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PRESTRESSEDCONCRETE PILES

A state of the art presentation on prestressed piling as used throughoutthe world is discussed in this General Report to the FIP Symposiumon Mass-Produced Prestressed Precast Elements in Madrid, Spain, May 1968.Historical information, design data and notes along with manufacturingtechniques, installation practices, discussion of problems and failuresand a complete bibliography are all covered in this comprehensive report.

Ben C. Gerwick Jr.PCI Representative to FIPPomeroy-GerwickSan Francisco, California

Prestressed concrete piles are ex-tensively employed throughout theworld in marine structures and foun-dations. The advantages they offerare their strength in bearing andbending, their durability and theireconomy. Piling initially appeared tobe one of the simplest applicationsof prestressing. As experience hasaccumulated over the years, how-ever, applications of prestressed con-crete piling have grown increasinglysophisticated, with piles acting un-der combined stresses and oftenserving as major structural elements.

The total annual world consump-tion of prestressed concrete piling isextremely large, approximately 57,-000,000 m (187,000,000 ft.) per year;it thus becomes extremely importantto examine anew all aspects of man-ufacture, installation, economics andtechnology.

The development of prestressedconcrete piling has coincided withthe expansion of the science of soil

mechanics, and piles are now beingrequired to withstand high dynamicstresses during installation in orderto meet the requirements of soil en-gineers. The huge number of pre-stressed concrete piles being in-stalled each year gives, by itself,an adequate testing sample fromwhich statistically valid results canbe drawn, even with the many vari-ables present.

In recent years, prestressed con-crete piles have broken through in-to new fields of utilization. This ispartly a result of their demonstratedeconomy in mass production, andpartly because of their technicalproperties and performance. Forthese reasons, it is important to in-vestigate the entire field of pre-stressed concrete piles, under theheadings of: utilization; design;manufacture; installation; economy;sheet piling; fender piles; splices;problems and failures; durability; re-search and development.

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National Reports have been re-ceived from Japan (Prestressed Con-crete Engineering Association), Ger-many (DDR), Australia (PrestressedConcrete Development Group), USA(Prestressed Concrete Institute) andthe USSR (Research Institute forConcrete and Reinforced Concrete).In addition, eminent engineers andspecialists in prestressed concretepiling from a number of differentcountries have contributed valuableassistance by furnishing data and de-tails on specific installations and as-pects of prestressed concrete piles.

Prestressed concrete piles havebeen extensively employed in atleast 21 countries, including bothhighly developed and less developedcountries, and in a variety of envir-onments including sub-arctic, trop-ical and desert.

In Japan alone, there are 45 fac-tories producing 1,000,000 tons offoundation piles annually, while inthe USA more than 2,000,000 m (6,-560,000 ft.) annual production isdivided between marine and founda-tion construction. Prestressed con-crete piles support major bridgesand wharves in Australia, New Zea-land, Peru, Ecuador, Venezuela,Netherlands, Kuwait, Norway,Spain, Italy, Singapore, Malaya,Sweden, USSR, Canada, Englandand USA, to name but a few coun-tries in order to illustrate the widerange of application in differenteconomies and climates.

Prestressed concrete piles havebeen constructed in the form of cy-lindrical piles up to 4 m (13.1 ft.) indiameter, as used on the Ooster-schelde Bridge in the Netherlands,and up to 70 m (230 ft.) long, as em-ployed in off-shore platforms in theGulf of Maracaibo, Venezuela. Pre-stressed piles have been driven inincrements, spliced together during

driving to form foundation piling upto 60 m (197 ft.) long to support mul-ti-story buildings in Honolulu andNew Orleans. They have beenspliced to lower sections of steel anddriven as composite piles to supportmajor bridges in California and NewSouth Wales, Australia. Frequently,prestressed foundation piles are be-ing extended up as structural col-umns to support a bridge deck oreven to support the second floor of aparking structure.

Fig. 1. Hollow cylinder pile beingmaneuvered into position by crane

Prestressed concrete cylinder piles(Fig. 1) may be used effectively asstructural columns and pier shafts.They may be set in pre-drilled holesand then seated by driving or bymeans of a concrete plug. Prestressedconcrete piles are being increasinglyused to resist uplift (tension), bend-ing and dynamic loads. They are act-ing as fender piling to resist ship im-pact on harbor structures in Kuwait,Singapore and California and as pro-tective fenders for major bridge

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piers. Prestressed sheet piles arewidely employed as retaining struc-tures for quay walls and bulkheads.

The Japanese National Report as-serts the following advantages forprestressed piling:

• Crack-free under handling anddriving

• Can be economically designedfor given Ioads and moment

• Uniform high quality, high-strength concrete

• Readily spliced and connectedThe USA report adds:

• High load-carrying capacity• Durability• Ease of handling, transporta-

tion and pitching• Ability to take hard driving

and penetrate hard material• High column strength and

ability to resist moment or com-bined moment and bearing

• Ability to take uplift• Economy.

Fig. 2. Long foundation pile (withoutsplices) being used for construction

in an American city

In some parts of the United States,there has been a trend towards man-ufacturing and driving prestressedfoundation piling in single longlengths without splices, even forbuildings in crowded cities (Fig. 2).Thus piles 40 m (131 ft.) long havebeen installed for major buildingsin San Francisco and Boston. Inother areas of the USA (New Or-leans and Honolulu), and rathermore generally in Japan, Scandina-via, etc., the main trend is towardsthe manufacture of standardizedsegments, spliced during driving.

When necessary, prestressed con-crete piles can be readily followeddown below the existing ground sur-face, permitting a more economicalsequence of pile driving, excava-tion and concreting instead of hav-ing to perform the driving in a deephole with maximum excavation ex-posed.

High-capacity prestressed con-crete piles are particularly advan-tageous for deep foundations withheavy loads in weak soils. They canbe driven successfully through rip-rap and debris, or through hardstrata such as coral, and can pene-trate soft or partially decomposedrock.

DESIGN

The design of prestressed concretepiles requires consideration of han-dling, transportation, pitching, driv-ing, permanent loads (both axial andbending), and transient bendingloads. Each of these requires an an-propriate consideration of allowablestresses and, in certain cases, of al-lowable deflections.

As a general rule, prestressed pilesshould be basically designed for thepermanently installed condition andthen checked for transient bendingloads, e.g. seismic or wind. A cer-

68 PCI Journal

taro minimum prestress and a certainminimum amount of spiral bindingare required for successful installa-tion. These minimum values can thenbe checked against the conditions ofhandling, transporting, pitching anddriving, and modified as necessary.The design for resisting axial andbending loads may be made as fol-lows:

1. Concentrically loaded shortcolumn

a. Design load based on ultimateload

N = Nu/S(Kf'_— 0.60f f ) AC /S

where N = allowable bearing loadfor design

N,, = ultimate bearing loadS = factor of safety, usually

2.5, 3 or 4K = coefficient of uniformity,

assumed by some to be0.85, especially for pilescast vertically and up to1.0 for piles cast hori-zontally

f. = effective prestress afterlosses

A, = cross-sectional area ofconcrete

f' -= 28-day strength (6 x 12-in. cylinders)

The Japanese on the other hand,use the following formula for theultimate strength of piles prestressedwith bars:

N,, _ (f,. — f) (A, + nAg)where n = secant modular ratio,

about 6.5 at ultimateconditions.

b. Design load based on allow-able stress in pile. Most codes spec-ify stress values ranging from 0.20fto 0.33f'. Based on recent studies.n-1 eva'uation, the author recom-mends

N = (0.30f f — 0.25 f e) A.For the usual case, this reduces toN = 0.275f A,,.2. Concentrically loaded long

columna. Ultimate load. For long columns

where buckling may occur, Euler'sformula may be used. For pin-endedconditions, the effective length, L,shall be the full length of the pile.For piles fully fixed at one end andhinged at the other, L can be takenas 0.7 of the length between hingeand point of fixity. For piles fullyfixed at both ends, assume L to be0.5 of the length.

h. Design load. A safety factor of2 is generally considered sufficientfor buckling load. Design loadsshould be based on the short columnvalue up to L/r = 60 and a straight-line function used between L/r = 60and L/r = 120. For L/r greater than120, the pile should be investigatedfor elastic stability, taking into ac-count the effect of creep and deflec-tion.3. Moment-resisting capacity

a. Ultimate. This is approximatelygiven by these formulae (for the typ-ical prestressed pile design and ma-terials'):

M„ = 0.37 dAs f c for solid squarepiles

M„ = 0.32 dA8 f c for solid circularand octagonal piles

M„ = 0.38 dA8f s for hollowsquarepiles

M„ = 0.32 dA8j, for hollow circularand octagonal piles

where A3 = total steel area of alltendons

f8 = ultimate strength of ten-dons

d = diameter of pileMu = ultimate moment capac-

ity

October 1968 69

A safety factor of 2 should be ap-plied for normal loading; for seismicor wind loading, the factor is 1.5.

b. Elastic theory. Determine al-lowable tension, ft, in the concrete.This may be taken as 0, as 0.5 mod-ulus of rupture, or as 0.8 modulusof rupture, depending on frequencyof load, exposure, corrosive condi-tions, etc.

M= (fe +ft)Zwhere M = allowable moment

Z = section modulus of pile

4. Combined moment and directload

The existence of direct load gen-erally increases the moment capacitywithin the elastic range, by addingto the available compressive stress(prestress plus stress due to directload) to resist tensile fiber stress. Theultimate moment capacity of the pileis reduced by direct load.

Typical load-moment interactiondiagrams are shown in Fig. 3. It canbe roughly estimated that, for a typ-ical foundation pile carrying 60 percent of the allowable axial load, the

foundation pit, ^♦ \\

AXIAL LOA-

Fig. 3. Typical load-moment interac-tion curves for prestressed concrete

piles

ultimate moment capacity is givenby Mu = 0.29dA&f8 for solid squarepiles and by Mu = 0.25dAj8 forround piles. For hollow piles, thelever arms are 0.304 and 0.26d, re-spectively.

At the pile head, where combinedmoment and direct load may be crit-ical, the favorable effect of transferlength (i.e. prestress varying fromzero to full over the transfer length)may be taken into account.

Permanent bending loads may beimposed by:

• Dead weight of a raking pile• Lateral load of soil• Eccentricity of axial load• Rigid frame action (i.e. struc-

tural bending moments im-posed).

Transient bending moments maybe imposed by:

• Waves and currents• Ice• Lateral forces from ships or

barges• Wind on structure• Seismic forces.

For many of these transient loads,allowable stresses may be increasedby one-third.

For the handling of prestressedpiles from manufacture, the value off' should be the ultimate compres-sive strength at the age in question.Impact must be considered in han-dling and transporting, with valuesof 50 and 100 per cent being applied,depending on the techniques em-ployed. The 50 percent value ismost common. In the USSR, a factorof 1.5 is applied to calculations forstrength in handling, and a factor of1.25 is applied to calculations forcrack resistance during handling.During handling and transporting,values of allowable tension rangingfrom 0 to 0.4 modulus of rupture areused.

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During pitching of the pile, thestresses rapidly diminish as the pileis raised to the vertical. However,the stress conditions in the pile aregenerally quite complex, dependingon the rigging and the angle madeby the lines with the pile. The usualpractice, which assumes verticallines leading from the horizontal pilecan be grossly erroneous.

A raking pile goes through a com-plex series of stress reversals. Fre-quently, the critical design condi-tion is the overhang of the pilebelow water until it touches bottom.There will be a high negative mo-ment at the main support until thepile head has been driven to thislevel. There is a tendency for thetoe to "kick out" during driving. Inits final position, the pile will be insimple positive bending due to deadweight.

In general, prestressed piles donot necessarily require full prestressat head and toe. Therefore, a longertransfer length may often be toler-ated and release strengths need notbe quite as rigorous as, for example,for short beams.

For prestressed piles which haveto resist axial tension (uplift), a de-sign tension load of Nt = f eAo maybe employed. This gives a factor ofsafety against cracking, since theload at cracking,

N= (f^ -I- ft) A0For ultimate strength,

to =fAs.where f = ultimate tensile strength

of concreteUnder seismic and wind loads, ax-

ial tension and bending moment mayoccur at the head of piles serving asa foundation for chimneys, stacks,etc., and can generally best be pro-vided for by means of auxiliary mildsteel bars in the pile head.

The Japanese National Report

tells of three standard classes of pile,classified according to the effectiveprestress:

Foundation piles have f e = 40 kg/cm2 (565 psi)

Moment piles have f e = 80 kg/cm2(1130 psi)

Special moment piles have f e = 100kg/cm2 (1420 psi)

In the USA, foundation piles over12m (40 ft.) long generally have aneffective prestress f e of 50 to 60kg/cm2 (710 to 850 psi). With greaterdesign moment requirements, valuesof fe up to 85 kg/cm2 (1200 psi) areused. For fender piles subject topure bending, values up to 100 kg/cm2 (1420 psi) are employed. Thesemoment-resisting piles suggest newapproaches to many structural andfoundation problems.

DESIGN FOR RESISTANCE TODRIVING STRESSES

Earlier in this paper, reference wasmade to empirical minimum valuesof longitudinal prestress and spiralbinding. These are, of course, ofgreat importance because, empiricalas they may be, the regularity ofstatistical performance indicates thatfundamental behavior is involved.The empirical minimum values oflongitudinal prestress adopted in alarge number of countries all rangefrom 45 to 65 kg/cm2 (640 to 925psi). This is the effective prestressafter losses. When less prestress hasbeen used, excessive difficulty hasbeen encountered with horizontalcracking during driving.

I would like to suggest the follow-ing hypothesis. The dynamic mod-ulus of rupture is about 0.8 of thestatic modulus of rupture. For a typ-ical prestressed pile, this may be 34kg/cm2 (480 psi). The effective lon-gitudinal prestress will normally beabout 50 kg/cm`' (710 psi). Thus atotal of 84 kg/cm2 (1190 psi) is avail-

October 196E 71

able to resist a dynamic tensionwave before horizontal cracking.However, tensile stresses of this or-der of magnitude are frequentlymeasured during driving. The ten-dons to produce a unit prestress of50 kg/cm`' (710 psi) have a steelarea of 0.5 percent and, at yield, canresist a force of approximately 70kg/em" (995 psi) before stretching.However, the existence of the crackhas probably dampened the tensilewave to something Iess than thisvalue, so progressive failure does notnormally result. If less than thisamount of effective prestress is used,the steel at the crack will stretch, theconcrete will be destroyed in com-pressive fatigue, and brittle fractureof the tendons will soon result.

Therefore, I recommend a mini-mum steel area of 0.5 percent forprestressing steel with a yieldstrength of 15,000 kg/cm= (213,000psi). By way of corroboration, manyrecommendations for conventionallyreinforced (mild steel) concrete pilescall for 3 percent steel area, with asteel of only 2600 kg/em 2 (37,000psi) yield strength. Thus 0.5/31/6 is inversely proportional to theratio of yield strengths, 15,000/2,600= 6/1. Exceptions to this rule existfor short piles (less than 12 m (40 ft.)long) and for piles which are in-stalled by means other than driving.Empirical minimum values of 30kg/cm2 (425 psi) (about 0.3 percentsteel area) have been set in somecountries for such short piles.

No general theory has been de-veloped for the design of spiral bind-ing for piles. Considerably less spiralis used than is required for similarstructural columns. Steel areas of0.06 to 0.2 percent are widely em-ployed and many thousands of pileshave been successfully driven withthese small amounts.

In the USSR, for piles up to 9 m(30 ft.) long, transverse reinforce-ment is used only at head and tip.However, since their behavior atsub-zero temperatures has not yetbeen thoroughly evaluated, their useis confined to residential construc-tion where they will be completelyburied in soil.

However, longitudinal cracking isoccurring with disturbing frequencyin hollow-core and cylinder piles.The British Prestressed ConcreteDevelopment Group (now incorpo-rated into The Concrete Society)recommends a greater area of spiralfor hollow-core piles than for solidpiles. My tentative recommendationwould be to use 0.3 to 0.4 percentsteel area for cylinder piles.

During driving, the bursting effectat the head and tip of the pile ismuch greater. There are tensilebursting forces due to the prestressand radial bursting pressures fromthe hammer blow. The lateral bind-ing requirement for most existingprestressed pile designs gives a steelarea of 0.5 percent for 120 cm (47in.) from the head. The British Pre-stressed Concrete DevelopmentGroup recommends 0.6 percent vol-ume in a top section of length equalto three times the least dimension.The USSR increases the steel areaby placing 5 layers of mesh at thehead, spaced at 5 em (2 in.).

The requirements for lateral bind-ing for prestressed beams generallyset values at A8 /AC of 0.4 percentsolely to resist tensile stresses fromprestressing in the transfer zone. Inpiles subjected to hard driving, thereare bursting tensile stresses of thesame order of magnitude as the ten-sile strength of the concrete. Thiswould seem to require a steel area of0.8 to 1 percent in the top cm (10in,) for proper resistance to spalling.

72 PCI Journal

This will properly balance yield ofsteel with initial cracking of the con-crete. Empirically, it has been foundthat the provision of about 1 per-cent steel area in the top 30 cm (12in.) of cylinder piles prevents thehead splitting. Sometimes this is pro-vided in the form of a band aroundthe top. It is interesting to note thatthis band occasionally breaks in harddriving, indicating that the burstingstresses are real. I prefer to usehoops of mild steel placed as closeas possible to the outside of the wall.Cover for durability is normally ofno importance at this location.

Chamfering of the circumferentialedges and top corners of the pilehead reduces the tendency to burst-ing and spalling.

DESIGN NOTES(see also Fig. 4)

1. Toggle and lifting holes are un-desirable because of stress concen-tration. Their use has resulted inpile failures during driving in NewZealand and elsewhere.

2. Pile-to-cap connections maytake one of the following forms:

a. Mild-steel dowels aregrouted into holes in the pilehead. These holes may be pre-formed or drilled after driving.

b. The prestressing tendonsmay be extended into the pile cap.For design, embedments of 45 to72 cm (18 to 28 in.) are usually em-ployed for strands, and they aretreated as the equivalent of mild-steel reinforcement. For tensionpiles, embedments are generally72 to 100 cm (28 to 39 in.).

c. The pile head may be ex-tended up into the cap a distanceof 45 to 72 cm (18 to 28 in.) . Thesurface of the pile head should beroughened and cleaned beforeconcreting.

d. For hollow-core piles, rein-

forcing steel cages may be con-creted into a plug in the pile head.To ensure bond, the inside surfaceof the pile head should be cleanedand roughened (e.g. by sand-blast-ing), epoxy bonding compoundapplied, and low-shrinkage con-crete placed in the plug. Precastplugs containing the reinforcementhave been employed, with the an-nular space between cylinder pilesand precast plug filled with grout.

e. Also for hollow-core piles,structural steel members may beconcreted in a plug instead of re-inforcing bars.

f. Post-tensioning of the pilehead to the cap has been frequent-ly proposed but infrequently usedowing to site labor requirements.It remains a technically attractiveapproach, using either the exten-sions of the tendons from the pileor separate prestressing barsgrouted or concreted in the pilehead.3. The matter of distribution of

tendons has been given considerablestudy, particularly in Japan for hol-low-core piles. The Japanese havefound that the ultimate bending mo-ment is not appreciably affected aslong as there are at least four ten-dons around the periphery of thecylinder. Recently, in the USSR, thetension reinforcement for shortfoundation piling up to 9 m (30 ft.)long has been grouped in the centerof the cross-section.

4. Pile shoes are unnecessary forthe great majority of piles; the tipsmay be square or blunt. For drivingin sands, silts and clays, a square tipis satisfactory; a blunt tip is general-ly best for driving in gravels, boul-ders, etc. Pile tips are generallygiven additional spiral reinforcementand this is especially important whenpiles are driven onto rock, as corn-

October 1968 73

Shape Advantages Disadvantages

Highest ratio of skin-fric- Low bending strength.tion perimeter to cross-sec-tional area. Low manufac-turing cost.

Good ratio of skin-frictionperimeter to volume. Lowmanufacturing cost. Goodbending strength on majoraxes.

Approximately equal bend- Surface defects on toping strength on all axes. sloping surfaces as castGood penetrating ability, are hard to avoid.Good column stability (l/r

ratio).

Equal bending strength on Manufacturing cost gen-r^ all axes. No sharp corners erally higher. Surface de-

r aid appearance and dura- fects on upper surfaces asbility. Minimum wave and cast are hard to avoid.

ti current loads. Good columnstability (l/r ratio).

Q May be used where greater Difficulty in maintainingbending strength is re- orientation during driving.

or quired around one axis, es-pecially if minimum surfaceto lateral wave and currentforces is desired.

High bending moment High cost of manufacture.about axes in relation to Difficulty of orientation.cross-sectional area.

High bending moment High cost of manufacture.X x about axis X—X in relation Difficulty of orientation.

to cross-sectional area.

NOTE: Hollow cores may be employed with most shapes. Varying cross-sections may be employed along the length of the pile, such as en-larged tips for bearing, enlarged upper sections for moment, or achange to a circular upper section in order to eliminate corners, etc.These changed sections may either be cast monolithically or splicedon at any stage.

Fig. 4. Cross-sectional shapes for prestressed concrete piles

74 PCI Journal

pressive stresses at the tip may thenbecome very high (theoretically upto twice the head compressive stressif there is no overburden above therock, reducing to about equal withnormal overburden). Pile shoes areof advantage in helping piles to pen-etrate buried timbers, coral androck. They may take the form ofplates, points or stubs. Steel stubscan best be fixed by embedding 1.25to 1.6 m (4 to 5 ft.) into the concreteand then enclosing this by heavyspiral binding. Means must be pro-vided to ensure thorough consolida-tion of concrete around the stub andto prevent air being trapped. In Nor-way, piles are seated in sloping hardrock by means of the "Oslo point", asteel pipe dowel which is drilledthrough the pile after driving andgrouted into the rock. In Singapore,prestressed tension piles were an-chored to granite by drilling throughthe hollow core, grouting a tendonin, and prestressing the pile to therock.

5. Attention is called to the needfor adequate vents in cylinder pilesto prevent the build-up of hydro-static head inside when driving insoft material.

6. Adequate cover must be pro-vided to ensure durability and toprevent corrosion of the spiral andtendons. European practice appearsto be from 2.5 cm (1 in.) in fresh wa-ter and soil to 4 cm (1.5 in.) in saltwater. American practice calls for4 cm (1.5 in.) in fresh water and upto 7.5 cm (3 in.) in salt water.

7. Many countries and authoritiesemploy square piles with either around or a "square" spiral reinforce-ment pattern. Other countries pre-fer octagonal and round piles with around pattern. Observation of manythousands of piles of both patternsand sections has failed to produce

any evidence for the assertion thatthe corners of square piles withround patterns are more liable tocrack or break off.

8. Structural lightweight aggre-gate concrete has been successfullyemployed for prestressed concretebearing piles, sheet piles and fenderpiles. It offers lighter unit weight inair (for handling, transporting, andpitching); lighter mass (for driving);and greatly reduced net dead weightwhen submerged in water or water-bearing soils. Experience indicatesgreater flexibility and a slightlygreater tendency for the pile headto spall; the latter should be coun-tered by increased spiral wrappingat the head and perhaps several lay-ers of cross-bars or mesh.

9. For hollow-core piles, with sol-id head and/or tip, the transitionalchange in cross-section should bemade gradually. Additional mildsteel, both longitudinal and circum-ferential, may be necessary at thistransition. Similarly, at splices orhead zones where mild steel rein-forcing bars are embedded, the loca-tions of the ends of the bars shouldbe staggered longitudinally.

MANUFACTURE

The manufacture of prestressedconcrete piling has developed alongseveral different lines:

1. Long line pretensioning meth-od, producing single pile lengths upto 65 m (213 ft.) and also sectionsfor splicing; diameters up to 130 cm(51 in.) for both solid and hollowcore; all shapes and cross-sections.

a. Fixed formsb. Sliding forms and extrusion

2. Segmental construction, withsegments joined together and post-tensioned. Joints are dry, concretedor epoxy-glued.

October 1968 75

a. Spun segmentsb. Segments cast vertically

3. Centrifugally spun pile sectionsfor lengths about 12 m (40 ft.) . Pilesections may be joined by welding.

a. Pretensioned against mouldsor internal core

b. Post-tensionedMaterials for prestressed piles are

selected on much the same basis asother prestressed concrete elements.There is one additional requirement,namely, durability and protection ofthe steel from corrosion in adverseenvironments such as sea-water andalkali soils, which usually dictates ahigher cement content than that re-quired for strength alone. For sea-water exposure, aggregates must besound against sulphate attack andnon-reactive. The alkali content ofthe cement is usually closely re-stricted. When piles will be exposedto freezing and thawing, both withand without sea-water present, theconcrete should be air-entrained.

Method 1: Long-line pretensioning.The long-line pretensioned manufac-turing process has been widely de-veloped for the mass-production ofpiles of all shapes and sizes. Themost economical mass-production isachieved with multiple fixed forms,producing square pile sections. Thesides of the forms are given a slightdraft, permitting the piles to bestripped directly from the forms. Atypical production sequence is asfollows:

a. Set out pre-cut bundles ofspiral coil.

b. Pull strands down bed,through the coils; take up slackand stress.

c. Spread out coils to properspacing and tie to strands.

d. Insert segmental end gatesand clamp.

e. Cast and vibrate concrete.f. Cover and keep damp for 3

to 4 hr., then raise temperature1/z deg. C/min. (1 deg. F/min.) to65°C (150°F) and hold for 10hours.

g. Release prestress.h. Allow temperature to fall at

½ deg. C/min to within 20°C(36°F) of ambient temperature.

i. Lift from forms to storage.j. Cure with water (optional).

The supplemental water curingshould be employed for piles whichwill be exposed in service, for large,thin-walled hollow-core or cylinderpiles, and in a drying atmosphere.When the pile is first removed fromthe forms, it is still hot and moistfrom the steaming, and the moistureevaporates very quickly. It is duringthis first two or three hours that sup-plemental water curing is mostneeded. Some factories spray on amembrane curing compound tohold in the moisture.

Step (h) is important primarily forlarge, solid piles. If they were to beexposed to cold atmospheric condi-tions while the inside cores werestill hot, thermal stresses mightcause cracking or crazing. Thermalstrains and shrinkage strains are ad-ditive. Steam covers should be tightenough to provide protection againstcold winds, and to insure relativelyuniform temperatures inside. If, be-cause of wind, for example, there isa substantial difference in curingtemperature, then when the pre-stress is released into the member,the modulus of elasticity will vary,plastic flow will occur and the re-sulting pile will have a decidedsweepFor piles with circular and oc-

tagonal cross-sections, the side formsmay be hinged or the top sectionsmade removable. The top sections

76 PCI Journal

may be removed shortly after con-creting (i.e. before steam curing) inorder to permit finishing of the topsurface. Where the side forms areleft on during curing, there will beinevitable minor surface defects onthe upper surfaces due to the bleed-ing of water and air. Proper vibra-tion and mix design will help to min-imize these but cannot prevent them.The problem is more acute with thedrier mixes of lower water/cementratio. These minor surface defectswill consist of numerous "blisters"about 5 mm (0.2 in.) deep and l cm(0.4 in.) in diameter and, of some-what more serious consequence,air voids 3 to 4 mm (0.12 to 0.16in.) in diameter and up to 1 (0.4 in.)and even 2 cm (0.8 in.) deep. The lat-ter should be filled throughout thetidal or freeze-thaw exposure zones.

Because there appears to be notechnique for completely eliminatingthese surface defects with fixed sideforms, the use of removable topforms appears to be the preferabletechnique.

Forms for long-line pretensioningmust be designed so as to preventtransverse fins or obstructions, suchas at the end gates. Any such finswill lock to the freshly set concretewhile the form is expanding understeam curing and cause transversetensile cracks, which may later beaggravated during driving.

Another method of forming theseupper surfaces of octagonal and cir-cular piles is to use sliding forms. Asection of form surmounted by ahopper and fitted with several formvibrators, is pulled along rails at thespring line (top of fixed bottomforms) at from 0.3 to 1 m/min. (1.0to 3.3 ft. /min.). The sliding sectionis about 7 m (23 ft.) long. The con-crete consistency, the vibration, andthe speed of travel must be carefully

controlled in order to preventsloughing. Sometimes the hopper isequipped with a screw-type feeder.Occasionally, a second small hoppertrails the first, feeding mortar to thesurface. Such slip-forms have a tend-ency to drag the surface and pro-duce minor crazing and tensile mi-cro-cracks. It is therefore customaryto specify that, if slip-forms are used,they shall be so designed and oper-ated that they do not produce anyvisible cracks or significant residualtensile stresses.

Hollow-core and cylinder pilesproduced on a long-line pretension-ing bed have their inside "cores"formed by fixed, collapsing man-drels. These are collapsed either me-chanically or hydraulically. Forsmaller cores, up to 30 cm (12 in.) di-ameter, these cores or voids may beformed by expendable tubes of pa-per (cardboard) or by pneumaticallyinflated rubber tubes which shouldbe well-reinforced so as to maintaintheir circular shape and providelongitudinal rigidity between pointsof support. A British innovation is tostretch temporary prestressing ten-dons, wrap paper or close-spacedwire mesh around them, and usethis to form the core. After curing,the temporary tendons are releasedand re-used.

All such mandrels or core formsmust be held accurately in place toprevent sag and, particularly, to pre-vent flotation as the concrete is vi-brated. Hold-down devices must befrequent enough to hold the corewithin the specified tolerance, usual-ly about 1 cm (0.4 in.). The use ofthe stressed tendons to hold downthe core is generally most unsatis-factory because, as the core floatsduring casting, the tendons are sim-ilarly displaced.

Sliding internal core forms have

October 1968 77

been used to form cores up to 110cm (43 in.) in diameter. These mustbe guided and supported, ahead ofthe concrete being cast, by beams,rollers or the prestressing strands.The system must be designed so thatthe spiral coils are not displaced asthe mandrels move. The tail supportmay be the freshly cast concrete.These sliding mandrels are fittedwith numerous form vibrators.Sometimes heating units aremounted in the mandrel in order togive early strength to the fresh con-crete.

The internal surfaces of a slip-formed core must be kept moist, es-pecially in the period before regularsteam curing. Hot moist air and low-pressure steam have been employed.Extrusion machines have been de-veloped to form both inside and out-side surfaces of hollow-core piles.With one proprietary machine,U-shaped staples are automaticallyinserted behind the concreting in-stead of spirals.

In Malaya and Fiji, large-diam-eter cylinder piles have been manu-factured using precast rings as spac-ers and formwork supports. Theserings are placed every 3 m (10 ft.),and the tendons run through pre-formed holes. The forms are thenplaced and the concrete cast. Partic-ular care must be taken to get fullbond between the cast concrete andprecast segments, so as to insure fullconcrete tensile strength for driving.Method 2: Segmental construction.Segmental construction involves themanufacture of short segmentswhich are joined and then pre-stressed. The best-known applica-tion is to cylinder piles. By one tech-nique, segments are produced bycentrifugal spinning. The core-form-ers are stressed so as to prevent theiroutward displacement. Special

means are employed to ensure per-fect ends, with no honeycombing,water pockets, etc. The segments arecured and then placed on an assem-bly bed. After the joints have beencoated with an epoxy glue, the ten-dons are inserted, stressed andgrouted. Finally, the anchorages arereleased.

Segments may also be cast verti-cally, as was most dramatically dem-onstrated on the 4 m (13.1 ft.) diam-eter piles for the OosterscheldeBridge. These segments were assem-bled, the joints were concreted andthen the entire pile was post-ten-sioned.Method 3: Centrifugally spun sec-tions. In Japan, a number of largemodern mechanized factories man-ufacture pile sections by a thirdmethod, that of centrifugal spinning.The plants there use bar, wire orstrand for prestressing tendons. Insome plants the tendons are ten-sioned against the mould, and theconcrete placed while the mould isspun at low speed. The speed is thenincreased for consolidation and themould finally reaches a peripheralspeed of 350 m/ min (1150 ft./min.).This whole process takes only 8 min-utes. Some plants cure in steam at65°C (150°F) for 12 to 24 hoursfollowed by water curing for 3 days.

During spinning, there is a tend-ency for the bars to move outward,leaving a small void inside. This canbe overcome by proper mix design,by prestressing bars before concret-ing, and by controlling the rate ofspinning at the various stages.

INSTALLATION

Prestressed piles may be installedby a variety of techniques, alone orin combination. These include driv-ing, jetting, drilling and weighting.Vibration, which has been extensive-ly applied to steel piling in recent

78 PCI Journal

years has, as far as I know, been ap-plied to prestressed foundation pileson an experimental basis only. It hasbeen applied to prestressed sheetpiles in Eastern Europe. Large con-ventionally reinforced concrete pileshave been installed by vibration inthe USSR and there is no reason tobelieve that this method would notbe fully satisfactory and give evenbetter performance with prestressedpiles. The absence of shrinkagecracks should minimize the dampingof vibration.

Most prestressed piles are in-stalled by driving, either alone or incombination with jetting. The hun-dreds of thousands of prestressedpiles successfully installed by driv-ing under the widest possible varia-tions in sub-surface conditions attestto the general efficacy of this meth-od. Some general rules have beendeveloped to ensure successful in-stallation. These rules have emergedin remarkably similar form in themany countries which developedthem (Japan, Britain, Australia,USA).

1. The pile and hammer shouldbe in alignment.

2. The pile head should besquare and free from protruber-ances or projecting wire or strand.Binding should be avoided in thepile helmet.

3. Avoid excessive hammer(ram) velocity. Reduce ram veloc-ity in soft soils.

4. Provide adequate cushion-ing at pile head. 15 to 35 cm (6 to14 in.) of softwood has consistent-ly proved most effective in reduc-ing internal stresses in the pile andin providing maximum penetrationof the pile. Provide a new cushionfor each pile. The Japanese usecorrugated paper board packing.Other materials used are rubber

(including crude rubber sheets),felt, sacking, asbestos fibre, coiledhemp rope and plywood.

5. Protect pile splice rings, orother fittings, with a ring cap.

6. Avoid excessive restraint topile, either in torsion or bending.The pile position and orientation,once the pile is well embedded inthe soil, cannot be corrected froma position at or near the headwithout causing distress.

7. For cutting off, clamp on asteel or wood band and cut withchisel and hammer or small pneu-matic drill. Prior cutting of a cir-cumferential slot with a concretesaw cut or rotary drill, will facili-tate a neat, unspalled head.

8. When pre-drilling or jetting,the tip must be well seated beforefull driving energy is applied.The greatest driving efficiency is

obtained by using a heavy ram, asoft head cushion, and low-velocityimpact.

With very long slender piles,guides or supports should be em-ployed to prevent "whip", vibrationand buckling during driving. Withraking piles, supports must be pro-vided for the overhanging lengths—both those above ground and, whendriving in water, those below.

The lifting and handling points forpitching must be properly markedand used. With long piles and mul-tiple (two to six) pick-up points, theangle which the slings make with thepile affects the stress, and pick-uppoints may therefore be differentfrom those used for handling at theplant and in transport.

In construction over water, pileheads are sometimes pulled into cor-rect alignment. With long unsup-ported lengths, it is easy to over-stress the pile. Thus, specificationsshould limit the amount or force of

October 1968 79

pulling to ensure against over-stress.A common limitation for a 10 m (33ft.) water depth is to limit the forceto 200 kg (440 lb.) by using a winchpreset to this maximum force.

After piles have been driven inrivers or harbors, they may requiretemporary lateral support againsttidal current, wave forces, etc., and,in the case of raking piles, againstdead-weight deflection.

Diesel hammers are widely usedto install prestressed piles. Becauseof generally light ram and high ve-locity of impact, particular care mustbe taken to provide a proper headcushion.

Jetting is often employed to aiddriving. This may be pre-jetting (pi-lot jetting) to break up hard layers,or may be jetting during driving. Inthe latter case, it is hazardous to jetat or below the tip during drivingas this may create a cavity, and pro-duce a "free end" condition, leadingto excessive tensile stresses andcracking in the pile.

Pre-drilling—dry, with water, orwith bentonite slurry—may be effec-tively used to aid penetrationthrough hard and dense upper lay-ers. This is increasingly employed assoil engineers are requiring deeperpenetration for settlement control.

The pulling and removal of pre-stressed concrete piles is very likelyto produce tensile cracks. If pullingis essential, extensive jetting shouldprecede the pull and special careshould be taken to insure a truly ax-ial pull.

Repairs to damaged or crackedprestressed piles in marine installa-tions sometimes become necessary.The following methods have beensuccessful:

• Underwater-setting cement(which must not contain CaC12)

• Underwater-setting epoxy

• Steel or concrete jacket, con-nected to pile with tremiegrout.

Any repairs should be preceded bythorough cleaning and the patchmust be protected until it has ade-quate strength and resistance to thewater.

On the Oosterschelde Bridge inthe Netherlands, the piles were sunkby internal dredging and weighting.The derrick barge was so rigged thata part of its weight could be imposedon the pile. After the proper eleva-tion had been reached, a bottomplug of underwater concrete wasplaced.

On the San Diego-CoronadoBridge in California, the piles wereinstalled through dense clay andsand strata to a suitable bearingzone in three stages. In stage 1, theywere pre-jetted, and driven to apoint about 2 m (6.5 ft.) above de-sign tip. In stage 2, side jetting wellabove the pile tip plus driving withthe hammer brought the pile to apoint 1 m (3.3 ft.) above design tip.Stage 3 employed the hammer alone,with a specified number of blows, toensure thorough consolidation of thesand around the pile.

STRESS-WAVE THEORY

The stress-wave theory explainsmany of the observed phenomenaconnected with the driving of pre-stressed piles. When a prestressedpile is struck by a hammer blow, theimpact lasts from 4 x 10-3 to 8 x 10 -3

seconds. The compression wavetravels at the speed of sound in con-crete—about 4000 m/sec. (13,100 ft./sec.) for a typical prestressed con-crete. The wave-length is normally16 to 50 m (52 to 164 ft.) dependingon the duration of impact. The maxi-mum compressive stress in the pile

80 PCI Journal

head will be from 70 to 240 kg/cm2(1000 to 3400 psi), but can reach 300kg/cm2 (4300 psi). When this com-pressive stress wave reaches the piletip, if the tip is on a hard bottom, thereflected wave is compressive; if on asoft bottom, it is a tensile wave.

The toe stress may thus reach acompressive stress of twice the headstress when driving through wateronto rock. Conversely, when drivinginto very soft material, a tensilestress of up to 50 per cent of thecompressive head stress may be re-flected. Actual tensile stresses, asmeasured by strain gauges, run upto 150 kg/cm`' (2000 psi).

In practice, the condition of softdriving and reflected tensile waveshas often proved to be serious. Hor-izontal cracks appear suddenly, al-most mysteriously, about one-thirdof the length down from the headand are spaced about 0.5 m (1.6 ft.)apart. Puffs of concrete dust are pro-duced, followed by spalling andwidening of the crack. Repeateddriving produces fatigue in the con-crete adjacent to the crack includinga definite rise in temperature, andeventually brittle fracture of the ten-dons. The actual point or points ofcracking tend to occur initially at apoint of discontinuity, such as a lift-ing point, insert, form mark or hon-eycomb; this tends to confuse thediagnosis but is generally irrelevant.

There are some 21 variables in-volved in computing driving stresses.A number of computer programshave been set up. Among the vari-ables are weight of pile, length,modulus of elasticity, damping prop-erties, weight of ram, velocity of im-pact, stiffness of cushion, soil re-sistance, and end condition.

In my opinion, the stress-wavetheory is basically valid but has twolimitations. First, it over-estimates

the tensile stress because it fails totake fully into account the effect ofside friction and dissipation of ener-gy into the surrounding soil. Second,it often predicts maximum tensilestresses over the entire middle thirdof the pile Iength, whereas crackinghas been observed only near the topthird point.

This theory does emphasize the im-portance of head cushioning, whichreduces the maximum stresses up to50 per cent. It explains why, for thesame energy, a heavy ram with lowvelocity produces much lowerstresses than a light ram with highvelocity.

The amplitude of the stress wavecan be reduced by reducing the ve-locity of impact (shorter stroke orfall), and by increasing the headcushion thickness. This is particular-ly important in soft driving and,most of all, when a pile breaksthrough a hard stratum of fill or sandand thus has its sides held by fric-tion with its tip free.

Lightweight concrete has a lowermodulus of elasticity. This reducesthe velocity of the stress wave toabout 3000 m/sec. (9800 ft./sec.)and the maximum compressive andtensile stresses are reduced by 18and 22 percent respectively. There isapparently greater internal dampingwithin the lightweight concrete,which is partly offset by the lowertensile strength of the concrete.

Theoretically, with a pile driventhrough water onto rock, a highcompressive stress can reflect fromthe toe, travel to the head and,if the ram has left the the pile head,be reflected as a tensile wave fromthe head. This appears to be a ratherrare condition in practice, owing todamping, skin friction, etc.

ECONOMY

The true economy of prestressed

October 1968

81

concrete piles is found in an eval-uation of their ability to perform theassigned structural function, withadequate safety, over a long period,at minimum cost. Thus, the structur-al behavior of the pile, its ability tobe transported, handled, driven andconnected, as well as its durability,are just as important as the costs ofmanufacture.

Prestressed piles have proved tobe the most economic solution for awide range of piling installationswhenever their inherent structuralproperties can be well utilized,whenever a large volume is required,and whenever durability becomes amajor consideration.

For marine structures, the criteriaare a life of 25 to 50 years, good col-umn strength, high load-bearing ca-pacity, bending strength, ability tobe handled and driven in longlengths, and low first cost. Pre-stressed piles are generally superiorto other types of piling and havetherefore been widely adopted formarine installations.

For foundations, the criteria arelow first cost, structural strength asa short column, and ability to behandled and driven to required pen-etration. In comparing various com-petitive types and sizes of piling forfoundations, an analysis should bemade which considers the structuralcapacity of the pile, the soil capacityfor the size and penetration consid-ered, the costs of furnishing, trans-porting, and driving, and the size,depth and cost of the footings. Thetime of construction should also beevaluated.

Prestressed piles prove quite com-petitive in most such analyses, andoften turn out to be the most eco-nomical solution. They must, ofcourse, be directly compared withother forms of concrete pile. As com-

pared with precast conventionallyreinforced piles, the major savingslie in column strength, durabilityand steel cost. Prestressed piles re-quire about one-sixth as much steelweight as conventionally reinforcedpiles. Since the unit cost of prestress-ing steel installed is about double,the net cost is only 1 x 2 = 1/a.

One major economic factor in fa-vor of prestressed piles for founda-tions (as compared to steel or timberpiles) is that, for many soil condi-tions, they develop the requiredbearing at substantially less pene-tration. This is often true for bothskin-friction and end-bearing condi-tions.

Labor requirements for manufac-ture vary substantially, dependingon method and volume of manufac-ture. Where a high sustained vol-ume is maintained, the manpowercan be properly allocated to ensurefull utilization. A number of coun-tries reported on man-hour require-ments, but it would appear unfairto compare them as all factors arenot known. However, the lowest re-ported man-hour requirements wereon long-line pretensioning beds us-ing multiple fixed forms.

PRESTRESSED CONCRETE SHEET PILING

Prestressed concrete sheet pilespresent the advantages of durabilityand excellent appearance. They arenot always directly economical infirst cost as compared with steel be-cause sheet piles must usually workin both positive and negative bend-ing. To meet this structural problem,a reasonable amount of eccentricityof prestressing force may be selectedand mild steel may be added atpoints of high moment, especiallyhigh negative moment. The amountof allowable tension in sheet pile de-sign is a question of exposure (dur-

82 PCI Journal

: : :iT

: : : : : :

iiiii •LIIIII

eenporary head extensionfor driving`

1

Fig. 5. Typical cross-sections and details of prestressed concrete sheet piles

ability) and type and duration ofload. Values of zero and one-halfmodulus of rupture are commonlyused. Typical sections in general useare shown in Fig. 5.

Some difficulties have occurred inthe breaking off of the wings of thegroove when driving sheet piles.This may sometimes necessitate de-tailing the mild-steel bars so that

they extend into the wings. Pre-stressed sheet piles may be installedwith gang jets or with pile hammers;generally, both are used together. InEast Germany, vibratory hammersare used. A special helmet must beused to enable one pile to be drivento the same grade as the adjoiningpile. Several countries report castingon the head, at time of manufacture,

October 1968 83

a short narrow extension whichserves as a driving block for thehammer. After driving, this extensionmay be cut off and the tendons tiedinto a capping beam, or the spacebetween extensions used for tie-backs.

In driving, the majority opinion isthat the male end (tongue) shouldlead, to prevent the plugging thatwould occur if the female end(groove) was leading. When a dou-ble-groove joint is used, a steel pipesplice is usually driven with the pileand later withdrawn.

In some building foundations, pre-stressed concrete sheet piling hasbeen driven as a perimeter wall. Thisthen serves as a bulkhead during ex-cavation, and is incorporated in thefinal construction as the buildingwall.

Joints have to be grouted to makethem tight against sand leakage.When accessible, they may bepacked by hand; expansive cementhas been used. In grouting throughwater, a canvas or polyethelene bag(tube) may be pushed down on agrout pipe, then the grout injectedas the pipe is withdrawn.

One recent study gives the follow-ing relative figures for cost per unitarea per unit of moment-resistingcapacity:

• Steel sheet piles, yield point2500 kg/cm2 (35,600 psi),unpainted............... 92

• Steel sheet piles, yield point3500 kg/em2 (50,000 psi),unpainted............... 90

• Prestressed sheet piles, withf ' = 550 kg/em2 (7800 psi)cylinder strength .........100

• Steel sheet piles, painted forcorrosion protection ......105

PRESTRESSED CONCRETE FENDER PILES

Prestressed piles have many ad-

vantages to offer as fender piles, dol-phin piles, and other piles resistingprimarily lateral forces. They aredurable and economical. They canbe designed to maximize deflectionand energy-absorption by keepingthe moment of inertia low in rela-tion to strength (by choice of sec-tion), by use of concrete with a lowmodulus of elasticity (such as struc-tural lightweight aggregate concrete)and by proper choice of prestresslevels.

One use of fender piles is forwharves and piers, where ships mustconstantly berth without damage toship or fender system. Prestressedfender piles were employed onwharves at Kuwait, Singapore andLos Angeles, and on a test installa-tion in San Francisco (see Fig 6).

Structural lightweight aggregateconcrete appears ideal for fenderand dolphin piles because of itsgreater deflection and energy ab-sorption. The greatest problem withprestressed dolphin piles is their lackof ultimate strength when designloadings are greatly exceeded. Theytend to snap off under excessive im-pact. Therefore it is suggested thatgreater reserve ultimate strength beprovided in the form of additionalunstressed strands.

Prestressed fender piles are usual-ly designed for a maximum tensionof up to one-half the modulus of rup-ture. It is important to preventcracking that will reduce durability;on the other hand, it would bewrong to make them too stiff. Thedesign should emphasize energy ab-sorption. As regards durability, theycan be economically replaced at in-tervals during the life of the struc-ture itself, so they do not require asgreat a factor of safety as do founda-tion piles.

Another type of fender is the pro-tective fender for bridge piers (Fig.

84 PCI Journal

lubber I I compactedfill

._ ...eyebohi iN`

gro t ^^ _

milar band chain

im

pre a d co--hnderepne

b)

MSS

Fig. 6. Fender systems for general cargowharves utilizing prestressed concretefender piles—a) Port of Los Angeles,California; b) Port of Jurong, Singapore;

c) Port of Kuwait

7). This is designed to protect thepier, and the prevention of damageto ships and the cost of repairs arenot of primary concern. Here, ulti-mate strength plus durability areparamount. Prestressed concrete fen-der piles have been used for protec-tion of piers on all major bridgesrecently constructed in California.

SPLICES

Many of the recent developmentsin prestressed concrete piling have

centered on techniques of splicing(see Fig. 8). The ideal splice wouldbe capable of being effected withoutsignificant interruption to driving, beas strong and as durable as the pileitself, and be cheap. The splice mustdevelop adequate strength in com-pression, tension, shear, torsion andmoment, as required, both during in-stallation and in service.

The epoxy-dowelled splice hascome closest to meeting all require-ments when high moment capacity is

October 1968 85

M

required. In Norway, small insulatedwires are wrapped around the tubesand electric heating is used to ac-celerate the hardening. In the USA,external steam heat has been usedfor the same purpose.

Mechanical splices are most eco-nomical when only direct compress-sion must be transferred. A numberof proprietary mechanical splices aremarketed and no attempt is made toillustrate all of them here.

PROBLEMS AND FAILURES

Proper design, manufacturing

techniques and installation practicecan only be achieved by a carefulanalysis of problems and failures.These represent an extremely smallproportion—considerally less than 1percent—of the total number of pre-stressed piles which have been suc-cessfully installed. These knownproblems are presented as specificcases, with assigned cause and cor-rective action taken.1. Horizontal cracking under driv-ing. This is the most common prob-lem. Puffs of "dust" are seen aboutone-third of the length from the top

■

Fig. 7. Bridge pier fenders utilizing prestressedconcrete fender piles—a) Eureka SloughBridge; b) Sacramento River Bridge; c) Bene-

cia-Martinez Bridge85 PCI Journal

X11

pileeztenslon

mlld.aaeclI Idowels ten[ral II II

Me In pile, bearleteztanslon I I

^

pla[e I I I

I

weldII I

drivenICI

I II I I I

II^I

I IIpile

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anchorea=rn, II

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Mick-walled I II

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prevent packing I I`

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1 1 IIu— (h)

Fig. 8. Typical splice details for prestressed concrete piles—a) epoxy-dowelledsplice, USA, Norway; b) welded splice; c) Hercules splice, Sweden; d) can anddowel splice; e) welded splice, Japanese patent; f) "Brunsplice" joint, USApatent; g) post-tensioned splice, Great Britain; h) steel pipe splice for hollow

piles, Norway

October 1968 87

and horizontal cracks appear at half-meter (1.6 ft.) intervals, with consid-erable spalling at the surface. Ifdriving continues, the concrete willdisintegrate in fatigue failure, andgrow sensibly hot just above thecrack. Eventually, the tendons willbreak in brittle fracture.

Cause: Tensile rebound stressesfrom free-end condition attoe.

Cure: a. Increase amount of soft-wood cushion.

b. Reduce height of stroke(fall) of ram.

c. Do not jet or drill belowtip of pile during driving.

d. Increase weight of ram.

2. Inclined cracking with extensivesurface spalling.

Cause: Torsion plus rebound ten-sile stress.

Cure: Steps a, b and c in (1) and,in addition,d. Make sure driving head

cannot restrain pile fromtwisting.

e. Do not restrain pile inleads or template.

3. Vertical splitting of cylinder pilesor hollow-core piles during instal-lation.

Cause: a. Soft mud and waterforced up inside to a levelabove outside levels, cre-ating an internal hydro-static head.

Cure: Provide adequate vents inpile.

Cause: b. Hydraulic ram effect ofhammer hitting water col-umn.

Cure: Do not drive pile head be-low water level or provide aspecial driving head withvery large openings (greater

than 60 per cent of voidarea).

Cause: c. Shrinkage cracking inmanufacture.

Cure: a. Adequate water cure.b. Re-design mix to mini-

mize shrinkage.c. Increase area of spiral.

Cause: d. Piles split while beingfilled with concrete, be-cause of hydrostatic headof liquid concrete.

Cure: a. Reduce rate of pour toensure it will obtain in-itial set without exceed-ing tensile strength ofpile concrete and alsowithin acceptable strainsof spiral, whichever maybe lower.

h. Increase area of helix.

Cause: e. Vertical splitting at topduring driving due tobursting stresses.

Cure: a. Increase spiral at head.b. Increase cushioning at

head.

Cause: f. Vertical splitting at orabove toe due to plug ofsoil wedging in void.

Cure: a. Prejet or predrill to breakup plugs.

b. Jet or drill inside tobreak up plugs duringdriving.

c. Increase spiral.d. Use solid tip.

Cause: g. Jetting inside producesinternal hydrostatic head.

Cure: Provide vents in pile walls.4. Vertical cracking of cylinder piles

after installation.

Cause: a. Freezing of water or mudinside.

Cure: a. Provide sub-surface ventsto allow circulation of wa-ter.

88 PCI Journal

b. Place wood or styro-foam log inside.

c. Fill pile with frost-proofmaterial.

Cause: b. High river level sudden-ly fell, leaving water in-side higher than out,combined with dryingshrinkage on outside andcolder temperature out-side.

Cure: a. Ensure adequate vents.b. Fill piles with concrete

core.C. Increase spiral.

Cause: c. Piles cracked from logs,debris, etc. carried byriver.

Cure: Fill with sand or concrete.

5. Breakage of prestressed verticaland raking piles in wharf by icemasses during a spring thaw.

Cause: Ice masses slid down rak-ing piles, wedged againstadjoining vertical piles,broke both.

Cure: In such an environment, de-sign pile layout so icemasses cannot wedge be-tween piles.

6. Excessive spalling at head underdriving.

Cause: Hammer impact on unre-strained concrete.

Cure: a. Chamfer head.b. More cushioning.c. More spiral at head.d. Make sure head is square

and plane.

7. Disintegration in service of cor-ners of prestressed piles belowwater line.

Cause: Reactive and unsound ag-gregates.

Cure: a. If caught very early, sealentire pile surface with

underwater-setting epoxy,or chip to undamagedconcrete and patch withepoxy-mortar or under-water-setting cement mor-tar.

b. Jacketing, with a struc-tural jacket.

c. Replacement.

8. Vertical hair-line cracks alongcenter of faces of solid piles.

Cause: Differential shrinkage of out-side of pile plus differentialcooling, especially in cold,dry, windy weather.

Cure: a. Provide graduated cool-ing period during lastphase of steam curing.

b. Cure by water immedi-ately after removal fromsteam cure.

9. Horizontal hair-line cracks on topsurface at lifting points.

Cause: Negative moment exceedsconcrete strength plus pre-stress.

Cure: a. Provide mild steel at lift-ing points.

b. Use more lifting points.DURABILITY

Prestressed concrete piles are gen-erally chosen because of their inher-ent durability. It is important thenthat this durability be ensured andprotection provided wherever pre-stressed piles are used in an adverseenvironment.

The primary protection to the ten-dons and spiral against sea water,chloride and sulphate attacks is pro-vided by the concrete cover. Thismust be impermeable and non-por-ous; otherwise, salt-cells will be setup causing electrolytic corrosion. Anadequate cement content is neededto insure sufficient alkalinity at thesurface of the steel. The USSR re-

October 1968 89

ports that the porosity is reduced byprestressing, which maintains a per-manent compressive stress in theconcrete.

The factors are, therefore: thickenough cover; non-porous concrete;adequate cement content; sound,non-reactive aggregates; mainte-nance of a high level of compressivestress (prestress).

Freeze-thaw, combined with saltwater, is perhaps the severest expos-ure for concrete piles. In Norway,such piles used to be lagged with 4to 5 cm (1.6 to 2.0 in.) of treated tim-ber, but now unprotected pre-stressed piles with 8 to 10 percententrained air are used, and specialattention is paid to achieving an im-permeable concrete cover. Freeze-thaw sea water tests by the U.S.Army Corps of Engineers in Maineshowed that air-entrained preten-sioned concrete was more durablethan any other concrete. The USSRspecifies three types of frost-resist-ant concrete, the choice dependingon the exposure in service.

There is a general belief thatcracks, even hair-line cracks thathave closed again under prestress,are a major path for corrosion. Re-search by Dr. P. W. Abeles has indi-cated that very small cracks do notnecessarily impair the durability ofthe pile. My examinations confirmthat hair-line cracks are not nearlyas serious a matter as permeabilityof the concrete cover. Under contin-ual prestress, and in the presence ofwater, autogenous healing of hair-line cracks may take place.

RESEARCH AND DEVELOPMENT

The following areas appear fruit-ful for further research and devel-opment.1. The amount of spiral reinforce-ment needed at all points in the pile.

The relation between this and thetensile strength of concrete, thelongitudinal driving stresses, thebearing capacity and the combinedstresses at the pile head.2. Further correlation of the stress-wave theory with actual strain gaugemeasurement and with the proper-ties of soils.3. An investigation of long-termdurability. Whenever piles whichhave been exposed to adverse envir-onments, such as sea water, are re-moved (as for repair of ship dam-age), then a detailed investigationshould be made of chloride concen-trations and of whether or not anyelectrolytic corrosion has begun.4. The behavior of prestressedpiles under combined loadings atultimate load, e.g., under seismicloading.5. The post-cracking behaviour ofprestressed piles under driving:amount of steel required, energy ab-sorption, damping, low cycle fatigueof steel and concrete, coordinationwith research on seismic behaviourof prestressed concrete.6. The effect of prolonged drivingon the properties of a prestressedpile. It has been assumed that, ifinstalled without cracking, its prop-erties remain unchanged despite per-haps a thousand cycles of compres-sion and tension. Is there a reductionin compressive or tensile strength orchange in modulus of elasticity dueto fatigue or does a "work harden-ing" take place? The latter may welloccur at the head of the pile, as somefield observations indicate a possiblegain in strength.7. The use of very high strengthconcrete for foundation piles; thelimitations, if any, on unit loads; thelong-term behaviour under sustainedload plus prestress; economics.8. Further development of fender

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piles: lightweight concrete, rubberstrips, use of epoxy surfacing to re-sist abrasion, additional steel for ul-timate behaviour.9. Development of a fully-pre-stressed splice that is economicaland quick to make.10. Grout injection through piles toincrease skin friction; correlationwith soil mechanics research andapplication to tension piles.

BIBLIOGRAPHY

General

Lin, T.Y. and Talbot, W. J., Jr., "Preten-sioned Concrete Piles—Present KnowledgeSummarized," Civil Engineering (USA),Vol. 31, No. 5, May 1961, pp. 53-57.

Gifford, F. W. and Williams, N.S., "DraftNotes for Guidance on Pretensioned Pre-stressed Piles," Symposium on Piles andFoundations, London, Reinforced ConcreteAssociation and Prestressed Concrete De-velopment Group, 27 April 1964, 11 pp.,with discussions by Harris, J. D., New,D. H. and Boonstra, G. C.

Weller, N.H., "Prestressed Concrete Piles,"a paper presented at a Symposium on Pre-stressed Concrete, Sydney, Cement andConcrete Association of Australia, August1961, 13 pp.

Gerwick, B.C., "Precast Prestressed Con-crete Piles," San Francisco, 1964. (Designprocedures, properties of typical sections,typical details, general notes, specifica-tions.)

Li, Shu T'ien, "Technological Frontiers ofPrestressed Concrete Piling," Journal ofthe Prestressed Concrete Institute, Vol.12, No. 6, December 1967.

Marine structuresGerwick, B. C., "Prestressed Concrete inMarine Structures," Civil Engineering(USA), Vol. 29, No. 11, November 1959,pp. 782-785.

Gerwick, B. C. and Talbot, W. J., "Pre-stressed Concrete Piling and Marine Struc-tures," Proceedings World Conference onPrestressed Concrete, San Francisco, July1957, pp. A25-1—A25-12.

Li, Shu T'ien, "Installation Practice forPrestressed Concrete Piling In Dock andHarbor Work," Dock and Harbour Au-thority, Vol. 48, No. 562, August 1967, pp.102-108.

Fornerod, M., "Prestressed Concrete Cyl-inder Piles," Proceedings World Confer-ence on Prestressed Concrete, San Fran-cisco, July 1957, pp. A26-1—A26-14.

Robertson, W. T., "Prestressed ConcreteCylinder Piles," paper presented at a Sym-posium on Concrete Construction in Aque-ous Environment, Detroit, American Con-crete Institute, 1964, Publication No. SP8,Paper No. 6, pp. 67-77.

Gerwick, B. C., "Experience In The UseOf Prestressed Concrete In Wharf andHarbour Construction," ProceedingsFourth International Harbour Conference,Antwerp, 1964.

Hoving, H. T., Krijn, A. C. and Van Loen-en, J. H., `Krug over de Oosterschelde",(Bridge over the Eastern Scheldt) CementBeton, Vol. 16, No. 11/12, November/December 1964, pp. 685-693, 760-764.

Launay, P., "Harbour Works in Indonesia,"Travaux, July/August 1966, pp. 1054-1070.

UtilizationAnon., "Prestressed Concrete Piles," PCItems, Vol. 7, No. 3, March 1961.

Gerwick, B. C., "High-Capacity Pre-stressed Concrete Piles for Deep Founda-tions", Journal of the Prestressed ConcreteInstitute, Vol. 9, No. 6, December 1964,pp. 32-43.

Altmann, W., "Spannbeton-Spundbohlenin der DDR", (Prestressed concrete sheetpiles in the USSR and East Germany),Bauplanung-Bautechnick, Vol. 19, No. 10,October 1965, pp. 480-482.

Manufacture and testsHewitt, F. M., "Extruded Prestressed Con-crete Piles," Spiroll Corporation Ltd., Win-nipeg, August 1967, 7 pp.

Tokyu Concrete Industries Co. Ltd., Pre-stressed concrete piling brochure, Tokyo,1967.

Tokyu Concrete Industries Co. Ltd., Re-

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ports of tests, Tokyo.Crushing test on post-tensioned centrif-ugal prestressed concrete piles, 1963.Durability test (under driving) on post-tensioned prestressed concrete piles,1963.Anchor length on pretensioned pre-stressed concrete pile under driving.Concrete temperature adjacent towelded splices, 1965.Repeated bending tests on prestressedconcrete moment piles, 1966.Bending tests on welded splices, 1966.Rupture in bending test on prestressedconcrete piles, 1966.Position of Neutral Axis and StressedState Of Prestressed Concrete Pile Sub-ject To Axial Force and Bending Mo-ment Within Elastic Range, Makita, H.Calculation Method For Ultimate LoadOn Centrifugally Cast Prestressed Con-crete Pile.Bending Resistance Of PrestressedConcrete Piles on Various Axes RelativeTo Tendon Pattern, Yoshinari, M., 1967.Welding Standards for Cylindrical PipeSplice.Production Methods and Quality Con-trol, July 1967.

StandardsJoint AASHO-PCI Committee, "Standardsfor Prestressed Concrete Piles—squarecross-section, octagonal cross-section, cyl-inder piles," Prestressed Concrete Institute,Chicago.

American Railway Engineering AssociaLion, "21-in. Prestressed Concrete Piles,"Manual of Recommended Practice, Chap-ter 8, Part 9, Chicago 1960.

Texts

Abeles, P. W. and Turner, F. H., "Pre-stressed Concrete Designer's Handbook,Concrete Publications Ltd., London, 1962,pp. 221-224.

Preston, H. K., "Practical Prestressed Con-crete, McGraw-Hill Book Co. Inc., NewYork, 1960, First edition, pp. 215-232.

Leonhardt, F., "Prestressed Concrete De-sign and Construction", Wilhelm Ernst andSohn, Berlin, 1964, Second edition, pp.575-577.

Lin, T. Y., "Design of Prestressed Con-crete Structures," John Wiley & Sons Inc.,New York, 1963, Second edition, pp. 431-444.

Driving stresses and installation problems

Smith, E. A. L., "Tension In ConcretePiles During Driving", Journal of the Pre-stressed Concrete Institute, Vol. 5, No. 1,March 1960, pp. 35-40.

Smith, E. A. L., "Pile-driving Analysis bythe Wave Equation," Proceedings of theAmerican Society of Civil Engineers,Structural Division, Vol. 86, 1960, pp.35-61, Paper No. 2574.

Zollman, C. C., "Investigation Of FailureOf Three Prestressed Concrete Piles,"Journal of The American Concrete Insti-tute, Vol. 58, No. 1, July 1961, pp. 107-112.

Gerwick, B. C., "Torsion in Concrete PilesDuring Driving," Journal of the Pre-stressed Concrete Institute, Vol. 4, No. 1,June 1959, pp. 58-63.

Gardner, S. V. and New, D. H., "SomeExperiences With Prestressed ConcretePiles," Proceedings of the Institution ofCivil Engineers, Vol. 18, January 1961,pp. 43-66; discussion, Vol. 21, April 1962.

Glanville, W. H., Grimes, C. and Davies,W. W., "The Behaviour Of ReinforcedConcrete Piles Under Driving", Proceed-ings of the Institution of Civil Engineers,Vol. 1, December 1935, pp. 150-234.

Hirsch, T. J., Samson, C. H. and Lowery,L. L., "Driving Stresses In PrestressedConcrete Piles"—"Computer Study Of Dy-namic Behaviour Of Piling"—"Fundamen-tal Design and Driving Considerations ForConcrete Piles," Texas A & M University,College Station, Texas, 1963-1965.

Strobel, G. C. and Heald, J., "Theoreticaland Practical Discussion of Design, Test-ing and Use of Pretensioned PrestressedConcrete Piling," Journal of the Pre-stressed Concrete Institute, June 1962;discussion, October 1962.

Prestressed Concrete Institute Committeeon Prestressed Concrete Piling for Build-ings, "Recommended Practices for DrivingPrestressed Concrete Piles," Journal of thePrestressed Concrete Institute, Vol. 11, No.4, August 1966, pp. 18-27.

Brown, T. E., "Handling and Driving Pre-stressed Concrete Piles," paper presented ata Symposium on Concrete Construction inAqueous Environment, Detroit, American

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Concrete Institute, 1964, Publication No.SP 8, Paper No. 7, pp. 79-87.

Blake, J. B., "Recommended Practice Forthe Driving Of Prestressed Concrete Piles,"Constructional Review, Vol. 36, No. 4,April 1963, pp. 27-31.

Portland Cement Association StructuralBureau, "Driving Conditions With Pre-stressed Concrete Piles," Portland CementAssociation, Chicago, 1962, 5 pp.

Splicing

Stormbull, A. S., Piling brochure, Oslo,Norway, November 1967.

Johnson, L. C., "Cylindrical Concrete PilesUsed in Fraser River Bridge at McBride,British Columbia," Report of Bridge Engi-neer's Office, Department of Highways,Victoria, 1967.

Durability

Gvozdev, A. A., "Research On PrestressedConcrete In USSR and Neighbouring Eu-ropean Countries," Proceedings of the Fifth

Congress of the Federation Internationalede la Precontrainte, Paris, 1966, pp. 12-19.

Abeles, P. W. and Filipek, S. J., "CorrosionOf Steel In Finely Cracked Reinforced andPrestressed Concrete," Journal of the Pre-stressed Concrete Institute, Vol. 10, No. 2,April 1965, pp. 36-41.

Related references

Federation Internationale de la Precon-trainte, "Report of the FIP Commission onPrestressed Lightweight Concrete," Pro-ceedings of the Fifth Congress of the FIP,Paris, 1966, pp. 131-148.

Zia, P. and Moreadith, F. L., "UltimateLoad Capacity Of Prestressed ConcreteColumns," Journal of the American Con-crete Institute, Vol. 63, No. 7, July 1966,pp. 767-788.

Laszlo, G., "Combined Bending and AxialLoad In Prestressed Concrete Columns,Journal of the Prestressed Concrete Insti-tute, Vol. 12, No. 6, December 1967, dis-cussion, pp. 77-88.

Discussion of this paper is invited. Please forward your discussion to PCI Headquartersby April 1 to permit publication in the June 1969 issue of the PCI JOURNAL.

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