Swelling Tests

In order to effectively utilize steel slag in both bound and unbound applications, it is important to assess its swelling potential. For this purpose, several swelling test methods

have been developed and used to assess the expansion potential of steel slag. These test methods can be grouped into two main groups: i) long-term swelling tests and ii) accelerated swelling tests. In the long-term swelling tests, steel slag samples are typically immersed in water, and swelling is monitored at room temperature for a long period of time (a minimum of 3-6 months). In accelerated swelling tests, compacted steel slag samples are exposed to hot water or steam in order to accelerate the swelling rate, and swelling is monitored for a shorter period of time (typically ranging from 2 to 14 days).

In both types of swelling test (long-term and accelerated) methods, steel slag samples are typically compacted in cylindrical molds, and the one-dimensional volume change of laterally constrained samples is measured. Both long-term and accelerated tests commonly used to assess the swelling behavior of steel slags are briefly described in the following.

Long-term Swelling Tests (ASTM D1883)

Several researchers have performed swelling tests on steel slag samples at room temperature to assess the rate of expansion (Crawford and Burn 1969; Juckes 2003; Poh et al. 2006). Different testing equipment and samples sizes have been used by different researchers, however, the principle of the test is the same for all the studies available in the literature. Samples are initially compacted to the desired dry unit weight in cylindrical molds of different sizes (5-cm-, 10-cm-, 15-cm-, 38-cm- diameter); typically the molds used for standard Proctor or CBR tests (ASTM D1883 recommends the use of the standard CBR mold). After sample preparation is completed, the samples are immersed in water tanks and maintained at room temperature with free access to water from both the top and bottom of the sample. One-dimensional swelling of the compacted slag samples is measured by LVDTs or dial gauges mounted at the top of the sample. Swell measurements are taken for a period varying from days to several months depending on the rate of swelling; specifically, for steel slag samples with free MgO content, the tests should be monitored for months as the conversion of MgO to periclase (Mg(OH)2) takes place at a slow rate. For unbound applications, swelling tests performed at room temperature seems to represent the field conditions better than other swelling testing

methods that involve application of high temperature or/and high pressure (ASTM D1883; Juckes 2003).

Water-Bath Swelling Test (ASTM D4792)

The test procedure described in ASTM D4792 (Standard Test Method for Potential Expansion of Aggregates from Hydration Reactions) was developed based on the test procedures developed by Emery (1974) and the Pennsylvania Test Method (PTM-130).

In the U.S., this testing method is commonly used to measure the expansion of industrial by-products that are used as aggregates. In ASTM D4792, samples are compacted in standard CBR-molds that are equipped with dial gauges, following the same procedures described in ASTM 1883. In order to accelerate swelling, molds are placed in a hot-water bath maintained at a temperature of 70 ± 3^oC for 7 days. The rate of swelling usually stabilizes in a period of 7 days. However, if there is no pronounced decrease in the rate of swelling after 7 days, tests are continued for longer periods (up to 2 weeks) to obtain additional data. The percent expansion is calculated from each day’s measurements; a graph of percent expansion versus elapsed time (in days) is then prepared once the test is completed (ASTM D4792; Emery 1974; Rohde et al. 2003).

Autoclave Expansion Test (ASTM C151-05)

The autoclave test is a very quick and common expansion test that uses both elevated temperature and pressure to accelerate expansive reactions. Aggregate samples are first compacted in molds, and initial sample height measurements are taken. Next, samples are placed in the autoclave machine and kept at a pressure of (2068 kPa) 300 psi and at temperature of 420^oC for 3 hours. After 3 hours, samples are cooled, and height measurements are taken again using a micrometer assembly. The percent expansion is calculated using the initial and final height measurements. Figure 3.4 (a) and (b) show the photographs of the autoclave machine and the mold with the dial gauge used for the swelling measurements, respectively.

Figure 3.4 Autoclave expansion test assembly: (a) autoclave test machine (b) mold and dial gauge used for height measurements (courtesy of John Yzenas 2008)

In general, the expansion rate measured with the autoclave tests is typically 10 times higher than that obtained with the water-bath test procedure (ASTM D4792). The severe autoclave conditions imposed in this test can cause disintegration of the steel slag particles. For this reason, results obtained from this test do not correlate well with actual field performance. However, the autoclave test is advantageous because it provides results for volumetric stability and integrity of aggregates quickly (ASTM C151-03, Yzenas 2008).

European Steam Test (EN 1744-1)

The European steam test is an accelerated swelling test which is widely used in Europe to check the quality of steel slag used in aggregate applications. The method is incorporated in the British standard BS EN 1744-1:1998, “Tests for chemical properties of aggregates-chemical analysis” that was published officially by the European Committee for Standardization (CEN) in 1998. According to this test procedure, a steel slag sample with a grain-size distribution in a given range (particle sizes ranging from 0 to 22mm) is compacted in a cylindrical mold of 21cm (~8inch) in diameter and 10cm (~4inch) in height. Next, a flow of steam at a temperature of about 100⁰C is applied to the compacted

a) b)

sample from the bottom in a steam unit. The expansion of the compacted sample is measured with a dial gauge located at the top of sample. Figure 3.5 shows a schematic representation of the apparatus used in the European steam test.

Heating jacket

Figure 3.5 Schematic representation of steam test (BS EN 1744-1:1998)

The sample volume change typically stabilizes in 24h to 168h, and the maximum expansion is recorded. Results of steam tests have shown that for BOF slag with an MgO content less than 5%, a testing time of 24 h is sufficient (Motz and Geiseler 2001). EAF and BOF slags that have an MgO content higher than 5% should be tested for 168 h since the hydration of MgO takes more time than that of free lime. The steam test procedure avoids some of the unrealistic test conditions present in the other test methods. For example, the steam test avoids the unrealistic conditions induced by high pressures in autoclave tests. In addition, the steam test also eliminates the wash off effects (dissolving of expansive compounds in water) that may be present in water-bath swelling tests.

Swelling measurements from the steam test are expected to be higher than those of long-term tests. (BS EN 1744-1:1998; Motz and Geiseler 2001).

Juckes et al. (2003) discussed the different types of expansion tests performed on steel slag and pointed out that the field performance of steel slag may be very different from that expected based on the results obtained from laboratory testing. Both long-term and accelerated swelling test methods have advantages and disadvantages associated with them. Short-term accelerated swelling tests are quick and provide the maximum expansion for a given steel slag sample. However, as the rate of swelling is expected to be much lower under ambient atmospheric conditions than that observed during the short-term accelerated testing performed on slag samples in the laboratory, the expansion values measured in the field for unbound applications are likely to be much lower than that measured during laboratory testing. In contrast, long-term swelling tests can better simulate the in situ conditions and hence provide a more accurate swelling rate for unbound applications. However, long-term swelling tests require test monitoring for several months to be able to predict the swelling rate. We can take advantage of both testing procedures to assess the overall expansive behavior of the samples by determining the rate of swelling from long-term laboratory tests and coupling this rate with a limiting absolute volumetric strain (%) obtained from the accelerated tests. In order to better correlate the results of laboratory testing with field performance it is essential to choose test methods and conditions that are consistent with the intended application for the steel slag. As an example, steam test procedure represents the field conditions better if the steel slag aggregate is used in bound asphalt pavements (Motz and Geiseler 2001; Juckes 2003). On the other hand, long-term swelling tests complemented with real scale field data seem to be the more appropriate to assess the swelling of steel slag used in unbound applications.