CHAPTER 4: LABORATORY EVALUATIONS

4.1 OVERVIEW

This chapter summarizes a comprehensive laboratory evaluation focusing on the frost resistance (with and without salts) of SRW blocks. The chapter is based upon the research described in Chan (2006), Hance (2005), and Haisler (2004), as well as the other technical documents produced under this FHWA project (see section 1.4). Because of the extensive amount of laboratory work performed in this project, efforts have been made to synthesize the key research and to present only the most important and relevant results with an eye toward which ones can or should be implemented into accelerated test methods and specifications to ensure long-term durability of SRW blocks in aggressive, cold environments.

Much of the research described in this chapter is related to ASTM C 1262 (2003), which was the most commonly used test method for assessing freeze-thaw durability of SRW blocks when this project was initiated. Because this method was being used and/or specified by a number of SHAs, including those States that reported durability problems associated with SRWs, the decision was made from the onset of this project to focus on refining and improving reliability and reproducibility of the test. To elaborate on this point, it should be noted that ASTM C 1262 (2003) test had received significant criticism from end users due to concerns over the repeatability of test results, even within one set of samples from the same lot of SRW blocks. There were also questions as to the actual relation between the test results and field exposure conditions and performance. Given the shortage of previous published research on this general topic (frost resistance of SRW blocks) and on this specific issue (test methods to predict field performance), fundamental research was initiated to improve the understanding of the distress mechanisms associated with SRW blocks and in properly testing potential SRW blocks for use in aggressive environments. Figure 27 shows the overall scope of research that was performed on the various aspects of ASTM C 1262 (2003), with the basic information on each of the areas of concentration. It is not feasible to discuss in detail all of these aspects of the research program; figure 27 provides the reader with the overall scope of the efforts launched under this project. It should also be noted that some of the research focusing on damage assessment methods was performed in a separate research project funded by National Concrete Masonry Association (NCMA), but this work was performed by essentially the same research team as the FHWA project, and the work was done parallel to the research described in this project report. Only a brief synopsis of these NCMA-funded efforts is presented herein.

Figure 27. Diagram. Concentration groups for the evaluation of ASTM C 1262 test method.

The remainder of this chapter is organized as follows:

• Sampling of SRW blocks for laboratory evaluation, including studies on spatial variations of properties within SRW units. Goal was to provide better guidance on prudent sampling and testing of SRW units.
• Issues related to ASTM C 1262 (2003), including variability in exposure conditions within chamber, temperature control and monitoring, and relation between testing regime and damage mechanisms. Goal was to provide better guidance on testing methodology to improve repeatability and to better capture mechanisms that relate to field performance.
• Evaluation of SRW properties that best relate to frost resistance in the laboratory and field. Goal was to develop frost indices that can be used to predict durability, based in part on key SRW block properties.
• Role of various deicing salts (and other compounds) on durability of SRW blocks. Goal was to compare damage observed in SRW blocks when exposed to common deicing salts, other salts found in nature (e.g., sodium sulfate), or fertilizers.
• Evaluation of full-scale approach to testing full SRW blocks in simulated exposure conditions. Goal was to determine if more realistic testing conditions in the laboratory would better relate to actual distress mechanisms and manifestations in the field.

4.2 SAMPLING CONSIDERATIONS FOR SRW BLOCKS

One of the initial steps in the freeze-thaw testing of SRW units involves extracting test specimens from representative units. These steps are covered in sections 6 and 7 of the ASTM C 1262 (2003) standard. The specific manner in which a laboratory technician selects the units and extracts test specimens from them may influence the outcome of test results, as will be described in this section.

Before proceeding further, a brief overview of the manufacturing process is provided. SRW units are manufactured in block plants typically by compacting (while simultaneously vibrating) concrete mixes into steel molds, followed by immediate removal of the molds. The shaped units are subsequently conveyed to curing chambers maintained at elevated temperatures and humidity where the units are kept for a certain time period (which may be variable) before being withdrawn. In one plant visited in early 2004, residence time of SRW units in the curing chamber was understood to last anywhere from 1 to 3 days, depending on production schedule. In many cases, units are cast as sets of conjoined pairs, which are then split in the middle to produce a natural-looking or "split face" fractured surface. Following splitting, the units are stacked on pallets and stored in a nonstandard manner. These steps are illustrated in figures 28 through 31. From this description and in reference to figure 32, the following locations on the units are identified for the solid and hollow units shown in the figure:

• Front face (split face): natural-looking fractured surface that is used as the exterior face of the wall.
• Back face: surface opposite and parallel to split face.

Figure 28. Photo. Units immediately after demolding.

Figure 29. Photo. Units prior to entering curing chamber.

Figure 30. Photo. Splitting of conjoined units.

Figure 31. Photo. Split face of units.

Also shown is the height, H, of the units for which 200 mm (8 inches) and 150 mm (6 inches) are common dimensions. For the rest of this report, the term unit will be used to refer to a whole block as produced in manufacturing plants, as shown in figures 32 and 33. Specimens or coupons are typically cut from the units for testing such as ASTM C 140 (2000) tests for compressive strength, absorption and density and ASTM C 1262 (2003) freeze-thaw tests. ASTM C 1262 (2003) uses the words specimens and coupons interchangeably, as does this report.

Figure 32. Photos. Definition of terms: SRW units (or blocks).

Figure 33. Drawing and photo. Definition of terms: test specimens (or coupons).

4.2.1 Current Sampling Guidelines

In the procurement of test specimens from SRW units, there are two levels of sampling. The first is sampling of SRW units from lots (defined below) and the second is extraction of test specimens from individual units. This research project did not investigate the first level of sampling in detail; however, some general comments are provided. As for sampling units from lots, section 6 of ASTM C 1262 (2003) states the following:

Clause 6.1:

"Select whole units representative of the lot from which they are selected. The units shall be free from visible cracks or structural defects."

Clause 6.2

"Select five units for freezing and thawing tests."

Meanwhile, ASTM C 140 (2000) defines "lot" as follows (Clause 4.1.2):

Any number of concrete masonry units of any configuration or dimension manufactured by the producer using the same materials, concrete mix design, manufacturing process, and curing method

While this clause provides a generalized statement of what a "lot" encompasses, various details of this definition still remain unclear. The length of production time (1 year, 1 month, one project or one batch) during which "the same materials, concrete mix design, manufacturing process, and curing method" were used is not clear. The manner in which units are to be selected from the lot also remains vague. For example, are units to be randomly sampled as they come out of the production line, or are units to be sampled from pallets during storage at the block plant or at a jobsite? ASTM C 140 (2000) includes curing method as one characteristic of a lot, but it does not necessarily imply curing time or duration. Production plant logistics play a role in determining how long SRW units are kept in the curing chamber. This is critical because the overall quality of concrete varies with early-age curing; and whether units are cured for 1 day or 3 days plus curing conditions (temperature and relative humidity) impact the quality of the material. Furthermore, depending on ambient weather conditions, the storage condition of the SRW units is critical, as illustrated by the picture in figure 34. Units sampled from within a pallet that is shrink-wrapped may be of different quality than ones from unprotected pallets, as shown in this figure.

When extracting test specimens from SRW units, ASTM C 1262 (2003) requires "saw-cutting solid coupons (test specimens) from full sized units" (Clause 7.1), and for units with exposed nonplanar surface which could be split, fluted or ribbed, the coupon should be cut "from another flat molded surface" (Clause 7.1.1). Aside from these statements, there is no further indication of where or how these specimens should be sampled within parent units. As will be discussed in the next sections, material properties within SRW units vary systematically with location, and thus a simple random scheme for specimen extraction (whereby specimens are extracted from random locations over a unit) may not work. An alternate method known as stratified random sampling is shown to reduce variability. The challenge in sampling equally applies to ASTM C 1262 (2003) specimens extracted for freeze-thaw testing or ASTM C 140 (2000) specimens extracted for material property determination.

Figure 34. Photo. Possible exposure condition of units in winter weather.

Two different philosophies with respect to sampling are possible depending on the intended purpose of the tests. These are:

• Tests to evaluate the performance of as-manufactured units for comparison of mixture designs, manufacturing methods or quality of raw materials.
• Tests to evaluate the performance of units to be installed in projects.

In the first case, it may be desired to sample whole units and extract specimens from these units in such manner that variability between test specimens is reduced. Ways in which this can be achieved are described in this chapter. For site acceptance of units, however, it is more sensible to sample units that are most vulnerable (i.e., of lowest quality among the population of units) and to extract specimens from the most vulnerable locations within the unit. The following sections describe how quality varies over a unit and how knowledge of this variation helps with the decision on choosing samples from units.

4.2.2 Spatial Variability of Material Properties

4.2.2.1 Within-Manufacturer Variability

Studies were conducted to investigate spatial variation in selected material properties in SRW units obtained from a single block manufacturer (Chan et al., 2005a and b). These units are depicted in figures 35 through 38. Figures 35 and 36 were referred to as large wall unit, and figures 37 and 38 were referred to as small wall unit. Specimens from each of these types of SRW units were extracted in the manner shown in the figures and tested for flexural strength to ASTM C 78 (ASTM 2002), flexural elastic modulus, 24-hour absorption to ASTM C 140 (2000), and boiled absorption to ASTM C 642 (ASTM 2002). An example of the spatial distribution in the 24-hour water absorption observed in these units is shown in figures 35 through 38. The values shown in this figure are average values across each of the rows of specimens shown (i.e., four specimens per row in the front face of large wall units, two to three specimens per row in the back face of large wall units, and two to three specimens per row in the small wall units). Three units were tested for each type of wall unit (large or small), and each of these units showed similar patterns in the measured properties, as follows:

• Along the casting direction, material at the bottom layer of a block (i.e., material first deposited during casting) displayed the lowest absorptions out of all sampling locations on that particular face (front or back face). On the other hand, material in the middle layer displayed the highest absorptions of all sampling locations on that particular face. Material in the top layer exhibited absorption values that were intermediate to those of the bottom and middle layers.
• Along a direction from the front (split) face towards the back face, values of water absorption exhibited a decreasing trend away from the front face. This trend was evident in small wall units where specimens taken from nearest to the front face exhibited the highest absorption values, specimens on the back face exhibited the lowest values, and specimens in between exhibited intermediate values. In the case of large wall units, this trend was also evident from the lower overall absorption of back face specimens compared to front face specimens.

Figure 35. Photo. Percentages in spatial distribution of absorption in large wall unit.

Figure 36. Drawing. Distribution of absorption in large wall unit.

Figure 37. Photo. Percentages in spatial distribution of absorption in small wall unit.

Figure 38. Drawing. Distribution of absorption in small wall unit.

Flexural strength and flexural elastic modulus, followed the same trend, although inverse of absorption (i.e., locations exhibiting lower absorptions displayed higher flexural strength and flexural elastic modulus, while locations exhibiting higher absorptions displayed lower flexural strength and flexural elastic modulus). These patterns altogether suggested that the material in the middle layer on the front face was likely of lowest quality (highest absorption and lowest flexural strengths and moduli) in the unit, while material in the back face was likely of higher quality (Chan et al., 2005a). A general linear model (GLM) analysis (Ott 1993) was also performed on the absorption data to verify the statistical significance of trends at the 95 percent confidence level (Chan et al., 2005b). This model confirmed a parabolic distribution of absorption along the casting direction (along the y-axis shown in the figure with maximum absorption in the middle layer), and a linear distribution of absorption in a direction from front to back face (with maximum absorption at the front face and minimum at the back face). These distributions are depicted in figures 35 through 38. No statistically significant distributions were detected along the x-axis.

Systematic spatial variations of properties in the SRW units tested suggest that the specific method of sampling alone can lead to disparate test results. For instance, on the front face of the large wall unit shown in figures 35 and 36, water absorption values of specimens in the middle layer were 24 percent higher than those in the bottom layer; while on the back face, water absorption values of specimens in the middle layer were up to 60 percent higher than those in the bottom layer. This indicates that extraction of test specimens from random locations without consideration of the forms of distribution augments test variability. To reduce apparent variability due to the spatial distributions of properties, an alternate sampling scheme known as stratified random sampling (as opposed to simple random sampling) can be employed (Chan et al., 2005b). The difference between these two methods is shown in figure 39 for sampling from 12 possible locations on the face of an SRW unit. In simple random sampling, replicate specimens in a test set are randomly extracted from the various locations, tested for a particular material property and their results averaged. On the other hand, in stratified random sampling, specimen sampling is carried out in a more systematic manner reflecting the expected distribution of properties (Lohr, 1999). For the types of distribution observed here, an equal number of specimens is extracted from each of the rows or strata (three shown in Figure 39) to form the test set. Other size test sets consisting two or five specimens can also be selected under stratified random sampling, but the computation of average value of the measured property for the test set needs to be adjusted accordingly. Details on this calculation are covered in Chan et al. (2005b). This technique yields results that are more representative of the overall unit and is shown to reduce the apparent variability in test results. For the population of front face specimens, it is demonstrated that test variability (as measured by coefficients of variation) is reduced by approximately one-third when using stratified random sampling as opposed to Simple Random.

Figure 39. Drawings. Simple random versus stratified random sampling from the face of an SRW unit.

It should be emphasized that stratified random sampling yields results that are more representative of the entire SRW unit and with lower variability. This approach may not be applicable in cases where the quality of the entire SRW unit (i.e., at every location on the unit) needs to comply with minimum quality standards, such as ASTM C 1372 (2003) for compressive strength, absorption, and density. In these cases, sampling from the middle layers is preferred because material in this layer is typically of lowest quality in a given face of the unit. Compliance with specifications of this middle layer thus increases the likelihood that the unit overall is satisfactory.

4.2.2.2 Between-Manufacturer Variability

The studies described in the previous section focused on systematic spatial variations in material properties in SRW units from a single, residential-grade manufacturer. Further investigations were conducted to determine whether similar trends also existed in SRW units from other manufacturers producing commercial grade units. SRW units from four major block manufacturers (identified as Manufacturers A, B, C and D) were also evaluated for systematic spatial distributions of properties. Complete details and results of this investigation on spatial variability are provided in Chan et al. (2006c). A brief discussion is presented here.

Each of the four participating manufacturers were asked to provide two grades of SRW units:

• Units that complied with local DOT specifications.
• Units that did not necessarily satisfy DOT specifications but considered satisfactory for typical commercial use (non-DOT).

DOT units generally possessed denser internal structure with higher paste volume and lower compaction void volume, and freeze-thaw durability compliant with ASTM C 1372 (2003) or State specifications. On the other hand, most non-DOT units had a leaner internal structure and contained larger volume of compaction voids. The various types of SRW units were then labeled as follows: A-D (for Manufacturer A, DOT unit), A-N (for Manufacturer A, non-DOT unit), B-D, B-N, C-D, C-N, D-D and D-N. Mixture designs, production methods or curing, and storage details of these units were not available. Similar block samples from each manufacturer were also tested using ASTM C 1262 (2003) (in water and saline), and these results will be discussed later in this chapter.

For these units, only the front (split) faces were evaluated, and specimens were extracted at approximately 25 to 50 mm (0.98 inch to 1.96 inch) from the split surface, as shown in figure 40a. As in the previous studies, three layers along the casting direction were also considered. Figure 40b shows the features on the side faces of the SRW units that were used as position references to identify the exact locations of specimen extraction. These side faces were referred to as either lipped/grooved (containing intentional indentations, ledges, or both), and flush side (consisting of a smooth surface with no features).

Material properties evaluated in these SRW units included the following:

a. Location of test specimens.

b. Lipped/grooved side of SRW units evaluated.

c. Flush side of SRW units evaluated.

Figure 40. Drawing and photos. Sampling of test specimens from SRW units from different manufacturers.

Examples of the spatial distribution exhibited by some of these properties are shown in figure 41 for ASTM C 642 (2002) boiled absorption, figure 42 for volumetric paste content, and figure 43 for volumetric compaction void content. The values shown in these figures represent the average absorption value of specimens in each of the layers shown (typically three to four specimens per layer). As with the SRW units described in section 4.2.2.1, variations in the measured values of these properties could arise depending on where samples were extracted from. For example, for unit C-D, samples taken from the middle layer showed 23 percent higher boiled absorption, 3 percent lower paste contents, and 63 percent higher compaction void contents compared to samples extracted near the lipped/grooved side. Also, as with the SRW units described in the previous section, the middle layer showed highest absorption of all sampled locations on this particular face.

A perhaps more significant observation was the consistency in the locations where maximum or minimum values occurred for various properties. As demonstrated in figures 41, 42, and 43, boiled absorption was generally highest in the middle layer of the units, which is also where paste volume was generally lowest and compaction void volume highest. Oven-dried density was also lowest in this layer (Chan et al., 2006c). Although these observed relationships in the properties were as expected, the consistency in the locations where maximum values occurred indicated that the above distributions were not random, but systematic in nature. These trends occurred similarly for all manufacturers and SRW unit grades evaluated, suggesting that these distributions in properties were likely tied to the manufacture of SRW units. As before, stratified random sampling methods would be more applicable for these units compared to simple random sampling.

From the preceding sections, it is evident that spatial distributions of material properties in SRW units were systematic in nature, and their statistical significance was demonstrated for units from a single manufacturer. While it is suspected that these distributions are related to manufacturing processes such as compaction and curing (Chan et al. 2005a), the existence of these patterns lead to several consequences. First, from a mix qualification standpoint, there may be units of lower quality compared to the ones tested here where sampling location could make the difference between compliance and noncompliance. For example, if the overall average water absorption of a given unit was hypothetically right on the specification limit for this property, specimens extracted from the middle layers would be found noncompliant, while specimens extracted from the outer layers would be found compliant. Second, due to spatial distributions of properties, the interpretation of material variability, operator variability or test method variability will be affected by sampling location. For the units described here, a laboratory technician who consistently chose to sample from the middle layers would obtain different results from a technician who consistently chose to sample from the outer (top and bottom) layers. Finally, the observed spatial patterns in various material properties such as absorption, flexural strength, compaction void content, and density imply that other properties such as permeability and freeze-thaw durability will vary from location to location within an SRW unit.

a. A-D Blocks

b. A-N Blocks

c. B-D Blocks

d. B-N blocks

e. C-D blocks

f. C-N Blocks.

g. D-D Blocks

h. D-N Blocks
Figure 41. Graphs. Spatial distributions of ASTM C 642 boiled absorption on split face of SRW units
(values shown represent mass of absorbed water as percent of mass of oven-dried specimen).

a. A-D Blocks

b. A-N Blocks

c. B-D Blocks

d. B-N Blocks

e. C-D Blocks

f. C-N Blocks

g. D-D Blocks

h. D-N Blocks
Figure 42. Graphs. Spatial distributions of volumetric paste content.

a. A-D Blocks

b. A-N Blocks

c. B-D Blocks

d. B-N Blocks

e. C-D Blocks

f. C-N Blocks

g. D-D Blocks

h. D-N Blocks
Figure 43. Graphs. Spatial distributions of volumetric compaction void content.

4.2.3 Split Face Delamination

One advantage of SRW systems rests on the aesthetic appearance offered by the front split surface of the SRW units. A closer look at this surface, however, often reveals thin delaminations or cracked sections of varying sizes, as shown in figures 44 and 45. These cracked sections are referred to here as split face delaminations. Chan et al. (2006a) discuss the possible origins of these delaminations, as well as the significance these may have on the evaluation of SRW unit condition both in the field and in the laboratory. As for their cause, it is suspected that split face delaminations are created in the manufacturing of SRW units during the splitting process. As shown earlier in figures 28 through 31, splitting of conjoined units is accomplished by pressing steel knife edges at the preformed notch location to be split and forcing the units to crack at this plane. Cracks emanating from the edges of the units may merge as they approach one another, as shown in figure 46a, leaving behind fractured sections in the split plane. This interaction between approaching cracks was simulated using a linear elastic fracture mechanics model (using the software FRANC2D, Cornell University, 2002) shown in figure 46b. The fractured sections left behind after crack merging are believed to constitute split face delaminations. Further support to this suggested mechanism was obtained from visual observations at a local manufacturing plant. Figure 46c shows a view of the split face of one unit immediately after being split, where a detached split face delamination is seen.

Figure 44. Photo. Sample A of split face delaminations on SRW units.

Figure 45. Photo. Sample B of split face delaminations on SRW units.

During field inspection, split face delaminations can mislead an inspector to attribute this feature as environmental damage or deterioration of the SRW unit. This confusion is enhanced by the fact that such features occur at the split surface of SRW units, which is also the surface in direct contact with the environment. During actual freezing conditions, water can seep into the space between the delamination and the SRW unit and expand upon freezing, thereby "jacking" the delaminated piece out of position (figure 47). Figure 48 shows a detached delamination under frost conditions. This mechanism may thus be interpreted as being frost-related damage during routine field inspection. One way that split face delaminations can be distinguished from other forms of damage is by the nature of the cracked and/or broken off residues. As shown in the previous sections, delaminated pieces tend to be thin and slender sections on the SRW split surface. On the other hand, frost degradation typically consists of crumbly material which appears in addition to cracking in the units, as shown in figure 49. Another point of distinction involves the quantity of fractured material. While split face delaminations tend to occur as single and isolated pieces, frost damaged material usually appears as more than one broken piece.

a. Splitting process.

b. Fracture mechanics-based simulation.

c. Observations at block plant.
Figure 46. Drawings and photo. Suspected cause of split face delaminations.

Figure 47. Photo. Breaking off of split face delamination due to ice "jacking" action in field SRW units.

Figure 48. Photo. Detached split face delamination under frost conditions in field SRW units.

Figure 49. Photo. Freeze-thaw damage on SRW unit in field.

As for testing SRW units in the laboratory, perhaps the most critical issue with split face delaminations concerns the inclusion of these loose pieces in test specimens. While these delaminations are commonly observed on the surface of units as thin shallow pieces, it is not improbable that the cracks penetrate deeper into the units. Figure 50 shows a section through the split face of an SRW unit that was shipped directly from a manufacturer to Cornell without previous exposure in the field. The size of the cracked section is approximately 130 mm long (5.12 inches) and up to 20 mm deep (0.79 inch), and it is also possible that microcracks exist in the vicinity of the main crack shown. The concern is that, if a laboratory technician extracted specimens from the split face region of a unit containing these delaminations, results from tests (e.g., strength, absorption, freeze-thaw resistance) conducted on these specimens will be misleading. To prevent this, the technician must thoroughly inspect test specimens for cracks or loose pieces prior to testing them. If loose pieces are only prevalent on the specimen surface, these pieces can pried off; however, if the specimen is cracked, the cracked portions must be trimmed off from the specimen by saw-cutting; otherwise the test specimen should be discarded and another one extracted from the parent unit. An alternate and more reliable solution is to entirely avoid extracting samples from the split face region and sample from a different surface such as those shown by the dashed lines in figure 50a. Avoiding the extraction of specimens from the split face is currently a requirement in ASTM C 1262 (2003) as mentioned in section 4.2.1 of this chapter.

a

b

c
Figure 50. Photos. Section through region containing split face delamination.

4.2.4 Recommendations for Sampling

As mentioned in 4.2.1, two different approaches to sampling could be taken depending on the intended purpose of the tests (to evaluate production methods and/or mix designs or to qualify units for projects). In either approach, the overall goal is to select units representative of the "lot" from which they are sampled and extract specimens representative of these units. Recommendations are offered in this section for specimen sampling. While these recommendations are intended for ASTM C 1262 (2003) freeze-thaw test specimens, they also apply to sampling for ASTM C 140 (2000) tests.

4.2.4.1 Sampling of SRW Units

• Units intended to evaluate the performance of as-manufactured products (for comparison of mixture designs, manufacturing methods, or quality of raw materials) shall be sampled at the manufacturing plant. These units shall be randomly sampled from a given production run for each scheduled production run. ASTM C 1262 (2003) requires that five units be sampled for freeze-thaw testing. For example, if SRW units are produced over 1 full day (8 hours), one unit could be sampled every 1 to 2 hours. These units shall be sampled near the end of the production line (i.e., after splitting and before stacking on pallets). Sampled units shall have been exposed to similar curing time and conditions (temperature and relative humidity) during manufacture and similar storage conditions after manufacture. These conditions shall be recorded.
• Units intended to evaluate the performance of products for use in projects shall be sampled either at the manufacturing plant or from pallets at jobsites. When sampling from pallets, the location on the pallet of the sampled units shall be recorded (interior of stack, exterior of stack, top units, middle units, bottom units, see figure 51). Pallet storage conditions (i.e., indoors at room condition until installation in project, outdoors and shrink-wrapped, or outdoors and unprotected) and ambient conditions (temperatures and precipitation) shall also be recorded.

It is understood that many laboratories in the industry employ this sampling technique for freeze-thaw test specimens (personal communications, NCMA, May 2005).

Figure 51. Drawing. Sampling of SRW units from pallet.

4.2.4.2 Extracting Specimens From SRW Units

• Specimens intended to evaluate the performance of as-manufactured products (for comparison of mixture designs, manufacturing methods, or quality of raw materials) shall be extracted from SRW units in such manner that the specimens cover the entire height of the unit, as shown in figure 52a. These specimens are more representative of the overall unit since each specimen covers all variations in properties along the casting direction. This method of specimen extraction is preferred over that shown in figure 52b, which covers only part of the variations along the casting direction. For specimens covering the full unit height, possible specimen sizes are therefore as follows:
  • For 200-mm (8-inch) tall unit: 200 mm (8inches) by 80 to 113 mm (3.1 to 4.4 inches) by 32 mm (1.25 inch).
  • For 150-mm (6-inch) tall unit: 150 mm (6 inches) by 107 to 150 mm (4.2 to 5.8 inches) by 32 mm (1.25 inches).

a. Preferred sampling method.

b. Not recommended.
Figure 52. Drawings. Extraction of freeze-thaw specimens from SRW unit.

There are occasions when specimens shorter than the actual height of the SRW unit may be required. For instance, 150-mm-(6-inch-) long specimens need to be extracted from 200-mm- (8-inch-) tall units. Such situations could arise where available container sizes pose a constraint, or if the performance of 200 mm (8 inches) units were to be compared to that of 150-mm (6-inch) units using specimens of similar dimensions. Since the specimen is now shorter than the unit height, material along the full height of the unit will no longer be represented within each specimen. Representation thus needs to be done over several specimens. A recommended approach to attain full height representation of the unit is shown in figure 53 where three specimens of required length X are extracted from a unit of height H at different positions so that their combined result reflects all layers equally. It is noted that the rightmost specimen shown in this figure actually comprises two halves, which is permitted under ASTM C 1262 (the two halves are "tested as and considered as a single specimen," Clause 7.1.4). When tested as a single specimen, these two halves must be tested using the same container size as the other full specimens (i.e., those shown on the left and middle in figure 53). The goal is to maintain the same (mass of test solution) relative to (mass of specimen) in each test container.

It is noted that if shorter specimens are sampled in the manner shown in figure 53, it must be ensured that the total test area of all specimens in a set be within the range of total test area of all specimens as per ASTM C 1262 (2003). ASTM C 1262 (2003) requires five replicate test specimens with test area of 161 to 225 squared centimeters (cm²) (25 to 35 squared inches (inch²)) each. This adds up to 805 to 1125 cm² (125 to 175 inch²) of test area per set. If specimens with an area of 150 by 75 mm (6 by 3 inches) are needed (test area of 113 cm² (18 inch²)), nine specimens shall be sampled and tested (for a total area of 1013 cm² (162 inch²). The reason for using nine specimens is because, as shown in figure 53, a set of three specimens is required to represent the face uniformly, and hence, only multiples of three-specimen sets can preserve this uniform representation. In the same manner that ASTM C 1262 (2003) requires five specimens extracted from five separate units, smaller specimens (1, 2 or 3 shown in figure 54) should also be extracted from separate units.

Another important consideration regarding specimen extraction is that the actual size and shape of specimens cut from SRW units dictates the size and shape of container to be used (subject to the required clearance of surrounding test solution in ASTM C 1262 (2003)). The size and shape of container in turn influences the total number of containers that can be fit in a given shelf in freezer, which then influences the freezer air cooling pattern as discussed later in this chapter.

Figure 53. Drawing. Extraction of specimens shorter than the unit height (view into back face).

• Specimens intended to evaluate the performance of products for use in projects shall be sampled such that the specimens predominantly contain material from the middle layer (typically the lowest quality layer in a face) for the unit (figure 54). All specimens in a test set (e.g., five for ASTM C 1262 (2003)) shall be sampled in this same manner.

Figure 54. Drawing. Extraction of specimens from middle layer.

• Whenever it is necessary to sample specimens from near the split face of a unit, the laboratory technician shall thoroughly inspect test specimens for cracks or loose pieces prior to testing them. If present, cracked portions shall be trimmed off from the specimen by saw-cutting, or else the test specimen shall be discarded and another one extracted from the parent unit. Situations where sampling near the split face may occur include the following ones:
  • ASTM C 1262 (2003) Clause 7.1.1 states the following:
    • ...cut the coupon from the exposed surface of the unit as the unit is used in service unless the exposed surface is a split, fluted (ribbed), or other nonplanar surface. In the case of a unit with an exposed nonplanar surface, cut the coupon from another flat molded surface.
      
      

      Compliance with this clause precludes the possibility of sampling material from the split face region. This would certainly be the case for a solid unit such as the one shown in figures 55 through 57. On the other hand, for a hollow unit (figures 58 through 60), the shaded section of the unit in figure 58 also fits the above requirement ("flat molded surface"), and as such, sampling from this region is allowed under ASTM C 1262 (2003). Figure 50, however, shows an example of a crack extending almost halfway into the portion of the unit circled in figure 58.

    • Where it may be of interest to determine material properties in this region given that it is the outer exposed surface of the unit.
  • Generally, it is recommended that sampling from the split face region be entirely avoided and that specimens be extracted from the back face as shown by the dashed lines in figures 55 and 58.

Figure 55. Drawing. Solid unit showing recommended sampling locations (red dashed lines).

Figure 56. Photo. Sample of solid unit.

Figure 57. Photo. Second sample of solid unit.

Figure 58. Drawing. Hollow unit showing recommended sampling locations (red dashed lines).

Figure 59. Photo. Sample of hollow unit.

Figure 60. Photo. Second sample of hollow unit.

4.2.4.3 General Laboratory Practice

• SRW units shall be carefully inspected for flaws such as cracks and chips (figure 61) particularly along the edges, and sampling from these regions shall be avoided. At times, the surface of the units may be scratched or damaged (figure 62), and these areas shall be avoided as well, particularly since this surface will be immersed in water or saline solution during testing. An example of a surface that is sound and free of defects is shown in figure 63.

Figure 61. Photo. Defects along edges of SRW units.

Figure 62. Photo. Scratched surface.

Figure 63. Photo. Example of sound surface.

• During saw-cutting, sufficient water shall be provided such that saw-cutting grime and loose particles are continuously flushed away and prevented from building up on the specimens. The specimens shall also be stable and not move during saw-cutting. Specimens shall also be cut at least 25 mm (1 inch) from the corners of the unit as shown in figure 64. This is to ensure that both specimen edges parallel to the height of the unit are saw-cut to match those of specimens taken near the middle of the unit.

Figure 64. Drawing. Recommended clearance from edges.

• Clause 7.1.1 of ASTM C 1262 (2003) states that "immediately following saw-cutting, remove loose particles and residue from the coupon by rinsing in tap water and brushing with a soft bristle brush." The importance of this washing is illustrated in figures 65 and 66 showing a specimen before and after being washed immediately after saw-cutting. The wash water was collected, oven-dried and weighed to determine the mass of solid particles removed. This turned out to be approximately 0.2 percent of the dry mass of the specimen. While 0.2 percent of the specimen mass may not seem to be a large value, its significance is apparent when one considers that the commonly specified freeze-thaw mass loss limit is 1 percent of the specimen mass.
• Detailed records shall be kept of the following items:
  • Curing conditions (method, length, temperature and relative humidity).
  • Sampling method of units (from the production line in block plants or from pallets on jobsites, location of units in the pallet; and storage condition of units before arriving to test laboratory).
  • Specimen extraction locations in the units (face of unit, full height or not, together with a sketch).

Figure 65. Specimen before washing, following saw-cutting.

Figure 66. Photo. Specimen after washing, following saw-cutting.

4.2.4.4 Other Research

• This research primarily focused on 200-mm- (8-inch-) tall SRW units. There are 150-mm- (6-inch-) and 100-mm- (4-inch-) tall units also available in the market. Variations along the casting direction in these shorter units shall also be examined by evaluating properties at three layers along the casting direction, as shown in this chapter and in Chan et al. (2005a and b). If spatial distribution of properties such as the ones observed for 200-mm (8-inch) units are also observed in the shorter units, it implies that spatial variations may be related to manufacturing methods.
• A more detailed investigation on the relevance of manufacturing methods on distribution of properties in SRW units entail examining factors such as compaction (surcharge) load and energy, frequency and amplitude of vibration, duration of compaction, mix composition (stiff mix versus a wetter one), unit geometry, and curing conditions (method, chamber conditions, and duration).

4.3 VARIABILITY IN FREEZE-THAW EQUIPMENT USED IN ASTM C 1262 (2003)

The previous section (section 4.2) discussed sources of test variability that could arise from specimen sampling, and a set of recommendations was provided to reduce this risk. Now, given that a population of similar specimens (similar geometry, mass, and properties) has been procured for freeze-thaw testing, the next question that arises is: how certain can a laboratory be that each and every specimen in this population is exposed to the ASTM C 1262 (2003) temperature-time (T-t) requirements during freezing? This section briefly addresses this issue by first exploring the extent of spatial variability in temperature that can exist within freezers, the significance of this variability, and recommended solutions to manage this variability. More thorough coverage of this technical issue can be found in Chan (2006) and Hance (2005).

The clauses in ASTM C 1262 (2003) relevant to freezer equipment and to the freeze-thaw cycle follow:

• Clause 5.1.1:
  
  ...the chamber or chambers shall be capable of maintaining the air temperature throughout the chamber within the specified test ranges when measured at any given time.
• Clause 8.2.1:
  
  During the freezing cycle, maintain the air temperature in the chamber at 0 ± 10°F (-18 ± 5°C) for a period of not less than 4.0 h and not more than 5.0 h. The cycle time does not include the time required for the air temperature in the chamber to reach the prescribed temperature.
• Clause 8.2.2:
  
  During the thawing cycle, maintain the air temperature around the containers at 75 ± 10°F (24 ± 5°C) for a period of not less than 2.5 h and not more than 96 h. The cycle time does not include the time required for the air temperature around the specimens to reach the prescribed temperatures.

These requirements are illustrated in the freezer-air cooling curve (T-t response) shown in figure 67 where various terms are defined. Cold soak is the time period during which the air temperature is between −18 °C ± 5 °C (0° ± 10 °F), and Clause 8.2.1 requires that cold soak be maintained for 4 to 5 hours. The cooling ramp is the portion of the cooling curve between the point at which the temperature starts falling until it reaches -18 °C ± 5 °C (0° ± 10 °F). ASTM C 1262 (2003) has no requirements for this cooling ramp. Together, the cooling ramp and cold soak comprise what is shown as the cooling branch of the curve. Similarly, on the warming side, the warm soak is the time period during which the air temperature is between 24 °C ± 5 °C (75 °F ± 10 °F); the warming ramp is the portion of the curve between the end of cold soak and the start of warm soak. While ASTM C 1262 (2003) requires the warm soak to be between 2.5 and 96 hours, it states no requirement on the warming ramp. Together, the warming ramp and warm soak comprise what is shown as the warming branch of the curve.

Figure 67. Graph. ASTM C 1262 (2003) freeze-thaw cycle—definitions.

Strict interpretation of the ASTM C 1262 (2003) clauses shown above suggests that the T-t conditions stated in Clause 8.2.1 must prevail throughout the chamber during freezing and the conditions in Clause 8.2.2 must exist around the specimens during thawing. This is reasonable if uniformity in exposure conditions is to be maintained among all specimens. However, as discussed in detail in Chan (2006) and Hance (2005) and briefly in the remainder of this section, freezer air cooling curves can vary from location to location inside a freezer, which affects specimen exposure. Such variation is influenced by, and can be partially controlled by the manner in which freezers are operated.

4.3.1 Comparison Between Different Freezers

Three different types of freezers were evaluated for internal temperature characteristics as briefly described in this section. The first was a chest freezer (figures 68 and 69) that can typically be purchased from appliance stores. This type of freezer cools the air within the freezer through its walls, and there is minimal air movement within the enclosed air space. Thus, high temperature gradients are likely inside this type of freezer. Since there are no automated temperature controls in these freezers, freeze-thaw cycling needs to be done manually (i.e., freezing by placing specimens into freezers and thawing by removing specimens from freezer and placing in laboratory air). Four such freezers were available for this study.

Figure 68. Photo. Closed chest freezer used in study.

Figure 69. Photo. Open chest freezer.

The second type of freezer was a walk-in environmental chamber (figures 70 through 72). Unlike the chest freezers, the walk-in freezer has ceiling mounted, fan-driven cooling and heating units that circulate conditioned air throughout the chamber, thereby promoting more uniform air temperature distribution. It operates at 2400 watts and has a cooling capacity of 0.13 watts per liter (watts/L) of freezer volume. This freezer has a programmable control device into which specified cooling and warming T-t profiles can be input and thus, continuous freeze-thaw cycles can be run without human intervention.

The third type of freezer was a cabinet freezer commonly used in testing laboratories. This freezer, shown in figure 73, is also equipped with cooling and heating units which are programmable to allow uninterrupted freeze-thaw cycling. Fans are also built into the unit to move air through the cabin for better air temperature distribution. This freezer operates at 5200 watts with a cooling capacity of 9.8 watts/L of freezer volume.

Figure 70. Photo. Walk-in freezer used in the study.

Figure 71. Photo. View of inside of walk-in freezer.

Figure 72. Drawing. Environmental chamber of walk-in freezer.

Figure 73. Photo. Cabinet freezer, closed.

Figure 74. Photo and drawing. Inside of cabinet freezer.

For each freezer shown in figures 68-74, the temperature distributions throughout the chamber were accurately recorded while conducting test trials following ASTM C 1262 (2003). For each freezer, different tests were performed with varying numbers of test samples, up to the maximum sample number shown in table 2, which provides additional information on the freezers used in the study as documented by Hance (2005).

4.3.1.1 Chest Freezer

In his thesis, Hance (2005) showed results of experiments carried out to determine the internal temperature variation in a chest freezer containing six ASTM C 1262 (2003) specimens. A wooden frame was built onto which 18 calibrated thermocouples were mounted to measure temperature inside the freezer at various locations (figure 75). These thermocouples were placed at a distance of about 25 to 50 mm (1 to 2 inches) from the interior wall surface.

Figure 75. Photo. View of chest freezer with wooden frame and six specimens.

Figure 76 shows the T-t response in which the large spread in internal temperatures is shown. Hance's analysis of these temperatures was based on the ASTM C 1262-98 version which specifies a target cold soak temperature range of −17 °C ± 5 °C (−12 to −22 °C). A similar analysis is shown here but using the ASTM C 1262-98^ε1 version which specifies a target cold soak temperature range of −18 °C ± 5 °C (−13 to −23°C) (this is also the version used throughout this FHWA project.)

Based on the average temperature T_avg, cold soak started at 0.9 hours when T_avg reached −13 °C (8.6 °F) and ended at 4.9 hours for a 4-hour cold soak period (the minimum recommended in ASTM C 1262, (2003)) at T_avg of −16.6 °C (12.2 °F). At the start of cold soak, the range between minimum and maximum measured temperatures was 6 °C (42.8 °F), and this range gradually decreased to 4.2 °C (39.6 °F) after 4 hours of cold soak. Figure 77 shows the standard deviation (σ) of the temperature measurements as a function of time, where it is seen that standard deviation gradually decreased with increasing soak time (hovering in the vicinity of 1.5 °C (34.7 °F)). The temperature variations for this chest freezer are summarized in table 3.

At the start of cold soak, approximately half of the temperature measurements are warmer than -13 °C (8.6 °F), while the other half are colder than -13 °C (8.6 °F). To increase the proportion of locations below -13 °C (8.6 °F), the temperature recorded at a single, random location (T_random) must therefore be colder than -13 °C (8.6 °F). Assuming that the temperature inside the freezer follows a normal distribution (with mean T_avg and standard deviation σ), the value of T_random must be such that:

-13 °C = T_random + 1.645σ, to ensure that 95 percent of the temperature measurements are below -13 °C. Thus, T_random = -15.5°C (4.2 °F)       Equation 2

-13 °C = T_random + 2.325σ, to ensure that 99 percent of the temperature measurements are below -13 °C. Thus, T_random = 16.5 °C (2.3 °F)       Equation 3

[The above expressions are based on the standard normal distribution in which 95 percent of measurements is below a value of T_{95 percent} = T_avg + 1.645σ while 99 percent of measurements is below a value of T_{99 percent} = T_avg + 2.326σ (Miller and Freund, 1985). The aim here is to determine T_avg such that T_{95 percent} and T_{99 percent} are equal to −13 °C (8.6 °F). An average value of σ = 1.5°C (4.2 °F) over the cold soak duration was used in these calculations].

The values of T_random calculated above indicate that due to variability in freezer internal temperature, the spot location must record an increasingly cooler temperature than -13 °C (8.6 °F) to ensure that most measured locations (95 and 99 percent considered) meet the -13 °C (8.6 °F) requirement.

Figure 76. Graph. Internal temperature variations in chest freezer loaded with six specimens—T-t response.

Figure 77. Graph. Internal temperature variations in chest freezer loaded with six specimens—standard deviation-time response.

* Based on Avg. ± 2σ, where σ is about 1.5°C over the duration of the cold soak period.
C.I. = confidence interval.
** C.R. = confidence range = estimated range within which 95 percent of the population exists.

4.3.1.2 Walk-in Environmental Chamber

Spatial temperature variability in the 18-m³ (630-ft³) walk-in chamber was evaluated by Hance (2005) using a similar approach. Eighteen calibrated thermocouples were located throughout the interior of this chamber as shown in figure 78. Evaluations were carried out at specimen quantities of 2, 20, 40, 60, 80, and 100 specimens. (Hance pointed out, however, that from a cooling capacity standpoint, 80 specimens appeared to be a reasonable upper limit for testing in this chamber). To illustrate the temperature variations in this chamber, results from the 60 specimen tests are shown in figures 79 and 80. Figure 79 shows the T-t response from the various thermocouples where it is observed that the band of curves was tighter than that obtained in the chest freezer. Again, based on average temperature, a 4-hour cold soak started at 4.3 hours with T_avg at -13 °C (8.6 °F) and ended at 8.3 hours with T_avg at -14.6 °C (5.7 °F). The range between minimum and maximum measured temperatures was 2.6 °C (4.7 °F) at start of cold soak and 1.6 °C (2.9 °F) at the end of the 4-hour cold soak. These parameters are summarized in table 4. The σ-t response is shown in figure 80. During cold soak, this parameter remained at about 0.4 °C (compared to 1.5 °C for the chest freezer). During the warming ramp, values of the standard deviation were higher likely due to nonuniform temperature conditions resulting from the introduction of warm air (from the heaters) to an already cold environment.

Figure 78. Photo. View of walk-in chamber with thermocouples on shelving units and suspended from ceiling.

Figure 79. Graph. Internal temperature variations in walk-in chamber loaded with 60 specimens—T-t response.

Figure 80. Graph. Internal temperature variations in walk-in chamber loaded with 60 specimens—standard deviation-time response

* Based on Avg. ± 2σ, where σ is about 0.4°C over the duration of the cold soak period.
C.I. = confidence interval.
** C.R. = confidence range

Compared to the chest freezer, the walk-in freezer exhibited more uniform temperature distribution, as indicated by the smaller standard deviation over the duration of the cold soak period (0.4 °C (32.7 °F) in walk-in freezer versus 1.5 °C (34.7 °F) in the chest freezer). This reduced temperature variation in the walk-in chamber was probably related to better air circulation imparted by the fans. As with the chest freezer however, due to variability in freezer internal temperature, the temperature recorded at a single, random location (T_random) must be colder than -13 °C (8.6 °F) to increase the proportion of measurements below -13 °C (8.6 °F). Assuming a normal distribution for the freezer internal temperature, the value of T_random must be such that:

-13 °C (8.6 °F) = T_random + 1.645σ, to ensure that 95 percent of the temperature measurements are below -13 °C (8.6 °F). Thus, T_random = -13.7°C (7.4 °F)       Equation 4

-13 °C = T_random + 2.326σ, to ensure that 99 percent of the temperature measurements are below -13 °C. Thus, T_random = -16.5°C (2.3 °F)       Equation 5

[The rational behind these expressions is similar to the ones shown previously for the chest freezer. An average value of σ = 0.4 °C over the cold soak duration was used in these calculations].

As seen, due to the lower variability, the walk-in chamber required less "overshooting" (that is targeting of a spot temperature measurement lower than -13 °C (8.6 °F) to ensure that most locations are below -13 °C (8.6 °F)) than the chest freezer.

Figure 81 shows the average T-t plots for the freezer air with various numbers of specimens, where it is clearly seen that the performance of the freezer depends on the number of specimens. While the initial rate of temperature change was similar regardless of specimen quantity (at about 56 °C/hr or 100 °F/hr), the curves diverged by the end of the test. Further, with increasing quantities of specimens, the time to reach start of cold soak was substantially delayed. For instance, with 20 specimens, T_avg reached -13 °C (8.6 °F) at 2.0 hours, whereas with 40 specimens, T_avg reached -13 °C (8.6 °F) at 2.8 hours and with 80 specimens, T_avg reached -13 °C (8.6 °F) at 7.0 hours. This means that as the number of specimens is increased, total testing time is expected to increase, and the rates of freezing will decrease.

The temperature variations for each of these tests are summarized in table 5, in which the following trends were observed:

• As the specimen quantities increased, the average temperature at the end of cold soak (after 4 hours) was warmer. In comparing 2 and 60 specimens, the temperatures at the end of cold soak differed by about 2.0 °C (3.6 °F).
• As the specimen quantities increased, the range of measured temperatures at any given time also increased. This increase was particularly pronounced at the start of cold soak, where for two specimens, the range of temperatures was 1.5 °C (2.7 °F), while for 80 specimens the range was 2.5 °C (4.5 °F). Internal temperature variability in the freezer therefore increased with increasing number of specimens.

The overall significance of these results is that, as expected, freezer performance is dependent on specimen quantity in an interactive manner. As the number of specimens change, freezer performance is affected, which modifies the exposure condition of the specimens themselves. This freezer-specimen interaction must be taken into consideration in testing and will be discussed further.

Figure 81. Graph. Average temperatures in walk-in chamber with varying quantities of specimens (values shown are number of specimens).

n.a. = data not available

4.3.1.3 Cabinet Freezer

Spatial temperature variability in the 0.53 m³ (18.8 ft³) cabinet freezer was assessed by placing 23 calibrated thermocouples throughout the freezer cabin. Evaluation of this freezer was carried out as part of the NCMA Foundation Study. At the time the freezer was received, four shelves were available, as shown in figure 74. Thermocouples were placed at each corner and the center of each shelf, as shown in figure 82. These thermocouples were typically located at about 25 mm (1 inch) above the shelf level. In addition, thermocouples were placed near the ceiling and the floor of the cabin, as well as adjacent to the freezer internal sensors. In total, 23 thermocouples were employed, and their locations are illustrated in figures 82 and 83.

Figure 82. Photos and drawings. View of thermocouple (TC) placement in cabinet freezer instrument to tests.

Figure 83. Photo. View of thermocouple (TC) placement with specimens in cabinet.

Evaluations were carried out using 0, 18 and 28 specimens. To illustrate the internal temperature variations, results from the 28 specimen tests are shown here. This was also the number of specimens tested in the NCMA study. Figure 84 shows the T-t response from the thermocouples where it is observed that the band of curves was tighter than that obtained in the chest freezer but not as tight as the one obtained with the walk-in chamber, especially at the onset of cold soak. Based on average temperature, a 4-hour cold soak started at 1.9 hours with T_avg at -13 °C (8.6 °F) and ended at 4.9 hours with T_avg at -18.3 °C (-0.9 °F). The range between minimum and maximum measured temperatures were 3.2 °C (5.8 °F) at start of cold soak and 1.4 °C (2.5 °F) at end of the 4-hour cold soak. These parameters are summarized in table 6. The standard deviation-t response is shown in figure 85 where the standard deviation is seen to gradually decrease from approximately 0.8 °C to 0.3 °C (33.4 °F to 32.5 °F) from start to end of cold soak (compared to almost constant values of about 0.4 °C (32.7 °F) in the walk-in freezer and 1.5 °C (34.7 °F) in the chest freezer). As with the walk-in freezer, values of standard deviation increased during warming ramp. As with the chest and walk-in freezers, due to variability in freezer internal temperature, the temperature recorded at a single, random location (T_random) must be colder than -13 °C (8.6 °F) to increase the proportion of measurements below -13 °C (8.6 °F).

Figure 84. Graph. Internal temperature variations in cabinet freezer loaded with 28 specimens—T-t response.

Figure 85. Graph. Internal temperature variations in cabinet freezer loaded with 28 specimens—standard deviation-time response.

* Based on Avg. ± 2σ, where σ averages about 0.6°C over the duration of the cold soak period.
C.I. = confidence interval.
** C.R. = confidence range

Assuming a normal distribution for the freezer internal temperature, the value of T_random must be such that:

-13 °C = T_random + 1.6456σ, to ensure that 95 percent of the temperature measurements are below -13 °C (8.6 °F). Thus, T_random = -14.0°C (6.8 °F)       Equation 6

-13 °C = T_random + 2.326σ, to ensure that 99 percent of the temperature measurements are below -13 °C (8.6 °F). Thus, T_random = -14.4°C (6.1 °F)       Equation 7

As seen, the "overshooting" (that is the targeting of a spot temperature measurement lower than -13 °C (8.6 °F) to ensure that most locations are below -13 °C (8.6 °F)) is intermediate between that of the chest and walk-in freezers.

Knowing the locations of these thermocouples allowed mapping of the temperature inside the cabinet freezer to detect patterns within the freezer. This was done by selecting a time from the T-t record (3.5 hours arbitrarily selected), ranking the available temperatures at this time in order from coldest to warmest and splitting all the locations into six groups of three or four locations per group. These groups were then labeled from 1 at the coldest spots to 6 at the warmest spots. The result of this mapping is shown in figure 86 in which the overall coldest and warmest spots are also identified. The coldest locations generally were in the front part of the freezer (i.e., near the door) and on the higher shelves (including the ceiling), while the warmest locations were toward the back of the freezer in the lowest shelves (including the floor). The coldest overall location was right at the location of the freezer's built-in temperature sensor that is used to control the freezer cycles. This sensor is located near the fan. The warmest overall location was at the back of the bottommost shelf. This pattern of temperature distribution generally coincided with the flow of air within the chamber, illustrated in figure 87. As shown, the air coming out from the fan is the coldest air and reaches the top shelf and freezer front locations first. On the other hand, the back locations in the lower shelves are more or less sheltered, and as such, experience the warmest temperatures. The freezer's internal control sensors, positioned at the fan exit, were exposed to the coldest temperatures within the chamber. The T-t trace for this location is shown by the dark line in figure 84. This has important implications for test control as will be discussed later.

Figure 86. Drawing. Individual temperature locations (front view of freezer, plan view of each shelf).

Figure 87. Drawing. Temperature mapping in cabinet freezer (side view of the freezer cabin).

The effect of the number of specimens is seen in figure 88 showing the average T-t plots for the freezer air for 0, 18 and 28 specimens. Unlike the walk-in chamber, the initial rate of temperature change depended on specimen quantity from 61 °C/hr (110 °F/hr) with no specimens, to 27 °C/hr (49 °F/hr) with 28 specimens. The times required for T_avg to reach -13 °C (8.6 °F) were 0.7, 1.5 and 1.9 hours for 0, 18 and 28 specimens, respectively. The temperature variations for each of these tests are summarized in table 7. It is interesting that temperature variability at the start of cold soak in the cabinet freezer, even without specimens in it, was larger than that observed in the walk-in chamber. The 95 percent confidence range was 3.4 °C (38.1 °F) for the cabinet freezer with 0 specimens and 1.6 °C for the walk-in freezer with 60 specimens. Although the reason for the larger variability observed in the cabinet freezer at start of cold soak is unclear, it is suspected that at this time (start of cold soak when T_avg reaches -13 °C (8.6 °F)), the cabinet freezer was still experiencing rapid temperature drops (see figure 89) and a stable, more uniform air distribution throughout the freezer had not yet been reached. Once in the cold soak zone at more stable temperatures, variability decreased. Overall, increasing the number of specimens increased variability.

Figure 88. Graph. Average temperatures in cabinet freezer with varying quantities of specimens
(values shown are number of specimens).

n.a. = data not available

4.3.1.4 Recommendations To Reduce Freezer Internal Variability

As seen from the previous sections, each of the freezers evaluated was capable of complying with ASTM C 1262 (2003) requirements, although the shapes of the T-t responses were quite distinct from freezer to freezer. Results of ASTM C 1262 (2003) tests conducted on specimens in different freezers (Haisler, 2004) showed that these kinds of shifts in T-t response (shown in figure 88) can have a very significant influence on specimen performance. The focus in this section is on how internal variability within a freezer can affect compliance with the test standard, and more importantly, how one can modify the freezer, specimen storage, or freezer controls to meet compliance with ASTM C 1262 (2003) and to provide consistent exposure conditions for all the samples contained in the freezer. Below are some recommendations for modifying ASTM C 1262 (2003) to meet these objectives.

It is evident that cold and warm spots exist within each freezer. Specimens that remain in these cold and warm spots can conceivably be subjected to different numbers (and types) of freeze-thaw cycles and exhibit different durabilities. One way to minimize discrepancies in exposure conditions is to move samples periodically during the course, which is currently specified in ASTM C 1262 (2003):

• Clause 8.2.4:
  
  At 25 ± 5 cycle intervals, remove containers from the test chamber. Open containers to visually inspect the condition of the specimens and to adjust the water level to comply with 8.1.1 (13 mm or ½ inch depth).
• Clause 8.2.5:
  
  Every time a container is replaced into a multi-level freezing test chamber, the container shall be placed on the level immediately above the level on which it was previously located. If the container was previously located on the top level of a multi-level freezing chamber, replace it onto the bottom level.

Based on these clauses, specimens tested to 100 cycles would only be rotated four times through the entire duration of the test. Moreover, Clause 8.2.5 only mentions sample rotation in the vertical direction of the freezer. Hence, if a specimen is placed on the back side of the freezer, it is possible for it to remain on the back side through 100 cycles with the only difference being the shelf level. Similarly, there could also be specimens near the front of the freezer through the entire test. It is thus recommended that specimen rotation occur at intervals shorter than 25 cycles (say every 10 cycles), and that rotation be not only carried out in the vertical direction, but also within a shelf (e.g., front and back). This approach would allow for more uniform exposure of all specimens over 100 cycles.

As demonstrated earlier for the walk-in and cabinet freezers, the T-t characteristics in these freezers varied when loaded with different quantities of specimens. This variation is as expected due to the different thermal mass associated with different specimen quantities and due to the way that specimen loading may influence interval air flow and convective heat transfer to or from any given specimen. Variations in specimen quantities, however, could arise due to a number of possible reasons, which include (but are not limited to) laboratory testing demand and schedules, specimen or container size constraints, or the gradual removal of specimens that are considered failed.

On the issue of specimen and container size constraints, various possibilities exist which could ultimately result in different specimen quantities, with associated changes in T-t characteristics within the freezer. A simple but effective recommendation to address this issue is to standardize the container size to be used for ASTM C 1262 (2003) testing.

Removal of failed specimens (e.g., mass loss exceeded specification limit) during the course of a test also changes the total number of specimens in the freezer and thus changes the exposure for the balance of the specimens. While such situations are clearly dealt with in ASTM C 666 (2004) for rapid freeze-thaw of ordinary concretes, ASTM C 1262 (2003) has no provision for this. ASTM C 666 (2004) covers this situation as follows: "Whenever a specimen is removed because of failure, replace it for the remainder of the test by a dummy specimen," (Clause 8.3, ASTM C 666, 2004). It is recommended that ASTM C 1262 (2003) adopt a similar provision and require that a failed specimen, upon removal from the chamber, be replaced by a "dummy" specimen to prevent fluctuations in the total mass inside the freezer during the course of a test.

Overall, it is seen that total specimen quantity in the freezer can vary for a number of reasons, which alters the temperature environment inside the freezer. This in turn changes the exposure conditions of the specimen themselves. As such, it is important that laboratories conducting freeze-thaw tests survey their freezers before conducting tests, to identify these variations and the extent to which temperatures may be distributed in the freezer. Knowledge of this temperature variability should then be employed to plan tests cycles, the results of which can be used to optimize the testing regime for that given freezer. Complete details on this approach are provided in Chan (2006). This approach requires a test cycle to be run on a given freezer (with a given number of test specimens) with temperature monitored throughout the chamber, from which T-t and standard deviation-t curves can be generated. A reliability-based approach is then taken to optimize the control of the freezer to minimize locations within the freezer that are undercooled or overcooled, in other words to minimize the proportion of noncompliant locations within the freezer. This approach, which makes use of graphical reliability (R) curves, is described in appendix A, where it appears as an annex to a new proposed version of ASTM C 1262 (2003) based on the findings of this project. Additional information, including sample calculations, can be found in chapter 4 of Chan (2006).

It should be noted that this approach of performing trial tests and statistically optimizing actual tests based on these results has already been applied to the NCMA-funded study and other ongoing projects. It must also be emphasized that R-curves portray a specific interaction between the freezer and specimens. A given freezer loaded with a certain quantity of specimens of a particular size, mass and arrangement inside the freezer will possess a uniquely characteristic R-curve. Changing any of these variables will alter the form of the curve. Finally, R-curves cannot be determined a priori but must be determined individually for each freezer and specimen configuration.

4.4 CHARACTERISTICS OF THE FREEZE-THAW CYCLE

Section 4.3 focused on evaluating temperature variability inside a freezer during a typical freeze-thaw cycle. The objective of this evaluation was to enable maximizing the number of locations in the freezer that were compliant with ASTM C 1262 (2003) cold soak requirements, which consists of maintaining the freezer air temperature at -18 °C ± 5 °C (0° ± 10 °F) for 4 to 5 hours (note that this is irrespective of how fast the freezer cooled to this condition). However, as shown in figure 89, the T-t response of the SRW specimen itself (as measured from sensors embedded in the specimen, figure 90) follows a path that is distinct from that of the freezer air. This section discusses the ice formation process and damage mechanisms as the specimen undergoes the various stages in a freeze-thaw cycle.

Figure 89. Graph. T-t curves for freezer air, water surrounding specimen and specimen.

Figure 90. Photo. Water surrounding specimen and specimen (as graphed in figure 89).

The work described in this section involved calorimetric methods to trace ice formation during freezing of plain water and 3 percent NaCl solution (salt as percentage of solution mass). The concept of freeze progress (or percent ice formed) was introduced to describe and quantify ice formation rates. Concentration changes in the salt solution during freezing were also examined. The work also includes the use of glass vials such as those shown in figures 91 through 94 to simulate freezing of solutions in saturated, confined spaces. Events taking place during freezing such as supercooling and expansion damage were traced along the T-t response of the solution. Other relevant aspects investigated included varying concentrations of salt solution, varying saturation levels, effect of cooling environment and estimation of ice pressures.

Figure 91. Photo. Temperature-monitored glass vials used to characterize freeze-thaw cycles and impact on water and saline solutions.

Figure 92. Drawing. Location of thermocouples.

Figure 93. Photo. Vials in freezer.

Figure 94. Photo. Broken vials.

4.4.1 Significance of Freeze-Thaw Cycle

Differences in the nature of the freeze-thaw environment can have an impact on specimen performance as evidenced from results obtained under this FHWA project and in the NCMA-funded project. In summary, for the NCMA study (described in detail in chapter 6 of Chan (2006)), specimens extracted from SRW units from a single production run were tested in two separate freezers: the walk-in chamber and the cabinet freezer, which were described earlier in this chapter. When tested according to ASTM C 140 (ASTM C 140, 2000), properties of the specimens evaluated in the two freezers differed by no more than 8 percent in their compressive strength (average 37 MPa (5,370 psi)) for specimens in walk-in chamber versus average 34 MPa (4,930 psi) for specimens in cabinet freezer), 2 percent in water absorption (127 versus 129 kg/m³) and 2 percent in oven-dry density (2230 versus 2210 kg/m³). Of the various test sets (labeled A to G) evaluated in the cabinet freezer, test set A specimens were similar to specimens tested in the walk-in freezer, which comprised nominal 200- by 100- by 32-mm-(8- by 4- by 1¼- inches-) size SRW coupons placed in 13 mm-(½ inch-) deep saline solution inside plastic containers of size 310 by 210 by 108 mm (12.3 by 8.3 by 4.3 inches), as shown in figures 95 and 96.