#ConstructionIndustry #CivilEngineering #ConcreteQuality #Methodology #SelfCompaction #UnconventionalConcretingTechniques
Image Source - medium.com
What is SCC (Self Consolidating/Compacting Concrete)
As in the term of context, SCC is known to be a special type of concrete, which posses the ability to flow under its' own weight without much alterations. This flowing nature of this concrete allows it to be compacted without the presence of any external vibrations ( Poker or Induced Vibrations ). In addition to the property of self compaction, this specifically manured concrete presents deformability and adequate cohesive properties to prevent segregation and bleeding, respectively.
SCC commonly get misidentified with flow concretes. ASTM C1017 identifies flow concrete, as any concrete having a slump greater than 190 mm. So classifying SCC into the grade of flow concretes is equitable, but its significant to pin point that not all flow concretes can not be categorized as SCC.
When a slump of a concrete surpasses the value of 280 mm, the stability of the concrete diminishes. So in order to go over, the concrete has to be specifically batch mixed, in order to contain its' both dynamic and static stabilities.
In order to gain these preferable properties, material utilized in SCC, are well governed, along with the inclusion of certain self governing admixtures, which will be elucidated through out this article.
Why the Urgency for SCC
1. Not all the constructions environments are in bulk or conventional shapes nowadays. These unconventional shapes and surroundings has made it difficult for typical concrete (200mm slump) to be utilized due to various senses such as,
Difficulties to make the concrete flow
Source - Concrete Construction.com
Inadequate water retaining properties
Source - Wikipedia.org
Inability to exert external vibrations for compaction
Source - Concrete Construction.com
Higher in-place cost at tricky environments, etc.
Source - Concretedecor.com
Hence, the construction industry tried to overcome these foibles by Introducing and developing SCC with the ability to solve out all the deficiencies mentioned above.
2. In addition to ceasing the difficulties in conventional concrete, SCC has proven to be much quicker in utilizations. This tract has ensured much more faster completion stages along with higher salvages in cost and workforce as well.
3. Not to mention, due to its' self compacting enacted with cohesive, rheological and flowing properties, SCC has identified to be far more superior in quality with respect to conventional concretes in cases of Durability and Reliability.
Materials and Properties in SCC
Materials
Properties
1. Rheology
Rheology is the science which describes about the deformation and the flowing ability of a material. Since SCC is known and manufactured to be in a highly flowable state, the phenomenon of rheology should be assessed thoroughly to get the perfect sense about the science behind it.
As mentioned in above, without proper care, SCC tends to loose stability since the slump surpasses the value of 280mm in par. So in order to keep this Stability and the flowing nature, SCC is manufactured pertaining 2 physical properties which critically address the rheology of the composite. They are,
Yield Stress - SCC is manufactured in manner which allows it to flow under gravity conveniently. Which is to be secured by making the yield stress, very low. Achieving such low yield while keeping the stability is solved by addition of GGBFS and Superplasticizers in batch mixing.
Source - quora.com
Viscosity - Being at a highly flowable state, keeping the stability of the mixture during and transport is achieved by increasing the viscosity in considerable margins. Viscosity modifying agents as chemical admixtures plays a vital role during batch mixing to achieve the mentioned property.
Source - institution of civil engineers.com
2. High Powder Content
Since the SCC needs lofty powder contents as construed above, it is more susceptible to bleeding rather than conventional concrete, specially in concreting flat surfaces. Hence its more adequate, hence utilized in concreting vertical tight spaces and shapes.
GGBFS
Source - Exporters India
3. Passability
Passability of concrete refers to its' ability to flow through confined spaces under its own weight, which is known to be a recognized advantage in SCC. The deformability plays a dominant role in determining the passability but increasing deformability does not always result in passability incensement. Factors such as narrow openings, congested reinforcements, properties of aggregates, and paste volume also plays some aspect in addition to the deformability.
Congested Reinforcements
Source - The Engineering Community
4. Segregation Resistance
This can be categorized same as the stability of concrete, which could be deliberated as "Keeping the homogenous property of concrete over the whole volume". Segregation resistance addresses the static stability of the concrete composite.
Factors such as densities of the utilized aggregates, their shape and grading along with elemental characteristics (eg :- Length, Width, R/F congestion, etc.) performs vitally in deciding the segregation effect.
Due to these complications, SCC is usually batch mixed to meet with the exact requirements which the site conditions desire, rather than sticking to a general blueprint.
Quality of SCC
Quality of SCC is measured by evaluating major properties of it, under different tests accordance with verified protocols, such as ASTM and European Guidelines. 2. Such common tests can be illustrated as below,
In this Article, the most general and common experiments conducted under local site conditions will be illustrated thoroughly. Testing approaches, conditions and all other protocols governing SCC, will be covered in a future piece.
Slump Flow Test
Slump Flow test for SCC is the most common and effective way of measuring its' Deformability/Passing ability in both laboratory and field conditions. This test procedure follows the Standards of ASTM C1611/C 1611M-05 and European Standards 2005.
T 500mm Slump Flow Test can also be conducted from the same apparatus as for slump flow test, which gives an inkling on speed of the flow along with its' viscosity. Being standardized under European Guidelines, this test also expected to deliver indications on consistency and unconfined flow potential of unhardened, fresh SCC as well.
Apparatus
Procedure
Step 01
Setting the base plate and the slump apparatus/mould properly.(It's important make sure that the base plate is horizontal)
The flow concrete slump apparatus positioning is different from conventional, since the appliance is kept upside down (100mm side - downwards) .
Mark the center of the base plate by cross which is drawn in parallel to base edges. Draw a circle of having a diameter of 500 mm.
Make sure the base plate and the mould are dampened before proceeding to the next step.
Source - readymix2.go.co.uk
If the underlying surface is not even, it is advisable to use sand/gravel type material to achieve a horizontal surface.
Step 02
Take the concrete into the remixing container/wheelbarrow from the delivery truck. (Make sure, not to use the initial discard from the truck.)
Normal Site Practice - Wheelbarrow filled in 2nd or 3rd.
Step 03
Give a good mix to the concrete in the remixing container/wheel barrow using the square mouthed scoop. (Scoops' square shaped mouth ensures proper mixing of material in the concrete)
Step 04
Start filling the mould continuously without any agitation or tampering by the rod. Fill the mould allowing for a little overflow and smoothen up the top surface using a square trowel.
Clean out the spilled surface using a mob or a moist cloth. Make sure not to keep the concrete in the mould for more than 30 seconds.
Step 05
Lift the mould from the base plate in steady upward motion allowing the concrete to be flowed on the base plate. Make sure to turn on the stopwatch as the mould looses the contact with base plate.
Step 06
Allow the concrete to flow freely, without any disturbance to the base plate. Make sure to mark the time, which concrete reaches the 500 mm diameter circle. - T 500 (to the nearest 0.1s)
Once the concrete stop flowing, take the largest diameter of the speeded circle, to the nearest 10 mm. Use the tampering rod as a reference point for accurate reading. - D1
Take the diameter perpendicular to the initially measured direction - D2
Step 07
Calculate the average between D1 and D2 which is taken as the slump of the measured concrete.
Check the concrete for any segregation.
The following video contains the procedure of conducting proper Slump Flow Test under field conditions.
Categorization
Classification under the Slump Value
2. According to the T 500 Value
Visual Stability Index test
Once the the concrete stops flowing on base plate after lift of the slump mould, the visual representation of the concrete surface can be utilized as a index to identify the level of segregation and stability of the operated concrete.
VSI follows scale from 0 to 3, in identifying the stability of a given concrete. The respective shapes of the mentioned scale can be illustrated as followed,
Source - RMCAO, January 2009
In Addition
V Funnel Test - To test the Dynamic stability and and flowability
J Ring Test - To test the Dynamic stability and and flowability of SCC in a environment surrounded by reinforcements.
L Box Test - Measures the ability of the concrete to flow through a confine spaces created by the formwork.
Sieve Stability Test - To measure the resistance by concrete to segregation.
are also conducted on SCC to gain the most desirable qualities in batch mixing. All the tests mentioned above are conducted on controlled laboratory conditions and the standard protocols to be followed on acknowledged tests, will be discussed thoroughly on a future article.
REFERENCES
BS 1881 : Part 101 : 198
BS EN 12350-2:2009
Lab Manual - Building and Construction Materials - M.L. Gambhir, Neha Jamwal
RMCAO, January 2009
***(Content of this article is protected under intellectual property Act No.36, 2003 - Sri Lanka. Copying, Reproducing or Extracting any part of this publication is strictly prohibited without the consent of the author.) ***
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