ROCK-FILLED CONCRETE IN CHINA - SELF-COMPACTING CONCRETE FOR MASSIVE CONCRETE

Transcription

1 ROCK-FILLED CONCRETE IN CHINA - SELF-COMPACTING CONCRETE FOR MASSIVE CONCRETE Xuehui An (1), Miansong Huang (1), Hu Zhou (1) and Feng Jin (1) (1) State Key Laboratory for River Dynamics and Hydraulic Engineering, Tsinghua University, Beijing, China Abstract Rock-Filled Concrete (RFC) is a new type of concrete for massive concrete construction works which was developed in China. Based on the technology of Self-Compacting Concrete (SCC) technology, it is produced by pouring ready-mixed SCC into the voids of large blocks of rock with the minimum size of 300mm in the formwork. The SCC fills the void space between the blocks due to its good fluidity and segregation resistance, and thereafter the mix sets to form the RFC mass. Since it was first developed in 2003, various investigations has been carried out in both the laboratory and the jobsite to study the properties of RFC in both fresh stage and after hardening, strongly shows that RFC satisfies the required properties as a material of massive concrete. And it has been employed in a number of hydraulic engineering structures already, in China. Besides, cost and environmental impact assessments of RFC have also been carried out to evaluate the social impacts, which aim to develop a new RFC construction system. All of these studies and applications showed that RFC had great advantages in massive concrete construction, such as low heat of hydration, fast construction activities, high construction quality, low cost, low energy consumption and low emissions of the greenhouse gas. And all of them contributed to simpler construction management and easier quality control, which made RFC become potentially promising. 1. What Is RFC? 1.1 Introduction of SCC For several years beginning in 1983, the reduction in the quality of construction work caused by the similar reduction of skilled workers made the durability of concrete structures become a social problem in Japan. To solve this problem, Self-Compacting Concrete was first advocated by Okamura et al. in 1986 and the prototype was first completed in 1988 [1], which can fill every corner of formwork purely by its own weight without vibrating compaction. Since then SCC has been used in various kind of practical structures all over the world. And the immediate cause for the employment of SCC was summarized as follows [2] : 615

2 (1) To shorten the construction period for large scale construction (2) To assure compaction in the confined zones of reinforcing bars where vibrating compaction is difficult or impossible (3) To eliminate noise or vibration due to compaction in concrete products (4) To assure durability with no initial defect of concrete or sure compaction. 1.2 Development of RFC However in spite of these benefits, compared with conventional vibro-compacted concrete, SCC displays a lower E-modulus, higher hydration heat and is also more costly in seeking to achieve the same compressive strength [3]. Thus, yet, SCC has still been used as a kind of special concrete rather than standard concrete in practical structures, especially in massive concrete in dam engineering. Yet, scope exists to employ SCC in some degree, especially in light of the further challenge faced by dam engineering of the need to pay more attention to reducing costs and environmental impacts in future projects. To overcome such challenges of the limitations of the use of SCC, and to improve the economics and environmental performance of dam projects, Rock-Filled Concrete (RFC) was developed as a new type of concrete for structures [4], especially massive concrete structures such as dams, in China. Since it was first developed in 2003, RFC has rapidly grown and have been put to practical use for several years beginning in Two types of construction technologies of RFC, conventional RFC [5] and Dump-type RFC (DRFC) [6], have also been proposed to use RFC in different types of structures. This paper mainly describes the comprehensive investigations of RFC carried out in both the laboratory and the jobsite, and the applications in hydraulic engineering projects in China. As Fig.1 shows, RFC is a combination of consolidated SCC and large blocks or rock with the minimum size of 300mm. The different types of RFC construction technology are shown in Fig.2 [7]. In general, RFC is produced by filling the working space with large-scale blocks of rock to form a rock-block mass first; then, either pump SCC into the working space or pour it directly on to the surface of rock-block mass; and thereafter, SCC flows down to fill all the void spaces by merit of its own weight and given its good fluidity and high segregation resistance. And with regards to DRFC, it is produced by pouring SCC into working space first and then dumping rock blocks into SCC. And the mix would also set to form the RFC mass. As a result, 55%-60%of the space is filled by rock blocks in RFC mass and therefore only about 40%-45% of the volume needs to be filled with SCC, which leads to a great reduction of cement content in RFC. With regard to massive concrete structures, in particular concrete dams require a reduction of the unit cement content to lower the financial cost as well as the heat of hydration. And RFC performs satisfactorily since the unit cement content of RFC with a strength grade of C15 is only 80kg~90kg/m 3. [8] SCC Rock blocks RFC Rock block SCC 616

3 Figure 1: Composite of RFC Conventional RFC Dump-type RFC Rock block SCC RFC Mass Figure 2: Two Types of RFC Construction Technology Figure 3: Compaction Tests of RFC Figure 4: The Cross Section of Plain RFC 2. Investigations of RFC Before RFC was put to practical use, various investigations had been carried out, and described as follows. 2.1 Compaction Tests The compaction tests of RFC were first carried out in laboratory to study the filling ability of SCC in rock-block mass [9]. After being washed, the rock blocks with the minimum size of 150mm were put into an acrylic form of internal dimensions 200cm by 50cm by 50cm. The slump flow value of SCC used in this experiment was 65cm and the strength index was C50. It was found that SCC did fill the void space effectively in rock- block mass from the liquidity of SCC in rock block. After hardening, it was also found that good compactness of RFC specimen was obtained and the SCC flowed into every space between block in the section of rock-block mass, as shown in Fig.3. The results showed that SCC has enough flow ability to fill all the voids in a rock-block mass. And, furthermore, the results can be 617

4 extrapolated to larger dimensions; since the dimensions of void space would be greater in rock-block mass of larger dimensions. The specimen obtained in the compaction tests was used to carry out the bending test. The cross section is shown in Fig.4, and it is clearly seen that the voids between rock blocks are fully filled with SCC where aggregate distribute equably and the all the rock-fill fragments or blocks on the section are broken, which indicates that the bond between rock blocks and SCC is quite strong. 2.2 Compression Strength Tests In order to employ RFC in the practical dam construction, a large RFC block of dimensions 2000mm by 1000mm by 1800mm was constructed in the laboratory, by simulating the real construction processes in dam engineering [10]. As shown in Fig.5, the RFC block was casted with three concrete lifts with the thickness of 600mm, and there were cold and hot joints between these lifts. Then, after curing, specimens with different sizes would be cut from the RFC block to study the compression strength and permeability of RFC. Nine prismatic specimens of dimensions 15cm by 15cm by 30cm were cut from the RFC block for axial compression strength tests. The results of the compressive tests are shown in Fig.6, and show the relationship of strengths between RFC and SCC. The average strength of SCC used in RFC construction was 13.2MPa, while the average axial compression strength of nine RFC specimens was 16.7MPa, which was 1.27 times the value for SCC, which showed that the strength of RFC is higher or at least not less than that of SCC used in the RFC construction. Bucket: simulation of pumping SCC 1h RFC block Remove the form Cure: 3d Cure Figure 5: Simulation of Dam Construction Hot joint Cold joint 2.3 Permeability Tests In hydraulic projects, as well as compressive strength, permeability is one of the most important properties concerned by engineers since it is one of the most important potential weaknesses. Structural weaknesses, such as cold and hot joints created by the limited thickness of concrete lifts, great affect permeability of the dam body. In order to study the permeability properties of RFC in different area, eighteen specimens in three cases (six specimens per case) were cut from RFC body, cold joint area and hot joint area, respectively. It is difficult to cut the specimens with standard size, which is 175mm top diameter, 185mm bottom diameter and 150mm height, directly from RFC block. As shown in 618

5 Fig.7, specimens of dimensions 120mm by 120mm by 150mm were cut from the block and then fixed to standard size by mortar with much higher strength of C60 to ensure higher permeability. The permeation resistance indexes of RFC in different areas were shown in Table 1. Since the required permeation resistance index of the dam concrete in hydraulic engineering is from W2 to W10 in China [11] and the minimum index of RFC is W14, it is indicated the permeability of RFC is high enough to satisfy the requirement of the dam concrete in hydraulic engineering. Axial compression strength fc/mpa % 166% 164% 160% 127% 113% 108% 98% 87% 78% SCC 100% Mortar RFC specimen Fixed to standard size CN-1 CN-2 CN-3 CN-4 CN-5 CN-6 CN-7 CN-8 CN-9 Average Specimens cut from Number of RFC specimens RFC block Figure 6: Comparison of Strength of RFC and SCC Permeability tests Figure 7: Preparation of Specimens for Permeability Tests Table 1: Permeation Resistance Index of RFC in Different Areas Specimen position RFC body Hot joint Cold joint Permeation resistance index W35 W31 W Compaction Tests Before practical application of RFC, not only laboratory tests but also in-situ tests were necessary to ensure the properties of RFC. Thus, two in-situ tests of RFC were carried out at the jobsite of Henan Baoquan pumped-storage projects and Sichuan Xiangjiaba hydropower project, respectively, to simulate construction reality. In the in-situ test of Baoquan project, the formwork is made of masonry wall, and SCC was poured directly on the face of rock-block mass by excavator. After hardening, it was found RFC with good compactness from the exploratory test. While in that of Xiangjiaba in-situ test, RFC was constructed with DRFC construction technology. SCC was poured into workspace from mix truck directly and then dump rock block into SCC with bulldozers and excavator. RFC perform satisfactorily in apparent density test, core compression test, water pressure test, and temperature rise of hydration heat test [12][13]. And the results of these tests are shown in Table

6 Table 2: Results of In-situ Tests at Baoquan and Xiangjiaba Projects Apparent density Core compression Adiabatic thermal Water pressure test test test rising Baoquan project 2415 kg/m MPa 1.33Lu Xiangjiaba project 2508 kg/m MPa Compaction Tests Besides the experimental investigations of RFC in both laboratory and jobsite, social impacts of RFC, such as cost and environmental aspects, were also assessed [14]. Generally speaking, the cost of materials is one of the most important considerations on projects. Based on the date from contractor of Baoquan project, a cost assessment was finished. The data showed the volume ratio of SCC and rock blocks were 42.8% and 53.2%, respectively, and the other 4% of volume is composed by air. In addition of a cost of Rmb160-Rmb180/m 3 for SCC, and of Rmb40/m 3 for rock blocks, the cost of materials was calculated to be approximately Rmb90-Rmb98/m 3. The cost of materials is about 70% of total cost, and then the total cost of RFC may be approximately Rmb /m 3. A similar assessment was also finished in Xiangjiaba project, which showed that the cost of RFC was 10Rmb lower than conventional vibro-compacted concrete. With regards to the environmental aspects of RFC, studies on the environmental impact assessment of RFC, conventional vibro-compacted concrete (abbrev. Covn. C) and Roller Compacted Concrete (RCC) were also carried out. As shown in Fig.8, the results suggested that RFC has better environmentally-friendly grading compared to conventional concrete and RCC, through the quantitative calculation in environmental impact of concrete in the entire life cycle. In the gravity dam construction, the employment of RFC can mitigate the negative effects that will inflict on the natural environment, such as CO 2 emission and energy consumption. materials manufacturing materials transportation concrete mixing concrete transportation concrete placing RFC Conv.C RCC (a)energy consumption of concrete in entire life cycly(unit: MJ/m 3 ) RFC Conv.C RCC (b)co 2 emission of concrete in entire life cycly(unit: kg/m 3 ) Figure 8: Environmental Impacts of Three Types of Concrete in Entire Life Cycle 3. Applications of RFC Based on all the investigations outlined, RFC has been put to practical use in the following hydraulic projects. 620

7 3.1 Dam Construction Experimental Dam Construction RFC was first used in a gravity dam in a reservoir project in Beijing. The 13.5m high, 2,000m 3 gravity dam was finished in As the on-site process of producing RFC shows in Fig.9, after the transportation and placement of rock blocks being finished by labor, SCC was transported from batching and mixing plant by mixer truck. It was then pumped into the working space containing rock blocks by pump truck, and the compacted RFC was obtained without any recourse to vibro compaction. At the dam, lifts of 1.2m were executed, and good quality RFC was revealed by later tests. Rock block Transportation Placement.. Figure 9: On-site Process of Producing RFC Pouring SCC RFC Workspace Rock blocks transportation Rock blocks placement Rock blocks transportation Rock blocks placement SCC casting SCC transportation SCC casting SCC production Figure 10: RFC Construction at The Jobsite of Baoquan Project 3.1.2Auxiliary Dam in Baoquan Pumped-storage Project After successfully employed in the experimental gravity dam, RFC was applied in part of the auxiliary dam of the upper reservoir in Henan Baoquan pumped storage project. The dam 621

8 was designed as a 50,000m 3, masonry gravity dam of 42.6m high. However, about the top 3m of the dam, with a volume of about 4,500m 3, was constructed with RFC to solve the problems in the practical masonry construction, such as low construction efficiency and low construction quality [15]. It was finished in 2006 and the picture of the jobsite during RFC construction are shown in Fig.10. The masonry wall was chosen as the formwork of RFC in this project, since it could satisfy the requirement of RFC construction and had already been successfully used in the in-situ test. As Fig.10 shows, rock blocks were transported to the working space by dump truck, and then were constructed to rock-block mass by excavators and bulldozers in the working space. According to the design, SCC with the index of C15 was used in RFC construction. Since the batching and mixing plant was set up near the dam heel, SCC was transported to the working space directly by tower crane and bucket after mixed in the plant, and then casted on the surface of rock-block mass. Since there was no need for vibro-compaction in the RFC execution, there were only up to 6 labors in the working space to assist by cleaning the working space, building masonry wall, and so on. The employment of RFC should offer great simplification of execution and speed up the construction of more massive concrete projects. 3.2 Backfill Construction Besides used in dam construction, RFC was also been employed in backfill construction in hydraulic engineering Gully backfill in Baoquan Pumped-storage Project After finishing the RFC construction on part of the auxiliary dam at Baoquan, most of the engineers involved, including those with the owners, designers and constructors, knew more about the benefits of RFC and came to an agreement on using RFC instead of conventional vibro-compacted concrete in the gully backfill project. The total volume of the backfill project was 130,000m 3, and approximately 50,000m 3 of them was constructed with RFC, finished in The designed index of conventional vibro-compacted concrete was lower than that in the auxiliary dam, and as a result, SCC with the index of C10 was used here to construct RFC. The transportation of rocks and construction of rock-block mass was similar to that in the auxiliary dam. Due to the large working space and significant drop in elevation at the particular location on the site, as shown in Fig.11, a chute was used to transport SCC from mixer truck to placement area and an excavator was used to pour SCC in the working space. And the statistics data suggested that the highest construction intensity of RFC was 1,000m 3 per day, which was as twice as that of conventional concrete. 622

9 Figure 11: Pour SCC by Chute and Excavator in Gully Backfill Project 3.2.2Caisson backfill in Xiangjiaba Hydropower Project In 2007, RFC was also employed in the caisson backfill in Xiangjiaba hydropower project, which would be the third largest hydropower station in China [16]. The open caisson group, composed by 10 caissons with a total volume of 80,000m 3, was the largest caisson group in China, and the maximum depth of caisson was 54.7m. Since the caisson backfill construction was hard to construct by conventional concrete due to the difficulty of concrete transportation to more than 57m down to the bottom of caisson and vibro-compaction executed by labors at the bottom, the DRFC construction technology was introduced to solve these practical problems. In total, approximately 70,000m 3 of RFC had been constructed in the caisson backfill project and it was finished in the end of The picture of DRFC construction at the jobsite of Xiangjiaba caisson backfill project was shown in Fig.12 and the working space of 9# caisson was shown in Fig.13. SCC was transported from batching and mixing plant by mixer trucks and then cast into caisson directly. And then the construction of rock blocks was similar to that of SCC, transported to the working space by dump trucks and bulldozers and then dumped into the caisson containing SCC. SCC and rock blocks could be cast into caisson at the same time, which leads to a great speed up of the construction. Pictures of caisson in different periods of construction are shown in Fig.14. By employing RFC, the backfill construction of one caisson could be finished in one week, while the designed construction period backfilled with conventional concrete was about 20~30 days. 623

10 SCC production SCC transportation Rock blocks preparation Rock blocks transportation Workspace of 9# Caisson Rock blocks pouring SCC casting Figure 12: DRFC Construction at the Jobsite of Xiangjiaba Caisson Backfill Project Figure 13: The Workspace of 9# Caisson Under DRFC Costruction Before backfilled Under backfilled After backfilled SCC Figure 14: Pictures of Caisson in Different Periods of Construction 3.3 Cause for Employment of RFC With the benefits of RFC known by more and more engineers, the practical application of RFC has been accelerated in China. Currently, there are immediate causes for the employment of RFC in practical structures. These needs can be summarized as follows [17] : (1) Using low unit cement content in the composite material results in low heat of hydration, which makes it much easier to ensure temperature control; 624

11 (2) Simplifying the placement of RFC by allowing for the use of general purpose machinery, eliminating the surface roughening process and also allowing for continuous pouring of SCC, all contribute to faster construction activities and shortening of the overall construction period; (3) Eliminating the need to vibrate concrete by using SCC results in compaction being ensured independent of the quality of construction work; (4) Simplifying the aggregate production and concrete mixing machinery contributes to cost reduction; (5) Using the rock-block mass as the skeleton of concrete results in relatively little drying and shrinkage; and, (6) Reducing noise as well as energy consumption contributes to lower emissions of the greenhouse gas (GHG) carbon dioxide (CO 2 ), and also sulphur dioxide (SO 2 ). 4. Construction System of RFC With the adoption of SCC and large scale rock blocks in RFC construction, there is no need for any kinds of vibro-compaction during execution. The construction systems previously based on conventional vibro-compacted concrete could be greatly improved, and a new construction system for RFC was proposed. As illustrated in Fig.15, it is composed by 6 parts and explained as follows: (1) Preparation of rock blocks Rubble and cobble stone with a size not less than 30cm are all permitted to use as blocks of rock during the RFC construction. (2) Clean working space and rock blocks Before RFC construction, the working space and rock blocks should be cleaned first, and the requirements of conventional concrete dam construction need to be satisfied. (3) Formwork Comparing with conventional vibro-compacted concrete, RFC needs a more stable, stiffer and closer formwork, due to the high deformability of SCC. However, if there is no special requirement in term of visual appearance, a 1.5m high and 30cm thick stone-wall also could be used as form. And the use of stone wall as form is very effective to ensure the stability, stiffness and closure without the need of form removal. (4) Construct rock-block mass in working space The working space needs to be filled with the prepared rock blocks in a manner of natural placement or packing. The thickness of rock-block mass should be less than 1.5m due to the fluidity limitation of SCC. It should be noted, though, that if laborers are employed to assist the packing process then the available void space of rock-block mass could be reduced with consequent further benefits for financial cost reduction as even less SCC is needed. (5) SCC production and placement It is recommended that SCC should be mixed at batching and mixing plant to ensure that the necessary characteristics of the concrete are maintained, such as self-compactability at fresh stage and the required compressive strength after hardening. The transportation and pouring of SCC should be finished in ninety minutes after mixed. General purpose 625

12 machinery such as pump, excavator and bucket could be used to place SCC, though pumping is proposed. (6) Continuous pouring and cyclic construction Cyclic activity is possible given fast speed of construction: after finishing the first lift of RFC, the next cycle of construction could be carried out and should be finish in the initial setting time of the first lift. Generally speaking, the initial setting time of SCC is four hours. As a result, the continuous pouring and cyclic activities using RFC could greatly speed up construction work. Back to step 1 and start construction of the ext lift 1. Rock blocks preparation: quarried blocks with the grainsize larger than 30cm. Transport to jobsite 2. Wash rock blocks and clean working space, to satisfy the requirement of dam concrete Figure 15: Construction System for RFC 3. Formwork 4.Fill the working space with rock blocks 6. Pump SCC into working space and finish a lift of RFC construction. SCC mix-proportion designing 5. SCC Production and transportation to placement area However, with regard to DRFC construction, it is only to exchange the position of (4) and (5). It should be noted that the minimum thickness of SCC casted first would be determined by the drop distance of rock blocks, and it was 2.0m in the DRFC construction at Xiangjiaba. 5. Prospect of RFC in Future Since the properties of RFC in different stages have already been studied in both the laboratory and the jobsite, and the construction systems for different types of RFC have also been established base on the practical applications, the main obstacles for the wide use of RFC can be considered to have been solved. The next task is to promote the rapid diffusion of this potential technology employed in more massive concrete projects. And rational training and qualification systems for engineers should also be established. There are also plans to build several more RFC gravity and arch dams, with heights in the 30m-50m range, within the next few years in China, such as, a 44.9m high arch dam at the Shilonggou project, in Guizhou province, and a 38.3m high gravity dam at the Qingyu reservoir project, in Shanxi province. However, the long-term behaviors of RFC shouldn t be ignored. In the next few years, areas to be investigated include drying, shrinkage upon 626

13 hardening, and freeze-thaw resistance of large-scale specimens. Following those areas of further investigation, a new integrated system of RFC from design to maintenance would be established and it is anticipated that RFC would be used in some higher dams, of approximately 100m in height in the near future. REFERENCES [1] Okamura, H. and Ouchi, M. Self-Compacting Concrete. Journal of Advanced Concrete Technology, 1(1)(2003)5-15. [2] Ouchi, M., Sakai, E., Sugiyama, T., Mitsui, K., Shindo, T., Maekawa, K. and Noguchi, T. Self-Compacting Concrete in Japan. Proceeding of 8th International Symposium on Utilization of High-Strength and High-Performance Concrete, Tokyo, 2008, [3] Leemann, A. and Hoffmann, C. Properties of Self-Compacting and Conventional Concrete - Differences and Similarities. Magazine of Concrete Research, 57(6)(2005) [4] Jin, F. and An, X., Construction Method of Rock-Filled Concrete Dam. Chinese Patent No. ZL (in Chinese). [5] Jin, F., An, X., Obara, T., Meada, M. and Zhou, H., Construction Method of Conventional Rock-Filled Concrete (in Chinese). [6] An, X., Jin, F., Obara, T., Meada, M., Zhou, H. and Okamura, H., Construction Method of Dump-type Rock-Filled Concrete. Chinese Patent No (in Chinese) [7] Zhou, H., Liu, Q., An, X. and Jin, F. Application of Rock-filled Concrete Technology in Highway Project of Xinjiang. Science and Technology Information, 25(2008)40-41.(in Chinese) [8] Jin, F., An, X., Shi, J. and Zhang, C. Study on Rock-filled Concrete Dam. Journal of Hydraulic Engineering, 36(11)(2005) ( in Chinese) [9] An, X., Jin, F. and Shi, J. Experimental Study of Self-Compacting Concrete Filled Pre-Packed Rock. Concrete, 1(2005)3-6.(in Chinese) [10] Huang, M., Zhou, H., An, X. and Jin, F. A Pilot Study on Integrated Properties of Rock-Filled Concrete. Journal of Building Materials, 11(2)(2008) (in Chinese) [11] SL , Design Specification for Concrete Gravity Dams, 2005, 37 pages. (in Chinese) [12] Song, D. and Liu, J. Application of Self-Compacted Rockfill Concrete in Baoquan Pump-Storage Power Station. Water Power, 33(9)(2007) (in Chinese) [13] China Gezhouba (Group) Corporation. Experimental Report of RFC.2007.(in Chinese) [14] Huang, M., Zhou, H., An, X. and Jin, F. The Environmental Impact Assessment of Rock-Filled-Concrete Technology in Dam Construction. Proceeding of Hydropower 2006, Kunming, 2006, [15] An, X. and Sato, F., The Application of Self-Compacting Concrete in Dam Construction; The Activity of Japanese Technology in China. Nikkei Construction, 6(2006) (in Japanese) [16] Huang, M., An, X., Zhou, H. and Jin, F. Rock-Fill Concrete, A New Type of Concrete. In: Walraven, J.C. and Stoelhorst, D. (eds.), Tailor Made Concrete Structures, Proceeding of International fib Symposium 2008, Amsterdam, 2008, [17] Huang, M., An, X., Zhou, H. and Jin, F. Rock-Filled Concrete development, investigations and applications. International Water Power and Dam Construction, 60(4),pp:20-24,