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ASTM Puts Self-Consolidating Concrete to the Test
by Martin Vachon
ASTM Committee C09 on Concrete and Concrete Aggregates has begun work toward developing standards for self-consolidating concrete. Martin Vachon describes how a special type of concrete self-consolidates without the use of vibration, its market share around the world, and the work of C09 in writing standards for it.
A Little Bit of History
Self-consolidating concrete (SCC) or self-compacting concrete, as it’s sometimes known, arrived as a revolution in the field of concrete technology. The concept was proposed by Professor Hajime Okamura of Kochi University of Technology, Japan, in 1986 as a solution to the growing durability concerns of the Japanese government. During his research, Okamura found that the main cause of the poor durability performances of Japanese concrete in structures was the inadequate consolidation of the concrete in the casting operations. By developing concrete that self-consolidates, he eliminated the main cause for the poor durability performance of their concrete. By 1988, the concept was developed and ready for the first real-scale tests.
The first paper on SCC was presented at the second East-Asia and Pacific Conference on Structural Engineering and Construction (EASEC-2) in 1989, followed by another presentation at an Energy Diversification Research Laboratories (CANMET)/American Concrete Institute (ACI) meeting in 1992. (1) In 1997, a RILEM committee (TC 174) on SCC was founded. Today, SCC is studied worldwide with papers presented in almost every concrete-related conference.
How Does It Work?
Some people may say that concrete that self-consolidates on its own weight without the use of vibration has been used for years in the construction field. But at that time fluidity was obtained by adding water, superplasticizer, or a combination of both. In addition, that earlier type of concrete was not used in civil engineering structures with normal durability requirements because they either contained too much water or were not stable and led to unacceptable heterogeneity in the structure.
Then what is different with SCC? Even if there is still no official definition of SCC in the United States, the concept could be defined as follows:
A self-consolidating concrete must:
• Have a fluidity that allows self-consolidation without external energy,
• Remain homogeneous in a form during and after the placing process, and
• Flow easily through reinforcement.
To achieve these performances, Okamura redesigned the concrete mix design process. His mix design procedure focused on three different aspects:
• Reduction of the coarse aggregate ******* in order to reduce the friction, or the frequency of collisions between them, increasing the overall concrete fluidity;
• Increasing the paste ******* to further increase fluidity;
• Managing the paste viscosity to reduce the risk of aggregate blocking when the concrete flows through obstacles.
Because conventional concrete is placed using external energy, there is no need for specific rheological characteristics. As a matter of fact, the intensity of energy applied in the consolidation process is adjusted to compensate most plastic property variation. That is probably why, up until now, a simple slump window requirement was enough, in conjunction with good consolidation practices, to lead to dense concrete elements in most applications. With SCC, other rheological requirements are needed to obtain good consolidation since no additional placing operations will compensate for any lack of rheological performances.
(In rheological terms, even though a significant amount of research tends to show that SCC’s viscosity varies with the shear rate and acts as a pseudoplastic material, SCC is often described as a Bingham fluid (viscoelastic) where the stress/shear rate ratio is linear and characterized by two constants—viscosity and yield stress (Table 1). This latter model will be used in the following paragraphs because of its simplicity.)
Back to the performance-based definition of SCC, the self-consolidation is mainly governed by yield stress, while the viscosity will affect the homogeneity and the ability to flow through reinforcement (Table 2). As the SCC viscosity can be adjusted depending on the application, the yield stress must remain significantly lower than other types of concrete in order to achieve self-consolidation.
At this time, various mix design methods have been proposed in order to obtain the unique rheological performances of SCC and may lead to different mix designs for a given application. However, the performance criterion remains the only objective means to evaluate the adequacy of an SCC mix. Since criterion relates to measurements, test procedures need to be developed. ASTM Committee C09 on Concrete and Concrete Aggregates undertook this task in June 2001 in its Subcommittee C09.47 on Self-Consolidating Concrete. Exploratory work has looked at existing SCC characterization procedures around the world. Most of the work still has to be completed, but with over 40 participants, this team should be able to do it in a timely fashion
Several tests have already been developed worldwide to characterize the performance of a fresh self-consolidating concrete. A few of them are briefly presented in the following paragraphs. The dimensions may vary from one country to the other.
A rheometer is a device that applies a range of shear rates and monitors the force needed to maintain these shear rates in a plastic material. The force is then converted to stress. A few concrete and mortar rheometers are available on the market and have been and are still used for measuring the yield stress, viscosity and other rheological characteristics of SCC. They are of tremendous help in understanding SCC’s behavior.
However, this type of equipment is fairly expensive and not easy to use on a job site. Therefore, numerous test methods have been developed for SCC with ease of use in mind. None of these test methods measures yield stress or viscosity, but they all simulate more or less real-scale casting environments.
This procedure, which is readily used today, relies on the use of the Abram’s cone (Figure 1). The cone is filled in one layer without rodding and the diameter, instead of the slump of the concrete sample, is measured after the cone has been lifted . This test is mostly used for evaluating the SCC’s self-compactibility as it mainly relates to its yield stress.
Monitoring the time it takes for the concrete to reach a spread of 500 mm can also be used as an evaluation of SCC viscosity.
These two tests (Figure 2) simulate the casting process by forcing an SCC sample to flow through obstacles under a static pressure. The final height H and H2/H1 for the U-box and the L-box respectively are recorded. They provide indication on the static and dynamic segregation resistance of an SCC as well as its ability to flow through reinforcements. They are frequently used in the field as acceptance test methods.
V-Funnel and Orimet
By monitoring the time it takes for the SCC to flow through an orifice under its own weight, these two test methods give an indication of its viscosity. Both tests are used in the field and are sometimes used as acceptance tests. The V-funnel is shown in Figure 3.
This apparatus is used to force the SCC to flow through reinforcement (Figure 4). It must be used in conjunction with an Abrams cone or the Orimet setup. The concrete flows from the inside to the outside of the ring. The size and the spacing between the bars can be adjusted to simulate any reinforcement configuration. The differences between the spread with and without the ring or the height difference between the concrete inside and outside the ring is measured.
German studies showed that with a bar spacing equivalent to 2.5 times the maximum aggregate size, the spread difference with and without the J-ring must be smaller than 50 mm.
This procedure is used to evaluate the resistance to static segregation of an SCC. A sample of concrete is poured over a 5-mm sieve and the amount of mortar passing through the sieve in a two-minute period is measured. The French Civil Engineering Association published a complete procedure (in French and English) in July 2000. (2)
SCC in the Field
SCC has already been used in several countries. In Japan, major construction projects included the use of SCC in the late ’90s. Today, in Japan, efforts are being made to free SCC of the “special concrete” label and integrate it into day-to-day concrete industry production. Nevertheless, the SCC market share is still under one percent in ready-mixed concrete (RMC) as well as precast concrete (PC). In Sweden, the market share was at three percent in RMC and PC in 2000, and was expected to double in 2001. Housing and tunneling as well as bridge construction for the Swedish National Road Administration were the main areas of use for SCC. In the Netherlands and Germany, the precast industry is mainly driving the development of SCC, with an expected six percent of market share in 2001 in Netherlands.
In the United States, the precast industry is also leading SCC technology implementation. The Precast/Prestressed Concrete Institute (PCI) is very active, with the creation in 2002 of a Fast Team. This team’s task is to draw recommendations on the use of SCC in precast/prestressed operations by October 2002. Meanwhile, the author estimates that the daily production of SCC in the precast/prestressed industry in the United States will be 5000 m3 in the first quarter of 2002. Furthermore, several state departments of transportation in the United States (23 according to a recent survey) are already involved in the study of SCC.
With such a high level of interest from the construction industry, as well as manufacturers of this new concrete, the use of SCC should grow at a tremendous rate in the next few years in the United States. However, even if it is made from the same constituents the industry has used for years, the whole process, from mix design to placing practices, including quality control procedures, needs to be reviewed and adapted to make the most of this new technology and prevent growing pains
HOW to Test SCC?
Several test procedures have been successfully employed to measure the plastic properties of SCC. The slump flow test (see Figure 1), using the traditional slump cone, is the most common field test and is in the process of being standardized by ASTM. The slump cone is completely filled without consolidation, the cone lifted and the spread of the concrete is measured. The spread can range from 18 to 32 inches (455 to 810 mm). The resistance to segregation is observed through a visual stability index (VSI). The VSI is established based on whether bleed water is observed at the leading edge of the spreading concrete or if aggregates pile at the center. VSI values range from 0 for “highly stable” to 3 for unacceptable stability.
During the slump flow test the viscosity of the SCC mixture can be estimated by measuring the time taken for the concrete to reach a spread diameter of 20 inches (500 mm) from the moment the slump cone is lifted up. This is called the T20 (T50) measurement and typically varies between 2 and 10 seconds for SCC. A higher T20 (T50) value indicates a more viscous mix which is more appropriate for concrete in applications with congested reinforcement or in deep sections. A lower T20 (T50) value may be appropriate for concrete that has to travel long horizontal distances without much obstruction.
The U-Box and L-Box tests are used for product development or prequalification and involve filling concrete on one side of the box and then opening a gate to allow the concrete to flow through the opening containing rebar. The Jring test is a variation to the slump flow, where a simulated rebar cage is placed around the slump cone and the ability of the SCC mix to spread past the cage without segregation is evaluated. The U-box, L-box and J-ring tests measure the passing ability of concrete in congested reinforcement. Another test being standardized is a column test which measures the coarse aggregate ******* of concrete at different heights in a placed columnar specimen as an indication of stability or resistance to segregation.
HOW to Order or Specify SCC?
When ordering and/or specifying SCC, consideration must be given to the end use of the concrete. Ready mixed concrete producers will generally have developed mixture proportions based on performance and applications. The required spread (slump flow) is based on the type of construction, selected placement method, complexity of the formwork shape and the configuration of the reinforcement. ACI Committee 237 is completing a guidance document that will provide guidelines to select the appropriate slump flow for various conditions. The lowest slump flow required for the job conditions must be specified. This will ensure SCC can be attained easily with required stability and at the lowest possible cost. The hardened concrete properties should be specified by the design professional based on structural and service requirements of the structure. For the most part, hardened concrete properties of SCC are similar to conventional concrete mixtures. Based on the requirements of each project, SCC concrete designs can be submitted by the producer only after specification provisions regarding the performance of the concrete in its plastic and hardened state are clearly defined