters that define the characteristics of the entrained air-void system, namely the spacing factor and the specific surface, are more appropriate for this purpose. These parameters are defined in ASTM C 457,4 which is the standard test method for microscopical determination of the parameters of air-void systems in hardened concrete. The spacing factor is the most significant indicator of the durability to freezing-and-thawing exposure of concrete. It is the average maximum distance from any point in the paste to the edge of a void. The spacing factor and the air voids should be small to produce durable concrete at relatively low air contents. Void sizes are not uniform, and the specific surface is defined as a single quantity that represents the surface area of air voids divided by their volume. ASTM C 457 provides two procedures for quantification of the spacing factor, specific surface (that is, the modified point count), and the linear-traverse methods.

The ASTM standard requires preparation of the concrete sections by careful lapping of the surfaces prior to the microscopical examination. The linear-traverse method involves measurement of the chords intercepted through the air voids on the section, and in the point count method, the operator records the frequency with which a series of regularly spaced points are superimposed on the air voids as well as between air voids. Both procedures could greatly benefit from automation. The automated procedure facilitates the process and removes the subjectivity of the operator, which is considered a shortcoming of the manual operations.5 Moreover, automation will be very effective when the task requires repetitive examination of numerous samples within a short period of time.

A number of studies have used computer-based imageprocessing systems for characterization of air-void parameters in concrete.6-14 Such systems allow for spatial reconstruction of images and provide a multitude of alternatives for characterization of air-void parameters either by conventional means (that is, linear traverse and modified point count) or through planar analysis of the images. The system developed by Pleau, Pigeon, and Laurencot12 employed an image-processing board and a microscope with a moving platform to facilitate automation. Their void-detection routine required establishment of a color contrast between the voids and a concrete matrix by filling the voids with white ink. Shape factors were used to differentiate the circular air voids from the other cavities. The drawback of the method was the inability of the system to differentiate the air voids when they were spaced within close proximity of each other. The overlapped voids were either considered as larger air voids or disregarded as a surface defect.