Drying Shrinkage Characteristics of Concrete Reinforced With Oil Palm Trunk Fiber

Transcription

1 Drying Shrinkage Characteristics of Concrete Reinforced With Oil Palm Trunk Fiber ZAKIAH AHMAD*, AZMI IBRAHIM Faculty of Civil Engineering, Universiti Teknologi Mara, 40450, Shah Alam, Selangor, Malaysia PARIDAH MD TAHIR Faculty of Forestry, University Putra Malaysia, 43400, Serdang, Selangor, Malaysia Abstract Concrete is subject to some form of restraint, such as steel reinforcement, forms or adjacent members. As concrete begins to lose volume, the restraint inhibits movement, which then induces tensile stress in the concrete. Once the tensile capacity of the concrete has been exceeded, it will crack. Therefore this paper reports on a study of shrinkage of plain and concrete reinforced with bio-waste fiber namely oil palm trunk fiber (OPTF). Metallic rings are the most widely used devices to test the restrain shrinkage of concrete. When the concrete deforms due to shrinkage, the ring restrains the material and tensile stresses are induced. However in this investigation, square column with PVC ring was used to determine the time for concrete specimens to crack under restrained shrinkage. Forty five test specimens of square columns sized 400 mm x 400mm with height of 150 mm were manufactured using Grade 30 concrete with 25 mm length OPTF at different volume percentage of OPTF; 0%, 1%, 2%, 3% and 4% of the total concrete volume. Thick PVC pipe with different diameters; 150mm, 250 mm and 350 mm were cast at the centre of the specimens to act as stiff cores which provide the internal restraint. It is shown with the ring setup that the cracking potential of concrete under restrained shrinkage can be classified on the basis of how long it takes the material to crack or the rate of stress development in the material at various percentages of fibers. The contributions of fibers were evaluated by using crack width and length. It was found that by increasing the fiber content has resulted in reduction in drying shrinkage as well as controlling the cracking. Keywords: Oil palm trunk fiber, fiber reinforced concrete, crack width, restrained shrinkage 1. Introduction Drying shrinkage is the reduction in volume caused principally by the loss of water during the drying process. If concrete members were free to shrink, without restraint, shrinkage of concrete would not be a major concern to structural engineer. However, the contraction of concrete members is often restrained by its supports, the adjacent structure or bonded reinforcement. Each of these forms of restraint involve the imposition of a gradually increasing tensile force on the concrete which may lead to time-dependent cracking, increases in deflection and widening of existing cracks. Restraint to shrinkage is probably the most common cause of unsightly cracking in concrete structures. The advent of shrinkage cracking depends on the degree of restraint to shrinkage, the extensibility and strength of concrete in tension, tensile creep and load induced tension in the member. In large sized concrete members as a column, the differential shrinkage between the surface and interior concrete causes tensile stresses to develop at the surface. The larger shrinkage at the surface causes crack to develop that may, with time penetrate deeper into the concrete. Extensive longitudinal cracks are initiated at the point of maximum tensile stress due to shrinkage, usually at the midpoint of the faces of the rectangular columns. With further drying, the cracks spread along the length and increase in width. ISSN:

2 The amount of shrinkage depends on many factors including the cement content, aggregate properties, the mixture composition, temperature and the relative humidity of the environment, the age of the concrete and the size of the structure. If concrete is restrained from shrinking, tensile stresses develop and once the tensile stresses exceed the tensile strength of concrete, the concrete cracks. One possible method to control or reduce the adverse effects of cracking due to restrained shrinkage in concrete members is the addition of fiber in the concrete (1). However the shrinkage of fiber reinforced concrete is influenced by a combination of factor such a specimen size, fiber type, fiber content and age of concrete when drying begins (2,3). Result obtained from studies on the effects of fibers on cracking due to restrained shrinkage using the ring type specimens that the presence of fibers considerably reduce shrinkage and the average crack width and delay the formation of the first crack (4). The amount of fiber reinforcement was found to be the most significant factor in controlling and reducing cracking due to shrinkage (5). Currently, Malaysia is the largest palm oil producer and contributes about 57.6% of the total supply of palm oil in the world. The increase in housing development also causes a lot more plantations being cleared. A large supply of elemental fragments of oil palm biomass can also be found through oil palm replanting. Tremendous amount of trunks and fronds are generated during the replanting since the economic life span of oil palm tree is about years. Apart from trunks and fronds, empty fruit bunches is another important fibers component generated during the extraction of oil palm fruit into palm oil. Therefore, there should be an effort to fully utilize the oil palm waste such as its leaves, trunks and bunches significantly to the other industry. The trunk of oil palm tree can be processed to form a good source of fiber. The process of extraction of fiber does not require a sophisticated engineering process like the synthetic fiber. With regard to the previous research on oil palm trunk fibers (OPTF), MDF board from OPTF was stronger and had better fiber-to-fiber strength recorded than the frond and empty fruit bunch (EFB) MDF (6). Therefore, this study investigates the suitability of the OPTF as concrete reinforcement and the effect of using different amount of OPTF in reducing the drying shrinkage and cracks development in concrete. 2. Material and Experimental Methods 2.1 Fiber The OPTF were taken directly from the plantation when the trees are newly fallen using special excavator machine. Freshly shredded fibers were obtained at random from trees aged between years. After that, the fibers were undergone cleaning process to remove the parenchyma, as this parenchyma contains carbohydrates, which can retard the concrete hardening process. Since the effective technology in processing oil palm trunk fibers has yet to be accomplished, the methods used were restricted to conventional processing methods that are manually. This process can be improved later especially when there is collaboration work with other company to invent such machine Determination of Physical Properties and Chemical Composition of OPTF Preliminary measurements on the fibers were made e.g., bulk density of the fibers, diameter of the fiber, fiber morphology study and chemical analysis. Natural fibers come in varying sizes and textures to the extent that it becomes very difficult to determine a proper estimate for their dimensions. Different methods have been used to obtain approximate values for the diameters of such fibers. Eichorn and Young (7) measured the diameter of hemp fibers using a calibrated FEG-SEM at an excitation voltage of 2 kev. An assumption was made by the authors that the fibers had a circular cross section. Devi et al (8) used à stereo microscope to obtain diameters of pineapple fibers based on a similar assumption of circular cross-section. Mwaikambo and Ansell (9) used SEM and image analysis techniques to obtain the diameters of sisal fiber bundles, assuming they had circular cross-sections. i. Determination Cross-sectional area and bulk density In this work, the diameters of the fibers were determined by; a. Taking six readings along the fiber length and the average was used as the diameter due to the variability of the fiber cross-section and also from Scanning Electron Microscopy (SEM) micrograph. ISSN:

3 b. Determining the cross sectional area of the fiber using the density of the fibers. For the density method, from m f and V = Al f, the expression for cross section area, A can be written as, V m f A [1] l f where ρ is the apparent bulk density of the fiber, m f is the mass of the fiber, V is the volume of the fiber and l f is the length of the fiber. The apparent bulk density of the as received oil palm fiber bundles was determined using the Archimedes principle. A bundle of oil palm fiber bundles was weighed and its weight recorded as M fa. The bundle was then immersed in benzene (a solvent which has density 875 kg/m3) until wetted. The weight of oil palm fiber bundles in benzene was recorded as M fs. The apparent bulk density of oil palm fiber bundles is subsequently computed as, M fa b [2] s M M fa fs where ρ b is the bulk density of oil palm fiber bundles, ρ s is the density of the solvent (benzene) M fa is the weight of the fiber in air and M fs is the weight of the fiber in solvent. The tensile strength of the sisal fiber bundle is determined using the cross-sectional area obtained from the weight of the fibers and the density as shown in Equation [3]. Fmax T [3] A Substituting for area A in Equation [3]. F max l T m f f [4] ii. Determination of tensile strength of oil palm fiber The oil palm fiber bundles were cut to lengths of approximately 70 mm, weighed and finally mounted on manila-card coupons using EVO-STIK super strong standard heavy-duty epoxy adhesive from Evode Industries. Fig. 1 shows a typical mounting of the fiber on the coupon. Epoxy hole fiber Manila card Fig. 1. Mounting of fiber for tensile testing on coupon. The tensile test was conducted according to ASTM D885(1995) using an Instron machine with a crosshead speed of 1mm/min. Twenty specimens were used The ends of the manila-card coupons were gripped by ISSN:

4 hydraulic clamps to align the fiber with the machine axis. The sides of the hole on the coupon were cut with a pair of scissors to allow load transfer to the fiber during tensile testing. The maximum load from this tensile test was substituted into Equation [4] for the fiber tensile strength. 2.2 Concrete Binders and Aggregates The concrete constituents used were ordinary Portland cement, fine aggregate, coarse aggregate and OPTF. The fine and coarse aggregate used satisfied the sieved analysis grading in accordance with BS 882:1992. Coarse aggregate has maximum size of 20 mm. The relative density for aggregate was assumed to be 2650 kg/m Concrete mix, casting and batching Design Grade 30 concrete was employed. The ratio of coarse aggregate and sand to cement was set at 1.5 and 3 respectively. Water-cement ratio was kept constant at 0.5. Length of OPTF used was 25 mm since from the previous studies by the author (10), for grade 30 concrete the optimum length of OPTF is 25 mm. The detail of mix proportion is shown in Table 1. Table 1. Mix Design (kg/m 3 ) Mix Fiber By Volume Cement Water Coarse Aggregate Fine Aggregate 1 0% % % % Note: a aggregate Batch quantities: kg/m 3 Assume R.D(relative density) for aggregate = 2650 kg/m 3 Cement: Sand: Coarse aggregate (1:1.5:3) Maximum size: 20mm Water cement ratio : 0.5 (constant) Design Grade 30 concrete was employed. The ratio of coarse aggregate and sand to cement was set at 1.5 and 3 respectively. Water-cement ratio was kept constant at 0.5. Length of OPTF used was 25 mm since from the previous studies by the author (10), for grade 30 concrete the optimum length of OPTF is 25 mm. The mixing method is critical to the properties of fiber reinforced concrete. The addition of fiber has to be uniformly dispersed in the concrete in order to obtain homogenous concrete mixture. Therefore the consistency of the matrix needs to be neither too stiff nor too wet to ensure that the fibers can be uniformly dispersed in the mixture. The concrete mixes were poured into the wooden moulds prepared earlier and compacted using external vibration. The casting and testing of the specimens was carried out inside laboratory at room temperature. 2.2 Specimen preparation and test methods Workability test Fresh plain and fibrous mixes were tested for workability by slump (BS 1881: Part 102: 1983) Splitting Tensile test The tensile strength properties of concrete were determined using splitting tensile test in accordance with ACI Manual-544.1R. The samples were cast in cylinder mould of size 150 mm x 150 mm. Seven (7) specimens were prepared for each parameters tested. Tensile strength was measured at 3, 7 and 28 days. ISSN:

5 2.2.3 Restrained drying shrinkage test i. Preparation of moulds To measure the restrained drying shrinkage that develops in concrete, the modification of ring test in accordance with AASHTO PP34-99 was used. A total of 45 specimens of 400 mm square column sections with ring at the centre were prepared as shown in Fig. 2. The number of specimens for each mixture is shown in Table 2. The mould is made from plywood where the thickness of the plywood is about 13mm. The mould have cross-section 400 mm x 400 mm and the height of column (H) is 150mm. Concrete is cast around the square plywood and a thick PVC ring. Because the PVC is stiffer than concrete, volume change of concrete is prevented to a certain extent, which depends on the ring s dimensions and properties of mix. PVC pipe with different diameters. D H= 150mm 50 mm 400mm 150 mm Demec point 400mm Fig. 2. Schematic diagram of the specimen for shrinkage test and the position of demec points Table 2. Number of specimens Concrete mix Dimensions of pipes Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Height (H) (mm) Core dia. (D) (mm) Fiber with (0 %) Fiber with (1 %) Fiber with (2 %) Fiber with (3 %) Fiber with (4 %) Total Total Therefore thick PVC ring of different diameters namely 150mm, 250 mm and 340 mm was used and glued at the centre of specimen as shown in Fig. 2. The radial drying of square column specimens was conducted in the laboratory atmosphere with 85%RH and at 20ºC. Drying was only allowed from the outer circumferential surface. The large depth-to-thickness ratio was chosen to provide uniform drying across the thickness (5). ii. Shrinkage Observation After 24 hours, the wooden moulds were removed but the pipes were remained. Two demec stainless steel tips were placed on the concrete surface using superglue at on each sides of the square column. ISSN:

6 The distant between the two points is 150 mm and 50 mm from the top surface as shown in Fig. 2. Prior to placing the demec stainless-tip, the surface of the columns was cleaned to make sure smooth surface. The strain measurement was made using demec strain gauge equipment. The strains were measured every day for the first week and once a week thereafter up to 10 weeks as well as recording the crack movement, the temperature and relative humidity of the environment. iii. Crack measurement All the cracks appeared were measured in terms of length and width. The crack lengths were measured manually using thread and measuring tape. The crack widths were measured using lens optic crack width equipment. 3. Results and discussion In this study the tensile strength of OPTF was found to be N/mm 2 which is considered high when compared with other natural fiber such as sisal ( N/mm 2 ) and Jute ( N/mm 2 ) (11). Bulk density of fibers is 1100kg/m 3. The diameter of clean fiber was found to be in between mm. In terms of color, the vascular bundles of OPTF were light yellowish in fresh state and turned to dark grey when dried. According to ACI 544.1R (12), the length of fibers to be added to the concrete may vary from mm. For this study the fiber length is fixed at 25mm. It was observed that the inclusion of OPTF in fresh concrete significantly altered the rheological behavior in terms of its mobility. Fig. 3 presents the results of workability study on the concrete reinforced with OPTF. From Fig. 3, the reduction in workability is evident with the increasing of fiber content where the workability ranged from 133 to 30 mm by slump test. The slump loss of about 73% from 130mm to 30mm (100 mm loss) when 3% fiber was included in the concrete. Fig. 3: The workability of concrete with different addition percentage of OPTF by slump test. When adding 1 to 2% fiber, the slump loss is not so significant. The inclusion of fiber stiffened the mixture resulting in an increased stability or cohesion but also to an apparent reduced of workability. Fig. 4 shows the result of splitting tensile strength of concrete reinforced with OPTF for different days of curing: the 25-day tensile strength is in the range of N/mm 2 It can be seen that the strength development of the strength was drastically increased at 1% of fiber volume. ISSN:

7 Fig. 4: Tensile strength (N/mm²) vs Percentage of fibre (volume) for variation days of curing 3,7 and 28. An analysis of variance was performed to determine if there were differences in mean tensile strength values among the different volume of fiber tested at 28 days. F-test indicated that there were significant differences in the mean tensile strength [p value = 0.02] at 5% significant level, which indicates that the different volume of OPTF influenced the tensile strength of OPTF reinforced concrete. At 28 days, concrete with 1% OPTF increased the tensile strength of the concrete by 26%. The addition of 2-3% of OPTF has no significant different in tensile strength with respect to the control specimens. However the addition of 4% of OPTF reduces the tensile strength. 3.1 The effect of OPTF on the restrained shrinkage behavior of concrete The results of restrained shrinkage test for different diameter rings are shown in Fig. 5. (a) (b) (c) Fig.5. Effect of volume of OPTF on shrinkage for; (a) D150, (b) D250 and (c) D340 core diameter ring. ISSN:

8 It can be seen that different diameter rings show different shrinkage behavior. For smaller diameter ring (D150) (Fig. 5a), the concrete experienced shrinkage only but for D250 (Fig. 5b) and D340 (Fig. 5c), the concrete shrunk and expanded within 70 days observation. It was observed that the higher the volume fraction, the lower the recorded final strains at the sides of the specimens. The rate of increase in shrinkage for all concrete was high during the 7-14 days which is a result of curing process. After that period the movement of concrete differs significantly depending on the restrained core diameter. However, the final values of shrinkage for the long duration of tests clearly illustrate the influence of fiber content. Also, it is evident that OPTF of higher volume fraction, 4%, have much greater effect in reducing shrinkage for different core diameters. When no cracks appeared, as for D150 and D250, the variations in shrinkage strains were limited. As soon as cracked developed, as for D340, the influence of OPTF became more pronounced. Concrete reinforced with 4% OPTF experienced the least shrinkage. Table 3 provides information on the age at which the first and second crack appeared. It was found that first crack occurred in interval of days of age. Table 3. List of specimens with the age of concrete when crack appeared OPTF Volume ( %) Age (Day) 2 nd Crack Height (mm) Core Dia. (mm) Age (Day) 1 st Crack % % % % % The appearance of cracks during this period was only observed on specimen with core diameter of 340 for all volume percentages of OPTF. Therefore the bigger surface area of restrained will leave smaller section for concrete which means that the concrete section is the thinnest/smallest. Large, thick concrete members dry out more slowly than small/thin sections. As a result, for the same drying period, shrinkage of large-size members is lower than the smaller-size ones which can dry out to their cores more quickly. Concrete column with the other 2 diameters of PVC rings; 150 and D 250, no cracks were observed. In order to represent the specimens with different core diameter and vol% of OPTF, the specimens are denoted as D,core diameter, vol%. For example, D1501% is for concrete with ring diameter 150 mm and 1 vol% OPTF. From Table 3, D3400% concrete had the first and second cracks at the age of 21 days. D3401% and D3402% concrete, the cracks appeared after 28 days. For D3403% and D3404% concrete, the crack appeared after 35 and 42 days respectively. The days at which the crack first appeared provides the evidence that the inclusion of fiber has a direct influence in delaying or even preventing the appearance of cracks. Varying the core diameter on shrinkage was found to have very little effect provided that is no cracking take place. But with the larger core diameter and therefore a much thinner shell, cracking is more likely and positive strains (expansion) are recorded. For the largest core diameter of D 340, the higher the fiber contents the lower the final strains. With the medium core diameter of D250, no cracking appeared, but shrinkage was appreciably reduced by the fiber. For the smallest core diameter D150, no cracking appeared and the inclusion of OPTF actually prevented cracking. Throughout the shrinkage test, the width and the length of the cracks were monitored and measured. The crack width reported here is an average of at least four to six measurements. Figs. 6 show the development of the ISSN:

9 crack width and length with progress of time for concrete with D340 diameter ring. It can be seen from Fig. 6 that the crack width and length increases with time meaning that once crack is initiated, it continued to open up with continued drying. This is due to the quasi brittle behavior of concrete material. However the dimensions of crack width and crack length of the concrete vary with the amount of OPTF added. The rate of increases of crack width and length in concrete with 4% OPTF was almost flat as time progress which indicates that the addition of 4% OPTF able to arrest the cracks from expanding. It can also be seen that, in general, the crack width and crack length was significantly reduced with the addition of fibers. CRACK WIDTH OF OPTF (D340) VS DAY CRACK LENGTH OF OPTF (D340) VS DAY Crack width (mm) % 1% 2% 3% 4% Crack length (mm) % 1% 2% 3% 4% Age - Days Age- days (a) (b) Fig. 6. Effect of OPTF on (a) crack width and (b) crack length for concrete restrained with D340 core diameter ring. The increased in crack width and crack length of the concrete with respect to time of ageing, explained the expansion in the concrete D340 as seen in Fig. 5c. During the 1 st and 2 nd week, the concrete shrank due to drying process of the concrete but the increased in strain values may be due to the widening of the crack width, the expansion of crack length and increasing number of cracks. Finally, the restrained shrinkage cracking of concrete containing OPTF appeared to be less than that of concrete OPTF. Cracking with OPTF was delayed to later ages and resulted in smaller crack widths and length. Fiber volume fraction of 1% of OPTF was sufficient to increase the tensile strength but not enough to control shrinkage cracks. Hence further studies are needed for optimal design of concrete reinforced with oil palm trunk fiber. Conclusions From the results of this investigation, the following conclusion can be drawn. 1. The fiber content has a direct influence on restrained drying shrinkage in concrete, as well as on the control of cracking. 2. Increasing the fiber content resulted in general reduction in drying shrinkage. 3. When the specimens cracked, the significance influence of the fiber content on the drying shrinkage can be observed. 4. The core diameter of the specimen had a direct effect on the cracking tendency. The bigger diameter which gives thinner shell has relatively higher crack sensitivity. The inclusion of fibers influenced this tendency significantly by preventing cracking in or controlling crack development. Acknowledgment Authors thank Institute of Research and Development, Universiti Teknologi Mara for financial support and Putrajaya Construction for the supply of oil palm trunk. ISSN:

10 References [1] S. Parviz, K. Ataullah, and J.W. Hsu. Mechanical Properties of Concrete materials Reinforced with Polypropylene or Polyethylene Fibers, ACI Materials Journal, 89(6): (1992). [2] R.N. Swamy, and H.Stavrides. Influence of fiber reinforcement on restrained shrinkage and cracking, ACI Journal, 76(3): (1979). [3] ACI Committee 207 Effect of Restraint, Volume Change and Reinforcement on Cracking of Mass Concrete, ACI 207.2R. ACI Materials J., 87(3): (1990). [4] J. G. Chern, and C.H. Young. Study of Factors influence drying shrinkage of steel fiber reinforced concrete. ACI Materials J., 87(3): (1990). [5] M. Grzybowski, and S. P. Shah. Shrinkage cracking of fiber reinforced concrete, ACI Materials J., 87(2): (1990). [6] L. Lionel. Properties of Medium Density Fiberboard From Oil Palm (Elaeis guineensis, Jacq.) Fibers, M.Sc. Forestry (Wood Industry) Project Report. Faculty of Forestry, University Pertanian Malaysia, Serdang, Selangor, Malaysia [7] S. J. Eichhorn, and R. J. Young. Composite micromechanics of hemp fibres and epoxy resin microdroplets. Composites Science and Technology, Vol. 64(5): (2004). [8] L. U. Devi, S. S. Bhagawan, and S. Thomas. Mechanical properties of pineapple leaf fiber-reinforced polyester composites. Journal of Applied Polymer Science, 64(9): (1997). [9] L.Y. Mwaikambo, M.P. Ansell, Mechanical properties of alkali treated plant fibres and their potential as reinforcement materials II. Sisal fibres. Journal of Materials Science, 41(8): (b). [10] M. J. Jelani, Z. Ahmad, H. M. Saman, P. M. Tahir. The Effect of Length of Fiber on Concrete, In: Proceedings of Seminar Science and Technology and Science Social, Kuantan Pahang May [11] M. A. Aziz, P. Paramasivam, and L. P Lee. Concrete Reinforced with Natural Fibers.In R. N. Swamy (eds.), Volume 2, New Reinforced Concrete,1984. [12] ACI 544.1R-96, State-of-the Art Report on Fiber Reinforced Concrete, ISSN: