International Concrete Abstracts Portal

The resistance of concretes to the transport of chloride ions is defined generally by the coefficient of diffusion. This, combined with a knowledge of the exposure of concrete to chlorides, can be used to estimate the depth of penetration of chlorides over a period of time. Therefore, an indication as to the likelihood of chlorides reaching reinforcement can be determined, and, if there is a risk of corrosion, preventive measures can be taken. The procedure to find the coefficient of diffusion from a standard diffusion test is well established. However, such a test may need several months to finish, depending on the quality of the concrete and the thickness of sample tested. As a consequence this is not a practicalmethod, and different organisations have conducted research to determine the coefficient of diffusion (D) more rapidly. The approach adopted has been to force chlorides through the test sample by applying a voltage, and such tests are known as accelerated chloride migration tests. By using this principle, and following from early studies by Whiting, a new test method for determining the chloride migration coefficient of the near surface concrete in-situ is being developed at Queen’s. This test makes use of a set up similar to the well established Autoclam permeability system. The results of an investigation carried out with this new apparatus is presented in this paper along with a description of the new test method. Early results indicated that this new test method could form the basis to determine the chloride diffusivity of the near surface concrete on site.

A test method, which uses pull-off test and partial coring techniques, was developed in this study. This test method is particularly suitable for assessing the durability of bonding new to old concrete subjected to freeze-thaw cycling and exposed to deicing salt. This test method also combines bond evaluation and ASTM C-672 test ( Standard Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals ) into one test method. Laboratory experimental research investigated important factors which influence the test results and their scatter. Test results show that this test method is very promising.

The contact electrical resistivity was found to correlate with the shear bond strength between steel rebar and concrete, between stainless steel fiber and cement paste, and between carbon fiber and cement paste. For the bond between steel rebar and concrete and that between stainless steel fiber (untreated or acetone washed) and cement paste, the contact resistivity increased linearly with increasing bond strength, due to an interfacial phase of high volume resistivity that helped the bonding. For the bond between stainless steel fiber (acid washed) and cement paste and that between carbon fiber (untreated) and cement paste, the contact resistivity decreased with increasing bond strength, due to the bond degradation by interfacial voids, which were high in volume resistivity. The acid washing of the stainless steel fiber decreased the contact resistivity, but had little effect on the bond strength. The high volume resistivity interfacial phase that enhanced the bonding between the untreated or acetone washed stainless steel fiber and cement paste apparently required for its formation the oxide layer on the stainless steel surface. The removal of the oxide layer by acid washing eliminated this phase, thus decreasing the contact resistivity and causing the contact resistivity to decrease with increasing bond strength. For a given interface at a given curing age, the correlation between bond strength and contact resistivity allows the bond strength to be nondestructively measured via contact resistivity measurement.

Two 16-story reinforced concrete apartment blocks founded on drilled shafts (CIP piles) were damaged in the 1994 Northridge earthquake. In order to assess the viability of the buildings for retrofit, it was considered necessary to evaluate the integrity of the existing concrete drilled shaft foundations, which were only partially reinforced. This paper describes the use of various nondestructive testing methods for the foundation evaluation, including ground penetrating radar, parallel seismic and impulse response tests. The selected test methods proved to be successfir!, and provided an economical approach while obtaiuing maximum informatron about the integrity and future performance of the hidden foundation.

Large concrete structures such as dams and bridge abutments and piers built over 50 years ago were usually constructed of mass concrete with no or little reinforcing steel. Most of these structures are in service today, and make up a considerable part of the infrastructure. Concern about their reliability and residual life now prompts owners to initiate structural inspections. The traditional approach of coring and visual inspection alone does not necessarily provide a full over for this evaluation. The more familiar nondestructive tests (NDT) such as rebound hammer, ultrasonic pulse velocity and similar surface tests are usually unsuccessfbl in providing information about bulk concrete quality at depth. This paper reviews two novel NDT methods that can address this problem: sonic ogging and parallel seismic, and presents three case histories that illustrate their application.