Title: Field Air Permeability Testing in Non-Destructive Condition Assessment of Damaged Concrete Structures

Transcription

1 Title: Field Air Permeability Testing in Non-Destructive Condition Assessment of Damaged Concrete Structures Submitted to Committee AFN10 Emerging Concrete Technologies Session Organizer: Dr. Mohammad S. Khan P.E. and Dr. David L. Gress AUTHOR CONTACT INFORMATION 1-Communicating Author: Ufuk Dilek Ph.D. P.E., Senior Materials Engineer, MACTEC Engineering and Consulting Inc Atlantic Avenue Raleigh, NC Phone: (919) Fax: (919) Keywords: permeability, air permeability, elastic (Young's) modulus, dynamic (Young's) modulus, pulse velocity, cracking, fire damage, non-destructive testing

2 Dilek, Ufuk 1 Field Air Permeability Testing in Non-Destructive Condition Assessment of Damaged Concrete Structures by: Ufuk Dilek ABSTRACT This article discusses the use of in-situ air permeability testing in evaluation of fire damage to a concrete structure. In-situ air permeability measurements were made on the surface of concrete members damaged by exposure to fire. Non-destructive in-situ field air permeability testing was effective in identifying damaged areas. The findings of the field air permeability testing were compared to the findings of a conventional non-destructive testing method, ultrasonic pulse velocity. The area of surface distress indicated by in-situ air permeability results was larger than the area of compromise indicated by conventional ultrasonic pulse velocity testing. Field air permeability results were also compared to the laboratory testing results performed on core samples removed from the structure adjacent to field air permeability locations. Laboratory testing of samples enabled a comparison between field and laboratory air permeability data and enabled validation of field air permeability results. Air Permeability Index (API) of 25 mm (1 in.) thick disks sawed from the cores was determined in the laboratory testing phase. A significant increase in API was detected in the damaged areas and API was found to be particularly sensitive to fire damage. Air permeability index results indicated significant potential for further use as a quantifiable laboratory test method. The study also demonstrated the applicability of field air permeability as a rapid field indicator of sustained damage to concrete and associated drying and cracking due to exposure to elevated temperatures. INTRODUCTION The subject fire damaged structure is a three level, pre-cast concrete parking garage. The bottom level is a concrete slab-on-grade while the upper level decks are constructed of 2.75 meters (9 feet) wide by 17.7 meters (58 feet) long pre-cast, pre-stressed, lightweight aggregate concrete double-tee members simply-supported on edge girders. An automobile parked on the lower level slab-on-grade caught fire and burned for 30 to 45 minutes. The fire completely destroyed the automobile. The heat from the fire visibly damaged two pre-stressed concrete double-tees directly above, at the second level (Figure 1). Four doubletee stems were affected, on two adjacent members. The two stems centered on and closest to the fire exhibited cracking, spalling and discoloration of concrete (Figure 2). The two stems farther from the fire were discolored. Visual observations found that spalling had occurred at the bottom of two of the four double-tee stems that were directly above the fire (Figure 2). The car was parked at an angle under stems B and C, as shown in Figure 1. The spalled areas were approximately 0.9 to 1.2 meters (3 to 4 feet) in length and began approximately 1.5 and 3.7 meters (5 and 12 feet) from the bearing end on stems B and C respectively. The two outside stems, stems A and D on the two affected pre-cast double-tees, did not exhibit any signs of spalling. Initial visual observations of the spalled concrete stems revealed a pink discoloration of concrete near the surface, continuing to a depth of approximately 12.5 mm. (½ in.). Pink

3 Dilek, Ufuk 2 concrete indicated the concrete temperature at these locations exceeded 300 ºC (570 ºF) and properties of concrete at these areas were significantly compromised (Figure 2). Cracks were observed at the bottom of the stems adjacent to spalled areas (Figure 2). An engineering evaluation of the structure was requested by the owner of the structure to evaluate the level of distress and necessary repairs or rehabilitation for continued safe operation of the structure. Engineering assessment included in-situ non-destructive evaluation techniques including ultrasonic pulse velocity of concrete, for identification of affected areas for removal of core samples. The laboratory testing of cores included Air Permeability Index of 25 mm (1 in.) thick disks sawed from the surfaces of the cores and Dynamic Young's Modulus of Elasticity of the same relatively thin disk specimens. Based on the pronounced response in API of damaged concrete from affected areas of the members, a non-destructive field air permeability study was performed in an effort to identify the potential of the field air permeability technique as a rapid field assessment tool. Air permeability measurements were made on the surface of affected concrete members. The study enabled a comparison of findings of the non-destructive, in-situ, field air permeability testing to conventional ultrasonic pulse velocity testing. Field air permeability measurements adjacent to core locations enabled a comparison of laboratory air permeability measurements to field air permeability results. PROBLEM STATEMENT Concrete elements subjected to exposures in excess of those intended in service can be damaged, but still retain structural capacity adequate for the expected loads. Severe exposures can cause cracking and microcracking, affecting both the mechanical (or structural) properties and the permeability, which is often critical for durability. When severe exposures are localized, such as during a fire, techniques are needed which permit evaluation of in-situ properties and establishing the limits of affected areas. Techniques are also needed in measurement of both the mechanical properties and the permeability of the affected concrete elements, and the presence and extent of any damage gradients. The evaluation should be based on physically meaningful measures of important concrete properties. This paper discusses the findings of a two-phase study examining the use of air permeability as a field and laboratory tool for identification of sustained fire damage. This study should be of value to both researchers and practicing engineers concerned with the development and use of in-situ and laboratory damage assessment techniques. The field air permeability technique discussed in this study enabled in-situ determination of an important property: permeability of damaged members that may be accessible on one side only. The disk test approach used in the laboratory phase of this study employed the same specimen to determine both an important mechanical property, the elastic modulus, measured non-destructively, and the permeability of concrete. The use of the same thin disk specimen in determination of modulus and permeability enabled identification of possible gradients, and the relationship between these properties, which both provide a deeper understanding of the relationships between fundamental engineering properties of concrete and improve evaluation capabilities for existing structures.

4 Dilek, Ufuk 3 METHODOLOGY- PHASE I: ULTRASONIC PULSE VELOCITY TESTING AND LABORATORY EVALUATION OF CONCRETE Ultrasonic Pulse Velocity Testing Through the Affected Stems Ultrasonic pulse velocity technique through the affected double-tee stems was used to determine the extent of likely distressed concrete and to identify affected areas for removal of concrete core samples. Pulse velocity testing was conducted in accordance with ASTM C 597 "Standard Test Method for Pulse Velocity Through Concrete" (1). Equipment used in the field assessment consisted of a commercially available pulse velocity meter with a digital readout, 5 cm (2 in.) diameter transducers with a frequency of 54 khz, shielded coaxial connection cables and a reference bar with a marked transit time of 26.1 µsec for calibration. A commercially available, water-soluble jelly was used as the acoustic coupling agent. Pulse velocity measurements were taken through the double-tee stems at 0.3 meter (1 foot) grid intervals and compared to values collected from unaffected members away from the fire site. Pulse velocity testing was done in an attempt to determine the extent of fire damage along the double-tee stems as indicated by a decrease in pulse velocity. Based on visual examinations and results of pulse velocity testing, locations for removal of concrete core samples were selected. Radiographic exposures were performed to precisely locate the pre-stressing tendons prior to coring of the stems to avoid damage to tendons. Through cores were taken from the four affected stems and from an unaffected area away from the fire site for reference. Laboratory Analysis of Concrete Cores Laboratory quantitative analysis of the fire damage to concrete was performed by determining the Dynamic Young's Modulus of Elasticity (Dynamic Modulus, E d ) and Air Permeability Index (API) of 25 mm (1 in.) thick concrete disks sawed from cores extracted from the structure. Analysis of concrete properties at 25 mm (1 in.) depth increments permitted assessment of damage gradients in smaller depths compared to conventional strength specimen or core sizes. A brief description of the Dynamic Modulus and Air Permeability Index test methodology is provided below. Air Permeability Index (API) Thin Concrete Disks Chloride ion permeability, water permeability and air permeability of concrete are related to the pore structure and the presence of cracks or microcracks in the concrete. Chloride ion permeability is often assessed using ASTM C 1202, Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration (2), or by determining the apparent diffusion coefficient after long term ponding of a slab with a chloride solution. These tests, however, can be time consuming, or subject to difficulties in interpretation. Testing in accordance with ASTM C 1202 consumes approximately 3 days including preparation time and results can be biased when the sample contains certain ionic species, such as calcium nitrite or soluble chloride compounds. The air permeability index described in this study can be determined relatively quickly in 6 to 8 minutes for conventional strength concretes, and provides a physically meaningful metric, although it is not a conventional D Arcy type permeability

5 Dilek, Ufuk 4 measure but a measure of gas flux through the concrete specimen. The literature contains work by others on the relationship between rate of gas permeation and cracking or damage in concrete. Bremner et al. (3) conducted tests to study the effect of stress on the nitrogen gas permeability of structural lightweight and normal weight concrete. Cylindrical hollow concrete specimens were loaded in axial compression at the same time as a nitrogen pressure differential was maintained across the cylinder wall. It was found that rapid increases in permeability occurred at approximately 54% to 62% of the ultimate strength for the normal weight concrete. Sugiyama et al. (4) used a very similar setup and found the stress level at which nitrogen gas permeability increased significantly was 76% to 79% of the ultimate strength for normal weight concrete. Timofeyev (5) described the relationship between the pressure, volume and time of a constant gas flow through porous media for a circular disk. A measure of the permeability of the concrete disks was determined using the Air Permeability Index (API) described by Schonlin and Hilsdorf (6). Whiting and Cady (7) discussed the use of air permeability measurements taken at the surface of bridge decks and an evaluation of corrosion potential. The study utilized a different air permeability gage than what was used in this study. The conclusions indicated the effective depth tested using the outlined methodology was limited to 12.5 mm (0.5 in.) of near surface concrete. The Air Permeability Index as described by Schonlin and Hilsdorf was used in evaluation of damaged concrete in various case studies involving different damage mechanisms (8,9). The test was conducted using 25 mm (1 in.) disk specimens and the test setup described in Dilek (10) (Figure 3). Schonlin and Hilsdorf s methodology to rapidly determine the air permeability of concrete disks based on gas permeation involves applying a vacuum to one side of the disk and recording the time for a given change in pressure. This method requires measurement of the time required for a volume of air to permeate through a concrete disk, due to differences in pressure on the opposite faces of the specimen. An equation for an air permeability index was developed on the basis of Boyle-Marriotte's law. This equation assumes a constant flow of gas and treats the concrete as a homogeneous porous material. API = ( 1 0) ( t t ) p 1 0 p p V a p s + p where API = the air permeability index (m 2 /s), p 0, p 1 = pressure inside the vacuum chamber at the beginning and end of the measurement, respectively (millibars), p a = atmospheric pressure (millibars), t 1 - t 0 = duration of measurement (sec), V s = volume of vacuum chamber (m 3 ), L = thickness of specimen (m), and A = cross-sectional area of specimen (m 2 ). L A (1)

6 Dilek, Ufuk 5 The API has units of m 2 /s. Schonlin and Hilsdorf refer to this parameter as a permeability "index" avoiding confusion with conventionally defined permeability coefficients. The vacuum chamber used in this study is shown in Figure 3. The test chamber was constructed of metal and had a volume of 460 ml. The chamber was sealed to the sawed face of the concrete disk using a thin strip of soft clay. The original, outside surface of the cylinder was sealed with 2 layers of heavy, plastic tape to limit the flow of air through this face. This method of specimen preparation is quick and was found to provide the same measured air permeability index as an epoxy seal applied to the outside face. The specimen was fixed to the chamber using a wing nut clamp, which was not needed once the vacuum had been applied. Testing was conducted by applying a vacuum to the chamber which firmly seated the device to the disk. Once a stable vacuum was obtained, the valve to the vacuum pump was closed and the time required for the pressure to fall from about 980 millibars (29.0 inches Hg) to about 915 millibars (27.0 inches Hg) was determined. The test was conducted three times for each specimen. Dynamic Elastic Modulus (Ed) The dynamic, or low strain, elastic (Young's) modulus of the concrete disks was determined non-destructively using resonant frequency principles similar to those found in ASTM C 215 "Standard Test Method for Fundamental Transverse, Longitudinal, and Torsional Frequencies of Concrete Specimens," (11) adapted for thin circular specimens. Determination of the dynamic elastic modulus of a disk is based on the theory developed by Hutchinson (12) assuming axisymmetric flexural vibration of a thick, free, circular plate, including shear and rotary inertia effects. A method of determining the dynamic elastic modulus of circular concrete disks based on resonant frequency is discussed in detail by Leming et al. (13). The dynamic elastic modulus of a disk under free-free vibration is fd E = + d 21 ( ) where E d = the elastic (dynamic) modulus, = Poisson s ratio, = the mass density of the disk, f = the fundamental cyclic natural frequency in Hertz, d = the diameter of the disk, and 0 = the frequency parameter associated with the fundamental vibration mode. E d may be determined by measuring f, d and, estimating and obtaining 0 from an iterative solution, as described in Leming et al. (13). Splitting Tensile Strength Laboratory testing also included determination of splitting tensile strength in general accordance with ASTM C 496 Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens (14). 0 2 (2)

7 Dilek, Ufuk 6 METHODOLOGY- PHASE II: FIELD AIR PERMEABILITY TESTING Field Air Permeability Testing The vacuum chamber used in the first phase of the study was also used in field air permeability measurements. The chamber was directly sealed to the surface of the stems (Figure 4). A portable vacuum pump and a generator were also required for field air permeability testing. The formed surface of the sides of the double tee stems provided a suitable smooth surface for the application of vacuum. The vacuum was sufficient to support the weight of the chamber and a separate support assembly was not required once the vacuum was applied. Once a stable vacuum was obtained, the valve to the vacuum pump was closed and the time required for the pressure to fall from about 980 millibars (29.0 inches Hg) to about 915 millibars (27.0 inches Hg) was determined. Field air permeability determinations were made adjacent to previous core locations to provide a comparison of the field air permeability values to the surface disk air permeability values. Air permeability measurements were also made along Stem B to provide a comparison of areas indicated using field air permeability to pulse velocity. While the laboratory air permeability is based on air flow through a 25 mm. (1 in) disk, sealed on the perimeter, the field air permeability will measure the air flow through the near surface concrete. The stem is too thick to have any expectation of through stem air flow, except possibly in the presence of a flexural crack. In general however, the air flow can be expected to be highly influenced by the concrete at the surface. Any deterioration gradient starting with the surface such as with fire, was expected to have a significant effect on the measured air flow. PHASE I ULTRASONIC PULSE VELOCITY TESTING AND LABORATORY EVALUATION RESULTS: Results of Pulse Velocity Testing. The average pulse velocity measured horizontally through the unaffected stems away from the fire site (control) was 4080 meters per second (mps) (13,400 feet per second (fps)) and the lowest velocity measured was 3810 mps (12,500 fps). The average of 5 consecutive measurements immediately above the spalled areas on stems B and C were low compared to the rest of the stems as well as the control values (Figure 5). The lowest individual pulse velocity values in these areas were 3290 mps (10,800 fps) and 3220 mps (10,550 fps) for stems B and C, respectively. The pulse velocity measurements recovered and approached control values quickly when measurement points were moved away from the center of the fire as shown in Figure 5. This finding indicated that the damage, as indicated by pulse velocity, was likely localized to the concrete directly above the location of the vehicle. Based on the pulse velocity results, core locations in Stems B and C were selected in the affected areas for further laboratory analysis. Following radiographic exposures to locate the tendons, 100 mm (4 in.) diameter cores were taken horizontally through the four affected stems and from an unaffected stem (E, Control) for reference. Cores from stems B and C directly above the fire were discolored at both ends while cores from stems A and D were discolored at the end facing the fire.

8 Dilek, Ufuk 7 Results of Laboratory Testing on Concrete Core Samples. Air Permeability Index (API) and Young s Modulus (E d ) were measured on 25 mm (1 in.) thick disks sawed from the cores. Two disks sawed from the first 25 mm on each end of the cores were labeled surface disks, and the other two disks were sawed from the next 25 mm depth of the core were labeled interior disks. The remaining center portion of the core, with a nominal length of 75 mm (3 in.), was then tested in general accordance with ASTM C 496 (14). While the length-to-diameter ratio requirements for cored specimens outlined in the test were not met, performing this test provided a comparison of splitting tensile strength at the center of the stem between the affected and unaffected members. Splitting tensile strengths for the affected Stems B and C were measured as 2.76 Mpa (400 psi) and 3.00 Mpa (435 psi) respectively. Splitting tensile strength values for the center of stems A, D and E (control) were 3.93 Mpa (570 psi), 3.79 Mpa (550 psi) and 3.93 Mpa (570 psi) respectively and indicated that the center portion for stems A and D were not significantly affected. Compressive strengths were not determined. Laboratory test results are presented in Table 1. A significant loss in Young s Modulus (E d ) and increase in Air Permeability Index (API) were found in the surface and interior disks of the two stems directly above the fire (B and C). Compromise in material properties was greater in the surface compared to the interior of the stems. The two stems farther away from the fire (A and D) showed less of an effect, but effects were more pronounced at the surface facing the fire compared to the surface directed away from the fire. Air Permeability Index (API) values were higher for stems closest to the fire by as much as an order of magnitude. Figure 6 shows air permeability index values in relation to dynamic modulus values. Results from stems B and C, closest to the fire, plotted on the upper left of the graph, with low modulus values and high air permeability. Stems A and D plotted closer to control values while still showing some compromise in the measured properties. The results indicated that the Young s Modulus and API in damaged members were generally linearly related until damage was pronounced as in the case of Stems B and C which exhibited a rapid non-linear increase in API as indicated by the exponential curve fit to the data. The API was found to be very responsive to the presence of fire damage. DISCUSSION OF PHASE I RESULTS AND DEVELOPMENT OF PHASE II Non-destructive pulse velocity readings indicated a localized compromise in the mechanical properties that was limited to an area of visibly spalled concrete directly above the location of the vehicle. Subsequent testing of concrete core samples were in general agreement with the findings of pulse velocity. At affected areas where a reduction in pulse velocity was indicated, a reduction in Young s Modulus was also determined. This was expected based on the relationship between the elastic modulus and compression wave speed (pulse velocity). Splitting tensile strength values also confirmed that significant damage had occurred to stems B and C. In addition to the mechanical property effects, the Phase I testing program identified a pronounced change in air permeability in damaged stems. Air Permeability Index was found to be particularly sensitive to fire damage. The API values of the undamaged concrete specimens are often less than 1 x 10-6 m 2 /s. Observations during testing and analysis of the data found that

9 Dilek, Ufuk 8 an API in excess of a 3 x 10-6 m 2 /s indicated a specimen with damage or of poor quality and was the approximate point at which a non-linear relationship with dynamic modulus began. These limits, although certainly tentative, may be of value in non-destructive or in-situ testing of concrete elements in service. The potential for air permeability as an indicator of fire damage warrants further research. Based on the findings of Phase I, the second phase of the evaluation program was developed. The purpose of Phase II was to evaluate the potential for air permeability as a field indicator of damaged concrete. PHASE II FIELD AIR PERMEABILITY RESULTS Field air permeability determinations were made adjacent to previous core locations to provide a comparison of the field air permeability values to the surface disk air permeability values. Air permeability measurements were also made along Stem B to provide a comparison of areas indicated using field air permeability to pulse velocity. Figure 1 shows the times measured for the required pressure difference to occur when the vacuum chamber was attached to the surface of the concrete member. Figure 1 also contains the pressure differential times associated with surface disks with matching field air permeability data for comparison. Also included in Figure 1 is a graph showing the pulse velocity and the field air permeability measurements along Stem B for comparison of findings of the two test methods. Field Air Permeability Testing Adjacent to Core Locations The average pressure differential time associated with disks from Stem E (control) was 337 seconds. Field air permeability measurements were taken as control, at the unaffected end of the stem B to eliminate double tee to double tee variation. The pressure differential time at the end of Stem B was measured as 105 seconds. Readings taken adjacent to core locations indicated the following. Affected areas on Stems B and C had very small times for the required pressure differential to dissipate confirming the findings of the disk API testing. On Stem D, the side facing the fire was affected more than the side facing away from the fire, again, similar to the findings of disk API testing. The finding that the field air permeability pressure differential times were approximately 1/3 of disk times indicated that the effective thickness or section contributing to the field air permeability was less than the thickness of the disk specimen. This finding indicated that the field air permeability test is essentially a surface permeability test limited to the near surface concrete. The laboratory test on 25 mm (1 in.) disks was performed by sealing the outside surface of the disk with 2 layers of heavy, plastic tape to limit the flow of air through this face. This specimen preparation limiting air flow through the sides left the exposed circular surface of the disk nearly 25 mm (1 in.) away as the primary surface for air flow. This was not the case however for the field air permeability test. Considering the end of the member was more than 100 mm (4 in.) away the path of least resistance would likely be the immediate concrete areas adjacent to the perimeter of the vacuum chamber. Because of the geometry of the test and the direction of the air flow, an equivalent contributing thickness proportional to the ratio of disk and field measured times may not represent the air flow through the member towards the vacuum chamber. A flow net that uses

10 Dilek, Ufuk 9 the pressure differential between the outside and the chamber and using a contributing peripheral area may model the air flow more accurately than an equivalent thickness model. For the purposes of this paper evaluating the applicability of air permeability testing as a field device this model was not developed and an analysis was performed based on measured pressure differential times alone. Further research is warranted in this area. Field Air Permeability Testing Along Stem B Readings taken along Stem B indicated a compromise directly above the fire; while pressure differential times increased moving away from the fire. While the findings confirmed that the area directly above the fire was damaged, the field air permeability times approached control values after nearly 6.5 meters which was a recovery over a larger area compared to the findings of pulse velocity which recovered relatively quickly at about 2.7 meters from the bearing end (Figure 1). Air permeability measurements in general were found to be particularly sensitive to fire damage in the laboratory phase. The field air permeability readings taken along Stem B indicated field air permeability test was able to identify distress at the surface areas that were not directly above the fire and were not picked up using pulse velocity. Since the measured pulse velocity is based on the average time of transit of a compression wave through the entire concrete member, it may be difficult to determine whether damage in a relatively thin surface zone exists if the distressed zone is shallow compared to the rest of the pulse travel path. This finding pointed out a limitation of through member pulse velocity testing in identification of shallow damage gradients. Field Air Permeability Testing at Crack Location The subject members were load tested in accordance with ACI 318 Chapter 20 (15) based on the damage identified during the testing phase. The members were tested in flexure using approximately kgs (44800 lbs). During the load test, flexural cracks were observed at the mid-span of the members (Figure 7). These cracks recovered and closed upon removal of the load due to the pre-stressing force on the members. A field air permeability measurement was made on one of these crack locations at midspan of Stem C. Figure 7 shows field air permeability measurement on the flexural crack which was outlined using a pencil. The pressure differential time was 60 seconds at this location which was less than typical unaffected field air permeability times measured during the study. Further research is warranted in applications of field air permeability testing in other types of distresses and cracking patterns such as surface crazing, individual cracking, other damage mechanisms or locating poor quality low strength, high water-cement ratio concrete in a placement etc, in addition to the fire damage mechanism discussed in this article. CONCLUSIONS AND RECOMMENDATIONS 1. Air Permeability Index was found to be particularly sensitive to fire damage. Additional research is recommended in using air permeability index of disks as a laboratory method of fire damage assessment. Additional research is also recommended in air permeability index in laboratory assessment of other damage and deterioration mechanisms and correlations with other

11 Dilek, Ufuk 10 permeability measures. 2. Field air permeability measurements adjacent to laboratory sample locations confirmed the findings of the laboratory testing phase. Additional research is recommended in field air permeability testing as a rapid field indicator for identification of damaged areas and selection of core locations or as a means for reducing the number of necessary cores. The field air permeability technique required access to only one side of the member as opposed to both sides for determination of pulse velocity. 3. Field air permeability was effective in identifying distress to the near surface concrete which was not identified using through member pulse velocity. Further research is warranted in applications of field air permeability in other types of distresses and cracking patterns in the field such as surface crazing, individual cracking, other damage mechanisms or locating poor quality high w/c concrete sections in a placement etc, in addition to the fire damage mechanism discussed in this article. Further research is recommended in development of an air flow net model that evaluates the areas contributing to the field air permeability test and the sensitivity of the procedure to different finishes and moisture contents. 3. The use of Dynamic Elastic (Young's) Modulus and Air Permeability Index tests performed on concrete disks provided a method for easily quantifying important engineering properties of relatively thin layers of concrete in structural elements and, therefore, also provided a means to detect and quantitatively assess the extent of damage gradients. 4. The relationship between Dynamic Elastic (Young's) Modulus and Air Permeability Index may be associated with particular classes of concrete. Further research is suggested with additional classes of concrete and additional damage mechanisms and severity. The findings of such research would be useful not only in forensic investigations but improving understanding of fundamental relationships in concrete over time and under stress. Additional research is also needed to develop theoretical basis for the slopes as well as the linearity or non-linearity of the relationships between E d and API found in this study. ACKNOWLEDGMENTS The author would like to gratefully acknowledge the assistance of Stanley L. Michel in CAD drawing. REFERENCES 1. ASTM. ASTM C Standard Test Method for Pulse Velocity Through Concrete. Annual Book of American Society of Testing and Materials, ASTM West Conshohocken, PA. Vol , ASTM. ASTM C Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration Annual Book of American Society of Testing and Materials, ASTM West Conshohocken, PA. Vol , Bremner, T.W., T.A Holm, and J.M. McInerney. Influence of Compressive Stress on the Permeability of Concrete. Structural Lightweight Aggregate Concrete Performance, ACI SP-136, T.A Holm and A.M. Vaysburd, Ed., American Concrete Institute, Detroit, MI,1992, pp Sugiyama, T., T.W. Bremner, and T.A Holm. Effect of Stress on Gas Permeability in Concrete. ACI Materials Journal, Vol. 93, No. 5, Sept-Oct 1996, pp

12 Dilek, Ufuk Timofeyev, D.P., Adsorptionskinetik, VEB Deutscher Verlag Fur Grundstoffindustrie, Leipzig, 1967, 336 pp. 6. Schonlin, K. and H.K. Hilsdorf. Permeability as a Measure of Potential Durability of Concrete Development of a Suitable Test Apparatus. Permeability of Concrete, ACI SP-108, D. Whiting and A. Walitt, Ed., American Concrete Institute, Detroit, MI, 1988, pp Whiting, D. and Cady, P. D., Condition Evaluation of Concrete Bridges Relative to Reinforcement Corrosion, Volume 7; Method for Field Measurement of Concrete Permeability SHRP-S/FR Report, 93 pp Dilek, U., M.L. Leming, and E.F. Sharpe. Assessment of Cryogenic Fluid Spill Damage to Concrete. Forensic Engineering: Proceedings of the Third Congress, P.A. Bosela et al., eds. Reston, Va.: American Society of Civil Engineers, Dilek, U., T. Caldwell, E.F Sharpe, and M.L. Leming. Fire Damage Assessment, Pre-Stressed Concrete Double-Tees in a Parking Deck Forensic Engineering: Proceedings of the Third Congress, P.A. Bosela et al., eds. Reston, Va.: American Society of Civil Engineers, Dilek, U. Effects of Manufactured Sand Characteristics on Properties of Concrete, Doctoral Dissertation, North Carolina State University, Raleigh, NC., ASTM. ASTM C Standard Test Method for Fundamental Transverse, Longitudinal, and Torsional Frequencies of Concrete Specimens. Annual Book of American Society of Testing and Materials, ASTM West Conshohocken, PA. Vol , Hutchinson, J.R. Axisymmetric Flexural Vibration of a Thick Free Circular Plate. Journal of Applied Mechanics, ASME, Vol. 46, March, 1979, pp Leming, M.L., J.M. Nau, J. Fukuda. Nondestructive Determination of the Dynamic Modulus of Concrete Disks. ACI Materials Journal, Vol. 95, No. 1, January-February 1998, pp ASTM. ASTM C Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens. Annual Book of American Society of Testing and Materials, ASTM West Conshohocken, PA. Vol , American Concrete Institute Committee 318, Building Code Requirements for Structural Concrete and Commentary, American Concrete Institute, Farmington Hills, MI 2002.

13 Dilek, Ufuk 12 LIST OF TABLES TABLE 1. Young s Modulus (E d ), Air Permeability Index (API) and Splitting Tensile Strength of Concrete LIST OF FIGURES FIGURE 1 Plan View of the Fire Site. FIGURE 2 Spalling and Cracking on Stem B (left), Close up of Pink Discoloration in Spall (right) FIGURE 3 Air Permeability Index Vacuum Chamber. FIGURE 4. Field Air Permeability Index Measurement Adjacent to Core Location on Stem D. FIGURE 5. Affected Area indicated by Pulse Velocity Measurements on Stems B and C (moving average of 5). FIGURE 6. Dynamic Young s Modulus of Elasticity and Air Permeability Index of Disks. FIGURE 7. Field Air Permeability Index Measurement on a Flexural Crack Induced During Load Testing.

14 Dilek, Ufuk 13 TABLE 1. Young s Modulus (E d ), Air Permeability Index (API) and Splitting Tensile Strength of Concrete Cores at Stems A B* C* D E (Control)** (average of two disks) Dynamic Young's Modulus at surface, Gpa (million psi) 18.4 (2.67) 9.9 (1.44) 10.5 (1.52) 16.7 (2.42) 24.5 (3.56) % Loss with respect to Control 25% 59% 57% 32% - at interior, Gpa (million psi) 22.3 (3.24) 14.8 (2.14) 11.7 (1.70) 17.2 (2.50) 23.6 (3.43) % Loss with respect to Control 6% 38% 50% 27% - Air Permeability Index (µm²/sec) at surface at interior Splitting tensile, Mpa, (psi) 3.93 (570) 2.76 (400) 3.00 (435) 3.79 (550) 3.93 (570) % Loss with respect to Control 0% 30% 24% 4% - *Cores at Stems B and C were taken from stems directly above the fire and above the area of visible spalling **Unaffected core, (E/control) was taken as reference approximately 60 feet away from the fire site at a non-soot-stained concrete double-tee

15 Dilek, Ufuk 14 FIGURE 1 Plan View of the Fire Site.

16 Dilek, Ufuk 15 FIGURE 2 Spalling and Cracking on Stem B (left), Close up of Pink Discoloration in Spall (right).

17 Dilek, Ufuk 16 FIGURE 3 Air Permeability Index Vacuum Chamber. *disk not taped for demonstration of assembly and seal.

18 Dilek, Ufuk 17 FIGURE 4 Field Air Permeability Index Measurement Adjacent to Core Location on Stem D. Note: Spalled areas on Stem C

19 Dilek, Ufuk 18 FIGURE 5 Affected Area indicated by Pulse Velocity Measurements on Stems B and C (moving average of 5).

20 Dilek, Ufuk 19 FIGURE 6 Dynamic Young s Modulus of Elasticity and Air Permeability Index of Disks.

21 Dilek, Ufuk 20 FIGURE 7 Field Air Permeability Index Measurement on a Flexural Crack Induced During Load Testing* *Flexural cracks during load testing (top), cracks closed upon unloading due to pre-stress. Photograph shows crack later outlined with a pencil.