Feasibility
Study on Using of Steel Fiber Shotcrete as Slab Material for Shuibuya CFRD
Liu Zhaohui
Key words: Feasibility Study, Steel Fiber Concrete, Shot, CFRD, Slab
Summary: Steel Fiber Concrete, as a new type of enhancing
material, has being widely used in various fields such as hydroelectric and
water resources, transportation, offshore protection, port as well as military
projects due to its sound deformation property. Considering the deformation
features of the slab of CFRD, in order to improve the force distribution of
Shuibuya CFRD, enhance its seepage control capacity, and to facilitate
construction at the mean time, the paper raises a new concept of using steel
fiber shotcrete as the slab material. Through analysis on the physical
properties of steel fiber shotcrete as well as the slab deformation, a
suggested scheme is proposed for the actual application of steel fiber
shotcrete in the slab of Shuibuya CFRD.
Concrete Face Rockfill Dam (CFRD) is one of
the dam types with rapid development in the world dam structures in recent 30
years, due to its obvious advantages of fast construction speed, less cost,
reliable operationcompletion, overall
stability, less impact by weather, easier river diversion and flood control
during construction period, . iIt is widely developed all around the world.
Presently, it trends to the development of high dam. For instance,
Tianshengqiao I in China is 178m, Bakun in Malaysia 205m, and the proposed
Shuibuya dam in Qingjiang River of Hubei, China has will reached the a height of 233m, the
highest of its kind in the world. The development of CFRD mainly depends on the
improvement of seeping seepage control by the slab. However, it is
still not
quite clear to thedebate on slab deformation of CFRD, and measure measurement and solution
on slab cracking is quite limited, . tTherefore, measures on seeping seepage control as well
as the adaptability of perimetric joints on dam deformation is of highly
concerned
by people
in the engineering sectors.
Considering the sound physical properties of steel fiber shotcrete such as high tensile and bending strength as well
as good seepage
proofpermeability
performance, . iIt is of good adaptability on the slab
deformation. Meanwhile, sSteel fiber shotcrete is simple and
convenient for construction, and of good evenness distribution, which is
good for construction quality control. In addition, several favorable factors
such as high strength in earlier stage and speed up inhigher construction speed could improve the
force distribution of Shuibuya CFRD, and enhance its seepage control capacity.
Therefore, a new concept of using steel fiber shotcrete as the slab material in
Shuibuya is proposed[1].
1 Physical & mechanics Mechanical Features Of of Steel Fiber Shotcrete
Steel
fiber shotcrete is a kind of enhanced compound concrete placed by sprayshooting,. whichIt has both the features
of common concrete and many enhancedhigher physical performance due to the
introducing of steel fibers. When steel fiber is introduced into the base of concrete mix, the brittleness and
the strength of concrete base mix can be improved, especially for tensile
and bending strength. Compared with common concrete, it is more suitable for
the force distribution of CFRD slab. Meanwhile, anti-seepage, anti-scouring
erosion and anti-vibration performance of steel fiber shotcrete are significantly
improved. Detailed physical indices are shown in Table 1[2].
Table 1 Physical & Mechanical Indices of Steel
Fiber Shotcrete
|
Performance |
Compared with common concrete |
Performance |
Compared with common concrete |
|
Preliminary
split strength |
1.5~2.0 times |
Flexibility |
40~200 times |
|
Tensile
& bending strength |
1.5~1.3 times |
Seepage-proof |
Notably improved |
|
Extending
capacity |
About 2.0 times |
Anti-impulsion
|
5~10 times |
|
Compressive strength |
1.0~3.0 times |
|
Certain improvement |
|
Shear strength
|
2.5~3.0 times |
|
Notably
improved |
|
Fatigue
strength |
Certain
improvement |
Heat
resistant |
Notably
improved |
1.1 Physical Properties of Steel Fiber Shotcrete
(1)The introduction of steel fiber will reduce the use of cement,
and hydration heat accordingly, therefore cracking hereof will be reduced correspondingly,
which could effectively eliminate the occurrencering of
impenetrated crack; (2) Steel fiber could prevent concrete contract shrinkage in a certain
degree,
meanwhile, i.
It could better control
the expansion of any crack and prolong reduce the occurring of new cracks; (3) The adoption of
shotcrete could improve the density of compared to concrete, reduce porosity and block seal the connection between
small openings. Together with a lower water to /cement ratio, better
seepage-proof performance around about 2.0 Mpa water pressure or
even higher could be obtained[3]. Thus it can be seen that steel fiber shotcrete has better seepage-proof
capability compared with common reinforced concrete.
1.2
Mechanical Features of Steel Fiber Shotcrete
(1) As steel fibers is are evenly distributed in the shotcrete, it could
enhance the forcing on each direction of 3D space, and the load distribution is
more reasonable, which could take the full advantage of concrete base;
(2) Due to the unevenness in common reinforced concrete, sometimes the load on
reinforced part is higher and the part without rebar is smaller, hence the
function of concrete base is limited, crack may easily occur in the joints
between concrete and rebar as well as the parts without rebar. Therefore, steel
fiber shotcrete has more favorable forcing stress condition compared with common
reinforced concrete.
Through the above comprehensive analysis on
the physical & mechanics features of steel fiber shotcrete, it could can be seen that using use of steel fiber shotcrete
as the slab material in rockfill dam could better adapt accommodate the complex
deformation of slab, especially for the complex deformation of perimetric joints.
1.3 Construction Technique for Steel Fiber
Shotcrete
There are two kinds of construction method
for steel fiber shotcrete, namely dry
spray and wet spray. In view of the developing trends, more and more projects adopt
the method of wet spray. Key technical issues include good solution for
directional control of steel fiber as well as the control of revert ratio, and
effectively preventing
the occurring
occurrence of concentration of
steel fiber. Presently, many breaking through achievementsmuch progress have been
made in the study and practice, construction technology and equipment are well
proved[5].
2
Force and Deformation Features of CFRD Slab
Current The current design of CFRD is still at the
stage of half theoretical and half precedentexperience, and but with the a trend toward
theorization more recently. The difference between steel fiber shotcrete CFRD and common concrete CFRD is only in the
aspect of the slab
material,
. therefore Therefore the key issue on
theoretical study is to rely
on the proper establishing of stress-strain model for the rockfill. In this
paper, a 100m high steel fiber
shotcrete CFRD is assumed as the ideal dam, 3D non-linearity finite element
analysis is carried out for stress-strain in impounding period
and operationcompletion periods respectively[1][4]. Based on the calculation, the slab and
dam deformation of the CFRD in the impounding period and operationcompletion periods is shown in Fig.1 as
below.
2.1
Dam Outline and Slab Deformation in operationCompletion Period
Seen fFrom Fig.1, the dam outline in operationcompletion period is shrunk smaller toward dam at top
and slightly inflated higher toward the reservoir. Based on the statistic result
of slab cracks from many CFRDs, cracks are distributed in obvious law, most of
them occurred at horizontal in the middle of 1/3~2/3 dam height, and the main
reason, as believed by most of the people in engineering sector, is due to the
stress of temperature. In view of the calculation result and the law of actual
cracks in slab, the author believes that concrete shrinkage is not the main
factor but the large difference on the elastic modulus between concrete slab
and dam body, which bend the upstream slab in the middle of slab due to the
inconformity on the deformation between rockfill and slab.
2.2 Dam Outline and Slab Deformation in Reservoir Impounding Period
Provided that design requirement on compression is strictly followed during construction, the deformation on the dam outline might not increase significantly in the impounding period. Through calculation it could be seen that the impact of impounding on horizontal displacement is larger than vertical displacement, the action of impounding may increase the vertical displacement on filter and transient zone about 20%~40%, and about 40%~60% for horizontal displacement.
The action of impounding makes the stress
deformation of slab complicated, compressive in riverbed part, and tensile at
both sides near abutments.,
tThe nearercloser to the abutment,
the status of the stress
deformation is more complicated, especially for the parts at 1/3~2/3 of the dam height near
abutment at both banks, where the stress deformation is shown of extremely
complicated 3 dimensional deformations. And tThe perimetric joint here
is under compressiveon, hence requires high adaptability on
deformation.
3
Reasons and Feasibility on Using Steel Fiber Shotcrete as the Slab Material for
Shuibuya CFRD
Some reinforcing rebar configuration in
slab for CFRD of middle and low height in earlier stage is not quite reasonableless in certain
aspect.,
rRebars are arranged in
the middle and in the form of a single layer, which is quite reasonless in
view of the actual stress deformation in the construction period and
impounding periods.
Actually the rebar doesn’t play contribute too much function, . And the concrete slab
is neither a tensile axes structure, nor a tensile
eccentricity structure with, but a complicated stress deformation pattern along with the construction and
impounding process. In the construction of CFRD in the later stage, following the increasing of dam
height, more and more projects adopt double layers of rebar in the slab, such as the
first and third stages of Tianshengqiao I. This a great improvement made by
designers through exploring in
practiceexperience,
which is conform to the actual stress deformation requirement during the
construction period and impounding period.
For better adapting the featuresadjustment of stress
deformation in the slab, it is suggested to use steel fiber shotcrete as the slab material in Shuibuya CFRD. Presently, iIn spite that of the present higher unit price
of steel fiber shotcrete is a little bitslightly more expensive
than reinforcing concrete, the adoption of steel fiber shotcrete could: (1)
better control on the occurringthe occurrence of surface
tensile crack as well as its
development, also limit the random cracking due to concrete
shrinkage; (2) better adaptability on deformation compared with common reinforcing reinforced concrete; (3)
reduce the slab thickness, and
the usage of cement, and the lower requirement for temperature control is relative
low; (4) greatly avoid or reduce the potential risk on rebar operationreplacement, . therefore itIt is safer in
construction as the usingand usage of rebar could be eliminated or
reduced; (5) not only better control on construction quality, but also higher
productivity compared with common reinforcing concrete; (6) later stage construction
and maintenance cost could be greatly reduced due to the improvement on
anti-cracking capability and durability. Therefore, its comprehensive economic
and technical indices are reasonable and feasible.
Construction equipment: the current steel fiber shotcrete construction technology is mature well proven and its associated package of equipment
is available. It is fully feasible in construction.
To sum up, through physical & mechanics
study
onies of
steel fiber shotcrete, with
consideration to the feature of actual deformation in Shuibuya CFRD, the
adoption of steel fiber shotcrete as the slab material is more suitable to meet
the overall dam deformation requirement compared with common reinforcing reinforced concrete.
Moreover, it is simple and safe in construction, has a higher productivity and of highbroad market for future application value for
market spread.
4
Proposed Scheme
The slab
of Shuibuya CFRD is divided into 3 stages, and the following 4 schemes could be
considered when using compared withto common reinforcing reinforced concrete: (1)
completely cancel the use of rebar scheme, namely i.e. integral completely use
of steel fiber shotcrete from stage 1 to stage 3 without rebar; (2) low ratio
of reinforcing scheme, namely mainly use of steel fiber shotcrete from stage 1
to stage 3 with only about 10% of rebar for connection; (3) staged lowering
ratio of reinforcing scheme to gradually reduce the use of rebar from stage 1
to stage 3, namely gradually reduce the ratio of reinforcing along the
direction slab placing and increase the use of steel fiber shotcrete at the
meantime. The ratio of reinforcing at stage 1 remains without change to the
original design, 50% of reinforcing ratio of the original design for stage 2,
and 10% for stage 3; (4) steel fiber shotcrete in stage 3 scheme. The ratio of reinforcing
at stage 1 & 2 remains without change to the original design, and 10% of
the original design or complete cancel of rebar for stage
3. For the above 4 schemes, the proportion of steel fiber of about 1/5 part in
the middle of riverbed toward both abutments will be increased properlyproportionally.
In view of the height of Shuibuya CFRD,
with secured safety, active and meaningful exploring practice could be done by
selecting the 4th scheme for actual application together with site
test and research. Through practice, to one would verify the feasibility of using steel fiber shotcrete as the slab material
for a high CFRD, and
to pave a firm foundation for theoretical research and practice on the
construction of even higher CFRD in the future.
References
1．
Sun Yi. Structure Analysis on Steel
Fiber Shotcrete for Shuibuya CFRD. [Dissertation of Master’s Degree], Wuhan
University on Hydroelectric and Water Resources Engineering, 1990.6;
2．
Wang Huande. Steel Fiber
Shotcrete, Hydroelectric and Water Resources Press, 1985.3;
3．
Li Guilin, Cheng Liangkui, Luo Jingning. Construction Manual For
Concrete and Reinforced Concrete. Metallurgy Industrial Press, 1988.7;
4．
Gao Lianshi, Wang Zhaohua, Wu Mengxi, Song Wenjing. Plan and
3-Dimentional Stress/Strain Analyses for Shuibuya CFRD on Qingjiang River.
Hydroelectric Department, Tsinghua University, 1998.1;
5．
WWW.ugc.mbt.com.。
Author profile: Sun Yi (1966), PhD, Male, Senior engineer, born in
Jingxian County, Hebei, Deputy chief-engineer of Hubei
Qingjiang Shuibuya Project Construction Co.
Liu zhaohui (1972), Male, Engineer, International Cooperation of Hubei
Qingjiang Hydroelectric Development Co. Ltd.