Microstructural Aspects of Stress Corrosion of Cementitious Materials
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MICROSTRUCTURAL ASPECTS OF STRESS CORROSION OF CEMENTITIOUS MATERIALS
U. SCHNEIDER, E. NAGELE, AND N. DUJARDIN Department of Civil Engineering, University of Kassel, M6nchebergstr. D-3500 Kassel, F.R.C.
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ABSTRACT Stress corrosion phenomena, i.e. significant effects of mechanical stresses on the corrosion resistance of materials, have been reported for a large variety of materials, including metals, oxides and halides. Recently it has been shown [1,2] that cementitious materials are also sensitive to The time dependent decrease in the flexural strength of stress corrosion. chemically stressed concrete and mortar depends significantly on the mechanical stress acting simultaneously with the chemical attack. For cementitious materials a comprehensive study of the stress corIn this paper the latest results from rosion phenomena has been started. the current research program are presented and the interrelations between the microstructure of hardened cement mortar and the strength reduction caused by stress corrosion, are briefly discussed.
EXPERIMENTAL Apparatus The experimental procedure has already been described in detail The scheme of the elsewhere [1,2] and only a brief survey is given here. experimental device for stress corrosion tests on mortar is shown in Fig. 1. The loaded specimens were stored in small containers filled with salt solution. The types of solutions were chosen according to the test program. The For comparison, unloaded specimens are stored in the same containers. specimens, notched and unnotched mortar prisms 4x4x16 cm, were immersed 28 days after casting in the aggressive media and loaded up between 10 and 40% of their ultimate four point bending load at that date.
Test Program series of tests was performed using prisms of portland cement The first mortar to study the effects of different aggressive media. All specimens were stored 28 days immersed in solution saturated with Ca(OH) 2 prior to storage in the salt solutions. As stress corrosion of cementitious Table materials was not reported before, a great variety of media was used. I shows the combination of salt solutions, their concentrations and the water-cement ratios of the specimens that have been studied up to now. Additional tests concerning the effects of notch depth, type of notch and degree of salt concentration on the stress corrosion of cementitious materials, are currently being performed.
EXPERIMENTAL RESULTS Figure 2 A few important results of the investigation are presented. shows the flexural strength of loaded and unloaded mortars in 5% (NH4 ) 2 S0 4 Each point represents the mean of solution as a function of immersion time. A In Fig. 2 also is shown the experimental scatter. four specimens.
Mat. Res. Soc. Symp. Proc. Vol. 85- 11987 Materials Research Society
162
significant decrease of strength with increasing load levels can be observed, i.e. stress corrosion takes place in this type of solution. The increase in strength observed at short immersion times is entirely due to the continued hydration of the cement i
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