GRD Journals- Global Research and Development Journal for Engineering | Volume 4 | Issue 3 | February (2019) ISSN: 2455-5703
Flexural and Flexural Toughness of Fiber Reinforced Concrete- American Standard Specifications Review Ali Jihad Hamad Lecturer Department of Construction Materials Technology Engineering Northern Technical University / Engineering Technical College of Mosul / Iraq
Rami Joseph Aghajan Sldozian PhD Candidate Department of Applied Sciences University of Technology / Baghdad / Iraq
Abstract Fibers used to enhance the brittleness property of steel reinforced concrete and plain concrete, and modify the tensile strength by increasing work of fracture. Thus, the toughness measurements are valuable for assessing the post crack performance of fiber reinforced concrete (FRC). There are many international standard around the world, but this paper are focuses on review the American standard specifications via exclusive ASTM. The review involve the flexural strength testing methods and toughness testing methods for FRC. For flexural strength test, the ASTM C 78 and ASTM C 293 reviewed and compared between both, while for toughness the ASTM C 1018, ASTM C 1609, ASTM C 1399 and ASTM C 1550 reviewed. This paper also describes concisely the method for each testing and considerable advantages of these methods. Beside, abridged some limitation of these methods. All figures have been redrawn with inserted more details to be most obvious and more rich. Keywords- Flexural, Flexural Strength, Toughness, Flexural Toughness, Fiber Reinforced Concrete (FRC) __________________________________________________________________________________________________
I. INTRODUCTION Fiber reinforced concrete (FRC) is define as a concrete having dispersed randomly oriented fibers (ACI 116r) [1]. The addition of fibers to the concrete have wide range of usage due to their evident advantages over ordinary concrete. The function of the use of fibers in concrete is to enhance the mechanical properties of concrete. Fibers used to modify the flexural strengths, toughness, impact resistance, fracture energy, and restrain crack formation. The concrete with fibers as additives used widely in highways, tunnel linings, concrete pipes, reinforced concrete frames, reinforced concrete beam members, shell roof systems, skyscrapers, pre-stressed concrete, light shell constructions, domes and folded plates in recent years [2-5]. Plain concrete (unreinforced concrete) breakdown suddenly when the deflection corresponding to the ultimate flexural strength exceeded, oppositely FRC continues to withstand considerable loads even at deflections significantly in excess of the fracture deflection of the plain concrete [6]. The fractured specimens of FRC exhibits that failure takes place chiefly due to fiber pull-out or debonding. Therefore, FRC specimen does not break immediately after first crack as shown in Figure 1. This has the effect of increasing the work of fracture, which denoted to as toughness and represented by the area under the load-deflection curve [6].
II. FLEXURAL STRENGTH When the flexural strength test is characterize the evaluation of tensile strength of concrete; also, the flexural strength named as a modulus of rupture (MOR) or (MR) [6, 7]. The standard test methods of ASTM C 78 [8] and ASTM C 293 [9] used to determine the flexural strength. A. ASTM C 78 versus ASTM C 293 The standard test method ASTM C 78 [8] achieving simple beam by applying third-point loading (four-point bending) and ASTM C 293 [9] achieving simple beam by applying center-point loading (three-point bending). The specimen for a flexural strength test matching to ASTM C 78 [8] and ASTM C 293 [9] should be confirmed to the dimensions in Figure 2 (a and b), and shows the planning and arrangement for the flexural strength test. The common specimen used is 150×150×500 mm, but used 100×100×500 if the maximum aggregate size is less than 25 mm [6, 7]. In ASTM C 78 [8] the span length (L, distance between the two reactions or supports) of test specimen divided into three third L uniform and should be achieve the of ratio of any third to the depth of the specimen ( / d = 1) shall be equal 1. Beside, in ASTM 3 C 293 [9] the span length of test specimen divided in two portions and the ratio of the straight distance between the point of load
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Flexural and Flexural Toughness of Fiber Reinforced Concrete- American Standard Specifications Review (GRDJE/ Volume 4 / Issue 3 / 002) L
application and nearest reaction to the depth of the beam ( / d = 1.5) shall be equal 1.5. In general, the span of the specimen have 2 three times of the depth.
Fig. 1: Flexure toughness of FRC versus plain concrete
Maximum aggregate size, Size of specimen, preparation of the specimen, moisture content, load rate and reform (sliced beam to be standard size) are the most factors affecting on the result of flexural strength. The specimen turn on its side with respect to its position as molded and center it on the support blocks. Load-applying blocks (upper roller) should be in contact with the surface of the specimen before applied load, and should be vertical to the face of the specimen and applied without eccentricity. Also, the load is operated at rate 0.8 to 1.2 MPa/min [6] on the specimen constantly without shock. According to the mechanics of materials, the flexural strength can calculated as shown in Figure 3 (a) and (b). In ASTM C 78 [8] third-point loading, the middle third portion (distance between upper roller bearing) of the beam is subjected to pure bending if the fracture occurs inside the middle third, and the flexural strength can be calculated as presented in Figure 3. But, if the fracture occurs outside the middle third as shown in Figure 4, the specimen is involving bending moment and shear force, so the flexural strength can be calculated as following [8, 10]. â&#x20AC;&#x201C; If (L/3 - a) < 0.05L the flexural strength can be computed as Ă&#x2014;d M Ă&#x2014; y Pa 3Pa R= = 2bd32 = I bd2 12 â&#x20AC;&#x201C; If (L/3 - a) > 0.05L the result should be rejected Where: a = average distance between failure crack and the nearest support measured on the tension surface of the beam, mm. The results of the flexural strength test according to ASTM C 293 [9] (center-point loading) gives 13 percent higher than thirdpoint loading [11]. The flexure strength of normal weight concrete can estimated as [12, 13]: R = (0.62 to 0.83) â&#x2C6;&#x161;đ?&#x2018;&#x201C;â&#x20AC;˛đ?&#x2018;? Where: đ?&#x2018;&#x201C;â&#x20AC;˛đ?&#x2018;? = compressive strength of concrete
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Flexural and Flexural Toughness of Fiber Reinforced Concrete- American Standard Specifications Review (GRDJE/ Volume 4 / Issue 3 / 002)
Fig. 2: Schematic of apparatus for flexure test of concrete, (a) third-point loading method according to ASTM C 78 [8] and (b) center-point loading method according to ASTM C 293 [9].
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Flexural and Flexural Toughness of Fiber Reinforced Concrete- American Standard Specifications Review (GRDJE/ Volume 4 / Issue 3 / 002)
Fig. 3: Shear and moment diagrams, (a) third-point loading and (b) center-point loading
Fig. 4: Flexural strength occurred outside of the middle third of specimen
III. FLEXURAL TOUGHNESS Flexural toughness is define as a measure of the energy absorption capacity by another word the toughness is ability to withstand crack opening. It is performance of areas under the load-deflection curve or area under stress-strain achieved by testing a simply supported beam under third-point loading. In generally the beams of FRC are tested in flexure by using the third-point loading arrangement specified in Test Method ASTM C 78 [8]. The reading done at sufficiently frequent intervals to ensure accurate reproduction of the load-deflection curve. The toughness test are carried out in accordance to ASTM C 1018 [14], ASTM C 1609 [15], ASTM C 1399 [16] and ASTM C 1550 [17].
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Flexural and Flexural Toughness of Fiber Reinforced Concrete- American Standard Specifications Review (GRDJE/ Volume 4 / Issue 3 / 002)
A. ASTM C 1018 ASTM C 1018 [14] is the most public test for many years used to FRC toughness. Beam specimen 100×100×350 mm preferred to test in flexure under third-point loading according to ASTM C 78. The depth and width of test specimens shall be at least three times of the maximum fiber length. ASTM C 1018 [14] test method provides the determination of a number of ratios called toughness indices. These indices are determined by dividing the area under the load-deflection curve obtained by third point loading flexural test, the area up to the deflection at which first crack deemed to have occurred. The first crack strength represents the behavior of fiber reinforced concrete to the inception of crack in the matrix. But, the toughness indices (In) and residual strength (R) factors characterize the level of strength retained after first crack up to specified endpoint deflection, and represent the average post-crack load retained over a specific deflection interval as a percentage of the load at first crack are derived from these indices. Toughness indices I5, I10, I20, I30, I40, I50 etc. were assessed by dividing the area up to a deflection of 3, 5.5, 10.5, 15.5, 20.5, 25.5 times the first crack deflection by the area up to first crack, respectively, as shown in Figure 5. For plain concrete considered as a brittle material and, so the various toughness indices in its case are equal to one. Additionally, the residual strength factors represent the remaining strength in fiber reinforced concrete derived from the toughness indices [10]. Residual strength factors stated in terms RM,N, the residual strength factor between Indexes IM and IN (N > M) is expressed as RM,N = C (IN – IM) Where constant C = 100/(N – M) were chosen such that for an preferably elastic-plastic material.
Fig. 5: Definition of Toughness Indices (load-deflection curve) according to ASTM C1018 [14]
The residual strength factors are R5,10= 20(I10-I5), R10,20= 10(I20-I10), R20,30= 10(I30-I20), R30,50= 5(I50-I30), and R10,50= 2.5(I50-I10) etc. The residual strength factors for plain concrete, its brittle response, consequently, have residual strength factors equal to zero. Finally, concluding from ASTM C 1018 [14] some limitations that its influence on the efficiency of the test. Loaddeflection curve should be accurately due to toughness indices depended on it. The first crack deflection is difficult obtained it because is very small, and this often difficult as result of various extraneous deflections that possibly will occur due to machine deformations and placing of the specimen on the supports [18]. In year 2006 should be mentioned the ASTM C 1018 [14] is withdrawn and replaced by new standard that is ASTM C 1609 [15]. B. ASTM C 1609 This test method evaluates the flexural performance of fiber-reinforced concrete using parameters derived from the load-deflection curve. The beam specimens of fiber reinforced concrete having a square cross-section and tested in flexure using a third-point
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Flexural and Flexural Toughness of Fiber Reinforced Concrete- American Standard Specifications Review (GRDJE/ Volume 4 / Issue 3 / 002)
loading arrangement according to ASTM C 78 [8]. The two sizes of 100×100×350 mm and 150×150×500 mm are preferred specimen in the testing. The depth and width of test specimens shall be at least three times of the maximum fiber length. Points termed first-peak, and residual loads at specified deflections are identified on the curve, and used to calculate flexural performance parameters. T_150^D represent the absolute area under the load-deflection curve up to a net deflection of L⁄150, where D is nominal depth of the beam specimen in mm. Definitions of terms and determinations of the corresponding deflections are shown in Figure 6. The flexural behavior of the fiber reinforced concrete represented by the first-peak strength up to the onset of cracking, whereas the residual capacity after cracking represented by residual strengths at specified deflections.
Fig. 6: Parameters calculated according to ASTM C1609 [15] (sometimes PP equal P1)
C. ASTM C 1399 In ASTM C 1399 [16] obtaining average residual-strength of fiber reinforced concrete test method presents a quantitative measure useful in the assessment of the performance of FRC. In addition, it makes available for comparative analysis, among beams having different fiber types, including materials, dimension and shape, and different fiber contents. The average residual strength (ARS) is computed using specified beam deflections that are obtained from a beam that has been cracked in a standard manner and presents a assess of the post-cracking strength of the FRC. The schematic of dimensions and setup for this test stated in Figure 7. This test method uses a specimen size of beam 100×100×350 mm placed above of stainless steel plate and cracked by loading on the combination beam and plate. The aim of used the plate is to help control the rate deflection and inhibit failure when the beam begins to crack. Start loading the beam and plate combination at the set rate and continue loading until reaching a deflection of 0.20 mm as shown in Figure 8. The steel plate removed after the beam has been cracked, and the cracked beam of FRC is reloaded to get data to plot a reloading load-deflection curve. The average residual strength of the FRC among the loads fixed at reloading curve deflections of 0.50, 0.75, 1.00, and 1.25 mm as follows. PA + PB + PC + PD ARS = ×k 4 Where: ARS = average residual strength, MPa, PA + PB + PC + PD = Sum of recorded loads at specified deflections, N, k = L/bd2, mm-2, L = span length, mm, b = average width of beam, mm, and
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Flexural and Flexural Toughness of Fiber Reinforced Concrete- American Standard Specifications Review (GRDJE/ Volume 4 / Issue 3 / 002)
d = average depth of beam, mm. The results by this test method on average comparing with test method ASTM C 1609 [15] are lower [16].
Fig. 7: Appropriate apparatus according to ASTM C 1399 [16]
Fig. 8: Load-deflection curves according to ASTM C 1399 [16]
D. ASTM C 1550 In the first, the specimen beam of this test method is different comparing with previous test methods where used round panel. This test method covers the determination of flexural toughness of FRC expressed as energy absorption in the post-crack range using a round panel supported on three symmetrically arranged pivots and subjected to a central point load. The specimen dimensions of
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Flexural and Flexural Toughness of Fiber Reinforced Concrete- American Standard Specifications Review (GRDJE/ Volume 4 / Issue 3 / 002)
the panel are 75 mm in thickness and 800 mm in diameter and fixture supporting the panel during testing shall consist of any configuration that includes three symmetrically arranged pivot points on a pitch circle diameter of 750 mm as shown in Figure 9.
Fig. 9: Schematic of round panel test with compilation and reaction frame, in addition to detail of transfer plate and pivot support
The energy absorption considered between the beginning of loading and the specified central deflection. It is the area under the load-net deflection curve between the origin and the specified central deflection. Toughness in this test usually expressed at central deflections of 5, 10, 20, or 40 mm. This test method commonly used for evaluating the toughness of fiber reinforced shotcrete, and is mainly used in the tunnel linings and mining industry. The main advantage of ASTM C 1550 [17] it gives similar results to other toughness test, and with lower variability. Beside the disadvantage is that the specimen panel is too large and heavy (~90 kg) and is difficult to be handled. In addition, size specimen is not adequate into most common testing machines [18].
IV. CONCLUSIONS It can synopsized from the review as following: 1) The ASTM C 78 is common known and more application in specification compared with ASTM 293. The results of the flexural strength test according to ASTM C 293 (center-point loading) higher than flexural strength test according to ASTM C 78 (third-point loading). 2) In ASTM C 1018 the first crack deflection is difficult attained it because is very minor, and this often difficult as result of various extraneous deflections that possibly will occur due to machine deformations and placing of the specimen on the supports. 3) In year 2006, the ASTM C 1609 inserted as alternative of ASTM C 1018. Whereas the new standard is definitely, an enhancement over the previous one in certain details, and the ASTM C 1609 applied to high performance fiber reinforced concrete having very high volume fraction of fibers. 4) The ASTM C 1399 is most valuable for relatively low fiber volumes. The specimen beam cannot used to compute the flexural strength and the results by this test method on average comparing with test method ASTM C 1609 are lower. 5) The ASTM C 1550 commonly used for evaluating the toughness of fibers reinforced shotcrete. The main advantage it gives similar results to other toughness test, and with lower variability. The specimen panel is too large and heavy, and size specimen is not suitable into most common testing machines.
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ACI Committee, (ACI 116) (1999) Cement and Concrete Terminology, American Concrete Institute.
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