Research for Anti-cracking Ability of Mill Cut Steel Fiber Concrete

Transcription

1 Research for Anti-cracking Ability of Mill Cut Steel Fiber Concrete Shanghai Research Institute of Building Sciences Group Shanghai municipal Engineering Institute 1997

2 Research for Anti-cracking Ability of Mill Cut Steel Fiber Concrete 1 Introduction Compared with the traditional reinforced concrete, steel fiber concrete has many structural advantages, evenly distributed steel fiber makes the concrete stress is more evenly dispersed in the cross-section and longitudinal direction of the structural material to avoid the stress concentration and therefore it can effectively reduce cracks or prevent crack extension and expansion, as long as a small number of steel fibers can significantly reduce the cracking of concrete. Grzybowaki and Shah (1) pointed out that with the admixture of % of round straight steel fiber in concrete, it can reduce the crack width formed due to the air shrinkage for more than 80%, and with the increase of fiber admixture amount, the formation time of cracks has also been slowed, and Bentur (3) thinks that the cracking of cement slurry mixed with steel fiber has a great relationship with interface status between the fibers with the matrix and its binding power. This research compared and analyzed the anti-cracking abilities of different varieties and admixture amount of steel fiber concrete through a series of tests of the laboratory including the crack resistance of cement mortar with steel fiber, adhesive property and constant load shocks. 2 Anti-cracking test of the steel fiber cement mortar 2.1 steel fiber cement mortar Quick anti-cracking test method About Test method of cement concrete shrinkage under limiting conditions, there is no standard test method both at home and abroad. this research applies the flat plate type test method proposed by Kraai to use it to study and compare the dry crack change of surface of the cement mortar (concrete) with steel fiber and ordinary cement mortar (concrete), this method can be carried out in a laboratory or field environment, and when this test is conducted in the laboratory, low humidity need to be maintained in order to facilitate the formation of cracks in the specimen within a short period of time. In this test, shrinkage cracks are divided into four types including large, medium, small and fine by its width size, measurement index of the shrinkage cracks with a width of about 3mm is 3, and measurement indexed of those with a width of about 2mm, 1mm and 0. 5mm are 2, 1 and 0.5 respectively, each measurement index multiplied by its corresponding total amount of crack lengths shall be the total amount of shrinkage cracks of the test specimen. This test attempts to compare the reinforcing effectiveness of different fiber varieties and admixture amount through fixed mortar mix proportion and test conditions (such as humidity, temperature and air flow rate, etc.). Mortar test plate size is 6100mm 914mm 19mm, the test mold is wooden mold with plastic film laying on the bottom and steel flat enclosed around with dimensions of mm to produce constraining force, the rate of water loss of the test plates is controlled by fans and iodine arc lights and other facilities (see Figure 1). Place the test plate after pouring before the fan with a speed of 19.3 ~ 22.5Km / hr, crack width and length will be measured after 24 hours.

3 Figure 1 Device for the cement mortar Anti-cracking test 2.2 Raw materials for the test Cement 425 # mineral waste residue silicate cement manufactured by Shanghai Cement Factory Sand medium-coarse sand with the fineness modulus being 2.5 Steel fiber 1. HarexMill cut steel fiber (SF01-32) 2. Domestic wire steel fiber mm 2.3 Mortar mix proportion for the test The mix proportion of cement mortar in this test is fixed as below: Cement: Sand: Water: steel fiber= 1: 1.5: 0.5: SF The steel fiber (SF) mix in the mortar mix proportion calculated according to the percentage by volume of the mortar is respectively 0, 0.39%(30kg/m 3 ), 0.58%(45kg/ m 3 ), 0.77%(60kg/ m 3, 0.96%(75kg/ m 3 ) and 1.15%(90 kg/m 3 ) according to the. 2.4 Test result (I) The crack length and width of the Cement mortar with mill cut steel fiber (volume fraction V f=0. 39% or 30kg/m 3 ) and the reference cement mortar without fiber are shown in Table 1.

4 Crack length and width of the mortar test panel Table 1 Reference test panel Test panel with steel fiber crack width Measurement index Length Area length Area (MM) Total shrinkage shift crack Total shrinkage shift crack (CM) (CM 2 ) (CM) (CM 2 ) < Total Percentage % Note: 1. The dimension of the limited cracking mortar test panel is 61CM 91CM 1.9CM 2. Evaporation water loss speed of the mortar test panel is 15~20g/93cm 2 *0. 5h Figure 2 undersurface cracking distribution of the cement mortar test panel (II) Cracks comparison of steel fiber cement mortars of different types

5 the total amount of shrinkage cracks Reference mortar Sheared fiber mortar Mill cut steel fiber mortar Figure 3 Comparison diagram of the total amount of shrinkage cracks (fiber admixture amount is 30kg/m 3 ) It is indicated in the above diagram that admixture of different types of steel fiber can reduce the shrinkage cracks of the cement mortar, and the anti-cracking effect of mill cut steel fiber is especially obvious. (III) Anti-cracking effect of different types of the fiber admixture amount In the cement mortar with the same water cement ratio, respectively mix cement mortar with HAREX mill cut steel fiber of 30, 45, 6075 and 90kg/m 3. The ratio of the total amount of shrinkage cracks of the cement mortar and that of the reference mortar is shown in Figure 4. It can be clearly seen that the total amount of cracks of the cement mortar is decreased with the increase of the steel fiber admixture amount. Note: M total amount of cracks of cement mortar with mill cut steel fiber. Mo total amount of cracks of reference mortar. Fiber admixture amount (Kg/m 3 ) Figure 4 anti-cracking effect of different fiber admixture amount

6 3 Adhesive property of the mill cut steel fiber and the matrix 3.1test material Cement: 525 # normal silicate cement manufactured by Shanghai Cement Factory Sand: standard sand Steel fiber: (l) HAREXMill cut steel fiber (SF01-32) (2) wire steel fiber of mm 3.2 Steel fiber cement mortar and test method Mix the normal silicate cement with standard sand with the cement-sand ratio of 1: 1.7 to prepare mortar with water cement ratio of 0. 5, and make into 8-shape test specimen (5 as a group). Precisely put a steel fiber in the central part of the minimum section. The test specimen after the molding shall be stored in damp air for 28 days, and measure the tensile strength of each mortar test specimen, illustrate the bonding capacity of steel fiber and cement mortar matrix with bonding strength. Bonding strength of the steel fiber Table 2 Test Specimen Name Pull-out load (N) bonding strength (MPa) Mill cut steel fiber wire steel fiber Anti-cracking test of steel fiber concrete under shock load 4.1 Test method introduction The impact test applies home-built equipment with the following method: 8kg steel ball freely falls from a height of 25cm, and the dimension of the test specimen is ø1 5.0 cm, thickness 5. 0cm, 6 test specimens in a group, the test result will apply average value of 4 data among them. 4.2 Raw materials for the test and mix proportion 1. Cement: Zhejiang Changxing Cement Factory Three Lions Brand 525 # silicate cement; 2. medium-coarse sand: fineness modulus of 2.65; 3. Macadam: 5~20mm macadam; tap water 3. Shanghai HAREX steel fiber (H) and domestic wire steel fiber (M). Mix proportion of concrete Shock test concrete Table 3 No. Mix proportion of concrete (kg/m 3 ) Water Cement Steel fiber Sand Macadam HA HA (H) 628 me HA (H) HA S 60(H) HA (M) HA (M)

7 4.3 Test result Anti-cracking test result under shock load of the steel fiber concrete is shown in the following table. Shock test of the steel fiber concrete Table 4 No. of test specimen Shock times when the test specimen cracks Shock anti-cracking performance improvement multiple HA HA HA HA HA HA Note: HA 5 and HA 6 are concrete with sheared fiber test result indicates that, anti-cracking ability of the steel fiber concrete under the effect of shock load has obvious improvement than that of the normal concrete. 5 Discussions 5.1 mechanism of action for the fibrous reinforcement material to improve the anti-cracking ability of the concrete The history for human to mix reinforced materials in brittle materials to improve the performance can be traced back to thousands of years ago, there was records of mixing straw to make bricks in ancient Egypt, and in the ancient mud wall of our country the grass fibers and other fibers can be seen for enhancement. However, awareness, research and rational application of scientific principle that how the admixture of the brittle material in the fiber can improve the fracture property only started in the middle of this century, such as the use of glass steel in the aviation industry promoted the development of this study which has formed a research field and subject. N G Nair, after studying the mechanical property of the cement reinforced by fiber, thought that the cement rigidity was very high and the brittleness is also rather large, the admixture of fiber can prevent the crack propagation, and increase the tensile strength and fracture properties to make the material have a certain degree of ductility which may obtain the effect to ease the local stress. Local cracking of concrete components produces the necessary energy consumption process, because the fiber has not been cracked and therefore it also maintains the integrity of components, and the partial cracked material can still bear, at the same time it also prevents the rapid expansion of cracks, avoids sudden rupture of components to bring catastrophic damage. Effect of the fibers in the above concrete has a great relationship with its shape and its bonding with the matrix. 5.2 Anti-cracking effect of cement mortar mixed with steel fiber As we all know that the shrinkage of concrete is mainly due to the water evaporation which leads to concrete volume shrinkage, while the reinforcing steel bar fixed surrounding the wooden mold limit this shrinkage, thus internal stress is generated in the concrete test specimen; on the other hand, concrete strength increase with the growth of age, tensile strength of the plasticity shrinkage cracks occur in the concrete is greater than the tensile stress. The water cement ratio in the mix proportion used in this test is 0.5, to make the test specimen crack, the test used the method of controlling

8 water loss, i.e., make the water failure ratio of the test specimen from 0.5 down to 0.4 within 4 hours, and in the test through weighing a small test specimen every half an hour to record the water loss situation, and by adjusting the wind speed and light to control each test plate to have an equal loss of water, Figure 5 is the water loss per unit area per hour of cement mortar at different conditions including temperatures and humidity, etc. Figure 5 indicates that water loss of the cement mortar has a very close relationship with the wind speed, humidity and temperature of its environment, fans and small solar lights used in the test are designed to control water loss of concrete test specimen to be substantially equal; conduct according to the shaded parts of the two curves in Figure 6 will be, so as to increase the comparability effect between every experiment. It can be seen from Table 1 that the shrinkage cracks length and width of the cement mortar after being mixed with steel fiber were significantly reduced, listed are width and length of each crack of the cement mortar mixed with mill cut steel fiber (30kg / m 3 ) and the reference cement mortar test panel, the maximum crack width formed in the reference cement mortar test panel is 2.2mm, while the crack width is 1.2~2. 2mm in length accounted for 64% of the total amount of cracks, after the admixture of 30kg/m 3 mill cut steel fiber, not only the crack length is reduced, and the largest width in the formed cracks is reduced from 2.2mm to 1.0mm. In addition, it can also be seen from Figure 2 that the depth of cracks in the test panel is also reduced due to the admixture of steel fiber, on the back of the reference test panel, a lot of through cracks due to the cracking of the test plate surface still can be seen; while on the back of the test panel mixed with steel fiber no trace of cracks is detected; it can be verified by the above test that, the cement mortar mixed with a certain amount of steel fiber (30kg/m 3 ) can effectively prevent the cracking of cement mortar, and even the cracks are formed, the length, width and depth of the crack all have obvious improvement compared to that of the unmixed cement mortar. Environment Temperature ( ) Transient Temperature ( ) Concrete temperature ( ) Water cement ratio (W/C) Cement mortar (kg/m 3 ) Hours (h) Figure 5 Water loss curve of the concrete surface Figure 6 Water loss curve of test control

9 Anti-cracking effect of the steel fiber toward the cement mortar board is strengthened with the improvement of fiber admixture amount, the greater the steel fiber admixture amount, the better the anti-cracking effect of mortar (concrete). Figure 4 is the Anti-cracking test results of cement mortar test plate with different Mill cut steel fiber admixture amount, and the steel fiber admixture amount were respectively 30, 45, 60, 75 and 90kg/m 3, and their number of cracks respectively amount to 41.1%, 27.25%, 16.8%, 3.8% and 57% of the reference mortar test plate. Thus, as long as the steel fiber admixture amount is adjusted, it might be possible to meet different design requirements. It must be noted that the above test result is obtained in the very severe test conditions, in fact, due to the implementation of various maintenance measures, cracking condition of the cracks should be far better than that in this test result. 5.3 influence of different steel fibers toward the adhesive property Cement matrix in the concrete and aggregate interface is the weak link of the concrete. For the steel fiber concrete, interfaces of the cement matrix and steel fiber are also inclined to be the weakest link of the steel fiber concrete, interface bonding between the cement stone and steel fiber is mainly physical bonding. The physical bonding results from the adherence between the interfaces of cement hydrate and steel fiber and mechanical meshing effect. therefore, the shape, surface condition and sectional form of the steel fiber and material of the steel fiber have great influence toward such physical bonding. The most commonly used steel fibers at present include the following types: 1. Short fiber made by cutting the steel wire, which has simple craftsmanship with the steel fiber tensile strength obtained is high and the deficiency of a smooth surface that has a low bonding strength with concrete matrix. Wire cutting into short fibers, such fibers production process is relatively simple, the resulting high tensile strength steel fiber, the inadequacy of its smooth surface, therefore bonding strength concrete matrix between the low. 2. Sheet wire steel fiber, which has the similar craftsmanship as the first type. The raw material is the sheet steel with a width of the length of the fiber needed; then shear to complete; during the shearing, steel fiber is a little curve and twisted, and this can increase the bonding strength. Thin wire steel fiber, the kind of fiber production technology and the first similar materials are the same as wide as required fiber length of thin steel plate, and then cut into, cut in steel fiber slightly bent and twisted, soyou can increase the bond management degrees. 3. Mill cut steel fiber, which is the fiber applying steel slab as the raw material and cut and made by milling cutter; the characteristics of this fiber is that it is twisted at axial direction, and by improving the cutting tool the fiber surface can form rough surface, in addition, the previous two kinds of fiber in the production process will have more or less impurities including oil stick on the surface, and the production method of the fiber will ensure clean surfaces. Therefore the bonding strength of the steel fiber and concrete matrix obtained by the method is better than the previous two kinds of steel fiber. The enhancement mechanism of steel fiber concrete is based on the presence of the directed steel fiber limits the formation and expansion of concrete cracks, adhesive property of the steel fiber and concrete plays a leading role toward this restriction, and with different shapes the steel fiber will also have different effects and roles. For steel wire and wire steel fiber with smooth surface, since its surface binding power is small it can only provide passing effect for the cracks internal stress. It is specifically indicated that after the visible cracks are formed, the anchor end of the steel fibers prevent the cracks from expansion and disperse and spread the stress acted on the crack to other directions, so that the steel wire or sheared fiber can only restrict the extension of visible cracks. Difference from them, there is a strong binding power between mill cut steel fiber and cement matrix, because it has a larger contact area, in the hardened cement mortar, the entire length of the mill cut steel fiber is presented in anchor shape (see Figure 7), so it can prevent the formation of cracks within the concrete to the maximum extent. test result of steel fiber and matrix bonding strength in Table 2 indicates that, the bonding strength of mill cut steel fiber

10 and matrix is larger than that of the wire steel fiber which can further indicate the reason that the anti-cracking ability of mill cut steel fiber concrete (mortar) is superior than that of the wire steel fiber concrete (mortar). 5.4 Influence of matrix toward the adhesive property of the fiber surface There are two ways to increase the bonding strength of steel fiber and concrete matrix, one is to start from the shape feature of the steel fiber; and the other is to start from binding materials. Figure 7 microscopy morphology of mill cut steel fiber in the concrete matrix Studies have shown that the improvement of the concrete matrix material strength is not in direct proportion to that of the bonding strength, and its effect is not obvious. Steel fiber in the concrete can be seen as coarse aggregate, in the pouring and molding process, the steel fiber is inclined to be horizontally arranged, especially in the upper and lower surface layers, some bubbles and water film will be gathered on the lower surface of the steel fiber, which will greatly affect the bond of the steel fiber and concrete matrix. Therefore, practice has proved that reducing the water consumption of concrete and adding some mineral admixture is an effective method. By using admixture such as the water reducing agent the water consumption can be greatly reduced, the layering and bleeding of the concrete can be reduced, the density of concrete matrix can be increased. Furthermore by adding mineral admixtures such as silica fume and high quality fly ash can also make grain composition of the concrete more reasonable and improve the density of transition layer of the steel fiber and concrete matrix, thereby improving its bonding strength. 5.5 The anti-cracking of steel fiber in the concrete. The contraction and expansion deformation of the concrete has a great relationship with the fact that whether the concrete itself has constraint conditions. When the concrete is under unconstrained conditions, if steel or fiber materials or very small size specimens are not equipped, this shrinkage is a free shrinkage, at this time, the internal material points

11 of the structure will approach by contraction which is called opposing deformation, in the compression status it will not cause cracking; if more steel bars are configured or large-size contraction of concrete surface was limited, it is the restrictions contraction, at this time, the internal material points of the structure reversely separated due to shrinkage, it is called dorsad deformation and is in tension. When the limited shrinkage stress exceeds the tensile strength, it will cause cracking. Similarly, free expansion is the dorsad deformation of material points, and is in compression, cracking will not occur. Cracks in the concrete can be roughly divided into two categories: one is the cracks formed due to the concrete shrinkage and the environment climate impact; the other is cracks generated by an external force which are the structural cracks. As an engineering material, the former will be mainly considered. Concrete cracking can be caused by many factors, so it is not possible to completely avoid the formation of cracks in concrete, the cracks once formed, due to the internal stress, the cracks will constantly develop along the weak parts of the concrete. The crack resistance of steel fiber in the concrete is the mainly expressed in the following two aspects: firstly, the steel fiber concrete can significantly improve the plastic shrinkage resistance ability. When the concrete pouring is completed, the main reason for cracks will be the inconsistent moisture evaporation within the concrete which results in uneven volume shrinkage, and at this time, the concrete is not yet hardened and cannot produce enough strength to resist the shrinkage stress, tensile stress caused by shrinkage of the cement slurry can lead to concrete cracking, and the presence of steel fiber can disperse or partially offset the internal stress, thereby inhibiting the formation and development of microcracks. Secondly, steel fiber can reduce the stress concentration at the crack tip to prevent the further development of microcracks. As we all know, once crack is formed, its tip will produce stress concentration, and the cracks will therefore continue to expand, but when the development of its tip intersects with the fiber, the steel fiber can offset some or all of the stress. Because the steel fiber in the concrete is distributed in a three-dimensional status, it can effectively prevent the cracks from developing into through cracks It is just because of these reasons, the incorporation of steel fiber can greatly reduce the number of macrocracks within the concrete (crack width is greater than 0. 05mm). Since the crack has been effectively controlled, the concrete durability will also be significantly increased. 6 Conclusion 6.1 The steel fiber has obvious effect for prevention the drying and cracking of concrete and mortar. 6.2 When the admixture amount of the steel fiber is30kg/m 3, the total amount of shrinkage cracks of the wire steel fiber mortar amounts to 50~65% of the test specimen. While mill cut steel fiber mortar is 40~50%. 6.3 The anti-cracking ability under shock load of the steel fiber concrete is 3~4 times of the normal concrete. 6.4 The anti-cracking effect of the steel fiber in the concrete has a significant relationship with the geometrical morphology and surface cleaning and roughness, the twisted shape, rough surfaces and no oil contamination are the root causes for the enhancement of bond strength between the mill cut steel fiber and interface. 7 Bibliography 1. Grzybowski etal Shrinkage Cracking of fiber Reinforcement concrete ACI mat. J. V. 87 No P P. PKraai A proposed test to determine the cracking potential due to drying shrinkage of concrete concrete construction V p A. Bentur Interfaces in fiber reinforced cements Materials Research Society V P G. LVondran Applications of steel fiber reinforced concrete Concrete International Nov P44-49