TheDock &:e:arbour · Authority VOL. LXXHNo . 831, SEPTEMBER 1991

North American Survey

SEPTEMBER 127

Underwater concrete construction of '

Phuket Port quay wall foundation by AR Cooke, Works Superintendent and J Simpson, Project Manager, Sir William Halcrow and Partners

Observation has led the authors to the view that ordinary concr ete plac ed underwater is not regarded with the confidence which it deserves. The successful placing of nearly JOO bays of underwater concrete for the foundation of a blockwork quay at Phuket Port, Thailand , all of which it was possible to inspect visually or by touch, suggests that the mix , the shutter and the placing m ethod used on that proj ect are worthy of examination.

Meth ods of plac ing, co ncrete mix de sign , th e use of additives and th e influ­ence of the shutt er are rev iewe d in thi s a rticl e, drawing princip a lly on the expe ri­ence gained in construction of th e founda­tion for the qu ay wall at Phuket Port.

In par ticul a r , of th e meth ods of placin g ava ilable , th at of pumped co ncrete proved in the case of Phuk et to be emin entl y suited to placing und erwater. since the mix prop erties for pumpin g are compat ible with th ose desired to underwater concrete an d th e co ntinuit y of supply minimi ses interrupti on in placing and thus the risk of disturbance o f th e concrete.

Co mp ariso n is made between th e mix used at Phuk e t and special co ncrete mixes dev ised in Japa n for und erwater ap plica­tion s.

Precep ts of good practice are propo sed and fea tur es to be reco mm ended for shutt ers for und erwater concre te a re pre­sent ed. Quality assura nce and me ans of

Fig I. Qu ay f 011nd(l{io11 al Ph11ke1 Porl.

sampling and tes ting including a nov e l yet simpl e met hod of re tri ev ing sa mpl es of the co ncr ete as place d are pre se nted .

The lack of up-to-d ate inform at ion on the ap plicat ion of mod ern technology in th e placing of ord inary co ncrete under­water has led the authors to the view that their experience on forming th e fou ndation fo r the quay wall at Phuk et Port may be of interest. .

Whilst recognising th at proprietory underwater concre te products have their place, they are not the subje ct of thi s article. Since it was apparent th at there is co nsiderab le literat ure on these pro­ducts . 25 ·7 some compari sons are made between the concr ete mix used at Phuk et and th ose special concretes.

Thus the procedures followed in placin g the co ncr ete at Phuket are described , and those factors thought to be influenti a l are iso lated.

The use of concr ete underw ate r was

:'.!" ,. '· .... ; .-_, . •·:.:':, ' ·-~·.; :", .-~·

=

known to the Romans and, mor e recently , it is possible to rea d in the Proc ee dings of the Institution of C ivil E nginee rs3 of efforts durin g th e first half of thi s centur y to overco me th e sa me probl ems encount­ere d underw ate r as were faced at Phuket . The only areas of new techn ology to have emerged since then a re the advent of the modern concrete pump and the use of chemical add itives to improv e the worka­bility of th e concrete.

The princip al function s of th e Phuket qu ay wall foun dat ion were to provide a leve l base on which to lay the blocks and to serve to distribut e the toe pressure from the blocks to the und er lying rock.

In this case , th e decision was mad e to have a concrete base since it was mor e compatible with th e rock foundation to have a strong un yielding base rnate rial. Al th ough nomin ally only 0.40 m thick , Fig . I , in practice the depth of the foun­da tion concrete was genera lly closer to

TIIE DOCK & HARBOUR AUTHORITY

Underwater concrete construction

1.0 111. and over 120 111 the foundation had to be deepe ned by up to 2.50 m in order to reac h a suitab le quali ty o f rock on which to found.

It was the dec ision to ~1s~ pumping for co ncret ing thi s de epe ned sec tion which led to th e adopt ion ther ea fter of co ncre te dcl i":Cry by pumpin g directly into th e sh utte rs for thl i norm al found atio n. On completion of th e deepen ed sectio n. a trial to check the feasib ility of pumpin g conc rete to the nor mal, shallower found a­tion slab was successful.

The co11crctc st ructur e co nsider ed in this article is a concrete slab . Nevert heless. the problem of this type of stru ctur e. in which a greate r proportion of the co ncrete surface is exposed to the sea than , say, in the case of a column or a tower and for which a leve l surface is req uired makes the expe rience part icula rly relevant to under­wa ter co ncreting gc nera llv.

Quality cont;ol of th e ~mdcrwatcr con­crete was achieved by conve ntio nal samp ­ling and testing above water. and inspec­tion o f eac h finished bay. In addit ion. a simple method of samp ling, which a llowed the retrieval of a testabl e cylinde r of w<;:t concrete from the foundation was deve loped .

Th e fact that construc tion cond itio ns were not ideal. ie near zero visibilit y. tidal range of 2.7 m on spri ngs with s trong current s . exposure to seasonal winds and waves . makes the experience relevant since some or all of th ese condit ions arc likely to appl y in most harbour works.

The contractor for th e Phuk et Port Project was a consortium of Tokyo Con ­st ruction Co. Waka ch iku Construc tion, bo th of Japa n . and Ch Karnchan g of Tha iland . The a uth ors were works supe r­intcn<.lcnt and proj ect manage r respe ct ive­ly for th e co nsulting group co mpri sing Sir William I lalcrow & Partner s, Ma unsc ll Consu ltant s Ltd. and Sindhu Pulsirivong & Associates.

Cement and additives Although it is not the purpose of thi s articl e to exte nd the arg um e nt of which

· type of cement is most suited for use in the sea. ther e see ms litt le doubt from the man y yea rs o f its use in the sea and from researc h . that Ordinary Por tland Cemen t (OPC) is suitabl e for, use in sea water. Hence for no rmal und e rwater concrete in th e sea there see ms no reason to use special ceme nt s.

Th e initi al se ttin g tim e of the Type I cement whic h was used throug hout was I JO minute s. The fineness was around 3 ,000 cm2/g ramme . Notw ithstanding the long time to initia l set. a retarder was used. Thi s was essentia lly a plasticiser . CO RMI C P45, wit h th e property of ex­tending the period of time over which the concrete can be placed. which therefore ensu red that th c·conc rete remained highl y workab le th ro ughout pumpin g.

A supcr plasticiser, CO RMIX SP4. was also used for all the und erwate r concre te. Thi s was dispensed auto mati ca lly by a gauge al the batching plant at 2,640 gram mes per batch. Thi s was eq uiva lent to 300 ml pe r 50 kg of ce ment. For use as a supe rpla sticiscr_ th e mak ers recomme nd 350-500 ml pe r 50 kg of ceme nt.

Th e dosage used at Phuk et did not , th erefore, take the mix into th e sup er ­plastici sed range. When the 950 ml per cubic metr e of P45 is taken into accou nt . the rate of addition is 400 ml per 50 kg o r cem<;:nt .

Concrete mix design Und cri.vatcr conc rete is expec ted to lose cement when be ing placed. Thu s we find e nh anced cement con tents traditionall y specified for co ncretes for underwat er use. Much str ess is also laid by deve lopers of special proprie tary conc rete mixe s for use underwater on the problem of segre­gatio n of co ncr ete without these pro­prietary ingredi e nts .

The approac h in desig ning a pumpin g mix for unde rwater place ment is similar to tha t for normal co ncrete fo llowing Road Note-+ (10) or ACT Report 2 II (9) with the additional aim of achieving a st rength greate r than nominall y required and higher workabi lity than would norma lly be re­quired.

Fortunatel y. these two aims are com­plementar y. T he maximum size of coa rse aggregate selec ted emerges as an import ant factor since thi s affects the ceme nt and wa ter co ntent.

The approach in desig ning a mix is:. a) To find the water-cement ratio - aggre­

gate -cement and sand- aggrega te ratios to give the req uired mea n str ength , the characteristic strengt h plu s twice the expec ted standard devia tion and then consider th e likely performa nce.

b) To see k a high cem ent an d sand content to ho ld the ma trix together.

Table 1

c) To selec t th e agg regate size appropriate to the requ ired mobilit y of the concr ete . In this respect, for all but large sing le pour s, not requ iring scree din g. 20 mm aggregate is esse ntial.

cl) To find wat er content sufficien t to give near total collapse of slu mp (ic in excess of 200 mm ), but not such as to allow segrega tion .

e) To select additive s to ass ist in achieving th e above perfor ma nce plu s a retard e r, where amb ient temperatures are high. T he procedures of CP 110 were fo llowed

with regard to trial mixes of und erwater co ncret e as was also the case for th e other co ncrete mixes on the projec t , which was carr ied out under the direction o f th e Harbour Depa rtm ent, The Kingdom of Thai land.

A com pact ing facto r of 0.99 and a slump of 23 cm ± 2.5 cm was sought . Th e resulti ng mix is shown in Table I and the mix used in the trials in Tab le 2.

T he aggregates were a limestone coarse aggregate suppli ed in two sizes and a tin minin g sand which we assu me from the geology o f th e island to have o rigina ted as a grani te . The gradings and other data on the aggregates are show n in Figs 4a and 4b.

Specific gravi ty or the materials: cement 3 . 15; sand 2.66; and coarse aggregate 2 .72. Flak iness index of coarse agg regate was 3.96. There was no shell in the sand. A typical den sity of the wet co ncrete was 2,374 kg .

At first. beca use 40 mm aggregate was allowed by the spec ification. a mix using this size o f aggregate was tried. but it pro ved to be too hars h and would have bee n difficult to place and screed . Also . this wou ld have ne cessi tat ed th e use of a lar ger pipe for pumpin g.

Eve n so , when the 20 mm maximum s ize aggregate was used in the skip. initially there was difficulty in ope nin g th e skip . As a consequence . the m ix had to be further adj usted. This mix was even tuall y also used for the concre te pumped directl y to the foundation. All the concrete was produced by a pan -type horizontal mixer (ELBA EMC 55 B) of I.OOO litre batch capac ity.

Some interesting observat ions can be made by comparing in T ables 3a and 3b the cube results for P2 (U /W mix) and P3 for the quay blocks: P2 ha s 440 kg o f cement pe r cubi c metr e of co ncret e co m­pared with P3's 330. yet the average stre ngth s are very close. The mix for the

Phuket Port Underwater Concrete Mix Proportions , Initial Design

Maxagg Cement W/C Sand /Agg

20mm Sand Water

Mix size content ratio ratio

, Aggregate kg/m:i kg/m3

mm kg/m3 kg/m 3

P2 20 400 0.5 0.41 1055 745 200 P2 20 400 0.55 0.40 1070 700 220

SEPTEMBER

quay blocks had 40 mm maximum size of aggregate, which allows the cement to be used more effectively than with the smaller aggregate. This and the greater workability of the underwater mix prob­ably explain the similarity in strength, notwithstanding the difference in cement content.

Behaviour of concrete underwater For practica l purpo ses, we can treat con­crete as a fluid which is denser than the surrounding water. It is clea r , both from consideration of .the nature of the mixture and from observations that the finest particles are likely to be moved at the interface between the two fluids , ie from the surface of the concrete.

It is standard practice to include a greater amount of cement than is required for the design strength requirement for the concrete , in order to compensate for this loss.

Consideration and observation would again tell us that the loss from the surface represents a very small proportion of the cement since, after some movement of cement particl es, those remaining in th e matrix are shielded by the sand in the matrix.

Unless the disturbanc e - ie the relative velocity of the surrounding water and the concrete is sufficient to remove sand par­ticles (about 0.30-0.50 /sec), only the out er film of cement is likely to be lost. The mobile advancing face and top surface during placing are the zones most at risk.

On occasions at Phuket , laitance was observed to gather at the shutter end as the concrete rose in the bay. This demon­strated that some condition , for exa mpl e, diver disturbance has agitat ed the surfaces. However , no adverse effect occurred as a conseque nce of the laitanc e since laitan ce was absent from the finished exposed concrete.

An aspect of the use of greatly in­creased cement content in underw ater concrete that is often overlooked is that , in the event of poor workmanship, a sig­nificant layer of lait ance can be quickly formed on the concrete . Since , where poor workmanship can be anticipated , extra cement is used , the important re­lationship between that of an experienced team and the mix design becomes evident. Given that the cement loss is, in practice, likely to be insignificant , particularly with well, ie carefully placed concrete , it would be pr efe rable to refine mix design for underwater concrete to reduce the cement content from these possibly excessive levels.

In some respects , concrete resembles a saturated soil mass and it is instructive to examine the concrete from that point of view. The distinction is that the voids of an underwater concrete are filled to a greater extent than most soils. Just as with a soil, the inter-granular pressure is unchanged for increase in head of water. Hence the

129

Underwater concrete construction

fluid concrete has , for all practical pur­poses , the same physical prop er ties and behaviour at any depth.

The mobility of concrete poured under­water can be seen to vary according to: i) the method of placing. For instance ,

with pumped concrete there is addi­tional motive power from the pumping action, and

ii) the internal lubric at ion intrinsic in the mix. The properties desired in a concrete

underwater vary from those on the surface where the availability of mechanical com ­paction leads to the use of stiffer mixes being regarded as good practice.

Underwater, with the restraints of achieving ho,mogeneity and a good surface finish , the concrete must be as close to being self-levelling as possible.

Japanese concrete mixes Yamaguchi et al7 address the problem of segregation of underwater concrete. It is as well to define segregation for the pur­pose of this article. Segregation: the removal of fine materials - cement and sand - from the matrix of the concrete so that either of these constituents exist there ~ after as a separate layer on or within the original concrete mass or they are removed

Table 2 Phuket Port - Underwater Concrete

Actual Mixes: the results of three trials of the mix used

Pi-8 1 Pi-82 Pi-8 3

Cement (kg/m 3) 440 440 440 Sand (kg/m 3) 700 700 700 20 mm Aggregate (kg/m 3) 1070 1070 1070 Water (kg/m 3) 220 220 220 Admixture ( cc/m3) Corm ix P45 950' 950 950 Admixture (cc/m3) Cormix SP4 2640''' 2640 2640 Slump (mm) 23 24 25 Density (kg/m 3) 2388 2395 2374 3 days 22.40 22.28 21.30 Cube N/mm 2 20.22 21 .79 20.22 Strength 21.56 21.59 20. 71

,,, Th ese proportions were changed in December 1986. after the occurrence of an exce ption ally long time to set , to: P45 400 cc/m1

SP4 1850 cc/m3

Table 3a Concrete Strength Results

Statistic Evaluation of Concrete Mix P2 and P3

April 1986 P2 P3

7Days 28Days 7Days 28 Days

11 26 52 43 86

x (N/mm 2) 26.10 30.90 27.60 31.10 MAX(N/mm 2

) 31.10 37.80 36.90 37.40 MIN (N/mm 2

) 19.50 21.40 21.40 23.90 SD n-1 (N/mm 2

) 2.82 4.96 4.10 3.31 Y( %) 10.80 16.05 14.86 10.64

Table 3b Statistic Evaluation of Concrete Mix P2 and P3

May i986 P2 - - -- -P3

7Days 28 Days 7Days 28 Days

11 13 26 57 113

x (N/mm 2) 26.50 31.00 26.00 29.50

MAX(N /mm 2) 29.60 36.50 34.20 35.10

MIN (N/mm 2) 22.70 25.30 19.90 20.90 SD n-1 (N/mm 2 ) 2.42 3.64 3.08 3.32 Y(%) 9.13 11.74 11.85 11.25

13() THE DOCK & HARBOUR AUTHORITY

Underwater concrete construction

e ntir ely, leavin g a hon eycombe d appear­ance.

In the abse nce of turbulence , there should be no segregation as has been demonstrated in the main Phuket pours. A ll method s of plac ing concrete und er water have strived to minimise turbulence. Howe ver , Yamaguchi et a l concentrate on the deve lopment of a segregrat ion-res istant concrete and highlight th e difference in behav iour , when dropp ed through water , o f their segregat ion-res istant "K-cr ete " and concre te without admixture.

This is a remarkabl e quality becau se concrete as'a rule is never dropp ed through wate r and, ind ee d , need not be dropped since mean s of co nductin g the concrete to its final position already exist. However , skip placing could be mor e safe ly ca rried out using a segregation-resistan t concre te .

It is worthy of no te th a t in the ac tu a l construct ion on which th e "K-crete '· was used , the co ncrete was place d by pump ing to a re lative ly co nfined space with a plan area of 22 m2 where ordinary concrete wo uld have performed adeq uate ly.

A "drop test" was performed on th e mix used in the Phuket qu ay foundation with th e expected res ult s of segregation. Th e simpl e test was perform ed by filling cylind ers from th e co ne of a compacting factor apparat us; testable cylinders were formed .

Kawa i et a12 condu cted tests on the ir spec ial mix in which the y clearly demon­strat ed the abi lity of the mix to flow around reinforcement and structura l memb ers. The fact that they se lec ted a 20 mm maxi­mum size of aggrega te is significant. Exper ience at Phuket was th at the mix only becam e feasib le with the use of 20 mm aggregate.

Observation both visua lly and by fee l of concr ete in th e qua y wa ll foun dation

ve rified that it flowed easi ly around irregu­larities including over hangs in the exposed rock and the bagwork profile from previous pours. It was ev ident that the Phuket con­crete had the potential to encase reinforce ­ment a lthou gh it was not required to do so.

Sogo 5 who also report s on a segregation­resistant concrete makes the point that higher pumping press ures are required because of the cohesiveness of the special concrete. This seems to be a cons iderab le disadvantage. Ease of pumping of the con­crete is an attribute to be designed into th e concrete , particul arly wh en operating in high ambient temperatures and over lon g distances.

At Phuk et , one short period wh en pumping pipeline blockages were prevalent was traced to a slight deposit of harden ed mortar within the pipe. The prop erti es making a concrete suit ab le for pumpin g are also to be found in a good und erwa ter concrete.

The main conclusion draw n from the above comparisons is that a clos e simi la rit y ex ists in mix proportions between the con­crete used at Phuket and th e speci al mixes. The ability of the spec ial segregation­res istant concre tes to withstand droppin g through water is a doubtful advantage since th is situ at ion would not occur in norm al practice , and with tremi e or pumping . The other merits of good underwater concrete such as mobility and abi lity to flow aro und objects are shared by the ord inary (Phuket ) and th e special segregation -res istant mixes.

The segregation -resistan t underwater co ncrete was use ·d in practi ce in an appli­cation, filling within a caisson, where the spec ial attributes of the concrete were not required nor were they put fully to the

Table4a

test. For instanc e , the top surface of the concrete pour did not have to be finished , and it was not possible to expose the side of the placed concrete for inspection.

The potential usefulness of th e non­segregating concrete is acknow ledged in situations wher e access "is difficult or lack of confidence exists in the placing method or personnel.

Review of placing procedures As with all civil engineering operations , carefu l plannin g for und erwater concret ing is essen tial. The equ ipment must be reli­able and bo th the operatio n and the equ ip­ment should be as simpl e as po ssible . The personn e l invo lved should have been thoroughly briefed and , if possible , had the opportunity to practic e the opera tion.

Tradit ional met hods of placing con­crete i1.1 situ und erwate r have be en by diving bell , various types of skip , tr emie pipe, bagwork and gro ut ed aggregate.

Co ncrete can be place d as precast ob jects such as caisso ns , b locks o r i11 situ which co nce rns us he re . In par ticular , th e adva nt age of the use of in situ concrete is th at the concrete can mould itself to irr e­gular shape s suc h as a rock fou ndation , as was the case at Phuket Port. The advan ­tage of pr ecast ing is that the qu a lity and integrit y of th e concrete can be checked befo re it is plac ed underwater. The provi ­sion of comp arab le quality control in in situ concrete is discussed late r in thi s a rticle .

Here we limi t ourse lves to the con­sidera tion of tremie , sk ipped and pump ed concrete. We also discuss co ncre te bag­work alt hough this , lacking as it does th e int egri ty of an in situ pour , is so mewhat outs ide the main field of interest of this articl e. Grout injec tion of aggregate is not discussed. It has limited app lications, eg in

Comparison of the Phuket Underwater Concrete Mix with the Segregation-res istant Mix of Kawai2

w . Coarse S/A Super "Special" Mix Water Cement C ratio Sand agg ratio Plasticiser ingredient

kg/m3 kg/m3 kg/m 3 kg/m3 . kg/m3 kg/m3

Kawa i 190 430 0.422 691 991 0.44 117 7

Phuk et P2 220 440 0.50 700 1070 0.40 400 -

~ m 1/50 kg cem ent

Table4b Comparison of the Phuket Underwater Concrete Mix with Sogo5

Target w . S/A slu_mp C ratio ratio Cement Water Sand Agg Admixt ur e SCA

,

mm kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3

Sogo 240 0 .56 0.4 400 224 600 1014 4 .0 2.8 using blast furnace slag cement

Sogo - 0.50 0.4 370 185 700 1070 4.0 2 .8

Phuket 230 0 .50 0.4 440 220 700 1070 400/ml 50 kg cement

132

Underwater concrete construction

4. Co nsistent qua lity of concre te as placed is difficult to maintain .

5 . Each insert io n of the skip into th e poured co ncre te crea tes laitance.

6. Laitance is like ly to be. trapped within the pou r.

7. Th e ope ration holds potentia l danger for the divers since th e skip swings dangerous ly in stron g tida l conditions.

8. Dir ect super vision of the operation und erwa te r by engineer 's staff is diffi­cult and even d angerous.

9. Not suit able for struc tur a l grade co n­crete .

Pump With the advent of powerful and reliable co ncre te pump s the ir adoption for use in plac ing co ncrete underw ater was soo n rea lised .

The propertie s so ught in a concre te to be pumped are similar to thos e for a co n­crete to be placed und erwater. It is a natu­ral consequence. therefor e , that pumping

should be used as a means of placing co n­cr ete direct ly int o position un derwater .

Pumpin g has fea tures of the tremie pipe method. It has the advantage of con­tinuit y of flow wh ile not suffering from the principa l disadva nta ges of trcmi c , the danger of loss of th e concrete from th e bo ttom and flood ing of the pip e. When pumping within the range of the pump hydraulic arm , this will also provide re sis­tance to th e upward thrust against th e already placed co ncre te. For thi s reason it is not pra ctical to have a final len gth of flex ib le pipe held or cont ro lled by the diver. The thrust and the whipla sh effect with the consequ e nt disturbance of th e co ncret e, loss of an y available visibilit y and of co ntrol of th e placing of the con ­cre te, preclude such a method.

Pumped concre te has the foll ow ing attributes:

i) Th e co ncre te ca n be plac ed from a cen tral po sition from which th e con­cre te can flow con tinu o usly to its fina l

Fig 2. Shuuer prep ared ro receive concrete in "hit " bays I and 2. ( Note: bag work on from face removed for clarity.

n n 11 :~ -

EXISTING - PREVIOVSLY - CAST COHCRETI

BAY.

H (i) :J ® C C J

,___ ,-,--

(j) I cb l I

1k ' ' ' I ~ I u u

1---1"'500""--- +--1"'->oo=-_,_,.....-'f!-oo-'""15:.:00.:.._-1,---~'" =o~o --, ~ SEE DETAIL A.

FRONT ELEVATION

THE DOCK & HARBO UR AU THORITY

position forced on by the continuou s ar riva l of fres h concrete.

ii) The position can readi ly be changed within the pour.

iii) Should co ncr etin g be interrupted the dow npip e ca n be removed and re­plac ed with minimal disturbance to the placed co ncrete .

Phuket quay wall The 360 m lon g qua y wa ll compri ses nin e co urses of pr eca st , 20-25 t mas s concre te block s laid on th e in si1u co ncrete base . Th e qua y is completed by an in siw re.in­fon;ed co ncrete ca pping block .

Th e e nds of the qu ay are formed by wing walls of the same structural cros s­section as the main wall. except that in the wing walls no toe o r heel concre te was req uir ed since the wall is esse ntiall y buried in rock fill. T he accurac y and th e qualit y of th e top surfa ce of the foundation co ncrete was paramount to the success of the sub­seq uent block lay ing.

Quay wall concrete foundation The foundation or blinding concrete is nominall y 400 mm thi ck. In th e eve nt , thi s was close r to 900 mm thick on avera ge exce pt at the deepened section to which we have already referred. The rea son for th is was over-break in the rock and the contractor 's excavation method.

I\ co ncrete plus was requir ed betwe e n th e toe o f the lowest cour se of blocks and the face of the excava tion wh ich it was hoped wou ld be in mat e rial sufficientl y st ro ng to pr ese nt a ve rtical face . Thi s was referred to as the toe co ncrete. T ypical c ross-sectio ns of a no rmal bay and of a dee pened bay respec tive ly arc shown on Fig I.

Foundation construction conc ept The found at ion was for med in a trench which was exca vated by a large grab dr ed­ger. When the found atio n was reached , excava tio n co nt inued by chiselling and grabb ing.

One metr e above the intended founda­tion leve l (- 12 .40) chi se lling proc eeded in a mo re prec ise mann er using a hea vy steel tube the foot of which was ar med with picks and inside which a 6 t hammer cou ld be dropp ed , th e deb ris remaining to be a ir-lift ed .

T he foundation was forme d in four separa te operation s : the main sec tion of base slab which requir ed two operat ions , the front sec tion or toe and rear sec tion of heel concrete. The main slab was cast in transverse bays 6 m wide by 1.5 m lon g, two alternate bays be ing cast with eac h main frame (see Fig 2). Thus , on remo ving the main frame, two infill bays were for­med. For eac h main frame cast, a po tenti a l adv ance of 6 m wa s made. When a suffi­cient number of main frame ba ys had been cast, the int er mediat e bays were con ­creted. The sequen ce was at time that intermediate ba ys were cast ·immediatel y fo llowing eac h pair of main frame bays.

SEPTEMBER

Th e to lera nce allowed o n the finished screeded co ncrete was ± 6 mm with no forward tilt across the foundati o n . Rear­ward tilt was allowed wit hin defined limit s. In practice , the frame was wet with a rear­ward tilt of 6 mm.

The sequ ence of operations for founda­tio n pr eparat ion and co ncreting for th e main frame was as follow s :

1. Co mp lete excavatio n . 2. Shutt er set accurately to line and

roughly to level. 3. Formation cleaned by a ir-lift , soft

po ckets ident.ified and removed. 4. Sand -filled bagwork placed to sea l

aro und lower edge of shutter. 5. Final a ir-lift ing . 6. Final leve lling and a lightm cnt chec k .

{These la st two task s we re joint contra cto r-e ngin ee r checks.)

7. Inspection of formati o n for clean liness and soundne ss.

8. Reco rd of depth to rock surface be low the top of the frame .

9 . Toward s the end of the inspec tion , preparation s for co ncreti ng we re put in hand ; the pipe lin e co nn ected and the pump moved into po sition o n th e shore bund (Fig 3).

IO. Co ncre ting . l l . Screeding . 12. Ea se fram e four hou rs after th e pour . 13. Remove fram e (c lea n a nd prepare for

next use). Two identical main frames were used:

thi s allowed a clean a nd maintain ed frame to be available for placing immedi ate ly a used fram e was used.

With the a lte rnat e a nd infill bay s cast an d the ba se cour se blocks placed , th e fro nt and rea r concrete were cast. This was basi ­ca lly an infi lling operation between th e main slab, base cou rse blocks. and th e front and rear faces of th e exc ava ted tr enc h.

Main frame foundation shutter Th e importan ce of shutt er des ign ca nn ot be over -emp hasised. The shutt er designed for the quay wall foundation co ncrete a t Phuk et is desc rib ed in de tail. The co ncep t was that alternate bays 1.5 m wide wo uld be cas t transverse to the line o f the quay; these are referred to as " hit" bays with subsequen t infilling of the "missed" bays .

A stee l framed sh utter was used which incorporated two " hit" bays , th ereby giv­ing an effective adva nce of 6 m per frame. Th e main sequen ce of operat ion s after cleaning of the rock foundation wa s set­tin g, a ligning and levelli ng the frame followed by sea ling aro und th e base of the shutt ers with sa nd-fill ed bagwork .

The decision to cast bays 1.50 m wide was ta ken for th e following re aso ns:

i) Co ntrol o f th ~ quality of th e co ncre te in po sition. Th e narro w shutt er width wo uld give a rapid build-up of poured concre te and concentr ate the direct ion of flow across th e bay . This would give a continuous supply of conc rete ,

133

Underwater concrete construction

pr event co ld joints, and mm1m1se laitanc e ; for mati on of laitan ce is inh erent in sk ipp ed concret e. A ny la itance formed wo uld be moved across the formation to the perimeter of the shutt er. Then , being displaced upwa rds by the de nse mass of con­cre te , wou ld be moved ove r th e lop edge of the shutte r , thus ensuring o nly good quality concrete remained within the bay . Th e top finish achieved on all pours bear s witnes s to the absence of surfa ce lait ancc.

.ii) A 1.50 m wide bay gave a very manageab le width which dive rs co uld scree d successfully.

iii) On strikin g th e shutter , all four side s of the " hit " bay , plus the top surfac e were available for inspection to assess the qualit y of the as-poured conc rete. Infill bays pres ent ed two short sides and the top surfa ce for inspectio n .

To meet th e above requir eme nts , th e shutte rin g fram e needed th e follow ing attr ibut es:

Fig 3.

nt'" ""'

CV

1. To be of a strong rig id construction. 2 . To have no mov ing or removcablc

par ts. 3. To be capab le of being plac ed easily

into po sit ion and on correct align­me nt.

4. To be ab le .to land firmly on to the prepar ed formation , bearing on the f ack ing points and to be easily adjus­ted to the correct level.

5. To have the jack ing points set bac k as far as poss ibl e fro m th e shu tt er face to avo id the ir being inadve rt antly cast i1J with the co ncrete.

6. Th ere should be intermediate jacking po int s to assist in striking th e sh utt er.

7. The two shuttered bays incor porat ed into th e fr ame to have their sides ta pered inward s for ease of striking (see Fig 3) .

8. The top edges of the shutt er to be such as to allow surplus an d poor qualit y co ncrete to flow over without remaining on th e top edge and thu s inhibitin g the screed ing operat ion.

~ \J]

SIDE VIEW

t ZQJ

i ~ r 112

J SECTION

;, = """'

DETAIL A.

SECTION (i) - (i)

IJ4 THE J;)OCK & HARBOUR AUT HORI TY

Underwater concrete construction

A knife edge was therefore prefer­ab le .

9. Bottom of shutt er to be so form ed as to minimi se suction -fri<ltion on str ik­ing and to allow the s,md-filled sea ling bagwork to be placed effective ly (see Fig3) .'

10. A ll rough or flame-c ut edges to be ground smooth to obviate inju ry to divers ' hand s.

Th e frame was slung from four lifting eyes on the o uter main beam s. The tra iling

- e nd was slightly lower to a llow the dive rs (two - one' a t eac h corne r) to locate the dista nce p iece arm an<l brack et on to the corne rs of the pre vious concrete bay (F ig 3). With these located , the frame was then lowered on to the formation and the trailing slings re leased . With the trailing end secu re , it was a simpl e matter for the diver to swing the lea ding end of the frame on to the correc t alignment and lower into its final position.

The trailing e nd , as we ll as giving long i­tudinal offset from the previous pou r, assis ted in alignment and also gave a roug h leve l such that initia lly it was only neces­sa ry to level the two lead ing ja cks to set the fram e on a reasonab ly level pla ne.

Whe n the frame had been set to line and ro ugh ly to level, air- liftin g inside the shutter s took place draw ing loose seabed mater ial from under the sides of the shut ­ter. Th e placing of partly fille d sa nd bags the n complete d the sea l. Par tly filled bag s were used as these were more pliabl e and better ab le to be wor ked into position , and were also mor e eas ily removed after str ik­ing the shutt er.

Followin g comple tion of the bagwork seal a furth er br ief final air-lift inside the bays was carri ed out. This was done prior to the fina l precise levelling to ens ure that the frame was not subsequentl y disturb ed .

Infill bays With a sufficient number of main or "hi t" bays cast. the infill or "miss" ba ys were then concreted. Th e infill bays were cast using a one-piece shutt er that spa nn ed the

Underwater shuuer fmme on barge.

pr evious ly cast bays across the 6 m width. Th ese consisted of a steel plate 2.5 m wide at eac h side tape ring inwards by 100 mm ove r its 400 mm depth ; the se were con­nected by two 100 mm steel box sec tions .

T he connec ting box sect ions we re set apart by over 2 m to give a good clearance forscr eed ingth e 1.50 m wide infill bay . The box sectio ns were also ra ised by 100 mm above the adjace nt cast bays. Thi s allowed any surplus concrete from the infilling to flow clear. The con necting box sec tions fulfilled a not her important function in that they allowed the divers to hold on to them , thu s providing the purchase neces­sa ry to pull the screed ra il.

To allow for easy positioning of the shutt er , the two end plate s were 6 ,200 mm apart , thus giving a nomi nal 100 mm clear­ance at eac h end. Thi s facilitated their be ing placed into position. Voids between the forma tion , the existing co ncre te and the s teel shutter we re filled in the nor mal way with sand-filled bagwork.

As the previously c.ast bays were used for screeding the infill ba ys, there was no req uir~ment to set the infill shutt er to level. The width was a lso controll ed by the previous ly placed concrete. It was , there ­for e , a relative ly straig htfor war d opera­tion to shutte r and concrete the infill bays , whereby as many as fou r cou ld be prepar ed and cas t in seq uence in a day.

Th e sequence for op e ration for the infill bays was: I. Set infill frame in position. 2. Ini t ial clea ning by air-lift. 3. Place sa nd-filled bagwork to sea l under

shutt er. 4. Inspection. 5. Con creting .

Placing concrete by skip Ini t ially, concrete was place d by skip by a bar ge-mou nted crane. The concre te was suppli ed to the barge by a la nd-ba sed , tru ck-mo unt ed concrete ' pump feeding two spec ially des igned und erwa ter skips. To span the 20 or 30 m between the con­crete pump and the barge the concre te pip e was se t into the pan of Larssen sheet piles

which were , in turn , supp ort ed on pairs of 40 gallon oil drums. This arrangem ent wo rk ed we ll durin g the ca lm weather per iod , but with the change of season and wave cond ition s , dredge pipeline flotatio n unit s had to be used.

The skips were used alternately: filling one whilst the other was lowe red under­water and discharged.

The diving team initia lly proposed by the cont ractor , although experi enced and competent , had no actual previ ous und er­water conc ret ing expe rience. Accordin gly, a tea ch-in was arranged at which one of the aut ho rs (Cooke) e ndeavoured to cross four languages and natio nal wo rking prac­tices to exp lain the principles of under­water concre ting by skip.

Trial bays were cast , first und erwa te r then on land. Th e und erwa ter tria l bays were ex·posed at low tide; the two bays had the following fault s - co ld j oints , honey ­comb ing , laitance , segrega tion. uneve n scrccding a nd divers· foo tprint s. A furth er trial in the dr y was ca rried o ut which was aga in less than sat isfacto ry.

The cont ractor sub seque ntl y called in a fini1 from the Midd le Eas t which specia lised in underwat er concrete and block placing. Th ereafte r , an acceptab le standard of workman ship was achieved and progre ss was made.

ll pro ved d ifficult. howe ver. to main­tain a high standard of po ured and screeded concrete beca use the mix was se nsitive to a ny de lay and frequent ly failed to leave the skip . This was not only wasteful and tim e-co nsumin g, but also di sturb ed and affected the qualit y of the concre te.

The divers had to conte nd with poor working condit ions . in ter ms of zero visi­bility, curre nt s, and skip mov ement ca used by wave action on the barg e. Whilst it was a me thod that worked , it was no t enti rely satisfacto ry . Some 34 ba ys out o f the total of 280 were cast by skip.

A further disadvan tage o f the skip method was that it was pote nti a lly dang er­ous for the engine er's staff to attend the un derwat er concret ing operatio n .

Close-up of jacking point on underwater shutter fram e.

SEPTEMBER

Pumped concrete in deepened section O ve r a 60 m le ngth of trench it was neces­sary to de epen the foundation by 2.50 m . Thi s need arose because th e harder silt­sto ne, which it was sought to found on , dipped over this le ngth .

For the m ass concre te infill of thi s sec­tion it was app arent that an alternative to sk ip- placed conc rete had to be found .

With a ll th e req uir ed eq uipm ent on site , an obviou s meth od for consideration was di rect pumping , if the pr edi cta ble dif­ficu lties cou ld be ov ercome .

The se difficulti es were : I . The need to mod ify th e mix for the ex tra

length of pumping and the vertica l drop of some 17 m .

2. Th e need to separate the co ncrete from water in th e pip e line .

3 . Co ntrol of the pumpin g forc es which react aga inst the co ncrete as it builds up aro und the disc harge e nd of the pipe.

4. A meth od of clos ing th e disch arge e nd of the pip elin e as it is withdrawn from the co ncre te.

5. Arrangement s for supp orting the flex i­ble and ve rti ca l sect ion s of pip elin e .

A ll of these problem s were successfully overcome. In order to regulate th e co n­creting operation over this sec tion it was di vided into 11 mana gea ble sized ba ys; mana gea ble in ter ms of surfa ce a rea, depth-volume and hence duration of pour. Bay vo lum es were between 90 and 120 m~. A di ve r was required to atte nd o n the operation at all tim es durin g pumpin g , but with incr ease in th e depth of water di s-

135

Underwater concrete construction

Corner of bay accidentally broken off showin g surface finish achieved. (The wrist watch gives an imprel·sion of scale.)

rupting the co ntractor s diving sched ule s, this was not a lways possible.

Revised pumped method For the co nstructi o n of the blo ckwork qua y wall concrete fo undati on , sk ipped concrete had been specified. In practice , it was found that with the available plant and the parti cular co ndition s , thi s met hod co uld be improved upon by pumping th e co ncrete directly into position under­wa te r.

Th e probl em was to plac e concrete within a shallow shutt er in 12-15 m of

water over a tidal range of up to 3 m. The fo un da tion was so me 80 m distan ce and parallel to the adjacent construction bund . Sea conditions varied from calm to 0. 8 m high waves.

The dire ct pumping met hod was deve l­oped for th e de epened sectio n of founda­tion where pours of up to 1200 m3 vo lume and 2.5 m deep were ca rri ed out. T his was refined for the cas tin g of sta ndard founda­tion bays which were 6,000 mm x I ,500 mm x 400 mm deep . In practice , this depth was about 1 m due to over-excavation.

With th e deve lopment of the direct

Fig 4a. Arrangement fo r concreting firs t bays of und erwater concrete foundation.

RE\NF'OACED JCEPT COOL aY WAn:R Sr'~T I AUIH A HOS(

l till.I§:

SECTION A VOLUME OF CONCRETE N MIXER LORRY cu m

VOLUhE OF CONCRETE IN SKIP 1.2 cu.m

RATE OF PLACING FROM SKIP MINUTES

Underwater concrete construction

discharge pumping met hod. the following advantages were realised: 1. Th e co ncret ing time was reduc ed from

at least 60 minute s to, 30 minut es per ba y.

2. Th e qualit y of th e as-p laced and scre ­eded concrete sur face was ext reme ly gpod.

3. Concret ing was abl e to proceed during sea co nditi ons in which previou sly it wo uld not ha ve been pos sible.

4. By informed observa tion o f the dow n pip e, monit oring of pro gress und er­water was possi ble from the surface.

5. Dir ect superv ision by the enginee r's staff was possible und erwate r without da nger and without impeding progr ess.

The co ncre te pump with hydrauli c arm was set up on the adj acent parallel bund 80 m be hin d the quay wall . To spa n the 20 o r 30 m between the barge and th e pump arm the 5 in (125 mm) concrete pipe line was laid in the pan of a conti nuou s length of Lar ssc n sheet piling wh ich was sup­ported on floa ters. A flexible lengt h of pipe ex ten ded the line up from water level on to the deck of the barge , which was orientated pe rpendicul ar to the foundat ion a nd rear bund line.

Th e pipeline was laid out acros s th e barge dec k and via a seco nd flex ible section ra ised up on an articulated arm and held by th e auxiliary crane hoist. Th e main

THE DOCK & HARBOUR AUTHORITY

hoist of th e cra ne th e n supp orted the 17 m vertical section of 5 in ( 125 mm ) rigid steel downl ine to which, at the lower e nd , a coun ter-ba lancing weig ht had been added to resist th e forces applied by the pumpin g action . Thi s was held in pos ition by a sliding yoke that allowed mo vement of the barge crane -hook witho ut tra nsferrin g this moveme nt to the do wnlin c position ed in the shutter and sitt ing on th e formation .

To sta rt concreting th e pip eline was spl it immediately prio r to the final sec tion of flexible hose and a temporar y section of pip e co nnected such th at the leading e nd was abl e to discha rge th e lining mortar overboard in the conve ntional pumping

Fig 4b. Disposition of cons truction p lant for quay co11srrucrio11 at Phuk er Port as of 8 November 1986.

CH. 0 CH. SO CH .10 0 CH.150 CH .Z OO CH.Z50 CH . 300 ~ . 360

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Fig 4c. Disposition of co11strucrion plallf as of I March 1987.

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CONCRt:TE PUMP

SEPTEMB ER

situ a tion. When good concret e was being discharged th e pipelin e was reconn ected , ha ving first inserted a loose bag of pol y­styre ne chip s i.n the pip eline to form an interface plug betwe en concrete and wat er as the concr ete was pump ed verti cally down. Th e bottom e nd of th e dow npip e was maintain ed , 200 mm clea r of the forma­tion by stee l distan ce pieces.

Pumpin g start ed , with initi a lly only sufficient for ce applied to mov e th e con­crete. When th e con cret e di schar ged the pla stic plu g usually bur st .and a llowe d the pol ystyrene chip s to float awa y. As the co ncrete built up around the discharge pip e, pumping pr ess ur e - ic strok e - was increase d. Pumping then co ntinu ed a t a ra te jud ged accept able by the diver. and th e co ncre te rose bo th loca lly and alo ng th e pour , th e do wnpip e was move d as nece ssar y acro ss th e pour by the cran e under dir ec tion from the diver. Th e con­cre te progr esse d acro ss the ba y to the fa r end an d in rising it di splaced any la itance over the side o f th e shutt er .

Pumpin g was stopp ed whe n th e first bay had bee n filled and th e do wnpip e was ra ise d so that th e e nd was still immer sed by abou t 200 mm inside th e ve ry fluid co ncre te. A plate was th en inse rt ed by th e diver int o slo ts in the pipe end to clos e it off. Th e dow npip e was the n ra ised slowly clear of the bay and move d into posi tion in the nex t ba y. Th e clos ur e plate was remo ve d , pumpin g was res tarted and the process repe a ted .

O n co mplet ion of the final bay. the plate was again inserted to close off the pip e e nd and the pipe lifted clear of the wo rk s. th e plate re moved a nd the concrete re mainin g in th e dow npi pe was a llowed to run away clea r o r th e work .

A n initia l ro ugh screed ing of th e con ­crete was carried o ut short ly af te r filling th e bay . Th is was to remove the over-fi lled and poor quality sur fm.:l' c.:om:re te. Beca use of its ve ry fluid na tur e as the screed ra il passed over the sh ut1e r to remove the bulk o f the co ncrete, the la tt e r ros e up be hin d th e screed rail and le ft the po ur 10-20 111111

high . Th e conc re te then stood for abou t o ne

hour to a llow it to firm up a littl e but we ll be for e an y initi a l se t too k place. Th e bay was th en re-scre eded . and the rail was passed ove r as many times (in one dir ec ­tion on ly) as was considered necessa ry to e nsure th at a ll co ncre te above th e leve l o f the shutte r was rem ove d .

A n ho ur or so af ter the ini tial se t had taken pla ce th e frame was rai sed by a few millimet res using a ll jack ing point s. Thi s sepa ra ted the shu tter from th e concre te with out disturbin g th e gree n co ncre te and a llowed, in du e cour se, th e shutt ering fr ame to be re move d with out damaging e ith er th e shutt er o r . th e co m:re te . Com­ple te strikin g of the shutt er too k place after 12 hour s .

Anot her impo rtant facto r that e merge d fro m th e use of pump ed con cre te was the

137

Underwater concrete construction

STEa CE:LtvER'r' P1PE

GENERAL ARRANGEMENT

LEGEND G) ARTtCU.ATEO -ANO SWIY'ELING STEEL CHAN N~l HELD BV CRANE

AUAIL...ARV HOIST lO 9..PPCRT ANO STEAOY CO<R:TE CElMR'I' PIPE. (l) st.lDI NG ¥0KE. ,

G) STEEL 8.t ll AST TO COVNTER PlMP REACTION FCACES.

© U · BAA WELDEO TO BOTTOM OF Co.dlETE DELIVERY OOWNPIPE TO All ON CON:REJE lO FL()N !)JT WHll..E WEK;HT TAXEN Of'F Pff:.

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U·BA.=I WELD TO eo n CM Of STEEL PI PE. p,.(;JTE LU::.S C Jflb.l TO FECE.'\'£ O.~ PJ..TE.

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CO'JCRETE DELIVERY TO UNDERvVA TER SHUTTER

Fig 5.

ease with which it was poss ible to stop an d restart th e flow o f con cre te thu s permittin g relo cati o n o f the discha rge pip e. A simpl e plat e va lve was dev ised to clos e o ff the dischar ge point. In fact , when pum p ing was not ac tu a lly in oper at ion , it was pos ­sib le for the diver to place his ha nd ove r the discha rge poin t of the pipe to re tai n the concre te. T his pract ice was discouraged on safety gro unds. bu t it served to illustra te th e ease wit h which th e co ncrete co uld be co ntro lled.

A lth o ugh in the co ur se o f pourin g a bay al Phuk et the pipe was ge nerall y moved o nly o nce. the imp o rt ant po int to e merge from thi s experie nce was th at with pump ed co ncrete a change of locat ion o f the d ischa rge ca n be ta ck led with con ficlence.

Sampling and testing the concrete Th e re arc well-es tablished pra ct ices in a ll countri es for sampli ng and testing concrete . T he pro pert y of chief intere st is st rengt h , an d thi s is meas ur ed as th e cru shin g load of 150 mm cub es or 150 mm diameter , 300 mm high cylinde rs.

Th e result s fro m such testin g or co ncre te in no rmal o n-la nd co nstru ctio n is ta ken to re aso nably re liabl y rep rese nt th e st re ngth o f th e concr ete in th e structure. In th e ca se of co ncre te to be pla ce d und erwate r th e no rm al samplin g of cub es and cylind ers ca nnot be take n to repr ese nt the concre te in place. Th e difference is th at the con­cre te in cubes and cylind ers is co mp ac ted in a standard way wherea s comp act ion of co ncre te und e rwa ter is se ldom ca rri ed out .

A t Phuk et Port , six cub es we re taken from each pour which usuall y co mpr ised two ba ys. In the ea rly stages , slump co ne tests we re condu cted at interv als to assess

th e loss of slum p to ensur e tha t th e co n­cre te was still wo rk ab le wh e n placed .

Tri a l mix es we re te sted for loss of slump . On ce th e mix was esta bli shed as show n in T able 5. chec king for slump loss was not necessa ry.

It is appr ec iate d th at clie nts and 0th e rs sho uld particularl y wis h to ha ve so me ev ide nce of the qu a lity st re ngth an d inte­gri ty o f the in si 11t underwa te r concre te. sinc e they a re un able to see for the mse lves the success of th e und erw at er co ncreti ng. Cori ng is a posi tive met hod of proving th e co ncrete . but seldo m ca rr ied o ut as a mat ter of rout ine. Cast ing a recoverab le slab und e rwa ter at th e sa me tim e as th e main stru ctur e co uld give an indi ca tio n , but may not be he ld as suffficie nt ly con ­c lusive p ro of th at the stru ctur e is so und since it is not t he act ua l stru ctur e .

Sampling concrete underwater An alt e rna tive to co ring o f u'nderw at er _ co ncre te is to ob tain rep resent ative in si11t sa mple s of we t concret e. Th e meth o d is an ada pta tio n o f o ne pr eviously used success ­full y und erwate r to obtain undi sturb ed sampl es o f silt . Th e cylind e rs recove red in th is way. subse quentl y capp ed and crusficd . gave consistentl y good res ult s th e re by demo nst ra ting both the e ffectiv eness of th e sampl ing method and the qu a lity o f th e as-p lace d concre te.

Th e equipment requir ed is quit e simpl e and ob tainable from any g roce ry sho p; in our case a loca lly produ ced powde red milk t in. T he thin wall ed tin. me asur ing 17 cm x 12.6 1 cm diam ete r was pr epar ed by remo ving both end s to give a cle ar un­obstru cte d s leeve. Tw o plast ic caps (o ne suppli ed with eac h tin ) were all that was necess ar y to co mpl ete th e eq uipm ent.

138 THE DOCK & H ARB Ot:R .-\l.ITHO RITY

Underw ater concrete construction

C 011cre1e sa111ple af ll'I' re111m•al f rom 1i11.

Con creting hav ing comme nced and prog ressed such tha t a n area has bee n raised to a suitable level. sam pling may procee d . T he samplin g sleeve sho uld be slow ly inse rted into the fluid concr ete. the diver ho lding the tin in one hand and the lower pla stic cap in the ot her. With the sleeve fully imm ersed in the con crete the bo ttom plastic ca p should be snapped secur ely into position . with the diver 's hand then held und e rnea th to supp ort th t: weight of the contr e te , the top lid is then pla ced in position.

Th e sampl e is then re moved from tile concrt:te ; this must be done ve ry slowly to allow the concr ete within the pour to reco ver the void and heal over. No water must be allowe d lo pt:netrat e the are a sampl ed . On remova l the sa mple should be place d on an adjace nt flat surface. and the convex concrete at the top removed slowly with out disturb a nce a nd the top plastic cap secured. It should t he n be moved clea r of the con struction activity . maint aine d in the up right as recove red position and allowe d cure to for thre e days.

Aft er curin g . the cylinder is removed from the water , the pla stic caps ta ken off . the meta l sleeve cut away , taking care not to cut into the samp le. Th e cylinder may then be capp ed and crushed as req uired or

· cut ope n to expose the qualit y of the con ­crete for inspection (see photo graph s).

Disturbance to the work s concret e is minimal if all due car e is take n . Should the rec ove red sampl e sho w fault s su ch as partial recovery , hon eycombing , troes or ceme nt washout and distu rban ce then the effe ctive ness of the samp ling techniqu e by the operati ve shou ld first be considered . The reco very of good samp les will show the qua lity of the as-placed concr ete and the effectiv e ness of the placing method . Howe ver , since the method described takes place in the fluid concrete , it will not detect ''co ld " j oints . The se wou ld be checked for on st riking the ''hit" shutter or at the front and rear face of the infill ba ys .

Th e stre ngths and densitie s achieved by the non-standard cylinders made under­water and the cubes made at the bat ching

Concr ete sampl e obwi ned unden varer and cw rhrough 10 show density of 1he mix.

plant a rc comp ared in T ab le 5. T he mean stren gth o f the non-standard cylinde rs is 67 per cent o f that of the cubes. Thi s is assumed to be due to the fact that the cubes are compacted, which raises the qu es tio n o f whet her cubes for unde rn ater concret e should be compacted.

An important fea tur e of the construc ­t ion method was that it allo wed 1-10 bays of concrete to be examined on four sides . No fault s of an y conseq ue nce were found on the exposed s ides o r top sur­faces when using the d irec ted pump ed concrete me thod .

Th e fac ility to exam ine (by to uch and by sight ) the perimete r of eac h main frame bay and the end s of each infill bay is a maj or cont ribution to the kno wledge gain ed from this project. In e ffect , o f the 140 "hit" bays that were exa min ed only tho se plac ed by skip had faults. Ha d the re bee n an y fault s in the concrete it is highly probab le that th ese would ha ve bee n e vide nt on the sides. The top surfac e of

the concre te left by screedin g was also exa mined . A piece of concrete broken off by a bay in the north return found ation show n in the pho tog raph gives a repr esen­ta tive impr ess ion of the qu a lity o f the sur face.

Underw ater concrete check list Th e following observa tion s of wha t can go wro ng apply chiefly lo skipped con crete , but ge neral co mment s dea rly can also apply to pumped or trem ie concrete : I . Addi tives omitted from con crete mix. 2. Bay poured in layers beca use the bay

is of too large plan area to b ring up in one laye r. With the consequ ence th at laitance is trapp ed within the pour.

3. Mix no t suitab le for me thod used . 4. Mix aggregates. 5. Bay s ize such that concrete can tak e

initial se t befo re scre edin g can be start ed o r bay complete d .

6. Inappropri a te aggrega te size for

Table 5 Compres sive stre ngth of underwater concrete sampled

12.61 cm dia meter , height 16.83 to 19 cm

MixP2

Mea n cube strengt h 28 day strengt h 30N /mm 2 Strength from non-standard

cylinder (N/mm 2)

23.68 20.01 20.01 23.68 21.61 20 .41 22 .01

Crus hing strength me an 21.63 Nlmm 1

One cylind er which was tested at thr ee da ys, had an ultimate strengl.b of l .9

Thi s gives an indication of the improved strength which cou ld be achieved if it wer e po ssible to use vibration .

SEPTEMBER

type and size of pour and method of delivery .

7. Aggregate size for screeding too large, should qe 20 mm or less.

8. Concrete, supp lied in skips from the batching plant , and compacted in transit.

9. Skip with obliqu e discharge causing skip to react on opening and mov e away and out of poured concrete.

10. Skip not covered at th e top. 11. Skip not filled 10 the br im , on lowering

into the water the. inflowing rush of water washing ou t cement and fines from the co ncr ete.

12. Slewing the skip into positio n when underwater. It should be correctly loca ted ver tica lly before lowe ring into into the water.

13. Discharging co ncrete from th e skip ab ove the pour ed co ncrete .

14. Diver wearin g fins whi le concreting . Water turbul ence causes washout.

15. Concrete poured on to unblinded rock rubbl e fo rma tio n, concrete run s away through voids , segregates. cemen t wash es out.

16 Shutter de s igned such that friction or suction will not allow shutter to be st ruck.

17. Shutt er formed fro m memb ers which ar e too light.

18. Shutternot prop erly cleaned. 19. Jac ks for leve lling and eas ing shutt e r

not suitable . 20. Shut ter des igned such that no scope

for laitan ce and surplus o r disturbed co ncre te to flow ove r and out of t he shu tter.

2 1. Skip dim ens ions such that it will no t fit insid e the shutt e r.

22. Skip fits into shutt er , but insufficient roo m for skip opening arrangem ent to operat e .

23. Skip and shutt er designed so. that diver disturbs pour ed co ncr ete while posi­tioning and discharging skip.

What to do l'ight 1. Select correct. appropr iat e me thod of

plac ing co ncr ete und erwat er , skip s (various) , trcmi c o r pumped.

2. Design correct mix co mpatible with supply, placing meth od , and required performance of co ncrete under water.

3. Use appropriate add itives to allow for suppl y. plac ing method . overa ll t ime scale and type of pe rformance requ ired of th e co ncrete in placing in its final position.

4. Desig n shutt e r of size and strength to accommod ate concr ete delivery rate within time sca les having con ­s ideratio n as to type of cons tru ction , sea, tide, and other co ndition s pecu­liar to the site.

Although in thi s art icle stress is laid on the co ntribu t ion of add itives, mention is mad e here of th e succcsf ul forming und er­wa ter of co ncrete ribs ( of cross -section of

139

Underwater concrete construction

about 1 m2) for th e repair of a qua y in the Middle East using a mix co mpl ete ly with ­out addit ives . Two grades of con crete used o n the project were supplied by a local re ad y-mix concr et e supplier fro m his no rma l range of concr ete mixe s. No ad ­mixture s were used in either mix , both of which perform e d sa tisfactori ly.

Underwater concre te can thu s be used in any location in the wo rld to which cement can be brought. It see ms unlikel y that the nature of the aggregate s unl ess the y are tota lly unsuitabl e for co ncrete will rend er underw a ter concrete imprac ­ticable. The on ly provi so being tha t the size should be 20 mm . T he experien ce re lated her e is in tropi ca l water , but ther e see ms to be reaso n to expect diff erent behaviour in cold er waters .

Co ncrete hagwork Co ncrete bagwork should normall y onl y be used for temporary wo rks, or when cover ed or sec ured by other mea ns. In exposed wave, scour loca tion s. whe re po ssible it should be avo ided , or alter­native mea ns devised . How eve r , co ncre te bagwork does have an important part to pla y in underwater cons tru ction.

Depending on the particu lar use to which the bag work is to be put th e follow­ing sho uld be tak en into co nsiderat ion. I. He ssian bag s will eve ntu ally ro t , the

th icker the bag , the greater separatio n between th e sub sequent b lobs of con ­crete - converse ly, the thinn er the bag the tighterthe remaining co ncr ete.

2. Cheap, thin , ope n weave hess ian bag s allow weep ing of grout in hand ling into pos ition which gi ves a ce rtain amount of bonding betwee n bag s .

3 . Pla stic woven bags may o r ma y not degrad e de pendin g on th e parti cular materi a l. I lowever , betwee n plast ic bags there is not the same co hesion that there is with hess ian bags. and allowance should be mad e for the lower coeffic ient o f friction between bag s when used in a stru ctu ral situ­at ion.

4. For placing bonded bagwork. the bags shoul d be no mor e than two­thirds full. thi s will allow them to be laid in tight with the bag mould ed to shape as requir ed. _

5. Stee l pin s may be used to sec ur e th e bagwork into posi tion , but allowance shou ld be made for the corrosion of the steel pins. In this respect the pin size ca n be increa sed , or eve n ga lva nised.

6. For conventio nal co ncrete-fi lled bags the co ncrete should be a stiff mix , as for norm a l concr e te , such th a t a cohesive m ater ial is produ ced. Dry mixed concrete for bagwor k is not reco mm ended , on handl ing into place by divers cement washout may be such that all remain s is a bag of aggreg ate.

7. If on placing the bag it requir es work­ing , shap ing and bond ing into posi tion a 3/4 in~!}ggrega te mix or smaller is recomm ended. Th e sma ller the aggre ­gate size th e easier, better and quicker the diver will be ab le to work the bags into place. Pinnin g will a lso be more easil y effected with a sma ller aggre­ga te .

Vibration of unde r water concr ete Th is should not be immediat e ly discounted and given th e righ t com bination of circum ­stan ces is en tirely pra ctical.

There are two basic situati ons for th e use of vibrated co ncret e und erwa te r ; 1 . To assist in moving concrete into a

right or difficult corne r or loc ation. 2. Wh ere high qualit y, well-compacted

dense concrete is requ ired and pre­casti ng of the element is not pr actic ­able. To success fully vibrate conc re te th e

followi ng poin ts must be ad hered to: a) Concret e must be of the rig ht co nsist­

ency not to imme di ate ly segregate . Vibra tion of an extre mely fluid mix wi ll caus e ra pid segre gati o n and laitanc e.

b) A co nventiona l co mpress ed air vibra ­tor should be used with the air dis­charg e well clea r of the co ncrete.

c) Th e size of vibrat or should be com­pa tible with the pe rfo rm ance of the mix. the purp ose for which it is be ing used (ie moving or comp acting) a nd the depth of con crete being vibrated .

d) In operati on. the vibrator sho uld be turned off and inserted deep inside the co ncrete and operated in brief bursts. On comp letion of the vibrat ion it should be turned off and then very s lowly withdrawn , allowing the con­crete to recove r the void formed by extract ion and prevent ing th e ingr ess of water into th e pour.

e) It may be necessar y to increase the ai r pressure to obtain the correct ambient operating pre ssure for the vibra tor.

f) Careful clea ning , oiling and stor ing a fter use a re , of cou rse,J iece ssa ry.

Pumped concrete: Sharjah power station A prior instan ce o f the use of pum ped con ­cre te occurre d at Sharjah steam power sta tion coo ling wate r pumphou se intake. where th e co ncrete floo r was cast : under ­wate r .

One metre of blinding co ncret e was requir ed for a pumphouse floor measu ring some 20 m2•. A stee l sheet piled coffe rda m was driven and braced and the fill excavated to 6 m be low the water table . De wateri ng was difficu lt with the appare nt ground condi tion , a!1d there was co nsequ ent con­cern abo ut overload ing the steel shee t · piled walls .

Placing underwater co ncrete would allow the fo rm at ion to be sea led a nd the toe of th e piles to be secured. Obvious

140

Underwater concrete construction

op ti ons for pl ac ing th e conc rete were co n­venti o na l skip o r tr e mie pi pe . H oweve r , with a concr e te pump on site and hydraulic arm re ach of suffi cie nt lengtµ to cover th e a rea dir ect pumpin g wa s a tt e mpt ed .

In itia l d ifficulti es, 11s with a ny ne w op e rat io n, we re e nco unt er e d . T hese we re mainl y:

( i)' mi x design ; (ii) co mmun icat ion betwee n the d ive r

and concr e te pump ope rat o r , an d ( iii) int ro du ction o f cha rged dow npipe

int o th e shutt er. Co mmun ica tions we re o rgani sed

e ffect ive ly \~ith p rac t ice and co nc re te was pump ed thro ugh the p ipe until di scha rg ing smoo thl y o n th e sur fac e . Pumpin g was the n sto pp ed a nd a b la nk fla nge ca p with a qui ck release was place d over th e ope n pi pe e nd; the pip e was then placed in the requ ired posi tion in the underwate r shut­te r. T he dive r re moved th e bla nk ca p a nd concr ete pumpin g was res um ed.

Th e di schar ge pipe was always main ­ta ined in a verti ca l pos iti on . T his was imp or ta n t , as o nce co ncre te bu ilt up aro und th e en d of the pipe the pum ping act io n gave a reactio n a nd th e hydra ul ic ra ms co ntro lling th e p ipe had to a bsorb

these fo rces and ho ld th e e nd in po siti on within th e po ur ed co ncre te.

Co ntro l of th e p ip e was abso lu te a nd , as th e requirem e nt was for blindin g co n­cre te only , the di ver was ab le to ro ugh sc ree d th e co ncre te behind th e di scharge pip e lin e as it prog resse d ac ro ss th e pour. T he qu alit y of finish was good and allowed th e contractor a fter so me cle an ing to wor k strai gh t off th e co ncre te instead of plac ing a furth er blin din g laye r . Th e form at ion was effec tive ly sea led and th e p iles secu red to e nab le co nstru ctio n o f th e pumph ousc to p ro ceed apace.

Conclusion In genera l, th ose withou t civil co nstruct ion kno w ledg e a re sur pr ise d to learn th at co ncre te ca n be success full y pou red an d scree de d un derw a te r. T hose who have t he knowle dge, bu t not actua l pri o r ex per i­e nce, inva ria bly approac h th e subject with som e tr ep id atio n .

T he aut hors be lieve tha t by the d irect pumpin g of co ncrete many of th e un know ns ca n be rem oved fro m the exe rcise, a nd co n tro l of the opera tion in as fa r as may ever be possi ble can be re moved to th e sur face.

MARINE

ENGINEERS

AND CONTRACTORS

Ameri~n Dredging Company BOX 190 BEACH & FRIE STREETS CAMDEN . NJ 08101 (21:,) 925-8~68 (609) 963 -0963 INCORPORATED in 1867 2100 tR/\NKFURS' /\VF_ • RAI TIMORE' MD ? 1??6 (301) 355 -0033

TTIE DO CK & H A RB OU R AU THORIT Y

Co nsiderin g th e req ui re m e nt s o f th e po ur ed co ncrete in te rm s of vo lum e , qu a lity a nd sta ndard o f finish , a deci sion should be made as to th e necessa ry deg ree o f expe r ienced su perv isio n that is requir e d bo th above a nd be low wa te r. Thi s is to all ow th e e nginee r in charge and a ll o th e rs to p rocee d wit h co n fide n ce. Dili ge nt app lica tion o f th e di rect pumpin g me th od a lo ng with th e sa m plin g tec hniqu es sho uld pro vide t hat con fide nce.

Refer ences

1. D avies I3 A. Labora1ory Me1hods of Tes1i11g Concrere for Placemem Under-1vc11er, Ma rine Co ncrete "86 - Int e r­na t iona l Co nfe re nce on Co ncre te in the · Marine Enviro nm e nt. Londo n . 22 -24 Septe m ber 1986. T he Co ncre te Soc iety , Frosroc T ec hn ology , As to n , Birmin gham , UK pp 279-286 .

2. Kawa i T . Takegawa K and O kumura . T Dock Co11s1ruc1ion Work Using Newly Developed High Quality Unden va1er Concre te, M ari ne Conc rete '8 6 -Interna tiona l Confere nce on Co ncrete in the Ma rine Environme nt, pp 257 -266.

3. Mal co lm J R a nd Lewi s J A , Ci1·il F.11gi11eeri11g Co11srr11crio11 Under Wat e r. Wo rk s Con s truction Di v isio n Meeting.1954 . pp738-760 .

4. Si m pson , J , Undeiw arer Concrere Srmcr11res. Th e Do ck & Ha rbo ur Aulhorit} . \10152 1 o.6 18, April 1972 , pp516-51 8 .

5. Sago S . Properries of Undenv arer Con crere Contai ning Segregation Comrolli11g Ad111ix111re, Ma rin e Co n­crete "86 - Internat ional Co nfe re nce on Concrete in the Ma rin e E nviron­ment, Lo nd on . 1986. pp 247 -256 .

6. Undenvarer Concrering. Re por t o f a Conc rete Socie ty Wo rkin g Par ty, 13 pages .

7. Y a m ag uc h i .M , T suc hida T a nd T oyo izu m i 1-1, Developme m of High Viscosiry Underwater Concrere fo r Maritime Stmc/1/res. Mar ine Co n­crete '86 - Int e rn at ional Co nfe re nce on Co ncrete in the M ari ne E nviro n­me nt , Londo n .198 6 , pp 235-2 45.

8. CTRTA - Pumpe d Concrete.

9. A CT Re port 2-11 , Design of Concrete Mixes .

10 . Mini st ry of Tran spo rt - U K , Ro ad Note 4, Design of Concrete Mixes .