Akhter B. Hossain,P. Abstract

Transcription

1 Paper accepted for presentation and publication in the Proceedings of Conference on Fiber Composites, High-Performance Concretes, and Smart Materials organized by International Center for Fiber Reinforced Concrete (ICFRC), January 4, Chennai, India Using the Restrained Ring Test in Conjunction with Passive Acoustic Emission to Quantify the Role of Steel Fiber Reinforcement in Shrinkage Cracking Mitigation 1 3 Hardik R. Shah,P P Akhter B. Hossain,P P and Jason WeissP 1 Graduate Research Assistant, Purdue University Assistant Professor, University of South Alabama 3 Assistant Professor (Corresponding Author), Purdue UniversityB School of Civil Engineering Purdue University 55 Stadium Mall Drive, West Lafayette Indiana , USA HTwjweiss@ecn.purdue.eduTH, Phone: , Fax: Abstract Residual tensile stresses can develop in concrete when external restraint prevents the length changes caused by drying, chemical reaction, or thermal change. When these tensile stresses exceed the tensile strength of concrete, cracking may be expected to occur. Fibers have the potential to delay the age of visible cracking and reduce the width of the crack that develops. To better understand how fibers influence the behavior of fiber reinforced concrete (FRC) under restraint, ring tests were conducted using mixtures with varying fiber volume. An analytical solution is presented that enables the residual stress development in the ring specimen, and stress transferred across the crack, to be quantified. Acoustic emission activity was measured in the restrained concrete to assess the development of damage in each of the specimens.

2 INTRODUCTION When volumetric changes caused by temperature and moisture variation are prevented, residual tensile stresses can develop. If these tensile stresses exceed the tensile strength of the concrete, cracking can be expected to occur. Several recent studies have indicated that many factors other than free shrinkage and tensile strength may influence the potential for early-age cracking. These factors include the magnitude and rate of shrinkage, the degree of restraint, time-dependent material property development, stress relaxation, geometry of the structure, and fracture resistance (Weiss et al. 1998, Weiss et al. 1999, Bentur ). Due to the low tensile strength and fracture toughness of cementitious materials, fiber reinforcement has been suggested as an effective method to mitigate earlyage cracking in concrete (Ramakrishnan and Coyle 1983, Balaguru and Shah 1985, Gryzbowski and Shah 199, Gopalaratnam et al. 1991, Shah et al, Cyr et al. 3). Fibers increase the toughness of concrete (Gopalaratnam et al. 1991) which manifests itself in a reduction in the crack width in restrained concrete. To improve models for the prediction and prevention of shrinkage cracking, it is essential that we develop a better understanding of how damage develops in restrained concrete and how fibers influence cracking. The main aim of this paper is to describe several experimental techniques which may be used to quantify the effectiveness of steel fibers in delaying the age of cracking, transferring stress across a crack, and reducing the width of crack. RESEARCH SIGNIFICANCE Experiments were performed using mixtures with various fiber volume fractions of steel. An approach is presented whereby residual stress can be estimated from the restrained ring. This can enable the estimation of how much stress fibers can transfer across the width of a crack. In addition, passive acoustic emission is used along with measures of residual stress to aide in quantifying the damage as it occurs in fiber reinforced concrete.

3 EXPERIMENTAL PROGRAM A series of fiber reinforced concretes were tested in this investigation. The proportions and mixing procedures were similar for all mixtures except for the volume of fiber that was added and the lengths of fiber used. A type I cement was used for all the mixtures with a water-to-cement ratio (w/c) of.5. The specimens were prepared with a constant volume (55%) of non-shrinking components (i.e., fine aggregate and fiber). Three different lengths of crimped steel fibers were used (5, 38, 5 mm). The nomenclature, volume of fiber, and mixture proproprtions was varied as shown in the Table 1. Mixing was performed in a pan mixer in accordance with ASTM C 19. The materials were prepared by combining the fine aggregate with one-third of the total water. After two minutes of mixing, cement was added to the mixer along with an additional third of the mix water. An additional four minutes of mixing was performed during which time the remaining water was added. The fibers were added to the mixer to enable all the fibers to be dispersed properly in the mixture. After mixing, the mortar was placed in the forms, rodded, vibrated, finished with a steel trowel, placed under a plastic sheet to prevent moisture o loss, and maintained at 3P PC for 4 hours. The specimens were demolded and o placed in a 3P PC, 5% relative humidity environment for remainder of the test. The ring specimens used in this research consisted of a mortar annulus (75 mm wide) that was cast around a steel ring with a 3 mm outer diameter and a wall thickness of 9.38 mm (except for the specimens with acoustic emission sensors, which used 19 mm thick steel ring). The height of each ring was 75 mm. The specimens were allowed to dry from top and bottom surface of the ring by sealing the circumference with two layers of aluminum tape. The geometry of the ring specimen used in this study is shown in Figure 1. Each ring specimen was equipped with four strain gages at the mid-height on the inner circumference of the steel ring. The strain gages were considered to be zero at the time of initial set (i.e., approximately 5 hours - ASTM C 38). Three acoustic sensors (375 khz broad band) were placed on the outer circumference of the concrete ring at mid-height (1 degrees apart). A small hole was cut in the aluminum tape on the circumference of the ring to allow the

4 is acoustic emission sensor to be coupled to the specimen surface using vacuum grease. While the data obtained from the AE sensor provides valuable information about microcracking, it should be noted that any information on localized cracking is dependent, in part, on the distance from the sensor to the crack. It should also be noted that this does not quantify any cracking that may have occurred in the first 4 hours since the AE sensors were not on the specimens at that time. Restrained Ring Analysis Recent work has described an approach for using the measured steel strain to determine the residual stress that develops in the ring specimen (Shah et al. 3, Hossain and Weiss 3). In this approach the strain was measured at the inner surface of the steel ring and used to determine the fictitious interface pressure that was exerted by the shrinking concrete on the steel ring. This fictitious pressure can be applied to the concrete ring enabling, the tensile stress in the concrete ring to be calculated as shown in Eq. 1. R IC + ROC RIC RIS σ = actual max ε steel ( t) ES -- Eq. 1 ROC RIC RIC where εbsteelb is the strain measured in the steel ring, EBSB the elastic modulus of the steel ring, RBICB is the inner radius of the concrete (i.e., outer radius of the steel ring), RBISB is the inner radius of the steel ring, and RBOCB is the outer radius of the concrete ring. Fig. shows an example of how the residual stress could be obtained from the measured steel strain using Eq. 1. For further details on the derivation and application of this formula, the reader is referred to existing literature (Hossain and Weiss 3). It should be noted that this formula is strictly only applicable to linear visco-elastic systems that are axi-symmetric. These assumptions are violated when a localized crack develops, however this paper uses this analysis technique to approximate the post-cracking response of these concretes, since it is assumed that the lack of bond enables the interface pressure to be redistributed relatively uniformly throughout the ring. Damage Detection In addition to resulting in the development of residual stress, restrained shrinkage has been suggested to cause microcracking and damage. Recent work

5 has suggested that the development of this damage can be monitored using passive acoustic emission (Chariton and Weiss ). In this paper only acoustic events that exceed 35 db are recorded to avoid background noise. To account for the magnitude of each acoustic event, acoustic emission energy was used (Puri and Weiss in press). EXPERIMENTAL RESULTS, ANALYSIS AND INTERPRETATION Effect of Fiber Volume on Residual Stress Development An example of the experimentally measured strain from the steel ring and resulting residual stress is shown in Fig.. It can be seen that an abrupt change in strain was observed (without fibers or with low volumes of fibers) which corresponds to the age of visible cracking in the specimen. The residual strain in the plain (i.e., % fiber) ring specimen was found to be reduced to almost zero after the visible crack developed. It has been noted that the mixtures with a higher volume fraction of fiber reinforcement show a much less substantial decrease in strain at the time of visible cracking. Fig. 3a illustrates the influence of fiber volume on the age of cracking (as defined as the time of abrupt stress decrease). It can be seen that as the volume of fiber (VBFB) increases an increase in the age of cracking (tbcrb) occurs. This may be attributed to the ability of the fibers to arrest small surface cracks as they develop thereby arresting cracks and delaying unstable cracking. Fig. 3b illustrates the stress that had developed before visible cracking occurred in the specimen (an age of 7 days was chosen). It can be seen that the stress level was fairly similar for all the specimens. This shows that before the specimen cracks the fibers do not substantially alter the residual stress development in the specimen. This suggests that the fibers are relatively inactive until the time at which a crack begins to localize. Fig. 3c shows a plot of stress after cracking as a function of steel fiber volume. The value of residual stress in the specimen after cracking increases with an increase in fiber volume suggesting that fibers enable the transfer of stress across the crack. The difference in stress before and after cracking for varying fiber volumes is shown in Fig 3d. The magnitude of the drop in stress (after the first crack is observed) was found to decrease with an increase in fiber volume. It should be

6 .5 noted that multiple cracking was observed in the specimens with a higher fiber volume (U>U %) and as a result, multiple abrupt drops in stress were observed. Effect of Fiber Volume on Crack Width Steel fiber reinforcement is beneficial for limiting the width of the crack that develops in these mortars. Fig. 3e provides an illustration of the reduction in crack width that can be attributed to the addition of fibers, by plotting the width of the crack at an age of 8 days for various fiber volumes. (It should be noted that the width of the crack had stabilized by this time.) The addition of a low volume of fiber (.6 %) decreases the crack width by approximately 35%. As the volume of fiber is increased the width of the crack is further reduced but at a diminishing rate. Acoustic Emission Results and Discussion The results of acoustic emission measurements are shown in Fig. 3f for a specimen with and without steel fiber reinforcement. Previous research by Kim and Weiss () has shown that the acoustic energy release rate is similar for all the specimens at early ages. It was noticed however that as the age of visible cracking approached, the behavior of each specimen began to diverge. In this paper the acoustic emission rate was measured on two specimens-a plain and 1% FRC specimen (19 mm steel thickness as degree of restraint). It can be seen that three distinct regions become evident. Region 1 corresponds to the time where the plain and FRC specimen exhibits a similar rate of stress development and acoustic energy release. This likely corresponds to the fact that substantial cracking had not occurred at this time to engage the fiber reinforcement. Region begins when the rate of stress development in both specimens begins to diverge. The FRC specimen continues to have an increasing residual stress; however the plain specimen begins to plateau. The plain specimen then shows an abrupt drop in the residual stress at a time that corresponds to visible cracking. A continual increase in acoustic energy was observed for the FRC specimen, corresponding to continuous microcracking damage occurring in the specimen as the residual stress rises (Region 3, Fig. 3f). This indicates that the fibers were effective in absorbing the energy formed inside the specimen and prohibiting the crack from widening.

7 CONCLUSION The restrained ring specimen geometry was used to quantify the behavior of fiber reinforced concrete. It was observed that the stress that develops prior to visible cracking is similar at very early ages irrespective of fiber volume. The age of visible cracking is slightly delayed by the inclusion of randomly distributed steel fibers presumably due to the fibers ability to arrest cracking before the crack propagates across the specimen unstably. The residual stress that remains in the specimen after cracking is estimated based on the axisymmetric analysis procedure. Passive acoustic emission was used to determine the rate of acoustic activity thereby providing some information in the rate of crack development. Fiber reinforced and plain specimens typically demonstrate similar acoustic activity at early ages, however as the residual stress level increases the acoustic activity in the plain specimen begins to increase as compared with the fiber reinforced specimen. The lower toughness (i.e., plain or low fiber volume mixtures) mixtures demonstrate an abrupt increase in acoustic activity at the age of visible cracking thereby suggesting unstable through cracking. ACKNOWLEDGEMENTS The authors gratefully acknowledge support received from the Center for FRC Industries and the National Science Foundation (NSF) through Grant No This work was conducted in the Charles Pankow Concrete Materials Laboratory; as such the authors gratefully acknowledge the support that has made this laboratory and its operation possible. REFERENCES Weiss, W. J., Yang, W., and Shah, S. P., "Shrinkage Cracking of Restrained Concrete Slabs. Journal of Engineering Mechanics Div., ASCE, 14(7), 1998, pp Weiss, W.J., Yang, W., and Shah, S. P., Factors Influencing Durability and Early-Age Cracking in High Strength Concrete Structures SP 189- High Performance Concrete: Research to Practice, Farmington Hills MI, 1999, pp

8 Bentur, A., Chapter 6.5: Early-Age Cracking Tests, RILEM State of Art Report- Early-Age Cracking In Cementitious Systems, Ramakrishnan, V., and Coyle, W. V. Steel Fiber Reinforced Super-Plasticized Concretes for Rehabilitation of Bridge Decks and Highway Pavements, DOT/RSPA/DMA-5/84-, 1983, 48 pp. Balaguru, P., and Shah, S. P. Alternative Reinforcing Materials for Developing Countries, International Journal for Development Technology, Vol. 3, 1985, pp Grysbowski, M., and Shah S. P., Shrinkage Cracking of Fiber Reinforced Concrete ACI Materials Journal, March/April, Vol.87, No., 199, pp Gopalaratnam, V. S., Shah, S. P., Batson, G., Griswell, M., Ramakrishnan, V., and Wecharatana, M. Fracture Toughness of Fiber Reinforced Concrete, ACI Materials Journal, Vol. 88, No. 4, 1991, pp Shah, S.P., Wang, J., Weiss, W. J., Shrinkage Cracking-Can it be Prevented, Concrete International, Vol., No. 4,, pp Cyr, M., Ouyang, C., and Shah, S. P., Design of hybrid-fiber reinforcement for shrinkage cracking by crack width predictions, Brittle Matrix Composites 7, ed A. M. Brandt, V. C. Li, and I. H. Marshall, Woodhead publishing limited, 3 pp Shah, H. R., Hossain, A. B, Mazotta, G., and Weiss, W. J., Time-Dependent Fracture in Restrained Concrete: The Influence of Notches and Fibers, Advances in Cement and Concrete, eds. Lange, D., Scrivener, K. L., and Marchand, J., 3, pp Hossain, A. B, Pease, B. and Weiss, W. J., Quantifying Early-Age Stress Development and Cracking in Low w/c Concrete Using the Restrained Ring Test with Acoustic Emission Transportation Research Board 3, (In Press) Chariton, T., and Weiss, W. J., Using Acoustic Emission to Monitor Damage Development in Mortars Restrained from Volumetric Changes, Concrete: Material Science to Application, A Tribute to Surendra P. Shah, ed. P. Balaguru, A. Namaan, W. Weiss, ACI SP-6,, pp Puri, S., and Weiss, W. J., Assessment of Localized Damage in Concrete Using Acoustic Emission Journal of Engineering Materials, ASCE, 3,(Under Review) Kim, B., and Weiss, W. J., Using acoustic emission to quantify damage in restrained fiber-reinforced cement mortars Cement and Concrete Research, Volume 33, Issue, February 3, pp

9 Table-1: Mixture Proportions Type of Fiber Specimen Fiber Addition Weight (lb/yd3) Mixture Volumes (%) Water Cement Aggregate Steel S S S S S S S S S S S (Note: The Specific Gravity of Cement is 3.15 and Fine Aggregate is.64) 75 mm thick Concrete wall RBOCB = 5 mm RBICB = 15 mm RBOCB RBICB Sealed 9.38 mm (or 19 mm) Steel Ring UTop View Direction of drying Sealed 75 mm Direction of drying UCross Sectional View Figure 1: Geometry of the Ring Specimens

10 Average Strain (µε) S S Residual Stress (MPa) S S (a) (b) Figure : (a) Measured Residual Strain and (b) Residual Stress calculated from the Measured Steel Strain Age of Visible Cracking t cr (Days) Age of Specimen (Days) S S S S S S S Fiber Volume (%) Fiber Volume (%) (a) (b) Figure 3: (a) Age of Visible cracking as a function of fiber volume (b) Residual stress at an age of 7 days (i.e., pre-cracking) as a function of fiber volume Residual Stress (Pre 7 days in MPa) Age of Specimen (Days) S S S S S-.-.5 S S S S

11 Residual Stress (Post 5 days in MPa) Crack Width (mm) S S S S-.-.5 S S S s Fiber Volume (%) (c) control S S S S S-.-.5 S S S S Fiber Volume (%) Difference in Stress at cracking (MPa) Residual Stress (MPa) (d) day and day Residual Stress () Residual Stress (S-1.-1.) Cumulative Acoustic Energy () Cumulative Acoustic Energy (S-1.-1.) Region 1 Region S S S S-.-.5 S S S S Fiber Volume (%) Region Age of Specimen (Days) Cumulative Acoustic Energy (nvs) (e) (f) Figure 3: (c) Residual stress at an age of 5 days (i.e., post cracking) as a function of fiber volume (d) Difference in stress at cracking (e) Crack width as a function of fiber volume (f) Stress development and Cumulative Acoustic Energy as function of age for control and 1 % fiber specimen