Shear strength of concrete beams reinforced with steel bars milled from scrap metals

MATERIALS & DESIGN, 27(10), 928-934

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Shear strength of concrete beams reinforced with steel bars milled from scrap metals

Charles K. Kankam, Mark Adom-Asamoah

Department of Civil Engineering, University of Science and Technology,

School of Engineering, Kumasi, Ghana

Abstract The paper reports a study on the shear resistance of concrete beams reinforced with mild steel

bars that are milled from scrap metal such as old vehicle parts and obsolete machinery. It has

been previously reported that because the chemical compositions of carbon, sulphur and

phosphorus in these reinforcing steel bars exceed the maximum allowable limits, the

characteristic tensile strengths are too high and ductility too low for standard mild steel.

Concrete beams reinforced with such bars to resist flexural tensile and shear stresses were tested

under a two-point loading system to provide a central constant moment region and outer shear

spans. Tested beams exhibited little deflection and very low ductility prior to collapse.

Experimental failure loads for the beams aver-aged 123% of the theoretical failure load, which

was generally governed by either shear or yielding of the tension steel. Shear failure was mostly

initiated by diagonal tension cracks, followed by either crushing of the concrete, or splitting of

the concrete over the longitudinal tensile bars near the supports. Failure of the beams was brittle

and the post-cracking strain energy absorption averaged 357.9 Nm. At failure the maximum

crack width in the beams ranged from 1.12 to 5.0 mm, the largest sizes forming in the diagonal

shear cracks.

Keywords: Shear strength; Steel bars; Concrete; Ductility; Crack; Deflection


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1. Introduction

Reinforced concrete, one of the most widely used construction materials in modern times, is a

composite material of steel bars that are embedded in a hardened concrete matrix. Concrete has a

high compressive strength, but is very weak in tension with a tensile strength being of the order

of 10% of the compressive strength. Rein-forcing steel bars to enhance the load-carrying

capacity of the structure must resist the tensile stresses that develop in a structural concrete

member due to flexural, axial or shear forces. Reinforcing steel bars for concrete construction are

generally of two types, namely, mild steel of nominal strength 250 N/mm2, and high-yield steel

from 425 to 460 N/mm2. Standard low carbon mild steel exhibits relatively large ductile

deformations compared with high-yield steel. However, it has been found in an earlier study that

mild steel bars milled from metal scraps in Ghana generally have a tensile strength far above the

standard 250 N/mm2 and very low ductility because of their high carbon, sulphur and

phosphorus contents [1].

Such high strength, low ductility mild steel is undesirable in reinforced concrete structures

because it defeats the philosophy of under-reinforced design as no warning of any impending

failure of the structure will be given by way of excessive deflections and cracks. However, the

bond strength of these bars that are generally milled with surface lugs is high and of the same

order as that of high-yield ribbed bars, but much greater than for normal mild steel bar [2].The

present study investigated the shear capacity of concrete beams reinforced with steel bars that

were milled from scrap metals. Shear stresses accompany a change in the bending moment in

beams and give rise to diagonal tension in concrete and also bond stresses between the

reinforcing bars and the concrete. Shear stresses in reinforced concrete beams without adequate

shear reinforcement produce cracks at inclined planes near the support and may cause collapse of

the beams.

2. Shear resistance of reinforced concrete beam

The mechanism of shear transfer in a cracked concrete beam acted on by a shear force V is

illustrated in a free-body diagram of Fig. 1. An element in the beam will be subjected to shear

stresses and to horizontal nor-mal stresses due to bending as shown in Fig. 2. Near the neutral


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axis of the flexural cracked region where the bending stresses are comparatively small, the

element may be neglected without serious loss of accuracy. The shear stresses then are

equivalent to the principal stresses which are traditionally called diagonal tension. Prior to

diagonal cracking the external shear force V produces practically no stress in the shear/web

reinforcement. When the diagonal crack forms any web bar that intercepts the diagonal crack

suddenly carries a portion of the shear force V. Web bars that do not intercept any cracks remain

essentially unstressed.

An equilibrium equation for shear in a cracked beam may be expressed as

(1)

Where Va is the shear resistance of the aggregate inter-lock across the diagonal crack; Vcz is the

shear carried by the uncracked concrete in compression; Vd is the shear carried by the dowel

action of the longitudinal reinforcement; Vs represents the shear force carried by the web bars

crossed by the diagonal crack (Fig. 1). As the external shear V is increased in a typical beam the

web steel yields so that Vs remains constant at the yield value and subsequent increase in the

shear force V must be carried by Va, Vcz and Vd. As the diagonal crack widens Va becomes less

effective since aggregate interlock reduces. Vcz and Vd have to take up the extra load and they

therefore increase rapidly [5]. Failure of the beam therefore occurs in one of two ways:

1. Dowel splitting of the concrete along the longitudinal reinforcement, or

2. Crushing of the concrete in the compression zone resulting from the combined shear and

direct stresses.

In providing shear resistance by allowing re-distribution of external forces across any inclined

crack that may form, the shear reinforcement has the following primary functions:

1. Carry part of the shear;

2. Resist growth of the inclined crack and thus help maintain interlock (that is, interface

shear transfer);

3. Tie longitudinal bars in place and thereby increase their dowel capacity.

In addition to the above primary functions dowel action on the stirrups may transfer a small force

across a crack and the confining action of the stirrups on the compression concrete may slightly

increase its strength. Little shear reinforcement may cause it to yield immediately at the


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formation of the inclined crack and the beam fails. Excessive shear reinforcement on the other

hand leads to shear compression failure before yielding of the web steel. The optimum amount of

shear reinforcement is such that both the shear reinforcement and the shear compression zone

continue to carry increasing shear after the formation of the inclined crack until yielding of the

shear reinforcement, thus ensuring ductile failure.

The following assumptions are made in the design of reinforcement for diagonal tension [3]:

1. diagonal tension is equal to shear at section;

2. diagonal tension crack is inclined at 45L to the longitudinal axis of the member;

3. crack extends over maximum horizontal distance equal to the effective depth of the beam;

4. normal tension is zero.

Fig.1.Shear transter in beam with web reinforcement

Fig.2. An element in one-half reinforced concrete beam


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3. Experimental programme

3.1. Materials and specimens

Samples of steel bars were obtained from three steel manufacturing companies in Ghana,

namely, Tema Steel Works, Wahome and Ferro Fabric for the experimental study. The chemical

and physical properties of the steel have previously been reported [1]. Concrete beams were

reinforced with different percentages of shear and tension reinforcements from the different

millers to investigate the shear capacity of these bars in concrete. The concrete consisted of

ordinary Portland cement, natural river sand and crushed granitic rock of 12 mm maximum size

in different proportions and different water–cement ratios. The shear reinforcement was formed

from 6 mm vertical steel stirrups along the entire length of the beam while the main tension

reinforcement consisted of 12 mm bars. The concrete for the beams and control cubes was

mechanically mixed in a paddle mixer, placed and compacted by means of a shutter vibrator.

They were cured at 100% relative humidity and approximately 25 LC room temperature for 28

days. The details of the beams are presented in Table 1.

Table 1: Description of beams

Beam

no. B x D Span/eff shear span/eff Percentage of steel Concrete strength

Steel

strength

depth

ratio depth ratio Tension Compression fcu (N/mm2) fy (N/mm2)

B1 140 x 230 6.9 1.72 1.19 0.20 26.42 390

B2 140 x 230 6.9 1.72 1.19 0.20 20.83 390

B3 140 x 230 6.9 1.72 1.19 0.20 28.45 340

B4 140 x 230 6.9 1.72 1.19 0.20 26.17 340

B5 140 x 230 6.9 1.72 1.19 0.20 26.4 490

B6 140 x 230 6.9 1.72 1.19 0.20 20.28 490

B7 100 x 230 8.7 2.31 1.78 0.30 26.42 390

B8 100 x 230 8.7 2.31 1.78 0.30 26.17 390

B9 100 x 230 8.7 2.31 1.78 0.30 20.83 340

B10 100 x 230 8.7 2.31 1.78 0.30 20.28 340

B11 100 x 230 8.7 2.31 1.78 0.30 20.28 340

B12 100 x 230 8.7 2.31 1.78 0.30 28.45 490


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3.2. Test procedure

The beams were simply supported at their ends on steel beams that formed part of a rigid steel

frame. A two-point symmetrical loading was applied through a spreader beam that formed part

of the loading system to produce a constant moment in the central span of the beams. Deflections

at mid-span were measured using a dial gauge reading to 0.01 mm mounted beneath the beam.

Crack width was measured on the concrete surface using a crack microscope reading to 0.02

mm. Load was applied in increments, and at each load increment all required measurements were

taken.

4. Flexural theory

For a simply supported beam that is subjected to a two-point symmetrical loading system with a

constant moment in the central region, the ultimate flexural load Pult is given by

(2)

where Mult denotes the ultimate moment of resistance; x is load per unit length due to self-weight

of the beam; L is span of beam; z is distance from point load to nearest support.

5. Shear strength

In accordance with the British Standard BS 8110: 1985 method of design [4], the theoretical

shear strength of the beam was calculated by considering the concrete section, the tension steel

reinforcement and the steel stirrups.

6. Theoretical and experimental results

6.1. Material properties of beams

Details of the concrete and reinforcing steel bars used in the investigation are given in Table 1.

The tensile strength of the steel was obtained from tensile tests per-formed on samples of the

steel that were obtained from the three manufacturers in Ghana [1].


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6.2. Behaviour of beams

The cracking, theoretical and experimental failure loads of the 18 beams are presented in Table

2. The theoretical failure load was calculated by considering: (i) yielding of the steel in tension;

(ii) crushing of the concrete in compression; (iii) shear failure. Figs. 3–5 show typical load–

deflection curves for three beams that were continuously loaded to failure. The post-cracking

strain energies of the beams are presented in Table 4.

Table 2 : First crack and failure load

Beam

no.

First

crack Experimental

Theoretical failure load Pult0

(KN) Pcr Pult

load failure load steel Concrete Shear Pult P'ult

Pcr(KN) Pult(KN) yieding crushing failure

B1 16 106.0 105.1 144.7 95a 1.12 0.15

B2 16 102.0 99.6 116.0 94.4a 1.08 0.16

B3 32 149.1 99.3 153.8 92.6a 1.61 0.21

B4 16 102.0 97.8 142.1 91.1a 1.12 0.16

B5 12 102.0 130.7 147.3 111.4a 0.92 0.12

B6 24 94.8 129.1 115.8 110.3a 0.86 0.25

B7 6 64.8 70.9a 74.2 83.4 0.91 0.09

B8 16 70.6 70.7 73.6 56a 1.26 0.23

B9 16 88.3 61.6 58.9 78.4 1.5 0.18

B10 10 62.8 61.0 57.5 53a 1.18 0.16

B11 8 90.3 87.2 81.3 69.6a 1.3 0.2

B12 18 66.7 84.3 76.1 63.9a 1.04 0.27

B13 10 48.0 32.4a 41.4 182.8 1.48 0.21

B14 6 46.0 31.7a 39.4 182 1.45 0.13

B15 7 42.0 33.3a 44.0 184 1.26 0.17

B16 6 42.0 33.3a 38.6 181.6 1.34 0.14

B17 4 43.0 32.8a 42.4 183.2 1.31 0.09

B18 6 42.5 31.1a 38.0 181.6 1.37 0.14

7. Discussion of test results

The load–deflection curves show a linear relationship prior to cracking, and a nonlinear response

after cracking of the beams. The relatively short range of deflections of the beams prior to failure

indicates the low ductile behaviour of the steel. The experimental failure loads Pult averaged


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approximately 120% of the theoretical failure load P 0ult. The cracking loads Pcr averaged 17%

of the experimental failure loads. Theoretically, failure of beams B1–B6, B8 and B10–B12 was

expected to be through shear, B9 by crushing of the concrete, B7, B13–B18 by yielding of the

steel in tension as presented in Table 2. The shear strength of beams B13–B18 was

approximately four times greater than their strength in compression or tension. While beams B1–

B12 (including B7 and B9) failed in diagonal tension which was characterised mostly by

splitting of the concrete over the horizontal tension steel bars and crushing of the concrete in

compression, failure of beams B13–B18 was predictably preceded by the yielding of the tensile

steel bars and crushing of the concrete in compression after flexural cracks had extended deep

into the compression zone.

Due to the low ductility of both main and web steel bars, the failure of the beams through

diagonal tension with yielding of the web steel was not preceded by large inelastic and plastic

deformations (Figs. 3–5). The post-cracking strain energy of the beams ranged from 206.4 to 810

Nm, and these values indicate low absorbed energies. Crack widths varied from 1.12 to 5.0 mm.

The largest cracks were produced in the beams that failed in diagonal tension.

Fig. 3Load-deflection curve for B4 Fig.4 Load-deflection curve for B12 Fig.5 Load-deflection curve for B13


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8. Mode of failure of beams

Table 3 shows the theoretical and actual modes of failure, number and maximum width of cracks

in the beams. The mode of failure of the beams was influenced by the concrete strength, ratios of

main and web steel, and the shear span–effective depth ratio (av/d) of the beams, which varied

from 1.72 for beams B1–B6, 2.31 for B7–B12 and 6.25 for B13–B18. With reference to Fig. 6, a

flexural crack a–b propagated towards the loading point as the load P was increased, gradually

becoming a flexural-shear crack (or diagonal crack a– b–c). As P further increased failure

occurred in one of two modes [6]:

1. For relatively low av/d ratios as in beams B1–B12, the diagonal crack would stop at point

j and a number of cracks developed randomly in the concrete around the longitudinal

tension reinforcement. The diagonal cracks mostly formed independently and not as a

result of flexural cracks. The beams remained stable after such cracking. As the load was

increased the diagonal crack widened and propagated along the level of the longitudinal

tension reinforcement. The increased shear force pressed down the longitudinal steel and

caused a destruction of the bond between the concrete and steel leading to splitting of

concrete along a–h. The diagonal cracks also propagated into the concrete compression

zone of the beam and the loading point and eventually crushed the concrete.

2. A high av/d ratio for a beam without adequate shear reinforcement would cause the

diagonal crack to spread rapidly to point e, resulting in collapse by splitting the beam into

two parts in a diagonal tension failure. Beams B13–B18 had a high av/d = 6.25 and were

provided with adequate shear reinforcement to resist diagonal tension failure. They,

therefore, expectedly failed in pure bending.


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Table: 3 Beams after failure

Beam Mode of failure Number and type of cracks Max. crack

no. Predicted Actual width (mm)

B1 Flexural shear Splitting shear bond 1 Diagonal+2 flexural+6 pure flexural 1.12

B2 Flexural shear Splitting shear bond 1 Diagonal+1 flexural+5 pure flexural 1.20

B3 Flexural shear Flexural shear crushing 2 Diagonal+2 flexural+9 pure flexural 1.86

B4 Flexural shear Flexural shear crushing 1 Diagonal+1 flexural+8 pure flexural 1.50

B5 Flexural shear Diagonal shear crushing 1 Diagonal+5 pure flexural 1.20

B6 Flexural shear Shear bond and crushing 3 Flexural shear+5 pure flexural 3.60

B7 Steel yielding Diagonal tension, shear bond 1 Diagonal+1flexural shear 5.00

B8 Flexural shear Flexural shear 1 Diagonal+5 pure flexural 2.00

B9

Concrete

crushing Concrete crushing, splitting shear 2 Diagonal+4 pure flexural 4.72

B10 Flexural shear Diagonal tension and crushing

2 Diagonal shear+1 flexural shear+11

flexural 4.00

B11 Flexural shear Shear bond and concrete crushing


flexural 3.50

B12 Flexural shear Shear bond and concrete crushing


flexural 2.50

B13 Steel yielding Tensile failure and concrete crushing 4 Pure flexural+ 6 flexural shear 1.15






Fig. 6 Diagonal Tension crack in beam


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Table: 4 Post-cracking deflection and energy absorption of beams

Beam Deflection at Deflection at dmax/dcr Post-cracking

no. first crack dcr (mm) failure dmax (mm) strain energy (Nm)

B1 0.25 6.00 24 292

B3 0.60 9.80 16.3 810

B4 0.60 8.00 13.3 465

B5 0.20 6.60 33 335

B6 0.30 6.20 20.7 330

B7 0.10 6.90 69 258

B8 0.20 6.00 30 206.4

B9 0.60 10.80 18 545

B10 0.30 8.50 28.3 306

B11 1.00 11.00 11 510

B12 0.30 7.20 24 264

B13 1.00 19.50 19.5 405

B14 0.50 13.50 27 285

B15 1.00 17.00 17 345

B16 1.50 18.00 12 235

B17 1.00 15.00 15 280

B18 1.00 14.00 14 220

9. Conclusions

The flexural shear resistance of concrete beams reinforced with tensile and web steel bars milled

from re-cycled obsolete machinery and old vehicle parts in Ghana were investigated. The steel

bars, which are mostly milled with surface lugs and classified by manufacturers as mild, have

rather low ductility and high tensile strength, and develop better bond with concrete than

standard round mild steel bars. The results of the study have shown that beams provided with

large web shear steel reinforcement developed high shear resistance and did not produce any

diagonal tension cracks. A little nominal amount of web steel in the beam failed to control the

formation of wide diagonal tension shear cracks. It showed very little ductile behaviour at yield

as the diagonal crack extended and widened, leading to brittle failure of the beam. For practical

purposes, therefore, concrete beams rein-forced with such bars with limited ductile behaviour

should be sufficiently provided with relatively large web steel reinforcement to control and

reduce the widths of diagonal shear cracks and prevent brittle shear failure.


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References

1. Kankam CK, Adom-Asamoah M. Strength and ductility characteristics of reinforcing

steel bars milled from scrap metals. J Mater Des 2002; 23(6):537–45.

2. Kankam CK. Bond characteristics of reinforcing steel bars milled from scrap metals. J

Mater Des 2004; 25(3):231–8.

3. Syal IC, Goel AK. Reinforced concrete structures. 3rd ed. Wheeler Publishers; 1976. p70.

4. British Standards Institution: Structural use of concrete. BS 8110: Part 1: 1985.

5. Taylor HPJ. The fundamental behaviour of reinforced concrete beams in bending and

shear. In: Proc. ACI-ASCE shear symposium. Ottawa, 1973 (ACI special publication SP

42). ACI Detroit; 1974. p. 43–77.

6. Kong FK, Evans RH. 3rd ed. Reinforced and prestressed concrete, vol. 28. London:

Chapman & Hall Publishers; 1990. p. 119–210.

Shear strength of concrete beams reinforced with steel bars milled from scrap metals

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