3101 Conference on OUR WORLD IN CONCRETE & STRUCTURES: 16 -17 August 2006, Singapore

BOND STRENGTH OF REINFORCED LATERIZED CONCRETEBEAMS

F. Falade, University of Lagos, Akoka-Lagos, Nigeria.G.L Oyekan*, University of Lagos, Akoka-Lagos, Nigeria.

ABSTRACTThis paper presents the results of an investigation into the effects of reinforcementdiameter and embedment depth on the bond ,st~~J!9m:;,.Rf.reintorced , laterizedconcrete beams. Four different bar diameters (12min, 'l6"mm, 20mrri~a'hd '25m'm)were considered. Three embedment depths were used; namely, 3d, 4d and 5d (d =bar diameter). The specimens weretested at curing ages of 7, 14,21 and 28 days.The results show that (i) for a given depth, bond strength decreased with increase inreinforcement diameter (ii) the higher the depth, the higher the value of bondstrength and (iii) generally there was increase in bond strength with age.

Keywords: Bond Strength, Laterized Concrete, Beams, Reinforcement, Embedmentdepth, Pullout rods

1.0 IntroductionLaterized concrete is concrete having its fine aggregate content made of laterite. Theconcrete has been the subject of many investigations by many researchers in thelast three decades (1, 2, 3, 4, 5). Properties of laterized concrete such as its shearstrength, moment capacity, resistance to impact, its response to suddenly appliedloads, high temperature exposure among others have also been investigated (6,7,8).It has been established, that the compressive strength of laterized concretecompares favourably with that of plain normal concrete.The effectiveness of reinforced concrete depends on the interaction between theconcrete matrix and reinforcement rebars. Three mechanisms can be identified as,contributing to the bond between concrete and the steel reinforcement: (i) adhesionbetween the steel and concrete matrix (ii) friction between the steel and thesurrounding concrete matrix and (iii) mechanical anchoring of the steel to concretethrough the bearing stress that develops between the concrete and the deformationsof the steel bars.When a bar is pulled from a concrete matrix, the rib bears against the surroundingconcrete, friction and adhesion between the concrete and steel along the face of therib act to prevent the rebar from sliding. This force acts vectorially to the bearingstress acting perpendicularly to the rib to yield the bond strength. The bearing iscontrolled by the radial pressure that the concrete cover and lateral reinforcementcan resist before splitting and the effective shear strength capable of shearing theconcrete surrounding the rebar. Pullout failure occurs when the steel bar is wellconfined by the concrete cover or the transverse reinforcement prevents a splitting

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failure. The pullout failure is primarily due to bearing of the ribs against the concretecausing the key between ribs to shear from the surrounding concrete.Charman and Shan (9) conducted an investigation to determine the bond strengthbetween reinforcing steel and concrete at an early age. Their investigations revealedthat smooth bars did not exhibit any age effect, while the bond behaviour ofdeformed bats was highly aqedependent, Their conclusion was that adhesion andfriction had relatively small contribution to the bond strength compared to the bondstrength that derives from the bearing stresses that develop between thedeformations on the steel and the surrounding concrete.Brettman et al (10) studied the effect of superplasticizers that are extensively usedwhen producing high-strength concrete on concrete steel bond strength. In theirinvestigation, they included the effect of concrete slump, degree of consolidation,concrete temperature and bar position on the bond strength of deformed reinforcingbars embedded in concrete, with and without plasticizer. The bond tests wereconducted on concrete of strength between 28.2N/mm2 and 33.8N/mm2 using amodified cantilever beam. The experimental results indicated that high-slumpsuperplasticized concrete provided a lower bond strength than low slump concrete ofthe same compressive strength. It was also observed that vibration of high-slumpconcrete increased the bond strength compared to high slump concrete withoutvibration. Treece and Jirsa (11) on their part conducted experimental investigation onthe bond strength of epoxy-coated reinforcing bars embedded in normal and high-strength concrete and compared it to that of uncoated bars. The results showed thatepoxy coating significantly reduced the bond strength of reinforcing bars; for splittingfailure the bond strength was about 65% of the bond strength of uncoated bars whilefor a pullout failure, the bond strength was about 85% of that of the uncoated bars.The results also indicated that the reduction in bond strength was independent of barsize and concrete strength and also that the bond strength was not affected byvariations in the coating thickness when the average coating thickness was between0.075rnm and 0.35mm. Wilby (12) reported that for reinforcement to be utilisedsatisfactorily, it has to bond to concrete so that a reinforced beam bonds as though itis a homogenous member. Ezeldin and Balaguru (13) conducted experimentalstudies onthe .bond ~1rE!Q9!b,<b~h~viourof bars embedded in- high:..tensile concretewith and without steel fibres. Four bar diameters were used (3mm, 5mm, 6mm and8mm); they found that the addition of silica fume increased the bond strength ofconcrete.The bond strength of reinforcing bars in both high strength and normal strengthlaterized concrete has not been investigated. Therefore there is a need to study thebond characteristics of reinforced laterized concrete with a view to properly guide theusers of the material which has been widely acknowledged as a good substitute fornormal concrete for construction works.

The objectives of this study are: to establish the relationships between the bondstrenqth, the diameter of the bar, embedment depth and curing age of reinforcedlaterized concrete.

2.0 Materials and Experimental Procedure

2.1 MaterialsThe materials used in this study are: cement, aggregates (fine and coarse), waterand reinforcement. Ordinary Portland cement whose properties conformed to therequirements of 8S12 (14) was used. The fine aggregate was laterite with particle

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size range of 4.75-0.063mm (particles passing through sieve with aperture 4.75mmbut retained on sieve with aperture 0.063mm) while the coarse aggregate wascrushed rock having particle size between 20.00 and 14.00mm. The pullout inserts(rods) were prepared from high yield reinforcement from rods of different diameters(12mm, 16mm, 20mm and 25mm). Engineers generally recommend the use of highyield instead of mild steel. The advantage of using high yield bars is that the mass ofsteel required is reduced, and even though its cost per kilogram is slightly higherthan mild steel, the total cost of the reinforcement and its fixing can be reduced.

2.2 Preparation of SpecimensThe pullout inserts (rods) were cut into short lengths. They were threaded to aconstant depth of 30mm at one end to facilitate fastening to the loading frame of thepullout rig.

d = variable

.} L (variable) 1_ 120mml< 30mm"'f--~=----=1

Fig. 1: Pullout Rod

The concrete mixture was obtained by weighing the appropriate proportion of theconcrete components into the mobile rotating drum mixer type and intimately mixingthem together and adding the corresponding quantity of water to the mixture. Theconstituents were proportioned using 1:2:4 (cementlaterite:granite chips) mix. Whenthe mixture had been mixed to a homogenous state, the mixture was emptied fromthe mixer to a flat plate. 150mm cubes were cast. Each mould for the cube was filledwith concrete in three layers using a trowel. Each layer was compacted using 16mmdiameter tamping rod to tamp 25 times in accordance with BS 1881 (15). The pulloutrods were placed centrally and vertically in the mould at the time of casting the cubeswhile the embedment length was as predetermined. The moulds with the contentwere left with polythene cover for 24±Y2 hours to adequately set before demoulding.The specimens were then demoulded and transferred into a curing tank thatcontained clean water where they were stored till the respective testing ages atcuring temperature of 21±1oc .

2.3 Testing of SpecimensThe cube specimens were tested on 600 Avery Dension Universal testing machinewhile the pullout specimens were tested on pullout rig (Fig. 2).

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- r--

- - - -- - - -

~, ..... ,_/F= ../"

1s=2~11lJ

~

- - - -, I- - - -

A-

----1l

--- - - -

'--- -

Proving Ring

Pullout Rod

Loading Frame

Reaction Frame

150MM concreteCube SpecimenSupport

Specimen Seat

Main Frame

Hydraulic Jack

Fig. 2: The Pullout Rig (Front View)

The load was applied through an hydraulic jack. Readings were taken on the dialgauge attached to the proving ring. The specimens were tested at the curing ages of7, 14,' 21 and 28 days, The average of the loads at which agrqup of three specimensfailed was determined and used for "calculating lhe bond strength for each age.

3.0 Results and Discussions

3.1 ResultsThe compressive strength values are: 12.45N/mm2, 16.0N/mm2, 18.70N/mm2 and20.15N/mm2 at 7, 14, 21 and 28 days respectively. The variation of bond strengthswith embedment depths and curing ages for each bar size are presented in Fig. 3-6,while Fig. 7-9 show the variation of bond strength with different bar diameters.

3.2 DiscussionDuring the pullout test, when load is applied to specimens through the rods, theconcrete is in compression while the rods are in tension. On application of the load,the pullout inserts (rods) resisted the tendency to being pulled out. The resistance ofthe Inserted rod can be attrIbuted to mInute Irregularitie$ on the bar mechanicallylocking to the concrete. The applied load is gradually transmitted to the concrete byfrictional resistance. When the applied force and the adhesive resistance can nolonger overcome it then the iron rod is pulled out leaving a groove behind.

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The bond strength decreased with increase in the diameter of reinforcement. Forexample, in Fig. 3 at 14-day curing age, the bond strength is 4.62N/mm2 for 12mmdiameter bar and embedment depth of 3d.

10,

9

~8

~7

6s:D 5c:! 4iii"0 3c:0m 2

1

00 7 14 21 28

Curing Age (Days)

Fig. 3: Variation of bond strength with different curing C1Qes. and embedment'depths for 12mm diameter bar"c,'

In Fig. 4, for the same age and embedment depth of reinforcement, the strengthvalue is 3.14N/mm2 for 16mm diameter bar.

7

6

0+----o 7 14 21

Curing Age (days)

Fig. 4: Variation of bond strength with different curing agesand embedment depth for 16mm diameter bar

28

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While in Fig. 5 for 20mm diameter bar and embedment depth of 3d, the strength is2.17N/mm2 at 14-day curing age

..-----_ ......•5

o 7 Curing A~! (Days) 21

Fig. 5: Variation of bond strength with different curing agesand embedment depth for 20mm diametre bar

28

For the same age and embedment depth, the bond strength is 1.90N/mm2 for 25mmdiameter bar (Fig. 6). The effective pullout force increased in bar diameter but thebond strength decreased.

6[-+-3d . 4<1.--.-Sd I

5·

------~,---------,o 14

Curing Age (Days)

Fig.6 : Variation of bond strength with different curing agesand embedment depth for 25mm diametre bar

7 21 28

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The bond strength was determined for each group of three specimens that weretested by using the expression:

FfbS=

Uldfbs= bond strengthU = circumference of the barId = embedment depth

The increase in the pullout force is not proportionate to the product of circumferenceand embedment depth and therefore reduction in bond strength.The strength increased with increase in embedment depth for examfle, in Fig. 3, for12mm diameter bar at 7-day curing age, the strength is 2.26N/mm for embedmentdepth of 3d (36mm) while the values are 2.76N/mm2 and 3.44N/mm2 for embedmentdepths of 4d (48mm) and 5d (60mm) respectively for the same diameter and age.The" trend of increase in strength with embedment depth was also observed in otherspecimens containing 16mm, 20mm and 25mm diameter bars (Fig. 4-6). Thisbehaviour can be attributed to increase in surface area which provides higher bondsurface and therefore higher resistance to pullout force. This in turn results in higherbond strength.Generally, there was increase in bond strength with increase in age. For example inFig. 3 for 12mm bar and embedment depth of 4d, the strength values are2.76N/mm2, 5.13N/mm2, 6.92N/mm2 and 7.27N/mm2 for 7,14,21 and 28 daysrespectively. The same trend of increase in strength with age was observed in otherbar diameters and embedment depths. The bond strength of reinforced concretestructural elements among others depends on the bond between concrete matrixand the reinforcement to ensure effective stress transfer from steel to thesurrounding concrete. The bond is higher when the compressive strength of concreteincreases because the grip effect on reinforcement is enhanced. Therefore, at laterages the bond strength increased because the compressive strength of concreteincreased under normal curing conditions.Figures 7-9 show the strength values for different bars for a given embedment depthat different curing ages.

1

o+-------~--------~------~~------~o 7 14 21 28

Curing Age (Days)

Fig. 7: Bond strength for different bar diametre at differentcuring ages for embedment depth of 3d

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8 ; - --- ------ ----- ---- ------ ----- ----]--

"

--+-12mm 16mm

-...- 20mm 25mm------_.- ---------.----- --

7

:

7 21 28o 14Curing Age (Days)

Fig. 8: Bond strength for different bar diametre at differentcuring ages for Embedment depth of 4d

10

9[

_12rnm

-'-2Omm8-

~ 7E~ 6.cCI 5·~en 4-0c~ 3

2

~L-.------,------o 7 14 21 28Curing Age (Days)

Fig. 9: Bond strength for different bar diametre at differentcuring ages for embedment depth of 5d

Figures 7-9 indicate that for the same embedment depth, the strength reduced forhigher diameter bars. For example, at 3d for 12mm diameter bar and 7-day curing,the bond strength is 2.26N/mm2 while at the same depth ratio the values are1.70N/mm2, 1.51N/mm2 and 1.46N/mm2 for 16mm, 20mm and 25mm diameter barsrespectively

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4.0 CONCLUSIONSFrom the results obtained in this study, the following conclusions are made:(i) The larger the diameter of reinforcement, the lower the bond strength for a givendepth.

(ii) Bond strength increases with embedment depth and curing ages.

(iii) Smaller diameter bars provide higher bond strength.

5.0 REFERENCES1. L. A. Balogun & D. Adepegba (1982), 'Effect of Varying Sand Content in

Laterized Concrete', The International Journal of Cement Composites andLightweight Concrete, Volume 4, Number 4, pp 235-240.

2. D. A. Adepegba (1978), 'A Comparative Study of Normal Concrete withConcrete which Contained Laterite instead of Sand', Building Science,Volume 10, pp 135-141.

3. M. A. Salau & L.A. Balogun (1990), 'Shear'Resistance of Reinforced LaterizedConcrete Beams without Shear Reinforcement', Building and Environment,Vol. 25, No.1, pp 71-76.

4. L. A. Balogun (1986), 'Effect of Temperature on the Residual Strength ofLaterized Concrete', Building and Environment, Vol. 21, No. 3/4, pp 221-226.

5. F. Falade (1994), 'Influence of Water/Cement Ratio and Mix Proportion onWorkability and Characteristic Strength of Concrete Containing Laterite FineAggregate', Int. Journal of Building and Environment, Edinburgh, Vol. 29, No.2, pp 237-240.

6. D. Adepegba (1977), 'Structural Strength of Short, Axially Loaded Columns inReinforced Laterized Concrete', Journal of Testing and Evaluation, Vol. 5, No.2, pp 134-140.

7. F. Falade (1991), 'Behaviour of Laterized Concrete Beams under Moment andShear', Ife Journal of Technology, Nigeria, Vol. 3, No.1, pp 7-12.

8. G. L. Oyekan and L. A. Balogun (1997), 'Impact Resistance of Plain LaterizedConcrete', Proc, International Conference on Building Envelope Systems andTechnology, University of Bath, UK, pp 141-153.

9. Chapman, R. A. and Shah, S. P. (1987), 'Early Age Bond Strength inReinforced Concrete', ACI Journal, 84, No.6, pp 501-:·510.

10. Brettman, B; Darwin, D and Donahey, R (1983), 'Bond of Reinforcement toSuperplasticized Concrete' Proceedings ACI Journal, pp 98-107.

11. Treece, R. A. and Jirsa, J. O. (1989), 'Bond Strength of Epoxy-CoatedReinforcing Bars', Proceedings, ACI Materials Journal, 86, 2, March-April, pp167-174.

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12. C. B. Wilby (2000), 'Concrete Materials and Structures', 3rd Edition,Cambridge University Press, London.

13. Ezeldin, A. S. and Balaguru, P. N. (1989), 'Bond Behaviour of Normal andHigh-Strength Fibre Reinforced Concrete' ACI Materials Journal, Proc. 86, 5,September-October, pp 515-524.

14. British Standard BS12, Portland Cement (Ordinary and Rapid-Hardening),Part 2, British Standard Institution.

15. British Standard 8S1881 (1970), Methods of Testing Concrete, Part 2, BritishStandard Institution, London

6.0 ACKNOWLEDGEMENTThe work contained in this report was supported by a grant from University of LagosCentral Research Committee. The support is gratefully acknowledged.

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