As explained in Chapter 5, aggregates can themselves be frost susceptible and break under repeated cycles of freezing and thawing. However, sound aggregates (i.e. aggregates not damaged by freezing and thawing cycles) can also be deleterious if they expel a significant amount of water during freezing, which will induce disruptive hydraulic pressures in the surrounding cement paste. The role of aggregates can be particularly important in relation to surface scaling, and the phenomenon of pop-outs is one type of common frost damage due to poor quality aggregates (Chapter 3). The interface between aggregate and cement paste is generally more porous than the bulk of the paste and this can also account for the influence of aggregate on the frost resistance of concrete.
4.2.1 Petrographic Examination
Basically, the petrographic examination of aggregates (ASTM C 295) aims at visually identifying the types of rock or the mineral constituents of an aggregate and determining their relative amounts. This objective is mainly achieved by the examination of a polished section of the aggregate (or of a piece of concrete containing this aggregate) under a stereoscopic microscope at a magnification ranging from 6× to 150×. The examination of thin sections under a polarizing microscope is also commonly used and, in some instances, complementary information can be obtained by other procedures such as X-ray diffraction analysis, differential thermal analysis, infrared spec troscopy or scanning electron microscopy.
Petrographic examination provides valuable information about the chemical composition of the main constituents of the aggregate as well as about their physical properties such as particle shape and size, surface texture or pore structure. This examination can also indicate the presence of coatings, of clay or of other contaminating substances, and can be used to determine the portion of the coarse aggregate which is composed of weathered or altered particles as well as the extent of that weathering or alteration (severe, moderate or slight). The correct interpretation of this test is not easy and requires a great deal of experience from the petrographer. Some minerals, such as chert or shaly materials, are well known for their generally poor frost resistance. However, the frost behaviour of an aggregate is related to so many parameters (some of these parameters, such as the exposure conditions which are of paramount importance but on which very little data is generally available, not being related to the intrinsic properties of the aggregate) that it is often difficult to pass a reliable and definitive judgment on its frost resistance. When available, the service record of an aggregate still remains one of the most reliable ways to predict the field performance.
4.2.2 Sulphate Soundness Test
In this test (ASTM C 88) a representative sample of an aggregate is subjected to repetitive cycles of immersion in a saturated solution of sodium sulphate (Na2SO4) or magnesium
sulphate (MgSO4) and oven drying at 110 °C. During the re-immersion, the rehydration
of Na2SO4 (or MgSO4) produces salt crystals in the pores of the aggregate particles. The
growth of these crystals results in internal expansive forces which are considered similar to the disruptive pressures due to frost. The sulphate soundness test can be performed on both fine and coarse aggregates. Prior to the immersiondrying cycles, the aggregate sample is divided into individual fractions corresponding to each of the different sieve sizes. After the completion of the final cycle, each fraction of the sample is dried at 110 °C and sieved over the same sieve (for the fine aggregate) or over the sieve below, i.e. the sieve with openings 5/6 of those of the initial one (for the coarse aggregate). The material retained on the sieve is then weighed and the weight loss, i.e. the difference between this weight and the initial weight of the fraction of the sample tested, is computed and expressed as a percentage of the initial weight. Of course this weight loss provides only an indication of the potential frost resistance of the aggregate. According to the ASTM C 33 Standard
(Standard Specifications for Concrete Aggregates) the aggregate is considered as unsound
if, after 5 cycles, the weight loss for any fraction is higher than 10% (for fine aggregate) or 12% (for coarse aggregate) when sodium sulphate (Na2SO4) is used. Since the test is
usually much more severe when magnesium sulphate (MgSO4) is used instead of Na2SO4, the acceptance limits are significantly higher and correspond to a weight loss of 15% (for fine aggregate) and 18% (for coarse aggregate) after five cycles. The visual examination of particles larger than 19 mm can also provide valuable qualitative information about the nature of the deterioration (disintegration, splitting, crumbling, cracking, flaking etc.).
The main advantage of the sulphate soundness test is that it is simple and, contrary to the ASTM C 666 freezing and thawing test (which can also be used to study the durability of concrete made with a given aggregate), the results can be obtained in a relatively short period (only a few days compared to a few months for the ASTM C 666 test).
Unfortunately this test, even if it is widely used and relatively simple, has some major drawbacks. The first one is related to the fact that the deterioration of aggregates by frost is most probably due to hydraulic pressures and not to pressures due to crystal growth (Chapter 5). The fact that no account is taken of the severity of the exposure conditions is, of course, another important limitation of this test method. The frost resistance of aggregate, as well as that of concrete as a whole, is not an intrinsic property of the material, but it is strongly dependent on the exposure conditions (such as moisture content, minimum temperature, cooling rate or length of the freezing period).
The sulphate soundness can be considered as an indication of the overall quality of the aggregate. The weight loss of good quality aggregate is usually very low and that of highly porous and poor quality aggregate is very high. It is possible, however, that aggregates considered as unsound according to the requirements of ASTM C 88 could be frost resistant when used in concrete under natural exposure conditions, and that aggregates considered sound turn out to be deleterious.
4.2.3 Critical Dilation Procedures
As explained in Chapter 5, coarse aggregates can have a detrimental effect on the frost resistance of concrete even if they are intrinsically frost resistant (i.e. if a bulk sample of these aggregates would not be damaged when exposed to freezing and thawing cycles). Concrete can be damaged if the cement paste cannot accommodate the volume of water that is expelled from the aggregate during freezing, or if excessive internal stresses are generated inside the aggregates due to the low permeability of the surrounding cement paste which restricts water expulsion from the aggregates (this does not occur when the aggregates are not confined during freezing). According to the ASTM C 682 Standard, the frost resistance of coarse aggregates can be evaluated by measuring the critical dilation of air-entrained concrete specimens made with these aggregates. The concrete mixture has to be prepared in accordance with the ACI Recommended Practice 211.1 (ACI Manual of
Concrete Practice—Part 1, 1991), with 307±3 kg/m3 of type I Portland cement and a total
air content of 6±1%. Whenever possible, the aggregates should be kept at a moisture level similar to that corresponding to the field conditions before preparing the concrete specimens (this is difficult to achieve and, when in doubt, aggregates should be used in a saturated state which represents the most harmful condition). The concrete specimens are stored in saturated lime water for 14 days and, whenever possible, should be brought to a moisture level similar to that expected to occur under field conditions at the time of initial freezing (if it is not possible, the Standard again describes a conditioning procedure to achieve a
high moisture content). After the conditioning period, the concrete specimens are subjected to a critical dilation test in accordance with the requirements of the ASTM C 671 Standard. A frost immunity period is then determined, this period being defined as the longest period of water immersion for which critical dilation does not occur (section 4.1.1). The aggregates are considered frost resistant if this frost immunity period is longer than the maximum period for which the concrete is expected to be water saturated under field conditions.
Field experience has indicated that this test method is valid to classify aggregates with various frost susceptibilities (Larson et al., 1965; Larson and Cady, 1969). Nevertheless the significance of the test results is highly dependent on the degree to which the conditions reproduced in the laboratory are representative of the conditions occurring in nature. For aggregates of intermediate quality, minor changes in the exposure conditions in the laboratory can have a significant influence on the test results. The reproduci-bility of the test conditions can also be affected by the difficulty of achieving and maintaining a given moisture level (experimentally, it is much easier to provide a fully saturated moisture condition).
The aggregates are often made up of many different fractions, some of them being more susceptible to frost action than others; care must then be taken to ensure that the sample used for the test is representative of the aggregate under study. The characteristics of an aggregate may vary significantly with the location from which it is extracted from the quarry and its frost durability can change accordingly. The frost susceptibility of an aggregate can also be related to the presence of a small amount of a highly unsound component. In this case ‘beneficiation’, i.e. the action of removing unsound particles from a given source of aggregate, could be an acceptable solution but it requires a very careful investigation.
4.2.4 Other Tests
Although it is not directly recommended in an ASTM Standard as such, one of the best ways of determining the durability of concrete made with a given aggregate is simply to prepare a good quality air-entrained concrete with this aggregate (in a saturated state) and to subject specimens of this concrete to the usual ASTM C 666 rapid freezing and thawing test.
Mercury intrusion porosimetry is not a standardized test but can be used to determine the pore structure of an aggregate. Data exist in the technical literature which allow the evaluation of the potential frost durability of concrete made with a given aggregate on the basis of the characteristics of the pore structure of the aggregate as measured with mercury intrusion porosimetry (Kaneuji et al., 1980) or with other methods (Shakoor and Scholer, 1985).
Direct freezing and thawing tests on samples of an aggreagte cannot evaluate the performance of this aggregate in concrete, but can be useful in determining whether the aggregate is itself frost susceptible.