Table 2. Empirical mixture proportioning methods for SCC
| Proposed by | Maximum CA volume ratio | Maximum proportion of sand in mortar, percent | Paste composition (w/p ratio) | Remarks |
| Okamura and Ozawa27 | 0.5 | 40
(empirical) | Mortar flow and V-funnel tests | Originally developed using moderate heat or belite rich cement |
| Edamatsu et al.28 | 0.5 | Determined by V-funnel test using standardised coarse aggregate | Mortar flow and V-funnel tests | Enables determination of stress transferability of mortar |
| EFNARC7 | 0.5 – 0.6 | 40 – 50 percent
(empirical) | Mortar flow and V-funnel tests | Allows more freedom in coarse aggregate content |
Rheology-based methods
Conventional methods of measuring concrete workability such as the slump test provide a broad an indication of the amount of work required to compact the concrete mixture. With the advent of more fluid concretes (pumpable concrete, self-levelling concrete), it was necessary to measure the flow properties of concrete. The rheological methods of characterization of workability are described on the Rheology page.
Particle packing models
Particle packing has been suggested by some researchers as a scientific approach to mixture proportioning of concrete29-32. A review of the common particle packing methods is provided elsewhere15. The concept of particle packing is borrowed from the ceramic industry. Here, the principle is to minimise the void content of a dry granular mixture of all ingredients (including cement, fly ash and microsilica). This is done by the choice of appropriate sizes and gradation of aggregate. While some models adopt a discrete particle-size approach, others assume the granular mixture to possess a continuous gradation. These two models are discussed next.
Discrete models
These refer to packing of systems containing two or more discrete size classes of particles. In this type of model, the coarsest particles form the base skeleton and its voids are filled by smaller particles and these in turn by finer particles and so on, in the order of decreasing particle size. The fundamental assumption of the discrete model is that each class of particles will pack to its maximum density in the volume available. The discrete models may be classified as binary, ternary and multimodal mixture models.
Sedran and de Larrard14 demonstrated the use of a discrete particle model (compressible packing model) to design self-compacting concrete mixtures (without VMA). This model optimized the granular skeleton of concrete, and used the results from rheology measurements on fresh SCC, filling ability (using L-box test), and resistance to segregation. Interrelationships between these parameters and the packing density of the skeleton were then established. For example, mathematical functions linking the viscosity and yield stress with packing density were derived; the confining effect of rebars was simulated by the boundary wall effect in packing. The proportions of fresh SCC were then found by using software which optimised the mixtures with respect to its properties and cost.
Continuous models
Continuous approach assumes that all sizes are present in the particle distribution system, that is, it can be described as a discrete approach having adjacent size classes ratios that approach 1:1 and no gaps exist between size classes. Andreassen worked on the ideal size distribution for maximum particle packing with a continuous approach and proposed the “Andreassen equation”33.
The Andreassen equation connects the percentage passing for a particular size to the maximum particle size in the system through an exponent ‘q’. The smooth line in Fig 6 shows the resulting distribution, or the ‘ideal packing curve’. Empirically, the exponent q in the Andreassen equation can be varied from 0.21 to 0.37 depending upon workability requirements. If the exponent increases, it means an increase of coarse materials, and if it decreases, the amount of fine materials is increased. As the water demand and water holding capacity of the mixture is controlled by the volume of fines, this exponent gives a reasonable basis for adjusting the dry materials, so that the required flowability is reached with minimum water demand. The exponent value q = 0.25 to 0.3 may be used in conventional concretes depending upon the slump range. For highly flowable mixes like self-compacting concretes, q < 0.23 may be used15.
This model has been developed into easy-to-use software called LISA, which can be downloaded from website [www.silicafume.net]. This model, as most others, is based on the assumption that the particles are spherical. The validity of this model for angular crushed aggregates needs to be ascertained in the laboratory. Fig 2 shows the actual overall particle size distribution with respect to the ‘ideal’ gradation (smooth curve) as calculated by the software for a q value of 0.22. The SCC obtained using this approach had a slump flow of 69 cm15.
Fig 2 Ideal grading curve for q = 0.22 and actual overall particle size distribution for SCC15
Particle packing in combination with paste rheology can be effectively used for the design of SCC, as shown in Table 3. The use of particle packing suggested in this table is from a continuous model approach.
Table 3. A combined effect for proportioning SCC using the principles of particle packing and rheology
| Property | Direction of change (with respect to normal concrete) | Rheological control | Control by particle packing |
Yield stress,
Ï„o | Usually decrease | Use superplasticisers |
Use low value of ‘q’
(< 0.23) |
Plastic viscosity,
µ | Usually increase | - |
Use low value of ‘q’
(< 0.23) |
| Dynamic control of segregation | - | Use pseudoplastic VMA | - |
Statistical methods
Khayat et al.34 proposed a mixture design procedure based on statistical models using a factorial design of experiments. The advantage of such an approach is that one can evaluate the effects of critical factors using minimum number of experiments. Another advantage is that only an approximate idea of the variables that affect the response is required, and not the exact relationships.
In Khayat’s study, five parameters – cementitious materials content (cm), water-to- cementitious materials ratio (w/cm), HRWRA concentrations, VMA concentration, and volume of coarse aggregate – at five different levels, were chosen. The response variables were the slump flow, relative flow resistance (analogous to torque measurement), and relative torque (viscosity). In addition, the V-funnel time, filling ability, and settlement were also measured. A total of 32 SCC mixtures were prepared to obtain the required relationships.
This method was useful in establishing interrelationships among mixture parameters for performance optimization. Trade-offs among various parameters for the same response were studied, such as: lowering of w/cm and increasing HRWRA dosage, keeping w/cm constant, and changing the cm content and adjusting HRWRA dosage. This model could predict the self-compactability of different mixture designs.
What is the appropriate choice for the design procedure?
Table 4 presents a summary of the common methods for mixture design (based on the review of existing literature presented earlier along with their applicability to conventional concrete and SCC. Although all the methods are applicable to both concretes, it would be ideal if mixture design tables were available for SCC based on guidelines drawn from empirical procedures. A strong support for this reasoning is that there is already a large database of experimental results available from all over the world. Developing design tables for SCC is now a viable proposition.
In the absence of mixture design tables, the question arises whether there could be one generalized method that will be applicable for the design of SCC. Such a method would have to incorporate essential parameters, viz. differences in aggregate morphology and gradation, and cement paste rheology. It is possible that the particle packing models in combination with the rheological models could provide a solution to this problem (see Table 3). However, further studies are necessary demonstrating the use of these models in designing successful SCC mixtures.
Table 4. Summary of mixture design procedures
| Type of concrete | Empirical methods | Rheology-based methods | Particle packing | Statistical design |
| Conventional | Applicable;
design tables available | Difficult to characterise by rheology alone | Applicable;
validation required | Applicable;
not widely used |
| SCC | May be applicable | Applicable - rheological characterization possible | Applicable;
validation required | Applicable |
Test methods for self-compactability
Filling ability, passing ability and stability of mixtures can be considered as the distinguishing properties of fresh SCC35. These requirements are not common to conventional concrete and, therefore, are handled through special tests. These tests should be done carefully to ensure that the ability of SCC to be placed remains acceptable. The flow properties of SCC have been characterized7,36,37. Based on their experience with SCC, researchers have suggested limits on test values. Table 5 lists the common testing methods and recommended values, as drawn from some research articles7,38. Brief descriptions of some of the less common methods, particularly the three segregation potential tests, are described below.
Self-compactability tests
Flowability is measured mostly using ‘slump flow’ test, which is simple and reliable. An estimate of the viscosity and the ability to parts through the narrow-opening can be obtained using the V-funnel test. However, it is reported5 that a number of factors, in addition to the viscosity, (namely, the deformation capacity (slump flow), size distribution and amount of coarse aggregate, and the shape of coarse aggregate) affect the V-funnel flow time5. These effects have not been quantified, particularly the effect of aggregate shape. As stated earlier, the study of aggregate shape and its influence on various SCC properties could be helpful in improving the scope for SCC with marginally unsuitable aggregates.
Table 5: Summary of common testing methods and limiting test values for SCC
| Property measured | Test method | Material | Recommended values |
| Flowability / Filling ability | Slump flow | Concrete | 650 – 800 mm
Average flow diameter |
| T50 | Concrete | 2 – 5 sec
Time to flow 500 mm |
| V – funnel | Concrete / mortar | 6 – 12 sec
Time for emptying of funnel |
| Orimet | Mortar | 0 – 5 sec
Time for emptying of apparatus |
| Passing ability | U – box | Concrete | 0 – 30 mm
Difference in heights in two limbs |
| L – box | Concrete | 0.8 – 1.0
Ratio of heights at beginning and end of flow |
| J - ring | Concrete | 0 – 10 mm
Difference in heights at the beginning and end of flow |
| Segregation potential | Settlement column test | Concrete | > 0.95 Segregation ratio |
| Sieve stability test | Concrete | 5 – 15% sample passing through 5 mm sieve |
| Penetration test | Concrete | Penetration depth < 8 mm |
Blocking (passing ability) tests
The resistance to blocking of concrete can be assessed by using the L-box test. This test indicates the one-dimensional flowability in a restrained condition (as opposed to slump flow, which shows two-dimensional unrestrained flow). This test is useful in two ways - both blocking and lack of stability can be detected visually. Further modifications in this test could be helpful in analyzing the full flow behaviour of concrete. For example, the size of the opening and its relative distance from the concrete could be varied to obtain a better understanding of the potential for blocking at a lower velocity of flow.
Passing ability of concrete can also be measured using the U-box apparatus, which has obstacles in the concrete flow path similar to those in the L-box test.
Settlement and stability tests
The high flowability of SCC makes the aggregates prone to settlement. Aggregate settlement depends on the viscosity of the cement paste. Tests for settlement39 enable quantification of the effect of mixture proportioning and height of placement on the stability of concrete.
In early stages of SCC development, tendency for settlement was assessed using visual analysis of plane surfaces cut out of hardened concrete. The relative distribution of aggregates in the concrete provided information about its potential for segregation and settlement. Apart from this, there have been some attempts to develop test methods to assess the stability of SCC in the fresh state itself.
Cussigh et al.38 have described three tests to characterise the segregation potential of SCC. These tests - settlement column test, sieve stability test, and penetration test, were found to have acceptable repeatability and sensitivity.
In sieve stability test, a fresh SCC sample is left undisturbed (static condition) for 15 minutes in a bucket. The top layer of the sample is then poured onto a 5 mm sieve, and the mass of the mortar passing through the sieve is determined. Segregation potential is expressed as the ratio between the mass of mortar collected through the sieve and the original mass collected from the top portion.
The settlement column in the second test is a mould of height 400-500 mm, into which fresh SCC is poured. The test involves the collection of concrete samples from the top and bottom parts of this column after a controlled agitation (this simulates an additional disturbance) and settlement period. The segregation potential is expressed as the ratio of the mass of coarse aggregates in the top and bottom parts.
The penetration test measures the segregation potential as the depth of penetration of a standard mass (54g) into the concrete. If segregation is high, then the top part of the concrete would be mainly mortar, and the resultant depth of penetration would be high. For good SCC, penetration should not be more than 8 mm.
Combination of methods
In spite of the large number of test methods, no single method or combination of methods has achieved widespread acceptance. Similarly, no single method has been found which characterises all the relevant workability aspects of SCC, viz., flowability, passing ability, and segregation resistance. Various combinations have been used to evaluate SCC behaviour. For the initial mixture design of SCC, all three workability parameters such as filling ability (flowability), passing ability and stability (segregation resistance) should be assessed. For site quality control, two test methods are generally sufficient to monitor production quality. Typical combinations are slump-flow and V-funnel, or slump-flow and J-ring. In addition, a critical portion of the proposed concrete structure can be tested in a mock-up trial.
Correlation between rheometer-based measurements (of the shear yield stress and plastic viscosity) and the values obtained from the empirical tests can be useful in predicting flow properties. Nielsson and Wallevik40 indicate that the plastic viscosity has a good correlation (almost linear) with the T50 (in the slump flow test) and the flow time in the Orimet and V-funnel tests. Good correlation was also obtained between the slump flow and yield value of the mixtures. Using such analyses, the scientific (rheological) measurements can be related to the empirical measurements. In combination with such understanding, further research that throws light on the connection between the paste and concrete rheology would help in refining the mixture proportioning methods, particularly in setting appropriate limits for the empirically determined values.
It is essential to have an acceptance test for SCC for field applications. An acceptance protocol could be a combination of the above-discussed test methods. For example5, in Japan, the slump flow test, V-funnel test, and the box shape (or U-box) test are used for this purpose. In Sweden, slump flow and L-box test are used as a combination. At present, guidelines for field acceptance test are largely based on experience. It would, however, be of benefit to use a single ‘all-in-one acceptance test’ for characterizing SCC for field applications. Ouchi et al.41 have proposed a simple all-acceptance test for use in the field, which has been used at the construction site of the Osaka Gas LNG tank42. In this test, the testing apparatus is installed between the concrete truck and the pump at the job site. The entire concrete from the mixer truck is passed through this apparatus, which consists of a box with openings (with reinforcing bars as obstacles) on the sides. If the concrete flows through the apparatus, it is considered as self-compactable for the structure. If it gets blocked in the apparatus, it is considered unsuitable.
Table 6 presents a new scheme for classification and use of the SCC test methods. Here, the methods are classified into tests that (i) determine basic rheological properties, (ii) can be used for fixing the proportion of constituents, and (iii) can be used as quality control tests at the jobsite.
Table 6: Classification of SCC test methods
| Basic tests | Tests for adjusting mixture proportions | Tests for quality control |
Rheology
- Shear yield stress
- Plastic viscosity
| Flowability
- V-funnel Passing ability
- U-box
- L-box
- Segregation control
- Settlement column
- Sieve stability
|
- Slump flow and T-50
- Slump flow and J-ring All-in-one acceptance test41
|
Walraven43 indicated that the type of application should determine the properties of SCC necessary for the job. Based on experience, it was found that various consistency classes could be defined using a combination of V-funnel time and slump flow distances. The application – walls, floors, ramps – would then indicate the requirements from these two tests (see Fig 3). In the case of ramps, for example, a V-funnel time of 9 – 25 sec and a slump flow of 470 – 570 mm are suggested. With experience gained from further studies, it may be possible to even set limits on the water content, powder content (or water-to-powder ratio), mortar and coarse aggregate content for a particular type of application. In other words, based on the application, one would be able to choose the required consistency class, which can be built into the mixture design procedure of SCC for appropriate selection of ingredients. This can only be possible if mixture design guideline tables for SCC, on the lines of the conventional concrete design procedures, are created using available database.
Fig 3 A schematic from Walraven43 linking SCC properties with applications
Construction issues
Use of SCC has been demonstrated in a number of structures in Japan and Europe. A frequently cited case is the construction of anchorages for the Akashi-Kaikyo bridge in Japan44. Examples of other applications include: construction of a wall for a large liquefied natural gas tank in Japan42, viaduct in Yokohama City45, and a number of bridges in Sweden46,47.
Experience in these projects brings to light certain construction issues relating to the use of SCC. One issue is that of understanding the limit of flow distance of the concrete, in order to avoid segregation of coarse aggregate. Results from Japan indicate that for distances less than 10 m, segregation does not occur. Arima et al.48 proposed the use of automatic gate valves for discharging the concrete at many different points, at intervals of 6-20 m.
Another issue is that of lateral pressure of the SCC on the formwork, due to the highly fluid nature of SCC49 . Higher rates of casting with SCC could compound the problem of excess formwork pressure. Prima facie, it may appear that more robust formwork and falsework will be required. However, available results indicate that SCC exerts about the same pressure as conventional concrete. This can be attributed perhaps to the inherent thixotropy of SCC, or in other words to, the significant build up of viscosity following a period of rest. Research from Sweden has shown61 that the use of SCC actually resulted in pressures less than the design values for conventional concrete, and only slightly more than the conventionally-vibrated concrete. For example, at the same casting rate of 1.5 m/hour for a 3 m high wall, the form pressure developed at the base was 25 kPa for normally-vibrated concrete and 29 kPa for SCC, while the calculated design value was more than 40 kPa. Difference in form pressures of the two concretes was not significant, given the vast differences in mixture design and compaction. In the same study, form pressure was found to be proportional to the casting rate.
Hardened concrete properties of SCC
The major difference between self-compacting and conventionally-vibrated concrete is the higher flowability of SCC, and consequently a higher proportion of fine materials. Given this difference, the available knowledge of concrete properties would suggest the differences in performance between these two concretes shown in column 2 of Table 7. However, the reality could be sometimes different, as shown in the last column of that table. Results from relevant studies outlining these performance characteristics are discussed later.
Table 7: Differences in performance of SCC and normally-vibrated concrete
| Property of SCC | Expectation | Reality |
| Variation in strength across depth of structure | Can take place for SCC | No difference (between SCC and vibrated concrete) |
| Creep and drying shrinkage | Higher for SCC | No significant difference |
| Early age shrinkage and cracking | Higher for SCC | Higher for SCC |
| Strength and elastic modulus | No difference for same grade of concrete | No difference |
| Durability | Better for SCC | Better for SCC |
Uniformity
Studies on the uniformity of SCC have revealed that the performance of SCC is comparable to a well-compacted conventional concrete. Khayat et al.50 showed that the variations in in-situ strength (determined from cores) along the height of experimental walls and columns were similar for the SCC and conventional mixes. Zhu et al.51 improved upon this work by using full-scale beams and columns for their study on the uniformity of SCC. The in-situ concrete properties were assessed by testing cores for in-situ strength, pull out of pre-embedded inserts and rebound hammer for near surface properties. SCC and conventional concretes showed similar results.
Creep and shrinkage of SCC
Creep and shrinkage of concrete is primarily governed by the amount of hydrated cement paste (hcp) or gel in the concrete mixture. It may be conjectured that the higher paste content of SCC (as a result of using supplementary cementing materials such as fly ash) could lead to a higher tendency to creep under sustained loads, and also more shrinkage. However, a comparative study52 of the mechanical properties – strength, elastic modulus, creep and shrinkage - of SCC and conventional concrete showed that the properties did not differ significantly52. According to this study, the creep, shrinkage, and elastic modulus of SCC compared well with normal concrete when the strength was kept constant. The tendency to creep was seen to be higher at early ages for SCC, just as in the case with the normal concrete.
An understanding of the distinction between ‘fresh cement paste’ and ‘hydrated cement paste’ is necessary to comprehend the deviation from expected behaviour of SCC in respect of creep and shrinkage. Table 8 lists the paste and ‘gel’ compositions for different systems that use fly ash as supplementary cementing material. The amount of fines content in fresh paste is increased in SCC compared to both pozzolanic and plain concrete. However, the content of the hydrated gel need not be very different from plain concrete. Some of the fly ash simply acts as a filler in the system and does not participate in the hydration process. Similarly, when other fillers such as limestone powder are used, they do not convert to hydrated gel, but remain as solid particles. If the cement content can be kept at levels similar to normal concrete, then there is not much possibility of higher creep and shrinkage.
Table 8: Distinction between fresh and hydrated paste
| Type of concrete | Fresh paste | Hydrated paste | Creep and drying shrinkage
(arbitrary units) |
| Plain concrete | Cement + water | Hydrated gel + Water | 100 |
| Pozzolanic concrete | Cement + ~20 percent added fly ash + water | Hydrated gel (cementitious and pozzolanic) + water | Marginally higher
(~110) |
SCC
(with fly ash) | Cement + ~ 40 percent added fly ash + water | Hydrated gel (cementitious and pozzolanic) + fly ash + water | Marginally higher
(~110) |
SCC
(with limestone powder) | Cement + ~ 40% added limestone powder + water | Hydrated gel (cementitious) + limestone powder + water | 100 |
The low water-to-binder ratios adopted in SCC (at its early development stages) could also contribute to the problem of autogenous shrinkage. The higher fines content of SCC can also increase capillary pressures causing shrinkage. SCC is vulnerable to cracking at early ages53 (2 – 8 hours). Turcry and Loukili54 have reported that at the same evaporation rate, the plastic shrinkage of SCC was at least two times higher than the corresponding ordinary concrete. However, it was seen that autogenous shrinkage was only a small fraction of the overall shrinkage in the plastic stage (<15 percent). With lower powder contents in concrete, it may be possible to lower the potential for such cracks. In any case, SCC should be treated similar to conventional high performance concrete systems (with high cementitious materials content), and curing should be started early (within two hours from casting).
Durability of hardened SCC
Bridges built using SCC in Sweden55 have shown promising results. High strengths and adequate durability were obtained using SCC. In a study of the permeation properties of concrete, Zhu and Bartos56 found that SCC showed lower water sorptivity and oxygen permeability compared to reference concrete (of the same grade). A Swedish study on core samples taken from tunnel linings, bridges and retaining walls57 indicated that SCC had a higher resistance against chloride penetration than conventional concrete (at equivalent w/c57). Investigation of freeze-thaw and scaling also confirmed better results for SCC. After microstructural investigations, the improved performance of SCC was attributed to the increase dispersion of cement and filler, and a denser ITZ compared to conventional concrete.
A study of frost durability by Persson58 indicated that at the same air content, the internal frost resistance of SCC was better than the corresponding conventional concrete, while the salt scaling was similar in the two concretes.
Summary
Self-compacting concrete is a recent development that shows potential for future applications. It meets the demands placed by the requirements of speed and quality in concrete construction.
Based on current research and available knowledge about SCC presented in this paper, the following trends are emerging:
