flexural behavior of high strength reinforced concrete … · 2019-01-31 · of steel, basalt, and...

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 10, Issue 01, January 2019, pp. 1147-1158, Article ID: IJCIET_10_01_106

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=10&IType=01

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

FLEXURAL BEHAVIOR OF HIGH STRENGTH

REINFORCED CONCRETE BEAMS

STRENGTHENED BY HYBRID FIBERS

Qais F. Hasan

Kirkuk Technical College, Northern Technical University, Iraq

Maan A. Al-Bayati

Civil Engineering Department, College of Engineering, Tikrit University, Iraq

Dler A. Al-Mamany

Kirkuk Technical College, Northern Technical University, Iraq

ABSTRACT

An experimental program is achieved to study the enhancement gained from adding

different volumetric ratios of hybrid steel, basalt, and glass fibers to high strength

concrete mixes, and to investigate the flexural behavior of rectangular reinforced

concrete beams produced from these fibrous mixes. Ten rectangular beams, which are

identical in geometry and reinforcement, are tested. The first one is a control beam,

group A consist of three beams strengthened by different ratios of steel fibers only,

group B consist of three beams strengthened by different ratios of hybrid steel-basalt

fibers, and group C consist of three beams strengthened by different ratios of hybrid

steel-glass fibers. Results and comparative studies expressed in the form of graphs and

tables show variable enhancement values in crack, yield, and ultimate loads capacities

for the tested beams. Standard high strength concrete cubes and cylinders test results

indicate that compressive strength for all the fibrous mixes is slightly worsened,

especially for hybrid steel-basalt fibers, due to the increase in matrix porosity whereas

the splitting tensile strength is enhanced. Generally, yield and ultimate load capacities

for most of the fibrous beams are reduced, especially for hybrid steel-basalt fibrous

beams due to the smooth texture of basalt fibers. In addition, adding glass fibers

hybridly with steel fibers enhances crack formation and propagation without a

noticeable reduction in yield and ultimate load capacities.

Key words: Hybrid fibers, High strength, Fibrous beams, Ultimate loads.

Cite this Article: Qais F. Hasan, Maan A. Al-Bayati and Dler A. Al-Mamany, Flexural

Behavior of High Strength Reinforced Concrete Beams Strengthened by Hybrid Fibers,

International Journal of Civil Engineering and Technology, 10(01), 2019, pp. 1147-

1158.

Qais F. Hasan, Maan A. Al-Bayati and Dler A. Al-Mamany


http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=01

1. INTRODUCTION

The concept of adding fibers to improve material properties is ancient [1], for example the

Mesopotamians used a straw to reinforce sunbaked bricks. Nowadays, fibers are produced in

many forms such as steel, glass, basalt, carbon, and others, however, steel fibers are the most

common one. Experiments using glass fibers have been conducted in the United States since

the early 1950's as well as in the United Kingdom and in Russia [2]. Enhanced spalling strength,

ductility, and toughness of concrete is found by adding 1.5 % volumetric ratio of steel fibers.

Using this percentage, different studies show that concrete compressive strength is enhanced

up to 15 % [3], 37 % [4], and 10 % [5]. Experimental results gained by [6] show that the

inclusion of basalt and glass fibers, separately, in concrete mix reduces the workability of this

mix. These studies observed that the splitting tensile strength of concrete increases up to 40 %

when adding 1.0 % basalt fibers, and no improvement in strength is observed after a dosage of

0.50 % glass fiber. Hybrid steel-polyethylene fibers are used in reinforced cementitious

composites by [7]. It is found that polyethylene fibers improve strain capacity while steel fibers

improve ultimate tensile strength. Hybrid polyvinyl-alcohol fibers are used to strengthen

mortar matrix with different types of light weight sand by [8]. It is observed that the ultimate

load capacities of the strengthened composites are almost the same irrespective of volume

fractions of light weight sand. The effect of adding hybrid steel-cellulose fibers on flexural and

direct shear capacities of concrete elements is studied by [9], while the impact of adding hybrid-

sized carbon fibers on the mechanical properties of Portland cement mortar is investigated by

[10]. Effect of adding different percentages of steel, basalt, and glass fibers separately on the

behavior of reinforced concrete beams, are presented in many researches [11-15], however, the

impact of adding hybrid fibers [16], by combining different types and volumetric ratios of these

three fibers, on high strength concrete compressive and splitting tensile strengths as well as on

beam flexural load-deflection behavior is still under discussion. In addition, lack of

experimental data available in literature for the behavior of high strength reinforced concrete

beams strengthened with different volumetric ratios of these hybrid fibers makes designers

reluctant to opt such fibers as a high strength concrete matrix additive. This study provides

designers with experimental database including tables, graphs, comparisons, and discussions

needed to support their decision in choosing the appropriate hybrid steel, basalt, and glass fibers

for high strength reinforced concrete beams strengthening.

2. RESEARCH SIGNIFICANCE

An experimental program is suggested in this study to give a database and to show and compare

the results and the enhancement gained from adding different volumetric ratios of hybrid fibers

of steel, basalt, and glass to high strength concrete mix through testing sixty standard concrete

cube for compressive strength, sixty standard concrete cylinders for splitting tensile strength,

and ten rectangular reinforced concrete beam specimens for flexural performance. Crack, yield,

and ultimate load-deflection capacities are to be denoted, compared, and discussed to illustrate

well the enhancement gained, in addition, cracks that developed and propagated at beams' faces

are to be followed and marked to show and explain the failure modes.

3. EXPERIMENTAL PROGRAM

To get high strength concrete a mix proportion of 1:1.5:2.8 for cement type (II/B-LL 32.5R),

natural river sand, and crushed stone of 10 mm maximum size, respectively, are used with

water/cement ratio of 0.48 and grade 50 super-plasticizer of 1.8 kg/m3 to produce adequate

Flexural Behavior of High Strength Reinforced Concrete Beams Strengthened by Hybrid Fibers


workability. Hooked-end shape steel fiber (SF), chopped basalt fiber (BF) and S-type glass

fiber (GF) shown in Figure 1, with the properties given in Table 1, are used hybridly for high

strength concrete mix strengthening.

Figure 1 Fibers used (a) Basalt, (b) Glass, and (c) Steel

Table 1 Specifications of the fibers used

Fiber

type

Diameter(µ

m)

Length(m

m)

Elasticity modulus

(GPa)

Specific gravity

(kg/m3)

Tensile strength

(MPa)

Basalt 20 12 89 2800 4100

Glass 13 12 77 2600 3400

Steel 550 30 200 7850 1500

Ten reinforced rectangular high strength concrete beam specimens of the same properties

and reinforcement details shown in Figure 2 are tested, following ASTM specifications [17]

under four points loading, and compared to study the enhancement gained from adding

different normal and hybrid volumetric ratios of the three mentioned fiber types. The first one

is a control beam (CB), as shown in Table 2. Group A consists of three beams strengthened

with different volumetric ratios of steel fibers. Group B consists of three beams, also,

strengthened with different volumetric ratios of hybrid steel-basalt fibers. Group C consists of

three beams, also, strengthened with different volumetric ratios of hybrid steel-glass fibers. In

addition, the data of sixty standard 100 x 100 x 100 mm3 concrete cubes sampled, compacted,

cured, and tested following ASTM specifications [18, 19, and 20], and sixty standard 100 mm

diameter by 200 mm length concrete cylinders sampled, compacted, cured, and tested

following ASTM specifications [18, 20, and 21] (as 6 cubes and 6 cylinders for each concrete

batch), are used to get both compressive (σ) and splitting tensile (fts) strengths for normal and

fibrous concrete mixes at 28 days age. To well visualize the failure modes and crack patterns,

the tested beams are painted with white emulsion and a grid of 50 mm x 50 mm lines is made

(c)



for each one, as shown in Figure 3. The cracks initiation is shown in alphabetic order and their

propagation is marked with multi-color pen and the total applied load is denoted at each stage

of crack propagation.

Figure 2 Geometry and reinforcement details of the tested beams

Figure 3 Sample beam under test machine

Table 2 Beams labeling and the corresponding fibers volumetric ratios

Group name Beam or batch label SF (%) BF (%) GF (%)

- CB - - -

A

SF0.5 0.5 - -

SF1.0 1.0 - -

SF1.5 1.5 - -

B

SF0.75BF0.25 0.75 0.25 -

SF0.5BF0.5 0.5 0.5 -

SF0.25BF0.75 0.25 0.75 -

C

SF0.75GF0.25 0.75 - 0.25

SF0.5GF0.5 0.5 - 0.5

SF0.25GF0.75 0.25 - 0.75



4. RESULTS AND DISCUSSIONS

4.1. Compressive and Splitting Tensile Strengths

Summary and comparisons of compressive strength and splitting tensile strength results for

high strength normal, and fibrous concrete cubes and cylinders are shown in Table 3. These

results show that the compressive strength for all the fibrous batches (σFRC) is decreased, in

general, when compared to control batch compressive strength (σCB), and a maximum reduction

by 23% is noticed when testing concrete batch SF0.25BF0.75 in group B when the basalt

contributes with steel to make a hybrid fibrous concrete mix. These results show that adding

basalt fibers hybridly with steel fibers has a deleterious effect on concrete compressive

strength, while adding steel fibers only (group A) has no noticeable effect. Although the cause

of reduction in compressive strength is not exactly clear, but poor interface of the fibers with

cement paste as well as the increase in porosity of the matrix due to fibers addition could be

the main contributing factors, especially because the concrete is of high strength type.

Splitting tensile strength is increased by adding different fibers, especially steel fibers, as

shown in Table 3. It means that fibers addition contributes well in enhancing bond

characteristics of concrete mix. Discrimination of steel fibers in this field makes them the better

choice when splitting tensile strength is a matter of concern. Maximum gain is noticed for batch

SF1.0, with 1.0% steel fibers, by 147% when compared to CB batch. Adding basalt fibers to

hybrid beam batches of group B shows the minimum gain in splitting tensile strength among

the other strengthened batches, and this is due do the smooth texture for basalt fibers which

weakens interlock forces between concrete materials, therefore, hybrid steel-basalt fibers may

be a worse option when the splitting tensile strength is needed to be enhanced. Hybrid batch

SF0.5GF0.5 results, at group C, represent the best fiber combination to enhance both

compressive and splitting tensile strengths. It suffered a reduction only by 4 % in compressive

strength and gained an increase by 128 % in splitting tensile strength compared to control beam

batch.

Table 3 Summary of compressive and splitting tensile strengths result

Group Batch σ (MPa) σFRC/ σCB Change in σ

(%) fts(MPa)

(fts)FRC/

(fts)CB

Change in

fts (%)

- CB 63.66 1 - 2.85 1 -

A

SF0.5 61.71 0.97 - 3 6.85 2.40 140

SF1.0 62.89 0.99 - 1 7.03 2.47 147

SF1.5 62.04 0.97 - 3 6.94 2.44 143

B

SF0.75BF0.25 50.37 0.79 - 21 3.66 1.28 28

SF0.5BF0.5 52.94 0.83 - 17 3.70 1.30 30

SF0.25BF0.75 49.15 0.77 - 23 3.89 1.36 36

C

SF0.75GF0.25 58.72 0.92 - 8 5.56 1.95 95

SF0.5GF0.5 61.09 0.96 - 4 6.51 2.28 128

SF0.25GF0.75 55.42 0.87 - 13 4.42 1.55 55



4.2. Beams Crack, Yield, and Ultimate Values

Crack load values and comparisons for all the tested beams, shown in Table 4, illustrate the

effect of adding different normal and hybrid fibers. In general, most of crack load capacities

are increased, with maximum increment by 60% for beam SF1.0 when compared to control

beam (CB), because of the enhancement done in tension zone due to fibers existence which

improves delay of crack formation and propagation through the beam depth. Adding hybrid

steel-glass fibers by different percentages in group C shows a noticeable enhancement in crack

load capacities, which means that this combination represents a good choice of hybrid fibers

when the crack load capacity is a matter of concern. On the other side, using and increasing the

percentage of the smooth texture basalt fibers in hybrid steel-basalt worsen the beam capacity

for crack loads, with a maximum reduction by 26% for beam SF0.25BF0.75. Post cracking

load behaviors shown in Figures (4-6) show different stiffness reductions for most of the

fibrous beams, and hence, reductions in their yield load capacities compared to CB. This is due

to the increase in porosity of these high strength concrete mixes, which led to reductions in

compressive strength, and hence, reductions in beams flexural strength. A valuable reduction,

by 24%, in yield load capacity is noticed for hybrid steel-basalt beam SF0.75BF0.25 at group

B, as shown in Table 4. A review of the yield load capacity reductions for beams of this group

indicates that the combination between steel and basalt fibers may be a worse choice for hybrid

fiber addition when the yield load capacity is required to be maintained. A 9% increment in

yield load capacity is noticed for beam SF1.0, which has also the maximum increment in crack

load capacity among all the fibrous beams. This fact makes the mix with 1.0% steel fiber has

the optimum gain in both crack and yield load capacities.

Table 4 Load-deflection values and comparisons for the tested beams

Group Beam Pcr

(kN)

change

in Pcr

(%)

Δcr(mm) change

in Δcr(%) Py(kN)

change

in Py(%) Δy(mm)

change in

Δy(%)

Pu

(kN)

change in

Pu (%)

- CB* 17.6 - 2.5 - 50.1 - 5.7 - 54.9 -

A

SF0.5 19.4 10 3.4 36 44.3 -12 6.5 13 50.5 -8

SF1.0 28.1 60 3.1 24 54.8 9 5.9 2 55.2 0.6

SF1.5 15.4 -13 2.5 0 50.3 0.4 6.5 13 52.6 -4

B

SF0.75BF0.25 18.9 7 4.8 92 38.1 -24 8.2 42 40.3 -27

SF0.5BF0.5 17.4 -1 2.5 0 45.7 -9 6.1 5 46.5 -15

SF0.25BF0.75 13.1 -26 2.6 4 44.5 -11 6.6 15 50.3 -8

C

SF0.75GF0.25 19.6 11 3.9 56 45.3 -10 7.4 29 48.0 -13

SF0.5GF0.5 21.9 24 3.0 20 52.4 5 6.5 14 55.8 2

SF0.25GF0.75 23.9 36 4.2 68 47.4 -5 7.2 25 50.0 -9

*Reference beam



Figure 4 Load-deflection curves for CB and group A beams

Ultimate load capacities for all the fibrous beams are also reduced in general. The worst

reduction is by 27% for beam SF0.75BF0.25 in group B, which has also the worst reduction in

yield load capacity. These reductions are accompanied with the maximum increments in both

crack and yield deflections, which make it relatively the worst choice for maintaining yield and

ultimate load and deflection capacities. The presence of basalt fibers, which affects concrete

compressive strength and slightly enhances splitting tensile strength as mentioned earlier,

worsens flexural capacities of the related tested beams. It is obvious from load-deflection

curves that the strain energy and the ductility ratio for the fibrous beams are, to some extent,

maintained except what is noticed when testing beam SF0.75BF0.25, which has significant

reductions in these two characteristics.

In the present study, all the hybrid volumetric fiber ratios have a sum of 1.0% for each

beam, therefore, another load and deflection comparisons are made for the hybrid beams

relative to the new control beam SF1.0 with 1.0% steel fiber, as shown in Table 5. As the steel

fiber percentage is reduced and basalt fibers are replaced hybridly the crack load capacities are

reduced, with a maximum value by 53% for beam SF0.25BF0.75, while increasing glass fiber

percentage at the expense of steel fibers has a praised effect on crack load capacity reductions

with a minimum value by 15% for beam SF0.25GF0.75. Yield and ultimate load capacities are

also reduced for all the hybrid fibers beams compared to SF1.0, and the maximum reduction in

yield capacity is noticed for beam SF0.75BF0.25 by 30%. Maximum reduction in ultimate load

capacity is noticed for beam SF0.75BF0.25, also, by 27%, which means that this hybrid fiber

combination represents the worst case in yield and ultimate capacities loss. Beam SF0.5GF0.5

gives yield and ultimate load capacities close to reference beam (SF1.0), and this makes the

replacement of steel fibers by glass fibers is useless in this case.



Table 5 Comparisons of hybrid beams relative to beam SF1.0

Beam Pcr (kN) change in

Pcr (%) Δcr(mm)

change

in Δcr

(%)

Py(kN) change

in Py (%) Δy(mm)

change

in Δy

(%)

Pu

(kN)

change in

Pu (%)

SF1.0* 28.1 - 3.1 - 54.8 - 5.9 - 55.2 -

SF0.75BF0.25 18.9 -33 4.8 55 38.1 -30 8.2 39 40.3 -27

SF0.5BF0.5 17.4 -38 2.5 -19 45.7 -16 6.1 3 46.5 -16

SF0.25BF0.75 13.1 -53 2.6 -16 44.5 -19 6.6 12 50.3 -9

SF0.75GF0.25 19.6 -39 3.9 26 45.3 -17 7.4 26 48.0 -13

SF0.5GF0.5 21.9 -22 3.0 -3 52.4 -4 6.5 12 55.8 1

SF0.25GF0.75 23.9 -15 4.2 35 47.4 -14 7.2 22 50.0 -10

*Reference beam

Figure 5 Load-deflection curves for CB and group B beams

Figure 6 Load-deflection curves for CB and group C beams

4.3. Crack Patterns and Failure Modes

Crack pattern of control beam CB, shown in Figure 7 indicates that the crack is initiated at the

tension face at beam mid-span. With load increment, other cracks are developed within the

middle third of the span and propagated successively. Crack patterns and failure modes shown

in Figures (7-10) indicate that failure of all the tested beams goes to be a flexural failure, and

this is done after yielding of main bottom reinforcement and before reaching ultimate load

which accompanied by successive compression concrete crushing. A quick review of fibrous



beams’ crack patterns, shown in Figures (8-10), illustrates that the formation of cracks is

delayed and the propagation of these cracks are reduced, and hence, all the fibrous beams are

enhanced compared to the control beam (CB). The crack patterns shown in Figure 8 for group

A, which has different volumetric ratios of steel fibers, indicate that the first main crack which

is formed in the middle of the tested beam still propagate with a limited formation of new

cracks. This is due to the enhancement in high strength concrete bond characteristics when

hooked-end steel fibers are added. The failure mode of this group is, also, done by yielding of

tension steel reinforcement followed by concrete compression failure with delamination of

multilayers due to enhanced tensile properties of fibrous high strength concrete.

Figure 7 Crack pattern and failure mode for the control beam (CB)

Beam SF0.5

Beam SF1.0

Beam SF1.5

Figure 8 Crack patterns and failure modes for group A beams



Beam SF0.75BF0.25

Beam SF0.5BF0.5

Beam SF0.25BF0.75

Figure 9 Crack patterns and failure modes for group B beams

Beam SF0.75GF0.25

Beam SF0.5GF0.5

Beam SF0.25GF0.75

Figure 10 Crack patterns and failure modes for group C beams



5. CONCLUSIONS

An experimental program is followed in this study and consisted of one hundred twenty

standard high strength concrete cubes and cylinders, in addition to ten reinforced concrete

beams, to investigate the behavior and the enhancement gained from adding normal and hybrid

steel, basalt, and glass fibers. Test results indicate reductions in the compressive strength for

all the fibrous concrete mixes compared to control mix (CB) in the time that steel fibers have

no noticeable effect. Splitting tensile strengths are increased for all the fibrous concrete mixes,

especially for steel fibers mixes. Hybrid steel-glass fibers mixes show better enhancement in

splitting tensile strength compared the hybrid steel-basalt fibers mixes. Results of load-

deflection curves indicate that most of crack load capacities of fibrous beams are increased,

especially when adding steel fibers only, which means better characteristics in cracks formation

and propagation for the fibrous beams, and the case is also when replacing a percentage of steel

fibers with glass fibers without a noticeable reduction in yield and ultimate load capacities.

These capacities are slightly worsened when a percentage of steel fibers is replaced by basalt

fibers because of the basalt's smooth texture which weakens concrete interlock forces

especially the concrete is of high strength type which is susceptible to any deficiency in its

interlock. To some extent, strain energy and ductility ratio for the fibrous beams are maintained.

Additional comparisons of the hybrid fibrous beams with 1.0% steel fibers beam show that

crack load capacities for hybrid steel-basalt fibers beams are reduced when adding relatively

high basalt percentages more than what noticed for hybrid steel-glass fibers beams, and this is

due to smooth texture for basalt fibers. Crack patterns and failure modes of all the tested beams

assure that the failure is of flexural type, by yielding of tension steel reinforcement followed

by concrete compression failure. In addition, cracks formation and propagation are enhanced

for all the fibrous beams. The enhancement in some fibrous mixes are extended to the

compression zone, which is clear when adding hooked-end steel fibers only. Extending the

present study to include more types of hybrid fibers, different beams cross sections, deep

beams, and time-dependent effects, are suggested as future works.

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flexural behavior of high strength reinforced concrete … · 2019-01-31 · of steel, basalt, and...

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