Research on Direct Tensile of Concrete by Method of Middle Directly Embedded Rebar

Civil Engineering and Technology March 2015, Volume 4, Issue 1, PP.9-14

Research on Direct Tensile of Concrete by Method of Middle Directly Embedded Rebar X.X. He, Y.H. Zhang # School of Civil and Transportation Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, China #

Email: zhangyanhe900@126.com

Abstract This paper introduces a kind of independent research and development of the direct tensile test template. Research on direct tensile strength of fly ash concrete by center directly embedded rebar in tensile method. Analysis of the impact on the direct tensile strength by the exposed length of embedded rebar and concrete strength. Obtain the experimental results: the select about the size of different exposed sizes of rebar impacts the direct tensile strength, and existing the best exposed sizes which can obtain the real the direct tensile strength.

Keywords: Fly Ash Concrete; Direct Tensile Strength; Embedded Experiment; I-Shaped Experiment; Exposed Size

1 INTRODUCE Concrete direct tensile strength is an important index of high concrete mechanics performance because it has a significant influence on concrete crack resistance and durability. In China, splitting strength and flexural strength are widely adopted to indirectly estimate the concrete direct tensile strength. But, it cannot reflect the actual concrete tension condition because of the splitting specimen’s stress state [1] and bending specimen’s stress gradient [2], which make the specimen out of the axis tensile state [3]. I-shaped direct tensile experiment is widely used internationally for the high precision [4][5], but the requirement for the testing equipment is very high, and some special tension devices[6][7] are needed. What’s more, under this method, the formwork is very complicated and concrete dosages are very large. On the other hand, embedded direct tensile experiment was used for a while, but it’s not been widely introduced because it’s hard to center the rebar and the exsertion of the rebar would generate added moment to influence the test results[8]. By comparisons, embedded direct tensile experiment is easy to test the concrete direct tensile strength [9][10][11]. So if this method can be improved to make sure the rebar’s centering and diminish the added moment, it would be a simple and rational method.

This experiment designed a new experiment mould for direct tensile method which accented on the embedded rebar centering. Four kinds of direct tensile specimens with different rebar exsertion length were made to analyze the influence to the concrete direct tensile strength of different rebar exsertion length, strength grade and the fly ash.

2 EXPERIMENT DESIGN 2.1 Tensile Specimen Mold All the molds are made by steel. The concrete direct tensile mold[12]with embedded rebar is showed as the Fig.1. The embedded rebar is fixed by the steel plates with holes on the both ends, and the embedded rebar length is the 6.25 times the rebar diameter which is calculated according to the binding power. The width of the mold middle section without rebar is as the specimen cross section length. I-shaped mold is as the Fig.2 showed. The flange and the web plate will be transited by a quadrant and the length of the web plate middle is the same as the cross section length.

2.2 Experiment Materials The materials are the P.O42.5# ordinary Portland cement from the Diamond Company, natural river sand, limestone -9http://www.ivypub.org/cet

gravel with two gradation of 5~10 mm and 10~20 mm, II-level fly ash and KSM-830 polycarboxylate superplasticizer with 38% water-reducing rate.

1

2 6 3 5

4

1．End plate；2．Spile；3．Side plate；4．Baseboard； 5．Connecting bolt；6．Embedded rebar

FIG.1 SKETCH MAP OF THE DTM MOLD

FIG.2 SKETCH MAP OF THE DTE MOLD

2.3 Experiment Mix Proportion The specimens are with three different strength grade. The ‘Design regulations of mix ratio of ordinary’ (JGJ55-2000) [13] is referred to for the experiment mix proportion design. The mix proportion without fly ash is the base

and the fly ash concrete is made by changing the cement ratio with the fly ash. Show in Table 1.

TABLE 1 CONCRETE MIX PROPORTION

Group 1 2 3

W/ kg/m3 160 165 140

C/ kg/m3 311 437 560

FA/ kg/m3 0 0 0

W/B 0.51 0.37 0.25

S/ kg/m3 780 734 645

G/ kg/m3 1169 1100 1051

βs 0.41 0.4 0.38

FA/(FA+C) 0, 0.3 0, 0.3 0, 0.3

Notice: W/B- Water-binder ratio, βs - Sand coarse aggregate ratio, W-Water, C-Cement, FA-Fly ash, S-Sand, G- Gravel.

2.4 Experiment Method Embedded direct tensile method is shown as the Fig.3 (a). The flexible connection of the exposed rings made of the rebar exsert part on both ends and testing machine chuck by the tightwire is used instead of the traditional rigid coupling. Shown in the Fig 3 (b). This method is approximate to the spherical hinge. In the traditional method the exsert rebar on the concrete specimen ends is clapmed directly by the testing machine chuck[14] which leads to the fix of the specimen. And along the direct tension and crack increasing, the concrete eccentric tension will be more severe. So this tensile method is better. The I-shaped test specimen will be hanged on the flange by the soft wire when it’s tested too, shown in the Fig3(c).

The plumb will be used to keep the specimen symmetric axes and the loading direction parallel. And the loading rate is 0.4Mpa/min [15].

In this article, the exsert rebar length is 5cm, 7.5cm, 10cm, 12.5cm respectively in the embedded direct tensile method, and the I-shaped direct tensile experiment is the matched group. The experimental methods are concluded below, shown in the table 2.

There are four exsert rebar lengths between 5cm to 12.5cm respectively in the direct tensile experiment with the centered embedded rebar. And there are three different strength grades of square concrete specimens which side length a=100mm with different fly ash replacement rate. And the direct tensile specimen’s effective length is the same as the cross section length, shown in the table 2.

The matched experiment includes cube compressive strength testing, splitting tensile strength testing and three point flexural strength testing of the same size and standard size. The concrete specimens are made by the standard method - 10 http://www.ivypub.org/cet

and the age is 28d.

(a) The DTM experiment

(b) Sketch map of the DTE experiment

(c) Load figure of DTE experiment

FIG.3 LOAD FIGURE OF THE TENSILE EXPERIMENT TABLE 2 GENERAL TABLE OF THE DIRECT TENSILE EXPERIMENT METHOD

Method the middle embedded rebar I-shaped direct tensile

Method Code

Tensile Strength Code

DTM

ftm

Strength Grade Code

Rebar exsert length /cm 5,7.5,10,12.5

1, 2, 3 DTE

Effective FA replacement Length in the rate % tensile section/mm 100 0, 0.3

fte

0

100

Notice: DTM(Direct Tension Middle)- Direct tension strength of middle embedded rebar; DTE(Direct Tension Eight)- Direct tension strength of I shape.

TABLE 3 AVERAGE OF STRENGTH /MPA

group 1 2 3 1 2 3

FA replacement rate /% 0 0 0 0.3 0.3 0.3

5 3.6 3.1 3.8 2.8 3.3 3.8

ftm/ MPa 10 12.5 2.1 2 3.2 3.2 3.7 3.7 3.2 3 3.2 3.1 3.7 3.6

7.5 2.3 3.1 3.7 3 3.2 3.7

Average 2.5 3.2 3.7 3 3.2 3.7

fte/MPa DTE 3.6 3.7 4.2 2.8 3 3.4

fcu/MPa a=100 39.6 45.7 50.8 31.4 32.4 38.6

3 EXPERIMENT RESULT ANALYSIS The average of the two tensile strength and correlated cube compressive strength are shown in the table 3.

3.1 The Influence of the Rebar Exsert Length to the Concrete Direct Tensile Strength 4.0

1-F0 2-F0 3-F0 1-F0.3 2-F0.3 3-F0.3

f tm/MPa

3.5 3.0 2.5 2.0

steel exposed length/cm

1.5 2.5

5.0

7.5

10.0

12.5

15.0

FIG.4 THE INFLUENCE TO THE FDTM FROM THE EMBEDDED REBAR EXSERT LENGTH

The relationship between the rebar exsert length and the concrete direct tensile strength of the DTM experiment is - 11 http://www.ivypub.org/cet

shown in the Fig.4. The imaginary lines stand for the group without fly ash, and the solid lines stand for the group with fly ash. Five groups data of six show that the concrete direct tensile strength will decrease significantly along the embedded rebar exsert length’s increasing.

The direct tensile strengths of the second & third group from the three different strength grades are almost the same.

Same as the the F0.3 group which means the FA replacement rate is 0.3 & the F0 group which means the FA replacement rate is 0. At the same time, the fcu of F0.3 group and F0 group decreased sharply shown in the table 2.

3.2 Comparison of the Two Direct Tensile Strength The changes of the two direct direct tensile strength ratio f tm / f te along the change of the embedded rebar exsert length and the cube compressive strength fcu are shown in the Fig.5 & Fig.6. As the figures showed, the tensile strengths of the DTM and DTE are quite different under the condition of different FA replacement rates. To the F0 group, f tm is smaller than f te when the f tm / f te belongs to 1~0.69 along the increasing of concrete strength grade; And to the F0.3 group, f tm is bigger than f te when f tm / f te belongs to 1.01~1.11.

The experimental value difference between DTM and DTE indicates that fly ash influence the tensile test, so a large number of further experiments on this aspect will be needed.

f tm/f te

1.0 0.8

1.1

f tm/f te

1-F0 2-F0 3-F0 1-F0.3 2-F0.3 3-F0.3

1.2

F0-5 F0-7.5 F0-10 F0-12.5 F0.3-5 F0.3-7.5 F0.3-10 F0.3-12.5

0.9

0.7

0.6 steel exposed length/cm

f cu100 /MPa

0.4

0.5

2.5

5.0

7.5

10.0 12.5 15.0 FIG.5 RELATIONSHIP BETWEEN FTM /FTE AND

20

40

60

80

FIG.6 RELATIONSHIP BETWEEN FTM /FTE AND FCU

THE EMBEDDED REBAR EXSERT LENGTH

3.3 Relationship of Direct Tension Strength and Tension Strength The relationship between the direct tensile strength and the compressive strength is shown in the Fig.7, namely to the DTM specimens with different exsert rebar lengths, the trend of direct tension strength increasing is approximate along with the compressive strength increasing. When the compressive strength grades are the same, the DTE direct tension strengths in F0 group are apparently bigger than the DTM ones. And in the F0.3 group, the DTM direct tension strengths are larger than DTE ones, with tiny difference.

4.5 0.12 DT M-0-5 DT M-0-7.5 DT M-0-10 DT M-0-12.5 DT E-0 DT M-0.3-5 DT M-0.3-7.5 DT M-0.3-10 DT M-0.3-12.5 DT E-0.3 0.395fcu^0.55

3.5 3.0 2.5 2.0 1.5 25

35

45

55

65

f cu100 /MPa 75

y = 1.1806x-0.7306

0.10

f t /f cu

f t /MPa

4.0

0.08 0.06 0.04 f cu100 /MPa

DT M-0-5 DT M-0-7.5 DT M-0-10 DT M-0-12.5 DT E-0 DT M-0.3-5 DT M-0.3-7.5 DT M-0.3-10 DT M-0.3-12.5 DT E-0.3 乘幂 (DT M-0.3-12.5)

0.02 25

FIG.7 RELATIONSHIP BETWEEN FTM , FTE AND THE EMBEDDED REBAR EXSERT LENGTH

45

65 FIG.8 RELATIONSHIP BETWEEN FTM/FCU , FTE/FCU AND FCU

The relationship between f tm / fcu , f te / fcu and f cu is shown in the Fig.8, namely the f tm / fcu , f te / fcu decrease along with the f cu increasing. And the trends in F0 group and F0.3 group both present declinations as power - 12 http://www.ivypub.org/cet

function.

3.4 The Variable Coefficient of Strength The comparison between the changes of the variable coefficient of testing strength along with the change of the cube compressive strength and the tensile strength is shown in Fig.9. In Fig.9, the dot dash lines represent F0 group and the solid lines represent the F0.3 group. We can conclude that when the fly ash is used (F0.3), the variable coefficient with the same concrete strength grade decreases; and the variable coefficient decrease with the strength increasing; the variable coefficients of the strengths obtained by the method used in this article are approximate to the cube compressive strengths, and they are even smaller than the latter ones. For the reason of the 100mm size specimens use, all the variable coefficients are bigger in the reasonable range.

0.40

F0-Cv/ftm F0-Cv/fte F0-Cv/fcu F0.3-Cv/ftm F0.3-Cv/fte F0.3-Cv/fcu

Cv

0.30 0.20 0.10

f cube/MPa

0.00 25

35

45

55

65

FIG.9 RELATIONSHIP BETWEEN FTM/FCU , FTE/FCU AND FCU

4 CONCLUSIONS The paper draws the main conclusions as follows: (1) To the specimens without fly ash, the tensile strength obtained from the DTM experiment are bigger than the DTE experiment ones, and the specimens with fly ash per contra.

(2) Along with the the exsert rebar lengths increasing, the concrete direct tensile strengths decrease. Under the tensile failure pattern, the additional bending moment will decrease the concrete direct tensile strength. So it’s can be deduced that the experiment results will be more approximate to the true value if the exsert rebar lengths get shorter.

(3) The DTE method presents some instability, and the size and weight of the specimens used are big, what’s more, the strength variable coefficients are bigger than the DTM ones. As to the failure mode, the cracks spread over the junction between the I-shape web plate and the flange.

(4) The fly ash has a systematic influence to the experiment method, which is stated in Fig.5&6. This phenomenon indicates that the further experiments on the fly ash concrete are still needed.

(5) The concrete direct tensile strength variable coefficients are small with the DTM mold and method used in this article. And the cracks spread over along the central randomly. And when the rebar exsert length is not that big, the experiment results are approximate to the true value. And this method can be generalized easily or is is economical and not complicated.

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AUTHORS 1

2

nationality, Master’s degree, Professor,

degree, postgraduate, the research field: Research on fly ash

The research field: Concreted and high

concrete tensile properties and size effect.

performance

Email: zhagnyanhe900@126.com

X.X. He (Born-1961), Famale, Han

concrete

mechanical

Y.H. Zhang (Born-1990), Male, Han nationality, Master’s

properties and damage mechanism the concrete structure.

Email: 68322510@sina.com

- 14 http://www.ivypub.org/cet