Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing
Recycling of concrete can provide an effective solution for waste management. This study investigates the interfacial bond stress versus slip response of steel bars embedded in recycled aggregate concrete. The bond tests were conducted using concentric pullout specimens. The specimens made from natu...
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Інститут проблем міцності ім. Г.С. Писаренко НАН України
2018
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| Цитувати: | Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing / M.M. Rafi // Проблемы прочности. — 2018. — № 6. — С. 131-145. — Бібліогр.: 46 назв. — англ. |
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irk-123456789-1740172020-12-30T01:26:13Z Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing Rafi, M.M. Научно-технический раздел Recycling of concrete can provide an effective solution for waste management. This study investigates the interfacial bond stress versus slip response of steel bars embedded in recycled aggregate concrete. The bond tests were conducted using concentric pullout specimens. The specimens made from natural aggregate concrete were used as control specimens. Three rebar diameters, two rebar types (hot-rolled deformed and cold-twisted ribbed), and three levels of replacement of recycled aggregates (RA) were considered. The bar embedment length was taken as 5 times the bar diameter. The 12 mm-diameter bar exhibited the highest interfacial bond strength, which decreased with the bar diameter. The bond behavior of bars was nearly unaffected by the concrete type for the levels of RA replacement considered. The cold-twisted ribbed bars demonstrated a stiffer post-peak interfacial bond stress-slip response, as compared to hot-rolled deformed bars. Переработка бетона является одним из эффективных решений проблемы утилизации строительных отходов. Исследуется изменение адгезионных напряжений на стыке стальных арматурных стержней с матрицей из переработанного бетона в зависимости от величины проскальзывания между ними. Адгезионные испытания на растяжение проводились на концентрических образцах. Образцы с наполнителем из стандартного бетона использовались в качестве контрольных. Исследовались арматурные стержни двух типов (горячекатаные деформированные и холоднокрученные ребристые) с разными диаметрами и разной долей добавки переработанного бетона. Длина заделки стержней пятикратно превышала их диаметр. Стержни диаметром 12 мм имели наивысшую прочность межфазного сцепления/ адгезии, которая уменьшалась с увеличением их диаметра. Величина доли добавки переработанного бетона практически не влияла на адгезионную прочность стержней. При этом реакция холоднокрученных ребристых стержней по достижении пика адгезионных напряжений в зависимости от проскальзывания оказалась более жесткой, чем горячекатаных деформированных стержней. Переробка бетону є одним з ефективних розв язків проблеми утилізації будівельних відходів. Досліджується зміна адгезійних напружень на стику стальних арматурних стрижнів із матрицею з переробленого бетону залежно від величини проковзування між ними. Адгезійні випробування на розтяг проводились на концентричних зразках. Зразки з наповнювачем зі стандартного бетону використовували як контрольні. Досліджувались арматурні стрижні двох типів (гарячекатані деформовані і холоднокручені ребристі) із різним діаметром і різною долею домішки переробленого бетону. Довжина защемлення стрижнів у п ять разів перевищувала їх діаметр. Стрижні діаметром 12 мм мали найвищу міцність міжфазового зчеплення/адгезії, яка зменшувалась зі збільшенням їх діаметра. Величина долі домішки переробленого бетону практично не впливала на адгезійну міцність стрижнів. При цьому реакція холоднокручених ребристих стрижнів після досягнення піка адгезійних напружень залежно від проковзування є більш жорсткою, ніж гарячекатаних деформованих стрижнів. 2018 Article Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing / M.M. Rafi // Проблемы прочности. — 2018. — № 6. — С. 131-145. — Бібліогр.: 46 назв. — англ. 0556-171X http://dspace.nbuv.gov.ua/handle/123456789/174017 539.4 en Проблемы прочности Інститут проблем міцності ім. Г.С. Писаренко НАН України |
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Научно-технический раздел Научно-технический раздел |
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Научно-технический раздел Научно-технический раздел Rafi, M.M. Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing Проблемы прочности |
| description |
Recycling of concrete can provide an effective solution for waste management. This study investigates the interfacial bond stress versus slip response of steel bars embedded in recycled aggregate concrete. The bond tests were conducted using concentric pullout specimens. The specimens made from natural aggregate concrete were used as control specimens. Three rebar diameters, two rebar types (hot-rolled deformed and cold-twisted ribbed), and three levels of replacement of recycled aggregates (RA) were considered. The bar embedment length was taken as 5 times the bar diameter. The 12 mm-diameter bar exhibited the highest interfacial bond strength, which decreased with the bar diameter. The bond behavior of bars was nearly unaffected by the concrete type for the levels of RA replacement considered. The cold-twisted ribbed bars demonstrated a stiffer post-peak interfacial bond stress-slip response, as compared to hot-rolled deformed bars. |
| format |
Article |
| author |
Rafi, M.M. |
| author_facet |
Rafi, M.M. |
| author_sort |
Rafi, M.M. |
| title |
Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing |
| title_short |
Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing |
| title_full |
Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing |
| title_fullStr |
Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing |
| title_full_unstemmed |
Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing |
| title_sort |
study of bond properties of steel rebars with recycled aggregate concrete. experimental testing |
| publisher |
Інститут проблем міцності ім. Г.С. Писаренко НАН України |
| publishDate |
2018 |
| topic_facet |
Научно-технический раздел |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/174017 |
| citation_txt |
Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete. Experimental Testing / M.M. Rafi // Проблемы прочности. — 2018. — № 6. — С. 131-145. — Бібліогр.: 46 назв. — англ. |
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Проблемы прочности |
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AT rafimm studyofbondpropertiesofsteelrebarswithrecycledaggregateconcreteexperimentaltesting |
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2025-07-15T10:53:34Z |
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1837709986211299328 |
| fulltext |
UDC 539.4
Study of Bond Properties of Steel Rebars with Recycled Aggregate Concrete.
Experimental Testing
M. M. Rafi
Department of Earthquake Engineering, NED University of Engineering and Technology, Karachi,
Pakistan
rafi-m@neduet.edu.pk
ÓÄÊ 539.4
Ýêñïåðèìåíòàëüíîå èññëåäîâàíèå àäãåçèîííûõ ñâîéñòâ æåëåçîáåòîííûõ
èçäåëèé ñ äîáàâêàìè ïîâòîðíî óòèëèçèðîâàííîãî áåòîíà
Ì. Ì. Ðàôè
Óíèâåðñèòåò èíæåíåðèè è òåõíîëîãèè NED, Êàðà÷è, Ïàêèñòàí
Ïåðåðàáîòêà áåòîíà ÿâëÿåòñÿ îäíèì èç ýôôåêòèâíûõ ðåøåíèé ïðîáëåìû óòèëèçàöèè ñòðîè-
òåëüíûõ îòõîäîâ. Èññëåäóåòñÿ èçìåíåíèå àäãåçèîííûõ íàïðÿæåíèé íà ñòûêå ñòàëüíûõ àðìà-
òóðíûõ ñòåðæíåé ñ ìàòðèöåé èç ïåðåðàáîòàííîãî áåòîíà â çàâèñèìîñòè îò âåëè÷èíû ïðî-
ñêàëüçûâàíèÿ ìåæäó íèìè. Àäãåçèîííûå èñïûòàíèÿ íà ðàñòÿæåíèå ïðîâîäèëèñü íà êîíöåíò-
ðè÷åñêèõ îáðàçöàõ. Îáðàçöû ñ íàïîëíèòåëåì èç ñòàíäàðòíîãî áåòîíà èñïîëüçîâàëèñü â
êà÷åñòâå êîíòðîëüíûõ. Èññëåäîâàëèñü àðìàòóðíûå ñòåðæíè äâóõ òèïîâ (ãîðÿ÷åêàòàíûå
äåôîðìèðîâàííûå è õîëîäíîêðó÷åííûå ðåáðèñòûå) ñ ðàçíûìè äèàìåòðàìè è ðàçíîé äîëåé
äîáàâêè ïåðåðàáîòàííîãî áåòîíà. Äëèíà çàäåëêè ñòåðæíåé ïÿòèêðàòíî ïðåâûøàëà èõ äèà-
ìåòð. Ñòåðæíè äèàìåòðîì 12 ìì èìåëè íàèâûñøóþ ïðî÷íîñòü ìåæôàçíîãî ñöåïëåíèÿ/
àäãåçèè, êîòîðàÿ óìåíüøàëàñü ñ óâåëè÷åíèåì èõ äèàìåòðà. Âåëè÷èíà äîëè äîáàâêè ïåðå-
ðàáîòàííîãî áåòîíà ïðàêòè÷åñêè íå âëèÿëà íà àäãåçèîííóþ ïðî÷íîñòü ñòåðæíåé. Ïðè ýòîì
ðåàêöèÿ õîëîäíîêðó÷åííûõ ðåáðèñòûõ ñòåðæíåé ïî äîñòèæåíèè ïèêà àäãåçèîííûõ íàïðÿ-
æåíèé â çàâèñèìîñòè îò ïðîñêàëüçûâàíèÿ îêàçàëàñü áîëåå æåñòêîé, ÷åì ãîðÿ÷åêàòàíûõ
äåôîðìèðîâàííûõ ñòåðæíåé.
Êëþ÷åâûå ñëîâà: àãðåãàòíîå ñîñòîÿíèå, àäãåçèÿ, ïðî÷íîñòü ïðè ñæàòèè, ðàçðóøåíèå,
èñïûòàíèÿ íà ðàñòÿæåíèå, àðìèðîâàíèå, íàïðÿæåíèå ñäâèãà.
N o t a t i o n
� max – maximum interfacial bond stress
c – concrete cover
c0 – clear distance between the ribs of the reinforcing bar
db – diameter of bar
fc – concrete compressive strength
ld – rebar embedment length
smax – maximum rebar slip
CSA – crushed stone aggregates
RAC – recycled aggregate concrete
RCA – recycled concrete aggregates
SD – standard deviation
© M. M. RAFI, 2018
ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2018, ¹ 6 131
Introduction. Concrete is one of the popular construction materials around the world
and is employed in both structural and non-structural applications. Goldstein [1] estimated
that one ton of concrete is produced annually for each person on earth. Nonetheless, not all
concrete, which is produced during the construction, renovation and demolition (CRD)
activities, is fully utilized. As a result, large amount of concrete waste is generated in
addition to other waste materials. The disposal of CRD waste is considered as one of the
challenges in big cities [2–4]; this waste is most often utilized in landfilling application.
Most of the CRD waste comprises of concrete which is about 75% by weight of all
building materials [5]; this concrete has, therefore, become a burdening waste [6, 7].
Recycling of concrete has attracted attention of the researchers around the world in an
attempt to solve the problem of waste concrete [8–14]. The use of recycled aggregates (RA)
as a substitute of natural aggregates may reduce burden on natural deposits in addition to
solving the waste management problem.
Use of recycled aggregates in structural concrete has so far received lesser attention of
the researchers. Adequate bond between steel and concrete is considered as the most
important aspect in reinforced concrete (RC) design. It allows transfer of stresses to the
steel rebars and influences the performance of RC in terms of crack width, deflection,
plastic hinge rotation capacity, strength of end anchorages and energy dissipation. This
paper presents the result of studies which were conducted to investigate the bond behavior
of steel bars embedded in concrete made with RA, termed as recycled aggregate concrete
(RAC). Pullout tests were carried out to study the rebar bond strength loaded in tension.
Parameters of the study included rebar type and diameter, and level of replacement of
recycled concrete aggregates (RCA) keeping the rest of all factors the same. Different
combinations of RCA and natural crushed stone aggregates (CSA) were tried in RAC. The
bond stress-slip response of the steel bars with RAC was compared with the control
specimens, which were made with CSA concrete.
1. Background and Scope. Concrete is a weak material in tension and is reinforced
with steel bars to enable it to resist tensile stresses. Adequate bond between steel and
concrete is required for safe transference of forces to the steel bars. Friction and adhesion
are the two important factors which are responsible for providing strength to the steel–
concrete interface bond. Therefore, the properties of both the steel and concrete can
influence the bond. The interfacial bond stress (�) is proportional to the normal confining
pressure which is exerted on the bar by concrete cover and transverse reinforcement. The
higher the normal pressure, the higher the frictional force required for pullout [15]. As a
result, the bond strength is increased. � is calculated with Eq. (1) which is the average
stress assuming constant tangential stress along the embedded rebar:
�
�
�
P
d lb d
, (1)
where P is the load db is the diameter of bar, and ld is the embedment length.
Two types of round reinforcing bars are produced in Pakistan [16]. These include
cold-twisted ribbed and hot-rolled deformed bars. The former type has oblique indentations
and is manufactured in accordance with BS 4449 [17] by cold working of an ordinary grade
of steel in which the round bar is subjected to a simultaneous cold rolling and cold ribbing.
The usual method of cold working of reinforcing rebars involves stretching and twisting of
mild steel between two fixed chucks. These bars are usually available with the trade name
of TOR bars. The hot-rolled deformed bars are manufactured by re-rolling of high strength
billet in accordance with ASTM A615 [18]. Both the longitudinal and circumferential
indentations are provided on these bars which are generally considered of superior quality
compared to the cold-twisted ribbed bars. Figure 1 illustrates typical stress–strain curves
M. M. Rafi
132 ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2018, ¹ 6
both for hot-rolled deformed and cold-twisted ribbed bars available in Pakistan. It is noted
in Fig. 1 that while the former bar curve exhibits typical characteristics expected of steel
reinforcing rebar it is hard to identify the yield point on the latter bar curve. Similarly, the
cold-twisted ribbed bar has a small strain hardening zone compared to the hot-rolled
deformed bar. Nonetheless, the cold-twisted ribbed bars are also demanded by the local
construction industry due to their lower prices, as compared to the hot-rolled deformed
bars. Both these bars have been employed in this study.
The studies related to performance of structural members made with RAC are limited
[19–26]. Satisfactory bond behavior is essential for the performance of an RC structure.
This behavior can be studied by carrying out pullout tests on steel bars embedded in
concrete specimens. The available research indicates that these tests can provide reliable
estimates of the bond efficiency of deformed reinforcing bars [27, 28]. While different
aspects of RC made with RAC have been investigated by the researchers [20–26], there
have been limited studies in the technical literature on the investigation of its bond behavior
with steel bars [28–34].
A review of the aforementioned studies indicates several gaps. For example, Prince
and Singh [28, 34] employed 8, 10 and 12 mm-diameter bars, which are normally required
for stirrups. As a result, the results of the study may not apply to the bars required for
flexure. The study conducted by Kim and Yun [32] employed 16 mm-diameter bars, which
were non-conforming to ASTM A615 [18] as the yield strength ( f y ) of these bars was
383 MPa. Xiao and Falkner [30] used 10 mm-diameter bar with 50 percent and 100 percent
RCA replacement. Not only that this diameter bar is suitable only for stirrups (as mentioned
in the above), the acceptance of such high replacement ratios of RCA in structural concrete
will be difficult by the regulatory authorities. As a result, the results of the above study may
not be used in design applications. Butler et al. [31] employed 25 mm-diameter bars; no
information, however, was provided on the properties of the bars. Finally, Lima et al. [33]
used 10 mm-diameter bars for the pullout specimens, which were casted using 30, 60, and
100 percent levels of replacement of RCA. Similar to the aforementioned studies, the major
shortcomings of this study include high level of RCA replacement, bar size which is not
suitable for longitudinal reinforcement and unavailability of steel bar properties. The study
presented in this paper is an attempt to fill these identified gaps by considering a wide
range of variables. Pullout test specimens were employed to study the bond behavior of
steel bars with the normal strength RAC made from ordinary Portland cement. The test
program included three levels of RCA replacement, different bar diameter, and both the
cold-twisted ribbed and hot-rolled deformed bars. The embedment length of the bar was
Study of Bond Properties of Steel Rebars ...
ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2018, ¹ 6 133
Fig. 1. Typical stress–strain curves of bars.
taken as 5db . The selection of this embedment length is attributed to the fact that for short
ld , a uniform distribution of � along the embedded surface can be assumed [29, 35], which
can be calculated by Eq. (1). Other embedment lengths, cement types, bar types, RCA
replacement levels and splitting tests are beyond the scope of this study. Similarly,
economic and durability aspects of RAC are not considered.
2. Experimental.
2.1. Materials.
2.1.1. Aggregates. Two types of coarse aggregates were employed in this study: CSA
and RCA. CSA and fine aggregates (sand) were purchased from a local supplier. The
recycled aggregates were obtained from the concrete waste which was generated from the
commercial testing of concrete specimens in the Material Testing Laboratory of the
Department of Civil Engineering at NED University. Note that the employed waste is of
unknown origin, quality and/or composition as these specimens were sent in the laboratory
for testing by the local construction industry. This could be regarded as an important aspect
in the perspective of practical use of RA in structural concrete. The aggregates were sieved
using sieves of required sizes and were blended in the proportion to provide grading similar
to CSA. Tests on aggregates were carried out in accordance with the relevant ASTM
standards to determine their properties. A summary of the test results is given in Table 1. It
can be noted in Table 1 that the specific gravity and density of CSA is higher than RCA,
whereas impact values, LA abrasion and absorption of the latter are significantly higher
than those of the former aggregates. This could be attributed to the presence of cement-sand
mortar in RCA, which is lightweight and porous.
The sieve analyses for both fine and coarse aggregates (RCA and CSA) were
performed, in accordance with ASTM C-136 [39]; both aggregate types complied with the
requirements of ASTM C33 [41]. The fineness modulus of sand is given in Table 1.
2.1.2. Cement. Ordinary Portland cement (OPC) was employed in this study. The
cement, which complied with ASTM C150 [42], was purchased from a local supplier.
2.1.3. Steel Rebars. Two types of steel rebars were employed in the test program, as
mentioned earlier. These include hot-rolled deformed and cold twisted ribbed bars. The
bars of diameter 12, 16, and 20 mm were employed. The steel bar properties are given in
Table 2, which were determined by carrying out tensile tests on the bars in the laboratory, in
accordance with ASTM E8/E8M [43].
134 ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2018, ¹ 6
M. M. Rafi
T a b l e 1
Properties of Aggregates
Property Standard CSA RCA Sand
Bulk specific gravity ASTM C127/C128*
(ASTM 2001a,b) [36, 37]
2.67 2.38 2.65
Absorption (%) ASTM C127/C128*
(ASTM 2001a,b) [36, 37]
0.67 5.13 2.3
Loose density (g/cc) ASTM C29 (ASTM 1997) [38] 1.6 1.33 2.08
Dry rodded density (g/cc) ASTM C29 (ASTM 1997) [38] 1.83 1.53 2.18
Fineness modulus ASTM C136 (ASTM 2001) [39] – – 2.86
Impact (%) ASTM C131 (ASTM 2014) [40] 7.05 28.49 –
L. A. abrasion (%) ASTM C131 (ASTM 2014) [40] 27.21 34.70 –
* ASTM C128 for sand.
As mentioned before, the embedment length of bar was taken as 5db ; embedment
lengths (ld ) of 60, 80, and 100 mm were used, respectively, for 12, 16, and 20 mm diameter
bars. The remaining length of the bar was debonded using three layers of plastic tape
applied over a layer of polythene sheet.
2.2. Specimen Details. Pullout test specimens of 200 mm cubes were cast, in
accordance with RILEM technical recommendations [44]. The target 28-day concrete
compressive strength ( fc ) was taken as 21 MPa and the mix design procedure was carried
out in accordance with ACI 211.1-91 [45] using a water/cement ratio of 0.5. The concrete
mix used is described in Table 3. Based on some trial and error calculations, the amount of
cement with each of the aforementioned level of RCA replacement was arbitrarily increased
by 5 kg/m3, in order to keep the concrete slump and fc nearly the same for all mixes
(Table 3). Cylinders of 100 200� mm size were cast to determine the concrete strength in
compression and tension. Splitting tests were conducted to determine the concrete tensile
strength. The concrete type in Table 3 was designated based on percent replacement of
RCA.
Mixing of concrete was performed using a mixer. The maximum aggregate size was
12 mm. Potable tap water was mixed in the concrete and the exact amount of water varied
depending on the moisture contents of aggregates. The slump of concrete ranged from 40 to
50 mm; it was measured at each casting of batch and is given in Table 3. The bars were cast
keeping them in the center of the cube with the aforementioned embedment lengths. The
concrete in the steel mold was vibrated using a mechanical vibrator of shaft diameter 16 mm.
ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2018, ¹ 6 135
Study of Bond Properties of Steel Rebars ...
T a b l e 2
Tensile Properties of Steel Bars
Bar type Bar
diameter
(mm)
Rib
spacing
(mm)
Rib
height
(mm)
Elastic
modulus
(GPa)
Yield
strength
(MPa)
Ultimate
strength
(MPa)
Strain
at yielding
(%)
Cold-
twisted
ribbed
12 7.2 0.83 207 439 549 0.21
16 8.9 1.10 203 420 488 0.21
20 9.7 1.10 206 423 495 0.21
Hot-rolled
deformed
12 7.2 0.83 203 461 623 0.23
16 8.9 1.10 205 516 645 0.25
20 9.7 1.10 204 523 595 0.26
T a b l e 3
Details of Concrete Mixes
Concrete
type
Cement
(kg/m3)
Sand
(kg/m3)
Coarse
aggregate
(kg/m3)
Slump
(mm)
28-day
strength
(MPa)
Splitting
tensile strength
(MPa)
0% RAC 315 791 1135 45 23 2.10
10% RAC 320 791 1135 45 26 2.80
20% RAC 325 791 1116 50 26 2.85
30% RAC 330 791 1116 40 25 2.17
The concrete specimens were removed from the mold after 24 hours and were cured in a
water tank for 28 days. The 28-day compressive and splitting tensile strengths of concrete
are given in Table 3 for all the mix types which is an average of 3 cylinders each. It is seen
in Table 3 that the 28-day strength for all mix types is close to each other. The specimens
were moved in the laboratory environment at the end of curing period and were kept there
until testing. The testing of specimens started after 28 days and was completed in nearly
one and a half month.
A total of 72 pullout specimens were cast and tested for the test program. The details
of these specimens are given in Table 4. The notation of the specimen is as follows: the first
two numbers indicate percentage of RCA replacement, the second letter (P) represents a
pullout specimen, this is followed by the bar diameter and the letter in the end is the type of
bar, such as D for hot-rolled deformed and T for cold-twisted ribbed bar. For example,
10P12T is a pullout specimen made with 10 percent replacement of RCA with a cold-
twisted ribbed bar of 12 mm-diameter.
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T a b l e 4
Details of Pullout Specimens
Specimen Nos. Bar type ld
(mm)
fc (MPa) � max
(MPa)
� max
fc
Bond
ratioMean SD
0P12D 3 Hot-rolled 60 24.09 0.46 24.36 4.96 1.00
10P12D 3 Hot-rolled 60 26.73 1.17 23.82 4.61 0.93
20P12D 3 Hot-rolled 60 25.81 0.59 24.04 4.73 0.95
30P12D 3 Hot-rolled 60 30.12 1.60 26.27 4.79 0.97
0P16D 3 Hot-rolled 80 24.09 0.46 20.93 4.26 1.00
10P16D 3 Hot-rolled 80 28.54 1.29 21.69 4.06 0.95
20P16D 3 Hot-rolled 80 25.81 0.59 19.67 3.87 0.91
30P16D 3 Hot-rolled 80 31.82 0.64 17.34 3.59 0.84
0P20D 3 Hot-rolled 100 21.64 0.54 20.10 4.32 1.00
10P20D 3 Hot-rolled 100 26.73 1.17 19.23 3.72 0.86
20P20D 3 Hot-rolled 100 33.09 1.27 21.59 3.75 0.87
30P20D 3 Hot-rolled 100 31.82 0.64 19.28 3.42 0.80
0P12T 3 Cold-twisted 60 21.64 0.54 23.70 5.09 1.00
10P12T 3 Cold-twisted 60 28.54 1.29 26.32 4.08 0.80
20P12T 3 Cold-twisted 60 25.81 0.59 21.84 4.13 0.81
30P12T 3 Cold-twisted 60 30.12 1.60 25.16 4.58 0.90
0P16T 3 Cold-twisted 80 24.09 0.46 22.14 4.51 1.00
10P16T 3 Cold-twisted 80 28.54 1.29 22.37 4.19 0.93
20P16T 3 Cold-twisted 80 33.09 1.27 22.95 3.99 0.88
30P16T 3 Cold-twisted 80 30.12 1.60 22.23 4.05 0.90
0P20T 3 Cold-twisted 100 21.64 0.54 21.11 4.54 1.00
10P20T 3 Cold-twisted 100 26.73 1.17 19.23 4.22 0.93
20P20T 3 Cold-twisted 100 33.09 1.27 22.14 3.85 0.85
30P20T 3 Cold-twisted 100 31.82 0.64 20.77 3.68 0.81
2.3. Instrumentation. Pullout tests were performed using a universal testing machine
(UTM). Linear variable differential transducers (LVDTs) were used to measure the bar
displacement. Two LVDTs were used on the loaded end whereas one was fixed at the
unloaded end. The test setup is shown in Fig. 2. The load was applied at a rate of 3 kN/s on
the bar, and the data of applied load and slip were constantly recorded through a data
acquisition system. Concrete strength on the day of testing and SD are given in Table 4,
which results are averaged from the data on 3 cylinders.
3. Results and Discussion. The relative displacement between the loaded end of the
bar and the concrete is referred as loaded-end slip whereas the relative displacement
between the rebar free-end and the concrete is referred to as the free-end slip in the
forthcoming discussion. The results in the forthcoming sections are based on an average of
three pullout specimens.
3.1. Failure Mode. All tested specimens typically exhibited the bond failure after the
concrete in front of bar lugs (ribs) was crushed and the bar was pulled out. Figure 3 shows
view of some of the failed specimens along with the pulled out bars and a schematic of the
failure mechanism. This behavior was the same for all specimens and was irrespective of
the bar type (hot-rolled or cold-twisted) and diameter, and the type of concrete mix.
Figure 4 illustrates representative plots of data of slip (s) for the loaded and free ends
of the bar versus the interfacial bond stress. The slip in the bar is a result of stress-strain
compatibility requirements between steel and concrete which are, respectively, in tension
and compression due to the applied force. It is noted in Fig. 4 that the free-end slip
precedes the loaded-end slip for all the specimens. These plots are typical for other bar
diameter and type, and the concrete type (Table 3). Possible factors to cause this difference
include initial settling of specimen on the plate, play in the support of LVDTs, elongation of
bar and wedging of concrete into indentations of the bar. Further, a considerably long
descending branch is seen in Fig. 4 without sudden failure, after the maximum interfacial
bond stress (� max ) is reached; this indicates a ductile bond failure for all the specimens
included in the testing program. This descending branch is a result of the resistance offered
by the frictional component of bond beyond the maximum bond strength. Note that the
adhesion is generally lost at low values of bar slip.
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a b
Fig. 2. Setup of pullout test: (a) lab specimen; (b) schematic arrangement.
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M. M. Rafi
a
b
Fig. 3. View of failed pullout specimens (a) and schematic of failure mechanism [46] (b).
a b
c d
Fig. 4. Free-end versus loaded-end slip: (a) 0P12D; (b) 10P16D; (c) 20P16T; (d) 30P20T.
3.2. Effect of Bar Diameter. Figure 5 illustrates the effect of bar diameter on the
interfacial bond stress of the rebar. The data of the unloaded ends of the bars have been
reported in Fig. 5 and in the forthcoming sections. The results of both the hot-rolled
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Study of Bond Properties of Steel Rebars ...
Fig. 5. Comparison of pullout behavior of different diameter bars.
deformed and cold-twisted ribbed bars have been included in Fig. 5. It is seen in Fig. 5 that
the 12 mm bar offers higher resistance to slip as compared to the other diameter bars (16
and 20 mm). As a result, the interfacial bond strength is higher for specimens employing 12
mm diameter bars. The observed bond stress is maximum for the specimens 30P12D and
10P12T (Table 4). The interfacial bond stress decreases with increase in the bar diameter
and the specimens with 16 and 20 mm-diameter bars exhibited lower interfacial bond
stress. This can be explained due to closely spaced ribs for 12 mm bars (average distance of
7.2 mm), as compared to 16 and 20 mm-diameter bars, which had average rib distance of
8.9 and 9.7 mm, respectively. As a result, 12 mm-diameter bars possessed better friction
and adhesion properties that increased the bond strength between the bar and the concrete.
Note that the rib spacing was measured using the tested bars between two successive ribs
along the same horizontal line on the bar circumference and an average of three
measurements was used. The same method was followed for both the hot-rolled deformed
and cold-twisted ribbed bars. Further, it is seen in Fig. 5 that the specimens made with
12 mm-diameter hot-rolled deformed bars exhibited less ductile behavior in the post-peak
region of bond stress–slip curve with a short descending branch. The length of descending
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Fig. 6. Effects of concrete type on interfacial bond stress-slip behavior.
branch increased with the bar diameter. This can be attributed to better bearing on the lugs
provided by the 16 and 20 mm-diameter bars, which had an average height of lugs of
1.1 mm, as compared to 0.83 mm lug height for the 12 mm-diameter bars. For cold-twisted
ribbed bars, the post-peak ductility of the interfacial bond is similar for all bars and all
specimens provided nearly the same level of ductility.
3.3. Effect of Concrete Type. Figure 6 presents a comparison of influence of RAC on
the interfacial bond stress-slip behaviors of the bars. It is seen in Fig. 6 that the behavior of
bar is similar for control specimens and those made with RCA replacement. Similar
pre-peak response, � max and post-peak behavior of the bond characteristics of the bar of
each diameter and type are seen in these plots. Xiao and Falkner [30] and Prince and Singh
[28, 34] also reported similar bond strength with specimens made with and without addition
of RCA. On the other hand, Butler et al. [31] found up to 19% lesser bond strength in
pullout specimens made with RCA. The values of � max for the bars with each of the
employed concrete types are given in Table 4. It can be noted in Table 4 that the differences
in � max are quite small (within 10%) and, thus, are negligible.
3.4. Effect of Bar Type. Figure 7 presents a comparison of bond-slip behaviors of the
hot-rolled deformed and cold-twisted ribbed bars. It is noted in Fig. 7 that both type of bars
attained similar � max at all levels of RCA replacement which was similar to the control
specimen. In general, the cold-twisted ribbed bars showed higher ductility in the post-peak
region with a long descending branch as compared to the hot-rolled deformed bars. This
may be partly attributed to better bearing on the lugs provided by cold-twisted ribbed bars
to the bond strength, which increased the frictional resistance of these bars to the bond
failure. The difference is more significant for smaller diameter bars and disappears as the
bar diameter is increased. As a result, the specimens made with 20 mm bars showed nearly
the same maximum slip for both the hot-rolled deformed and cold-twisted ribbed bars
before failure. In addition, the cold-twisted ribbed bars offered higher frictional resistance
to reduce the amount of slip in the post-peak region, as compared to the hot-rolled
deformed bars. This resulted in a stiffer post-peak bond stress-slip response of the
cold-twisted bars.
3.5. Interfacial Bond Strength. Figure 8 depict the normalized stress versus the bar
diameter curves. Here the maximum interfacial bond stress was normalized by fc . It is
seen in Fig. 8 that the normalized stress decreases with the bar diameter. As a result, the 12
mm-diameter bar has the highest normalized stress. The change in this stress between 16
and 20 mm-diameter bar is nominal. The behavior of both hot-rolled deformed and
cold-twisted ribbed bars are the same in this respect. The effects of concrete type on the
normalized stress are also seen in Fig. 8. In general, the normalized stress reduces with the
level of RCA replacement. Table 4 provides the data of bond ratio, which is the ratio of
normalized bond of the bars embedded in RAC specimens to that of the control ones. It is
seen in Table 4 that the difference in the bar interfacial bond strength with most of the RAC
specimens is within 15%, as compared to the control ones.
Conclusions. This paper reported studies related to the bond tests of the steel bars.
The bond behaviors of hot-rolled deformed and cold-twisted ribbed bars with RAC was
investigated using concentric pullout specimens. Three levels of RA replacement in RAC
were employed. The diameter of bars included 12, 16, and 20 mm. The embedment length
of the bar was taken as 5db . Specimens made from natural aggregate concrete were used as
control specimens. The following conclusions have been drawn from the studies presented.
1. All pullout specimens failed by pullout mode of bond failure, which was the
expected failure mode. The failure was caused by the pulling of the bar after a significantly
larger rebar slip. Smaller diameter bars demonstrated higher interfacial bond strength due to
closer ribs in these bars. The bond strength decreased as the bar diameter was increased.
This can be attributed to increased rib spacing in the larger diameter rebars.
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Fig. 7. Effect of bar type on interfacial bond stress–slip behavior.
Fig. 8. Normalized interfacial bond stress versus bar diameter: (a) hot-rolled deformed bars; (b) cold-
twisted ribbed bars.
a b
2. The bond behavior of a particular diameter bar was unaffected by the replacement
of RA in concrete and was similar to the control specimen. These are significant results
which indicate that the bond behavior of RAC remains similar as the concrete made with
virgin aggregates. Most of the bars achieved up to 85 percent of normalized bond strength
( )max� fc in RAC, as compared to the concrete made from natural aggregates.
3. The bond stress-slip response of both the hot-rolled deformed and cold-twisted
ribbed bars was similar in the ascending part of the curve. The bond characteristics of the
hot-rolled deformed bars indicated lesser ductility in the descending portion of the observed
bond stress-slip envelope as compared to the cold-twisted ribbed bars. This indicates that
the indentations on the cold-twisted ribbed bars provide better friction resistance to bond,
as compared to that of hot-rolled deformed bars.
4. Although the pullout specimens with different bar types and sizes, concrete strength
and level of RCA addition were used for the findings and suggestions made in this study,
these apply only to bars with the embedment length not exceeding 5db .
Ð å ç þ ì å
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Äîñë³äæóâàëèñü àðìàòóðí³ ñòðèæí³ äâîõ òèï³â (ãàðÿ÷åêàòàí³ äåôîðìîâàí³ ³ õîëîäíî-
êðó÷åí³ ðåáðèñò³) ³ç ð³çíèì ä³àìåòðîì ³ ð³çíîþ äîëåþ äîì³øêè ïåðåðîáëåíîãî
áåòîíó. Äîâæèíà çàùåìëåííÿ ñòðèæí³â ó ï’ÿòü ðàç³â ïåðåâèùóâàëà ¿õ ä³àìåòð.
Ñòðèæí³ ä³àìåòðîì 12 ìì ìàëè íàéâèùó ì³öí³ñòü ì³æôàçîâîãî ç÷åïëåííÿ/àäãå糿, ÿêà
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áåòîíó ïðàêòè÷íî íå âïëèâàëà íà àäãåç³éíó ì³öí³ñòü ñòðèæí³â. Ïðè öüîìó ðåàêö³ÿ
õîëîäíîêðó÷åíèõ ðåáðèñòèõ ñòðèæí³â ï³ñëÿ äîñÿãíåííÿ ï³êà àäãåç³éíèõ íàïðóæåíü
çàëåæíî â³ä ïðîêîâçóâàííÿ º á³ëüø æîðñòêîþ, í³æ ãàðÿ÷åêàòàíèõ äåôîðìîâàíèõ
ñòðèæí³â.
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