SELF COMPACTING CONCRETE

Introduction

Self-compacting concrete (SCC) is a flowing concrete mixture that is able to consolidate under its own weight. The highly fluid nature of SCC makes it suitable for placing in difficult conditions and in sections with congested reinforcement. Use of SCC can also help minimize hearing-related damages on the worksite that are induced by vibration of concrete. Another advantage of SCC is that the time required to place large sections is considerably reduced.

When the construction industry in Japan experienced a decline in the availability of skilled labour in the 1980s, a need was felt for a concrete that could overcome the problems of defective workmanship. This led to the development of self-compacting concrete, primarily through the work by Okamura¹. A committee was formed to study the properties of self-compacting concrete, including a fundamental investigation on workability of concrete, which was carried out by Ozawa et al². at the University of Tokyo. The first usable version of self-compacting concrete was completed in 1988 and was named “High Performance Concrete”, and later proposed as “Self Compacting High Performance Concrete”.

In Japan, the volume of SCC in construction has risen steadily over the years³. Data indicate that the share of application of SCC in precast concrete industry is more than three times higher than that in the ready-mixed concrete industry. This is attributable to the higher cost of SCC. The estimated average price of SCC supplied by the RMC industry in Japan was 1.5 times that of the conventional concrete in the year 2002. Research studies in Japan are also promoting new types of applications with SCC, such as in lattice type structures, casting without pump, and tunnel linings.

Since the development of SCC in Japan, many organizations across the world have carried out research on properties of SCC. The Brite-Euram SCC project⁴ was set up to promote the use of SCC in some of the European countries. A state-of-the-art report on SCC was compiled by Skarendahl and Petersson⁵ summarizing the conclusions from the research studies sponsored by the Brite-Euram project on SCC. A recent initiative in Europe is the formation of the project – Testing SCC⁶– involving a number of institutes in research studies on various test methods for SCC. In addition, an organization with the participation from the speciality concrete product industry – EFNARC⁷– has developed specifications and guidelines for the use of SCC that covers a number of topics, ranging from materials selection and mixture design to the significance of testing methods.

Current studies in SCC, which are being conducted in many countries, can be divided into the following categories: (i) use of rheometers to obtain data about flow behaviour of cement paste and concrete, (ii) mixture proportioning methods for SCC, (iii) characterization of SCC using laboratory test methods, (iv) durability and hardened properties of SCC and their comparison with normal concrete, and (v) construction issues related to SCC. These will be relevant to the immediate needs. In addition, the following questions also need particular attention, from a long-term perspective: (i) development of mixture design guideline tables similar to those for normal concrete, (ii) a shift to more ‘normal’ powder contents in SCC, from the existing high powder mixtures, (iii) better understanding of the problems of autogenous and plastic shrinkage in SCC, and (iv) development of site quality control parameters such as in ‘all-in-one’, acceptance tests.

Materials for SCC

Mixture proportions for SCC differ from those of ordinary concrete, in that the former has more powder content and less coarse aggregate. Moreover, SCC incorporates high range water reducers (HRWR, superplasticisers) in larger amounts and frequently a viscosity modifying agent (VMA) in small doses. The questions that dominate the selection of materials for SCC are: (i) limits on the amount of marginally unsuitable aggregates, that is, those deviating from ideal shapes and sizes, (ii) choice of HRWR, (iii) choice of VMA, and (iv) interaction and compatibility between cement, HRWR, and VMA. These are discussed below.

Aggregates

Aggregates constitute the bulk of a concrete mixture, and give dimensional stability to concrete. Among the various properties of aggregate, the important ones for SCC are the shape and gradation. Many researchers have been able to produce self-compacting concrete with locally available aggregate. It is observed from these studies that self-compactability is achievable at lower cement (or fines) content when rounded aggregates are used, as compared to angular aggregates. Although there have been several studies on the effect of coarse aggregate content on the flow behaviour of SCC^8,9,10, enough attention has not been paid to quantify the effect of the shape of the aggregate.

In the case of SCC, rounded aggregates would provide a better flowability and less blocking potential for a given water-to-powder ratio, compared to angular and semi-rounded aggregates. Moreover, the presence of flaky and elongated particles may give rise to blocking problems in confined areas, and also increase the minimum yield stress (rheology terms are discussed in the next section). Incorporation of aggregate shape in the mixture design would enable the selection of appropriate paste content required to overcome these difficulties. It is possible that the highly flowable nature of SCC could allow a higher proportion of flaky aggregates compared to normal concrete. However, this aspect needs to be checked.

O’Flannery and O’Mahony¹¹ have devised a method for shape characterisation of coarse aggregate, which could assist in designing SCC mixtures having marginally unsuitable aggregates. The purpose of the study was to determine dimensional parameters for ‘fingerprinting’ any given coarse aggregate sample. The overall idea was to overcome local deficiencies in aggregate shape and to arrive at required packing characteristics irrespective of the aggregate. Incorporation of aggregate shape in mixture design, based on the method developed by O’Flannery and O’Mahony, is explained in Table 1.

Another deficiency in aggregates is poor gradation. Use of fillers (either reactive or inert) has been suggested as a means of overcoming this problem^12,13. At present, a trial and error approach is used to fix the type and amount of filler. Alternatively, particle packing models could be used to reduce the number of experimental trials^14,15. Such models are discussed later.

In view of on increased awareness of the environmental impact of mining river sand and depleting supplies of the same, use of manufactured sand and other alternative fine aggregate has become essential in some parts of the world. In fact, river sand is simply not available in many areas. Although there are studies that have shown that quarry run could be used as a filler instead of limestone for SCC¹⁶, there has not been sufficient documentation of the use of manufactured sand, either as fine aggregate or as a filler, in SCC. Further research on this topic will be useful.

Table 1. Use of shape characterisation of aggregate

	Conventional method	Suggested method
Aggregate shape	Rounded versus crushed (subjective assessment) Flakiness index Elongation index	•Cubicity, sphericity, and roundness indices (based on Flannery and Mahony) - Cubicity (Cubicity index Ic = 100XYZ/X3, where X, Y, and Z are dimensions of the aggregate) to indicate the likely degree of potential compaction; values of index between 60 and 10 are cubic, between 30 and 60 indicate flakiness (100 for perfect cube), while values below 30 suggest elongation - Sphericity index to describe the polyhedral shape of the particle, and - Roundness index to describe the degree of angularity; values between 84 and 100 are angular, and between 52 and 68 are rounded
Adjustments in mixture proportioning	Adjustment in water content by type of aggregate – rounded or crushed No recommendation for flaky aggregate	• Use index value for adjustment of mixture; also, quantify the degree of flakiness or elongation by a weighted value for the index

Admixtures

SCC invariably incorporates chemical admixtures - in particular, a high range water reducing admixture (HRWRA) and sometimes, viscosity-modifying agent (VMA). The HRWRA helps in achieving excellent flow at low water contents and VMA reduces bleeding and improves the stability of the concrete mixture. An effective VMA can also bring down the powder requirement and still give the required stability. Moreover, SCC almost always includes a mineral admixture, to enhance the deformability and stability of concrete. Issues linked with the use of chemical admixtures are discussed in this section.

High range water reducers

A number of studies have been conducted on the use of different types of HRWRAs with or without viscosity modifying agents in self-compacting concrete^17,18,19. These studies seem to indicate those that HRWRAs that work on the principle of ‘steric hindrance’ require a lower dosage compared to those based on ‘electrostatic repulsion’. Stated in other words, acrylic copolymers (AC) and polycarboxylate ethers (PCE) are effective at lower dosages compared to sulfonated condensates of melamine (SMF) or naphthalene (SNF) formaldehyde²⁰. At present, SNF-based admixture is priced lower (in India) than that based on AC and PCE In the opinion of the authors, SNF-based admixture seems to be preferable that based on PCE.

Viscosity modifying agents

The conventional method of improving the stability of flowing SCC is to increase the fines content by using a large amount of filler, reactive or inert. Of late, however, attempts are being made to reduce the fines content (and paste content) to the levels of normal concrete (in doing so, reducing the potential for creep and shrinkage) and use viscosity modifying agents (VMAs) to improve the stability. Current research shows that SCC produced with low powder content and VMA had similar fresh concrete properties as SCC with high powder contents produced without VMA²¹.

VMAs have been in use for a long time²². They were mainly used for underwater concreting in the past, but are now also used in self-compacting concrete. Most VMAs have polysaccharides as active ingredient; however, some starches could also be appropriate for control of viscosity in SCC ^23,24.

The sequence of addition of VMA and superplasticizer into the concrete mixture is important. If VMA is added before the superplasticizer, it swells in water and it becomes difficult to produce flowing concrete. To avoid this problem, VMA should be added after the superplasticizer has come into contact with the cement particles. Another method of addition is to disperse the superplasticizer in mixing water, and then add VMA to this mixture.

Effective addition of VMA in concrete is an application-related issue, because of the relatively low proportions of VMA needed to stabilize the superplasticised concrete. Unless the VMA is uniformly dispersed across the entire volume of concrete, it cannot perform the intended function. At present, VMA is packaged in water-soluble bags that can be added directly at the concrete mixer. The other alternative is to prepare a suspension of VMA in water (saturated with superplasticizer) before adding into the concrete mixture. Addition of microsilica, sepiolite or attapulgite improves the stability of suspensions of these polysaccharides.

Admixture compatibility

A large amount of superplasticisers, typically SNF-based, is added to SCC to make it flowable at a reasonable water contents. There exists the problem of incompatibility between cement and HRWRA, which is generally felt acutely for mixtures having low water content. Jolicoeur and Simard²⁵ have studied the interaction between SNF and cement. In concretes having low water content and high superplasticizer dosage, gypsum (present in cement) may precipitate out, causing a premature stiffening of the paste and consequent loss of slump²⁶. However, SCC mixtures typically may have a water content of 170 – 200 litres/m³ and the compatibility problems associated with low water contents may not arise.

Sometimes superplasticizers are blended with retarders or lignosulfonates (which may have sugar in them), for slump retention in hot weather conditions. When a VMA is used along with such blended superplasticizers, concrete may not set for nearly twenty hours. This problem may be avoided by using pure SNF-based superplasticisers. The retarding effect of the VMA itself will be adequate for extending the slump retention time.

Mixture proportioning methods

Self-compacting concrete mixtures should be designed for a combination of filling ability, resistance to segregation, and ability to pass through and around reinforcement without blockage. The principles of producing SCC are shown in Fig 1. Correct selection of aggregate size and gradation, along with adjustments in paste rheology is essential for SCC.

Fig 1 Principles of SCC mixture design

In the past, SCC mixtures have had high cementitious materials contents, providing a high degree of stability to the mixture. As a result, water contents of SCC mixtures were about 190 – 220 litres/m³. With the development of viscosity modifying agents specially suited for SCC applications, however, it has been possible to reduce the content of cementitious materials, bringing down the water contents to values closer to conventional concrete (160 – 190 kg/m³).
A number of procedures for designing self-compacting concrete mixtures have been proposed. These can be broadly classified into four categories: (i) Empirical methods, (ii) Rheology based methods, (iii) Particle packing models, and (iv) Statistical methods.

Empirical methods

A customary method for design of SCC is to follow the recommendations of Okamura and Ozawa²⁷. In the method, 50 percent of the solid volume is taken up by coarse aggregate, while 40 percent of the mortar volume is fine aggregate. Paste composition (that is. the water-to-powder ratio) is then determined using flow tests on mortar. This method was derived from numerous experiments using aggregates specific to the researchers’ area. A survey of literature indicates that a number of researchers use this method as a starting point for their investigations. Adjustments in coarse and fine aggregate contents are then made to achieve desired flow properties.

Modifications to the above approach have been proposed by Edamatsu et al²⁸. In the Edamatsu’s method, the limiting coarse aggregate volume ratio is kept at 0.5. The fine aggregate content, in this case, is then fixed using V-funnel test with standardised coarse aggregate (glass beads). Water-to-powder ratio and superplasticiser dosage are determined from mortar flow and funnel tests.

The guidelines recommended by EFNARC⁷ are also based on Okamura’s method. The difference is that instead of fixing the coarse aggregate limit at 0.5, a higher amount is permitted in the case of rounded aggregate (up to 0.6). The proportion of sand in the mortar is varied between 40 and 50 percent, and water-to-powder ratio and superplasticiser dosage are determined through mortar slump flow and V-funnel tests. A comparison of the three methods discussed in this section is presented in Table 2.

It can be inferred from Table 2 that the Edamatsu’s method provides a more scientific basis for fixing the mortar content of SCC, once the coarse aggregate content is decided. The method used by EFNARC, on the other hand, allows for including more coarse aggregate when rounded particles (as opposed to crushed particles) are used.

Given the variability of the concrete raw materials, and the associated unpredictability of the resultant concrete properties, empirical methods have been, and possibly will be, the choice for mixture design of SCC until a more rational method comes about, either based on rheology principles or particle packing models (discussed later). In any case, with enough data available on mixture design and properties of SCC, it may be possible to construct mixture proportioning tables for SCC, on the lines of what is now available for normal concrete.

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 concrete^29-32. 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, τo	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 characterized^7,36,37. 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 articles^7,38. 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 Sweden^46,47.

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:

1. Use of viscosity modifying agents (of the pseudoplastic variety) compiled with high-range water reducing agent for dynamic
   control of flow and segregation is increasing
2. A better understanding of the rheological parameters – yield stress and plastic viscosity – has made it easier to describe the role of superplasticizer, particle packing (increased fines content etc.) and pseudoplastic VMA in SCC. It has also given the user a tool to prescribe variants of SCC based on the type of application and placing conditions
3. There is now accumulated evidence that properties of SCC in hardened state are similar to those of conventional concrete
   The wide variety of test methods for SCC can be classified for simplicity into ‘basic tests’, ‘proportioning tests’ and ‘control/verification tests’
   

The following topics require further investigation:

• Use of basic rheological measurements to establish empirical or arbitrary test parameters;
• Determination of yield stress and plastic viscosity for different placing conditions;
• Establishing the role of fines, superplasticizers, and VMA in SCC, with respect to compatibility between these systems;
• Development of criteria for using marginally unsuitable aggregates (in respect of shape and grading) as well as alternative aggregates (such as manufactured sand) in SCC;
• Feasibility of combining rheology and particle packing models for proportioning SCC mixtures (long term goal);
• Preparation of a set of design tables for mixture proportioning of SCC on the lines of ACI Committee 211.

SOURCE :http://www.theconcreteportal.com/scc.html

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