Durability of concrete and cement composites

Identification of frost-susceptible aggregates

A key step in designing durable concrete in cold weather applications is to identify whether an aggregate of interest may be susceptible to frost damage, which may manifest itself as internal distress within concrete or surface damage, such as D-cracking (especially for pavements) and pop-outs. The process of identifying salt-susceptible aggregates may involve a review of past field performance or, more likely, of relevant laboratory data.

Past field performance of a given aggregate source may provide very useful information, but one must exercise caution in relying too heavily on past field performance to predict future performance of aggregates from the same source used in new construction. When reviewing past field performance of a given aggregate source, one must ensure that the materials, mixture proportions, environmental conditions and exposure (especially deicing salts) and other relevant parameters are similar for the existing structure (or pavement) from which field performance information is available and the new structure planned for construction. Even if the parameters highlighted above for the existing and new projects are similar enough to warrant a valid comparison, one must still consider the following issue. Owing to inherent changes in aggregate mineralogy and properties from one portion of a quarry or pit to another, it is not uncommon for the frost-resistance aspects of aggregates from a specific production facility to vary considerably with time. Thus, information obtained on aggregates from any single quarry must be used with some caution. However, general trends in behavior can often be gleaned from quarry- or pit-specific data, especially when historic performance has been either excellent or poor. For aggregate sources with mixed or marginal performance histories, laboratory evaluations are essential for determining frost susceptibility.

There are a variety of laboratory tests for evaluating the frost resistance of aggregates, ranging from those that test solely the aggregate to those that test concrete containing the subject aggregate. There does not appear to exist a single test that can be used to clearly delineate a frost-resistant aggregate from a frost- susceptible aggregate, especially given the complexities of the underlying mechanisms of frost damage and the variety of manifestations of damage in field concrete, such as D-cracking and pop-outs. This difficulty in predicting aggre- gate performance from a single laboratory test is compounded by the fact that the specific exposure conditions to which concrete is subjected play a major role in determining the frost resistance of a given aggregate. Factors such as element type (pavement, slab, etc.), design and construction details (joint spacing, width, etc.), and degree of environmental severity (number of freeze-thaw cycles, presence of deicing salts, availability of moisture, etc.) vary significantly from one application to another. Despite these complexities, there are still several laboratory tests that can provide excellent insight into the physical, mineral- ogical, and chemical characteristics that govern a given aggregate’s frost resistance. Some of these approaches are described briefly next; a more detailed discussion of these methods is available in National Cooperative Highway Research Program Research Results Digest 281 (NCHRP, 2003). Tests performed solely on aggregate samples are discussed first, leading to discussions on testing aggregates in concrete. Some of the direct aggregate tests do not actually evaluate frost resistance, but rather they measure other relevant aggregate properties that affect frost resistance, such as absorption or porosity. Aggregate absorption often has an important impact on frost resistance of aggregates and its manifestation in field-damaged concrete. For certain aggre- gate types, there is a good correlation between absorption capacity and frost resistance, but for others, the correlation is quite weak (mainly because some high porosity aggregates show good frost resistance). Pigeon and Pleau (1995) have suggested that a maximum absorption capacity of 2% be imposed on aggregates to prevent aggregate-related damage from freezing and thawing cycles. Various agencies and highway departments have imposed absorption capacity limits (e.g., 1.7% maximum imposed by Minnesota Department of Transportation). Aggregates of higher porosities and absorption capacities may suffer damage upon freezing since water is expelled from the aggregate. Although certain aggregates exhibiting higher absorption may be prone to D- cracking or pop-outs, absorption should not be considered an absolute index of frost resistance because many aggregates with relatively high absorptions are quite durable. Thus, local experience and familiarity with a given aggregate type or source should be used to determine if absorption is related to the durability of that aggregate (NCHRP, 2003). Standard aggregate absorption tests (e.g., ASTM C 127 and 128) are typically used in specifications. More advanced tests, such as nitrogen absorption or mercury intrusion porosimetry, can be used to obtain information on internal porosity, pore structure, and absorption capacity, but these are not as commonly available or economically viable for most commer- cial and highway department laboratories. Other general aggregate tests that may ascertain information about frost resistance are specific gravity (because it tends to relate to absorption) and gradation. As noted previously, the size of an aggregate tends to affect its frost resistance and certain non-durable aggregates can be rendered durable by reducing the size below its critical value. Besides measuring basic aggregate properties (e.g., absorption, specific gravity, grading), more specific aggregate testing that aims at triggering distress (and typically measuring mass loss) is also widely used. Unconfined aggregate tests, that is, aggregates tested by themselves, are fairly common measures for assessing frost resistance. These tests are quicker than testing of concrete and can be used more economically to track aggregate quality and performance. There are a variety of different unconfined aggregate tests. Most involve freeze- thaw cycles of aggregates in water or salt solutions, but there are dozens of permutations on how the test can be performed. Some tests, AASHTO T 103-91 (Procedure B), for example, use immersion of the aggregates in alcohol-water solution to increase the penetration of water during the soaking period. The use of a salt solution has been reported to improve the correlation between laboratory freezing and thawing results and D-cracking in concrete pavements (Rogers et al., 1989), especially because deicing salts tend to exacerbate damage due to D-cracking. The Canadian Standards Association test (CSA A23.2-24A ± Test Method for the Resistance of Unconfined Coarse Aggregate to Freezing and Thawing) is one version of this test and involves soaking aggregates in 3% sodium chloride solution for 24 hours prior to the start of the test, with five cycles to follow in that same salt solution. The CSA specifies a maximum mass loss in this test of 6% for severe exposure conditions and 10% for less severe conditions. The CSA version of this test was found by Senior and Rogers (1991) to have good precision and correlation with freezing and thawing damage in pavements.

Sulfate soundness tests (e.g., ASTM C 88) are fairly common methods used in routine quality control of aggregates. They involve subjecting aggregate samples to repeated soaking in either magnesium or sodium sulfate solution, followed by oven drying, and recording of the mass loss after various test cycles; rather than inducing freeze-thaw cycles within the aggregates, these tests trigger crystallization and/or hydration pressures in the pores of aggregates, which can lead to significant damage. They are not regarded as reliable indicators of frost resistance but can provide a rapid means of acquiring comparative data for a given aggregate source.

The wet attrition of fine and coarse aggregates has gained in popularity in recent years, with the Micro-Deval test emerging as the most common method (NCHRP, 2003). Rogers et al. (1991) found a good correlation between mass loss in the Micro-Deval test and magnesium sulfate soundness testing, but better reproducibility for the Micro-Deval test (mainly due to lower sensitivity to aggregate grading). CSA specifications limit the loss for fine aggregates to 20% and the loss for coarse aggregates to between 14 and 17% (CSA, 2000). In addition to the laboratory methods just described, two tests have been developed specifically to address D-cracking potential of aggregates. These methods, the Iowa Pore Index Test (IPIT) and the Washington Hydraulic Fracture Test (WHFT), are discussed next.

The IPIT was developed in an attempt at quantifying the volume of micropores in aggregates, which has been found to correlate with the potential for D-cracking (Dubberke and Marks, 1992). The test involves placing a dried  aggregate sample in a pressure meter, filling the vessel with water, and subjecting it to a pressure of 241 kPa. The volume of water forced into the aggregates in the first minute, referred to as `primary load’, is aimed at defining the macropores in aggregates. The volume of water injected into the sample between one and 15 minutes, known as `secondary load’, is intended to generate quantitative data on the micropores. Some researchers and practi- tioners have specified values for the IPT that correlate well (either by themselves or in combination with other tests) with field performance of aggregates in their localities. For example, the Iowa Department of Transportation specifies a maximum secondary load value of 27 ml, and field performance in Iowa has shown that aggregates that exceed this load value may be prone to D-cracking (NCHRP, 2003). Winslow (1987) reported that the IPIT was a good indicator of D-cracking in Illinois for crushed carbonate aggregates, but not for carbonate gravels, most likely due to the rapid early absorption exhibited by gravels under the test conditions. Rogers and Senior (1994) reported that aggregates with greater than 2% absorption (measured in water after 24 hours) and with IPIT secondary loads greater than 27 ml generally exhibited poor field performance.

The Washington Hydraulic Fracture Test (WHFT) was developed by Jannsen and Snyder (1994) and later modified (Embacher and Snyder, 2001) to assess aggregate frost resistance. Aggregates are put under pressure but, rather than measuring the volume of penetrating water, the degradation of the aggregate sample is measured as the pressure is removed from the saturated aggregate, dispelling the internal water and causing hydraulic pressures. The damage to the aggregate sample is estimated by the change in gradation of the aggregates as larger particles are fractured to form smaller ones. More work is needed to determine whether this method accurately predicts frost damage of aggregates, but it is promising in that the test actually triggers water expulsion and causes aggregate fracture. The shortcoming of this test, and any other test that involves solely aggregates, is that it does not actually involve a cementitious matrix, thereby making it impossible to assess the effects of materials and mixture proportions on external penetration of water or solution or on the resistance to stresses from water expelled from aggregates.

Performing freezing and thawing tests on concrete containing an aggregate of interest is a common method of assessing the frost resistance of aggregates. Accelerated freezing and thawing tests, such as ASTM C 666, involve subjecting concrete specimens to rapid thermal cycles (e.g., 0 to 40ëC), while tracking damage by measuring mass loss, length change, or changes in dynamic modulus (using resonant frequency or pulse velocity, etc.). To assess the D-cracking potential for a given aggregate in concrete, researchers have used different testing regimes, with variations of the freezing and thawing cycle time and exposure conditions of the beams (in water or air, in rigid containers or cloth wrap, etc.). Koubaa and Snyder (1996) proposed soaking carbonate aggregates  in chloride solution before casting the concrete and test concrete to highlight the salt susceptibility of aggregates.

Stark (1976) found that length change was a better indicator of D-cracking susceptibility than other common test outputs (e.g., resonant frequency). Stark’s recommendation was to use a 0.035% expansion limit after 350 cycles of ASTM C 666 (modified to reduce the number of freeze-thaw cycles to two per day).

When an aggregate has been identified as being frost susceptible (through field and/or laboratory performance), it should either be avoided in certain applications or used prudently to ensure durability. When local sources are the only options, and these sources are non-durable, efforts should be made to improve the frost resistance of the aggregates in the concrete of concern as much as is possible. Options include reducing the aggregate size (below the threshold size), optimizing the concrete mixture (with reduced permeability and tight air void structure), ensuring good drainage and good structural detailing, or blending the poor aggregate with a more durable one. How a given aggregate responds to these attempts at mitigation dictates how suitable it will be for the intended application. Unfortunately, in some cases, frost-susceptible aggregates are difficult to control and will result in field distress, regardless of the quality of the surrounding concrete. In these cases, just as in the case for alkali-carbonate reactive rocks, selective quarrying is needed to identify the poor-performing aggregate and avoid its use in aggressive, cold-weather applications.

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