All about self-compacting concrete (from A to Z)

Introduction

SCC is the concrete that is able to flow under its own weight and completely fill the formwork, even in the presence of dense reinforcement, without the need of any vibration, whilst maintaining homogeneity.

There is no doubt that understanding the concrete rheology concepts and terminologies will play a crucial role in determining your competency in proportioning and designing self-compacting concrete.

Rheology as a science is simply defined as the study of fluidity and deformation of matter while focusing on the interrelation between force, deformation and time. Hence, concrete rheology science deals with the fresh properties of concrete considering concrete as a fluid or a semi-fluid materials explaining the development of these properties such as viscosity and flowability and how they are changed with time.

Rheological parameters are many such as flow resistance, torque viscosity and structural build-up, however the most effective parameters in controlling concrete workability are the shear yield stress and plastic viscosity.

The yield stress is defined as the stress that must be applied to the sample before it starts to flow. Simply, you can consider yield stress as the boundary between two material states. below the yield stress, the material will be solid-like and will not flow, however above the yield stress the materials will flow like a liquid. It can be determined via testing such as the compression test.

In the case of Bingham fluids such as concrete, plastic viscosity is defined as the slope of the stress versus shear rate in the high shear rate limit. Simply, it is defined as the resistance of concrete to flow resulting from the mechanical friction within concrete.

As plastic viscosity increases, the stability of concrete increases, and the flowability decreases and this is always the most important challenge while designing SCC as you should decrease the plastic viscosity to the extent that allows the concrete to flow smoothly while keeping an eye on the stability which is decreased as viscosity decreased. Decreasing stability of concrete means to be highly vulnerable to segregation.

Recommended volume of coarse aggregate in SCC according to ACI 237R

- The basic rule is the same for both conventional and self-compacting concrete: The target is to use the maximum volume and N.M.S of coarse aggregate while still maintaining appropriate passing ability and stability, and for that purpose (enhancing passing ability and stability), the N.M.S for SCC may be reduced by one size compared to that recommended in ACI 301 for conventional concrete.

-?The exact volume of coarse aggregate and S/A ratio in the SCC mix design will be precisely estimated based on the trials you will conduct with your own materials for the proposed mix design, however the values mentioned here by the author are a recommended guideline according to ACI 237R to be a starting point for proportioning the mix:

1- If the coarse aggregate N.M.S equals ? inch ( 12.5mm ) or greater, 50% of the total volume of concrete should be filled with the bulk volume of the aggregate ( in terms of absolute volume, 28 to 32% ) for congested formwork, however this percentage may be increased if there is no concerns about blocking or passing ability.

For example: Given the maximum aggregate size 12.5mm, the dry rodded united weight of the aggregate is 1650, Sp.gr = 2.65

The aggregate weight = 0.5 * 1650 = 825 kg/m3

Absolute volume = 825 / ( 1000 * 2.65 ) = 0.31 ( 28 – 32% )

2- When the coarse aggregate N.M.S is less than 12.5mm, it is recommended to start the first trial batch with initial proportion 50% sand and 50% aggregate by volume which means the S/A ratio ( Sand to Total aggregate ) = 50

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Basic properties & concepts of Self Compacting Concrete ( SCC )

Unlike conventional concrete, SCC has its own concepts for workability and Rheological properties which make it unique. The basic concepts of SCC could be summarized as the following:

1- Passing ability

It is the ability of SCC to flow smoothly through tight openings such as spaces between reinforcing bars without segregation or blocking.

Enhancing passing ability could be achieved through precisely determining the Suitable volume and NMS of coarse aggregates in the mix design.

Generally, passing ability is assessed through the following tests:

- J-ring test

- L-box test

2- Filling ability

It is the ability of SCC to flow into and fill completely all spaces within the formwork, under its own weight.

Enhancing filling ability could be achieved through determining the right dosage of the PC admixture and suitable water content to attain the required flow over the specified time.

Generally, filling ability is assessed through the following tests:

- Flow test

- T50

- V-Funnel

3- Stability ( Segregation Resistance )

It is the ability of SCC to remain homogeneous in composition during transport and placing.

Enhancing stability could be achieved either by increasing the fines through increasing cementitious materials content and incorporating extremely fine materials such as Micro Silica or limestone powder, or by depending on special chemical admixtures which enhance the concrete viscosity such as VMA ( Viscosity Modifying Admixture ).

Generally, the SCC stability is assessed through the following tests:

- VSI ( Visual Stability Index )

- Column Segregation.

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SCC filling ability assessment tests

We can assess the filling ability of SCC through three fresh concrete tests could be summarized as detailed below:

1-?Slump flow test

In this test, the traditional slump cone is completely filled without consolidation then lifted to measure the spread of the concrete which normally ranges from 550mm to 750mm.

2- T50 value

- During the slump flow test, the viscosity of the SCC mixture can be estimated by measuring the time taken for the concrete to reach a spread diameter of (500 mm) from the moment the slump cone is lifted up.

- This is called the (T50) measurement and typically varies between 2 and 7 seconds for SCC. Results below 5 seconds reflect appropriate flowability of the concrete especially when concrete will have to travel long horizontal distances.

3- V-Funnel test

- During the test, the V-shaped funnel is filled with about 12 liter of concrete without any compaction or tamping.

- Open the trap door within 10 sec after filling allowing the concrete to flow out under gravity noting that the whole test should consume no more than 5 minutes.

- Results below 10 seconds reflect appropriate viscosity and ease to flow of concrete, however results beyond 12 seconds may be a sign of high viscosity of concrete.

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SCC passing ability assessment tests

Passing ability is one of the most unique features of SCC and so the following tests have been designed to assess it:

1- J-Ring test ( ASTM C1621 )

- Simply two slump flow tests will be conducted, one with the J-Ring and one without it and the following values will be measured

1- J-Ring slump flow

2- slump flow

- The difference between the two tests will be measured in mm and this value will represent the J-Ring test result.

- Differences below 25mm would reflect a satisfactory passing ability of the SCC, while differences more than 50mm is a sign of poor passing ability of the SCC.

2- L-box test ( BS EN 12350 )

- Fill the vertical part of the apparatus with about 14 L of concrete and leave it for one minute.

- Lift the sliding gate and allow concrete to flow out into the horizontal section. - Measure H1 ( height of concrete at the vertical section ) and H2 ( height of concrete at the horizontal section ).

- Calculate the blocking ratio ( H2/H1 ) and report it as the final result of the test. - Generally, the blocking ratio should not be below 0.8 to ensure proper passing ability of the SCC.

3- U-box test

- Fill the apparatus with about 20 L of concrete and wait for one minute.

- Lift the sliding gate and allow the concrete to flow into the other compartment of the apparatus.

- Measure H1 ( height of the concrete at the first compartment which has been filled ) and H2 ( height of concrete in the other compartment ).

- Calculate the filling height ( H1 - H2 ) and report it as the test result.

- The nearest of the filling height value to zero, the better passing ability of the SCC.

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SCC stability ( segregation-resistance ) assessment tests

?In fact, i believe that stability is the real challenge of SCC because it's the ability to retain and keep concrete homogeneous without segregation while having slump flow up to 700 mm for at least two hours. Hence, the following tests have been designed to assess this important feature of SCC:

1- Column Segregation ( Static Segregation ASTM C 1610 )

- Fill the column with the SCC and allow it to sit for 15 minutes.

- After that, wash the concrete in the top and bottom sections of the column over a no. 4 sieve, and the retained aggregate is weight.

- Calculate the static segregation percentage from the following equation:

S = 2 ( (CAb - CAt) / (CAb + CAt) ) * 100 if CAb > CAt , and

S = 0 if CAb <= CAt

where

S is the static segregation percent

CAb is the weight of coarse aggregate in the bottom section

CAt is the weight of coarse aggregate in the top section

- Values below 10% for segregation percent reflect a satisfactory stability, however values more than 15% is a sign of poor stability and high vulnerability for segregation.

- Finally, the test may not be suitable for field assessment because it consumes more than 20 minutes, instead it is suitable for plant trials and initial proportioning of the mix design.

2- Rapid assessment of static segregation using penetration test ASTM C 1712

- The test does not directly measure the static segregation resistance, instead it provides assessment of whether static segregation is likely to occur or not.

- Fill the inverted slump mold with SCC without tamping and strike off the surface of the concrete.

- Allow the concrete to sit for 80 +/- 5 seconds.

- Place the penetration apparatus on the top of the inverted slump mold making sure that the hollow cylinder in the center.

- Lower the hollow cylinder carefully to just touch the surface of concrete and then tighten the set screw.

- Take the initial reading on the reading scale at the mark that is in line with the top of the metal rod.

- release the set screw to allow the hollow cylinder to freely penetrate the concrete.

- Wait for 30 +/- 2 sec after releasing set screw and then take the final reading on the reading scale at the mark that is in line with the top of the metal rod.

- calculate the penetration depth ( Pd ):

Pd = d2 - d1 where

d1 is the intial reading (mm)

d2 is the final reading (mm)

- Assess the result according to the following:

1- Pd <= 10 mm ( SCC is resistant )

2- 10mm < Pd < 25mm ( SCC is moderately resistant )

3- Pd >= 25 mm. ( SCc is not resistant )

3- Visual Stability Index test ( VSI ) ASTM C1611

- This test is used to qualitatively assess the stability of the SCC based on visual evidence of segregation and bleeding.

- VSI = 0 or 1 is accepted , however values of 2 or 3 may be a sign of poor stability.

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