105
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
EVALUATION OF CONCRETE’S RESISTANCE TO PHYSICAL
SULFATE SALT ATTACK
Semion Zhutovsky
(1)
, R. Douglas Hooton
(1)
(1) University of Toronto, Canada
Abstract
Physical sulfate salt attack (PSA) is one of the most severe and rapid deterioration
mechanisms that can take place in concrete. Yet there is no standard method for evaluation of
concrete resistance to PSA. Testing of concrete’s resistance to PSA has two main aspects –
exposure conditions and evaluation of deterioration. Evaluation of concrete’s resistance to
PSA often requires a long time, which is inappropriate for evaluation of concrete mixtures for
a construction project. Many studies report sulfate resistance based on subjective visual
ratings, which is inadequate as a durability design criterion. The objective of this research is
to identify exposure conditions and deterioration evaluation methods suitable for standard
testing that can be used for rapid comparison of mixture compositions, durability design, and
analysis of life cycle cost. Various methods for assessment of deterioration were applied using
selected exposure. It was found that 100 thermal cycles between 5 and 30 °C immersed in
30% sodium sulfate solution were sufficient to assess the resistance of a range of mortar
mixtures. The most suitable techniques for evaluation of deterioration rate was mass loss,
while fundamental resonance frequency and ultrasonic pulse velocity were found to be
unsuitable.
1. Introduction
Physical sulfate salt attack (PSA) is one of the most severe and rapid deterioration
mechanisms that can take place in concrete. Salt scaling, salt weathering, or salt hydration are
the terms that are often used for PSA [1]. PSA is sometimes confused with chemical sulfate
attack [2], [3]. Unlike in sulfate attack of chemical origin, no chemical interaction between
sulfate salts and cement minerals is involved in PSA [3]. Sulfate attack of concrete has been
the subject of extensive research, though the mechanisms of PSA were often overlooked [4].
A recent Portland Cement Association report on the results of testing concrete resistance to
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
sulfate attack declares that the PSA damage of concretes can be significantly more extensive
than the damage caused by chemical sulfate attack [5].
It has been demonstrated by previous research that water to binder (w/b) ratio is the most
important parameter for the resistance to sulfate attack [5]. The current North American
standards limit w/b ratio to 0.40 and 0.45 in Canada [6] and USA [7], respectively, for the
severe conditions of sulfate exposure. However, the reduction of w/b ratio does not prevent
deterioration, though it enhances the resistance to sulfate attack [8]. Supplementary
cementitious materials (SCM) are often used to make concrete tolerant for sulfate rich
environment. While SCMs improve the resistance to chemical sulfate attack by diluting
aluminates in the binder, some literature sources report that they may even increase the
susceptibility to PSA [5], [9], [10]. So there is a controversy about the resistance of different
binders and mixture designs to PSA.
Currently, there are no standard methods for testing concrete resistance to PSA [2]. Existing
standard test methods ASTM C452 [11] and ASTM C1012 [12] were prepared for testing the
resistance of binders to chemical sulfate attack. In these standards, the expansion of mortar
bars is used as a measure of deterioration, while PSA is typically not associated with
expansion. The form PSA damage is usually the gradual surface scaling, much like the
damage caused by freezing and thawing [3]. This is because PSA causes damage by means of
the cycles of crystallization, dissolution and phase transitions of sulfate salts [13]. Testing of
concrete’s resistance to PSA has two main aspects – exposure conditions and evaluation of
deterioration. Historically, field testing was used for testing sulfate resistance of concretes [5],
[9], [8], [14]. Such tests could take from 8 to 40 years. Such long time is inappropriate for
evaluation of concrete mixtures for a construction project. Many such studies report sulfate
resistance based on visual ratings, which are subjective and inadequate as a design criterion
for durability. Thus it is of great interest to identify exposure conditions and deterioration
evaluation methods suitable for accelerated standard testing that can be used for rapid
comparison of mixture compositions, durability design, and analysis of life cycle cost.
Different accelerated exposure conditions for the testing of PSA resistance of concrete are
reported in the literature [13], [15], [16]. Typically, the most significant damage in PSA is
caused by phase transitions between sulfate salts: thenardite (
Na2SO4) and mirabilite
(
Na2SO4·10H2O) [13]. To activate this mechanism, either relative humidity (RH) or
temperature variations are needed. Thus, PSA testing exposures can be categorized into three
groups: wetting and drying cycles, partial submerged samples, and fully submerged samples.
In first group, changes in RH are triggered by wetting and drying cycles, inducing the phase
changes between mirabilite and thenardite. However, change of RH inside concrete or mortar
samples is slow, and significant time is required to reach RH equilibrium inside a sample. For
this reason, there will always be a moisture gradient through the sample cross-section, which
results in partial conversion of thenardite to mirabilite, and the testing time required to
achieve the damage level needed to quantify PSA resistance in a wide range of mixtures is
long. In partially submerged samples, the rate of deterioration may be fast, but the damage is
localized in the evaporation zone, which makes it difficult to quantify the resistance to PSA.
On the other hand, thermal cycling of fully submerged samples seems to be a very promising
PSA exposure, because thermal equilibrium can be achieved quickly. When temperature is