Self Compacting Concrete Or Self Consolidating Concrete (SCC) is such type of concrete which is having self compacting characteristics and additional strength benefits.
Self-compacting concrete (SCC) compacts itself without any vibration. Well proportioned SCC can flow under its own weight through and around congested reinforcement, filling forms completely and producing a void-free mass with little or no mechanical vibration.
Buildings made with self compacting concrete
Properties of SCC
- Self-compactability
- Avoidance of bleeding and segregation
- Low shrinkage
- Low permeability
- Strength as needed
The use of self-compacting concrete, SCC, (including steel fibre reinforced self compacting concrete, SFR SCC) will improve the quality of concrete structures by eliminating some of the potential for human error. It will replace manual compaction of fresh concrete with a modern semi-automatic placing technology.
However, this type of concrete needs a more advanced mix design than conventional concrete and a more careful quality assurance with more testing and checking, at least during the start-up period.
Before SCC is produced and used the mix has to be designed and tested to be sure that the mix fulfils the demands regarding among others workability, segregation and passing ability. Suitable test methods and criteria are presented below.
As SCC is very sensitive to water content the humidity of the aggregates have to be measured carefully. If this is not possible the mix design must be more tolerant regarding the water content. Instructions is made showing that water not unplanned is added to the SCC e.g. at cleaning of the concrete mixer, the truck mixers or the concrete pumping system.
Transportation of SCC and SFR SCC has is made by trucks with rotating drums from production plant to construction site if it is produced outside the construction site.
Mix design
Before any SCC is produced at the concrete plant or in a truck-mixer and can be used at the construction site the mix has to be designed and tested to find suitable test results and other criteria. During this evaluation stage the equipment at the plant and the local material used have to be tested to find new recipes with the right mixing sequences and mixing times valid for the plant and the material used at this specific plant. One way to improve the mix design is to use the Compressive Packing Model developed at LCPC and presented in the End Product.
Note that different admixtures have various effects on hardening time and also can have influence on the temperature development why this has to be checked especially under winter conditions. It is also necessary to check that the admixtures not will cause any emission injurious to the health.
We have also found that different kinds of fines have various influences on the
strength, shrinkage and creep.
The following observations and tests need to be carried out for SCC and SFR SCC in addition to tests for traditional vibrated concrete in the laboratory and, in the beginning of the use of SCC, on full-scale mock-ups or tests before a new mix design can be approved and used:
· Homogeneity
· Workability
· Concrete workable for the required time (possible to use the concrete up to more than 90 minutes after mixing)
· Passing ability - depending on product, decide if needed. L-box is suitable for SCC and J-ring is suitable for SFR SCC.
· Slump-flow
· T50
· Segregation by e.g. Stability Sieving Test (GTM)
· Early strength - depending on product, admixtures and temperature, decide if
needed · Temperature development - depending on product and admixtures, decide if needed
· Sensitivity to changes of material included in SCC
· Form pressure, decide if needed
· Possibility to pump the SCC, decide if needed. Check that the designed mix can be used, as the slump-flow will decrease depending on the length of the piping.
The results from these tests shall be filed at the concrete plant that produces the SCC.
The different mix designs, including relevant tolerances or allowable variations, used at concrete production shall be documented and saved by the actual concrete supplier and should at least contain:
· Type and amount of: cement different aggregates fines admixtures
· Amount of total water
· Mixing times and sequences
· Batch volume
· Strength class
· Slump-flow
· Tests to be carried out during concrete production
· Allowable variation of total water content in ready mix
· Type and amount of steel fibers (when used).
Production at concrete plants or in truck-mixers
Before SCC is produced for delivery to construction sites it is necessary to carry out a training program for all involved persons at the production plants so everybody has got education and experience how to handle SCC and/or SFR SCC.
Receiving inspection of material
Receiving inspection and tests shall be carried out according to the quality plans valid for the concrete supplier. Regarding admixtures the concrete supplier shall ask for and keep certificates regarding the chemical content and showing that they not are hazardous.
Production of concrete
During the production at the concrete plant or in truck-mixer the normal quality control according to the quality plans has to be fulfilled. When a concrete supplier starts to produce SCC for the first time it is necessary to make detailed work instructions and educate all the operators at the plant or truck-mixer and also the truck drivers and other involved in the process about this new type of concrete so that everybody is aware of how to handle SCC during the production and delivery.
Before starting to mix SCC, the mixer has to be clean and moisten but no free water is allowed as only some few liters can change the consistence of the mix. The type of mixer has effect on the requirements of amount of water and superplasticiser.
When SFR SCC is produced the fibers must be introduced in the mixer taking precaution that they are evenly distributed in the mix and no balling of fibers occur.
Therefore bundle-collated fibers will be used or the fibers will be introduced by means of fibre separators.
Tests at concrete production
When tests on SCC are carried out according to norms and standards for traditional vibrated concrete vibration shall not be used if this is mentioned in the procedures. During the first period (3 months or 1.000 m2 ), when producing SCC, more tests must be performed than normal until the process is stable and the involved people are used to SCC.
There are also some tests that have to be carried out more often during the production of SCC or are specific for SCC:
· Moistness of sand and gravel has to be measured with an accuracy of measurement of = 0.5% to be able to adjust the water added during the mixing. If the moistness of the aggregates is very stable it is enough to measure the three first batches every shift and after that every tenth batch. If it not is possible to measure the moisture content with this accuracy the mix must be designed to be more tolerant regarding water content.
· Grading curves for filler and fine aggregate have to be checked according to
specifications that usually have less spread than for traditional vibrated concrete.
· Wattmeter readings for every batch to be able to know when the batch is ready. It is to day not possible to use the wattmeter to adjust the water content in the SCC to reach the said slump-flow.
· Slump-flow tests shall be made for every batch and compared with the wattmeter readings until the mix is stable (5 consecutive batches). After that slump-flow tests shall be made at every shift for 3 consecutive batches at the start of production of SCC and thereafter for every 10th batch. Tolerance for slump-flow is the decided concrete diameter ±5cm. T50 shall be within tolerances according to the mix design.
· Segregation test for new mixes has to be made e.g. according to GTM´s Stability Sieving Test until the mix is stable for 3 consecutive tests and thereafter for every 10th batch.
· L-box test shall be made for the same batches as for the slump-flow tests until the mix is stable. After that the L-box tests shall be made at every shift for the first batch at start of production of SCC.
· J-ring passing ability test is found better for testing on SFR SCC than L-box test. Suitable gap is 1 - 3 time the fibre length or representative for actual application.
· Fibre distribution and fibre content of SFR SCC by Fibre Content Test shall be
made at the first batch at every shift on the fresh concrete.
Transportation
Before using SCC as a new product it is necessary to inform and teach all the involved truck-drivers about SCC and how to handle this new type of concrete. It is also recommended to have a written instruction for the truck-drivers describing how to handle SCC and SFR SCC.
The SCC shall be transported to the construction sites by trucks with rotating drums if it is produced outside the construction site. The trucks used for transportation of SCC shall be suitable and tested for this operation so that the SCC will keep the quality during the transport.
The truck-driver shall check the concrete drum, before filling SCC in it, and make sure that the drum is clean and moist but without free water. Also concrete pumps and concrete bins shall be moist but without free water.
SCC truck should be always rotated with low speed (not less than 1 rotations per minute) during transport and waiting time. And it should be rotated with full speed (for modern trucks 10 – 20 rotations per minute) for at least 3 minutes at the construction site before delivery.
If the distance that the SCC has to be transported is so long that the workability will be influenced than the admixtures have to be added at the site. This must in that case be tested and approved during the mix design.
The delivery note shall contain the following information in addition to the normal information:
· Slump-flow - target value
· Remarks if any admixtures shall or may be added at the site before use
· Production time (time when it was produced)
· Type and amount of steel fibers (when used)
If the truck-driver shall or may add some admixtures or steel fibers at the site there shall be a written instruction how to do this, which tests that shall be performed and how to complete the delivery note.
If the steel fibers shall be introduced in the truck-mixer at site it must be checked that they are evenly distributed in the mix and no balling of fibers occur. The same testing that normally is made at the concrete plant has then also to be done at the site (J-ring, distribution of fibers and fibre content).
It is the truck-driver’s responsibility to inform the site if the time from production to delivery at site is longer than expected and in that way the SCC can be too “old”. Time for production shall always be written on the delivery note.
Formwork
Normal quality control for formwork has to be taken care of according to the
contractor’s quality system.
As we have found that different kinds of form-oils have various influences on the surface quality of the hardened concrete the type which is intended to be used need to be checked if the quality of the surfaces is of importance.
As we have got different experience regarding form pressure the formwork has to be designed for hydrostatic pressure if testing not has shown that the form pressure will be less than hydrostatic pressure.
The site manager or the appointed responsible has to check the form regarding the possibility for the concrete to reach all places without too long distance to flow or too high free fall and inform the responsible person for the casting about the suitable placing points for the concrete.
Placing of concrete
1.General
When casting SCC or SFR SCC on construction sites the normal quality control according to the contractor’s quality plans, procedures etc is valid.
When SCC or SFR SCC is used for the first time on a construction site by the labour
and staff it is necessary to perform a program with information and training about this new product and to introduce the procedures so that everybody is fully informed of how to handle SCC during receiving, casting and treatment after casting.
2.Receiving of SCC
When a truck arrives to the site and before the SCC is used one responsible person at the site has to check the delivery note to see that the SCC is according to what is ordered and also check the SCC visually to see if any segregation can be observed.
The responsible person shall also make a slump-flow test to find out if the SCC fulfils the demands (T50, slump-flow, segregation etc) on the three first deliveries every day and after that on every tenth delivery or as soon as he/she suspects that the slump-flow not is within the tolerances. After this the SCC can be released for casting.
If the delivered SCC does not fulfill the demands the responsible person at the site contacts the concrete plant to decide what to do with the delivery (reject and send it back or add some admixtures and perform new tests).
3.Placing of concrete
Placing of concrete may not start until the test of slump-flow is carried out with acceptable result and/or the SCC is released by the responsible person.
During the casting of SCC the operator responsible for placing the concrete has to check that the opening of the pump hose or bucket is not more than 3 meter above the cast surface, limit the distance that the concrete can flow to less than 10 meters from the placing point, if there not are any obstacles as dense reinforcement or embedded items, and check that no segregation is observed.
If there has been an interruption in the placing of concrete and the SCC has started to stiffen it can be necessary to “wake up” the concrete, to avoid pucker, by e.g striking a stick or a board into the concrete several times to get it flowable again before continuing.
When the casting is big and covering a large construction it can be a good idea to use several preplaced concreting pipes possibly with valves to control the flow and with an internal distance of 6 - 8 meters for the distribution of the SCC.
4.Curing and treatment after placing of concrete
The same type of treatment of the ready cast surfaces can be made as for traditional vibrated concrete but steel-trowelling or vacuum sucking of the surface of slabs is difficult to do.
In the case of SFR SCC used in flooring applications, trowelling or other methods should be used to ensure that no fibers would protrude from the surface. When using SCC plastic shrinkage can occur due to lack of water separation, specially when the weather is warm and windy. Due to this it can be necessary to shower the
Test methods
The following tests are, in addition to normal testing, useful during the development, the production and the use of SCC:
· Slump-flow and T-50
· L-box
· J-ring (SFR SCC)
· Stability Sieving Test (GTM)
· Fibre Content Test (SFR SCC)
