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The roller-compacted con-crete (RCC) technique in-troduced to
dam construc-tion in the 1980s has
achieved significant time and cost sav-ings compared to
conventional meth-ods of concrete placement and con-solidation.
International interest inRCC dam design and constructioncontinues
to increase, and new tech-niques for design and construction
arebeing developed, particularly to accom-modate higher dams. RCC
dams arereaching 200 meters in height: the 192-meter, 396-MW Miel 1
Dam in Co-lombia was completed in 2002; the192-meter, 4,200-MW
Longtan Dam inChina is under construction; and the
227-meter, 6,400-MW Ta Sang Dam inMyanmar is under design. In
dams thishigh, lift joint strength and imperme-ability are
critical.
The sloped layer method of placingRCC was recently developed at
the131-meter high, 300-MW Jiangya Damowned by the Lishui Hydro and
PowerCorporation in Hunan Province,China.The method since has been
used at sev-eral other dams in China as well as atthe recently
completed 60-meter-highTannur Dam in Jordan.The method alsowas used
on a trial basis in placing thetop 6 to 8 meters of the
43-meter-highLajeado Dam, which is part of the902.5-MW Luis Eduardo
MagalhaesHydro Project in Brazil.The methodachieves increased RCC
placement ratesand largely overcomes concerns aboutlift joint
bonding.
Paying attention to lift joints
A significant difference between conven-tionally placed concrete
dams and RCCdams is the number of horizontal con-struction joints,
otherwise referred to aslift joints. In order to achieve
requireddensities by roller compaction, RCC liftthickness typically
is maintained at about0.3 meter, while 1.5 meters is typical
forinternally vibrated conventional con-crete.Thus, an RCC dam may
have fivetimes as many lift joints as it would have
had if constructed in conventionallyplaced concrete.
There is no doubt that the bondingof lift joints is the key to
achieving amonolithic structure. Failure of a con-ventional
concrete or RCC gravitydam is far more likely to occur along
itscontact with the foundation or along alift joint than through
the concreteitself. At lift joints, tensile and shearstrengths are
always lower, and perme-ability is always higher, than within
theconcrete.This is due to several factors:the aging of the lift
surface concrete; thetendency for segregation of the con-crete
overlying the joint; and the poten-tial for lower density toward
the bottomof the lift. Experience has shown thatseepage through RCC
dams generallyoccurs along lift joints, which is clearevidence of
non-monolithic construc-tion and defective lift joint bonding.
RCC dams that are more than 100meters high require significant
verticaltensile strength capacity and horizontalshear resistance
throughout, including atthe lift joints. Dams of this height
typi-cally require shear strengths of 1 to 1.5megapascals and
cohesion strengths of0.5 to 1.0 megapascals.The difficulty
inconsistently achieving these strengths isemphasized when one
considers thatRCC dams now being planned to ex-ceed 200 meters in
height will requirenearly 700 lift joints. Furthermore, forexample,
placing Ta Sang Dams 8 mil-lion cubic meters of RCC will
requirepreparing and bonding 2,700 hectaresof lift surface, with
nearly 350 kilome-ters of lift joint line exposed at theupstream
face! In any RCC dam, occa-sional sub-standard work on lift
jointswill be inevitable due to high place-
C I V I L S T R U C T U R E S
Using Sloped Layers to Improve RCC Dam Construction
By Brian A. Forbes
Using the sloped layer method to construct a roller-compacted
concrete (RCC) damenables each layer of RCC to be placed within the
initial set time of the previouslayer.This improves horizontal lift
joint strength and impermeability, significantlyenhancing the
safety of the dam.
Brian Forbes, C.P. Eng., MIE Australia,is manager of dams
engineering forGHD Pty Ltd., Consulting Engineers,Brisbane,
Australia. He is interna-tionally recognized for his work inRCC dam
design and construction,and has served on the review boardsfor some
of the worlds highest RCCdams.
This article has been evaluated and edited inaccordance with
reviews conducted by two ormore professionals who have relevant
expertise.These peer reviewers judge manuscripts for technical
accuracy, usefulness, and overallimportance within the
hydroelectric industry.
From HRW, July 2003 - HCI Publications, www.hcipub.comReproduced
with permission.
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ment rates, the need for night work, andthe urgency to prepare
lift surfacesahead of the rapidly advancing nextRCC lift.The
inspecting engineer willsimply not be able to ensure that everylift
joint is properly constructed, with-out inherent
weaknesses.Therefore, thegoal must be to minimize the risk of
adefective lift joint.
To achieve homogeneous,monolithicRCC across a lift joint
requires the over-lying lift to be placed within the initialset
time of the lower lift. Using retardersin the concrete can easily
extend settimes to 5 to 8 hours, compared to 1.5 to2.5 hours
without retarders.However, inlarge dams, it usually takes 15 to 30
hoursto complete a horizontal lift from oneabutment across to the
other. Com-pletely homogenous, monolithic RCCin large dams with
horizontally-placedlift joints cannot be achieved by the
usualprecautionary measures such as minimiz-ing the tracking of
equipment over com-pleted lift surfaces, repeated compressedair
clean-off, application of curing water,and use of high contents of
fly ash orslow hydrating pozzolans in the RCCmixture. As a result,
zones of lowerstrength in the lift joints of such damswill be
inevitable.
Developing the sloped layermethod
RCC placement at Jiangya Dam began
in 1996, with the RCC initially placedin horizontal layers. The
dam wasdivided into monoliths or blocks about60 meters wide and
extending betweenthe formed upstream and downstreamfaces. Each
monolith was constructed in3-meter lifts of RCC, with each
liftconsisting of ten 0.3-meter-thick hori-zontal layers.The area
of placement wassuch that successive layers of RCCcould be placed
within 5 to 6 hours,which was within the initial set time ofthe
retarded RCC.When the lift in onemonolith was completed, the form
atthe transverse joint was moved and thelift in the adjacent
monolith was con-structed.The cold joint at the top of the3-meter
lift was green cut and thor-oughly cleaned. Then a thin layer
ofbedding mortar was applied when thenext lift was placed on it, as
in any con-ventional concrete dam.This methodachieved excellent
bond and homo-geneity of the RCC across the lift joint;however,
construction was slow andcostly owing to the effort required
toerect the transverse form, the difficultyof maintaining a ramp
access throughthe form for the RCC trucks, and thecost of the set
retarder.
In 1997, when Jiangya Dam hadreached mid-height, the chief
engi-neers of Lishui Hydro and Power Cor-poration, Liaoning
Construction Bu-reau (contractor), and Changjiang
Water Resources Committee (con-struction supervisor) conceived
thesloped layer method on site, andadopted the method principally
toaccelerate the overall RCC placingrate.The method succeeded, not
onlyensuring that lifts were placed withinthe initial set time of
the RCC, butalso enabling the dam to be competedon schedule. By
placing the 0.3-meter-thick layers of RCC on a slope, fromone
abutment to the other betweenthe formed upstream and
downstreamfaces, the same 3-meter lift could bebuilt up across the
entire dam as a con-tinuous process, without the need for
atransverse form.When a new 3-meterlift commenced, the slope of the
0.3-meter layers could be changed tomaintain the desired volume of
RCCin each layer.
Applying the method,realizing the advantages
For example, suppose one were using 3-meter-high forms and
producing RCCwith an initial set time of two hours at arate of 500
cubic meters per hour; then,for the sloped layer to be placed
withinthe initial set time, the slope S (horizon-tal on vertical)
of a 0.3-meter-thickRCC layer would have to be:
S < 2x500Wx3x0.3
i.e., approximately 1,000W
where W is the width between up-stream and downstream faces as
shownin Figure 1.
Hence, a slope of 1 vertical on 10horizontal would be required
at lowerelevations where the width of the dammight be 100 meters;
at upper elevations,where the width might be 25 meters, aslope as
flat as 1 on 40 could be used.Initial trials at Jiangya Dam
confirmedthe feasibility of slopes as steep as 1 on 8with the
smooth double drum steelvibratory rollers operating in an up-down
slope direction. Slopes are con-trolled during placement by paint
linesmarked on the upstream and down-stream forms and by survey
methods.
2 HRW / July 2003
This figure shows how a 3.0-meter lift in an RCC dam can be
constructed in 0.3-meter-thick sloped layers, which can be placed
within the initial set time of the RCC in the previous layer.
Figure 1
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HRW / July 2003 3
Using this method, the final clean upand surface preparation of
the lower lift including application of beddingmortar is restricted
to a narrow stripalong the toe of the sloped layer.Thesurface of
the completed 3-meter liftcan be green-cut while the RCC is
stillyoung.The upstream and downstreamface forms can be erected in
anticipa-tion of placement resuming at this posi-tion in about 10
days, assuming aplacement rate equivalent to one 0.3-meter
horizontal lift of RCC per day inthe traditional method.
If sloped layers are placed within theinitial set time of the
RCC, no surfacepreparation or bedding mortar isrequired prior to
placing the overlyingsloped layer. For 3-meter formed lifts,this
potentially reduces the requiredsurface preparation by 90 percent
com-
pared to conventional placement. It alsoreduces the number of
lift joints (andpotential failure surfaces through theRCC dam) by
90 percent.
In the sloped layer method, the thinwedges or feathered edges at
the toeand top of each sloped layer requirespecial treatment. At
the toe of eachsloping layer at Jiangya Dam, whereaggregate in the
RCC could easily becrushed under the steel roller, a 4- to
5-meter-wide, 0.15- to 0.3-meter-thickhorizontal layer was first
placed alongthe top of the previous lift as a footafter the contact
area with the lower lifthad been thoroughly cleaned and cov-ered
with a thin layer of bedding mor-tar.The foot was placed and rolled
in anupstream-downstream direction. Thesloping layer then commenced
fromabout the center of the foot. (See Figure
2.) Later, any feathered edges weretrimmed back 0.1 to 0.2 meter
to firmRCC and covered with the beddingmortar just prior to placing
the foot forthe start of the next sloping layer.Anyfeathered edges
at the top of the slopinglayers were cut back to a thickness of
50to 70 millimeters as part of the greencutting and lift joint
preparation proc-ess; this was easily achieved using high-pressure
air-water jetting to lift off anypoorly bonded material.
Besides ensuring improved lift jointquality, the sloped layer
method allowsthe surface preparation, curing, and lift-ing of
formwork to be performedlargely independent of the RCC place-ment.
In addition, the amount of timeavailable for lift surface clean up
andpreparation is increased tenfold whenusing 3-meter lifts.
Because the sloped layers of RCCcan be placed before the lower
liftreaches final set condition, the form-work and its anchorage
into the RCCmust be designed to resist a greaterload.
For inclined downstream faces, thebest arrangement appears to be
to usevertical steps.When formwork is used,the step height is equal
to the liftheight. For example, at Tannur Dam, alift height of 1.2
meters using four 0.3-meter sloped layers per lift was adopted.This
matched the design height of thesteps on the downstream face,
suitingthe formwork system that had beenoriginally supplied for
horizontal layerconstruction.
At Jiangya Dam, reusable, precastconcrete blocks formed the
1-meter-high, stepped downstream face.This sys-tem was retained
when the change wasmade to sloping layers. Blocks weresimply added
ahead of the advancinglayers as the horizontal RCC steps
wereconstructed and a base for the blocksbecame available. Sheets
of steel plateagainst the blocks separated the blocksfrom the RCC.
The steel plates ex-tended some 1.5 to 2 meters beyondthe leading
block to provide interimsupport for the zone of sloping RCCjust
ahead of the horizontal surface of
At Chinas 131-meter-high Jiangya Dam, precast concrete blocks
were used to form the steps in the downstream face of the dam, with
steel plates separat-ing the blocks from the grout-enriched RCC
facing concrete. This system beganwith horizontal RCC placement at
the lower part of the dam and continuedwith the sloped layer method
adopted during construction.
Preventing a transverse feathered edge at the toe of a sloped
layer in an RCCdam requires a special approach. At Jiangya Dam, in
China, a foot was firstplaced from which to begin the slope, as
shown.
Figure 2
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the step formed behind. Using this step-precast block-steel
plate system wouldappear to be ideal where more than onestep is
required to match the selected liftheight. Relocation of the blocks
andsteel plate is simple and repetitive; it canbe easily achieved
with a few laborersand a small mobile crane or front-endloader.
The sloped layer method has beenshown to work well in
conjunctionwith grout-enriched RCC used for
facing the upstream and downstreamfaces of the dam and
connecting theRCC to the rock abutments. In partic-ular, RCC
progress is not delayed whileawaiting delivery and placing of
con-ventional concrete facing.
Placing a sloped layer generally in-volves commencing at the
downstreamface and moving across to the upstreamface, working over
the full height of thelift and compacting in the up-downslope
direction.The placement area can
be roughly divided into three sub-areas;placement begins by
dumping RCCfirst in the downstream sub-area, thenprogressing to the
upstream face, fol-lowed by the spreading and compactingoperations
accordingly.
When placing RCC in the narrowriver channel area, the sloped
layers canbe oriented in an upstream-down-stream direction.
Obviously, the layersmust slope upwards from upstream todownstream,
since otherwise the shearresistance along a cold layer jointwould
be severely reduced by the re-duction in the effective angle of
fric-tion.When the distance between theupstream and downstream
faces of thedam equals the distance between abut-ments, i.e.,
forming an essentiallysquare placement area, the orientationof the
slope can be redirected and place-ment can proceed from abutment
toabutment as shown in Figure 1.
In the traditional horizontal layermethod, the RCC layers
typically areplaced with a 2 to 3 percent crossfalltoward the
upstream to drain rainfall, aswell as wash water generated from
liftsurface green cutting, clean up, and gen-eral lift surface
preparation.This cross-fall can be retained in the sloped
layermethod.
Comparing the pros and cons of the method
The most important advantage of usingthe sloped layer method in
an RCCdam is undoubtedly the improved safetyachieved by producing
homogenous,monolithic RCC across the joints be-tween layers.This is
accomplished bylimiting the number of cold lift joints toone every
3 meters, thus reducing thenumber of horizontal lift joints by up
to90 percent, or by 50 percent comparedto a conventional internally
vibratedconcrete dam. Other significant advan-tages of the method
are:
Increasing overall placement rates,potentially by 30 to 50
percent;
Reducing clean up, lift joint treat-ment, and bedding mix
application byup to 90 percent, with associated costsavings;
4 HRW / July 2003
At Jiangya Dam, each sloped layer was placed first at the
downstream steppedform with the dumping-spreading-compacting
procedures progressing acrossthe dam to the upstream face. Here the
width of the dam is approximately 50 meters.
A sloped layer of RCC is being placed at the recently completed,
60-meter-highTannur Dam, owned by the Jordan Valley Authority, in
Jordan. At this dam, alift height of 1.2 meters using
0.3-meter-thick sloped layers was adopted. Thelift height matched
the design height of the steps on the downstream face,suiting the
formwork system that anticipated horizontal layer construction.
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HRW / July 2003 5
Taking green cutting and initial liftjoint preparation off the
critical path,allowing up to ten days for this work tobe done
before placing the next lift ofRCC;
Preventing wash-down/cleanupwaste from encroaching on
currentplacing areas;
Removing the final lift joint cleanup from the critical path and
allowingup to 90 percent more time for jointpreparation prior to
placing the nextlayer of RCC;
Eliminating the need for greencutting and layer surface
preparation ofthe grout-enriched RCC or the con-ventional vibrated
concrete facing; thematerial is often so fresh that pokervibrators
penetrate into the previouslyplaced layer of facing;
Allowing upstream and down-stream formwork to proceed off
thecritical path, potentially days ahead ofbeing required; this
permits formworkto be left in place longer for thermalprotection of
the concrete during cur-ing in colder climates or, by adopting
aleap frog procedure, achieving a 40 to50 percent reduction in the
quantity offormwork required;
Reducing the total surface area ofRCC required to be cured;
Reducing the area of exposedyoung RCC that could be damagedby
rainfall or freezing conditions and,consequently, the volume of
RCC
that may require removal and replace-ment;
Reducing the potential for cooledRCC to gain heat from hot
ambientconditions; and
Providing (most of the time) anotch in the crest of the dam
capableof passing floods in excess of the nor-mal diversion
capacity; this provides amore secure plant and equipmentparking
zone above flood water leveland potentially reduces the
clean-upwork following the flooding, permit-ting an earlier
resumption of RCCplacing.
Aspects of the sloped layer that mayappear disadvantageous
are:
Limiting downstream formworkoptions by requiring either high
steps(equal to the selected lift height) or aprecast concrete
block/steel plate stepconstruction system with some associ-ated
minor complications in progressiveblock placing;
Increasing the rate of rise againstthe formwork, requiring
additionalformwork anchorages;
Requiring care and attention of thetoe of the sloped layer to
prevent thinfeathered edges, where aggregate couldbe easily crushed
under the roller poten-tially creating preferred seepage paths;
Requiring the removal of thefeathered edges of RCC that can
occurwhere the sloped layer ends at the topof the lift;
Somewhat complicating surveycontrol, with slopes both parallel
andtransverse to the dam axis (althoughlaser survey systems can
accommodatethis complexity); and
Somewhat complicating the finish-ing of the downstream step
horizontallift surface concrete facing (grout-enriched RCC or
conventional inter-nally vibrated concrete).
Conclusion
Seeking better ways to prove the qualityand safety of RCC dams
and to reducetheir cost and effect on the local envi-ronment is
essential as RCC dams be-come more costly and time-consumingto
build.The recently developed tech-nique of placing multiple layers
on aslope to achieve a much thicker liftlargely eliminates the
vexing problem oflift joint bond, allowing monolithic,homogeneous
RCC to be achieved inlifts up to 3 meters thick. Eight- to
10-meter-long monolithic concrete coresshowing no evidence of lift
joints havebeen extracted from recently completedRCC dams in China
where layers havebeen placed within the initial set time.This sort
of evidence will give engineersthe confidence to design the
200-meter-high RCC dams of the future. Mr. Forbes may be contacted
at GHD Pty. Ltd, GPO Box 668, BrisbaneQ4001, Australia; (61)
7-33163601; E-mail: [email protected].
