Bond of deformed reinforcing steel bars embedded in steel fiber reinforced concrete: State-of-the-art report

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Bond of Deformed Reinforcing Steel Bars Embedded in Steel Fiber

Reinforced Concrete – State-of-the-Art Report

Keverling Buismanweg 4 Postbus 69 2600 AB Delft 015-2693793 015-2693799 info@delftcluster.nl www.delftcluster.nl

Delft Cluster verricht lange-termijn fundamenteel strategisch onderzoek

Contact person : dr.ir. C.R. Braam

Date : October 2001

Author(s) : dr.ir. A.J. Bigaj-van Vliet

:

Project name : Toepassing van nieuwe materialen bij geboorde

betonen tunnels Project number : 01.06.03 Number of pages : 65 Number of tables : 66 Number of figures : 44 Number of appendices :

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-Projectgroep

Tijdens de uitvoering van het project bestond de Delft Cluster-groep van thema Grond en Constructie uit:

Naam Organisatie

Thema Trekker dr. P. van den Berg GeoDelft

Thema Duwer Prof.dr.ir. J. Rots TU Delft, Bouwkunde

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Betrokken personen

Bij de totstandkoming van dit rapport waren betrokken:

Naam Organisatie

dr.ir. A.J. Bigaj-van Vliet TNO Bouw

dr.ir. C.R. Braam TU Delft, CITG

Dipl.-Ing. P. Schumacher TU Delft, CITG

dr.ir. A.H.J.M. Vervuurt TNO Bouw

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Management Summary

Title

Bond of Deformed Reinforcing Steel Bars Embedded in Steel Fiber Reinforced Concrete – State-of-the-Art Report

Author(s) dr.ir. A.J. Bigaj-van Vliet

Date October 2001

Project number 01.06.03

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Management samenvatting

Titel

Bond of Deformed Reinforcing Steel Bars Embedded in Steel Fiber Reinforced Concrete – State-of-the-Art Report

Auteurs dr.ir. A.J. Bigaj-van Vliet

Datum October 2001

Project nummer 01.06.03

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1 Introduction

1.1 Problem statement

The use of steel fiber reinforced concrete (SFRC) in civil engineering structures has been widely discussed in recent years. In the early stage of development it was hoped that the tensile strength of the concrete could be increased significantly through the addition of fibers. However, only an insignificant increase in strength could be achieved with the amounts of fibers customarily used in practice. The load bearing capacity of reinforced and prestressed concrete proved not to be increased markedly by steel fiber addition within the practical range

i.e. a volume fraction Vf≤3%. With fibers, however, the behavior of solid structural

components was found to be improved as far as ductility is concerned, in particular in the zones where tensile stresses occur in the concrete. After initiation of cracks, the crack-arresting effect of fibers provides a redistribution of the stresses within the structure and consequently, the overall structural toughness is enhanced. Furthermore, fibers durably and noticeably improve the behavior under service loads: addition of steel fibers significantly reduces both deformation and crack width. Note, that when using the Eurocode 2, the amount of steel reinforcement is to a high degree determined by the serviceability limit state (SLS) checks [34]. Another major - and primarily industrial – incentive in using fibers is to reduce the production costs by shortening the construction time and, where possible, reducing the amount of conventional reinforcement.

An overview of the practical applications of SFRC shows that depending on the type of structure, the use of the steel fibers can either reduce the required amount of conventional steel reinforcement or in some cases replace it altogether, while maintaining satisfactory performance of the structure [33]. Steel-concrete composite floors, reinforced concrete floors supported by columns or walls and floors on an elastic foundations belong to the category of structural elements in which the conventional steel reinforcement can be partially replaced by the use of SFRC. In these cases the use of steel fibers is intended to reduce opening of creep and shrinkage cracks and to increase the speed of construction works. Steel reinforcement is still needed there to guarantee sufficient deformation capacity and load carrying capacity at the supports. Besides the traditional use of fibers for controlling cracks in e.g. slabs and toppings, examples can be given of fiber application for load-bearing purposes. Research affirms the possibility of using fibers for structural repairs, ductile beam-column connections [32] or shear reinforcement, e.g. in order to replace conventional (web) reinforcement in I-shaped girders [23]. Also in case of prefabricated tunnels, it is possible to eliminate conventional (bending) reinforcement if SFRC is used, provided that the bending moments remain low. However, with respect to force distribution in tunnel structures it is important to note that, under some geological circumstances or exceptional loading situations, it is possible to find sections where stresses due to bending dominate the stress distribution and even absence of a normal force is possible. In such cases it is not feasible to apply steel fibers as main reinforcement. Therefore, in order to provide a general structural solution for future tunnel planning it is often suggested to combine the best properties of both steel fibers and ordinary steel reinforcement. This approach results in a combination of steel fiber reinforcement and conventional steel reinforcement. In such case bending moments in the tunnel lining are resisted by the reinforcing steel, while splitting forces due to thrust jacking and coupling forces resulting from the girder behavior of the tunnel lining are withstand solely by the SFRC. In case of extrusion tunnels, a multiaxial state of stress is present: tension is the result of truss forces, bending results from soil settlements and compression follows from the soil (and water) pressure. Also

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The number of applications of SFRC increased over the past few years. However, SRFC can be efficiently and safely applied in a wide variety of structures provided that it is fully clear in which way the use of SFRC contributes to the resistance of the structure to the applied load, both in terms of load carrying capacity and deformability. Considering that frequently a combination of both types of reinforcement is applied (e.g. conventional steel reinforcement used as main reinforcement and steel fibers used as detailing reinforcement or for durability purposes), the behavior of structural elements, which combine both types of reinforcement, should be thoroughly investigated. If the design of civil engineering structures has to be cost saving, an optimum between both types of reinforcement has to be aimed at. This can be achieved by keeping the main reinforcement of a conventional type as simple and as effective as possible and supplementing it with steel fiber reinforcement added in adjusted quantities and tuned to specific load cases.

When discussing deformation capacity of structural elements or civil engineering structures manufactured using SFRC, one must be able to describe thoroughly both the behavior of the concrete matrix reinforced with steel fibers and the interaction between this composite matrix and discrete steel reinforcement of the conventional type. Yet, even though the knowledge on bond behavior is essential for evaluating the overall behavior of structural components containing reinforcement and steel fibers, information is hardly available in this area. Within the Delft Cluster project DC 06.01.03 an extensive research is carried out aimed to clarify the behavior of SFRC under compressive state of stresses. A literature study as well as recently completed research at TU Delft [18] provide a significant amount of information with respect to the tensile behavior of SFRC. The complementary literature study described in this report aims at gathering and evaluating available information on bond of deformed bars embedded in SFRC.

1.2 Composition of SFRC

SFRC is a composite material, which is in general regarded as macroscopically homogeneous and isotropic. These assumptions are valid under the condition that the fibers are uniformly distributed in the concrete matrix. Mix proportions and mixing procedures must be adjusted accordingly, in order to prevent fiber balling and to achieve random orientation of fibers. The

fiber volume Vf applied in practice varies considerably. The following may serve as a

guideline [34]:

− Industrial floors 20 to 40 kg/m3

− Housing constructions 25 to 50 kg/m3

− Environmental structures 40 to 70 kg/m3

− Tunnel structures and civil engineering structures 40 to 100 kg/m3

− Special cases 120 kg/m3

The addition of steel fibers to concrete mixtures puts additional demands to the design of the mix composition. A number of adaptations need to be made with respect to the aggregate grading curve when fibers are added to the concrete mix. Firstly, continuous aggregate grading gives a lower probability of fiber balling than discontinuous grading. Secondly, from a workability point of view it is recommended to limit the maximum grain size to 1/2 - 1/3 of the fiber length [17, 34]. As the maximum grain size decreases, the fibers are more randomly distributed and orientated in the mix. Thirdly, the ratio of finest/total aggregate volume needs

to be adjusted to attain an optimum packing density. As the fiber volume Vf increases, the

packing density decreases. A similar effect is found when the fiber’s aspect ratio (Lf/df) is

increased. It is generally agreed that the Lf/dfratio should not be higher than 60. If higher Lf/df

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to limit the maximum aggregate diameter dmax to 16 mm [34]. In order to diminish the

decrease in packing density, a larger amount of fine aggregate is needed in SFRC, compared to non-fibrous concrete. Depending on the fiber volume the optimum packing density is reached when the fine to total aggregate ratio is between 40 and 60%. [17]. Besides, with higher fiber volumes, a higher cement content is required to guarantee sufficient workability. This is due to the fact that when adding fibers to the mix a larger internal surface needs to be moistened by the cement paste. Note that the amount of water should be increased proportionally to the amount of cement in order not to reduce strength and durability of the composite material due to a largely increased water/cement ratio.

Table 1.1 ACI guideline on SFRC mix design [5]

component dmax= 10 mm dmax= 20 mm dmax= 38 mm

water/cement ratio 0.35 – 0.45 0.35 – 0.45 0.35 – 0.55 cement [kg/m3] 360 - 600 300 - 540 280 - 420 fine/total aggregate [%] 45 - 60 45 - 55 40 - 55 air volume [%] 4 - 8 4 - 6 4 - 5 Vfstraight fibers [%] 0.8 – 2.0 0.6 – 1.6 0.4 – 1.4 Vfdeformed fibers [%] 0.4 – 1.0 0.3 – 0.8 0.2 – 0.7

The American Concrete Institute (ACI) developed a guideline to simplify the mix design of SFRC [5]. Though it does not take into account the effect and importance of additives such as superplasticizers, it is a very good indication when verifying the quality of the mix composition.

Generally, the workability of concrete gets worse if fibers are added to the matrix. With increasing aspect ratio and volume of fibers, the workability of the concrete mix decreases. The probability of fiber balling increases when the maximum fiber volume is reached for a given maximum grain size and aspect ratio or when the volume of coarse aggregate is too high with respect to the total aggregate volume. Moreover, production aspects such as elongation of the mixing time and time-discontinuity when adding fibers contribute to decreasing workability of the concrete. Addition of suitable superplasticizers shall help to improve the workability of the concrete mix to the desired level. Also compaction has an influence on fiber orientation. An internal vibrator for compacting the concrete will disturb the orientation of the fibers locally, creating weak spots. With the application of an external vibrator the fibers will tend to orientate in a direction perpendicular to the vibration direction. Yet, the compacting method by means of external vibrators is preferable. Good workability in combination with proper compaction is necessary to guarantee a homogeneous and isotropic structure of SFRC. As a result of non-uniform density and inhomogeneous orientation of the fibers the properties of the SFRC material after hardening may not be the same in every place and in every direction. Nevertheless, one must not forget the unavoidable deviations from random fiber orientation, which occur as a result of the geometrical influences (formwork, free surfaces, conventional reinforcement). Furthermore, the pouring direction as well as the flow direction tends to orient the steel fibers a well.

1.3 Concrete composition vs. bond behavior

Under increasing pull-out forces, the bond behavior in plain (non-fibrous) concrete is marked by: 1) the initiation of inclined cracks at contact points between the steel lugs and concrete at

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concrete, where excessive growth of the splitting cracks can be prevented. Nevertheless, splitting always occurs in some way prior to bond failure, in the form of either partial splitting (quite often undetected) or full splitting, the latter being the subject of investigations on cover splitting in RC elements. In other words, bond of reinforcing bars to concrete and concrete splitting are intertwined to such a degree that bond failure is always accompanied by extended splitting, which makes bar pull-out easier.

Figure 1.1 Mechanism of bond resistance in confined concrete [32].

Since the tensile strength of concrete is low, cracking in concrete occurs at relatively low loads. At the onset of cracking, the role of the randomly orientated steel fibers in SFRC is to retard the propagation of micro cracks in the matrix. It is conceivable, that the steel fibers are able to bridge the so-called Goto-microcracks arising at the ribs of the bar (see Fig. 1.2).

Figure 1.2: Effect of steel fibers in cracked reinforced concrete section [27].

Once cracking occurs, fibers strengthen the matrix by transmitting a substantial tensile force during fiber slipping, with a sufficient margin against fracture, prevent further opening of cracks and resist additional tensile forces, which the concrete matrix itself cannot sustain. Hence, while the fibers may not much delay the formation of the first crack, they may keep

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crack width small and prevent the sudden opening of splitting cracks. This would control the failure in the concrete matrix itself, thereby preserving the bond strength between the reinforcing bar and the surrounding concrete matrix. Hence, compared to plain concrete, fibers may enhance the bond behavior of deformed bars embedded in SFRC by arresting the bond and splitting cracks. It is plausible that fibers may also delay the crushing of concrete between the lugs of the reinforcing bars. If this were true then the fibers would enhance both bond stiffness and bond strength both for the splitting and for the pull-out type of bond failure.

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2 Literature overview

2.1 Pull-out tests with short embedment length (RILEM Standard RC6 or

similar test set-up)

2.1.1 S. Cattaneo [6], S. Cattaneo & G. Rosati [7]

In the experimental investigation presented in [6] and [7] the bond strength, failure development and failure type (splitting or pull-out) in case of ribbed steel bars embedded in high performance fiber reinforced concrete were studied. Figure 2.1 shows test set-up and geometry of the test specimens used.

Figure 2.1: Scheme of the testing device (left) and geometry of test specimens (right) [6].

Table 2.1: General test program [6], [7].

Type of tests Type of specimen Results Assumptions Variables short pull-out cylindrical specimen

concentrically placed bar no bond-free length concrete in compression - curves fracture process (acoustic emission) uniformly distributed bond stress Vf ds c/ds

Table 2.2: Materials characteristics [6], [7].

Fibers Concrete Steel

Shape Lf / df [mm / mm] Vf [%] fccm [MPa] fct [MPa] Gf [N/m] ds [mm] fR [-] Type 0 147 - 13 / 0.16 1 155 - - 14 18 0.078 0.083 FeB44K

Table 2.3: Specimens geometry [6], [7].

c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull/ds [-] Lb-freepush/ds [-] Additional confinement Loading vs. casting 1.5 2

20 – 36 concentric 3 0 0 none identical

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Tables 2.1, 2.2 and 2.3 give main characteristics of the test program. Note that the effective rib area of the bars is estimated on the basis of limited geometrical data. No bond-free length is provided at the loaded end of the bar. Hence it is very likely that boundary restrain at the supported specimen surface influences the results of the tests. Test results and major conclusions from this investigation are presented in Figures 2.2, 2.3 and 2.4 and in Table 2.4.

Figure 2.2: Bond stress – displacement curves for plain concrete and SFRC [6]: (a) BD0-1455: plain concrete fccm=147 MP, ds= 14 mm, c/ds= 1.5

BD1-1455: SFRC fccm=155 MPa, ds= 14 mm, c/ds= 1.5

(b) BD0-1870: plain concrete fccm=147 MP, ds= 18 mm, c/ds= 1.5

BD1-1870: SFRC fccm=155 MPa, ds= 18 mm, c/ds= 1.5

Figure 2.3: Bond stress – displacement curves for SFRC [6]: (a) BD1-1455: SFRC fccm=155 MPa, ds= 14 mm, c/ds= 1.5

BD1-1470: SFRC fccm=155 MPa, ds= 14 mm, c/ds= 2

(b) BD1-1870: SFRC fccm=155 MPa, ds= 18 mm, c/ds= 1.5

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Figure 2.4 Bond strength as a function of c/ds[6].

Table 2.4: Main conclusions and observations [6], [7].

Aspect (Variable) Characteristics Conclusion

Bond strength With Vf= 1% fibers splitting bond strength of SFRC is about 20% higher than for plain reference concrete, while√fcis only about 3% higher.

Pull-out bond strength of SFRC is higher than for plain reference concrete [6].

Bond stiffness (pre-peak behavior)

Bond ductility (post-peak behavior)

Post-peak behavior is more ductile for SFRC than for plain reference concrete.

Failure propagation Both in SFRC and in plain reference concrete the occurrence of cracks starts before the load reaches 50% of the peak load. Diffusion of internal splitting cracks in SFRC is less fast than in the plain reference concrete and internal cracking in SFRC is localized to the volume close to the bar nearly until reaching the peak load (in plain reference concrete already at 75% of the peak load extensive growth of internal splitting cracks takes place in the large volume around the pulled bars – widespread both in the longitudinal and radial direction).

Effect of fibers (Vf)

Structural response In contrast to the plain reference concrete splitting failure in SFRC does not lead to specimen separation due to the bridging effect of the fibers in the splitting cracks

Bond strength With increasing bar diameter bond strength of SFRC tends to decrease.

Attention: With increasing fRa tendency to increasing bond strength is generally expected. Unfortunately, in case of this review, estimation of fRis based on incomplete data.

Bond stiffness

Bond ductility With increasing bar diameter post-peak response in SFRC tends to be slightly more ductile.

Effect of bar diameter (ds)

Failure propagation

Bond strength Increase of c/dsfrom 1.5 to 2 increases the bond strength of SFRC by nearly 30%.

Bond stiffness Increase of c/dsfrom 1.5 to 2 does not change the bond stiffness of SFRC.

Bond ductility Increase of c/dsfrom 1.5 to 2 increases the ductility of SFRC in the post-peak range (larger bond stress for given slip values in the whole post-peak range).

Effect of concrete cover (c/ds)

Failure propagation c/ds≤2 leads to splitting failure both for SFRC and for the plain reference concrete.

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2.1.2 F.S. Rostásy & K. Hartwig [27]

The experimental investigation presented in [27] was intended to answer the question why crack widths are reduced in reinforced bending elements made of SFRC when compared to reinforced elements made of normal weight concrete without fibers. The possible improvement of bond of the embedded deformed bars due to fiber addition was seen as an explanation for this finding. Besides, the extent of longitudinal cracking and occurrence of bond splitting failure in case of SFRC was investigated. Figure 2.5 shows the types of test specimens and Tables 2.5, 2.6 and 2.7 give the main characteristics of this test program.

Figure 2.5: Test specimens [27]

Table 2.5: General test program [27]

Type of tests Type of specimen Results Assumptions Variables short pull-out cylindrical and cube

specimens concentrically placed bar

bond-free length concrete in compression - curves fracture process (longitudinal splitting crack opening wspl) uniformly distributed bond stress Vf fiber shape c/ds bar position

Table 2.6: Materials characteristics [27].

shape Lf / df [mm / mm ] Vf [%] fccm [MPa] fspm [MPa] Gf [N/m] ds [mm] fR [-] type 0 3.11 straight (loose) 25 / 0.4 0.75 3.22 1.5 3.32 hooked (collated) 30 / 0.5 2.25 44 3.43 - 16 0.072 hot-rolled

Table 2.7: Specimens geometry [27].

c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull_/ds [-] Lb-freepush_/ds [-] Additional confinement Loading vs. casting

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Note that internal vibrators were used for concrete compaction. Hence it is very likely that the orientation of the fibers was locally strongly disturbed and that weak spots were created, in particular in case of higher fiber contents.

Figure 2.6: Bond stress – slip curves for plain concrete and SFRC (hooked fibers) [27]: (left) concentric and edge bar position

(right) corner bar position

Figure 2.7: Mean bond stress at (theoretical) occurrence of first longitudinal cracking in the concrete cover, related to fiber content and bar position [27].

Typical bond stress displacement curves (

-

curves) are given in Figure 2.6. Figure 2.7 shows the bond stress at first longitudinal cracking, defined as the stress at 0.15‰ transverse strain, a value that is assumed to correspond to the mean failure strain of fiber reinforced

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concrete in axial tension. Main conclusions from this investigation with respect to bond behavior of ribbed steel bars in SFRC are summarized in Table 2.8.

Table 2.8: Main conclusions and observations [27]

Bond strength Bond strength is not significantly influenced by fiber content, irrespective of the failure mode.

Attention: Actually, with increasing Vfa slight tendency to decreasing bond strength is observed in the concentric tests (pull-out failure mode). No effect of Vfis found in the eccentric tests (splitting failure mode).

The bond stiffness is not significantly influenced by the fiber content. Attention: Actually, with increasing Vfunexpectedly a slight tendency to decreasing bond stiffness is observed in concentric pull-out tests. No effect of Vfis found in eccentric pull-out tests.

Bond ductility (post-peak

behavior)

With increasing Vfthe post-peak behavior becomes more ductile in case of bond splitting failure.

Failure propagation

The bond stress at (theoretical) occurrence of first longitudinal cracking in the concrete cover does not depend on the fiber content. Attention: Actually, with increasing Vfunexpectedly a slight tendency to decreasing bond stress at occurrence of the first longitudinal cracks is observed in the eccentric pull-out tests for the edge bar position.

Structural response

A reduction of the crack widths in bending elements in reinforced concrete is only due to transfer of tensile force by the fibers across cracks and not because of improved by fibers bond of the embedded deformed bar reinforcement.

Bond strength

Bond stiffness The fiber shape does not significantly influence the bond stiffness. Bond ductility

Effect of fibers (fiber shape)

Failure propagation

Bond strength An increase of c/dsfrom 1 to 2 increases the bond strength of SFRC by roughly 40%.

Bond stiffness The bond stiffness is not significantly influenced by the concrete cover thickness. Bond ductility Effect of concrete cover (c/ds) Failure propagation

c/ds= 5 leads to pull-out failure both for SFRC and the plain reference concrete.

c/ds≤2 leads to splitting failure both for SFRC and the plain reference concrete.

Increase of c/dsfrom 1 to 2 roughly doubles the value of the bond stress at the (theoretical) occurrence of first longitudinal cracking in the concrete.

Bond strength The corner position of the bar leads to a lower bond strength than the edge position.

Bond stiffness The bond stiffness is not significantly influenced by the bar position (concentric, edge or corner).

Bond ductility Effect of

bar position

Failure propagation

The bond stress at the (theoretical) occurrence of the first longitudinal cracking in the concrete is not significantly influenced by the bar position (edge or corner).

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2.1.3 M.H. Harajli [12]

An experimental investigation of local bond stress - slip behavior of ribbed bars embedded in SFRC is discussed in [12]. Whereas the primary emphasis was on the monotonic local bond-slip relationship, two tests are carried out under large bond-slip reversals in order to study the effect of fibers on the cyclic bond-slip behavior. A part of this investigation (i.e. beam tests) is discussed in chapter 2.4.1. Figure 2.8 shows the type of specimens used in the pull-out test series. Tables 2.9, 2.10 and 2.11 give the main characteristics of this part of the test program.

Type of tests Type of specimen Results Assumptions Variables short pull-out prismatic specimens

concentrically placed bar bond-free length with and without confining

reinforcement concrete in compression - curves uniformly distributed bond stress Vf fiber shape ds confinement

shape Lf / df [mm / mm ] Vf [%] fc’m [MPa] fspm [MPa] Gf [N/m] ds [mm] fR [-] type 0 22 20 0.091 1 23.5 hooked (loose) 30 / 0.5 2 26.0 0.9 21.5 hooked (collated) 50 / 0.5 1.4 -- -25 0.066 Grade 60

Table 2.11: Specimens geometry [17]. c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull/ds [-] Lb-freepush/ds [-] Additional confinement Loading vs. casting 1st direction 3 2nd direction 5.5 -7 60-140 concentric 3.5 5 5 confining reinforcement in some cases (4 bars ds=10 mm, stirrups ds=6 mm at 76 mm spacing) perpendiculair direction

Since the concrete strength was not intended to be a parameter in this investigation, the results

of the individual specimens were normalized to fc’ = 22 MPa, assuming that the bond

resistance is proportional to (fc’)0.5. Note that it is not quite clear how the strength of concrete

in the specimens was deduced, since only in some cases concrete cylinders for testing concrete compressive strength were taken after adding fibers and in general no more than the compressive strength of the unreinforced matrix was determined before adding fibers. In case of mixes with larger fiber aspect ratio (50 / 0.5), a decrease of the concrete compressive strength after adding fibers is reported, which suggests that workability of the mix and compaction of the specimens were very poor. Test results and major conclusions from this investigation are presented in Figure 2.9 and in Table 2.12.

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Figure 2.8: Pull-out test series: test setup, specimen dimensions and reinforcement details [12].

Figure 2.9: Mean bond stress – slip relationships for different reinforcement and fiber parameters [12].

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Bond strength Whenever bond failure is caused by pull-out, adding steel fibers to concrete matrix improves the bond resistance of the reinforcing bars. However, this improvement is not proportional to the volume fraction of fibers used.

With Vf= 2% fibers, pull-out bond strength of SFRC is about 15% higher than for plain reference concrete, while√fcis only about 8% higher. With Vf= 1% fibers pull-out bond strength of SFRC is not different from that of plain reference concrete.

Attention: Actually significant number of test results that have not been taken into account when formulating this conclusion. In case of Lf/ df= 30/0.5 and ds= 20 mm bond strength increase is found to be 75% and 30% for Vf= 2% and 1%, respectively (√fcincreases with 8% and 3%, respectively) On the contrary, 15% bond strength decrease is found for Vf= 1% in case of Lf/ df= 50/0.5 and ds= 25 mm – obviously due to poor workability and compaction of the concrete (√fc decreases with 1%). Note that data concerning concrete compressive strength are not quite reliable.

In case of pull-out failure, the slip value at which peak bond stress is mobilized is dependent on the fiber content, i.e. with increasing Vfa tendency to increasing bond stiffness is observed.

Attention: A bond stiffness decrease with respect to the plain reference concrete is found for Vf= 1% in case of Lf/ df= 50/0.5 – obviously due to poor workability and compaction of the concrete. Bond ductility

(post-peak behavior)

In case of the pull-out failure, the slip value at which bond resistance leveled-off is independent of fiber content.

In case of pull-out failure, the ratio between peak bond stress and “frictional” post-peak bond resistance is dependent on fiber content, i.e. in case of pull-out bond failure with increasing Vf post-peak behavior becomes more ductile.

Failure propagation Effect of fibers (Vf) Structural response

Bond strength The ratio between peak bond stress and “frictional” post-peak bond resistance is independent of the fiber aspect ratio.

On basis of equal fiber reinforcing index VfLf/ df, fibers with small Lf/ dfare more effective than fibers with large Lf/ dfin the improving bond strength.

Attention: This conclusion results from possible false interpretation of test data: in case of fibers with Lf/ df= 50/0.5 obviously poor workability of the concrete mix and wrong compaction caused bond strength degradation.

Bond stiffness The slip value at which the peak bond stress is mobilized is independent of the fiber aspect ratio.

Bond ductility The slip value at which the bond resistance leveled-off is independent of the fiber aspect ratio.

Effect of fibers (fiber shape)

Failure propagation

Bond strength With increasing bar diameter the bond strength clearly increases both in SFRC and in plain reference concrete.

Attention: With increasing fRa tendency for increasing bond strength is generally expected. Considering large difference in fRvalues for the tested bars, it is fully unclear what is the reason for the observed results. Perhaps it can be attributed to the relatively small spacing of the lugs in case of bars with higher fRvalue.

Bond stiffness Slip value at which peak bond stress is mobilized is proportional to the clear spacing between the lugs.

Bond ductility Slip value at which bond resistance levels-off is approximately equal to the clear spacing between the lugs.

Effect of bar diameter (ds) and bar geometry (fR) Failure propagation

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Bond strength With concrete cover thickness that leads to pull-out failure addition of confining reinforcement does not influence bond strength.

Bond stiffness With concrete cover thickness that leads to pull-out failure addition of confining reinforcement the slip value at which peak bond stress is mobilized is independent of additional concrete confinement.

Bond ductility With concrete cover thickness that leads to pull-out failure addition of confining reinforcement does not influence bond ductility.

Effect of concrete cover

(c/ds)

Failure propagation

c/ds= 3 leads to pull-out failure for SFRC.

With concrete cover thickness that leads to pull-out failure addition of confining reinforcement does not influence failure propagation.

2.1.4 M.H. Harajli, M. Hout&&&&W. Jalkh [14]

The experimental investigation presented in [14] was intended to extend the basis for a formulation of an analytical model suitable for describing the bond stress – slip response of reinforcing bars embedded in SFRC. In particular the experimental results of tests defined to investigate splitting-type bond failure were reported. Furthermore constitutive equations describing the characteristic bond properties were proposed, further discussed in chapter 2.5. Figure 2.10 shows the types of specimens used in this test series. Tables 2.13, 2.14 and 2.15 give main characteristics of the test program. Typical bond stress - slip curves obtained in this test series are given in Figures 2.11.

Specimen type P Specimen type P* (no plastic sheet provided) Specimen type PA (plastic sheet provided)

Figure 2.10: Pull-out test series: specimen dimensions and reinforcement details [14].

Since the concrete strength was not intended to be a parameter in this investigation, the results

for individual specimens were normalized to fc’ = 30 MPa (i.e. the average strength of

concrete matrix before adding fibres), assuming that the bond resistance is proportional to (fc’)

0.5

. Just as in [12], it is not quite clear in which way the actual strength of the concrete in the specimens was deduced.

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Type of tests Type of specimen Results Assumptions Variables short pull-out prismatic specimens with

and without notch concentrically placed bar

bond-free length concrete in compression - curves uniformly distributed bond stress Vf fiber shape specimen geometry

Table 2.14: Materials characteristics [14]

shape Lf / df [mm / mm ] Vf [%] fc’m [MPa] fspm [MPa] Gf [N/m] ds [mm] fR [-] type 0 22 1 23.5 hooked (loose) 30 / 0.5 2 26.0 0.9 21.5 hooked (collated) 50 / 0.5 1.5 -- - 25 0.066 Grade 60

Table 2.15: Specimens geometry [14] c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull/ds [-] Lb-freepush/ds [-] Additional confinement Loading vs. casting 1st direction 2.3 - 3 2nd direction 5.5

60-140 concentric 5 3 – 4.3 3 – 4.3 none perpendiculair direction

Before conclusions from this investigation are presented, some issues must be discussed. Firstly, it is stressed that in some specimens the splitting mode of specimen failure was introduced by placing a thin plastic sheet in the plane of the longitudinal axis of the bar, i.e. by making a notch. This type of specimens are absolutely unsuitable for determining the resistance of concrete to splitting forces originating from bond action of a pulled bar: in abovementioned specimens, due to specimen geometry (notch) and boundary conditions (rubber pad covered with grease acting as friction-free or outward-directed-shear-force support) additional splitting forces are acting. These splitting forces are superimposed on forces due to the splitting action of the embedded bar and, the consequently, splitting process is speed-up in comparison with cases where no notch is present, discussed in [12] and [14]

(compare case P1-unconfined, ds= 25 mm, Vf= 0%, 1%, 2% in Fig. 2.9 [12] and case P1A,

ds= 25 mm, Vf= 0%, 1%, 2% in Fig. 2.11 [14]; compare case P2-unconfined, ds= 25 mm,

Vf= 0%, 0,9% and case P2A, ds= 25 mm, Vf= 0%, 0,9% in Fig. 2.11 [14]). For this reason

test results of series of specimens with inserted plastic sheet should be excluded from the evaluation.

Secondly, unexpected differences are found in the response of specimens with nearly identical

geometry, as described in [12] and in [14]. While case P1-unconfined, ds= 25 mm, Vf= 0.9%

in Fig. 2.9 [12] shows the typical response for a pull-out type of bond failure, in case P1A,

ds= 25 mm, Vf= 0%, 1%, 2% in Fig 2.11 [14] a sudden drop of bond resistance is found,

which denotes occurrence of splitting cracks. Such differences in behavior can hardly be

linked to increase of embedded length of the bar from 3.5 dsto 5 ds, or to increases in concrete

compressive strength from 22 MPa to 30 MPa, in the former and in the latter case, respectively. Most likely this are test conditions, i.e. boundary conditions or eccentricities, that lead to different behavior in test series reported in [12] and [14]. Note that while in case of

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[14] it is reported that rubber pad covered with grease was placed between the loading plate and the specimen, no information concerning boundary friction is provided in case of [12]. For these reasons conclusions from references [12] and [14] are found to be not fully reliable.

Figure 2.11: Local bond stress – slip relationships for different specimens and fiber parameters [14].

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Bond strength Bond stress at which splitting occurs (bond strength in case of splitting bond failure) is practically independent of fiber reinforcement.

Attention: occurrence of splitting cracks in the discussed test series is cannot be solely attributed to splitting action of embedded bars. Additional splitting forces are introduced in the test specimens due to specimen geometry and boundary conditions.

Before splitting, the response in terms of local bond stress – slip relation is similar to that for pull-out type of bond failure.

behavior)

In case of splitting bond failure with increasing Vfpost-peak behavior becomes more ductile and approaches that for pull-out bond failure. Failure

propagation

When splitting cracks develop bond resistance descends suddenly to a post-splitting bond stress level. With further increase in bar slip beyond splitting bond resistance diminishes approximately linear until it reaches constant bond stress plateau analogous to the local bond stress – slip response for specimens with pull-out bond failure. Effect of fibers (Vf) Structural response Bond strength Bond stiffness Bond ductility Effect of fibers (fiber shape) Failure propagation

Bond strength Bond stress at which splitting occurs increases with increasing “concrete splitting area surrounding the reinforcement”.

Attention: bond stress at which splitting occurs that is larger if specimen is not provided with a notch, since in the latter case besides bar splitting action additional splitting forces are introduced due to member geometry.

Bond stiffness

Bond ductility With increasing “concrete splitting area surrounding the reinforcement” post-peak behavior becomes more ductile and approaches that for pull-out bond failure.

Attention: premature splitting is a result of specimen geometry. Effect of notch

(specimen

geometry)

Failure propagation

If the bond stress at which splitting occurs is small, the stage of diminishing bond resistance is preceded by a slight increase in the bond stress until the maximum interfacial shear stress between the fibers and matrix is mobilized.

Attention: tendency to regaining bond resistance is observed only in specimens with notch, where premature splitting is a result of specimen geometry and not of splitting action of embedded bar.

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2.1.5 F. De Bonte [9]

The experimental investigation presented in [9] focused on determining the influence of the fiber content and of concrete cover thickness on bond strength of ribbed bars and on bond failure propagation in SFRC. Figure 2.12 shows the types of test specimens and Tables 2.17, 2.18 and 2.19 give main characteristics of this test program.

Figure 2.12: Test set-up and specimens geometry; dimensions in mm [9].

concentrically and eccentrically placed bar

bond-free length concrete in compression - curves uniformly distributed bond stress Vf c/ds bar position

shape Lf / df [mm / mm ] Vf [%] fcm [MPa] fspm [MPa] Gf [N/m] ds [mm] fR [-] type 0 39 0.25 38 hooked 60 / 0.9 0.75 39 - - 20 > 0.065 BE500

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Table 2.19: Specimens geometry [9] c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull/ds [-] Lb-freepush/ds [-] Additional confinement Loading vs. casting 1.5 3.25 30-65 concentric edge 3 5 5 none perpendiculair direction

Note that the effective rib area was most probably not measured and only the minimum value guaranteed by the producer is provided. Furthermore, the dimensions of the test specimens are small (in some cases even very small) compared to the fiber length. In particular in case of specimen type 40/40 it is very likely that the fiber orientation is restrained by the geometrical boundaries of the specimen and by concentrically placed bar. It would not be surprising if in such a case fibers tend to orientate parallel to the bar and are not very effective in arresting bond and splitting cracks. This aspect is, however, not addressed in the test report.

Figure 2.13: Bond stress – slip curves for specimen type 40/40: (left) single test results

(right) mean test results

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(right) mean test results

Typical bond stress - displacement curves are given in Figures 2.13 –2.15. Main conclusions from this investigation are summarized in Table 2.20.

Bond strength For a small concrete cover (type 40/40) practically no difference is found with respect to splitting bond strength when changing fiber content within the range investigated.

For a larger concrete cover (type 75/40) with increasing fiber content splitting bond strength increases slightly.

Pull-out bond strength is not influenced by fiber content. Bond stiffness

(pre-peak behavior)

For a small concrete cover in case of splitting bond failure (type 40/40) practically no difference is found with respect to bond stiffness when changing fiber content within the range investigated. For a larger concrete cover in case of splitting bond failure (type 75/40) with increasing fiber content slip at maximum bond stress clearly increases .

Bond stiffness in case of pull-out bond failure is not influenced by fiber content.

behavior)

With increasing Vfpost-peak behavior becomes more ductile in case of bond splitting failure.

Post-peak behavior for pull-out bond failure is not influenced by fiber content. Failure propagation Effect of fibers (Vf) Structural response

Bond strength Increase of c/ds leads to similar increase of bond strength both in SFRC and plain reference concrete.

Bond stiffness Increase of c/dsleads to larger slip values at maximum bond stress both in SFRC and plain reference concrete

Bond ductility Increase of c/dsleads to similar more stable post-peak behavior both in SFRC and plain reference concrete.

Effect of concrete cover

(c/ds)

Failure propagation

c/ds= 3.25 leads in some cases to pull-out failure and in some other cases to splitting both for SFRC with Vf= 0.25% and plain reference concrete.

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2.1.6 P. Soroushian, F. Mirza & A. Alhozaimy [32]

The experimental investigation reported in [32] deals with the effects of steel fibre reinforcement on the local bond behavior of deformed bars under conditions similar to the ones in a of beam-column connection. Figure 2.16 shows the test set-up and geometry of the specimen used in the pull-out test series. Tables 2.21, 2.22 and 2.23 give main characteristics of this test program.

Figure 2.12: Test set-up and test specimen geometry [32] Table 2.21: General test program [32]

concentrically placed bar bond-free length with confining reinforcement

concrete in compression - curves uniformly distributed bond stress Vf Lf/ df fiber shape

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shape Lf/ df [mm / mm ] Vf [%] fcm [MPa] fspm [MPa] Tfl(* [kNm] ds [mm] fR [-] type 0 28.0 3.47 4.1 0.5 3831.0 4.57 23.0 1 3928.6 9.38 72.5 60 1.5 27.7 11.79 63.0 80 1 37.1 14.44 74.7 hooked 100 1 36.0 8.89 59.1 crimped 60 1 35.0 9.68 48.4 straight 60 1.4 38.9 6.07 37.8 25 0.099 Grade 60 ( * Tfl– flexural toughness

Table 2.23: Specimens geometry [32] c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull/ds [-] Lb-freepush/ds [-] Additional confinement Loading vs. casting 1st direction 3 2nd direction 5.5 75-140 concentric 4 5.5 5.5 confining reinforcement (4 bars ds=13 mm, stirrups ds=10 mm) perpendiculair direction

Note that the effective rib area of the bars is estimated on the basis of very limited geometrical data. Flexural toughness was measured on specimens with cross-section 102 x 102 mm. With respect to the geometry of test specimens for pull-out tests few issues must be discussed. It is reported that during casting a plastic film sheet was placed in plane of the longitudinal bar axis to represent the splitting crack caused by the pullout of a bar in actual joint conditions. It is debatable if such type of pre-defined crack is suitable for modeling cracks in SRFC. However, it is not of such a critical importance as in the tests discussed in Chapter 2.1.4, considering the amount of confining reinforcement provided and the fact that due to this confinement bond failure is caused by bar pull-out and not by splitting. Yet, there is another aspect related to the placing of the plastic sheet that needs further consideration.

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Figure 2.14: Effect of steel fiber volume fraction (Vf) on local bond behavior [32]:

(a) mean bond stress – slip relationships for various volume fractions Vf

(b) bond stress at peak versus fiber volume fractions Vf(regression analysis)

(c) slip at peak versus fiber volume fractions Vf(regression analysis)

Figure 2.15: Effect of steel fiber aspect ratio (Lf/ df) on local bond behavior [32]:

(a) mean bond stress – slip relationships for various fiber aspect ratio (Lf/ df)

(b) bond stress at peak versus fiber aspect ratio (Lf/ df) (regression analysis)

(c) slip at peak versus fiber aspect ratio (Lf/ df) (regression analysis)

As can be seen in Fig. 2.12, the opening in the plastic sheet is very small with respect to the

bar diameter (1 dsx 4 dson each side of the bar). It is hard to judge how this will influence the

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length of the fiber is not known. One could imagine that due to flow of the mix during casting, fibers tend to concentrate and orientate perpendicular to the plane of the opening, where they would be very effective in arresting bond and splitting cracks. Test results and major conclusions from this investigation are presented in Figures 2.13, 2.14 and 2.15 and in Table 2.24.

Bond strength Fiber reinforcement has a significant effect on improving the local bond strength of deformed bars in case of pull-out bond failure. For Vf= 0.5% there is an increase of about one-third in local bond strength over that of plain concrete, while √fc’ is only about 5% higher. Further increase in fiber volume fraction above 0.5%, however, did not have a significant effect on bond strength.

Attention: There may be a relation between observed leveling of the effect of fiber addition and possible unintentional concentration of fibers in the opening of the inserted plastic sheet.

Local bond strength is more strongly correlated with flexural toughness than with compressive or the flexural strengths of material (flexural toughness is defined as the area underneath the load-deflection curve determined on 102 x 102 x 356 mm prisms loaded at one-third points on a span of 303 mm, up to a mid-span deflection equal to the test beam span divided by 150).

Slip at peak bond stress for pull-out bond failure is reduced in the presence of fibers, on the average by 44%.

Attention: Bond stiffness increases with increasing fiber volume fraction.

behavior)

With increasing Vfpost-peak behavior becomes more ductile in case of pull-out bond failure.

Failure propagation Effect of fibers (Vf) Structural response Bond strength

No significant difference was observed in local bond strength for fibrous concretes with different aspect ratios, at 95% level of confidence.

Bond stiffness

The slip value at peak bond stress increases with increasing fiber aspect ratio, up to Lf/ df= 80.

Attention: Bond stiffness increases with increasing fiber aspect ratio. Bond ductility Post-peak behavior in case of pull-out bond failure is not influenced

by the fiber aspect ratio Effect of fibers

(Lf/ df)

Failure propagation

Bond strength Fiber type (straight or with different mechanical deformations) has no statistically significant effect on local bond strength.

Bond stiffness Fiber type has no statistically significant effect on slip value at peak bond stress. Bond ductility Effect of fibers (fiber shape) Failure propagation 2.1.7 G. A. Plizzari [25]

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while Tables 2.25, 2.26 and 2.27 give main characteristics of this test program. Note that the effective rib area has not been specified. Furthermore, steel angles have been inserted both at the loaded end and at the unloaded end of the specimen in order to localize a splitting crack. Earlier research [26] has shown that specimens with preformed cracks are in general characterized by a lower bond stiffness compared to those without preformed splitting cracks, where the effect of the confining contribution of concrete is not disturbed. It is not fully clear whether a similar effect is to be expected if only small notches are created by inserting stiff material in the top and bottom surface of the specimen, the latter being loaded by the distributed reaction force.

Figure 2.15: Specimen geometry and arrangement of measuring devices [25] Table 2.25: General test program [25]

concentrically placed bar bond-free length concrete in compression - curves fracture process (longitudinal splitting crack opening wspl) uniformly distributed bond stress Vf confinement fcm

shape Lf / df [mm / mm ] Vf [%] fcm [MPa] fspm [MPa] Gf [N/m] ds [mm] fR [-] type 0 40 4.2 -0.38 50 4.1 -0 77 4.7 -hooked 30 / 0.5 0.38 85 5.7 -20 24 - B500B

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Table 2.27: Specimens geometry [25] c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull_/ds [-] Lb-freepush_/ds [-] Additional confinement Loading vs. casting 1st direction 2.5 2nd direction 9.5

50-190 concentric 8.5 1 1 in some cases one two-legged stirrup (ds= 6, 8, 10 mm)

opposite direction

Figure 2.16: Bond stress – slip curves for specimens without stirrup reinforcement

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Typical test results are given in Figures 2.16 –2.18. In Figure 2.18 the stirrup index of

confinementΩis defined as the ratio between the global cross-section area of the stirrup legs

and the area of the pulled bar in the splitting plane and the concrete index of confinement B is defined as the ratio between the net area of the concrete in the splitting plane and the net area of the pulled bar in the splitting plane. Main conclusions from this investigation with respect to bond behavior of ribbed steel bars in SFRC are summarized in Table 2.28.

Table 2.28: Main conclusions and observations [25].

Bond strength Splitting bond strength (both in specimens with and without stirrup confinement) as well as pull-out bond strength are clearly increased due to fiber addition.

With Vf= 0.38% fibers splitting bond strength of normal strength SFRC is about 50% higher than for plain reference concrete, while

√fcis only about 7% higher. For high strength SFRC splitting bond strength is about 115% higher than for plain reference concrete, while√fcis only about 5% higher.

With Vf= 0.38% fibers pull-out bond strength of normal strength SFRC is about 12.5% higher than for plain reference concrete, while

√fc is about 7% higher. For high strength SFRC pull-out bond strength is about 10% higher than for plain reference concrete, while

√fcis about 5% higher.

Attention: The bars anchored in high strength SFRC yielded before reaching the maximum bond strength.

Attention: Relatively long embedded length does not correspond very well with an assumption of uniformly distributed bond stress, consequently leading to underestimation of local bond stress values. Bond stiffness

(pre-peak behavior)

Bond stiffness is significantly increased due to fiber addition.

behavior)

When fibers are adopted, bond ductility increases and more ductile failure propagation is observed.

The ultimate loaded end slip in SFRC with Vf= 0.38% fiber is three times the value observed in concrete without fibers.

Failure propagation

When fibers are adopted, more ductile failure propagation is observed.

Structural response

Steel fibers reduce the splitting cracks opening, thus improving concrete durability.

Bond strength The larger the stirrup index of confinement the higher the bond strength becomes. Bond stiffness Bond ductility Effect of additional confinement Failure propagation

Splitting crack opening decreases when a higher stirrup index of confinement is adopted.

Splitting crack opening is larger in high strength concrete, because of its more brittle behavior.

2.1.8 S. Hota & A.E. Naaman [16]

The purpose of the experimental study presented in [16] was to investigate the bond characteristics of reinforcing bars embedded in SFRC under various types of loading, i.e. monotonic, unidirectional cyclic and reversed cyclic loading. Four types of matrix were tested, i.e. SIFCON, fiber reinforced concrete, confined concrete and plain concrete. Figure 2.19 shows the loading setup and specimen geometry. Tables 2.29, 2.30 and 2.31 give main characteristics of this test program. Relative rib area of the reinforcing bar is not given and cannot be estimated due to too limited data on rib geometry. In this review attention is given mainly to results of monotonic pull-out tests.

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Figure 2.19: Loading set-up (left) and geometry of test specimen (right)[16]. Table 2.29: General test program [16].

concentrically placed bar no bond-free length concrete in compression - curves uniformly distributed bond stress Vf fc’ confinement loading type

shape Lf / df [mm / mm ] Vf [%] fcm [MPa] fspm [MPa] Gf [N/m] ds [mm] fR [-] type 33.8 0 61 44 2 61 5 hooked 30 / 0.5 9.6 60.5 - - 25 -

-Table 2.31: Specimens geometry [16]. c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull/ds [-] Lb-freepush/ds [-] Additional confinement Loading vs. casting

2.5 63.5 concentric 4 0 0 in some cases spiral

confinement (ds= 3.2 mm at 12.5

or 19 mm spacing)

perpendicular direction

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ultimate strength was reached and splitting failure occurred. In case of specimens confined by stirrup reinforcement failure was consistently caused by cone pull-out due to specimen geometry (no bond free length provided at loaded end of the bar). Since results of these tests cannot be directly compared with cases where (frictional) pull-out or splitting bond failure occurs, in this review tests with spiral confinement are not further evaluated.

(a) (b)

(c)

Figure 2.20: Bond stress – displacement curves [16]:

(a) comparison for plain concrete fc’m= 61 MPa, SFRC fc’m= 61 MPa and

SIFCON fc’m= 60.5 MPa

(b) comparison for SIFCON fc’m= 35 MPa and fc’m= 60.5 MPa

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Table 2.32: Main conclusions and observations [16]:

Bond strength For the range of fiber volume fraction tested, the bond strength increases with an increase in the volume fractions of steel fibers Attention: with increase in fiber content shift from splitting failure mode (for plain concrete and SFRC Vf= 2%) to (frictional) pull-out failure mode (for SIFCON Vf= 9%) is reported.

Attention: with Vf= 2% fibers splitting bond strength of SFRC is about 77% higher than for plain reference concrete, while fc’ remains constant, both for higher and lower strength matrix.

Bond stiffness is not meaningfully influenced by fiber content.

behavior)

Bond ductility increases with increasing fiber content. The inclusion of steel fibers slows down the post-peak degradation of the bond stress - slip curve, causing increase in pull-out energy absorption capacity (area under bond stress – slip response) and maximum slip.

Failure propagation

Load – displacement response linearly decreases after the peak, when pull-out occurs.

Structural response

Bond strength

An increase in compressive strength of the matrix increases the bond strength.

Attention: with increase of fc’ from 35 to 63 MPa (increase of√fc’ with 34%), the bond stress increases typically by 30% for SIFCON, by 85% for SFRC and by 40% for plain reference concrete.

Bond stiffness Bond ductility Effect of matrix strength (fc’) Failure propagation Bond strength Bond stiffness Bond ductility Effect of concrete cover (c/ds) Failure propagation

In SIFCON (Vf= 9%), c/ds= 2.5 leads to (frictional) pull-out failure. In SFRC (Vf= 2%), c/ds= 2.5 leads to failure, which starts with some frictional pullout, but eventually ends up with splitting type of failure, when the cracks width reach a certain critical level.

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2.2 Pull-out tests with short embedment length (Danish Standard DS2082 or similar test set-up)

2.2.1 A. Samen Ezeldin, P.N. Balaguru [30], [31]

The experimental study presented in [30] and [31] was designed to evaluate the bond strength and bond – slip behavior of reinforced concrete containing steel fibers. The tests were conducted using modified pull-out specimen in which the concrete surrounding the rebar was in tension. Such configuration was intended to simulate the behavior of the bars in the tension zone of beams and beam-columns. The investigated loading patterns included monotonic, half cyclic and reverse cyclic pull-out load. In this review, however, only part of research concerning monotonic loading is considered. 18 types of matrix were tested, varying silica fume content, fiber content and fiber aspect ratio. Figure 2.21 shows loading setup and specimen geometry. Tables 2.33, 2.34 and 2.35 give main characteristics of this test program. Note that for none of the bar diameters used the relative rib area is given. For the interpretation of test results it is important to note that the geometry of the test specimens is not related to the (variable) bar diameter. In particular, for each bar diameter an other ratio between concrete cover and bar diameter is chosen (i.e. other confining conditions). Furthermore, the

embedment length of the bars varies with bar diameter from 50mm to 180mm (Lb/ ds varies

from 5.3 to 8). Such a large variation of embedment length in combination with the assumption of uniformly distributed bond stresses does not enable objective comparison of (average) bond stress values. Test results and major conclusions from this investigation are presented in Figures 2.22 and in Table 2.36.

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Table 2.33: General test program [30], [31].

concentrically placed bar no bond-free length concrete in compression - curves uniformly distributed bond stress Vf Lf/ df ds c/ds fc’ silica fume content loading type

Table 2.34: Materials characteristics [30], [31].

shape Lf/ df [mm / mm ] Vf [%] fc’m [MPa] fspm [MPa] Gf [N/m] ds [mm] fR [-] type 0 35 80 - -0.40 40 75 -0.60 40 - -30 / 0.5 0.75 40 - -0.40 45 - -0.60 45 - -60 / 0.8 0.75 45 75 - -0.40 40 75 - -0.60 45 70 - -hooked (collated) 50 / 0.5 0.75 45 75 - -9 16 19 25 - ASTM A615, Grade 60

Table 2.35: Specimens geometry [30], [31].

ds [mm] c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull/ds [-] Lb-freepush/ds [-] Additional confinement Loading vs. casting 9 6 59 5.5 0 0 16 5 81 8 0 0 19 4 80 6.5 0 0 25 3 76 concentric 7 0 0 none

-It is remarkable that despite the in general large ratios between concrete cover on the bar and

bar diameter, frictional pull-out failure was obtained only for bars with diameter ds= 9 mm

(c/ds= 6). Unfortunately no information is provided with respect to deformations on the bars

(ribs and/or indentations), which would allow judging the magnitude of radial splitting forces introduced in the specimen due to the wedging action of pulled bars. One could presume that

bars with a very high effective rib areas were used. For all but ds= 9 mm splitting bond failure

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(a)

(b)

(c)

Figure 2.22: Normalized load – slip curves for reinforcing bar ds= 19 [30]:

(a) comparison for different fiber content (b) comparison for different fiber aspect ratio (c) comparison for different silica fume content

Note that splitting bond failure does not provide the pull-out bond strength and that obtained splitting bond capacity is directly linked to concrete confinement (which is not consistently chosen in the discussed test series, as mentioned above).

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Table 2.36: Main conclusions and observations [30], [31]:

Bond strength For the range of fiber volume fraction tested, the bond strength increases insignificantly with an increase in the volume fractions of steel fibers.

Attention: this conclusion concerns solely splitting bond strength, since it is based on tests in which splitting bond failure took place Contribution of fibers to bond strength seems to be slightly higher for silica fume concrete.

Attention: this conclusion concerns solely splitting bond strength, since it is based on tests in which splitting bond failure took place Bond strength in case of pull-out bond failure is not greatly improved by addition of fibers.

Attention: note that due to specimen geometry (no bond-free length) and boundary conditions (stress-free edge surface) cone pull-out at loaded bar end is likely to take place

The contribution of fibers to the load –slip behavior in the ascending branch is negligible in case of splitting bond failure.

The slip at maximum pull-out load in case of splitting bond failure consistently increase with the increase in fiber content.

behavior)

Inclusion of fibers has a softening effect on the post-peak behavior of the pull-out specimens both in case of splitting and pull-out bond failure.

Failure propagation

Presence of fibers makes the splitting of concrete more stable, resulting in more ductile splitting failure.

Structural response

Bond strength Both in plain concrete and in SFRC (with and without silica fume) bond strength increases with compressive strength of the matrix and within the investigated range it is in general proportional to√fc’. this conclusion concerns solely splitting bond strength, since it is

based on tests in which splitting bond failure took place

Attention: with increase of fc’ from 45 to 80 MPa (increase of√fc’ with 40%), the splitting bond strength increases by about 30% for SFRC with Vf= 0.75%. Note that increase in matrix strength is due to addition of silica fume.

Bond stiffness Bond ductility Effect of matrix strength (fc’) and silica fume content Failure propagation Bond strength Bond stiffness Bond ductility Effect of concrete cover (c/ds) and bar diameter (ds) Failure propagation

Both in plain concrete and SFRC (Vf≤0.75%), c/ds≤5 leads to splitting bond failure.

Both in plain concrete and SFRC (Vf≤0.75%), c/ds= 6 leads to pull-out bond failure.

Attention: It is very likely, yet not verifiable, that pulled bars were characterized by a very high effective rib area.

2.2.2 N. Krstulovic-Opara, K.A. Watson & J.M. LaFave [19]

The goal of this research reported in [19] was to determine the effect of a change in the material tensile strength and toughness on the pull-out response of deformed bars. Four material systems were therefore selected; plain concrete, two types of SFRC and high performance SFRC (HPSFRC), representing brittle, pseudo-brittle and pseudo-ductile material

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Figure 2.23: Layout of the pull-out specimen; dimensions in inches [19]. Table 2.37: General test program [19].

concentrically placed bar no bond-free length concrete in compression Axial load- curves, - curves uniformly distributed bond stress Vf fc’m ds c/ds Lb / ds Table 2.38: Materials characteristics [19].

shape Lf / df [mm / mm ] Vf [%] fc’m [MPa] fspm [MPa] Gf [N/m] ds [mm] fR [-] type 0 35 80 1.9 0.7 1 40 75 3.1 9.7 3 40 4.5 18.6 straight (brass-collated) 6 / 0.15 7 40 6.0 29.0 10 25 -

-Table 2.39: Specimens geometry [19].

ds [mm] c/ds [-] c [mm] Bar position Lb/ ds [-] Lb-freepull/ds [-] Lb-freepush/ds [-] Additional confinement Loading vs. casting 10 2.5 5 0 0 25 1 25 concentric 2 0 0 none

-Figure 2.23 shows the layout of the pull-out specimen used in this study. Tables 2.37, 2.38 and 2.39 give main characteristics of this test program. Test results and major conclusions from this investigation are presented in Figures 2.24 and 2.25 and in Table 2.40. A modified version of the Danish Standard DS2082 pull-out test specimen was used, in which the concrete surrounding the rebar was in tension. Note that for none of the bar diameters used the relative rib area is given.

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Figure 2.24: Average bond – global slip curves [19].

Bond of deformed reinforcing steel bars embedded in steel fiber reinforced concrete: State-of-the-art report