Fatigue Damage and Durability Assessment of MEMS

The 2nd International Conference on Composites: Characterization, Fabrication and Application (CCFA-2) Dec. 27-30, 2010, Kish Island, Iran IUST

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Plastic Shrinkage Cracking and Workability of Fibers SCC Composite *A. A. Maghsoudi1, M. Nouri 2, M. Maghsoudi3 1

Associate Prof., 2MSc. student, 3M.Sc. student, Civil Eng., Dept. Shahid Bahonar University of Kerman, Iran (*E-mail: [email protected])

Abstract Compared to fiber reinforced concrete (FRC), self compacting concrete (SCC) is the new generation type of concrete with high flowability and good resistance to segregation. This type of concrete can be further extended when combined with fiber and can create a type of composite material. In this article, polypropylene and a new type of steel fibers were used as resin in SCC as matrix. The experimental effect of volume, length, and aspect ratios of the fibers on plastic state of this composite material is reported. For this aim, different tests on plastic phase such as L-box, slump flow diameter, J-ring and column segregation were carried out on SCC fibers included and excluded of nano particles. The plastic test results were shown that this type of fibers can be successfully used to design SCC and nano SCC composite and reached acceptable values on plastic state.

Keywords: SCC and nano SCC composite, polypropylene and steel fibers, resin, matrix. 1. Introduction Self compacting concrete, SCC can be considered as a concrete which has little resistance to ow so that it can be placed and compacted under its own weight with little or no vibration effort, yet possesses enough viscosity to be handled without segregation or bleeding. SCC was developed in the late 1980s as a solution to achieve durable concrete structures in dependent of the quality of construction work [1]. The fresh mechanical properties of SCC are often determined through the use of rheometers, which measures the viscosity and the yield strength of concrete in plastic state. The term ber reinforced concrete, FRC can be defined as a concrete containing dispersed randomly oriented bers. Inherently concrete is brittle under tensile loading and mechanical properties of concrete may be improved by randomly oriented discrete bers which prevent or control initiation, propagation, or coalescence of cracks [2]. The character and performance of FRC changes, depending on the properties of concrete and the bers. The properties of bers that are usually of interest are ber concentration, ber geometry, ber orientation, and ber distribution. There is widespread agreement that nanotechnology has the potential to revolutionize the world of concrete materials science. The fundamental processes that govern the most pertinent issues to the study of concrete technology in plastic phase all are affected, if not dominated, by the performance of the material at the nano scale. This paper examines the relationship between nanotechnology and the study of self compacting fiber reinforced concrete, SCFRC and nano while is in plastic phase. In this article, nano particles, polypropylene and two different types of steel bers are used, in combination, and the effect of ber inclusion on the flow ability, pass ability, resistance to segregation and column segregation of SCFRC (such as; slump flow, J-ring, L-box and column segregation) tests are performed to assess the plastic phase on this new generation concrete type. There is as yet no universally accepted standard for characterizing of fibers and fibers nano SCCs. Nevertheless, a few testing methods seem to reappear several times in literature and tend to become internationally recognized as suitable methods to characterize the fibers and nano fibers SCCs. Hence, almost same procedure was employed to produce these types of SCCs too. Immediately after the mixing, the value of slump flow, J-ring, L-box and column segregation tests were determine by the following methods similar to the normal SCC. It is remind that, the effectiveness of polypropylene fiber in controlling the plastic shrinkage cracking of SCC is investigated separately and reported elsewhere [3]. However, it was concluded that, even at very low fiber volume fraction of 0.10%, an average crack reduction of 85 to 95% was achieved for sever environment.

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2. Experimental program

The experimental program consisted of batching different trial mixes on plain SCC to reach the acceptable values on plastic state, then for the selected one, four mixtures of SCFRC (Table 1), a plain control mix and three ber reinforced mixes are designed for further investigations. Table1: Mixture proportions Mix No.

C (kg/m3)

Aggregate (kg/m3) C.A.

F.A.

Nano Silica (cc)

LP (kg/m3)

Water (Lit)

PCE (Lit)

Steel ber (kg/m3)

w/c

spiral

straight

Poly. ber (kg/m3)

1

604

868

821

1330

105

213.4

8.4

0.35

0

0

0

2

604

868

821

1330

105

213.4

8.4

0.35

0

16.1

0

3

604

868

821

1330

105

213.4

8.4

0.35

13.3

0

0

4

604

868

821

1330

105

213.4

8.4

0.35

0

0

12.08

The properties of materials used in producing SCFRC are as follows: The cement used in all mixes was Type II cement, limestone powder (LP) was used as a mineral viscosity enhancing admixture, the LP was a by-product of marble extraction with a CaCO3, as for the aggregates, crushed limestone and crushed sand from the same local source were used. As can be seen from the gradation of the aggregates presented in Table 2, the maximum aggregate size was 19mm. The coarse and ne aggregate with a specic gravity of 2.70, and water absorptions of 6% and 17% respectively was obtained. A novel poly-carboxylic ether type super plasticizer, SP produced by a local manufacturer and colloidal silica with a specific gravity of 1.03 and 50.9% solid dust that are smaller than 50nm was used in concrete mixtures. Two steel ber types (with a length of 30 to 50mm), one in the form of spring and one as straight and one type of Polypropylene fiber with a length of 15mm were used. Table 2: Grading of coarse and ne aggregate Sieve size (mm) 19.25 12.5 9.5 4.75 2.36 1.18 0.6 0.3 0.15

Coarse (%passing) 100 96.14 63.85 6.71 1.5 -

Fine (%passing) 100 100 100 86.64 58.8 35 21 7.5 1.2

2.3. Test procedures The workability methods used in this study are given and standardized by the normal self compacting concrete committee of EFNARC and measured the free and restricted deformability (slump ow and Jring) and stability (slump ow diameter) of normal SCC mix are used for FRSCC too. These three test procedures are briey described below: 2.3.1 Slump flow and T50 test The slump flow test is used to assess the horizontal unconfined free flow of SCC, defined as the average distance of lateral flow of the concrete. This test method is intended to monitor the consistency of fresh SCC with coarse aggregate up to 2.54cm (1 in) in both laboratory and field settings (ASTM, 2005). Typical requirements for slump flow values are between 63 and 79 cm (25 and

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31 in) (EFNARC, 2002), but lower values are acceptable for applications that do not involve long members (i.e., long concrete flow distance) and highly congested sections. The velocity of concrete flow during the slump flow test provides an indication of viscosity (i.e concrete resistance to flow). During this test, the time taken for the concrete to reach a 50cm (20in.) diameter circle drawn on the slump flow table is measured, termed the T50 value, which provides a relative measure of viscosity and unconfined flow rate of SCC. A larger value normally corresponds to increased viscosity and stability. A time of 3-7 seconds is acceptable for civil engineering works (EFNARC, 2002). Here, the same procedure (Fig.1) is used for FRSCC too. 2.3.2 Column Segregation Test This test method is intended to provide the user with a procedure to determine the vertical segregation and stability of SCC. Because segregation was believed to be most prevalent within the top few inches of the apparatus, the apparatus was modified by dividing the top 6.5 in. section into two sections measuring 2.0 and 4.5 in. each in height. The 2.0 in. column section was placed at the very top as shown in Figure 2. The same procedure is used for FRSCC The mold was slightly overfilled in one lift. The surface of the concrete was then leveled to the top of the mold by means of both lateral and horizontal motion of a thin steel plate (less than 1/16 in. in thickness). The same steel plate and technique was used to separate the column sections after a rest of 10 to 15 minutes. The concrete for each column section was placed into individual containers and weighed. The concrete was then wet-washed through a No.4 sieve leaving the coarse aggregates on the sieve, which was then oven-dried and weighted for each column section [4].

Figure 1: J-ring test of FRSCC Figure 2: Schematic of column segregation test The vertical segregation resistance was evaluated by means of a Vertical Stability Mass Index (V_SMI) and Vertical Stability Volume Index (V_SVI), which are expressed as follows:

Where MCAi is the mass of oven-dried coarse aggregate from column section “i”; MCi is the mass of the fresh concrete in column section “i”; and hi is the height of column section “i”. The V_SMI index (and V_SVI index) represents the mass of fiber and coarse aggregate per unit mass (volume) of concrete in each section relative to the mass of fiber and coarse aggregate per unit mass (volume) of concrete in the base section (i.e., section S1 in Figure 2). This definition for segregation indices allowed comparing the test results from different mixes. If there is no segregation, then both V_SVI and V_SMI should be unity for all column sections. A value of larger/smaller than unity indicates that the section has more/less coarse aggregate relative to the base section per unit concrete mass (volume) and:

2.3.3 L-box Test The PCI guidelines (2003) and EFNARC (2002) propose that if the L-box apparatus is designed for disassembly after the concrete is allowed to harden, horizontal segregation of the concrete may also be detected by subsequent sawing and inspection of the SCC in the horizontal sections. However, no specific method has been included in these documents. In this study, a procedure analogous to the column segregation test procedure was adopted to evaluate horizontal segregation resistance of SCFRC

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(both with reinforcing obstacle and without) and the passing ability of coarse aggregate through the obstacle too. The L-box test assesses the flowability of SCC through obstacles (Khayat, et al., 2004; PCI Interim Guidelines 2003). Although, this test method has not yet been standardized by ASTM, it is one of the most common test methods in the literature. In this study, the vertical section of L-box was filled with fresh concrete and left to rest for 1 minute, then the gate was lifted to let the concrete flow into the horizontal section. The heights of the concrete left in the vertical section (h 1) and at the end of the horizontal section (h2) were measured to calculate the blocking ratio, h 2/h1, which is an indication of the self-leveling and blockage characteristics of the SCFRC. Different acceptable limits for blocking ratio with a range of 0.8-1.0 have been proposed by different researchers. In this investigation, SCFRC with h2/h1 larger than 0.80 was considered to have satisfactory self-leveling and concrete passing properties measured with L-box. In this study, a procedure analogous to the column segregation test procedure was adopted to evaluate horizontal segregation resistance of SCFRC (both with reinforcing obstacle and without) and the passing ability of coarse aggregate through the obstacle [4].

3. Overall Mix Rating for Fresh Concrete Properties An overall SCC rating system was developed that can be used to group SCC mixes as those with good, medium, and poor fresh properties. Similar method is used for each measured SCFRC and fresh property parameter such as flowability, h2/h1 ratios, and segregation indices, rating criteria were selected for good, moderate and poor ratings. Table 3 shows the selected parameters for each test method and the associated rating criteria. Table 4 also shows the overall fresh properties mix rating obtained for each mix. That is, for a given mix, an overall rating of “poor” was assigned if two or more of the individual tests indicated poor ratings for the mix; “good” was assigned if none of the individual tests indicated a poor rating, and if the number of good values from the individual tests exceed the moderate values from the individual tests; and all other mixes were assigned an overall rating of “moderate”. Table 3: Fresh properties rating criteria Test Method

Good

Rating Criteria Moderate

Poor

66

61≤P≤71

71