Scholarly article on topic 'Effect of tension lap splice on the behavior of high strength concrete (HSC) beams'

Effect of tension lap splice on the behavior of high strength concrete (HSC) beams Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Ahmed El-Azab, Hatem M. Mohamed

Abstract In the recent years, many research efforts have been carried out on the bond strength between normal strength concrete (NSC) and reinforcing bars spliced in tension zones in beams. Many codes gave a minimum splice length for tension and compression reinforcement as a factor of the bar diameter depending on many parameters such as concrete strength, steel yield stress, shape of bar end, shape of bar surface and also bar location. Also, codes gave another restriction about the percentage of total reinforcement to be spliced at the same time. Comparatively limited attention has been directed toward the bond between high strength concrete (HSC) and reinforcing bars spliced in tension zones in beams. HSC has high modulus of elasticity, high density and long-term durability. This research presents an experimental study on the bond between high strength concrete (HSC) and reinforcing bars spliced in tension zones in beams. It reports the influence of several parameters on bond in splices. The parameters covered are casting position, splice length as a factor of bar diameter, bar diameter and reinforcement ratio. The research involved tests on sixteen simply-supported beams of 1800mm span, 200mm width and 400mm thickness made of HSC. In each beam, the total tensile steel bars were spliced in the constant moment zone. Crack pattern, crack propagation, cracking load, failure load and mi span deflection were recorded and analyzed to study the mentioned parameters effect.

Academic research paper on topic "Effect of tension lap splice on the behavior of high strength concrete (HSC) beams"

HBRC Journal (2014) xxx, xxx-xxx

Housing and Building National Research Center HBRC Journal

http://ees.elsevier.com/hbrcj

Effect of tension lap splice on the behavior of high strength concrete (HSC) beams

Ahmed El-Azab, Hatem M. Mohamed *

Engineering Consultant Group, Cairo, Egypt Faculty of Engineering, Cairo University, Cairo, Egypt

Received 23 December 2013; accepted 22 January 2014

KEYWORDS

Bond; Concrete; Lap splice; High strength; Casting position; Reinforcement ratio

Abstract In the recent years, many research efforts have been carried out on the bond strength between normal strength concrete (NSC) and reinforcing bars spliced in tension zones in beams. Many codes gave a minimum splice length for tension and compression reinforcement as a factor of the bar diameter depending on many parameters such as concrete strength, steel yield stress, shape of bar end, shape of bar surface and also bar location. Also, codes gave another restriction about the percentage of total reinforcement to be spliced at the same time. Comparatively limited attention has been directed toward the bond between high strength concrete (HSC) and reinforcing bars spliced in tension zones in beams. HSC has high modulus of elasticity, high density and long-term durability. This research presents an experimental study on the bond between high strength concrete (HSC) and reinforcing bars spliced in tension zones in beams. It reports the influence of several parameters on bond in splices. The parameters covered are casting position, splice length as a factor of bar diameter, bar diameter and reinforcement ratio. The research involved tests on sixteen simply-supported beams of 1800 mm span, 200 mm width and 400 mm thickness made of HSC. In each beam, the total tensile steel bars were spliced in the constant moment zone. Crack pattern, crack propagation, cracking load, failure load and mi span deflection were recorded and analyzed to study the mentioned parameters effect.

© 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research

Center.

* Corresponding author. Tel.: +20 1223185801; fax: +20 2330424645.

E-mail address: hatem_amn@yahoo.com (H.M. Mohamed).

Peer review under responsibility of Housing and Building National

Research Center.

Introduction

Adequate bond between concrete and reinforcing bars in a splice is an essential requirement in the design of reinforced concrete structures. In the last 15 years, concrete with compressive strength exceeding 70 MPa and ranging up to 120 MPa has been achieved consistently and utilized in bridges and high rise building construction. This concrete was described as high strength concrete (HSC) since it has higher

1687-4048 © 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center. http://dx.doi.org/10.1016/j.hbrcj.2014.0L002

strength than the usual normal-strength concrete (NSC) that has been produced for almost a century with 28-days strength in the range of 20-40 MPa.

Many researches were reported on bond strength between concrete and deformed bars for both normal strength and high strength concrete. Experimental tests were done and analytical equations were proposed by some researchers such as Asfahani and Rangan [1] and Orangun et al. [2].

Asfhani and Rangan [1] studied the effect of several parameters on bond of splices. The parameters considered were concrete strength, splice length, concrete cover, ratios between sides, bottom cover, spacing between spliced bars, rib face angle of the reinforcing bar and admixtures in the concrete mix. Based on test results, the following equations were proposed to calculate the maximum cracking bond strength (i.e., bond strength when the concrete cover cracks) of short reinforcing bars embedded in concrete blocks in pull-out tests.

1. For concrete with compressive strength less than 50 MPa:

Uc = 4.9(c/db + 0.5)/(c/4 + 3.6)/ (1)

2. For concrete with compressive strength equal to or greater than 50 MPa (HSC):

Uc = 8.6(c/db + 0.5)/(c/db + 5.5/ (2)

where Uc is the cracking bond stress; C is the minimum of CX (side clear cover), CY (bottom clear cover & (CS + db)/2., db is the bar diameter, CS is the clear distance between two adjacent bars and /ct is the tensile strength of concrete taken equal to 0.55y/,', where /c' is the cylindrical compressive strength of concrete expressed in (Mpa). The factor 8.6 in Eq. (2) should be replaced by 7.3 for bars within rib face angle between 23 and 27 deg. since Eq. (2) was obtained based on bars with rib face angle between 40 and 47 deg.

Mostafa [3] studied the effect of different parameters on the HSC beams with tension lap splice. These include silica fume dosage, steel fiber volume (V/), splice length as a factor of bar diameter and the percentage of spliced reinforcement with respect to the total reinforcement. 30 High Strength Concrete (HSC) beams' specimens with tension lap-splices in the constant moment region were tested. The specimens were divided into 10 groups, three specimens each with a specimen with no splice as the control specimen.

Three different percentages of silica fume (10%, 15% and 20%) were used as an addition of Portland cement. It was found that silica fume dosage had no effect on either crack pattern or failure mode. It was also found that the cracking load increased by 18% and 53% when using silica of 15% and 20%, respectively. Also the ultimate load increased for the same ratios by 7% and 17%, respectively. In addition, the increase of silica fume dosage from 10% to 20% had a minor effect on beam stiffness. At load levels above cracking loads, the increase in silica fume decreases beam stiffness for the same concrete strength. The only gain when increasing silica fume dosage was the increase in the beam ductility represented by area under the load deflection curve.

Splice lengths 20, 30 and 40 times the reinforcing bar diameter were investigated in Mostafa's research. It was found that splice length had no effect on either crack pattern or failure mode, except that increasing splice length prevents splitting cracks to occur. It was noticed that the cracking load

increased by about 20% and 22% when increasing splice length from 20 to 30 and 40 times bar diameter respectively. Also the ultimate load increased with 19% and 20%, respectively. It was also noticed that cracking and ultimate loads for both spliced length 30 and 40 times bar diameter were approximately equal. It was also found that increasing splice length from 20 to 40 times bar diameter increased the beam stiffness. Also, trend of load deflection behavior for both splice lengths (30 and 40 times bar diameter) were approximately identical; this led to estimate the development length to be not less than 35 times bar diameter for concrete strength between 50 and 58 N/mm2.

In the same research variable ratios of spliced tension bars at mid span with respect to total tension bars (33%, 67% and 100%) were investigated. It was found that spliced reinforcement percentage had no effect on either crack pattern or failure mode such as splice length and silica fume dosage. Also it was found that the cracking load increased by about 12%, 19% and 49% for spliced percentage of 33%, 67% and 100%, respectively. However the ultimate load varied insignificantly by about ±2% only. The main conclusion was tension reinforcement may be spliced till 100% of total steel without any loss of beam capacity.

Farahat [4] proved that the new technique of using studs connected to the reinforcing bars along the spliced length results in avoiding the effect of splitting cracks and cover spalling. The studied parameters were the length of lap splice (20 and 40 times bar diameter), shape of bond studs (L, V and C shapes), height of bond studs (50mm,100mm and 150 mm) and spacing between bond studs along the spliced length (10, 20 and 40 times bar diameter). The contribution of using bond studs to the ultimate capacity, strength, deflection and cracking was precisely observed. The experimental test program consisted of 13 reinforced concrete beams with concrete compressive strength of 30 N/mm2 was classified based on the pervious studied parameters. The cut-off ratio for the spliced bars in all specimens was 100% in the middle part of the beam.

Reducing the tension lap splice to 40 and 20 times the bar diameter reduced the cracking load by 2.5% and 8.75%, the ultimate load by 18% and 33% and the ductility by 44% and 88%, respectively. However reducing the tension lap splice has no effect on the initial stiffness compared with that of the reference beam. The beam with lap splices of 20 times the bar diameter failed in brittle mode. The L and C-shaped bond studs were much better than the V-shaped studs in enhancing the tension lap splice. It can be concluded that the lap splice length can be calculated from the following equation:

Lp = Lh + ^ Lv (3)

Where: Lp = the required lap splice length according to design code. Lh = horizontal length of lap splice. RLv = the summation of vertical projection lengths for the provided bond studs.

The ultimate load capacity and the ductility were reduced with the increase of the spacing between studs. However, providing L-shaped studs even at bigger spacing significantly improved the initial stiffness. The change in the stud height had a minor effect on the test results. However, the smaller stud heights gave better results.

Hamad et al. [5], tested 16 HPC beams with the following variables:

1- The percentages replacement by weight of Portland cement by silica fumes were taken (0%, 5%, 10%, 15% and 20%).

2- Casting position (top or bottom).

3- Super plasticizer dosage (2 or 4 L/100 kg).

Hamad et al. [5] investigated the bond strength of reinforcement in HSC. They concluded the following:

1- Equations of Orangun et al. [2] provide a much better estimate of bond strength than equation of the ACI code [6-9]. It was only Olsen [10] in 1990 who reported results of 21 beams' splice tests and concluded that Orangun et al. [2] equations overestimated the splice strength of HSC.

2- The current code limit of 70 MPa on concrete compres-sive strength in computing the anchorage length appears to be unnecessary and unwarranted. However, it is recommended that the removal of ACI 318-95 [7] limitation on f be coupled with some ductility requirements on anchored bars in HSC.

Eight beams in four pairs were tested by Hwang et al. [11]. Each pair included a specimen with plain Portland cement concrete and one with concrete in which 10% of the Portland cement was replaced by equal weight of silica fume. Variables among pairs were water-to-cementitious material ratios of 0.28 and 0.33 were selected and two nominal beam cross sections were used: one had no transverse reinforcement over the splice; the other had No. 3 stirrups spaced 100 mm uniformly distributed along the region of constant moment.

Flexural cracks were first noticed at the ends of the splice, and generally three or more flexural cracks developed across the splice itself. From these cracks, longitudinal splitting gradually developed. For the beams with a known rib orientation, the earliest longitudinal cracks appeared directly over the bar splice. The crack patterns of specimens with stirrups over the splice were more abundant than those of specimens without stirrups. Transverse steel improved bond strength and ductility of the anchorage. Due to different stress levels developed in the reinforcing bars at failure, the final maximum crack widths of specimens with stirrups reached twice those without stirrups. The stiffness of silica fume specimens degraded more rapidly than that of plain cement specimen when more pronounced slippage of bars was found. The bond strength around the bar nominal perimeter was calculated from the following:

Utest — (f x db)/4Ls (4)

where: fs: is the steel stress of the spliced bar at failure. db: is the nominal diameter of the spliced bar. Ls: is the splice length.

Bond efficiency is defined as the ratio between measured and calculated bond strength for each specimen. Bond ratio is defined as the ratio between bond efficiency of the silica fume specimen and the bond efficiency of the plain cement specimen. The bond efficiencies of the specimens with silica fume were all less than those of plain cement counterparts. The average bond ratio of silica fume to plain cement efficiency was 0.90 with a standard deviation of 0.05.

The replacement of 10% cement by silica fume could increase both the compressive strength and tensile splitting strength by 12% and 23%, respectively. However, greater tensile strengths of concrete failed to follow the trends of increased bond strength expected from the expression of

Orangun et al. [2]. The ACI 318-95 [7] limit of 70 MPa on concrete compressive strength was appeared to be unnecessary and unwarranted in computing the anchorage length. A review of this limit was recommended.

The proposed ACI 318-B [9] bond provisions for the development or lap splicing of tensile reinforcement contain both a simple design approach and a refined design approach:

1- The simple design approach:

-The development length (Ldb) for No.7 deformed bars and

larger may be calculated using the following equation.

Ldb — (0.05 dbfy)/Pfz (5)

-The development length (Ldb) for No.6 deformed bars and

smaller is 80% of that calculated from Eq. (5).

The modification factors are simple lump sum constants:

a- A value of 1.00 for a clear cover to the bars not less than db and in addition; either the clear spacing must not be less than 2db or the clear spacing must not be less than db and minimum stirrups must be provided. b- A value of 1.50 when even less confinement is available.

2- The refined design approach:

Some economies on this length may be realized by using this design approach, for the influence of confinement.

The modification factor for confinement is defined by:

(a) For No.7 deformed bars and larger:

Ldb — (1.5 db)/k (6)

(b) For No.6 deformed bars and smaller:

Ldb — (1.5 db)/(.8 k) (7)

where k = the smaller of Cc + Ktr or Cs + Ktr 6 2.5db (in.)

Ktr — (Atrfyt)/(1500s.n) 6 2db (in.) (8)

where Atr = transverse reinforcing area intercepting the relevant bond splitting cracks, in2. fyt = yield strength of transverse reinforcement, psi. s = spacing of transverse reinforcement, in. n = number of developing bars confined by Atr for the splitting crack pattern considered. Cc = thickness of concrete cover measured from extreme fiber to center of bar, in. Cs = smaller of side cover to center of outside bar measured along the line through the layer of bars or half the center distance of adjacent bars in the layer, in.

Gjorv et al. [12] studied the mechanical behavior of the steel-concrete bond. The pullout strength at four levels of concrete compressive strength (35, 42, 63 and 84 MPa) was investigated. For these strength levels, three levels of condensed silica fume (CSF) were used (0%, 8% and 16%) by weight of cement, respectively.

The observed effect of CSF may be explained by the following mechanisms:

a- Reduced accumulation of free water at the interface during casting of specimens.

b- Reduced preferential orientation of calcium hydroxide (CH) crystals at the steel-past transition zone.

c- Densification of the transition zone due to pozzolanic reaction between CH and CSF.

The main objective of this research is to investigate the bond between high strength concrete (HSC) and reinforcing

bars in splices in beams in terms of flexural cracks, deflection, strains and ultimate loads.

Experimental work

This research is a part of an experimental investigation [13] which studies bond between high strength concrete (HSC) and reinforcing bars in splices in beams. The objective of this experimental program is to study the behavior of HSC beams with tension lap splices. Different parameters were considered such as casting position, splice length as a factor of the bar diameter, bar diameters and reinforcement ratio. The effect of these parameters on flexural capacity, crack pattern and crack propagation and mode of failure was observed during testing.

Tests were carried out on sixteen simply-supported reinforced concrete beams, which were subjected to incremental load up to failure.

Test specimens

In the experimental program, tests were carried out on sixteen high strength concrete beams reinforced with high grade steel bars spliced-if any- in the constant moment region and designed to start failure in tension zone (under reinforced sections).

All the tested beams had 200 mm x 400 mm cross-section and 1800 mm clear span. The beams were simply supported and subjected to two concentrated static loads (four node testing).

The details of the tested beams are shown in Table 1 and Fig. 1. A three-part notation system was used to indicate the variables of each beam. The first part of the notation indicates the casting position: B and T for bottom and top casting respectively. The second part indicates the splice length as a factor of the bar diameter with two different bar diameters: LM x N for splice length of M times bar diameter and N is the diameter of reinforcement bar. The third part is the reinforcement ratio: R.295 and R.424 for AS/(b x d) equal to

0.295% and 0.424%, respectively. The specimens with no splice are referred to as the control specimens.

Group (A): This group consists of four specimens having the same reinforcing ratio 0.295% and casting position (Bottom) but different in the splice length (0, 20, 30 and 40) times bar diameter 10 mm.

Group (B): This group consists of four specimens having the same reinforcing ratio 0.295% and casting position (Top) but different in the splice length (0, 20, 30 and 40) times bar diameter 10 mm. The main difference between group (A) and (B) is the casting position.

Group (C): This group consists of four specimens similar to those in group (A) except using bar diameter 12 mm instead of 10 mm.

Group (D): This group consists of four specimens similar to those in group (C) except using reinforcing ratio 0.424% instead of 0.295%.

Materials

The concrete mixtures used to cast the specimens were developed by trial batching in the concrete research laboratory at Cairo university. One mix was used through casting and was designed to develop cube strength of 75 N/mm2. Table 2 shows the weights required to cast one cubic meter of concrete.

Test procedure

Static hydraulic loading jack with an electrical load cell was used to apply the vertical load. A digital load indicator of (1 kN) accuracy was used to measure the applied load.

Each beam was centered on the testing machine. Loads were applied of specimens with load increment of 1 ton. Fig. 2 shows a photograph for the test instrumentation and Fig. 3 shows a schematic view of the test setup. Specimens' casts in a top casting position were turned upside down before being placed in the test frame.

At every load increment, the cracks were observed and marked and readings were taken for deflection and steel strain. Failure was considered to occur when the load could not be increased further.

Table 1 Details of tested beams.

Group No. Specimen designation Casting position Splice length Rft. bar diameter (mm) Rft. ratio (%)

A 1 B-L0x10-R.295 Bottom - 10 0.295

2 B-L20x10-R.295 Bottom 20 Ф 10 0.295

3 B-L30x10-R.295 Bottom 30 Ф 10 0.295

4 B-L40x10-R.295 Bottom 40 Ф 10 0.295

B 5 T-L0x10-R.295 Top - 10 0.295

6 T-L20x10-R.295 Top 20 Ф 10 0.295

7 T-L30x10-R.295 Top 30 Ф 10 0.295

8 T-L40x10-R.295 Top 40 Ф 10 0.295

C 9 B-L0x12-R.295 Bottom - 12 0.295

10 B-L20x12-R.295 Bottom 20 Ф 12 0.295

11 B-L30x12-R.295 Bottom 30 Ф 12 0.295

12 B-L40x12-R.295 Bottom 40 Ф 12 0.295

D 13 B-L0x12-R.424 Bottom - 12 0.424

14 B-L20x12-R.424 Bottom 20 Ф 12 0.424

15 B-L30x12-R.424 Bottom 30 Ф 12 0.424

16 B-L40x12-R.424 Bottom 40 Ф 12 0.424

Figure 1 Typical reinforcement details and concrete dimensions for all specimens.

Table 2 Design of the concrete mix (per m3).

Material Weight (kN)

Coarse aggregate (Gravel) 10.80

Fine aggregate (Sand) 5.40

Cement 6.00

Water 1.80

Silica fume 1.20

Superplasticizer (Sikament R2002) 0.22

The deflections were measured at mid-span by a dial gauge of 0.01 mm accuracy (LVDT instrument). The crack propagation was plotted on the concrete beams during loading.

The steel strains at mid-span were measured using 100 mm gauge length for one deformed bar in the splice region.

All measured values of deflection, load and steel strain had been continuously monitored through controlled data acquisition system. All test records were automatically saved on computer file for further data manipulation and plotting.

Test results

The design parameters taken into consideration include casting position, splice length as a factor of the bar diameter with two different bar diameters, and reinforcement ratios. Effect of the studied parameters on the splice length in high strength

Figure 2 Test instrumentation.

I Hydraulic jack Concrete beam ■'■Jjf

I-T-1 Steel beams

1 600 1 J

Dial gauge to ^H

measure deflection ^H

Figure 3 A schematic view of test arrangement.

concrete beams will be discussed. Also the effect of changing parameters on the following results is presented:

1. Crack propagation, crack pattern, and failure mode.

2. Cracking load and ultimate failure load.

3. Load-deflection relationship.

4. Equivalent uniform bond stress.

5. Ductility measure, stiffness measure, and strength measure.

Cracking pattern and mode of failure

Fig. 4 shows the crack pattern at failure mode for each specimen. At different load levels top cast beams showed greater average crack width than bottom cast beams for the same splice length, bar diameter, and reinforcement ratio. This is because of bleeding of concrete which made lower quality concrete underneath the reinforcement in the splice region. There were longitudinal cracks observed in top cast beams for all splice lengths (20, 30 and 40 times bar diameter).

For specimen (T-L20x10-R.295) failure occurred due to longitudinal splitting crack formed in the bottom cover on the tension side directly below the splice region and it was sudden and brittle. For specimens with bottom cast position, there were no longitudinal cracks observed except specimen with splice length 20 times bar diameter.

It was noticed that splice length had no effect on both crack pattern and failure mode except that increasing splice length prevents splitting cracks to occur. No longitudinal cracks were observed for beams with splice length 40 times bar diameter except beam with top casting position. It was also noticed that bar diameter and reinforcement ratio had no effect on either crack pattern or failure mode except that for reinforcement ratio of 0.424%, there was splitting crack for splice length 30 times bar diameter on contrary for reinforcement ratio 0.295% for the same splice length.

Cracking and failure loads

The cracking load (Pcr) for all specimens was recorded at the observation of the first crack. The failure load (Pu) which is the load at which the specimens could not carry any additional load was also recorded. Table 3 gives the cracking load (Pcr) and the failure load (Pu) for each specimen.

It was noticed that the average cracking load for group (A) (Bottom Casting) was larger than the average cracking load for group (B) (Top Casting) by 23%. Also the average ultimate load for group (A) is larger than the average ultimate load for group (B) by 68%. The reason in increasing the crack load and the ultimate load could be due to a slight reduction in the strength of the cement paste and the splitting tensile strength of concrete cover for top casting specimen.

Figure 4 Crack pattern and failure mode for tested specimens.

Fig 4. (continued)

Fig 4. (continued)

Fig 4. (continued)

Fig. 5 shows the average cracking and ultimate loads for specimens having 0, 20, 30 and 40 times bar diameter. It was noticed that the cracking load increased by 56%, 56% and

34% when the splice length became 20, 30 and 40, respectively compared with the control specimen (without splice). Also the ultimate load increased for the splice length 40 times bar

Table 3 Cracking and failure loads.

Specimen Cracking load (kN) Failure load (kN) Specimen Cracking load (kN) Failure load (kN)

B-L0x10-R.295 60 270 B-L0x12-R.295 80 233

B-L20x10-R.295 180 500 B-L20x12-R.295 90 153

B-L30x10-R.295 90 305 B-L30x12-R.295 90 230

B-L40x10-R.295 40 298 B-L40x12-R.295 100 243

T-L0x10-R.295 40 203 B-L0x12-R.424 50 305

T-L20x10-R.295 80 188 B-L20x12-R.424 100 173

T-L30x10-R.295 100 223 B-L30x12-R.424 80 95

T-L40x10-R.295 80 200 B-L40x12-R.424 90 300

300 250 200 ' 150 100 50 0

90 90 78

■ Cracking Load (KN) □ Failure Load (KN)

Splice Length

0 20D 30D 40D

Figure 5 Effect of splice length on cracking and ultimate loads.

diameter by 3%. The previous could be explained as the spliced reinforcement is effectively larger than that outside the splice region. It is also noticed that ultimate load decreased for splice length 20 times bar diameter by 32% compared with the ultimate load for the control specimen because of the effect of splitting cracks cover spalling on the splice resistance mechanism. Also the ultimate load decreased for the splice length of 30 times bar diameter by 14%. This decrease in ultimate load for splices 20 and 30 times bar diameter is due to longitudinal splitting crack failure.

The results show that the average cracking load for group (A) (bar diameter 10 mm) is approximately equal to the average cracking load for group (C) (bar diameter 12 mm). Also the average ultimate load for group (A) is larger than the average ultimate load for group (C) by 60%. The previous point could be explained as the use of small bar diameter with the same reinforcement ratio reduces the average crack width (crack control).

It is noticed that the average cracking load for group (C) (reinforcement ratio 0.295%) was larger than the average cracking load for group (D) (reinforcement ratio 0.424%) by 12.5%. However, the average ultimate load varied by 2% only.

Load-deflection relationship

As shown in Figs. 6-9, group (A) (bottom casting position) had larger stiffness compared with group (B) (top casting position). This is due to a slight reduction in the strength of the cement paste and the splitting tensile strength of concrete cover for top casting position.

As shown in Figs. 10-13, group (A) (bar diameter 10 mm) had larger stiffness compared with group (C) (bar diameter 12 mm) for the same reinforcement ratio 0.295%. This is due to decrease of crack width as the bar diameter decreases for the same reinforcement ratio.

i _ 1___'

iiMiiMiwim*

:7 -&-L0X1O-R 295 —••• T-L0X1O-R 295

• w » » m

DtAtcbon (mm)

Figure 6 Load deflection curve of beams without splice for group (A) and (B).

Figure 7 Load deflection curve of beams with splice length 20 U for group (A) and (B).

As shown in Figs. 14-17, for splice length 0 and 40 times bar diameter, group (D) (reinforcement ratio 0.424%) had larger stiffness compared with group (C) (reinforcement ratio 0.295%). For splice length 20 times bar diameter, group (D) and group (C) had the same load deflection curve and did not have ductile behavior. For splice length of 30 times bar diameter, group (C) had higher ductile behavior when compared to group (D).

Figure 8 Load deflection curve of beams with splice length 30 U Figure 11 Load deflection curve of beams with splice length

for group (A) and (B).

20 U for group (A) and (C).

Figure 9

for group (A) and (B)

Load deflection curve of beams with splice length 40 U Figure 12 Load deflection curve of beams with splice length

30 U for group (A) and (C).

Figure 10 Load deflection curve of beams without splice for group (A) and (C).

Figure 13 Load deflection curve of beams with splice length 40 U for group (A) and (C).

Figure 14 Load deflection curve of beams without splice for group (C) and (D).

Figure 16 Load deflection curve of beams with splice length 30 U for group (C) and (D).

L__ L J

i—r---- ------

-----B-C40X12R 295

—— B-L40X12-R 424

Figure 15 Load deflection curve of beams with splice length 20 U for group (C) and (D).

• »0 JO 10 «0 to

DaAtction (wtt)

Figure 17 Load deflection curve of beams with splice length 40 U for group (C) and (D).

Table 4 Ductility, stiffness, and strength measures.

Group No. Specimen designation D S K

A 1 B-L0x10-R.295 1.00 1.00 1.00

2 B-L20x10-R.295 0.08 0.74 1.85

3 B-L30x10-R.295 0.50 0.25 1.13

4 B-L40x10-R.295 0.83 3.01 1.10

B 5 T-L0x10-R.295 1.00 1.00 1.00

6 T-L20x10-R.295 0.24 0.60 0.93

7 T-L30x10-R.295 0.18 1.20 1.10

8 T-L40x10-R.295 0.85 0.80 0.99

C 9 B-L0x12-R.295 1.00 1.00 1.00

10 B-L20x12-R.295 0.12 1.42 0.66

11 B-L30x12-R.295 0.50 1.62 0.99

12 B-L40x12-R.295 1.03 3.04 1.04

D 13 B-L0x12-R.424 1.00 1.00 1.00

14 B-L20x12-R.424 0.13 1.33 0.57

15 B-L30x12-R.424 0.18 0.50 0.31

16 B-L40x12-R.424 0.73 1.11 0.98

Ductility measure, stiffness measure, and strength measure

The ductility measure (D) is defined as the ratio of the central deflection at the maximum load of the tested specimen to that of the specimen without tension lap splice.

The stiffness measure (S) is defined as the ratio of the initial slope in the load-deflection curve for the tested specimen to that for the reference specimen without splice.

The strength measure (K) is defined as the ultimate load of the tested specimen to that for the reference specimen without splice.

The summary of the results is given in Table 4. The results include the ductility measure D, the stiffness measure S, and the strength measure K.

It can be noticed that the use of different casting positions (bottom and top casting position) had no effect on the ductility. However, the initial stiffness is reduced for top casting position by 75% and 19% for 40 and 20 times bar diameter respectively. Group (B) (top casting position) had strength measure less than group (A) by 50%, 3% and 10% for splice length 20, 30 and 40 times bar diameter respectively.

It was noticed that the ductility increased by increasing the splice length where an average ductility measure is 0.14, 0.34 and 0.86 for 20, 30 and 40 times bar diameter respectively. The initial stiffness for splice length 20 and 30 times bar diameter was approximately equal but the initial stiffness for splice length 40 times bar diameter was increased by about 100% in average. It can be noticed that the use of different lengths of tension lap splice have a minor effect on the strength. It can be noticed also that the use of different bar diameters had no effect on the ductility. The initial stiffness for bar diameter 12 mm (group C) was larger than those of bar diameter 10 mm (group A) by 52% in average. However, the strength measure for bar diameter 10 mm (group A) was larger than the strength for bar diameter 12 mm (group C) by 52% in average for the same splice length.

The beams in group (C) (reinforcement ratio 0.295%) are more ductile than those of group (D) (reinforcement ratio 0.424%) where average ductility measure for group (C) is 0.55 but for group (D) it is 0.35. The initial stiffness for group (C) was more than group (D) by 6%, 69% and 63% for splice length 20, 30 and 40 times bar diameter respectively. Also, the strength measure was increased by 45% in average.

Conclusions

Based on comparison of modes of failure, cracking, ultimate loads and load-deflection curves of HSC beams with spliced bars in the constant moment region tested in this study, the following conclusions can be made:

(1) The development length required achieving bond stress between tension deformed steel and HSC should be larger than 30 times bar diameter for concrete having strength between 65 and 93 N/mm2.

(2) At different load levels top cast beams showed greater average crack width than bottom cast beams for the same splice length, bar diameter, and reinforcement ratio.

(3) Splice length, bar diameter, and reinforcement ratio had no effect on both crack pattern and failure mode.

(4) Bottom casting position has higher cracking and ultimate load compared to top casting position.

(5) The splice length up to 30 times bar diameter decreased the moment capacity of beam. The splice length of 40 times bar diameter results in the same capacity of the beam without any splice.

(6) Bottom casting position leads to larger beam stiffness than top casting position. No effect on the ductility was noticed due to changing the casting position.

(7) The ductility is increased by increasing the splice length.

(8) Different bar diameters have no effect on the ductility.

Conflict of interest

References

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