Ž .Construction and Building Materials 14 2000 73]78

Behaviour of precast reinforced concrete pile caps

Toong Khuan Chana,U, Chee Keong Pohb

aSchool of Ci il and Structural Engineering, Nanyang Technological Uni ersity, Nanyang A¨enue, Singapore 639798, SingaporebLand Transport Authority, Singapore, Singapore

Received 5 July 1999; received in revised form 1 November 1999; accepted 6 January 2000

Abstract

The objective of this investigation is to study the behaviour of precast reinforced concrete pile caps and the ultimateload-carrying capacity. Three pile cap units were cast and tested to failure. One unit was a control pile cap cast in situ and theother two were precast reinforced units with in situ concrete infill. The experimental results showed that the precast pile capbehaved in a similar manner as compared with the conventional cast in situ pile cap. Furthermore, all the three units failed atloads exceeding the failure loads predicted using conventional design methods and exhibited predicted failure modes. In addition,there was a substantial increase in productivity as the precast pile caps could be constructed quickly and thus reducing the risk ofexposing the excavated pit to rain and possible failure of the unsupported sides. Q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Precast; Pile caps; Ultimate load

1. Introduction

The current trend of increasing efficiency and pro-ductivity in the management of construction activitieshas placed considerable emphasis on the use of precastmembers where off-site manufacture, under controlledconditions, and uncoupled from site processes and de-lays, can provide a constant supply of precast elements.The use of precast elements is more crucial at locationswhere heavy rains can cause serious delays due to adifficult working environment. This is particularly evi-dent for foundation works in soft or slimy soils whereheavy rainfall can cause the sides of the excavation tofail and thus requires further time and effort to rectifythe excavation.

The construction of conventional cast in situ pileŽ .caps see Fig. 1 requires an excavation for the pile cap,

U Corresponding author. Tel.: q65-790-5283; fax: q65-791-0676.Ž .E-mail address: ctkchan@ntu.edu.sg T.K. Chan

base preparation with a layer of lean concrete, con-struction of forms, installation of a steel reinforcementcage and placing of fresh concrete. This sequence ofwork may easily take up to 2 days for a small pile capof 1]2 m width. The steel cage may be pre-constructedand lifted into the pit to speed up this process. Thiscurrent practice is vulnerable to heavy rains especiallywhen the surrounding soil is weak. Flooding followedby failure of the sides of the pit is not uncommon.

An innovative system of precast pile caps is proposedwhere no extensive ground preparation or externalforms are required. The steel cage can be constructedseparately and cast with a thin layer of concrete on thesides to form a precast reinforced concrete shell asillustrated in Figs. 2 and 3. This shell serves as apermanent form for the pile cap and rests directly onthe cut-off piles. The precast shell is then infilled within situ concrete to complete the construction of the pilecap. A lean concrete layer, which is normally requiredto provide a firm base, may not be necessary with thissystem.

0950-0618r00r$ - see front matter Q 2000 Elsevier Science Ltd. All rights reserved.Ž .PII: S 0 9 5 0 - 0 6 1 8 0 0 0 0 0 0 6 - 4

( )T.K. Chan, C.K. Poh r Construction and Building Materials 14 2000 73]7874

Fig. 1. Unit A: conventional cast in situ pile cap.

The objectives of this project are to compare theultimate load-carrying capacity of precast reinforcedconcrete pile caps with conventional cast in situ pilecaps and to study the behaviour of these precast units.No previous experimental work on precast pile capswas reported in the literature.

2. Design concept

The concept of this precast pile cap is to cast a thinconcrete shell together with the steel reinforcementcage to provide a permanent form to hold the freshconcrete. The sides of the steel cage are cast with athin layer of concrete of approximately 70 mm toprovide an outer cover of at least 50 mm to the steelbars. Inner cover to the steel is not required, as the insitu concrete will protect the bars. The bottom of thesteel cage is left open to enable it to rest on the top ofthe piles, leaving a small clearance between the precastshell and the ground. After proper alignment of theprecast element and the addition of the column starterbars, the pile cap can be infilled with fresh concrete.

The bottom steel bars provide all necessary anchoragebetween the precast shell and the cast in situ concrete.

Two approaches are available for the analysis of pileŽ . Ž .caps: i the beam method; and ii the strut-and-tie

analogy method. The British design code for the struc-w xtural use of concrete 1,2 allows both the beam ap-

proach and the strut-and-tie method to be used for thedesign of pile caps. When the pile cap is designed bybeam theory, it is assumed to act as a beam spanningbetween the piles and is designed for usual conditionsof bending and shear. The bending moment is taken asthe sum of the moments acting from the centre of thepile to the column face. Consequently, the reinforce-ment in the pile cap for bending is placed uniformlyacross the full width of the cap.

The pile cap can also be idealised as a strut-and-tiew xmodel 3]6 , with compression struts transferring load

from the column to the top of the piles, and tensionties equilibrating the outward components of the com-pression thrusts. The tension ties have constant forcesin them and must be anchored for the full horizontaltie force outside the intersection of the pile and thecompression strut. No curtailment of reinforcementwithin the cap is allowed and full anchorage must beprovided beyond the piles. The reinforcement bars are

Fig. 2. Unit B: precast reinforced concrete shell with cast in situconcrete infill.

( )T.K. Chan, C.K. Poh r Construction and Building Materials 14 2000 73]78 75

Fig. 3. Unit C: precast reinforced concrete shell with cast in situconcrete infill.

placed in concentrated bands in the direction of the tieforces to resist the tensile forces.

The precast pile cap conforms to the assumptions ofboth these methods of analysis as the embedding of thesteel bars into the precast shell provides the necessaryanchorage. The concrete in the compression zones isconfined by links, which are provided in the precastshell. The interface between the shell and the in situconcrete is subjected to only compressive forces andneeds no further ties.

3. Methods

Three pile cap units for a four-pile group werefabricated. The first unit is a conventional cast in situpile cap of 1000=1000=400 mm designed in accor-dance with BS8110 and to fail in flexure. Four 150-mmconcrete cubes were utilised to represent the piles. Thesecond unit was of similar dimensions and steel rein-forcements, but precast with a shell thickness of 70mm. The third unit of 1000=1000=300 mm was pre-cast and was constructed with a larger amount ofreinforcement to investigate failure in shear. Theseunits will be labelled as A, B and C, respectively.

Table 1Results of material tests

Ž .Material Type Strength MPa

Concrete Pile cap A, cast in situ 39.7Pile cap B, precast shell 33.4Pile cap B, cast in situ infill 38.3Pile cap C, precast shell 35.8Pile cap C, cast in situ infill 36.4

Steel 10-mm-diameter deformed bars 480.7

The details of the specimens are shown in Figs. 1]3.The precast shells were left to cure for at least 28 daysbefore the in situ concrete infill. Three 100-mm cubeswere cast for each batch of concrete made and weretested on the same day as the load test on the pile capunits. Each of the test results tabulated in Table 1 isthe average of three cube specimens. Tensile tests onthe reinforcing bars were also carried out to determinethe yield strength.

ŽA total of 20 electrical-resistance strain gauges TML.FLA-5-11 were installed at various locations on se-

lected reinforcing bars in the three units as shown inFigs. 2 and 3. Displacement transducers were used tomonitor deflections at various positions on the unitsduring the test.

A 2000-kN testing frame was used to apply compres-sive load onto the units. A load cell was placed on topof a 20-mm steel plate, which was used to transfer theload onto the column stump. The four concrete pilesupports were supported on 12-mm-thick steel plates,which in turn were supported on rocker bearing sup-ports as shown in Fig. 4. The load was increased atintervals of 20 kN until failure. Care was taken tocheck the unit for visible cracks. The vertical displace-ments and strain gauge readings were automaticallyrecorded during the tests using a computerised dataacquisition system.

Fig. 4. Test load arrangement.

( )T.K. Chan, C.K. Poh r Construction and Building Materials 14 2000 73]7876

Table 2Crack width observations at first crack

Ž .Unit Load Disp. Crack width mmŽ . Ž .kN mm North West South East

A 840 1.87 0.16 0.20 0.12 0.24B 900 1.92 0.30 0.24 0.18 0.26C 450 1.72 0.20 0.14 0.12 0.08

4. Results

The observed load-deflection relationships at the pilecap centre, for the three units, are shown in Fig. 5. Theobserved crack widths and ultimate loads of the threepile cap units are tabulated in Tables 2 and 3, respec-tively.

Pile cap A was predicted to fail at a total load of 890kN by the design equations of BS8110, with flexurebeing critical. However, the unit failed at 38% higherload of 1230 kN. The load-deflection curve was linearup to a point of more than 800 kN, where a definitesoftening occurred. This point coincided with the ap-pearance of the first crack in the unit. The load contin-ued to increase at a lower rate up to a maximum of1230 kN where the unit continued to deflect with nofurther increase in load. The load carrying capacitybegan to reduce noticeably after a deflection of morethan 6 mm. The strains in the reinforcing bars exhib-ited a sudden increase after the appearance of the firstcracks. The strains continued to increase as the loadwas increased and were all beyond the yield stress atthe maximum load.

Pile cap B, which has similar reinforcing steel ratioand layout, has the same predicted failure load. Theultimate load was also very similar at 1250 kN; anincrease of 41% over the BS8110 predictions. Theload-displacement behaviour was very similar to speci-men A with the first crack at a load of 900 kN and a0.05-mm difference in maximum centre displacementat ultimate load compared with pile cap A. There was asimilar increase in the strains in the reinforcing barsafter the appearance of the first cracks. At the maxi-mum load, the strains in the reinforcing bars haveexceeded yield stress.

Pile cap C, which was designed with a shallowerdepth and larger amount of steel reinforcement failedat a maximum load of 870 kN. The failure load was 7%higher than the prediction of BS8110. There was asignificant drop in the stiffness of the pile cap after thefirst crack at 450 kN. When the load reached a maxi-mum of 870 kN, a shear failure occurred with a punch-ing cone extending from the outside faces of the columnto the inside edges of the piles. The strains in thereinforcing bars exhibited a sudden increase after theappearance of the first cracks at 450 kN. At the maxi-

Table 3Comparison of experimental and predicted failure loads

Unit Failure Load ratioBS8110 predicted loadsŽ .load kN Exp.rPred.Ž . Ž .Flexure kN Shear kN

A 1230 890 1240 1.38B 1250 890 1240 1.41C 870 904 811 1.07

Fig. 5. Load-displacement behaviour of the pile cap units.

( )T.K. Chan, C.K. Poh r Construction and Building Materials 14 2000 73]78 77

mum load, all the measured strains in the reinforcingbars were observed to have exceeded the yield stressindicating that the pile cap was also close to its flexuralcapacity.

5. Crack behaviour

The pile caps typically had very few cracks prior tofailure. Units A and B failed in flexure with flexuralcracks extending diagonally between the piles. Failureof unit C was with a square crack pattern within thefour piles indicative of punching shear failure.

Fig. 6 shows the deformation pattern at the soffit ofthe pile caps at failure and Fig. 7 shows the crackpatterns at the sides of the pile caps. The flexuralcracks originated from the centre of the soffit of pilecap A, extending diagonally towards the piles and prop-agating outwards. Cracks were first observed on thevertical faces of the unit when the loading was approxi-mately 840 kN. At this loading, the largest crack widthmeasured was 0.24 mm. Many new cracks developed onthe four vertical faces of the unit just before the failureload was reached.

Cracks first appeared on the vertical faces of unit Bat approximately 900 kN and the largest recorded crackwidth measured 0.30 mm. As in pile cap A, the cracksoriginated from the bottom centre of the unit. Simi-larly, the flexural cracks extended diagonally towardsthe piles and failure was characterised by the rapiddevelopment of many new cracks on the vertical faces.The crack patterns for both units A and B were similar.

Cracks were first observed on the vertical faces ofunit C at a load of 450 kN with the largest crack widthbeing 0.20 mm. In contrast to pile caps A and B, therewere cracks at the bottom of pile cap C that ran

Fig. 6. Crack patterns at the soffit of the pile cap units.

parallel to the sides of the unit, indicating a ‘drop’ ofthe concrete mass due to the punching shear failure.Thus, a punching cone had extended from the loadedarea to the inside of the piles.

6. Discussion

The comparison of crack widths indicates that theprecast unit B has slightly larger crack widths com-pared to the conventional cast in situ unit A. However,

Fig. 7. Crack patterns at the sides of the pile cap units.

( )T.K. Chan, C.K. Poh r Construction and Building Materials 14 2000 73]7878

the first crack occurred at a marginally higher load inthe precast unit. The loads at which these cracks oc-curred in units A and B were higher than the estimatedworking loads that the pile caps were designed for. It isevident from the crack widths, loads at which thesecracks occurred and the crack patterns that the precastpile cap exhibits similar behaviour as a conventionalcast in situ pile cap. The first crack unit C occurred atapproximately the designed working load of the pilecap.

The failure loads of the precast pile cap can bepredicted using conventional design equations as re-ported in these tests. The results suggest that theprecast shell does not reduce the load-carrying capacityor cause a weak joint in the function of the pile cap.The interface between the in situ concrete and theprecast shell is not subjected to large stresses based onthe beam approach as the moment of resistance isassumed to fall off according to the bending momentdiagram and therefore only nominal steel is requiredbeyond the pile.

w xAccording to the strut-and-tie model 4]6 , the flowof forces is within the concrete unit and the concrete inthe shell does not contribute to the areas which com-prise the compression struts. The nodal zones of highcompressive stresses are entirely within the in situconcrete, which is effectively confined by the precastshell. Extending the steel bars into the precast segmentprovides full anchorage of the tension tie. The inter-face is therefore under compressive confining stressesand not expected to fail.

The durability of these precast units should notdiffer significantly from conventional pile caps as theinterface is not under tensile stresses. Pile cap Bcracked at a slightly higher load compared to theconventionally cast pile cap A, confirming that theprecast shell did not induce cracking at a lower load orat the interface. There were no cracks at the soffit ofpile caps B and C until the loads exceeded 0.7 and 0.5of the ultimate load, respectively. It should be furthernoted that more durable concrete could be providedfor the precast shell to provide additional resistance tochemical attack although it has been reported thatsmall cracks of less than 0.5 mm very rarely pose anyparticular corrosion risk, whatever the nature of the

w xenvironment 7,8 . No shrinkage cracks were observedat the bottom face of the precast units as the pile capswere provided with a reinforcement ratio of 0.0016.This was more than the recommended reinforcementratio of 0.0013 to be provided in two orthogonal direc-

w xtions on the top and bottom faces of pile caps 9 .However, a faint shrinkage crack was observed at the

top surface between the precast shell and the concreteinfill as no top steel was provided for these units. Theprovision of minimum steel at the top face wouldeliminate the shrinkage cracks.

7. Summary and conclusions

A comparison of the observed failure loads of thetwo precast test units with predictions from the Britishcode indicates that the failure load of precast pile capswas approximately 40% and 7% higher when the unitsfailed in flexure and shear, respectively. The behaviourof the precast unit is similar to the corresponding castin situ unit with only a slight increase in crack widths.It is therefore expected that current design equationsfor conventional cast in situ construction can be usedto predict the failure loads of the pre cast units al-though the predictions may be conservative in certaincases. These findings lead to the conclusion that theprecast pile cap is a feasible method of construction.

In addition, there was a substantial increase in pro-ductivity as the precast shells could be placed over thecut piles, aligned, levelled and infilled with concrete ina short time. The risk of exposing the excavated pit torain and possible failure of the unsupported sides wasalso reduced.

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w x3 Canadian Standards Association. CAN3-A23.3-M94 Design ofconcrete structures for buildings. Canadian Standards Associa-tion, 1994

w x4 Adebar P, Kuchma D, Collins MP. Strut-and-tie models for thedesign of pile caps: an experimental study. ACI Struct J

Ž .1990;87 1 :81]92.w x5 Adebar P, Zhou LZ. Design of deep pile caps by strut-and-tie

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beams and pile caps failing in diagonal splitting. ACI Struct JŽ .1993;90 4 :356]363.

w x7 Rowe RE, Sommerville G, Beeby AW et al. Handbook toBritish Standard BS8110, 1985: structural use of concrete.Palladian Publications Ltd, 1987.

w x8 Beeby AW. Cracking and corrosion. Concrete in the oceansreport No. 1 CIRIArUEG. London: Cement and ConcreteAssociation, Department of Energy, 1978.

w x9 The Institution of Structural Engineers, The Institution of CivilEngineers. Manual for the design of reinforced concrete build-ing structures. London, UK: The Institution of Structural Engi-neers, October 1985.