Detecting Solidification Using Moisture Transport from ...

SP-259—6

Detecting Solidification Using Moisture Transport from Saturated Lightweight Aggregate by R. Henkensiefken, G. Sant, T. Nantung, and J. Weiss Synopsis: The propensity for early-age shrinkage cracking in low w/c concretes has spawned the development of new technologies that can reduce the risk of cracking. One such technology is internal curing. Internal curing uses saturated lightweight aggregate to supply ‘curing water’ to low w/c paste as it hydrates. Significant research has been performed to determine the effects of internal curing on shrinkage and stress development in sealed samples. However, relatively little detailed information exist about how water is released from the lightweight aggregate to the surrounding cement paste. This study examines the timing of moisture release from saturated lightweight aggregate (LWA). Specifically this paper focuses on fluid transport around the time of set. X-ray absorption is used to trace the time at which water moves from the lightweight aggregate to the paste. X-ray observations are compared with results from the Vicat needle, autogenous shrinkage, and acoustic emission tests. These results are contextualized in terms of structure formation and vapor space cavitation in the cement paste.

Keywords: chemical shrinkage; internal curing; lightweight aggregate; setting; solidification; x-ray absorption

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ACI member Ryan Henkensiefken is a graduate research assistant in the School of Civil Engineering at Purdue University. He received his BSCE from Minnesota State University, Mankato and is currently pursuing a MSCE. His research interests include performance of concrete made with lightweight aggregate. ACI member Gaurav Sant is a research assistant in the School of Civil Engineering at Purdue University. He received his BSCE and MSCE from Purdue University and is currently pursuing a PhD. His research interests include early-age material behavior and moisture transport in cementitious systems. He is the former president of the ACI-Purdue student chapter. Tommy Nantung is the section manager of Pavement, Materials, and Accelerated Testing at the INDOT Office of Research and Development in West Lafayette, Indiana. He received his BCSE from Parahyangan Catholic University in Indonesia, his Master’s in Construction Engineering from the University of Michigan and his PhD from Purdue University. He is a registered professional engineer in the state of Indiana. ACI member Jason Weiss is a professor and associate head of the School of Civil Engineering at Purdue University. He earned his BAE from Penn State University and his MS and PhD from Northwestern University. He is a member of ACI Committees 123, 209, 365, 446 and 522. He is also the associate director of the center for Advanced Cement-Based Materials (ACBM). INTRODUCTION Internal curing is a term that is used to describe the use of water filled inclusions in cement paste, often in the form of saturated lightweight aggregates (LWA), to supply additional moisture to the cement paste as it hydrates1, 2. The key aspect of internal curing is that it can be used in a low w/c mixture to produce a relatively dense paste that exhibits the benefits of a low w/c (i.e., porosity, permeability), while negating the effects of autogenous shrinkage3. Significant research has been performed on the topic of internal curing4. Specifically, researchers have shown the benefits of internal curing including: 1) a reduction in shrinkage in low w/c sealed concrete, 2) an increase in the compressive or flexural strength (especially at later ages) and 3) improved durability4. However, relatively little is understood about other aspects of internal curing, including the timing of water release from the lightweight aggregate and the distance the water travels through the surrounding cement paste. RESEARCH SIGNIFICANCE This study provides information to better understand the timing of moisture release from saturated lightweight aggregates to the paste in concrete mixtures with internal curing. This paper focuses on moisture movement near the time of set. Specifically, the objectives of this paper are: x to discuss the mechanisms of internal curing in self-curing concrete, x to demonstrate the use of x-ray absorption to trace changes in density associated with moisture movement from saturated lightweight aggregate into the surrounding paste, and x to provide experimental measures of moisture transport from LWA that can indicate the time of solidification of the cement paste. BACKGROUND ON INTERNAL CURING AND MOISTURE TRANSPORT AROUND THE TIME OF SET IN CEMENT PASTES Before beginning to discuss the behavior of a paste system around the time of set it is important to define some fundamental aspects and terminology related to these materials. Chemical shrinkage is a term that describes the total volume change that occurs due to the reaction of cement and water during hydration. Chemical shrinkage occurs because the volume of the hydration products is smaller than the initial reactants5-7. In a mature paste the chemical shrinkage is approximately 8-9% of the overall system volume. Autogenous shrinkage on the other hand describes the external change in volume that occurs in a sealed system maintained under isothermal conditions3. Figure 1 shows a measure of chemical (total shrinkage) and autogenous shrinkage (external shrinkage) measured during the first 24 hours for a cement paste with a water-to-cement ratio of 0.308, 9. It can be noticed in Figure 1 that the behavior of these curves can be loosely divided into two regions, before set and after set (initial set and final set occur at 6 and 7 hours, respectively). Before set (alternatively called



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solidification), chemical and autogenous shrinkage are similar. At these early ages the system is a fluid and can collapse upon itself as it shrinks. Therefore the chemical shrinkage equals the autogenous shrinkage (i.e., the total shrinkage equals the external shrinkage). As the cement particles begin to contact one another (e.g., the system sets), the chemical shrinkage and autogenous shrinkage curves diverge10, 11. Since the paste can no longer collapse upon itself, vapor filled voids are created (or cavitated) in the system12. When the voids grow inside the cement paste due to a reduction in absolute volume, acoustic events (constituting acoustic energy release) occur, which can be detected using acoustic emission9. Figure 1 shows an increase in acoustic emission events right around the time the chemical and autogenous shrinkage curves diverge. It should be noted that if the paste is not sealed (the samples measured within Figure 1 were sealed) and the paste were ponded with water, cavitation would not occur and water would be drawn into the void spaces from the surface around the time of set9. This is similar to what is observed each day on a construction site as the ‘sheen of bleed water’ disappears around set. While the formation of vapor spaces in itself is not a concern, the formation of the liquid-vapor menisci in the pores is a concern. These menisci represent the fact that the suction pressure that has developed in the fluid is higher than the pressure in the vapor (resulting in a curvature in the meniscus). While this vapor filled space exists in all mixtures, it is especially problematic in low w/c mixtures, since mixtures with a low w/c have fewer large diameter pore spaces (i.e., capillary pores). As a result the radius of curvature of the meniscus (critical Kelvin radius) that can develop in low w/c mixtures is smaller (corresponding to a higher pressure). The Kelvin-YoungLaplace equations can be used to relate the size of the meniscus radius to the capillary pressure and the corresponding internal relative humidity respectively.

V



2J cos(T ) r

Equation (1)

where: ı (Pa) is the capillary pressure, Ȗ (N/m) is the surface tension of pore fluid, ș (radians) is the liquid-solid contact angle (assumed to be 0 radians), and r (m) is the radius of curvature of the meniscus. It can be argued that the smaller meniscus radius in the low w/c mixtures corresponds to a higher capillary pressure resulting in higher shrinkage. The capillary pressure can also be related to the internal equilibrium relative humidity as represented by Kelvin’s equation (Equation 2).

V

RT ln( RH ) Vm

Equation (2)

where: R is the universal gas constant (8.314 J/mol·K), T (K) is the thermodynamic temperature, RH (unitless) is the internal relative humidity, and Vm (§ 18×10-6 m3/mol) is the molar volume of pore solution. The capillary pressure that develops can be related to material behavior such as shrinkage using Bentz’s modification of Mackenzie’s formulation which is described in Equation (3)13.

Hp

S §1 1 · ¸ V¨  3 ¨© K K s ¸¹

Equation (3)

where: S (unitless) is the degree of saturation of cement paste, K (Pa) is the paste’s bulk modulus, and Ks (Pa) is the modulus of the solid skeleton inside cement paste. The continued consumption of water due to hydration results in the volume of vapor filled voids progressively increasing and penetrating smaller voids corresponding to smaller radii of curvature. This results in a continuous increase in the capillary pressure, a reduction in the equilibrium relative humidity and an increase in shrinkage14. The basic principle for internal curing is that the largest pores will lose water first. As the pores of the lightweight aggregate are generally larger than those of the surrounding cement paste, the capillary pressure that develops in the pores in the paste causes water to be drawn out of the pores in the lightweight aggregate. This implies that the water in the lightweight aggregate can replenish the volume of vapor filled space that cavitates during hydration15,16. This enables a larger liquid vapor meniscus to be maintained in the cement paste resulting in a lower capillary pressure and reduced shrinkage. However, for internal curing to be effective, water needs to be able to move from the lightweight aggregate to the cement paste. This includes a few inherent assumptions: 1) the pores in the lightweight aggregate need to be larger than the pore size of the paste to ensure preferential moisture evacuation from the saturated aggregate, and 2) the aggregate needs to be well-distributed in the paste to ensure that water evacuated from the aggregate is able to equally access the self-desiccating cement paste17. Some studies have

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used x-ray absorption to track the water movement from LWA or cement paste2, 18-20. This paper will investigate this water movement (from the LWA) with a fine resolution specifically near the time of set. It is the hypothesis of this work that water will not leave the saturated aggregate prior to the development of a capillary pressure. This is most significant as the system solidifies. As such the movement of water should be able to be used to detect the time of set. The following section provides a brief background on how x-ray absorption can be used to determine the movement of water from an aggregate while the sections after that describe the experiments and results of these tests performed on a mixture that was identical to that described in Figure 1. BACKGROUND ON THE DETECTION OF WATER MOVEMENT USING X-RAY ABSORPTION The addition or removal of a liquid from a porous medium changes the density of the system. This paper uses x-ray absorption techniques to detect this change in density in a composite cement paste-LWA system. As such, x-ray absorption can be used to assess the removal of water from a LWA. Simply stated, a less dense a material absorbs less x-ray radiation. Consequently, more x-ray radiation will be transmitted through the less dense sample resulting in more x-ray radiation being measured at the detector. Considering the case of the saturated LWA, as water is removed from the LWA, the local density of the material decreases, the x-ray radiation absorbed by the material decreases, and the transmitted x-ray intensity (i.e., the intensity measured at the detector) increases. Equation 4 describes the relationship between the intensity of the x-ray radiation that is incident on the specimen (I0) and the intensity of the radiation that passes through the lightweight aggregate specimen and is measured at the detector (IMeasured)21, 22.

I Measured

I 0 ˜ exp> P LWA ˜ VLWA  PW ˜ VW  PV ˜ VV ˜ t @

Equation (4)

where: PLWA and VLWA are the linear attenuation coefficient (cm-1) and the volume proportion (fraction) of the LWA, PW and VW are the linear attenuation coefficient (cm-1) and volume proportion (fraction) of water contained in the pores in the aggregate, and PV and VV are the linear attenuation coefficient (cm-1) and volume proportion (fraction) of vapor contained in the pores of the aggregate specimen, and t is the thickness of the sample. This equation is not used to calculate intensity, but rather is used to show what is happening in these systems from a fundamental perspective. It represents when different materials and different volume proportions of materials are used, different measured intensities will occur. For example, as water is removed from the lightweight aggregate, the density (and overall x-ray attenuation) of the medium decreases and the transmitted intensity of x-ray radiation measured at the camera (detector) increases. Consequently, the state and level of moisture at any time can be measured and quantified using this technique. EXPERIMENTAL METHODS AND PROCEDURES Materials and mixture proportions ASTM C150 type I ordinary portland cement (OPC) was used. The OPC had a Blaine fineness of 370 m2/kg and an estimated Bogue phase composition of 56% C3S, 16% C2S, 12% C3A, 7% C4AF and a Na2O equivalent of 0.68%. The LWA was natural shale that was expanded in a rotary kiln. The water absorption of the LWA was approximately 10.5% as determined from ASTM C128-07. Information on the desorption characteristics of this LWA can be found in previous work14, 15. All cementitious mixtures were prepared with a constant w/c of 0.30. A high-range water-reducing admixture (HRWRA) was added to the paste at a rate of 0.5 percent by mass of cement. For pastes provided with internal curing (i.e., containing saturated lightweight aggregate), to obtain similar mixture consistency and avoid bleeding concerns, the HRWRA dosage was reduced to 0.44 percent by mass of cement. Autogenous shrinkage measurement – The membrane method protocol Autogenous shrinkage was assessed for plain cement mixtures, and a mixture containing saturated aggregate using the membrane protocol. The membrane protocol involved the encapsulation of the fresh mixture in



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a polyurethane membrane which was suspended from a balance plate in a temperature controlled bath. The change in weight (density) experienced as the mixture hydrated was used to determine the time of solidification and the extent of autogenous deformation experienced by the system23. Due to space limitations, the details of the membrane method procedure can be found in previous work23. Specimen preparation for x-ray absorption Figure 2 shows a schematic illustration of the sample holder used in the experiments. The holder was formed using a 5 mm (0.20 inch) thick HDPE (high density polyethylene) sheet. The sample holder was 50 mm x 37 mm (2 inch x 1.5 inch) with two 12 mm (0.5 inch) openings cut out from the center. Two thin clear acetate sheets, approximately 0.5 mm (0.02 inch) thick, were attached to the front and back surface of the holder using epoxy to seal the sample holder. A prismatic LWA sample (2 mm x 5 mm x 12 mm) (0.08 inch x 0.20 inch x 0.50 inch) was cut from a larger specimen was then inserted into a notch on the holder. Before performing the experiments, the LWA was oven-dried at 110ºC (230ºF) for 24 hours to ensure a uniform moisture condition. The aggregate was then cooled and immersed in water for 24 hours. The cement paste was mixed in accordance with the procedure detailed in ASTM C305-06. To ensure little moisture loss from LWA, the paste was mixed first and the LWA removed from water just prior to placement in the sample holder. The LWA was patted dry using a cotton swab, and then fixed in position in the holder using epoxy. The paste was placed into both openings in the sample holder. One opening contained the test specimen, which included the saturated aggregate sample and the cement paste, while other opening contained just the plain cement paste. The plain paste sample was used as a reference to account for beam energy variations that may occur during the course of the experiment22. The design of the sample holder permitted simultaneous testing of the test specimen and the reference specimen. After the cement paste was placed, the sample holder was sealed on top with clear tape and the entire sample holder then sealed with aluminum tape to ensure no drying of the specimen occurred. The top surface was sealed with the clear tape first to prevent chemical reaction between the cement paste and the aluminum tape. At this point the sample holder was placed in the x-ray chamber maintained at 23.0 ± 0.5°C (73 ± 2°F) and measurements were obtained. Description of the X-Ray Absorption Equipment The equipment used is a GNI x-ray absorption system. The system consists of a directional x-ray source contained in an environmentally controlled (temperature and humidity) chamber24. X-ray imaging was performed at 50 KeV and 100PA, the spot size of the source was approximately 0.008 mm (0.0003 inch). The x-ray beam output by the source exhibits a conical spread24. The x-ray intensity was measured using a charge-coupled device (CCD) (i.e., x-ray camera). The 16-bit CCD camera records the cumulative x-ray intensity at each pixel, for a total surface of 252 pixels x 256 pixels24. The x-ray source and camera can be moved simultaneously using an X-Y positioning table with a precision of r 0.001 mm (0.00004 inch). The Focus to Detector Distance (FDD) of the equipment can be adjusted from 10 mm to 500 mm (0.40 inch to 19.7 inch); however, for this study the FDD was fixed at 500 mm. The Object to Detector Distance (ODD) was fixed at 100 mm (3.9 inch) for this study. The specimens analyzed in this study are mounted on a fixed stage, which allows the user to perform repetitive imaging, ensuring a fixed specimen position. An illustration of the experimental setup is provided in Figure 3. A single pixel size and spatial resolution of 0.08 mm (0.003 inch) has been determined for the geometry used in this investigation. EXPERIMENTAL RESULTS AND DISCUSSION Autogenous shrinkage measurements Previous research has shown that the rate of autogenous shrinkage changes at the time of set. As such the change in the rate of autogenous shrinkage can be used to identify solidification in cementitious mixtures23. Figure 4 shows the rate of autogenous deformation as a function of specimen age for a plain cement paste and a cement paste provided with internal curing (~22% saturated lightweight aggregate by volume), along with the Vicat time of set for the plain cement paste. The time of set from the autogenous shrinkage measurement (6 hours) corresponds to the time of set from the Vicat test (6 hours). This can be explained as follows: the formation of a solid skeleton and the system no longer being able to collapse freely results in cavitation of vapor filled space and a suction pressure. It

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should be noted that a slightly later time of solidification is noted for the plain cement paste and this may be due to a slightly higher WRA dosage which induces a greater extent of retardation in this system. This can be validated by x-ray absorption measurements which detect a decrease in density as water is removed from the lightweight aggregate as described in the following section. X-ray absorption measurements While three samples were tested and showed similar trends, Figure 5 shows a typical result from one sample with the difference in counts (transmitted intensities) normalized to the counts at 3.5 hour (transmitted intensity at 3.5 hour) as a function of horizontal position on the specimen. Here, the contact zone between the aggregate and the paste has been labeled as the ‘interface’ as was done by Lura19, 25-28. This refers to the portion of the sample where the beam passes through the LWA and the paste. The peak in this region represents the likely location of the edge of the LWA. It is observed (Figure 5) that as the specimen ages, the difference in counts in the LWA increases (becomes more negative), which relates to the removal of water from the saturated aggregate. During the first hours of measurement, the number of counts is observed to remain relatively constant. This is as expected; as at early ages the system is saturated (the paste system collapses on itself and does not create vapor filled space). However, as a solid skeleton forms in the cement paste vapor filled spaces begin to cavitate in the system. This causes a suction pressure to develop in the pore fluid which causes water to be drawn out of the aggregate causing an increase in the measured x-ray intensity (i.e., difference in counts). Further, the average of the difference in counts from a position of 2.0 mm to 2.4 mm on the specimen is shown as a function of specimen age Figure 6. Two key observations can be made from this figure: first, the counts remain stable until an age of 5.5 hours. This would indicate that water is not being drawn out of the aggregate at this time. However, after 5.5 hours, the transmitted x-ray intensity is observed to progressively increase. This would indicate a loss of water from the pores of the aggregate into the cement paste. This would indicate that solidification in the cement paste system provided with internal curing occurs at 5.5 hours. It should be noted that the time of moisture movement detected by the x-ray (5.5 hours) is similar to the time shown by the initial set as determined with the Vicat needle (6 hrs) and the time that a change in the rate of autogenous shrinkage is observed (6 hours) for the mixture provided with internal curing (Figure 4). This is an important observation as it verifies, a saturated LWA does not loose significant water until a solid skeleton forms and vapor spaces begin to cavitate. SUMMARY AND CONCLUSIONS This paper describes a series of investigations to demonstrate how moisture moves from LWA to the cement paste near the time of set. Specifically, the Vicat Needle, the divergence of chemical and autogenous shrinkage, the cavitation of vapor filled space as detected with acoustic emission, the rate of autogenous shrinkage and x-ray absorption techniques were implemented to determine the time of set. The results of this study suggest that these test methods yield a similar time of setting. This is because the test methods identify when the solid skeleton causes vapor to cavitate in the system which corresponds to the time of set. In this study, x-ray absorption was utilized to determine the time of set. This technique allows for monitoring of water movement in a hardening system. This technique is currently being used to determine the distance water can move in a hardened LWA system. This is similar to the approach used by Lura28, however the higher the resolution provided by the camera and the ability to align the sample may allow resolution of finer details. ACKNOWLEDGMENTS This work was supported in part by the Joint Transportation Research Program administered by the Indiana Department of Transportation and Purdue University (Project SPR 3211). The contents of this paper reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein, and do not necessarily reflect the official views or policies of the Federal Highway Administration and the Indiana Department of Transportation, nor do the contents constitute a standard, specification, or regulation. The authors gratefully acknowledge support received from the Purdue ERIC program, the Center for Advanced Cement Based Materials as well as material supplied by Northeast Solite Corporation.



Transition from Fluid to Solid: Re-examining the Behavior of Concrete 83 REFERENCES

1. Philleo, R. Concrete Science and Reality. in Materials Science of Concrete II. 1997. Westerville, OH: American Ceramic Society. 2. ACI Sp-220 Autogenous Deformation of Concrete. ed. O.M. Jensen, D.P. Bentz, and P. Lura. 2004, American Concrete Institute: Farmington Hill, MI. 181-198. 3. Jensen, O.M., "Autogenous Phenomena in Cement-Based Materials," 2005, Department of Building Technology and Structural Engineering, 4. 196-ICC, R.T.C., "Rilem Report 41: Internal Curing of Concrete," 2007, 5. Jensen, O.M. and P.F. Hansen, "Water-Entrained Cement-Based Materials: I. Principles and Theoretical Background," Cement and Concrete Research, V. 31, No. 4, 2001. p. 647. 6. La Chatelier, H. Sur Les Changements De Volume Qui Accompagent Le Durcissement Des Ciments. in Bulletin Societe de l'encouragement pour l'industrie nationale. 1900. Paris. 7. L'Hermite, R.G. Volume Changes of Concrete. in 4th International Symposium on the Chemistry of Cement. 1960. Washington D.C. 8. Sant, G., P. Lura and W. Weiss, "Measurement of Volume Change in Cementitious Materials at Early Ages: Review of Testing Protocols and Interpretation of Results," Transportation Research Record, V. 1979, No. 2006. 9. Couch, J., P. Lura, O.M. Jensen and W.J. Weiss. Use of Acoustic Emission to Detect Cavitation and Solidification (Time Zero) in Cement Pastes. in International RILEM Conference on Volume Changes of Hardening Concrete: Testing and Mitigation. 2006. 10. Barcelo, L., S. Boivin, P. Acker, J. Toupin and B. Clavaud, Early Age Shrinkage of Concrete: Back to Physical Mechanisms, in Materials aspects of concrete repair and rehabilitation. 2000: Mont-Tremblant. 11. Hammer, T.A., Test Methods for Linear Measurement of Autogenous Shrinkage before Set, in Autogenous Shrinkage of Concrete. 1998: Hiroshima, Japan. 12. Couch, J., P. Lura, O. Jenson and W.J. Weiss, "Use of Acoustic Emission to Detect Cavitation and Solidification (Time Zero) in Cement Pastes, Volume Changes of Hardening Concrete: Testing and Mitigation," 006. p. 393-400. 13. Bentz, D.P., E.J. Garboczi and D.A. Quenard, "Modelling Drying Shrinkage in Reconstructed Porous Materials: Application to Porous Vycor Glass," Modelling Simul. Mater. Sci. Eng., V. 6, No. 1998. p. 211-236. 14. Henkensiefken, R., T. Nantung and W.J. Weiss. Reducing Restrained Shrinkage Cracking in Concrete: Examining the Behavior of Self-Curing Concrete Made Using Different Volumes of Saturated Lightweight Aggregate (Accepted March, 2008). in Concrete Bridge Conference. 2008. St. Louis, Mo. 15. Radlinska, A., F. Rajabipour, B. Bucher, R. Henkensiefken, G. Sant, and W.J. Weiss, "Shrinkage Mitigation Strategies in Cementitious Systems: A Closer Look at Differences in Sealed and Unsealed Behavior," Accepted by Transportation Research Board, 2007. 16. Bentz, D.P., P. Lura and J.W. Roberts, "Mixture Proportioning for Internal Curing," Concrete International, V. 27, No. 02, 2005, 35-40 17. Bentz, D.P. and K.A. Snyder, "Protected Paste Volume in Concrete: Extension to Internal Curing Using Saturated Lightweight Fine Aggregate," Cement and Concrete Research, V. 29, No. 11, 1999. p. 1863. 18. Lura, P., O. Jensen and S.-I. Igarashi, "Experimental Observation of Internal Water Curing of Concrete," Materials and Structures, V. 40, No. 2, 2007. p. 211. 19. Lura, P., "Autogenous Deformation and Internal Curing of Concrete," 2003, 20. Bentz, D.P. and K.K. Hansen, "Preliminary Observations of Water Movement in Cement Pastes During Curing Using X-Ray Absorption," Cement and Concrete Research, V. 30, No. 7, 2000. p. 1157. 21. Sant, G., A. Eberhardt, D.P. Bentz and W. Weiss, "The Influence of Shrinkage Reducing-Admixtures (Sras) on Moisture Absorption in Cementitious Materials at Early-Ages," submitted to the ASCE Journal of Materials in Civil Engineering (2008), 2008. 22. Sant, G. and W. Weiss, "The Use of X-Ray Absorption for Assessing Moisture Movement in Cementitious Materials," submitted to the Journal of ASTM International (2008), 2008. 23. Sant, G., P. Lura and W. Weiss, A Discussion of Analysis Approaches for Determining 'Time Zero' from Chemical Shrinkage and Autogenous Strain Measurements in Cement Pastes, in Volume Changes of Hardened Concrete. 2006: Lyngby, Denmark. 24. Nielsen G., G., X-Ray Ct System - Users Manual' Gni/Xras/Um.003. 2007.

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25. Lura, P., D.P. Bentz, D.A. Lange, K. Kovler, A. Bentur, and K. van Breugel, Measurement of Water Transport from Saturated Pumice Aggregates to Hardening Cement Paste, in Advances in Cement and Concrete. Engineering Conferences International. 2003: Copper Mountain, CO. 26. Bentz, D.P., P.M. Halleck, A.S. Grader and J.W. Roberts. Four-Dimensional X-Ray Microtomography Study of Water Movement During Internal Curing. in Proceedings of the International RILEM Conference - Volume Changes of Hardening Concrete: Testing and Mitigation. 2006. 27. Bentz, D.P., P.M. Halleck, A.S. Grader and J.W. Roberts, "Water Movement During Internal Curing: Direct Observation Using X-Ray Microtomography," Concrete International, V. 28, No. 10, 2006, 39-45 28. Lura, P., D. Bentz, D. Lange, K. Kovler, A. Bentur, and K. van Breugel, "Measurement of Water Transport from Saturated Pumice Aggregates to Hardening Cement Paste," Materials and Structures, V. 39, No. 9, 2006. p. 861.

-6000 -9000

175 Final Set

w/c = 0.30 Chemical Shrinkage Autogenous Strain Acoustic Emission

Initial Set

Volume Reduction (ml/gcem)

200

-3000

150 125

Vapor Space Creation

100 75 50

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Acoustic Emission Events (Counts)

225

0

0 0

2

4

6

8 10 12 14 16 Age of Specimen (Hours)

18

20

22

24

Figure 1 – Chemical and autogenous shrinkage measurements and acoustic cavitation events as a function of specimen age for a high performance cement paste (w/c = 0.30)



Transition from Fluid to Solid: Re-examining the Behavior of Concrete 85

25 mm

LWA

Paste

Aluminum Tape

Mounting Screw Hole

5 mm

25 mm

Figure 2 – Schematic of sample holder with precut LWA and paste (1 mm = 0.0393 inches)

X-Ray Beam Source FOD Sample

FDD

X-Ray Source

Sample Holder

ODD Useful Beam Detector

(a)

Sample Holder Mount

Detector (Not Shown)

(b)

Figure 3 – Equipment Layout: (a) A schematic illustration of the geometry of the experimental setup (not to scale) and (b) A photo of the specimen in the x-ray chamber

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-100 -200 -300 -400

Final Set

Rate of Autogenous Strain (Pm/m/hour)

0

-500 -600

w/c = 0.30 Plain Paste Paste Internal Curing

-700 -800 0

2

4

6 8 10 12 Age of Specimen (Hours)

14

16

18

Difference in Counts from Initial Counts at 3.5 hrs

Figure 4 – The rate of autogenous deformation as a function of specimen age in a plain cement paste and cement paste provided with internal curing (The time of initial and final set denoted on the graph was assessed for the plain cement paste mixture)

3000 2000

Paste

LWA

1000 0

4.5 hr 5.0 hr 5.5 hr 6.0 hr 6.5 hr 7.5 hr 8.0 hr 8.5 hr 9.5 hr 10.5 hr 11 hr 26.5 hr

-1000 -2000 -3000

Interface

-4000 -5000 1.4

1.6

1.8

2.0

2.2

2.4 2.6 2.8 Position (mm)

3.0

3.2

3.4

3.6

Figure 5 – Difference in counts normalized to initial counts of the cement paste and LWA at different ages.

Transition from Fluid to Solid: Re-examining the Behavior of Concrete 87 0 Average counts from 2.0 - 2.4 mm Final Set

-1000 -2000 Initial Set

Difference in Counts from Initial Counts at 3.5 hrs



-3000 -4000 -5000

0

4

8

12 16 20 Age of Specimen (Hours)

24

28

Figure 6 –Difference in counts normalized to initial counts averaged from 2.0 mm to 2.4 mm in the LWA as a function of age

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