Plastic Viscosity And Flowability Of Self Compacting Concrete Engineering Essay

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Self-compacting concrete (SCC) was first developed in Japan in the late 1980's, as a high performance durable concrete. The aim was to develop a concrete mix with unique properties in the fresh and hardened state. In the plastic state, SCC can flow under its own weight and maintain homogeneity while completely filling any formwork and passing through congested reinforcement for different structural geometric conditions without the need of internal vibration, with showing a high bond and high resistance to aggregate segregation, which improves the quality of concrete work, the surface finish and the working environment.

By adding fibres to SCC, the result will be a Self Compacting fibre reinforced concrete (SCFRC), which combines the benefits of SCC in the fresh state and a high performance in the hardened state. Moreover, using SCC can offer many advantages for the precast, prestressed concrete industry and forecast-in-place construction.[1] Also it will simplify the placement process of concrete through densely reinforced bars and restricted sections without vibration; therefore, it is necessary to ensure an excellent deformability, good stability, filling and flow ability with low risk of blockage.

The composition and characteristics of SCC's constituents in the fresh state have a great effects and impacts on the hardened properties; therefore, it is very important to understand the flow behaviour and the viscosity of Self Compacting Concrete without adding fibres and the impact of adding fibres on its behaviour.

The slump test is widely used to evaluate the workability of concrete. Other flow characteristics such as viscosity, passing ability and filling capabilities are needed to understand the flow behaviour of SCC.

The objective of the literature is to present a background about self compacting concrete and its characteristics and study the flowability and the viscosity of self compacting concrete and describe the method used to measure them.

Mechanism of achieving self compacting concrete:

Self-compacting concrete is a two-phase particle, solid phase suspended in a viscous liquid paste phase. The main characteristics of SCC are the properties in the fresh state, which have a great influence on the flow ability, pass ability and homogeneity, that makes SCC able to flow and completely fill complex formwork under its own weight, and to pass through congested reinforcement with showing a high bond and high resistance to aggregate segregation without applying any vibration.

To maintain the flowability of the suspension and to avoid the segregation of the phases; Okamura and Ouchi (2003) have suggested the following criteria to achieve self compactability:

"Limited aggregate contents.

Low water-powder ratio.

And using superplasticizer."[2]

When the concrete is deformed, the relative distance between particles can decrease and then the internal stress can increase particularly near obstacles. Research has found that as the internal stress increases, the energy required for flowing is consumed, resulting in blockage, this can be overcome by limiting the coarse aggregates whose energy consumption is particularly intense, which will "reduces aggregate interlock and bridging when the concrete passes through narrow openings or gaps between reinforcement" and result in a good passing ability of the SCC mix [15].

Aggregates

High deformability and high segregation resistance

Rounded + reduction of max size

Mortar

- Compatible of deformability &viscosity

Low (w/p)

High superplasticizer dosage

-low pressure transfer

Limited fine aggregate content

Figure 1: Mechanism for achieving self compacting concrete[2]

Steel Bars

Blockage can also be avoided by using a highly viscous paste. Paste with a high viscosity prevents localised increases of internal stress due to the approach of coarse aggregate particles [23]. It is also recommended to replace some quantity of cement by filler agents such as lime stone, fly ash and GGBS (Ground Granulated Blast Furnace slag) to increase the cohesiveness and the flowability of the mix.

By using superplasticizer and reducing the water-powder ratio, self compacting concrete achieves a high deformability while ensuring the deflocculating of fine particles. (Figure 2) shows the basic principles for achieving self compactability.

Figure 2: Basic principles for achieving self-compactability [2][23]

Therefore, to achieve self compactability, all aggregate particles should be fully coated and lubricated by a layer of paste to reduce aggregates friction. Adding superplasticizer will reduce the water demand and increase the flowability of SCC, which means better workability. And finally, viscosity modifying admixtures can increase the viscosity of the mix to fill the interstices of the granular skeleton.

Materials and Mix Compositions:

In general, the basic ingredients used for self compacting concrete mix are similar to those used for the conventional high performance vibrated concrete as we can see in (Figure3), except they are mixed in different proportions, in addition to the admixtures used in Self compacting concrete and the control of aggregate.

10%

10%

18%

18%

8%

26%

25%

45%

36%

2%

2%

Cement Water Air Fines Fine Aggregate Coarse Aggregates

Regular concrete

SCC mix

Figure 3: proportion of constituents for SCC and regular mixes[2]

Fine fillers such as ground limestone can be used in addition to cement, which my increase the ratio of fine to coarse aggregates by up to 50 percent of the total aggregate fraction. Experimental program on different SCC mixes using slump test showed that replacing part of cement by Ground Granulated Blast furnace Slag (GGBS) had a good contribution to the slump value, which means increasing the flowability and the segregation resistance of the mix by making a good homogeneity, cohesiveness between material ingredients.

Also, the quantity of paste in the mix highly influences the workability of SCC and increases the value of slump. Moreover, as we can see in the (table1) [23], by introducing new type of superplasticizer (Glenium ACE 333) instead of naphthalene, we can get a better result in the slump test i.e. better flowability and segregation resistance.

Table1: composition of SCC(without fibres) and related slump values

(*naphthalene based SP,**adding fibres to the mix)[23]

Flow characteristics of self compacting concrete:

SCC is made from the same basic constituents as conventional concrete but with the addition of viscosity-modifying admixtures, which are a water-soluble polymers or inorganic substances with a very high surface area that bind and grab water upon mixing to control bleeding, and high levels of superplasticizer admixtures to impart high workability and reduce the water-powder ratio (w/p).

It is difficult to understand fresh SCC behaviour without fully comprehended its rheology. "Rheology can be defined as the science of flow and deformation of matter. The placing, spreading, pumping and compaction of any concrete depends on rheology[7]", which enable us to predict fresh properties therefore choosing the right materials to achieve the required performance.

The main rheological characteristics of self compacting concrete are yield stress representing the required shear stress or the minimum force required to initiate the flow, which tend to be nearly zero due to adding superplasticizer which ensure that SCC will flow under its own mass, and moderate plastic viscosity which helps in maintaining the stability of SCC and resist segregation, and can be described as the stiffness of fresh concrete. But it should be mention that the high amount of this value will makes concrete difficult to mix and place. [3]

The flow behaviour of SCC is governed by an adequate balance of the rheological parameters, i.e. shear stress and plastic viscosity [11], and can be described using Bingham model (equation 1).

τ = τo + μγ (1)

Ï„ - Shear stress applied to material.

Ï„o- Yield stress.

μ - Plastic viscosity.

Shear stress, Ï„ (Pa)

γ - Rate of shear [7].

Vibrated concrete

Bingham fluid

τ=τ0+µγ

Yield stress Ï„0

Self compacting concrete

Newtonian fluid

Τ=µγ

Shear rate, γ(s-1)

Figure 4: Concrete rheological models

In some cases where the yield stress appears to be negative, Bingham equation is no longer valid; therefore Herchel-Bulkley model in the equation (2) will be used to describe the flow behaviour of SCC:

τ = τo + Kγn (2)

Different equations can also be used such as Casson model and Modified Bingham model and can be found in the reference [12].

Wallevik [21] has proposed an acceptable region for the rheological properties of SCC, where the darker area in the (Figure 5) represents the optimum field.

Figure 5: Proposed area in yield value-viscosity diagram for SCC [21]

It becomes possible to understand the flow behaviour of self compacting concrete once the plastic viscosity is known, providing that it is best described by Bingham equation and the other parameter (yield stress) is constant over a wide range of plastic viscosity.[3]

Khayat (1999) has suggested that viscosity can be enhanced by reducing the water to cementitious material or by incorporating low to moderate dosage of viscosity modified agent (VMA)[15], which is not only increases the viscosity but also provides mixture robustness and overcomes effects due to poor aggregate shape and grading [5], and therefore reduces the risk of blockage. Other materials have a different contribution to the yield stress and the plastic viscosity (table 2).

Table 2: The contribution of different materials to the yield stress and the plastic viscosity.

The effect of fibres on the behaviour of self compacting concrete:

In the hardened state, the incorporation of steel fibres in self compacting concrete increases considerably toughness, durability, ductility as well as tensile strength and can also retard the propagation of cracks and increase the energy absorption [14].

While in the fresh state, steel fibres was regarded as very difficult due to the lack in workability of mixtures and the resistance to flow,[9] this is mainly caused by the elongated shape of fibres compared with aggregates size hence the surface area of fibres is higher for the same volume. Also, because those stiffer fibres push apart aggregates that are relatively large which will increases the porosity of the granular skeleton.

Figure 6: different types of steel fibres used for the production of FRSCC.[6]

And finally, fibres are often deformed with either hooked ends or wave-shaped (figure 6) to ensure the anchorage with the surrounding area.

Fibres need to be distributed homogeneously with limited dosage. Usually, it is recommended that they are 2-4 times the maximum aggregate size. To determine the maximum fibre content, a minimum slump flow of 600mm is acceptable with no segregation of SCC and no clustering of fibres along the flow. The observation of three tests using slump flow, show that using fibres with a large surface area (Figures 7) decreases the flowability of SCC with less than 600mm. Although there is no segregation but the flow diameter is not round [9]. By using Long fibres segregation occurs without affecting the flow diameter (figure8). The third test where low intermediate aspect ratio of fibres was used gives a combination of the two previous results i.e. segregation occurs and the flow diameter was not round (figure9). [9]

Figure 7: spread area for fibres having a large surface area (example:Darmix 80/30 BP

Figure 8:Spread area for long fibres(Dramix 80/60 BP)

Figure 9:Spread area for fibres having a low intermediate aspect ratio(Dramix 65/40 BN)

Fibres increase the yield value (Figure 10) and the plastic viscosity (Figure 11) of FRSCC.[9]

Figure 11: effect of the fibre factor and the mixture composition on the plastic viscosity (series OS1-9)[9]

Figure 10: rheological measurements on SCFRC (series OS1-9): slump flow versus yield value[9]

The maximum fibre factor (Vf·Lf /Df) can be determined using this equation (7):

Maximum fibre factor = (0.781 − MFC)/0.211 (7)

Where:

MFC: is the maximum fibre content volume.

Vf: the fibre volume (%).

Lf :fibre length (mm).

Df: fibre diameter (mm).

Predicting the plastic viscosity of SCC and FRSCC

Many attempts have been done in this regard by assuming that SCC consists of two phases, solid aggregates (solid phase) suspended in a viscous paste (paste phase). For a dilute suspension (low concentration of solid phase), the volume fraction of solids influences the viscosity of the mix [23],while for a high concentrated particles , the volume fraction is not the only parameter that influence the viscosity , but also the size, type of particles and their hydrodynamic interaction.

The viscosity for low concentration suspension based on Ford[23]can be calculated using equation (4), and equation(5) gives the general expression of the viscosity for a high concentrated suspension.[23]

ηr =(1-[η]Φ)-1 (4)

ηr =1+[η]Φ+BΦ2+CΦ3+... (5)

ηr is the relative viscosity i.e. ratio of viscosity of the suspension (mortar or concrete) to that of the liquid phase (cement paste).

Ï• is the volume concentration of particles.

[η] is the intrinsic viscosity which is a measure of the effect of individual particles on the viscosity.

B and C are parameters can be given from the reference [23].

Ghanbari and Karihaloo[3] have shown using micromechanical models how to predict the plastic viscosity of SCC and FRSCC .The effective viscosity of the suspension with fibres base on [23] can be calculated using equation (6):

(6)

η the plastic viscosity of the mix without fibres and ld is the fibre aspect ratio and Φ is the volume fraction of fibres.

Testing Self Compacting Concrete

Number of tests can be used to evaluate the self-compactability of concrete, including passing ability, filling capacity, and segregation resistance; some of those tests are slump flow, V-Funnel, J-Ring and L-box [15]

The slump flow is a simple rapid and on site test procedure, to evaluate the horizontal free flow (deformability) of SCC and FRSCC when there are no obstructions. It is very similar to the slump test used for conventional concrete except that, instead of the loss in height, two perpendicular diameters are recorded as slump flow. The higher the slump flow, the greater the concrete's ability to fill formworks. Also the time required for the concrete to reach a diameter of 500mm is also measured and recorded as T500, which represent the viscosity of concrete, and indicates how stable the concrete is. A lower time points to a greater flowability.

[16]

Figure12: slump flow test

J-ring can also be used on site; this test is used to determine the passing ability of the concrete. It is an extension of the slump flow test in which a heavy ring apparatus is used, the passing ability of concrete will be indicated via measuring the difference in height between the concrete just outside the ring and inside it. the time required for the concrete to reach a diameter of 500mm is also measured as in case of slump test and recorded as T50J, which represent the viscosity of concrete

[17]

Figure 13:J-ring test

The V-funnel test is used to determine the flowability or viscosity of concrete. After filling the funnel with about concrete (about 12 litre), the less the time takes to flow through the apparatus will indicates a good flowability and stability of concrete.

[18]

Figure 14 funnel test

L-box: measures the filling ability and passing ability of SCC, this is a lab test which describes the ability of concrete to pass reinforcement bars without blocking. After filling the vertical column of the L-box we lift the gate to allowed SCC to flow into the horizontal part after passing through the rebar obstructions. We then measure the concrete heights (h1, h2) at the beginning and end of the horizontal section respectively. The ratio h2/h1 represents the filling ability and typically, this value should be greater than 0.8, while the passing ability can be described visually by inspecting the area around the rebar, with an even distribution of aggregate indicating good passing ability.

[18]

Figure 15: L-Box test

Project proposal:

Introduction:

In the fresh state, workability has been tested using slump flow test which is the most common test for SCC, but when there is a reinforcement, the mix of self compacting concrete (SCC) and self compacting fibre reinforced concrete(SCFRC) should be tested using different test methods such as L-Box and J-Ring (J-fibre penetration) tests to measure the passing, filling and the flowability in congested reinforcement.

The aims of this project is to test the flowability, the passing and filling capacity of self compacting concrete and self compacting fibres reinforced concrete, using the two different experimental tests, L-Box and J-Ring, with different mix proportion, comparing results with Slump test results, and checking the mechanical properties of this mixes by carrying out compressive, tensile and bending test.

Proposed work.

Materials and mixing procedure of SCC and SCFRC:

The constituency of the mix will contain two ranges of quartz sand with the same proportion 9-300 μm and 250-600 μm (table 3) , the superplasticizer (SP) used to ensure the deflocculating of fine particles will be Glenium ACE 333. Cements, Microsilica, Ground Granulated Blast furnace Slag (GGBS) and viscosity modified agent (VMA) will also be used as well as tap water. 2.5% (of volume) of long Dramix fibres 30 mm will be added to mix.[23]

Table3: A typical mix design of SCC before adding fibre.

To get the mixes ready for test, the following mix sequences will be used:

The coarsest aggregates representing quartz sand type(B) from the (table 3) should be first mixed with Microsilica for 60 seconds, then the rest of aggregates type(A) will be mixed with cements for 60 seconds. After that we add GGBS and mix it for 60 seconds. Thereafter, half of superplasticizer and all water will be mixed together and half of this liquid will be added and mixed for 60 seconds.

Then the rest of the liquid will be divided to four parts and added each part in increment then mixed for 1 minute. The final step will be adding fibres in increment and mix it for 2 minutes then the rest superplasticizer will be added and mix it for 30 seconds [23].

J-Ring test:

J-Ring test used to determine the passing ability of SCC, i.e. the ability of the concrete to flow under its own weight to completely fill all spaces within the formwork. Usually, J-Ring can be used in conjunction with the slump-flow test and a comparison between results is needed to understand the effects of reinforcement on the flow behaviour of the mix.

[17]

Figure16: J-ring test equipments

The J-ring can also be used to check the resistance of SCC to segregation visually.

Using the J-ring test we can get results for three parameters:

"The flow spread indicates the restricted deformability of SCC due to blocking effect of reinforcement bars.

The flow time T50J indicates the rate of deformation within a defined flow distance.

The blocking step quantifies the effect of blocking."[20]

Equipments:

Based on the guidelines for testing fresh SCC, (Schutter,2005) the equipments needed are:

Rigid base plate (900 Ã- 900 mm) made of steel, plywood or hard plastic, with smooth and plane test surface. A circle of Ø500mm should be mark at the centre of the plate.

Abrams cone, with diameter equal to 100mm bottom and 200 mm for the top and the height of 300 mm, as shown in Figure17.

Open steel ring with a rectangular section 30mm, diameter 300mm and steel bars (Φ16mm) fixed in it with 100mm height.

Stopwatch to record the flow time T50 when the flow reach the diameter 500 mm with the accuracy of 0.1 second, and a ruler(graduated in mm).

Test procedure:

Before starting, we should insure that the base plate and the inner surface of cone are just wet.

After ensuring the stability of the base plate, we fill the bucket with fresh SCC and leave it for 1 minute to stand still.

Abrams cone

j-ring

Plain steel rods

Base plate

Concrete sample

132.5

132.5

16xΦ16

300

Δhy1

Δhy2

Δh0

Δhx2

We fill the cone with the concrete without any external action.

Top view

15mm

Δhx1

Δhx2

Δh0

H=100mm0

Bj

30mm

Figure 17: J-ring dimensions

After 30 seconds, in a single movement we lift the cone and start the stopwatch.

When the concrete reach the diameter 500 mm, stop the stopwatch and record the reading as the T50J value.

The measurements:

We should record the following measurements:

(Δh0) from the (figure 17) represents the differences between the lower height outside the ring and the central position.

Δhx1, Δhx2: in the x-direction (the largest spread diameter).

Δhy1, Δhy2: in the y-direction (perpendicular to x).

(dmax), the maximum diameter of the flow spread.

(dperp), perpendicular diameter to dmax.

Expression of results

The flow spread of the J-Ring (SJ) indicates the restricted deformability of SCC and can be expressed in the equation (8).

(8)

The guideline that the difference between slump test flow and J-Ring should be less than 25 mm to indicate good passing .

The flow time T50J has been recorded from test.

The J-ring blocking step BJ is calculated using equation (9).

(9)

If BJ is less than 10mm then, no blocking can be observed and value over 20mm is not acceptable.

This test should be repeated after adding fibres, and same measurements should be recorded.

L-box (Reference method for passing ability)

L-box test aims at investigating the passing or blocking ability of SCC through specified gaps of steel smooth bars and the flowing behaviour within a defined flow distance. [20]

Equipment:

L- Box (figure 18) with either two or three smooth bars, the gaps between bars are 41 and 59 mm, respectively.

Spirit level, to insure that the L-box is level.

Figure 18:L-Box test dimension

Test procedure

After ensuring that the L-box is stable and level position and the gate is closed, we fill the vertical part of it with the right amount of concrete (12.7 litres).

Then we can let the concrete rest for one minute.

We can lift the gate to allowed the flow of concrete into the horizontal part of the L-box, then we can measure (Δh) the average distance between the top edge of the box and the concrete.

800

Figure 19: L-box bars

Expression of results

The passing ratio PL can be calculated using this equation (10):

(10)

And the blocking ratio BL can be calculated using equation (11):

(11)

Where Δh=150-H2

The target is to reduce Δh value until it reaches zero value.

Segregation resistance will be judged visually.

Diagrammatic work plan:

The figure below represents the project time-line. As presented below, the project consists of 4 main experiments, J-Ring, L-Box, Compressive and Tensile tests. For each test analysing data is needed and we should perform a whole analysis for the entire project. Writing report will be carried out throughout the whole period.

Discussion :

By adding fibres to SCC the workability will be reduced. In order for the mixture to pass through the reinforcement successfully, the difference between the slump flow and the J-ring should not exceed 50mm, and the blocking step no more than 10mm without fibres and 20 mm after incorporating fibres. For L-Box, the acceptable limit for blocking should be no less than 0.80. [16] Segregations for both tests can be judged visually.

Further investigation should be done in the hardened state of SCC and SCFRC such as a compressive strength, tensile and bending tests, 3 types of specimens , Beams 500x100x100 mm, 100mm cubes and 100x200 mm cylinders, will be produced with standard cast steel moulds.

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