Plastic Shrinkage Cracking in Steel and Polypropylene Fiber Reinforced Concrete

Plastic Shrinkage Cracking in Steel and Polypropylene Fiber Reinforced Concrete Abstract Aminath Ali and Prasert Suwanvitaya Department of Civil Engineering, Faculty of Engineering, Kasetsart University E-mail: fengprs@ku.ac.th Plastic shrinkage cracking is an important factor in the ultimate life of a concrete structure. The cracks allow ingress of deleterious substances into the concrete, which in turn could lead to structural failure. In recent years, fiber reinforced concrete has become popular as a preventive measure for plastic shrinkage cracking. Low volume fiber reinforcement has been recommended as a crack control solution, but there are no standard specifications for the application of fiber reinforced concrete (FRC) as a crack control method. This research studied the effects of steel and polyvinyl alcohol (PVA) fibers on the shrinkage cracking of concrete and the effect of concrete strength on the performance of fibers. The investigations consisted of Ordinary Portland Cement concrete with two cement contents and three fiber volumes of 0.05%, 0.1% and 0.15%. Restrained shrinkage tests were conducted using uniaxially restrained specimens and unrestrained specimens were tested to determine the free shrinkage behavior with fibers. Results indicated that in low dosages both steel and PVA fibers reduced plastic shrinkage cracking. However, reduction in free shrinkage of concrete was not as significant. It was also observed that the steel fibers increased the tensile capacity of the concrete while the PVA fibers did not show any substantial improvement. Keywords Plastic shrinkage cracking, free shrinkage, fiber reinforced concrete, steel fiber, polyvinyl alcohol fiber. 1. Introduction Concrete is a brittle material by its nature with high compressive strength, limited by low tensile capacity. Normally to allow concrete to withstand tensile stresses, steel reinforcement is provided. Steel reinforcement does not prevent early age cracking that occurs primarily due to shrinkage. ฉบ บท 85 ป ท 26 กรกฎาคม - ก นยายน 2556 45

46 ว ศวกรรมสาร มก. Cracking occurs when the tensile stress developed in the member exceeds the tensile capacity of the material. The mechanisms that occur during the maturing of fresh concrete causes change in volume of the concrete itself. If the concrete is allowed to expand and contract without any form of restraint, it will not crack. However, fresh concrete, even at its earliest age has some internal restraints due to aggregates and reinforcement, which usually cause minute cracking. These early age effects can be broadly divided into two stages. Firstly, cracking that occurs before the concrete has set (i.e. concrete is in a fluid or semi-fluid state), and secondly, cracking that occurs after the concrete has begun to set [1]. In the plastic stage (or the early age), plastic shrinkage cracking and plastic settlement cracking causes the most significant problems. Initial cracking age is delayed when low percentages of fibers are added to the concrete. Fibers prevent cracks from widening and propagating throughout the concrete. However, there is no significant reduction in free shrinkage observed with polymer fibers [2]. Zhang & Li [3] used steel fibers to show that long-term shrinkage was reduced in steel fiber reinforced concrete (SFRC). The total plastic shrinkage crack area reduced with increased steel fiber volume [4]. Banthia et al. [5] confirmed that, at constant temperature and humidity, steel fiber reinforced concrete produced less cracking than normal concrete. PVA fibers are very effective at reducing early age cracking and drying shrinkage [6]. 2. Experimental methodology 2.1 Overview The main objective of the research was to establish the effect of steel and PVA fiber reinforcement on the plastic shrinkage cracking behavior of concrete, with varying dosage of fibers. To study this, free shrinkage tests and restrained shrinkage tests were conducted under normal temperature and humidity with accelerated wind velocity. Splitting tensile tests were used to determine the tensile capacity of the mixes, and uniaxially restrained shrinkage tests were conducted to determine the cracking potential in relation to fiber type and fiber content. 2.2 Materials & Mix Proportion Normal strength concrete, using commercially available ASTM Type I Portland cement was produced. There were two series of mixtures identified as M20 & M30 according to cement content. Table 1 shows the mix proportions used in the study. Locally available graded river sand with a fineness modulus of 2.8 was used as fine aggregate, and 9.5mm graded coarse aggregate was used in the mixture. Commercially available 60mm long hooked end alloy-steel monofilament fibers with elastic modulus of 210 GPa and 12mm long PVA monofilament microfibers with elastic modulus of 40 GPa were used in this study. The mixtures were designed with high waterto-cement ratio and high cement content to induce visible cracking.

Plastic Shrinkage Cracking in Steel and Polypropylene Fiber Reinforced Concrete 47 Table 1 Mix Proportions in kg/m 3 Series M20 M30 Cement Content 439 569 Fine Aggregate Content 822 717 Coarse Aggregate Content 691 691 Water/Cement ratio 0.63 0.49 Seven mixtures were evaluated for each series with fiber volumes of 0%, 0.05%, 0.1%, and 0.15%. Two free shrinkage specimens, one restrained shrinkage specimen and two splitting tensile test specimens were tested for each mixture. Plastic shrinkage cracking tests were conducted under normal laboratory conditions within a temperature range of 29ºC - 33ºC and 65% RH for 24 hours. Free shrinkage tests were conducted at 25ºC in a temperature controlled room. Splitting tensile tests were conducted after 28 days of water curing. 2.3 Mix Procedure Plain concrete was prepared according to ASTM C192/C 192M 02. Then, the fibers were gradually added to the fresh concrete, allowing the fibers to disperse thoroughly. When all the fibers were mixed in, the concrete was allowed to rest for two minutes and again mixed for three minutes. The concrete was then placed in the steel molds in two layers, smoothed with a steel towel. Final floating was done perpendicular to the length of the specimen. For each mix, workability of the mixture was determined using the slump test. Specimens were transferred to the drying environment within 30 min from when the water was added to the concrete. 2.4 Test Procedure In this study, a variation of the geometry used by Berke & Dalliare [7] with stress risers was used to realistically duplicate restraint conditions found in practice. The specimens used for restrained shrinkage test were 100 x 100 x 600mm in size. A 60mm high stress riser at the center of span and two 45mm high risers at the two ends were provided on the bottom surface of the mold to produce concentrated stress and reduce thickness at the center of the slab. The risers at the ends restrict the longitudinal movement and cracks were induced at the thin section above the middle riser. Figure 1 shows the geometry of the restrained mold. Figure 1 Geometry of restrained mold The specimens were measured to quantify the amount of cracked surface area using image analysis software. Results were compared with plain control specimens to calculate the cracking ratio of each mixture The procedure measures for the free shrinkage of concrete specimens described in ASTM C157 and ASTM C341 were used in this test. The size of the specimen had to be greater than 3 inches due to the fiber length, therefore specimens of size 100 x 100 x 300mm were cast and measured for change in length using a comparator. The specimens were conditioned in a lime bath at constant ฉบ บท 85 ป ท 26 กรกฎาคม - ก นยายน 2556

48 ว ศวกรรมสาร มก. temperature and moved to storage. Readings were taken on day 1, 2, 3, 7, and 14. In addition, standard 6-inch dia. cylinders were 3. Results and discussion Table 2 Test results of the experimental program loaded up to cracking point to find out the 28- day splitting tensile strength of each mixture. Series Fiber Crack Area (mm. 2 ) Tensile Strength (N/ Free shrinkage at 14 mm. (%Vol.) ) days (µm.) M20 M30 M20 M30 M20 M30 Control - 206.72 200.12 1.314 1.578-2523.70 1165.88 0.05 112.41 72.39 1.121 1.231-2315.17-1061.61 PVA 0.10 79.87 96.52 1.324 1.341-1507.11-853.08 0.15 69.14 17.81 1.301 1.585-1106.64-637.44 0.05 157.94 151.78 1.280 1.322-2327.01-1099.53 Steel 0.10 231.86 153.44 1.447 1.426-1789.10-808.06 0.15 131.14 154.29 1.298 1.514-1325.83-907.58 3.1 Plastic shrinkage cracking When the crack reduction behavior of PVA fiber was considered for both series, a very significant reduction in total cracked area was observed. M30 series showed the highest improvement, with 91% reduction for 0.15% fiber volume, 66% with 0.1% and 64% with 0.05% fiber volume. With 0.15% fiber volume in M20, 66% of the cracks were controlled and with 0.1% fiber volume 61% of the cracks were controlled. However, for 0.05% fiber volume only 24% cracks reduction was achieved. Figure 2 Influence of fiber volume on plastic shrinkage cracks in M20 & M30 series

Plastic Shrinkage Cracking in Steel and Polypropylene Fiber Reinforced Concrete 49 For steel FRC specimens the highest reduction was 40% and 30% with 0.1% fiber volume for M20 and M30, respectively. For concrete with 0.05% fiber volume, the crack reduction percentage was 24% and 25% and up to 0.05%, fiber volume the low strength M20 series had higher crack area. Between 0.05% and 0.1% fiber volume, this effect reversed and M30 series showed higher total crack area. At 0.15%, the crack reduction percentage was lower that of 0.1% for both concrete mixes. Plastic shrinkage cracking in SFRC reduced as fiber volume increased confirming the conclusions by Eren & Marar [4] and Jozsa & Fenveysi [8]. both series of mixes are plotted together in Figure 2. High coefficient values indicate a strong relationship between the two variables. The results agree with the general conclusions made by Passuello et al. [9] that increasing fiber volume leads to a reduction in plastic shrinkage cracking. 3.2 Tensile Properties The tensile splitting tests were carried out in accordance to ASTM 341. The 14 specimens were tested at the age of 28 days (see Figure 3). The loading rate was between 135 to 180 kn/min (0.032 to 0.042 MPa/s). Table 2 Regression analysis results between total plastic shrinkage area and fiber volume percentage Mix Type a b R 2 M20 PVA -44.525 228.35 0.844 M20 Steel -27.38 222.25 0.842 M30 PVA -55.145 227.41 0.838 M30 Steel -17.905 203.26 0.683 Regression analysis showed a linear relationship between total plastic shrinkage area (A) and fiber volume (V). The results of the analysis are given in Table 2 and the relationship obtained follows the equation: A = a (V) + b (1) Where a and b are regression coefficients, R2 is the correlation coefficient, A is total plastic shrinkage crack area in mm2 and V is fiber volume (%). Results show an inverse linear relationship between fiber volume and cracked area. The regression lines and experimental results for Figure 3 Influence of fiber on tensile strength The characteristic tensile splitting strength comparison for PVA and steel fiber used in the M20 series does not show an increased tensile strength with increased fiber volume. The steel fiber specimens were able to carry additional load even after cracking, making it difficult to determine the critical load. Slight increase in tensile capacity was measured for M30 series compared to M20 series, but there is no improvement compared with plain concrete. 3.3 Free shrinkage The free shrinkage curves in Figure 4 show the shrinkage for each series of specimens. The control specimen had the ฉบ บท 85 ป ท 26 กรกฎาคม - ก นยายน 2556

50 ว ศวกรรมสาร มก. highest shrinkage as expected. PVA and steel FRC with the lowest fiber content (PVA1 and SF1) did not show any significant shrinkage compared to plain concrete. The PVA3 mix with 0.15% fiber had the least shrinkage. In all the mixes, it was observed that with increase in fiber volume, shrinkage increased rapidly for the first 2-3 days and declined within two weeks. This rapid rate of shrinkage during the first 2 days can be attributed to the combined effect of drying, chemical (autogenous shrinkage) and plastic shrinkage. The behavior tallies with the results obtained by Altoubat & Lange [10]. a) b) Figure 4 Free shrinkage for 14-day duration with varying fiber volumes on a) M20 and b) M30 series. 3.4 Relationship between tensile strength and plastic shrinkage cracking. Correlation analysis showed that there is a strong inverse relationship between total plastic shrinkage cracking area and split tensile capacity of concrete. Both M20 and M30 series with PVA fiber has correlation coefficients of -0.946 and -0.974 respectively. Steel fiber specimens for M20 and M30 series had values of -0.657 and -0.776. Figure 5

Plastic Shrinkage Cracking in Steel and Polypropylene Fiber Reinforced Concrete 51 Figure 5 Correlation between total cracked area and split tensile strength shows that specimens with high tensile strength had lower cracked area. Although there is a correlation between the strength and total cracked area for different fiber volumes, this does not imply for all fiber types. Further studies using different fiber types need to done before any definitive conclusion is made. 4. Conclusion Experimental results indicate that FRC is more effective in crack reduction than plain concrete. It was found that plastic shrinkage cracks were significantly reduced with addition of short PVA fibers, compared to steel FRC and plain concrete. There was no significant difference between the behaviors of steel fibers in both series of concrete. At low volumes, PVA microfibers showed the most improvement. Change in length tests showed that at very low fiber volumes the reduction in shrinkage seen for both fiber types (less than 10%) was not significant enough to make a conclusion. Further research with slightly higher volumes of fibers need to be done to isolate the influence of fiber volume. 5. Acknowledgments The authors appreciate the financial support given by Thailand International Development Cooperation Agency (TICA) and material support given by Kasetsart University for this study. 6. References [1] Bentur A., Report 25: Early Age Cracking in Cementitious Systems - Report of RILEM Technical committee TC 181-EAS: Early Age Cracking, Shrinkage Induced Stresses and Cracking in Cementitious Systems, Vol. 25 of RILEM report, Bentur. RILEM Publications, 2003, 337 pages. ฉบ บท 85 ป ท 26 กรกฎาคม - ก นยายน 2556

52 ว ศวกรรมสาร มก. [2] Weiss W. J., Yang W., and Shah S. P., Restrained Shrinkage Cracking in Concrete, Sixth International Purdue Conference On Concrete Pavement: Design and Materials for High Performance. Purdue University, 197, pp. 159-175. [3] Zhang J., and Li V., Influences of Fibers on Drying Shrinkage of Fiber-Reinforced Cementitious Composite, Journal of Engineering Mechanics, Vol. 127, No.1, 2001, pp. 37-44. [4] Eren Ö., and Marar K., Effect of Steel Fibers on Plastic Shrinkage Cracking of Normal and High Strength Concretes, Materials Research, Vol. 13, No.2, 2010, pp. 135-141. [5] Banthia, N., Yan, C., and Mindess S., Restrained Shrinkage Cracking In Fiber Reinforced Concrete: A Novel Test Technique, Cement And Concrete Research, Vol. 26, No.1, 1996, pp. 9-14. [6] Corinaldesi, V., and Moriconi G., Characterization of Self-Compacting Concretes Prepared with Different Fibers and Mineral Additions, Cement and Concrete Composites, Vol. 33, No. 5, 2011, pp. 596-601. [7] Berke N.S. and Dallaire M.P., The Effect of Low Addition Rates of Polypropylene Fibers on Plastic Shrinkage Cracking and Mechanical Properties of Concrete, (pp. 19-41). In J.I. Daniel & S.P. Shah. (Eds.), Fiber Reinforced Concrete Developments an Innovations. American Concrete Institute. Farmington Hills, MI, 1994. [8] Jozsa Z., and Fenveysi O., Early Age Shrinkage Cracking of Fiber Reinforced Concrete, Concrete Structure, Vol. 11, 2010, pp. 61-66. [9] Passuello A., Moriconi G., and Shah S. P., Cracking Behavior of Concrete with Shrinkage Reducing Admixtures and PVA Fibers, Cement and Concrete Composites, Vol. 31, No.10, 2009, pp. 699-704. [10] Altoubat S. A., and Lange D. A., Creep, Shrinkage and Cracking of Restrained Concrete at Early Age. ACI Materials Journal, Vol. 98, No. 4, 2001, pp.323-331.