Self Compacting Concrete
Current Developments in Self-Compacting Concrete
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’,
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 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 SCC8,9,10, 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 problem12,13. 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 trials14,15. 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
Table 1. Use of shape characterisation of aggregate
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 concrete17,18,19. 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.
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.
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 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
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.
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 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.
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
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
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.
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
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
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
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
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
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.
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:
The following topics require further investigation:
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