Plastic Viscosity And Flowability Of Scc Mixes Construction 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 in its fresh state, 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 facilitate 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.

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Self Compacting Concrete (SCC) mixture is strongly dependent on the composition and characteristics of its constituents in the fresh state, which have 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 capability 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 without applying any vibration, and to pass through congested reinforcement with showing a high bond and high resistance to aggregate segregation.

To maintain the flowability of the suspension and to avoid the segregation of the phases; Okamura and Ozawa [2] have suggested the following criteria to achieve self compactability [2]:

"Limited aggregate contents.

Low water-powder ratio.

And using superplasticizer."

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, as "decreasing coarse to fine aggregate ratio will reduces aggregate interlock and bridging when the concrete passes through narrow openings or gaps between reinforcement and increases the passing ability of the SCC"[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 compactability [2]

Steel Bars

Blockage can also be avoided by using a highly viscous paste. When concrete is deformed, 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).

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 the production of SCC.

Figure 2: Basic principles for production of self-compacting concrete [13]

Therefore, to increase the fluidity and viscosity of paste and reduce aggregates friction, all aggregate particles should be fully coated and lubricated by a layer of paste, this can be also attained by careful selection of the cement and additions, adding superplasticizer and (or) viscosity modifying admixture and also by limiting the water/powder ratio.

Materials and Mix Compositions:

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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. In general the mixes have the following characteristics: [2]

- Viscosity modifying agents.

- Superplasticizer.

- Decrease the amount of coarse aggregates (31.2% by volume) and increase the powder contents (500 kg/m3).

- Low water /powder ratio (0.34 by weight).

- Paste content (34.8%).

- Fine aggregate /mortar (47.5% by volume).

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]

In addition to cement, fine fillers such as ground limestone can be used, 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 (anti-segregation) which grab and hold water 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]. Using the science rheology it is becoming possible to predict fresh properties, select materials and model processes to achieve the required performance.

The main rheological characteristic of self compacting concrete are yield stress representing a minimum force required to start concrete flowing or the required shear stress 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 a resistance to flow, or 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 behaviour of SCC is governed by an adequate balance of the rheological parameters, i.e. shear stress and plastic viscosity [11]. If the viscosity is too low, it is recommended to increase the shear stress to avoid segregation. On the other hand, if viscosity is too high, the shear stress should be low.[10]

Flow of fresh concrete is described by Bingham model in the equation (1), providing that a certain degree of flow can be achieved and the concrete maintains its homogeneity.

τ = τo + μγ (1)

Ï„ - shear stress applied to material,

Ï„o - yield stress

μ - Plastic viscosity

γ - Rate of shear [7]

Shear stress, Ï„ (Pa)

Vibrated concrete

Bingham fluid

τ=τ0+µγ

Self compacting concrete

Newtonian fluid

Τ=µγ

Yield stress Ï„0

Shear rate, γ(s-1)

Figure 4: Concrete rheological models

For a very low yield stress, segregation can occurs. When flow start, the plastic viscosity takes the control of the flow behaviour which helps to maintain the stability of the concrete (to avoid segregation).

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:

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τ = τo + Kγn (2)

Also, there are different equation can be used such as Casson model and Modified Bingham model. [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]

Khayat [15] 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) 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. Also, there is a correlation between the size, shape and the distribution of the aggregates and the passing ability for SCC [22].

As we mentioned before, SCC consists of two phases, solid aggregates (solid phase) suspended in a viscous paste (paste phase). For a dilute suspension (low concentrate solid phase), the volume fraction of solids influences the viscosity of the mix [23],while for a high concentrated paste , 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].

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

The use of fibres in self compacting concrete increases considerably the toughness, the durability of cement and can also retard the propagation of cracks and increase the energy absorption [14].The application of fibres in concrete was regarded as very difficult in the past, due to insufficient workability of fibres reinforced mixtures.

"Fibres affect the characteristics of SCC in the fresh state. They are needle-like particles, they increase the resistance to flow and contribute to an internal structure in the fresh state" [9]. Fibres need to be distributed homogeneously without being clustered.

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

The effect of fibres on workability is mainly caused by the shape of the fibres is more elongated than the aggregates hence the surface area of fibres is higher for the same volume. Also "the surface characteristics of fibres differ from that of cement and aggregates, e.g. plastic fibres might be hydrophilic or hydrophobic, and the stiff fibres push apart particles that are relatively large compared to the fibre length, which increases the porosity of the granular skeleton" [9] and Finally, steel fibres often are deformed with either have hooked ends or are wave-shaped (figure 6) to improve the anchorage between them and the surrounding matrix.

Figure 7: effect of the aggregate size on the fibre distribution [Johnston,1996]

We should also mention that it is recommended to choose fibres no shorter than the maximum aggregate size [Johnston, 1996; Vandewalle, 1993] and usually, 2-4 times.[9]

A minimum shear stress (yield value) has to be surpassed to initiate the flow. "Beyond this threshold, the shear stress is linearly related (with plastic viscosity being the slope) with the increase of the rate of deformation."[9] Fibres increase the yield value (Figure 8) and the plastic viscosity (Figure 9) of SCC.

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

Figure 8: rheological measurements on SCFRC (series OS1-9): slump flow versus yield value

[9]

The effective viscosity of the suspension with fibres base on [23] can be calculated using equation (6):

ηe=η (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.

The effect of fibres depends on the composition and characteristics of SCC in the fresh state. The fibre dosage for SCC is limited, In order to obtain a concrete with fibres that is self-compacting their effect has to be compensated for high filling ability and sufficient segregation resistance.

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 fibre types having a large surface area decrease the flowability of SCC (Figures 10). Although the fibres are homogenously distributed but the contour of the flown-out concrete is not round. With using Long fibres and/or large aggregates segregation occurs without affecting the flow diameter. [9]

Figure 11:Spread area for long fibres(Dramix 80/60 BP)

Figure 12:Spread area for fibres having a low intermediate aspect ratio(Dramix 65/40 BN)

Figure 10: spread area for fibres having a large surface area (example:Darmix 80/30 BP

"Coarse (relative to the fibre length) aggregates decrease the maximum possible fibre content and increase the bar spacing required to avoid blocking". [9]

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

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

Where:

MFC is the: maximum fibre content volume.

Vf: the fibre volume (%).

Lf :fibre length (mm).

Df: fibre diameter (mm).

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 in the absence of 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]

Figure13: 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 inside the ring and just outside 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 14:J-ring test

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

[18]

Figure 15 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 16: 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, the filling ability and the flowability in congested reinforcement.

The aims of this project is to test the filling ability, the flowability and the passing 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, and checking the mechanical properties of this mixes by implementing compressive and bending test.

Relevant experience and 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 , 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.

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

The coarsest aggregates (Quartz Sand, (B) 250-600 μm) first should be mixed with Microsilica for 60 seconds, then the rest of aggregates to be mixed with cements for 60 seconds. Then 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. Remaining half of the liquid will be divided to four more parts and each part will be added in increment and mix for 60 second.

After mixing for 2 minutes fibres are added in increment (This step is omitted in mixes of without fibres).

Finally half of the remaining Superplasticizer is added and mix for 30 seconds.

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]

Figure17: J-ring test equipments

The J-ring tests both the filling ability and the passing ability of SCC. It can also be used to check the resistance of SCC to segregation visually by comparing test results from two different portions of sample. [20]

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 self-compacting concrete, (Schutter,[20]) the equipments needed are:

Base plate (900 Ã- 900 mm) made of rigid and impermeable material either 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 Figure18.

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

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 inner surface of the cone and the test surface of the base plate are just wet by using the moist sponge or towel.

After placing the base plate in a stable and level position, we fill the bucket with fresh SCC and let the sample stand still for about 1 minute (± 10 seconds).

We can place the J-ring on the base plate around the cone and fill the cone with the concrete without any external action. And any concrete remaining on the base plate should be removed.

Abrams cone

j-ring

Plain steel rods

Base plate

Concrete sample

132.5

132.5

16xΦ16

300

Δhy1

Δhy2

Δh0

Δhx2

Δh0

Δhx2

Δhx1

30mm

H=100mm0

35

35

Top view

Δhx1

Figure 18: J-ring dimensions

After 30 seconds for, in a single movement lift the cone perpendicular to the base plate, at the same time 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 18) 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).

Then we should measure the largest diameter of the flow spread (dmax), and the one perpendicular to it (dperp).

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 (1 in.) to indicate good passing ability and a difference greater than 50 mm (2 in.) indicates poor passing ability.

The flow time T50J has been recorded from test.

The J-ring blocking step BJ is calculated using equation (9) and expressed in mm to the nearest 1 mm.

(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 19) 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 19: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 (± 10 seconds).

We can lift the sliding gate and let the concrete flow out 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, at three positions, one at the centre and two at each side.

800

Figure 20: L-box bars

Expression of results

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

PL= (10)

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

(11)

Where Hmax = 91 mm and H = 150 - Δh

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. Each experimental task will be further divided into another phase: performing the test and analysing the data.

Discussion :

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 .Also, the acceptable limit for blocking and segregation for the L-Box test should be no less than 0.80.[16]

By adding fibres to SCC the workability will be reduced, further investigation should be done in the hardened state of SCC and SCFRC such as a compressive strength, fracture and tensile strength tests.