SCC IN PRACTICE: OPPORTUNITIES AND BOTTLENECKS

F. Cussigh

VINCI Construction France, France

Abstract

This paper is the point of view of a French SCC user which sometimes produces its own SCC on housing or civil engineering job sites. It deals mainly with cast in place concrete (SCC development is much faster in the precast industry). Some opportunities and bottlenecks for SCC development in cast in place applications are exposed, based on some practical examples. In a first part, the great technical potential of SCC is illustrated: easiness of placing (allowing to build difficult concrete structures), surface finish quality, productivity enhancement and quality of hardened concrete. Other benefits when using SCC are not developed even if they are quite important and may be a sufficient reason for choosing SCC: noise reduction (for the workers and for the site surroundings) and improvement of working conditions (reduction of physical strain). Despite all the opportunities listed, SCC development for cast in place applications is quite slow (about 3% of the ready-mixed concrete in France) and some possible explanations for this are given in a second part: need for high quality concrete materials, in some cases (high vertical elements) need for special formwork, insufficient accuracy of production equipment, stringent quality control requirements and lack of specific standards. Soon, some of these drawbacks will be minimized and SCC development will go on and increase the possibilities offered in terms of architectural design.

1. INTRODUCTION

This paper is the point of view of a French SCC user which sometimes produces its own SCC on housing or civil engineering job sites. It deals mainly with cast in place concrete (SCC development is much faster in the precast industry). Some opportunities and bottlenecks for SCC development in cast in place applications are exposed, based on some practical examples.

2. OPPORTUNITIES WITH SCC In this chapter, relying on some practical applications, we intend to illustrate the great

technical potential of SCC: easiness of placing (allowing to build difficult concrete structures), surface finish quality, productivity enhancement and quality of hardened concrete.

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

Other benefits when using SCC are not developed even if they are quite important and may be a sufficient reason for choosing SCC: noise reduction (for the workers and for the site surroundings) and improvement of working conditions (reduction of physical strain).

2.1 Easiness of placing SCC filling ability in confined conditions allow to pour difficult concrete structures which

could not be cast with traditional vibrated concrete. For instance, some vertical elements of the Court House of Pontoise were cast in one single pour, including the bottom wall and the upper columns:

Photos 1 & 2: Architectural walls for the Court House of Pontoise

The stability of SCC (no bleeding and negligible settlement) is also very interesting for overhead casting (no need for grouting the surface contact zone): for instance SCC is used in tunnelling for the thin wall in the top part section of SOCATOP project near Paris.

For common housing construction, SCC is often chosen when large openings are present because special concreting systems (casting columns which have to be broken afterwards) are needed with conventional vibrated concrete (or a lot of repair product!):

Photo 3: Perfect filling below large window

SCC is also very easily transported: it can be pumped at high speed (low friction inside the pipes) and used for underground works. For the construction of ANDRA underground

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

laboratories (500m deep shafts), SCC was delivered by gravity through a 500m long pipe without segregation.

2.2 Surface quality With SCC, it is possible to obtain very high surface quality because concrete remains

homogeneous and contains an important amount of fines leading to very good moulding properties. Classical concrete surface defects like honeycombing or leakage stains are not encountered with SCC. SCC is particularly interesting for producing special finish surfaces with relief.

Photos 4 & 5: Examples of special surface finish with SCC

2.3 Productivity enhancement Since casting operation can be easier and faster with SCC and it is possible to reduce the

number of different pours, construction productivity can be improved. Depending on the construction site, construction time can be reduced by using SCC.

For instance, for the construction of sewer treatment plant in Achères (near Paris), Biostyr cells were cast with SCC injected at the bottom of the formwork. There was no problem of water-tightness of the formwork stop ends because precast columns were put in place in the corners of the cells before casting the walls.

Photos 6 & 7: Casting method for the Biostyr cell walls

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

2.4 Quality of hardened concrete For some special applications, access for concrete vibration is very difficult or impossible.

This was the case for the closure joint of the Øresund tunnel between Copenhagen and Malmoe. The top part of this joint (about 1 meter wide) was only accessible from the inside, and the filling had to be perfect in order to guarantee that the joint would be watertight. A SCC mix was developed and injected from the bottom (see section view herebelow), with very good results.

Figure 1: Closure join t top slab casting

As there was a durability requirement of 100 years for this project, it was necessary to perform pre-testing of this mix in order to compare its performance with the one of the ordinary vibrated mix used for the tunnel segments: quality of hardened SCC was even better.

3. BOTTLENECKS Despite all the opportunities listed herebefore, SCC development for cast in place

applications is quite slow (about 3% of the ready-mixed concrete in France) and here are some possible explanations for it.

3.1 Need for high quality concrete materials In some cases, materials used for ordinary (vibrated) concrete production are not suitable

for SCC production even if they are conforming to the standards. The problem may concern the aggregates (grading curve of the sand) or the available addition. It can be insufficient average quality or two large variations impeding production of consistent SCC.

Then, over-cost of SCC material is likely to be increased and this may be a reason to go on with traditionally vibrated concrete. This was the case for the pylon legs of Rion-Antirion bridge in Greece, even if initial trials were successful. Pre-testing of SCC mix included a mock-up, creep and shrinkage laboratory tests and durability tests. As it was difficult, especially for passing ability, to develop a self-compacting C60/75 mix with local 100% crushed aggregates, a rounded sand was added. Because there was no available local rounded sand with established suitability regarding alkali-silica reaction, price of SCC mix was significantly increased and vibrated concrete was used. Still, it may be highlighted from this experience that from a technical point of view, the SCC mix developed was very satisfactory. Creep was even lower than for vibrated concrete: that is because vibrated C60/75 was a mix without silica fume whereas silica fume was included in SCC mix. This difference in mix design was more important than the paste content parameter or SCC behaviour.

Another example is the sewer treatment plant in Achères and some problems encountered due to variations in limestone filler water demand. With some filler deliveries, even if the

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

product was still conforming with the standards, a higher clay content originated significant slump-flow loss and defects on walls surface. After this was discovered, a systematic check of cement, filler and sand water demands (using the Concrete Equivalent Mortar method) was performed in the site laboratory for every delivery.

3.2 Need for special formworks Classical formwork used in France for walls in housing construction (Outinord,

Hussor, …) are able to withstand a formwork pressure of 8 to 10t/m². This means that there is no problem for classical casting height up to 4m. For higher walls, special care is needed: formworks should be strengthened or casting speed should be low enough. The problem is that it is very difficult to predict what will be the formwork pressure depending on the casting speed: thixotropy of SCC should be assessed and verified (vibrations on site can break part of its effect). In practice, pressure gauges are used (to measure tensile strength in the tie rods). Some interesting behaviour of SCC was exploited on a site where 10m high walls were cast in winter time: 120 minutes after mixing, concrete was still flowable when kept in agitation but had no slump when at rest (see photo herebelow).

Photo 8: Thixotropic concrete

As far as formwork tightness is concerned, generally there is no major problem because SCC can withstand small defects without leakage. Anyway, for bigger defects, results obtained with vibrated concrete are not satisfactory (honeycombing). This matter may be a problem for some special applications like tunnel lining against irregular rock profile.

Another difficulty should be taken into account with inclined surfaces which are traditionally unformed with vibrated concrete (bridge segments bottom slab for instance). If top formwork is used for such surfaces, it should withstand high concrete pressure and should be designed in the site preparation phase.

3.3 Insufficient accuracy of production equipment Depending on SCC fresh state specifications which may include stringent criteria for

passing ability or segregation resistance, accuracy of production equipment should be in relation with the range of SCC mix acceptable variations: quite often, variation on water

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

content should not be more than plus or minus 5l/m3 and this is difficult to obtain with classical equipment.

The accuracy of moisture probes for aggregates is for micro-wave type (one of the most efficient) about 1%. This means that just from the sand fraction possible variation of SCC water content is about 10l/m3. Moisture probes are not reliable for coarse aggregates and most of the time batching plants use “typical values” for coarse aggregate moisture. So, it is very clear that a production survey is needed relying on indicators during mixing operation. In France, we have batching plants with forced action mixers and we use wattmeter value (energy consumption from the mixer engine) as an indicator, even if sometimes with very fluid and low viscosity SCC it is not sensitive enough. One load is made of different successive batches and it is possible to adjust water addition from one batch to the other. Of course, slump-flow measurements at the mixer output are necessary to deliver a consistent SCC.

There are possibilities to improve this situation by using more sophisticated equipment like moisture probes inside the mixer to directly measure concrete water content. This kind of equipment is already used on some precast batching plants, but it is not suitable for cast in place concrete because it slows down concrete production rate too much. Rotating probes of micro-wave type may be used with a special mixing sequence and reasonable impact on mixing time.

3.4 Quality control requirements This chapter is in very close relation with the previous one: the accuracy of the batching

equipment and possible variations on materials quality lead to special quality control organization. For the time being, SCC is not only produced under production team survey but also with systematic laboratory assistance and this is a significant part of SCC over-cost compared to vibrated concrete. Hopefully, when suitable initial tests have been performed and robustness of SCC mix has been established, slump-flow is the only routine test used to check SCC compliance. But slump-flow should be performed more often than slump-test or flow-test for vibrated concrete because consequence of bad slump-flow (too low or too high) cannot be corrected when placing the concrete.

Another problem, which can be an obstacle to SCC use, is the difficulty to define where SCC supplier responsibility stops: for instance for surface finish, final result is much more dependant on concrete quality in the case of SCC (no vibration). If formwork, demoulding product and casting procedures are well defined and checked, there could be a contractual specification on SCC surface finish quality (colour, number and size of blowholes, …).

3.5 Lack of standards For the time being, European standards on SCC are still in preparation. Even the definition

of SCC can be unclear and some very fluid concrete which still need some vibration are called SCC in some countries. Hopefully, there will be soon a set of relevant standards for SCC [1] and dialog between producers and users will be more simple. For the clients, this is also a kind of recognition: SCC can be used like any other vibrated concrete mix.

On the sewer treatment plant site in Achères, we have had to justify that SCC was fully compacted in its mass even if there were some blowholes on the surface. The fact that no vibration was used led to such questions, and the answer was to drill cores and to perform

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

density and permeability tests in comparison with vibrated concrete and then to demonstrate from those results SCC quality.

4. CONCLUSIONS SCC technical potential is very high, but there is a gap between what can be done in the

laboratory and practical possibilities at the batching plant and on site. Even if there are references [2] with very positive applications, SCC still suffers from heavy additional quality specifications and quality control. This leads to a significant over-cost when compared to ordinary vibrated concrete (about 30%). At the same time, using SCC is in relation with high quality level because SCC does not forgive defects: if you have a severe segregation problem on your mix, SCC will not come out of the skip or pump; if your formwork tightness is bad, you will not fill up the formwork because SCC will leak out; if the slump-flow is not high enough, surface finish will be of poor quality, etc. This can be considered as an advantage on the way towards an improved quality of concrete construction.

REFERENCES [1] Cussigh F., “A set of European standards for SCC”, SCC 2007 proceedings (2007). [2] ‘Monograph on SCC structures’, Collection Technique Cimbéton (April 2004).

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium