Mix design procedure, tests, and standards

Mix design procedure, tests, and standards

1

Mohammed Sonebia, Ammar Yahiab Queens’ University Belfast, Belfast, United Kingdom, bUniversity of Sherbrooke, Sherbrooke, QC, Canada

a

1.1

Mix design procedure of SCC

1.1.1 Background The mix design of SCC is chosen to achieve the required all performance criteria for the concrete in both the fresh and hardened states. As in the case of vibrated concrete, the water-to-supplementary cementitious materials (w/cm) is one of the fundamental mixture parameters governing the properties, including rheology, strength, and durability of SCC. As in the case of normal vibrated concrete, the w/cm is one of the fundamental key factor governing. However, in designing SCC, there are a number of factors that should be taken into consideration to a greater degree than conventional concrete. These include: (1) properties of locally available raw materials, including physical properties of supplementary cementitious materials (SCM) mineral additions and aggregates, (2) proper selection of chemical admixtures to ensure good compatibilities with the selected SCM, and (3) adapt the fresh properties given the casting method (pumping, etc.), geometry of cast element and reinforcing bars arrangements. Mix design of SCC should consider both the fresh and hardened properties according to the application on hand. The consideration shall include specifications for the content of SCM and fillers, the water content or w/cm, the volume of coarse aggregate, the sand-to-aggregate ratio (S/A), as well as the air content for durability specifications. Proper selection of the type and combinations of chemical admixtures is part of the mix design process and depends on the flow characteristics requirement given the application on hand. In principle, three basic mixture-proportioning approaches for designing SCC mixtures have been used: powder type (i.e. increasing the powder content and fine fractions in the form of fly ash, blast-furnace slag or limestone filler); viscosity agent type (i.e. using suitable viscosity-modifying admixture to improve stability of the mixture); and combined type (i.e. combining the abovementioned approaches).

1.1.2 Mix design approach In principle, three different approaches have been used for the production of SCC. The first approach consists in increasing the content of ultra-fines particles by the addition of SCM. This approach was mainly based on increasing the supplementary Self-Compacting Concrete: Materials, Properties, and Applications. https://doi.org/10.1016/B978-0-12-817369-5.00001-5 Copyright © 2020 Elsevier Inc. All rights reserved.

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Self-Compacting Concrete: Materials, Properties, and Applications

cementitious materials, including fly ash, blast-furnace slag, or silica fume, and fillers content (powder type) (Sonebi and Bartos, 1999; Skarendahl, 2001). The second approach consisted in decreasing powder content and incorporating a moderate dosage of viscosity modifying admixtures (VMA) to secure an adequate stability of the mixture. On the other hand, the third approach consisted in incorporating relatively moderate dosage of VMA and supplementary cementitious materials to design SCC. The first and second approaches were initially used in Japan and Asia, while the third one was employed in North America (ACI 237). For each of these design approaches, the water content is consequentially selected to ensure the required properties: (a) Minimum free water content: this correspond to the powder type SCC that consists in using a relatively higher content of fine materials and a lower water content to enhance rheology and passing ability of SCC. In such cases, the w/cm of 0.30–0.35 is typically used with a content of fines
80 μm of 500–600 kg/m3. This approach may require, however, a relatively high dosage of superplasticizer (SP) or high range water reducer admixture (HRWRA) to obtain the targeted deformability. In general, this approach can result in SCC mixtures with low yield value and moderate-to-high viscosity. The partial replacement of cement with a less reactive powder is necessary to enhance rheology and reduce heat of hydration. (b) Moderate water content and proper concentration of viscosity-modifying agent (VMA): This approach corresponds to VMA SCC type. In this approach, the w/cm can be maintained at the proper level to achieve the targeted mechanical properties and durability. A proper dosage of VMA is used to secure adequate stability, while reducing the powder content. Depending on the paste content, the use of VMA along with HRWRA can ensure good deformability and adequate stability, hence resulting in good filling capacity. (c) Low water content and low concentration of VMA: In this approach, the combination of a given content of supplementary materials and low dosage of VMA is employed. Such SCC mixtures are typically more robust than those proportioned with high powder content and low w/cm (Khayat, 1998; Sonebi, 2006). Robust mixtures are less sensitive to deviations in the mixture composition, characteristics of the raw materials, and water content.

1.1.2.1 Japanese method for designing SCC The Japanese method of designing SCC (Ozawa et al., 1992) is a relatively workintensive method, but provides a fundamental understanding of the interaction of mix ingredients of SCC. This method can be summarizing in three steps: performing paste test, optimizing the mortar paste and finally adjusting SCC mixes. This method is based on the assumption that moderate-heat Portland cement or belite-rich cement is the only source of powder materials. With this method, if the requirement for selfcompactability is satisfied, the required performance of the hardened concrete is generally achieved. The coarse aggregate content (<20 mm) should be 50% (volumetric coarse aggregate to solid volume in concrete excluding air). The other part should be the mortar phase, which is composed of 40% volume sand (<5 mm) and 60% cement paste (<0.09 mm). The water-powder ratio is assumed to be 0.9–1.0, by volume, depending on the properties of the powder and, finally, the superplasticizer dosage

Mix design procedure, tests, and standards

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as well as and the final water-powder ratio are determined in order to ensure selfcompactability.

1.1.2.2 CBI method for designing SCC The Swedish Cement and Concrete Research Institute (Petersson and Billberg, 1999) proposed a design method for SCC. This approach considers the concrete as a suspension of solid aggregate phase in a viscous paste phase constituted by the powder, water and admixtures and taking into account the void content of the aggregates, the effect of the aggregates on passing ability (risk of blocking) and the physical characteristics of fine mortar. The effect of a single sized fraction of aggregate on the passing ability is experimentally measured with L-box test (Bartos et al., 2002) and the blocking ratio should be <0.80.

1.1.2.3 Design of experiments (DoE) method Design of experiment method (DoE) has been used to optimize SCC mixes by investigating the influence of mix ingredients on the rheological parameters, the filling ability, the passing ability, the segregation and mechanical performances (Khayat and Ghezal, 1999; Sonebi et al., 2007a,b; Sonebi, 2004a,b). The optimization of SCC often necessitates carrying out several trial batches to achieve adequate balance between the rheological and fresh properties, as well as the mechanical performances. The factorial design method was successfully used to determine the influence of the key parameters and their interactions on the relevant properties of SCC, and to establish statistical models, which simplified the test protocol required to optimize a given SCC mix by reducing the number of trial batches needed to achieve optimum balance among various mix variables.

1.1.2.4 Other approaches for designing SCC Other approaches have been reported in the literature. The LCPC method from Laboratoire Central des Ponts et Chauss
ees in France has been based on the solid suspension models to predict the packing density of all dry granular constituents (Sedran and de Larrard, 1999). The University College London method has been developed based on mortar tests (Domone, 2006). On the other hand, the Icelandic Building Research Institute method (Wallevik and Nielsson, 1998) has been based on the rheological properties of SCC (yield stress and plastic viscosity).

1.1.2.5 Typical SCC mixture design The mix design of various SCC mixtures used in Japan, Europe, and North America is summarized in Tables 1.1–1.3. On the other hand, the ACI 237 committee recommend typical powder contents given the targeted slump flow of SCC (ACI 237) (Table 1.4).

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Self-Compacting Concrete: Materials, Properties, and Applications

Table 1.1 Typical SCC mixtures used in Japan. Constituents (kg/m3)

J-powder type

J-combined type

J-VMA type

Water Cement Fly ash Ground blast furnace slag SCM Sand Coarse aggregate HRWR VMA Slump flow (mm)

175 530a 70 – 600 751 789 9.0 0 625

165 298 206 – 504 702 871 10.6 0.0875 660

165 220 – 220 440 870 825 4.4 4.1 600

a

Low heat cement is used.

Table 1.2 Typical SCC mixtures used in Europe. Constituents (kg/m3)

E-powder type

E-combined type

E-VMA type

Water Cement Limestone Fly ash Ground blast furnace slag SCM Sand Coarse aggregate HRWR VMA Slump flow (mm)

190 280 245 – – 525 865 750 4.2 0 600–750

192 330 – – 200 530 870 750 5.3 0 600–750

200 310 – 190 – 500 700 750 6.5 7.5 600–750

Table 1.3 Typical SCC mixtures used in North America. Constituents (kg/m3)

E-powder type

E-combined type

E-VMA type

Water Cement Limestone Fly ash Ground blast furnace slag SCM Sand Coarse aggregate HRWR (mL) VMA (mL) Slump flow (mm)

174 408 – 45 – 525 1052 616 1602 0 710

180 357 119 – – 476 936 684 2500 0 660

154 416 – – – 416 1015 892 2616 542 610

Mix design procedure, tests, and standards

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Table 1.4 Recommended powder content ranges.

Powder content (kg/m3)

Slump flow < 550 mm

Slump flow between 550 and 600 mm

Slump flow > 650 mm

355–385

355–385

>458 mm

1.1.3 Procedures to adjust the mix composition of SCC If necessary, adjustments to the mix composition should then be made to fulfil the workability and mechanical specifications. Once all requirements are fulfilled, the mixture should be tested at full scale in the concrete plant and if necessary at site to validate both the fresh and hardened properties given the mixing and working conditions on hand. If the required performance is not achieved, consideration should be given to redesign the mixture. The following actions might be appropriate, depending on the source of problem (The European Guidelines, 2005): l

l

l

l

l

Adjust the water/powder ratio and test the flow and other properties of the paste; Use and test different types of mineral addition; Adjust the proportions of fine aggregate and the dosage of superplasticizer; Consider using a viscosity modifying agent to reduce sensitivity of the mixture; Adjust the proportion or grading of the coarse aggregate and the ration of fine aggregate to coarse aggregate.

Regardless of the mix design approach, laboratory trials should be used to verify properties of the initial mix composition with respect to the specified characteristics and classes. The performance of the proposed mix design can be validated by trial batches.

1.2

Tests and standards

1.2.1 Basic performance requirements of SCC The fresh performance requirements of SCC depend on several parameters, including complexity of the formwork, degree of reinforcement congestion, placement method, desired surface finish, labour skill, and level of quality assurance as well as quality control (Bartos et al., 2002; Skarendahl, 2001; ACI 237, 2007). The target fresh properties of the mixture are generally quantified by the workability test methods described. For highly flowable concrete, such as self-consolidating concrete (SCC) or self-levelling concrete (SLC), targeted fresh properties of the mixture are generally quantified by the workability test methods. Workability describes the ease with which concrete can be mixed, transported, placed, consolidated, and finished (ACI 237). Workability of SCC is described in terms of filling ability, passing ability and stability (resistance to segregation). It is characterized by various specific testing methods

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Self-Compacting Concrete: Materials, Properties, and Applications

(ACI Committee 237). The workability of fresh SCC can be divided into three key properties: – – –

Filling ability: this is the ability of the SCC to flow into and fill completely all spaces within the formwork under its own weight. Passing ability: this is the ability of the SCC to flow through tight openings such as spaces between steel reinforcing bars and between bars and formwork without hindrance and under its own weight. Resistance to segregation: the SCC must meet the required levels of properties filling and passing abilities while its composition remains uniform throughout the process of transport and placing.

In terms of filling ability, passing ability, and stability, some of the many variables that influence the desired fresh properties of SCC are shown in Tables 1.5–1.7 (ACI Committee 237).

1.2.2 Test methods 1.2.2.1 Filling ability Slump flow The filling ability, or deformability, describes the ability of SCC to flow into and fill completely all spaces within the formwork, under its own weight without any external mechanical vibration. This property is of importance to the casting technique and distance between filling points (ACI Committee 237). Slump flow test (ASTM C1611/1611M-18, 2018) is used to assess the free flow of SCC in the absence of obstruction. The test method is based on the test method for determining the slump of a normal concrete (ASTM C143/143M-15, 2015). It was first developed in Japan to characterize fluid concrete mixtures for placement underwater and in Canada (Sonebi and Khayat, 1999). The procedure is based on EN 12350-8 (2010) for determining the slump flow of SCC. A sample of freshly mixed concrete is placed in a mould shaped as the frustum of a cone as used for the EN 12350-2 slump test. The concrete is placed in one lift and not compacted by any means of Table 1.5 Variables influencing filling ability (Khayat and Daczko, 2002). Application variables

Influence

Reinforcement level Intricacy of the element shape Wall thickness Placement technique Element length

High reinforcement level inhibits flow Intricate shapes are more difficult to fill Narrow sections inhibit flow Slow, discontinuous pouring decreases placement energy Longer distances are more difficult to fill

Mixture variables

Influence

Fluidity (slump flow) level Viscosity level

High fluidity improves filling ability Viscosity that is too high can limit filling ability

Mix design procedure, tests, and standards

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Table 1.6 Variables influencing passing ability (Khayat and Daczko, 2002). Application variables

Influence

Reinforcement level Narrowing of formwork

Tight reinforcement can cause aggregate bridging and blocking of concrete Narrow sections in formwork can cause aggregate bridging and blocking of concrete

Mixture variables

Influence

Fluidity (slump flow) level Viscosity level Coarse aggregate size Coarse aggregate content

Low fluidity may not allow for enough deformability, while toohigh fluidity can cause instability and mixture separation Should be gauged in light of fluidity level Larger coarse aggregate size may increase blocking tendency Larger coarse aggregate content may increase blocking tendency

Table 1.7 Variables influencing stability (Khayat and Daczko, 2002). Application variables Placement technique Reinforcement level Element height

Influence High placement energy can cause materials to separate Flowing through reinforcement can cause separation of materials (in conjunction with loss of passing ability) Increased section height increases potential for aggregate settlement/ separation and bleeding

Mixture variables

Influence

Fluidity (slump flow) level Viscosity level

All else being equal, as fluidity increases, stability decreases (highly fluid SCC must be proportioned to be stable) All else being equal, as viscosity increases, stability increases

mechanical or manual agitation. The mould is raised, and the concrete allowed subsiding. The slump flow consists of measuring the mean base diameter of the concrete sample at the end of the slump test which is expressed to the nearest 10 mm (Fig. 1.1). The diameter of the concrete circle is a measure for the flowability of SCC. When the slump flow test is performed, the time needed for the concrete to spread 500 mm is also noted. This test is called T-500 flow time. Because of the viscous nature of SCC, the readings of slump flow measurement was determined after there was no more discernible movement of the concrete, approximately 60 s after the removal of the slump cone. The result is an indication of the filling ability

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Self-Compacting Concrete: Materials, Properties, and Applications

Fig. 1.1 Slump-flow test and T500.

of SCC. With the same test, the T500 slump flow time was measured when the concrete was slumping until it reached 500 mm of slump flow and is given as the nearest 0.5 s. T500 time is also a measure of the speed of flow and hence the viscosity of the SCC. The T500 time is a test to assess the flowability and the flow rate of SCC in the absence of obstructions. The test is easy to perform either at a concrete plant or on a job site. The European guidelines for SCC have classified three of classes of slump flow as follow: Class

Slump flow (mm)

SF1 SF2 SF3

550–650 660–750 760–850

V-funnel test The V-funnel test is used to assess the filling ability. The V-funnel value is affected by both yield stress and viscosity of SCC (Sonebi and Bartos, 1999; Bartos et al., 2002; ACI 237, 2007). The V-funnel test is described by EN 12350-9 (2010) and not suitable when the maximum size of the aggregate exceeds 20 mm. The V-funnel time is the period of a defined volume of SCC pass through narrow narrow-opening passing ability, which involves viscosity of freshly mixed self-compacting concrete from observation of the flow speed of the sample through the specially designed funnel under

Mix design procedure, tests, and standards

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self-weight. A shorter flow time indicates a lower viscosity and a longer flow time indicates a higher viscosity (ACI 237). If the flow is discontinuous, this can be an indication of blocking and/or segregation. It is important to note the flow-through time is only valid if the flow is continuous. The dimensions of V-funnel are given in Fig. 1.2. This test also covers evaluation of the segregation resistance of freshly mixed selfcompacting concrete by the observation of the variation on the flow speed due to the difference of the sample’s remaining period in the funnel. V-funnel equipment consists of a V-shaped funnel with an opening at its bottom. The funnel is filled with concrete, then the gate is open and the time taken for concrete to flow through the apparatus is recorded. According to the European guidelines for SCC, two viscosity classes with T50 and V-funnel time have been established. Class

T50 (s)

V-funnel time (s)

VS1/VF1 VS2/VF2


2 >2


8 9–25

Fig. 1.2 V-funnel test.

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Self-Compacting Concrete: Materials, Properties, and Applications

1.2.2.2 Passing ability The passing ability refers to the ease of concrete to pass among various obstacles and narrow spacing in the formwork without showing blockage that can result from local aggregate segregation near the obstacles. This can give rise to interlocking and blockage of the flow in the absence of any mechanical vibration (Bartos et al., 2002, ACI 237, 2007).

J-Ring The J-Ring test aims to investigate the passing ability of SCC to flow though tight openings including spaces between reinforcing bars and other obstructions without segregation or blocking (Sonebi and Bartos, 1999). J-Ring test is described by EN 12350-12 (2010) and ASTM C1621/1621M (2017). The J-Ring test is used to determine the blocking characteristics of SCC. The equipment consists of a ring placed on several rebars with adaptable gap widths, combined to the Abram’s cone (Fig. 1.3). The apparatus had a diameter of 300 mm with a fixed spacing of vertical bars. The apparatus has a ring of a rectangular cross-section into which narrow gap (bar spacing approximately 41 mm) with dimensions shown in Fig. 1.3. Wide gap J-Ring had bar spacing approximately 59 mm. It can be note that other combinations of gap spacing of J-Ring may be used. Note that slump cone may have the feet removed to fit inside the J-Ring. T500J time can be also measured when the concrete touches the circle of 500 mm diameter if it has been requested. The mean of two perpendicular diameters is calculated (dm and dr). The concrete surface at the central position Δh0 and at the four positions outside the J-Ring, two Δhx1 and Δhx2 in the x-direction and the other two Δhy1 and Δhy2 in the y-direction (perpendicular to x) are measured (Fig. 1.3). The J-Ring flow spread SFj is expressed to the nearest 10 mm given by the following equation: SFj ¼

ð dm + d r Þ 2

Fig. 1.3 J-Ring according EN 12350-12.

Mix design procedure, tests, and standards

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where: SFj: flow spread (mm) dm: largest diameter of flow spread (mm) dr: largest diameter at 90° to dm (mm)

The J-Ring blocking step Bj is calculated using the following equation:
Δhx1 + Δhx2 + Δhy1 + Δhy2  Δh0 Bj ¼ 4 where: Bj: Blocking step Δh: heights

The J-Ring test (EN 12350-12, 2010; ASTM C1621/1621M, 2017) is generally used to assess the restricted deformability through closely spaced obstacles of fresh SCC with nominal maximum size of aggregate of 25 mm (ASTM C1621/1621M, 2017; AASHTO T345, 2002). Fig. 1.4 shows the use of J-Ring in combination of inverted slump cone.

L-box test The L-box aims to investigate the passing ability of self-compacting concrete and its dimensions are shown in Fig. 1.5. The L-box test consists of an L-shaped apparatus. The vertical part of the box is filled of concrete and left at rest for 1 min. The gate separating the vertical and horizontal compartments is then lifted, and the concrete flows out through closely spaced reinforcing bars at the bottom. The time for the leading edge of the concrete to reach the end of long horizontal section is noted. The heights of concrete remaining in the vertical section and at the leaving edge are determined. The blocking value is calculated to evaluate the self-levelling characteristic of the concrete. In general, this ratio varies from 0.5 up to 1.0 for SCC used in precast, prestressed applications (NCHRP Project 18-12, 2008). A ratio of 0.7 and higher is indicative of good passing ability.

Slump cone J-Ring

Slump plate

Fig. 1.4 J-Ring with inverted cone.

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Self-Compacting Concrete: Materials, Properties, and Applications

100

200

gate 2 or 3 × 12 φ smooth bars Gaps between bars 59 or 41mm

600

H1

ΔH

150

H2 700

Fig. 1.5 L-box test.

The L-box is described by EN 12350-10 (2010). With the L-box it is possible to measure different properties such as blocking and segregation. The vertical part of the box is filled with 12.7 L of concrete which is left to rest for 1 min to allow any internal segregation to occur. After that, the gate is opened and the concrete flows out of the vertical part into the horizontal part through the reinforcement bars. The gap between the reinforcement bars was 41  1 mm and 59  1 mm for 10 mm and 20 mm coarse aggregate, respectively. Three smooth steel bars and two bars (12 mm) were used in the L-box for 10 and 20 mm coarse aggregate concrete, respectively. The height values H1 and H2 of concrete were measured and used to determine the passing ability ratio h2/h1 ratio to the nearest 0.05 (Fig. 1.5). The European guidelines for SCC have classified two classes according the passing ability. Class

Passing ability

PA1 PA2

0.80 with 2 rebars 0.80 with 3 rebars

The L-box blocking ratio can be defined as the ratio between H2 and H1 (H2/H1), or blocking ratio. This blocking ratio can be used to evaluate the passing ability of SCC. As in the case of other passing ability test methods, the minimum blocking ratio depends on the geometry of the cast element and application type and can vary with the nominal maximum size aggregate and use of fibres. Typical values of blocking ratios for SCC are varied between 0.6 and 1. The lower limits of 0.6–0.7 can be suitable for the casting of relatively shallow structural elements with low density of reinforcement (NCHRP Project 18-12, 2008).

Mix design procedure, tests, and standards

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Special issues Several advantages and limitations of slump flow and J-Ring flow test, L-box, and V-funnel methods are presented in Table 1.8.

1.2.2.3 Resistance to segregation Stability of concrete describes its ability to maintain homogeneous distribution of its various constituents during flow and setting. There are two types of stability characteristics that are important for SCC: static stability and dynamic stability. Dynamic stability refers to the resistance of concrete to the separation of its constituents during mixing and placement (free fall and flow) into the formwork. Adequate dynamic stability is required for SCC when the application has special requirements, such as long flow distance or flow through closely spaced obstacles and narrow spacing. Static stability refers to the resistance of concrete to bleeding, segregation, and surface settlement after the end of casting while the concrete is still in a plastic state.

Static stability Sieving segregation test The sieving segregation test is used to assess the resistance of self-compacting concrete (Bartos et al., 2002; Cussigh et al., 2003). This testis not applicable to concrete containing fibres or lightweight aggregate. Table 1.8 Advantages and limitations of J-Ring, L-box and V-funnel flow tests. Advantages

Limitations

V-funnel 

Possibility to correlate to the plastic viscosity

  

Difficult to carried out by a single operator No good reproducibility Interrupted flow concrete having high viscosity mixes



Roughness and moisture of the baseplate can affect the measurements Need to check the level of baseplate before starting the measurements

J-Ring     

Good reproducibility Convenient Can be carried out by a single operator Measurements are easy to conduct. Give an indication of risk of segregation



L-box 

Easy and quick measurements

 

Good reproducibility The flow time can give a good indication of filling ability Can be carried out by 1 operator





Need to check the level the L-box before testing



After testing it is difficult to clean concrete from L-box

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Self-Compacting Concrete: Materials, Properties, and Applications

The sieve segregation test is shown in Fig. 1.6 and standardized by EN 12350-11 (2010). The test aims to determine how likely a SCC mix can segregate by allowing a 10 L concrete sample to undergo static segregation for 15 min (in a bucket). Then the top layer of the sample (4.8 kg  0.2) is poured onto a 5 mm sieve and some mortar passes through the sieve. After 2 min the weight of material which has passed through the sieve is recorded. The segregation ratio is then calculated as the proportion of the sample and the material passing through the sieve. More mortar passing through the sieve indicates a greater liability to segregation. The segregation portion SR is calculated as follows:where: SR: segregated portion mps: mass of sieve receiver plus passed material (g) mp: mass of sieve receiver (g) mc: initial mass of concrete place onto the sieve (g)

Fig. 1.6 Sieve segregation test.

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The European guidelines for SCC have classified two classes according the segregation resistance (sieve segregation) as follows: Class

Segregation resistance (%)

SR1 SR2


20
15

Surface settlement test method The surface settlement test method can be used to evaluate the surface settlement of SCC from a plastic state until the time of hardening (Khayat et al., 1997). This test enables the quantification of the effect of mixture proportioning parameters on static stability of concrete. A maximum surface settlement lower than 0.5% or a rate of settlement after 30 min lower than 0.14%/h is recommended for SCC used in precast, prestressed applications (NCHRP Project 18-12, 2008). Fig. 1.7 presents the surface fresh settlement test method (Sonebi and Bartos, 2002). Column segregation test method Column segregation test (ASTM C1610/1610M17, 2017) consists in casting concrete in a column divided in four sections along the concrete sample. From each section, the concrete is weighed and washed out to determine the coarse aggregate content for each section (Fig. 1.8). The coefficient of

Fig. 1.7 Surface settlement test.

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Self-Compacting Concrete: Materials, Properties, and Applications

Fig. 1.8 Column segregation test.

variation of the aggregate among the column section is taken as a segregation index (Iseg). A segregation index (Iseg) lower than 5% is recommended for SCC used in precast, prestressed applications (NCHRP Project 18-12, 2008). Visual stability index (VSI) method involves the visual examination of the SCC just after the performance of the slump flow test. It is used to evaluate the relative stability of SCC by assigning a numerical rating. This consists in assigning values of 0–3, in 0.5 increments. The VSI test is most applicable to instable SCC mixtures. If a mix does not bleed, this test is useful in identifying a mix’s tendency to segregate (Daczko and Kurtz, 2001). In general, a VSI lower than 2 is recommended for highperformance SCC. Special issues Several advantages and limitations of surface settlement and column segregation tests are presented in Table 1.9.

Dynamic stability Dynamic stability refers to the resistance of concrete to separation of constituents upon placement and spread into the formwork. Adequate dynamic stability is required for SCC when flowing through closely spaced obstacles and narrow spaces to avoid segregation, aggregate interlock, and blockage (ACI Committee 237, 2007). Dynamic stability can be evaluated by determining the ability of concrete to pass among obstacles and narrow spacing in the formwork without blockage (passing ability). In general, Caisson filling capacity test is a good indicator of the dynamic stability of the SCC. However, due to intensive labour and relatively long testing time of this apparatus, an adequate combination of filling and passing ability test can be used in conjunction to assure proper dynamic stability of the SCC.

Mix design procedure, tests, and standards

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Table 1.9 Advantages and limitations of surface settlement test (NCHRP Project 18-12, 2008). Advantages

Limitations

Surface settlement test   

Easy to perform in the laboratory for mix design Reproducible Good correlation between total settlement and rate of settlement

   

Expensive apparatus Time consuming Difficult to perform by a single operator Require a large amount of concrete

Column segregation test    

Good indicator of the dynamic stability of the SCC Good correlations between column Iseg and S Results can be obtained quickly Not operator, moisture, and position sensitive

  

Require a large amount of concrete Difficult to perform by a single operator Require an electronic balance on the site

Visual stability index 

Easy to perform



 

Can be performed by a single operator Can be performed with slump flow

Bad reproducibility by multioperator



Require an experimented operator

Dynamic settlement column segregation test Testing fresh dynamic segregation resistance of self-compacting concrete (SCC) using the settlement column segregation test was reported by Rooney and Bartos (2001) and Sonebi et al. (2005, 2007a,b). The settlement column test which is filled with fresh SCC was subjected to a controlled jolting action followed by settlement period of 5 min in order to determine the dynamic segregation resistance. The settlement column segregation test comprises a small column of SCC mix with internal dimensions of 500 mm  150 mm 100 mm (Fig. 1.9) being subjected to a controlled jolting action followed by a 5 min settlement period. A sample is then taken from the top and the bottom of the column via hinged doors and the mortar is washed out through a 5 mm sieve. The theory behind the test is that, if the mortar is unable to uniformly suspend the coarse aggregate particles, gravity and the force caused by the impact of the jolting will cause them dynamic action to settle towards the bottom of the column. The mass of the coarse aggregate in the samples is then compared. If it is found that there is a significantly higher mass in the bottom sample than in the top, then it suggests that there is segregation. The result of the dynamic segregation test is expressed as: Segregation ratio of fresh concrete ðSRFCÞ Mass of coarse aggregate in top sample ¼ Mass of coarse aggregate in bottom sample

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Self-Compacting Concrete: Materials, Properties, and Applications

Fig. 1.9 Dynamic settlement column test.

A summary of the results of the segregation threshold is shown in Table 1.10 (Sonebi et al., 2007a,b). It was found that four different levels of dynamic segregation existed within the ranges of mixes tested. After mild dynamic segregation had been defined, it was possible to determine the range of results, which defined notable dynamic segregation. It had been established that the starting point for notable dynamic segregation was represented by a settlement column test result of 0.87 – concrete producing a result higher than this did not have segregation, which was extreme enough to be considered as notable. Tilting box test The tilting-box (T-Box) setup is comprised of a rectangular channel measuring 40 in. (1000 mm) in length, 7.9 in. (200 mm) in width, and 15.8 in. (400 mm) in height (Esmaeilkhanian et al., 2014) as shown in Fig. 1.10. The penetration resistance is determined before the initiation of the tilting cycles of the concrete sample and at the conclusion of the test. The dynamic segregation is quantified using the penetration depth index, PDI, calculated using Eq. (1.1):

Table 1.10 Segregation threshold of fresh SCC. Segregation category

Settlement ratio of fresh concrete SRFC

1. No segregation 2. Mild segregation 3. Notable segregation 4. Severe segregation

0.96 and above 0.95–0.88 0.87–0.72 0.71 and below

(5 in. 0.2 0

A

Φ

A

Φ 2.76 in. (70 mm)

Penetrometer

mm )

19

15.75 in. (400 mm)

Mix design procedure, tests, and standards

Steel nuts Section A-A 2.76 in. (70 mm)

Supporting Device

Plan View

2.13 in. (54 mm) Φ0 B .39 Φ2 in. .13 (10 in. m m (54 mm ) )

B

2.64 in. (67 mm)

7.87 in. (200 mm)

Side View

Hole Section B-B

Fig. 1.10 Configuration of T-Box test (note: 1 mm ¼ 0.0394 in.) (Esmaeilkhanian et al., 2014).

PDI ¼ Dpf  Dpi

(1.1)

where, Dpi is the initial penetration depth in the tilt up section measured immediately after pouring the concrete into the T-Box, and Dpf is the final penetration depth recorded on the same side after the completion of tilting cycles. The sieve-washing technique was also used to validate the results of the PDI. The segregation index derived from this method, referred to as volumetric index (VI), is defined as (Eq. 1.2): VIð%Þ ¼

Vtd  Vtu  100 Average ðVtd , Vtu Þ

(1.2)

1.2.3 Rheological properties of SCC Rheology is the science of deformation and flow of matter. Rheology is a means to characterize the workability of fresh SCC. For example, it can provide insights beyond what the slump-flow test alone can provide. Rheological parameters are related to concrete properties, such as filling ability, passing ability, and stability. The use of rheology for characterizing the properties of fresh concrete assumes that concrete can be considered as a suspension (solid inclusion and suspending fluid). Rheology is described based on the relationship between shear stress, τ (Pa), and shear

20

Self-Compacting Concrete: Materials, Properties, and Applications

rate, γ_ (1/s or s1). This relationship; namely the flow curve, indicates the shear stress needed to deform (i.e. flow) concrete at a certain rate. In general, a highly flowable concrete requires a smaller shear force to cause flow at a certain rate. Although, rheology is not yet an everyday method used for quality control, it has been of high importance when validating workability measurements, studying compatibility between new admixtures and cement-based materials.

1.2.3.1 Rheological models Rheological models describe the nature of the relationship between the shear stress and shear rate, i.e. how the shear stress varies with the rate of shear (rate of deformation). The flow curve can be rather linear or in some cases non-linear. However, only the mainly models relevant to particle suspensions, such as SCC are discussed in this chapter.

Bingham model The most commonly used rheological model for fresh cementitious materials is the Bingham model. This model is also linear, but contrarily to the Newtonian model, it is characterized by not passing through the origin. Its intercept with the shear stress axis is always at a positive shear stress value. This intercept is referred to as the material yield stress, τ0 (Pa) and the slope of the flow curve is the material plastic viscosity, μpl (Pa s) (Fig. 1.11). At least two stress values, measured at two different shear rates, are needed to describe the fresh properties of concrete. This is the disadvantage of most workability test, because only one point is actually measured. The equation describing the Bingham is given in Eq. (1.3): τ ¼ τ0 + μpl  γ_

(1.3)

Shear stress (Pa)

The yield stress determines when the flowing concrete stops; at the point where the stress in the material reaches the yield stress. It can also be described as the minimum shear stress that needs to be applied to initiate flow. The rheological measurements are usually determined using rheometer adapted for concrete. In general, there is a good

mpl t0 Shear rate (s-1)

Fig. 1.11 The Bingham model.

Mix design procedure, tests, and standards

21

relationship between yield tress and slump flow, as shown in Fig. 1.12 (Billberg, 2011; Svermova et al., 2003).

Herschel-Bulkley model The relationships between shear stress and shear rate are far from linear shape. In this case, one of the most used models to describe such nonlinear behaviour is the Herschel-Bulkley model. Eq. (1.4) describes this model: τ ¼ τ0 + K γ_ n

(1.4)

where K is the consistency (Pa sn) and n is the consistency index (unitless). Note that for the case of n ¼ 1, the model equals to the Bingham model with K ¼ μpl. For n-values higher than 1, the material is shear-thickening, and for n-values lower than 1 the material is shear-thinning. Shear-thinning is generally explained by a more favourable particle orientation in the flow direction. For example, elongated particles can form up (line up) in the direction of the flow. Also flow-induced breakage of flocculated particles can result in a shear-thinning behaviour. On the other hand, shear thickening is far more complex. In general, surpassing a critical shear rate (rate of flow) can cause turbulence leading to an unstable particle system, causing an increased resistance to the flowing material. This is also referred to as grain inertia. Cluster formation is reported to be one of the influencing factors (Ref ). Shear-thickening is generally limited to processes involving higher rates of shear, such as pumping or mixing. For shear thickening SCC, another model is available: the modified Bingham model shown in Eq. (1.5) (Yahia and Khayat, 2003). This model is developed merely to enable a more correctly determination of the yield stress when the flow curve is not linear and the yield stress is very low. If performing a linear regression, using the Bingham model for non-linear shear thickening flow curve will underestimate the yield stress (Fig. 1.13).

350

Yield stress (Pa)

300 250 R² = 0.925

200 150 100 50 0 300

400

500 600 700 Slump-flow (mm)

800

900

Fig. 1.12 Correlation between SCC slump-flow and yield stress (Billberg, 2011).

Self-Compacting Concrete: Materials, Properties, and Applications

Shear stress (Pa)

22

Linear regression Non-linear flow curve

Δτ0 = underestimation of the true yield stress

Shear rate (s–1)

Fig. 1.13 Underestimation of the true yield stress.

τ ¼ τ0 + μ  γ_ + c  γ_ 2

(1.5)

SCC is subject to various shear rates from mixing, transport, and casting. It is worthy to mention that the use of the same level of shear rates when measuring the rheology in a rheometer as in the actual process is important to ensure proper measurements. Examples of shear rates for different processes and rheometers are listed in Table 1.11 (Roussel, 2006). These values depend on type of mixer and shear history, etc., so they should be seen as approximate values. For example, a poured concrete with a flow speed of 1 m/s and a thickness of 0.1 m, can undergo a shear rate of 10/s.

1.2.3.2 Rheological target values The rheological parameters determined from the Bingham model, yield stress, τ0 (Pa) and plastic viscosity, μpl (Pa s), are important for the development of successful SCC mixtures. Wallevik (2002a) developed the target area shown in Fig. 1.14 using different SCC mixtures from various countries and different rheometers. However, the targeted box is complex because of the large range in slump-flow values for SCC (550–750 mm).

Table 1.11 Maximum shear rates in various steps of concrete flow (Roussel, 2006). Flow pattern Mixing (Chopin, 2003) Mixing truck Pumping (Kaplan, 2001) Casting Tattersall two-point device (MKIII) (Tattersall, 1991) BML rheometer (Wallevik, 2003) BTRHEOM rheometer (De Larrard et al., 1997)

Approximate maximum shear rate (s21) 10–60 10 20–40 10 5 10 15

Mix design procedure, tests, and standards

23

Yield value (Pa)

160

120

80

550 mm 600 mm

40

650 mm 700 mm

0 0

30

60

90

120

Plastic viscosity (Pa s) Fig. 1.14 SCC target area in terms of rheology (Wallevik, 2002a).

1.2.3.3 Thixotropy Thixotropy is a reversible, isothermal, time-dependent decrease in the apparent viscosity when a material is subjected to increased shear rate (Mewis, 1979). The reversibility of the process results that the viscosity recovers, i.e. structure builds up, again when the shear rate is eliminated or reduced. The most interesting engineering aspect of thixotropy is the part when the structure is building up after the material is at rest. This means that a highly flowable material, such as SCC wills build-up when left at rest. Thixotropic SCC can build-up at rest, which can improve stability and reduces static segregation. Thixotropic SCC can also present other advantages, such as reducing lateral pressure exerted on formwork systems. However, this property can present negatives aspects in the case of multi-lift castings of SCC, which result in lowering residual bond strength between the two layers. For most structures and especially for aesthetic, architectural structures, these lift-lines are more or less catastrophic. In hydraulic structures, the water permeability can be greatly reduced across these lift lines (Khayat et al., 2012). The methods developed to measure the structural build-up involve increasing the stress in the material slowly and measuring the peak shear stress when the structure breaks.

1.2.3.4 Factors affecting SCC fresh properties When optimizing and controlling the fresh properties of SCC, it is very important to know the effect of constituent materials on fresh properties. Rheological parameters (Bingham) can be varied using different materials and proportions. Fig. 1.15 describes the variation of rheological parameters using different materials and combinations.

1.2.3.5 Rheological tests Rheology is the science of the flow and deformation of materials and is concerned with the relationship between shear stress, shear strains and time. Its application to the flow properties of fresh concrete has been extensively documented (Tattersall

Self-Compacting Concrete: Materials, Properties, and Applications

Fig. 1.15 General presentation of how different concrete constituent materials can influence the rheology of concrete (Wallevik, 2002b).

Silica fume Time Yield stress (Pa)

24

VA/binder

Air SP+VA/binder Water/paste SP Plastic viscosity (Pa s)

and Banfill, 1983; Tattersall, 1991). There is general agreement that concrete behaves according to the Bingham model as follows: :

τ ¼ τ0 + μ γ

Shear stress (τ)

where τ0 is yield value which concrete can resist stresses not exceeding the yield stress τ0 without flowing (overcome interparticle attractive effects), μ is the plastic viscosity, and is the shear rate. There is linear relationship between τ and μ. Thus, the two rheological parameters τ and μ characterize the behaviour of concrete (Fig. 1.16). For example, two-point workability test used to determine the rheological parameters (Fig. 1.17). Measurements of shear stress at two points, or at two shear rates, were therefore required to define its behaviour (Tattersall, 1991). Other rheometers such as coaxial rheometer BML (Fig. 1.18), BTRHEOM (Fig. 1.19), and MKIII Tattersall-IBB (Fig. 1.20, Sonebi, 2004a) can be used to determine the yield stress and the plastic viscosity (Ferraris and Brower, 2003), and ICAR rheometer (Fig. 1.21) and Vikomat (Fig. 1.22) rheometers.

µ (plastic viscosity)

τo (yield value)

Rate of shear strain (γ)

Fig. 1.16 Variation of shear stress vs rate of shear strain.

Mix design procedure, tests, and standards

25

Fig. 1.17 Two-point workability test – Tattersall.

Fig. 1.18 BML-rheometer.

Fig. 1.19 BTRHEOM rheometer.

Fig. 1.20 Tattersall MKIII (IBB) rheometer.

Fig. 1.21 ICAR rheometer.

Fig. 1.22 Vikomat XL rheometer.

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Self-Compacting Concrete: Materials, Properties, and Applications

References AASHTO T345, 2002. Standard Method of Test for Passing Ability of Self-Consolidating Concrete (SCC) by J-Ring. ACI 237, 2007. Self-Consolidating Concrete (ACI 237R-07). American Concrete Institute, Farmington Hills, MI (34 pp.). ASTM C143/143M-15, 2015. Standard Test Method for Slump of Hydraulic-Cement Concrete. ASTM C1610/1610M-17, 2017. Standard Test Method for Static Segregation of SelfConsolidating Concrete Using Column Technique. ASTM C1611/1611M-18, 2018. Standard Test Method for Slump Flow of Self-Consolidating Concrete. ASTM C1621/1621M, 2017. Standard Test Method for Passing Ability of Self-Consolidating Concrete by J-Ring. Bartos, P.J.M., Sonebi, M., Tamimi, A.K., 2002. Workability and rheology of fresh concrete: compendium of tests. In: Report of RILEM Technical Committee TC 145-WSM: Workability of Special Concrete Mixes. RILEM Publications S.A.R.L, Paris (127 p.). Billberg, P., 2011. Influence of powder type and VMA combination on certain key fresh properties of SCC. In: 9th International Symposium on High Performance Concrete—Design, Verification and Utilisation, Rotorua, New Zealand. Chopin, D., 2003. Malaxage des b
etons a` hautes performances et des b
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Sonebi, M., Rooney, M., Bartos, P.J.M., 2005. Evaluation of the segregation resistance of selfcompacting concrete using different methods. In: The first International Symposium on Design, Performance and Use of Self-Consolidating Concrete, Changsha, China, pp. 301–308. Sonebi, M., Rooney, M., Bartos, P.J.M., 2007a. Test method to evaluate the dynamic segregation of fresh self-compacting concrete using the settlement column test. In: Proceedings of 5th Int. RILEM Symposium on SCC, Ghent, Belgium. vol. 1. pp. 43–48. Sonebi, M., Gr€unewald, S., Walraven, J., 2007b. Filling ability and passing ability of selfconsolidating concrete. ACI Mater. J. 104 (2), 162–170. Svermova, L., Sonebi, M., Bartos, P.J.M., 2003. Influence of mix proportions on rheology of cement grouts containing limestone powder. Cem. Concr. Compos. 25 (7), 737–749. Tattersall, G.H., 1991. Workability and Quality Control of Concrete. Spon, London. Tattersall, G.H., Banfill, P.F.G., 1983. Rheology of Fresh Concrete. Pitman, London. The European Guidelines, 2005. The European Guidelines for Self-Compacting Concrete Specification, Production and Use, May. (63 p.). Wallevik, O.H., 2002a. Practical Description of Rheology of SCC. SF Day at the Our World of Concrete, Singapore, p. 42. Wallevik, O.H., 2002b. Rheology of Coarse Particle Suspensions, such as Cement Paste, Mortar, and Concrete (Course compendium, Chapter 6). IBRI, Reykjavik. Wallevik, J.E., 2003. Rheology of Particle Suspensions; Fresh Concrete, Mortar and Cement Paste With Various Types of Lignosulfonates (Ph.D. thesis). The Norwegian University of Science and Technology (411 pp.). Wallevik, O., Nielsson, I., 1998. Self-compacting concrete—a rheological approach. In: Proceedings of Int. workshop of SCC, Kochi, Japan, pp. 136–159. Yahia, A., Khayat, K.H., 2003. Applicability of rheological models to high-performance cement grouts containing various supplementary cementitious materials and viscosity-enhancing admixture. Mater. Struct. 36 (260), 402–412.