Rock-Filled Concrete - development, investigations and applications

A new type of concrete has been under development in China called Rock-Filled Concrete (RFC). Based on Self-Compacting Concrete (SCC) technology, engineers have been working on the RFC system since it was first developed in 2003. It has been employed in a number of hydraulic engineering structures already, in China.

RFC is produced by pouring ready-mixed SCC into forms containing large blocks of rock at least 30cm wide, which are typically obtained easily near dam sites without the need for crushing. The SCC fills the void space between the blocks due to its good fluidity, and thereafter the mix sets to form the RFC mass (see Fig.1).

The properties of RFC are affected by the gradation of the rock blocks, unit rock block content, unit SCC content, and so on. To study the properties of RFC, tests carried out included compaction, static load, permeability and in-situ. In addition, cost and environmental impact assessments of RFC have also been carried out to evaluate the social impacts.

From the results of the tests, it has been confirmed that the RFC can satisfy the required properties as a material for dam structures. In addition, the results of the assessments show that employment of RFC could lead to cost reduction and improved environmental protection.

Development of RFC

What is RFC

Since its development in 1988, in Japan, SCC has been used in many practical structures in the world[1],[2]. However, compared with conventional vibro-compacted concrete, SCC displays a lower E-modulus, higher shrinkage, a greater rate of creep and is also more costly in seeking to achieve the same compressive strength[3]. Thus, SCC has seldom been used as a standard concrete in dam construction. 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, engineers in China began to develop RFC technology as a new type of concrete for structures, especially large-scale structures such as dams[4]. This paper mainly describes the new RFC technology that is being comprehensively studied and field-tested.

In general, as shown in Fig.2, there are two main sub-processes in RFC construction: filling the working space with large-scale blocks of rock (again, generally of size larger than 30cm in width) to form a rock-block mass; then, either pump SCC into the working space or pour it directly on to the surface of rock-block mass, and thereafter the SCC flows down to fill all the void spaces by merit of its own weight and given its good fluidity and high segregation resistance. It should be noted that the thickness of rock-block mass is proposed to be less than 1.5m to ensure effective compactness after the RFC sets.

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. Generally, using larger aggregate in concrete production is an effective way to reduce the unit cement content, and fully-graded aggregate is usually used in concrete for dam construction. The maximum grain-size of the fully-graded aggregate is 15cm because of the limited capacities of concrete mixers and vibrators. With regards to RFC, 55%-60% of the space is filled by rock blocks 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. As such, the unit cement content of RFC with a strength grade of C15 is only 80kg-90kg/m3 [5].

Construction processes of RFC

The RFC construction process is mainly composed by six steps, which are shown in Fig.3 [6].

Preparation of rock blocks, and Clean Working Space

Rubble and cobble stone with a size not less than 30cm are all permitted to be used as blocks of rock in RFC construction, but the requirements of conventional concrete dam construction still need to be satisfied.

Formwork

Compared with conventional vibro-compacted concrete, RFC needs a more stable, stiffer and closer formwork due to the high deformability of SCC. However, if there are no special requirements in terms of visual appearance, a 1.5m high and 30cm thick stone wall could be used as formwork. And, the use of stone wall as formwork is very effective to ensure the stability, stiffness and closure.

Construct rock-block mass in working space

The working space needs to be filled with prepared rock blocks in a manner of natural placement, or packing. The thickness of the rock-block mass should be less than 1.5m due to the fluidity limitation of SCC. It should be noted, though, that if labourers 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.

SCC production and placement

It is recommended that SCC should be produced at a mixing and batching 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 setting periods. The transportation and placement of SCC should be finished within 90 minutes of mixing. General purpose machinery such as pump, excavator and bucket could be used to place SCC, though pumping is proposed.

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.

Benefits of RFC

Since the development of RFC in 2003, the concrete has already been used in two hydraulic engineering projects and is planned to be used in a few more projects. The main reasons for the employment of RFC can be summarised as:

• Using low unit cement content in the composite material results in low heat of hydration, which makes it much easier to ensure temperature control;

• 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;

• Eliminating the need to vibrate concrete by using SCC results in compaction being ensured independent of the quality of construction work;

• Simplifying the aggregate production and concrete mixing machinery contributes to cost reduction;

• Using the rock-block mass as the skeleton of concrete results in relatively little drying and shrinkage; and,

• Reducing noise as well as energy consumption contributes to lower emissions of the greenhouse gas (GHG) carbon dioxide (CO2), and also sulphur dioxide (SO2).

Investigations of RFC

Four types of lab tests have been carried out with RFC. The results show that SCC can fill the space between rock-blocks well to form a compaction structure, and the strength and durability of RFC after hardening can meet the demands of dam body.

Filling ability tests

Experimental studies to evaluate the ability of SCC to fill the voids in the rock-block mass, shown in Fig.4, were carried out in an acrylic form of internal dimensions 200cm by 50cm by 50cm [7]. The rock-blocks were washed during preparation, the flow area of SCC in slump test was 65cm by 65cm and the strength after hardening was 50MPa.

It was found that SCC did fill the void space effectively in rock-block mass. After the RFC specimen set, it was also found that good compactness was obtained. The results showed that SCC flowed into all the voids between blocks in the section of rock-block mass, and it was concluded that the fluidity of SCC was good enough to fill the three-dimensional space in a rock-block mass. And, furthermore, the results can be extrapolated to larger dimensions; the dimensions of void space would be greater in rock-block mass of larger dimensions.

Static bending tests

The cross-section of the RFC specimen was obtained after the static bending test is shown in Fig.5. It can be seen that the voids in rock-block mass are full of SCC where aggregate is distributed evenly. Also, all rock-fill fragments or blocks in the section are broken, which indicates that the bonds between rock-blocks and SCC are quite strong.

Permeability tests

Permeability is one of the most important potential weaknesses in dam construction. Structural weaknesses, such as cold joints and hot joints created by the limited thickness of concrete lifts, can affect permeability of the dam.

In order to study the permeability properties of these weakness planes, a large RFC block of dimensions 2.0m by 1.0m by 1.8m, and with cold and hot joints, was constructed in the laboratory and is shown in Fig.6. The Chinese standard sizes for a specimen in a permeability test are: top diameter 17.5cm; bottom diameter 18.5cm; and, length 15cm. However, it was difficult to cut the standard specimens from the RFC block, as shown in Fig.7, and specimens of dimensions 12cm by 12cm by 15cm were cut from the block and then fixed to be the standard test size by mortar with much higher strength of C60 to ensure better permeability.

Three cases (six specimens per case) were tested in the experiment, which were cut from normal RFC, hot joint area and cold area, respectively. The results are shown in Table.1. The average permeation resistance index of normal RFC, hot joint area and cold joint area were W35, W31 and W14, respectively, which indicated that the permeability of RFC block is high enough to satisfy the requirement of the dam concrete (W2-W10) in hydraulic engineering.

Compression strength tests

It is proposed that the dimension of specimen for compressive strength test should be three times that of the biggest aggregate size. However, the size of biggest rock-block would be 15cm at least in RFC, and the dimensions of specimens for compression strength test should be 45cm by 45cm by 45cm at least. Due to the limitation of test equipment, it is difficult to test the specimen of such a big scale, then nine prismatic specimens of dimensions 15cm by 15cm by 30cm were cut from the normal RFC obtained above for axial compression strength tests. The results of the compressive tests are shown in Fig.8, and show the strengths of RFC and SCC.

The average strength of SCC with the same size used in the RFC construction was tested to be 13.2MPa, and the average axial compression strength of nine RFC specimens was 16.7MPa, which is 1.27 times the value for SCC. The results suggest that the strength of RFC is higher or at least not less than that of SCC used in the RFC construction.

In-situ tests

After the lab tests mentioned above, four kinds of in-situ tests, which were composed of apparent density test, core compression test, water pressure test, and temperature rise of hydration heat test, were carried out at the jobsites of Baoquan pumped-storage project, in Henan province, and Xiangjiaba project, in Sichuan province. RFC performed satisfactory in these in-situ tests, the results of which are shown in Table.2[8].

Applications of RFC in practical engineering

Based on all the tests outlined, RFC has been applied at the following projects:

Gravity dam in a reservoir project in Beijing

RFC was first used in a gravity dam in a reservoir project in Beijing, as shown in Fig.9. The 13.5m high, 2,000m3 gravity dam was finished in 2005.

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.

Auxiliary dam, Baoquan pumped storage project

Another application of RFC was in part of the auxiliary dam of the upper reservoir of the Baoquan pumped storage project, in Henan province, in 2006. The dam was designed as a 50,000m3 masonry gravity structure of 42.6m height and 196m in crest length. However, the top 3m of the dam, with a volume of 3,000m3, was constructed of RFC to solve the problems in the practical construction, such as low construction efficiency and low construction quality[4]. It was completed in 2006.

Fig.10 and Fig.11 are views of the Baoquan jobsite during construction. Dump trucks were used to transport rock blocks to the working space, and excavators and bulldozers were used to construct the rock-block mass in the working space. The batching and mixing plant was built at the lowest level of the auxiliary dam, thus the mixed SCC was transported from plant to placement area directly by tower crane and bucket. There were up to six workers in the working space to assist by cleaning the working space, building stone walls, and so on.

In general, since there is no need for vibro-compaction in the RFC execution, the employment of the material should offer great simplification of execution and speed up the construction of more massive concrete projects.

Gully backfill 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.

In this project, SCC with the index of C10 was used to construct RFC to reduce the cost, since the designed index of conventional concrete is lower than that in the auxiliary dam. 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.12, 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.

In total, approximately 40,000m3 of RFC was placed at Baoquan.

Cost and environmental assessment

Generally speaking, the cost of materials is one of the most important considerations on projects. After completing the RFC work at Baoquan, a cost assessment was done with the contractor’s data.

With an assumed void ratio of 45% for the RFC on the project, a cost of Rmb160-Rmb180/m3 for SCC, and of Rmb40/m3 for the rock blocks, the cost of materials was calculated to be approximately Rmb112-Rmb121/m3. If we assume that the cost of materials is 70% of total cost, then the total cost of RFC may be approximately Rmb160-173/m3.

As mentioned above, dams should be constructed with more attention paid to reducing environmental impacts in future. With regards to the environmental aspects of RFC, studies on the environmental impact assessment of RFC, conventional vibro-compacted concrete and RCC (Roller Compacted Concrete) were also carried out, and the results are shown in Table.3 and Fig.13[10].

Compared with conventional concrete and RCC, the innovative construction method of RFC leads to a great reduction in workload of aggregate crushing in materials manufacturing phase, as well as that of concrete mixing, transportation and placing. In addition, it needs no vibro-compaction and surface roughening in the concrete placing phase. The study shows 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 CO2 emission and energy consumption.

Conclusion

The concept of RFC was proposed in 2003, in China. Studies have strongly shown that RFC performs satisfactorily in early age and after hardening, which could satisfy the requirements of hydraulic engineering projects, especially for a massive concrete construction project like a dam. In addition, there are the benefits of simple execution, low heat of hydration and lower cost.

Besides the project applications mentioned above, RFC has been used on other sites. By the end of 2007, approximately 70,000m3 of RFC as been constructed in the Xiangjiaba caisson backfill project, in Sichuan province. The caisson was difficult to construct using conventional concrete as the material had to be transported more than 50m down to the bottom of the structure.

In 2008, RFC is about to be used to build a 44.9m high arch dam at the Shilonggou project, in Guizhou province, and for a 38.3m high gravity dam at the Qingyu reservoir project, in Shanxi province. 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.

Based on the applications of RFC so far, the next task is to investigate the long-term behaviours of the concrete. Areas to be investigated include drying, shrinkage upon hardening, and freeze-thaw resistance of large-scale specimens of more than 35cm in size. In addition, studies are required for further optimisation of RFC construction methods to establish a new integrated system.

Following those areas of further investigation, it is anticipated that RFC?would be used in some higher dams, of approximately 100m in height, in future.

Huang, M., An, X., Zhou, H. & Jin, F.

State Key Laboratory for River Dynamics and Hydraulic Engineering, Tsinghua University, Beijing, China