the investigation of shear behavior of r.c. beams...

Second International Symposium on Design, Performance and Use of Self-Consolidating Concrete SCC’2009-China, June 5-7 2009, Beijing, China

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THE INVESTIGATION OF SHEAR BEHAVIOR OF R.C. BEAMS MADE OF SELF COMPACTING CONCERTE WITH HIGH STRENGTH

H.R. Ebrahimi and M.H.A.Beygi

Msc graduate of Civil Engineering Faculty of Mazandaran University, Assistant Professor of Civil Engineering Faculty of Mazandaran University

Abstract : Self-Compacting Concrete has a unique characteristic which flows to place and around

obstruction under its own weight to fill the formwork completely and its self compact without any segregation.

This article outlines the effect of parameters such as stirrups spacing and variations of concrete strength on the behavior of reinforced concrete beams made of Self Compacting Concrete. In this study 9 beams were tested which 3 of these beams were made of ordinary concrete with the strength of 40 Mpa and 6 of the beams were made of SCC with high strength concrete having the strengths of 60 and 80 Mp. For each of these groups the stirrups were placed by the distances of 80, 100 and 133mm. the tested beams were simply supported and were loaded by two concentrated load.

It was found that using SCC with high strength in comparison with normal concrete will increase the stiffness and ultimate hear strength of beams and will reduce deflections. The cracking pattern at ultimate load of Normal and High strength self compacting concrete beams were the same, also the reduction in the distance of stirrups in shear span was the reason to increase ductility and ultimate load in all beams.

Keywords: Shear Behavior, Beams, Self Compacting Concrete, Stiffness, ultimate load

1- INTRODUCTION The development of self compacting concrete (SCC) is a desirable success in construction

industry in order to cope with related problem of casting of concrete. Self compacting concrete is not subjected to parameters such as skilled workers, shape and amount of bars or the form and shape of a structure and due to its high fluidity and resistance against the segregation can be pumped in long distance. The concept of self compacting concrete was suggested in year 1986 by Professor H. Okamura [1], but the first specimen was made by professor Ozewa from Tokyo University in year 1988 in Japan, and since then different research has been carried out.

Now, self compacting concrete can be classified as an advanced structural material, and in order to reach to a compacting state it does not need to be vibrated.


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This concrete in compare with normal concrete has many advantages such as advancement of quality of concrete, the reduction of repairs in site, faster construction periods, lower cost and the easy of automation processing, and also it will induce a better and safer and more hygienic position due to elimination of vibrators, and it will reduce the noise in the site.

The experimental work in this research includes the tests of reinforced concrete beams made of normal concrete mixture and SEC with high strength. The test includes 9 beams which 3 beams are made of normal concrete mixtures with 40Mpa strength and the other 6 beams are made of SCC with high strength concrete having strengths of 60 and 80Mpa.

In each of these groups the stirrups were placed within distances of 80, 100 and 133mm, after 28 days, static load test had been carried out in the structural laboratory of Mazandaran University until the shear failure took place. After the test, comparison between normal beams and SCC with high strength beams was started.

2- EXPERIMENTAL PROGRAM

2-1- Material Properties Three concrete mixtures had been tested in this research: One normal concrete with the

cubic strength of 40Mpa and two SCC with high strength concrete having strengths of 60 and 80 Mpa. The consumed gravel was crashed stone with nominal size of aggregate 6-20mm, and the consumed sand were chosen from the river in city of Amol with nominal size of aggregate of 0-6mm.

In SCC mixed design with high strength often the amount of lime stone powder material are used because it will reduce the segregation and settlement. Using the high amount of cement will increase the cost and the temperature will rise due to hydratation. That is why the other cement material such as lime stone powder (LSP) and blast furnace slag (GGBS) and fly ash instead of cement would be used in different SCC mixtures [2], in this research, Portland cement type I, lime stone powder (LSP) and micro silica powder as filler used for producing high strength SCC, and Portland cement type I used to make normal strength concrete.

One of the new super plasticizer known as viscocrete, copolymer (refined poly carbon silica) has been used for SCC concrete.

In order to easy the recognition of beams we will use the abbreviations as follow: Beams made of normal concrete are NC and beams made of SCC with strength of 60Mpa are shown as SCC60 and the beams made of SCC with strength of 80Mpa are shown as SCC80. The number used after S will show the spacing of stirrups along the shear span of beams. 2-2- Proportion of mixed design, SCC mixture with high strength and normal concrete

The proportion of mixed design of SCC mixture with high strength used in this research has been determined with mixture test according to complete reports of different projects.

The ratio of mixed design of mixtures NC, SCC60 and SCC80 in one cubic meter has been shown in table (1).


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Table 1- Proportion of mixed design of Normal Concrete and SCC mixture

Amount of materials consumed in different concretes (kg/m3) Proportion of mixed design

(kg/m3) NC SCC60 SCC80 Cement 450 450 495

Micro silica powder __ 50 55 Lime stone powder __ 150 165

Size of aggregate 6-20 mm 1002.5 862.5 817.9 Size of sand 0-6 mm 693.5 663.5 629.1

Effective water 204 165 177 Super plasticizer (Viscocrete 1) __ 9 11

Ratio of water to cement 0.47 0.37 0.36 Ratio of water to powder __ 0.26 0.25

SCC mixture with high strength containing lime stone powder and micro silica powder

which increase the bounding and flow ability and the limitation of heat production, generally this type of material shows less reaction to the cement and can reduce the problems due to slump of concrete. 2-3- Properties of fresh SCC

The method used for the investigation of properties of fresh SCC, Slump flow ability is L shape box. Slump test including the determination of average diameter of concrete specimen distributed on a base plate which after the slump test has no compaction.

This test will investigate the concrete deformability subjected to its weight against the friction of the surface of base plate without any external force. Due to the viscosity nature of SCC, the measurement of slump had taken place when a considerable displacement was not observed, in this test, also time of T50 was also measured. T50 is the time of 500mm longitudinal distribution on the base plate.

The size of L shape box is shown in figure (1). The L Shape box is used to measure different properties such as filling ability, settlement and separation of aggregate. Vertical part of box would be filled with 12.5 liter petrol and remain in this position for one minute which allow the separation of aggregate take place, after that the gate will open and concrete from vertical part will flow to the horizontal part through the bars.

The heights of h1 and h2 of concrete have been measured and to determine (h1 / h2) L shape box had been used [2].

Figure 1- L Shape Box test

2-4- Outline of test In this investigation 9 beams with equal dimensions were tested under static loading. The

length of all beams were 140cm with span length of 125cm (L=125cm), the width of cross


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section of these beams were 18cm (b=18cm), the height of cross section were 20cm (h=20cm) the effective depth 16cm (d=16cm) and the ratio of length of shear span to the effective depth are 2.5 (a/d=2.5), as shown in figure (2).

Beams made of normal concrete were reinforced with (3 Φ 16) as tension rebars and (2 Φ 10) as compression rebars, and also beams made of SCC with high strength which had strength of 60 and 80Mpa, each of them were reinforced by (3 Φ 16 and 3 Φ 18) for tensile area and (2 Φ 10) for compressive area.

Figure 2 – Details of RC beams used in test Stirrups alongside the shear span with 6mm diameter used with different spacings of 80,

100 and 133 mm for each group of test specimens. The test of all the beams carried out 28 days after the casting of concrete. Before the load

is being applied on the beams, in order to determine the cracks directions, the surface of the beams would be covered with white paint and that the place of LVDT on the center of beam,


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and the points related to the measurement of strain gage were determined and marked. After placing the beams on the supports, we made sure about the right position of supports

and the place of applied load. Figure (3) shows general design of loading framework and Figure (4) shows the picture of model of beams with position of applied load and position of buttons of strain gauges. As we can see total load was applied to the beams with a rigid steel beam as a two concentrated load calibrated load cell were positioned between jack and rigid steel beam.

Figure 3- Loading framework

Figure 4- The type of applied load on the tested beams and position of strain gauges It should be mentioned when we address the failure load, this is the same as the total

applied load from the load cell, which initially applied to the rigid steel beam and through it were applied to the concrete beam as a two concentrated symmetrical load. Strain gages B and C are placed in the area of shear spans on the left and right of beams respectively.

3- TEST RESULTS

3-1- Test results on the fresh concrete of SCC mixture In order to control mixed design of high strength self compacting concrete it was necessary

to carry on slump test and L-box test according to European of Japans code. Test results show the properties of fresh SCC mixture with high strength and NC in table (2). SCC mixture with high strength has more fluidity and application in compare with normal concrete mixture. Normal concrete mixture was evaluated by normal slump test and their fresh properties can not be compared with the same properties in SCC.

Due to main difference of SCC mixture properties they have a high fluidity and ability. The ratio of volume of large aggregate in reference mixture is considerably higher than the SCC


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mixtures. SCC mixture with high strength is designed to have more filler; also SCC mixture has

more cement mortar and less large aggregate, which is very effective to increase its ability. By the way the increase in the fluidity is subjected to the effect of super plasticizer Viscocretel.

Table 2- Test results on the fresh concrete

Experimental Test Group NC SCC60 SCC80 Slump flow ability (mm) 83 680 710 Slump flow ability T50(Sec) -------- 3 4

L Shape Box Ratio h2/h1 -------- 0.95 1 The amounts o fluidity of slump in SCC mixture with high strength is equal to 680 and

710mm respectively, and also the amounts of T50 of SCC mixture with high strength is 3 and 4 second respectively, the results of L shape box SCC mixtures with high strength shows good fluidity and deformability, and the ratio of h2/h1, was more than 0.8, which the amount of 0.8 would be considered as a smaller critical state [2].

The ratio of amount for SCC mixture with high strength was close to one. This high amount shows deformability between close barriers (bars) and high ability of fluidity of SCC concrete. 3-2- Compressive strength of samples

Cubic specimen for compressive strength with dimensions of 100×100×100mm were used one day after casting the specimen were cured n the water until the tests took place after 1,7 and 28 days. Figure (5) shows the increase strength until 28 days for SCC concrete with high and normal strength and ratios of relative increase in compressive strength and 28 day strength can be seen in figure (6).

As we were expecting compressive strength was highly subjected to the ratio of water to cement and used filler (Lime stone powder and Micro silica).

As SCC mixture with high strength in their early days had high strength which is due to the effect of rapid reaction of lime stone powder on the cement hydratation [2].

The results presented in Figure (6) shows that there is no considerable difference in the curve of increase of strength growth in non of studied mixture.

Figure 5-The development of compressive strength in normal concrete and SCC concrete


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Figure 6- Compressive strength in different ages relative to the 28 days standard strength

3-3- Indirect tensile strength results

Indirect tensile strength in 28 days on the cylindrical specimen with the diameters of 100mm and 200mm were investigated and the specimen was cued under the water. The comparison for indirect tensile strength for 28 days between high strength mixture and normal concrete has been shown in Figure (7). It has been observed that indirect tensile strength of SCC mixture with high strength is higher than NC concrete, also shows that compressive strength is almost similar to the indirect tensile strength in the SCC concrete with high strength and normal concrete.

Figure 7- Indirect tensile strength (Splitting test) SCC and Normal mixture

3-4- Load – reflection relationship (The experimental curves) We can evaluate the rigidity and reflection and deformation beams. Theses curves are

shown in Figure (8). In the beginning of loading under small loads all the beams were uncracked. By the

increase in loading the crack induced in mid span. Figure (8) shows the effect of using the SCC concrete with high strength on the reaction of load-reflection of beams with NC normal concrete in corresponding distanced stirrups.


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Figure 8- Experimental load-reflection curve in beams with normal concrete and high strength SCC

In this figure it is obvious that using SCC concrete with high strength will increase of slope

in load-reflection curve in compare with normal concrete. SCC beams with high strength have higher rigidity and lower displacement in compare with normal concrete under the same loading (this will say about the ability of absorbing more energy in SCC beams with high strength in compare with normal concrete beams). In load-reflection curve SCC beams with high strength in the linear proportional limit a considerable failure will occur, or by other terms when concrete stress reach the maximum point of stress-strain curves, due to combination of shear and compressive stress a sudden failure will occur. In SCC80 beam with the distance of 80mm between stirrups in addition to increase the strength, we can reach to more rigidity and ability to energy absorption. On the other hand, beam with normal concrete and stirrup spacing of 133mm has the minimum rigidity between tested beams.

As we were expecting, the results show that the reduction of distance between stirrups in each groups will lead to increase of slope in load-reflection curve. It is clear that the reduction of stirrups spacing will lead to the increase of rigidity and reduction of beams deflection under the identical loading. The results also show that the reduction of distance of stirrups in each group will restrict the diagonal cracking development and will increase the ductility of beams.

3-5- The investigation of behavior of load-strain at positions B and C

Strain gages B and C were considered to investigate the effect of type of concrete, stirrups spacing, and the strength of concrete on the opening of diagonal cracks. This should be mentioned the strain gage at position B and C are placed with angle of 45o to the longitudinal axes of beams, regarding this fact that, the mentioned strain gage were placed in the shear area.

The effect of using SCC with high strength, the distance of stirrups, and the strength of concrete are relatively considerable. Figures (9 and 10) show the effect of using SCC with high strength in compare with normal concrete on the load-strain curves related to beams to position B and C of having different stirrups spacing.


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Figure 9- Load-Strain curve related to position B with normal concrete and high strength SCC

Figure 10- Load-Strain curve related to position C with normal concrete and high strength SCC

It is clear that all SCC beams with high strength and normal concrete have similar behavior

in load-strain curve related to position B and C. in load-strain curves related to position B and C we can see that all beams in the beginning of loading, from rigidity point of view had similar behavior. By increasing the ultimate load when it reaches to ultimate capacity of beams failure occurs, which will show the opening of diagonal craks induced in shear spans related to B and C positions.

We can see this failure will occur with delay in SCC beams with due to its rigidity and higher compressive strength. After the diagonal cracks occur, shear bars will begin to act and will increase the shear strength by direct transfer of shear force and in the end diagonal cracks developed towards the compressive and shear stresses were crushed.

By comparing the load-strain curves of beams made of normal concrete and SCC with similar distance of stirrups we can see that SCC beams due to compressive strength and more rigidity will show more slope in load-strain curves related to position B and C relative to beams with normal concrete. Under identical loading, the diagonal cracks in SCC80 beam with stirrups spacing of 80mm, has the lowest width relative to other beams. By using SCC with high strength in beams, this is a reason to reduce B and C strain in compare with beams with normal concrete with similar stirrup spacing.

3-6- Cracking behavior and the cracks pattern

The results shows that using SCC with high strength in compare with normal concrete, has influence on the time of creation of first crack (shear and flexural) and beter performance of beams under service load. The results in table (3) shows, the average cracking moment in


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each group studied in this research for all the beams in low load, when tensile stress in concrete is less than tensile strength (modules of rupture) all section is effective in resisting of flexural moment, but with gradual increasing of applied load, cracking in flexural area of beams, mainly vertical cracks due to pure bending moment created. Cracking outside the pur flexural zone initiated with flexural cracking, but with increasing load other cracks due to shear stress were induced. This type of cracks appeared to be diagonal cracks, after that cracking developed toward concentrated load, which located at the compression face of the beam until this continuous shear cracks causes collapse of the beams.

Table 3- Load and average cracking moment of each group

Specimens Cracking loda (Ton) Cracking moment (Ton) NC 2.85 0.57

SCC60 3.85 0.76 SCC80 4.45 0.88

By comparing the cracking behavior in the tested beams we will reach to this conclusion

that patterns and cracking model in all beams under the ultimate load are the same. In SCC beams with high strength, the number of shear cracking relative to the normal concrete beams is higher. That is why using the SCC with high strength in beams will lead to a beter behavior in time of use and the initiation of first shear and flexural cracks and the reduction of width of cracks under the identical cracks. The type of cracking in beams in this research subjected to shear ultimate load are shown in figures (11 to 19).


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Figure 11- The type of cracking and test- of NCS80 beam



Figure 14- The type of cracking and test- of SCC60C80 beam

Figure 15- The type of cracking and test- of SCC60C100 beam

Figure 16- The type of cracking and test- of SCC60S133 beam





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3-7- Cracking moment By the gradual increase of load on the beam which will lead to the failure of beam,

different stages of behavior of beams are recognizable. In less load, when the tensile stress in concrete are less than tensile strength, in this state, both materials concrete and steel will behave as an elastic (linear) and in this stage variation of strain over the depth of beam is linear, and both material concrete and steel will follow Hooks law (stress is proportional to strain). By the increase of load, concrete will reach to tensile strength and in this stage tensile cracks will be formed in the tensile area. Finally with increasing load shear force in the beam reaches to the shear capacity of the beam. Regarding the assumptions related to the elastic analysis of cross section, for the calculation of stress in steel and concrete, we will use the idea of transformed cross section.

Transformed section includes total concrete section and n Time steel cross section area. (n) is the ratio of modules elasticity of steel to modules elasticity of concrete, which is known as modular ratio (n=ES/EC)[5].

Theory cracking moment is obtained from bending of homogeneous beam formula:

y

IFr

′=crM (1)

In which : fr: fr is modules of rupture of concrete and tensile stress of concrete is limited by modules of rupture of fr. I : I is effective the moment of Inertia of section which is equal to moment of Inertia of transformed uncracked section. y/- y/ : y/ Is the distance between ultimate tensile layer of cross section and surface center [5].

In experimental calculations, moment of Inertia of uncracked section can be assumed equal to moment of Inertia of Total cross section (Ie . Ig). As a result the cracking moment according to Iranian code instead of equation (1) will suggest a simpler equation:

tr

yIF g

Mcr = (2)

Modules of rupture of concrete (tensile flexural strength of concrete) in above equation will be used by this equation:

cr ff 63.0= (3) In which :

fc : fc is compressive strength of concrete (N/mm2) fr : modules of rupture of concrete (Tensile flexural strength of concrete) yt : The distance of tensile external fiber till neutral axis of concrete cross section (mm)[5].

ACI code for the modules of rupture of concrete with normal weight will suggest:

cff r 2= (4)

Which is the compressive strength ( 2kg cm )[4].

Experimental results and theory moment in the initiation of cracking are shown in table (4) Table (4) shows that prediction of Iranian code and ACI for the evaluation of cracking moment in each groups of NC and SCC with high strength concrete are conservative relative to the experimental results.


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We can see that this difference for the SCC beams with high strength concrete in compare with normal concrete is higher.

Table 4- Theory and experimental results of cracking moment in each group

Beams in the tested groups

Equation (1) and (4) (t.m)

Equation (3) and (4) (t.m)

ACI code (t.m)

Average results of test

NC 0.427 0.414 0.429 0.57 SCC60 0.608 0.586 0.592 0.76 SCC80 0.728 0.641 0.643 0.88

ACI code will suggest another equation for the high strength which is:

cff r 97.2= (5)

In above equation concrete strength is 2kg cm [4]. In table (5) cracking moment

obtained from equation (5) is shown for a comparison with cracking moment obtained from test of SCC beams with high strength concrete.

The prediction of ACI code for the evaluation of cracking moment of beams made of high strength concrete, are more accurate in compare with other method. That is why the equation suggested by ACI code for the estimation of cracking moment in high strength concrete can be used for SCC concrete with high strength.

Table 5- Cracking moment of SCC beams with high strength by using equation (5)

Beams of tested groups ACI code Average results of

test SCC60 0.878 0.76 SCC80 0.955 0.88

3-8- Ultimate shear strength

In addition to prepare the flexural strength, reinforced beams must be strengthened against the creation of diagonal tensile cracks which is the result of the combination of shear stress and vertical flexural stress. Often reinforced beams in area which shear force is excessive, would be reinforced by stirrups.

According to Iranian code, provided shear strength by the element subjected to shear and flexural would be calculated by this equation :

dbwcυ=cV (6) Which in above equation :

bw : The width web (mm) d : The distance of top (compressive) fiber to center of cross section of longitudinal bars (mm) vc : Resisting shear stress of concrete which will be calculated by this equation:

cc fΦ= 2.0Vc (7) Which , cΦ partial safety factor of concrete and it is equal to 0.6 suggested by Iranian

code. ACI code suggest for shear strength of concrete in this equation:


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dbc wf53.0Vc = (8)

Which is compressive strength of concrete 2kg cm. According to Iranian code shear

force resisted by vertical stirrup can be calculated by following formula :

sdfAv yvss Φ= (9)

In this equation , is partial safety factor for steel strength which is equal to 0.85 [5]. ACI code, give the following formula for calculation of ultimate shear related to vertical stirrups:

sdfAv yvs = (10)

In table (6) ultimate shear strength obtained from ACI and ABA and the experimental results are presented [6].

Table 6: Experimental and analytical results ultimate shear strength for the tested beams

Beams

Ultimate shear strength obtained from Iranian code

(ton)

Ultimate shear strength obtained from ACI code

(ton)

Ultimate shear strength obtained from test result

(ton) NCS80 4.65 5.88 9 NCS100 4.15 5.31 8 NCS133 3.65 4.75 7

SCC60S80 5.47 6.98 11.85 SCC60S100 4.97 6.41 10.75 SCC60S133 4.47 5.85 9.6 SCC80S80 5.78 7.34 13.25 SCC80S100 5.28 6.77 12 SCC80S133 4.78 6.21 11

Table (6) shows that the evaluated amount by the Iranian code and ACI for SCC beams

with high strength compared to the results of test are conservative. (ACI Code – Ultimate shear strength predicted by ACI code shows closer result to experimental results compared to Iranian code of practice) This could be due to the lack of experimental data and scientific experience for the Self Compacting Concretes with high strength.

4- Conclusion - By using SCC with high strength in addition to the increase of ultimate shear strength in

beams, it can reach to more rigidity and the ability to absorb energy in compare with normal concrete.

- By keeping constant to cross sectional longitudinal bars and spacing of stirrups using the SCC will lead to the increase of cracking strength and ultimate shear strength in beams. This means that SCC with high strength has influence on the time of creation of first crack.


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- By reducing the stirrups distance, under the identical loading, strains, reflection of beams will reduce. Under service load, these influences are more effective and beter, because in addition to reduce the cracks width, deflections and shear strength will increase. - In service load, more wider cracks in beams with normal concrete in compare with SCC beams with high strength in corresponding stirrup spacing under identical load could be related to compressive strength of concrete.

- By comparing the cracks pattern in ordinary reinforcing concrete beams and high strength SCC, it can be observed that patterns and models for cracking in all beams under ultimate load are the same.

- Cracking moment of high strength reinforced concrete beams predicted by ACI code could be used high strength reinforced SCC beams.

REFERENCES [1] Okamura, H, "Self-compacting High-Performance Concrete", Concrete International, pp.50-54

(1997). [2] M. Sonebi, P.J.M. Bartos, W.Zhu, J.Gibbs, A. Tamimi, "Properties Of Hardened concrete",

advanced Concrete Masonry Centre, University Of Paisley, Scotland, United Kingdom. [3] Pera, J., Husson, S., and Guilhot, B. 'Influence of finely ground limestone on cement hydration',

Cement & concrete Composites, 1999 (21), 99-105. [4] Building code requirements for structural concrete (ACI 318-02) and commentary (ACI 318 R02) [5] Designing of Reinforced concrete buildings according to ABA, Sh. Tahooni, Tehran, Tehran

University, Institue of Publication. [6] H. R. Ebrahimi, Th investigation of shear behavior and cracking of reinforced concrete beams

made of SCC with high strength, Msc Thesis, Mazandaran University September 2004.

the investigation of shear behavior of r.c. beams...

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