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Title no. 98-M46

New Methodology for Designing Self-Compacting Concrete by Aaron W. Saak, Hamlin M. Jennings, and Surendra F! Shah

Okamura'

yurgui et d.''

A new segregation-controlled design methodology is introduced for self-compacting concrete (SCC). The theory suggests that aggregate segregation is governed by the yield stress, viscosity, and density of the cement paste matrix. The concept of a rheological self- jlow zone (SFZ) for concrete is introduced where aggregate segregation is avoided, yet the concrete has a high workability.

The applicability of the theory is studied by systemtically changing the rheology of the cement paste matrix offesh concrete. The yield stress and viscosity of three different types of pastes incorporating silica fume and a cellulose thickening agent is measured as a function of density. A U-tube apparatus is then used to determine the SFZ for concrete made with the different cement paste compositions. The results suggest that the new segregation control design theory can be used to produce SCC. The slump of concrete produced using this new methodology was 29 cm (1 1 in.), with no segregation of coarse aggregate even at the periphery of the slumped material.

0.64 0.36 0.22 0.64

0.54 0.46 0.24 0.78

Keywords: cement paste; concrete; slurnp

INTRODUCTION Self-compacting concrete (SCC) has generated tremendous

interest since its initial development in Japan by Okamura in the late 1980s.' The combination of high fluidity and segregation resistance results in consolidation due entirely to the concrete's self-weight. The advantages of SCC include reduced labor costs and improved quality control.273

Severa1 different approaches have been used to develop SCC. One of the most common design methodologies is to divide concrete into two constituents; coarse aggregates and mortar.134,5 The rheology of the mortar is then adjusted to achieve self-flowing concrete via incorporation of a variety of mineral additives, plasticizers, and thickeners. Another approach is to optimize the particle size distribution of the binder (for example, cement, silica fume, limestone dust, etc.) and of the fine and coarse aggregates based on packing cons idera t i~ns .~~~ In general, the latter approach has provided a better understanding of the physical properties required to achieve SCC.

Developing self-flowing materials such as SCC requires careful control of other factors to ensure high workability. The morphology, particle size distribution, and interparticle spacing of the aggregates are also important factors dictating the flow properties of fresh concrete. Unfortunately, from a practica1 standpoint, the particle morphology is largely un- controllable. The particle size distribution of the aggregates, however, can be controlled by adjusting the percentage of fine and coarse paiticles.

Based on published data, it is unclear if high packing density alone should be used as a design criteria for producing highly flowable concrete.68 Recent results from experiments examin- ing the influence of the physical properties of the particles on the rheology of cement paste suggest that the interparticle separation (IPS) should be used in conjunction with particle packing density as the rheological design para meter^.^^^

Table 1 -Mixture proportlon guidelines for self- compacting conprete suggested by different authors

1 0.44 1 0.56 1 0.18 1 0.78 Ambrose, Rols,

and Pera"

'v, = voiume coarse aggregate: V, = voiuine fine aggregate; vasg = voiume of total aggregate; V, = volume of binder (solids); and V, = volume of total solids (aggregates + binder).

In concrete, the IPS is controlled by the particle size distribution of the aggregates and volume percent binder (that is. cement paste matrix). For a given particle size distribution of aggregate, the amount of binder must be sufficient to fill the interstitial voids between aggregates and produce the desired IPS. There has been little variation in the suggested guidelines presented by different authors for producing SCC (Table 1). The aggegate particle size distribution, controlled by the ratio of the volume of the coarse and fine aggregates to the total volume of aggregates, or Vc/Vag and V /Va,,, respectively, and the volume percent of bincfer of t l e total solids content (VbIVs) effectively sets the particle packing density and IPS, and thus, greatly influences the rheology of the material.

Whatever the design methodology, the key requirements for SCC are very high workability with no segregation of the aggregates during placement. Unfortunately, the widespread use of qualitative terms such as workability makes it difficult to determine the fundamental rheological properties that control the self-flowing ability of concrete. Additionally, there is a very limited understanding of what rheological parameters control the segregation of aggregates.12

In this paper, a new design methodology for SCC is introdud The theory proposes that the rheoiogy of the cement paste matrix largely dictates the segregation resistance and workability of fresh concrete, given a specified particle size distribution and volume fraction of aggregate. Thus, the rheology of the matrix can be enginezred to produce SCC.

Secondly, the applicability of the theory is tested by measuring the flow properties of fresh concrete. The yield stress, viscosity, and density of cement paste containing different admixtures are determined in an initial set of experiments. Next, concrete is made using the same cement paste compositions with the particle size distribution and volume fraction

ACI Matertals Joumal, V. 98. No. 6. November-December 2001. MS No. 00-222lOO-223 received September 20, 2000, and reviewed under institute publication policies. Copyright O 2001, American Concrete institute. Al1 nghts reserved, including the making of copies unless pennission i s obtained from the copyright proprietors. Pertinent discussion will be published in the Septemkr-October 2002 ACI Materials Joumal if received by June 1,2002.

ACI Materials JournaVNovember-December 2001 429

AMa W. SUL is employed by General Electnc, Wonhington, Ohio. He received his PhDfmm the Lkpattment of Maten& Science and Engineering at Northwestem Uni- ved^ E v ~ s r o n , 111.

ACI mcmber LIimlin M . Jexu~hgs is a pmfessor of Materials Science and Engineer- ing and Civil hgineering al Northwestem Universiíy. He is a member of ACI Com- ninco 236, Mntcriai Science of Concrete.

SoRadrp P. Shnb, FACI, is Walter P: Murphy Pmfessor of Civil Engineering at Nonhwestem Universiíy and Director of the Center for Advanced Cement-Based Materials. He ir a member of ACI Committees 125, Concrete Technology in Space Applicatwns; 215, Fatigue of Concrete; 363, High-Strength Concrete; 440, Fiber Reinforced folymer Reinforcernent; 446, Fracture Mechanics; 544, Fiber Reinforced Concrete; 548, Polymers in Concrete; ond 549. Thin Reinforced Cementitious Pmd- ucts and Fermcement. His research interests include high-performance cement-based materiols, fiber-reinforred composites, nondestructive testing, closed-loop testing, constitutive laws, and the fracture mechanics of cement-based system.

I Fig. 1-Representation of aggregate suspended in cement paste. Restonng forces muSt be at least equal to gravitational and buoyant forces to avoidparticle segregation.

of aggregate held constant. Thus, any changes in the flow properties of the concrete are directly related to rheology of the cement paste matrix. The self-flowing ability of the concrete is detennined by measurements taken using a U-tube apparatus. The results frorn the U-tube testing are then compared with theoretical predictions.

SEGREGATION CONTROL THEORY The most common way to produce workable concrete is

by simply adding a high dosage of high-range water-reducing admixture during mixing. As the dosage of high-range water-reducing admixture increases, however, the aggregates begin to segregate during hauling and placement. Although the rheology of the cernent paste has been altered to become more fluid, segregation of the aggregates results in an unac- ceptable construction material. The rheology of the more cornplex cornposite material (paste + aggregates) dictates the self-flowing capability of concrete. Thus, avoiding segregation of the aggregates is then a critical design parameter.

One of the most important requirements for any seif-flowing material is that the particles remain suspended while the material is at rest, with only minirnum segregation occuning due to creep. It is equally important that the particles move with the matrix as a cohesive fluid during flow. Consequently, segregation of the aggregates in concrete must be avoided under both static (at rest) and dynamic (flowing) conditions.

Static segregation control The balance between the buoyant and gravitational forces

acting on an aggregate and the restoring force due to the yield stress of the cement paste matrix provides an estimate of the size (and density) of a particle that will remain motion- less under static conditions (Fig. I).l3

The total downward force acting on an aggregate is the difference between the gravitational and buoyant forces l 4

The gravitational force Fgrav is given as

where g is the gravitational constant, pp is the density of the aggregate, and Vp is the particle volume. The buoyant force Fbuoy is expressed as

where pm is the density of the matrix. Combining Eq. ( 2 ) and (3), the total downward force acting on an aggregate is

Under static conditions, the restoring, or drag, force exerted by the cernent paste matrix is proportional to the yield stress.13 This can be expressed as

where zy is the yield stress of the paste, and Ap is the cross- sectional area of the particle submerged in the cement paste matrix. The yield stress is often defined as the stress required to initiate macroscopic f10w.l~

For a particle to remain stationary, the restoring force must be at least equal to the total downward force. Setting Eq. (4) equal to Eq. (3, the minimum yield stress of the cement paste matrix, assuming spherical particles, is given as

where r is the particle radius. It is important to note that the difference in density between the aggregate and the cement paste is also an irnportant factor determining segregation resistance under static conditions. Rearranging Eq. (6) to move the cement paste-controllable terms on the left gives

(7)

Thus, for a given aggregate, both the yield stress and density of the matrix are important in designing for segregation resistance. For rnaxirnum segregation resistance, the yield stress of the paste would be high, and the difference in density between the aggregate and the paste would be l o ~ .

430 ACI Materials JournaVNovember-December 2001

Dynamic segregation control During placement, the yield stress of cement paste is ex-

ceeded and the concrete begins to flow at a low shear rate. The ratio between the ' stress 2 required for the material to fiow at a given shear rate and the shear rate itself is called apparent viscosity, or simply viscosity q

Qualitativel , viscosity is a measure of a materials resistance to flow.

If the density of the particle is greater than the density of the matrix, segregation will occur to some extent. If the viscosity of the matrix is high enough, however, the velocity of the falling particle will be so slow that segregation, for al1 practica1 purposes, is avoided. In this dynamic case, the restoring force in Fig. 1 is replaced by a drag force given by

Y6

'

(9) V 2

Fdrag = C D p m T A p

where C, is the drag coefficient and Y is the constant terminal falling velocity of the particle (that is, the particle is not a~celerating).'~ By combining Eq. (4) and (9), the terminal velocity can be expressed as

To avoid segregation, the terminal velocity of the falling particle should be minimized.

The drag coefficient is related to the particle Reynolds number Re at a specific terminal velocity. The Reynolds number is a measure of the energy dissipated by viscous effects.I8 Classically, it is defined as the ratio of inertia to viscous forces.'' The Reynolds number of a sphere is defined as

where r\ is the viscosity of the matrix.I8 The relationship between the Reynolds number and the drag coefficient for a sphere is given in Fig. 2. At high C,, the terminal velocity of a falling sphere is low (that is, segregation is avoided). Based on Fig. 2, this condition results in a low Reynolds number, or equivalently, a high paste viscosity. Once again, it should be noted that the terminal velocity is also a function of the density difference between the cement paste matrix and aggregate. Thus, both the viscosity and density of the cement paste matrix control aggregate segregation during dynamic flow conditions.

Cement paste is ' shear thinning and viscosity decreases with increasing shear rate. Consequently, it is important to consider what shear rate range the material encounters in the field when designing for segregation resistance. The common concrete processing procedures of hauling, placing, and casting are low shear rate operations. Using the table given by Reed,20 the maximum shear rates experienced by typical concrete is most likely between 1 and 100 s-' (Fig. 3).

10'

10)

h

2 10'

5

8 3 loo 6

c

.- 2 10'

u

10'' t. --- * ' - I I .

1 O-' 10'' 10' 10) 10'

Reynolds Number (R 2 Fig. 2-Relationship between R nolds number (K) and drag coeflcient (C,) for a sphere. "58

o. 1 1 10 100 1,000 10,000 100,Ooo

Shear Rate (s - l )

Fig. 3 S h e a r rate range for dgerent processing procedures.20

The theory introduced previously for a single particle in an infinite matrix is obviously not an accurate picture of real concrete. The fine and coarse aggregates in concrete produce a continuous particle size distribution ranging from less than 100 to over 10,OOO pm (0.1 to over 10 mm). The fine aggregate particles will segregate at a lower paste yield stress and viscosity than the largest coarse aggregates. Thus, the fine aggregates place an additional upward force on the coarse aggregates, hindering segregation. Assuming the aggregates are packed to maximum density, the geometry of the system will also restrain the coarse aggregates from segregating. Finally, the aggregate particles are not spherical, leading to an even more complicated system to model from a fluid mechanics viewpoint.

Unfortunately, there are no fundamentally sound thmreticai models for determining the segregation of articles in highly concentrated suspensions like concrete.'7PThe infiuence of hydrodynamic interactions as well as particle size distnbution and morphology are among the effects that lead to modeling difficulties. The best way to determine the segregation resistance of such materials is by experiment.18 Nonetheless, the simple approach presented in this section explains, to some extent, the principles governing the segregation of aggregates in concrete. Based on these concepts, it should be possible to engineer the rheology of the cement paste matrix to produce self-fiowing concrete.

SELF-FLOW ZONE (SFZ) The theory introduced in the section entitled "Segregation

control theory" suggests that the density, yield stress, and


Optimum rheology A fw cegregation resistance

5 4 -

Poor workability

;S Particie segregation

0 Seif-flow zone

b i1

4 - \

Optimum rheology for celf-flow

Fig. 4-Keld stress versus viscosity diagram, normalized by difference in densiíy between particles and matrix. Rheology of suspension optimized for self7flowing applications at minimum yield stress and viscosity where segregation does not OCCUI:

Segregation resistance optimized at highest yield stress and viscosity that still leads to self-jlowing materials.

n

(a> (b)

Fig. 5+a) Glass sphere placed on top of fresh cement paste; and ( b ) j k a l resting position of sphere measured after paste hardened.

viscosity of the cement paste matrix controls segregation resistance for a given aggregate distribution in concrete. Based on Eq. (7), (lo), and ( 1 l), a minimum paste yield stress and viscosity is necessary to avoid segregation under static and dynamic conditions, respectively. The exact values for the minimum yield stress and viscosity, however, are dependent on the density difference between the paste and aggregates.

If the yield stress or viscosity is high enough, the particles wiíl never segregate; however, the material will also have poor workability. Thus, there is a critica1 range of yield stress and viscosity where segregation is minimized; however, the concrete possesses the desired workabiiity for SCC applications. These ideas are illustrated in Fig. 4.

The high workability region in Fig. 4 is designated the self- flow zone ( S E ) . The rheology of concrete is optimized at a cement paste yield stress and viscosity just high enough to avoid segregation. Segregation resistance, however, is optimized for SCC at the highest yield stress and viscosity within the SFZ.

EXPERIMENTAL CONFIRMATION OF STATIC SEGREGATION CONTROL THEORY

Experiments were performed using a spherical glass aggregate and cement paste to confirm the static segregation

10.0 mrn I.-ri

k-?

0) 10.8 mm

Fig. &(a) Vane used for yield stress measurements (cup inner radius = 22 mm); and (b ) concentric cylinder geometry used for viscosity characterization.

control theory. Pastes were mixed containing 100 volume c/c cement, cement and 30 volume % silica fume, and cement and 30 volume % fly ash. The watedcement ratio (H./c) ranged from 0.2 to 0.60, and a sulfonated naphthalene form- aldehyde (SNF) high-range water-reducing admixture was added in dosages from 0.0 to 3.8 weight % to each type of paste to generate a wide range of yield stress values.

The pastes were mixed in a Hobart paddle mixer. The water, silica fume, and high-range water-reducing admixture were placed in the mixing bowl. Cement and fly ash were added to the other materials over a 1 min interval with the mixer set to speed No. 1 (140 RPM). The paste was then mixed for 2.5 min on speed No. 2 (285 RPM). The mixer was then stopped, and the sides of the mixing bowl were scraped. Finally, the paste was mixed for an additional 2.5 min at speed No. 2. The total mixing time at speed No. 2 was 5 min.

After mixing, the paste was immediately poured into a 5.1 x 10.2 cm (2 x 4 in.) cylinder. The top of the cylinder was trowelled, and the sample was weighed. The bulk density of the paste was calculated by dividing the m a s of the sample by the volume of the cylinder.

Next, a glass sphere was gently placed on top of the fresh paste and allowed to fa11 freely. After the paste hardened, the cylinders were sectioned to determine the final resting position of the sphere. The distance frvm the bottom of the sphere to the bottom of the cylinder h was measured and divided by the original height of the cylinder h to quantify the degree of segregation in dimensionless units (Fig. 5) .

The density of the glass spheres was 2.49 g/cm3, and the average diameter of the spheres was 14 mm. The diameter of each sphere was measured to 0.01 mm before each test.

Yield stress measurement The yield stress of the paste was measured using a rheom-

eter with a vane attachment to avoid the possibility of wall slip (Fig. 6(a)). A constant rotational speed of 0.01 rad/s was applied, and the resultant stress was recorded as a function of time (Fig. 7). The peak stress in Fig. 7 corresponds to the yield stress of the material.*l

Experimental resutts Representative cross sections of hardened paste are shown

in Fig. 8. As expected, the final resting height of the spheres decreased as the yield stress decreased.


Table 2-Composition of pastes used to make concrete'

r,(Pa)

h' h -

'Values for each variable were coded from - 1 to + 1 for statistical analysis.

82.60 34.65 17.61 0.02

0.9 I 0.86 0.7 1 0.00

Time

Fig. 7-Stress response for constant rotational speed yield stress measurements. Peak stress corresponds to yield stress ( zy) of material.

Rearranging Eq. (6) in terms of material dependent variables yields

where the terms on the left side of the equation can be rnea- sured, and g is the gravitational constant. Based on the static segregation theory, the value on the left side of Eq. (12) must be greater than 413 for segregation to be avoided.

The results from al1 of the tests are plotted in Fig. 9. The maximum normalized height of the spheres is always less than unity due to the geornetry of the spherical particle sink- ing slightly into the pliable cement paste surface. The spheres start to segregate at zdrgAp values less than 4/3, in general agreement with the theory. Furthermore, there ap-

-

-

Silica Fume u

4 6 8 10

O jdl , , , ( , , , l . . , l . . . ]

o 4 2 3

T 5 y

rgAP

Fig. 9-Normalized height of glass spheres as function of paste yield stress, sphere radius, and diference in densi9 between sphere and paste. Segregation occurs at values less than 4/3, in agreement with theory.

pears to be no difference in the falling height based on the incorporation of fly ash or silica fume into the paste. Although the scope of these experiments is limited, it is encouraging that the simple theory presented in this paper predicted the rheological limits for coarse particle segregation under static conditions.

APPLICATION OF SFZ THEORY TO SCC The applicability of the SFZ theory for designing SCC was

tested by changing the rheology of the cement paste matrix in fresh concrete while holding the aggregate volurne fraction constant.

Three different types of cernent paste were used for this investigation, with corn ositions representative of paste used in SCC (Table 2).19p-6 The first type of paste consisted of cement powder, water, and a SNF-type high-range water-reducing admixture. The concentration of solids in the paste ranged from 50 to 54 volurne % (w/c = 0.27 to 0.32) and the hgh-range water-reducing admixture dosage from 0.2 to 1.8 weight % of solids. The solids volurne of the second type of paste consisted of 30% silica fume and 70% cement. The concentration of solids in the paste ranged from 50 to 54% by volurne. The amount of high-range water- reducing admixture added was 0.8 to 3.8 weight % of solids.

The composition of the third type of paste was identical to the second type (30 volume % silica fume, 70 volurne %


1 o o ~ " ' " i " " i ' " ' ~ " " ~ " ' ' ' " '

R2

t

0.9952 0.9987

1.

Intercept % solids

-1 -

Coeficient t Coefficient t 1.75 1 - 0.343 -

0.564 5.64 0.348 21.32

-1 O +1

Weight 8 High-range waterqeducing admixture

Fig. 10-Experimental design matrix for rheological characterization for cement paste. (For Paste 111, weight % high- range water-reducing admixture on x-axis should be replaced by weight % cellulose.)

Tabie 3-Regression equation coeff icients and reiated model fitting variables for Paste I (100 volume YO cement)

Paste 1

Deeree of freedom 1 3 I 3

l % high-range water- reducine admixtures -2.125 -21.29 1 -0.481 -33.58

W - i * % - ~ 3 .875 1 - 4 . 8 6 1 Oi55- 1 ~ 5 .72 ~

%solids*%HRwRAl - 1 - I -0.108 l -5.01

'Each variable coded from -1 to I and response data analyzed using log tnnsformation.

cement); however, the amount of high-range water-reducing admixture was held constant at 3.8 weight % of solids. Once again, the concentration of solids ranged from 50 to 54 volume %. A surface-treated hydroxypropyl methylcellulose powder was added to the paste as a thickening agent at dosages from Oto 0.03 weight % of solids.

' h e surface coating on the hydroxypropyl methylcellulose powder provides enhanced dispersion characteristics compared with typical cellulose based products that often require thermal cycling or high shear mixing for homogenous dispersion. The powder is easily dispersed in water at low shear rates without a significant increase in viscosity. As the pH is in- creased, the surface coating breaks down and the powder begins to hydrate. At the pH typical of most cementitious suspensions, hydration of the cellulose powder occurs in less than 1 min after,the addition of cement.22

The concentration of solids and weight % high-range water-reducing admixture (or weight % cellulose) were coded from -1 to + 1, and a statistical design program was used to generate an experimental test matrix (Fig. 10). Coding was required to better understand the influence of second-order interactions between the variables. The yield stress, viscosity, and density of the paste are determined at each of the

o 100 200 300 400 500 600 700

Skar Rate (SI)

Fig. 11-Test program used to measure viscosis of cement paste, Shear rate first ramped from O to 600 s-', then held at 600 S', andfinally ramped down to O s-'. Kscosity at 50 J-'

along ascending, or upper; jlow curve used for analysis.

points shown in Fig. 10. In some cases, additional testing points were also selected.

Mixing was performed using the process outlined in the section entitled "Experimental confirmation of static segregation control theory." The bulk density of the pastes was measured by weighing the samples in a plastic cylinder immediately after mixing, as discussed in the same section.

Rheoiogical measurements The viscosity of the paste was characterized using a con-

centric cylinder configuration on the rheometer (Fig. 6 (b)). Immediately after rnixing, the sample was placed in the rheometer and allowed to equilibrate for 60 s. The shear rate was then ramped from O to 600 s-l over a 10 s time interval. Next, the shear rate was heid at 600 s-l until h e stress either reached an equilibrium value, started to increase, or the time of the test surpassed 15 min. Finaily, the shear rate was ramped down from 600 to O s-' over a 30 s time interval (Fig. 11). This testing procedure was developed during a previous study for determining the general trends of a cement paste flow curve."

As mentioned in the section entitled "Static segregation control," the shear rate of normal concrete placement procedures is relatively low. It is also important to note that the time scale of deformation during testing in a typical SCC measurement apparatus (for example, U-tube, funnel cone. L-box) is on the order of seconds. Thus, the viscosity calculated using the ascending, or upper, flow curve is more representative of the actual viscosity of cement paste in concrete during placement.12 Based on the shear rate ranges provided by Reed for typical concrete placement and mixing processes, the viscosity of the paste at 50 s-l along the upper flow curve was used for comparison ~ l 5 0 (Fig. 1 1).20

The yield stress of the paste was measured using the procedure described in the section entitled "Yield stress measurement." A constant rotational speed of 0.01 rad/s was applied, and the resultant stress was recorded as a function of time (Fig. 7).

Regression modeling of cement paste rheology The yield stress at each testing condition was divided by the

difference in density between the paste and coarse aggegate


Table 4-Regression equation coeff icients and related model fitting variables for Paste II (30 volume % silica fume).

Paste 11

R2 Degree of freedom

0.9944 0.9837 8 5

Coefficient I t Coefficient I t Intercept 1 1.752 1 - ] 0.339 1 -

% solids 1 0.183 1 7.14 1 0.162 1 5.65

R2

Degree of freedom

Intercept % solids

% cellulose % high-range water- reducine admixtures

0.9988 0.9956 3 4

Coefficient t Coefficient t

2.189 - 0.808 -

0.146 8.91 0.154 9.85 0.781 47.78 0.43 1 27.61 -1.122

%soiids*%HFWRA( 0.083 1 2.08 1 -

1 -35.25 1 -

4.526

4.092 % solids * % cellulose

-16.47

-4.60 - -

%HRWRA*%HRhWd 0.345 1 6.29 1 0.348 1 5.90

Ap, giving units of m2/s2, to have the form of the lefí side of Eq. (7). (The density of the coarse aggregate was 2.73 g/cm3.) Likewise, the viscosity measured at 50 s-’ on the upper flow curve was also divided by the difference in density Ap, producing units of m2/s.

The z jAp and q5dAp values for each composition were analyzed using a statistical software program. The logarithm of each value was used for the regression analysis due to the wide range of yield stress and viscosity values.

Variables were included in the regression model if the t value was greater than 2.0. The t value is an indicator of the significance of an independent variable in determining the magnitude of a dependent variable. The t value is mathemat- ically the regression coefficient of the variable divided by its standard deviation.

Residual tables were also examined to make sure that the model was not overly affected by a single experiment. A residual is defined as the difference between the observed and fitted value for a given composition. If the largest residual was 1.5 times greater than the next largest residual, the experiment was considered an ~ u t l i e r . ~ ~ Based on these criteria, none of the experiments were eliminated from the data set. The regression coefficients and related model fitting statistics are provided in Table 3 to 5.

As indicated in Table 3 and 4, the amount of high-range water-reducing admixture is the most significant factor influencing both the viscosity and yield stress of cement paste. The second-order interaction of high-range water- reducing admixture (that is, %HRWRA * %HRWRA in Table 3 and 4) was also significant for both compositions. These results are in agreement with previous studies that have examined the influence of plasticizing agents on the rheology of cement paste and ceramic suspens i~ns . ” ’~ ’~~

The data from Table 5 indicate that cellulose has a large influence on the rheology of cement paste. By increasing the weight % cellulose, the yield stress and viscosity increase dramatically (as indicated by the positive sign of the regression coefficient). In al1 cases, increasing the volume % solids increases both the yield stress and viscosity of the paste.

Using the regression equations provided in Table 3 to 5, the viscosity at 50 s-’ divided by the difference in density Ap was plotted as a function of yield stress divided by Ap for various compositions of each type of paste (Fig. 12 to 14). The areas shown in the diagram represent the posible yield stress-viscosity combinations for the different pastes

Table 5-Regression equation coeff icients and related model fitting variables for Paste 111 (30 volume % silica fume + cellulose)t

% cellulose * % cellulose -0.442 1 -13.52 1 4 . 2 2 4 -7.18

‘Each variable coded from - 1 to 1 and response data analyxd using log uansformaoon.

(50-54voPh) 7

(x103 &/s’)

100

n O 2 4 6 8 10

(x103 m/s l )

AP

Fig. 12-qAp versus ~ 5 0 l A p response surface for Paste 1 (100% cement) at solids concentrations ranging from 50 ro 54 volume % and high-range water-reducing admixture dosages of 0.2 to 1.8 weight % of solids. Arrow indicates how rheology of pastes changes for constant high-range water-reducing admixture dosage as solids concentration increases.

used in this study. Severa1 response curves at a constant weight % high-range water-reducing admixture (or cellulose) dosage are highlighted in the figures for clarity. The arrows in Fig. 12 to 14 indicate the influence of the concentration of solids (from 50 to 54%) on the rheology of the paste for a given high-range water-reducing admixture (or cellulose) dosage.

INFLUENCE OF MATRIX RHEOLOGY ON WORKABILITY OF CONCRETE

The influence of the rheology of the cement paste matrix on the flow properties of fresh concrete was examined using the data obtained in the section entitled “Application of the SFZ theory to SCC.” For al1 of the batches, only the composition of the cement paste matrix was changed. The ratio of the volume of fine aggregate to total aggregate (Vf/Vhgg) was 0.55, and the ratio of volume of paste to total aggregate ( Vp/Vogg) was 0.60 for al1 compositions. The fine aggregate was standard silica sand, and the coarse aggregate was limestone grave1 with a maximum diameter of 9.5 mm. The particle size distribution of the aggregates is given in Fig. 15.


O 2 4 6 8 10

Fig. 13-zjAp versus q50lAp response surface for Paste II (30 volume % silica fume) at solids concentrations ranging from 50 to 54 volume % and high-range water-reducing additions from 0.8 to 3.8 weight % of soliak. A m w indicates how rheology of pastes changes for constant high-range water- reducing admixture dosage as solids concentration increases.

300

Ap 200

o o29 WP! cellulose as mdicated

/@ .-

increasmg soMs 0023 4 e concentratmn __z 1 (50-54 VOP!)

O i - O 2 4 6 8 10

Fig. 14-zylAp versus q50lAp response surface for Paste III (30 volume % silica &me with cellulose) at solids concentrations ranging from 50 to 54 volume % and cellulose powder additions from 0.00 to 0.03 weight % of solids. Arrow indicates how rheology of pastes changes for constant high-range water-reducing admixture dosage as solids concentration increases. (High-range water-reducing admixture dosage was 3.8 weight %)

Concrete mixing procedure Cement paste was mixed in a large planetary mixer in a

procedure similar to the process explained in the section entitled “Experimental confirmation of static segregation control theory.” Next, the fine aggregate was added to the paste over a 1 min interval. The paddle speed was then in- creased to the maximum, and the mortar was mixed for 2.5 min. Finally, the coarse aggregate was added to the mix over a 1 min time frame, and the concrete was then mixed for an additional 2.5 min at the highest speed.

Characterization of concrete workability A U-shaped apparatus (Fig. 16) was used to quantify the

flow properties of the fresh concrete. This test is the most widely used method for assessing the workability of SCC.24

- 0.80 E 4 C e> g 0.60 e e> > m ‘J 0.40 a E - u 0.20

1.1

100 lo00 ioax,

Particle Size (microns)

Fig. 15-Particle size distribution offine and coarse aggregates.

Filling Chamber

181 iiiiii

mm

Fig. 16-U-shaped apparatus. Maximum filling height = 191 mm. Self-compacting minimum = 168 mm.

At the bottom of the U-tube, three steel rods were spaced approximately 25 mm apart to simulate typical reinforcing bars. The bars act as obstacles, hindenng the cohesive flow of the material.

One side of the apparatus was filled with concrete immediately after mixing. After filling, the door separating the two chambers was lifted, and the equilibrium height was measured as the highest point of the concrete. Based on the criteria suggested by Okamura and Ouchi for their ~ y s t e m , ~ ~ concrete that filled the chamber to 88% of the maximum equiiibnum height or greater was considered self-compacting. Although the geometry of the U-shaped apparatus iüustrated in Fig. 16 is not exactly the same as the device used by Okamura and Ouchi, the same criteria were applied to determine if the composition was seíf-compacting. For the U-shaped apparatus used in this study, this cnterion corresponds to a filling height greater than 168 mm.

Experimental approach The largest coarse aggregate particles used to make the

concrete were approximately 9.5 mm in diameter. Using Eq. (7), the value of zJAp required to avoid segregation is

436 ACI Materials Journal/November-Decernber 2001

!

SeH Flow *c Self Aow Minimum Minimum

Fig. 17-Filling height of concrete in U-tube: (a) 127 mm; and (b) 188 mm. For this study, concrete with filling height greater than 168 mm is considered seij-compacting.

5 2 5 = (4/3)( 9.806m/s2)(4.75 x 10-3m) = (13) AP 3

62.1 x 10-3m2/s2

Using this value as a starting point, the filling height of concrete in the U-tube appmtus was studied by systematically changing the composition, and consequently, the rheology of the cement paste matrix. Several tests were conducted to determine the range of T / A ~ and qso/Ap values that led to SCC for each paste. In general, the testing started at the maximum solids concentration (that is, 54 volume % solids)

Assuming the U-tube filling height was greater than 168 mm, a new composition was chosen with a higher zY/Ap vaiue until the filling height was less than the defined self-compacting minimum. Additionally, the value of s / A p was decreased by changing the composition of the paste until noticeable bleeding occurred, indicating segregation. Once the minimum T,JA~ value had been estabiished at 54 volume % soiids, several other tests were conducted to determine the SFZ (that is, where the U-tube filling height was always greater than or equal to 168 mm).

at compositions where zy/Ap was near 62.1 x 10- 3 m 2 2 /s .

Experimental results and discussion Pictures of two concretes of different U-tube filling height

are shown in Fig. 17 as examples of typical results. The filling heights of ail of the tested paste compositions are shown in Fig. 18 to 20. ”he SFZ for each of the pastes was determined based on the filling height data provided in Fig. 18 to 20. The SFZ was drawn as a simple polygon for each type of paste. The SFZ is constrained by the q s ~ / A p limits for al1 of the pastes. Thus, the restilts suggest that the value of zy/Ap is a more important parameter for designing SCC.

The SFZ for each of the pastes is plotted together in Fig. 2 1. Both the Paste iI (30 volume % silica fume) and Paste 111 (30 volume % silica fume + cellulose) SFZs begin within the Paste 1 (100% cement) self-flow region. ”he silica fume and cellulose paste self-flow mgions extend to much higher zy/Ap values, however. Thus, paste containing silica fume and cellulose

400

300

- 7 Y :-.. ........ AP 200

(xi O’ m2d>

Ido

62 1

O 2 4 6 a 10

Fig. 18-U-tubefilling height of concrete for Paste 1. Filling height for each concrete labeled in figure (units = mm). Bleeding occumd in samples at very low VAp as indicated.

. . . . . . . . . . . , . _ . . m . . , . . . , . . . . . . . . . ................ - . . . . .- : .- _. . . . .

300

100

62.1

O O 2 4 6 8 10

Fig. l9-U-rube filling height of concrete for Paste II. Filling height for each concrete labeled in figure (units = mm). Bleeding occumd in samples at very low r,,/Ap as indicated.

has a greater segregation resistance while still providing the desired concrete workability in comparison to paste containing only cement. These results provide a quantitative insight into írends found in the field and in other studies.’*’0,22

Producing SCC using only cement, water, and high-range water-reducing admixtures for the paste could be dificult given the small size of the cement paste SFZ (Fig. 21). From an engineering perspective, the pastes containing silica fume and cellulose would be preferred since there is a wider range of rheological options and greater segregation control.

The minimum T ~ / A ~ value calculated in Eq. (13) for segregation resistance is higher than the minimum experimental value for al1 the pastes (Fig. 21). The theoretical minimum zY/Ap value, however, is within the SFZ for al1 of the pastes. This simple result leads to an immediate engineering benefít in terms of producing SCC.

As an example, assume that the aggregate distribution and paste volume fraction of the concrete are already determined. As indicated in Table 1, these values do not have a wide variation in practice. The remaining design parameter is


?,O

AP - (x1aYds')

250

200'

Fig. 2O-U-tubefilling height of concrete for Paste III. Filling height for each concrete labeled in figure (units = mm). Bkeding occumd in samples at very low c,J& as indicateú

Paste 111 (silica fume + cellulose) .

1 1 Paste I (cément oniy)

O 2 4 6 8 10

Fig. 2ISelf=flow zones for different paste compositions. Theoretical segregation limit calculated using Eq. (2).

the rheology of the cement paste. The value of z,,lAp can be estimated for segregation resistance using Eq. (7). A small amount of cement paste can be mixed, the density measured, and the yield stress estimated using a mini or cylindrical slump test.25326 Most likely, only a few batches of cement paste will be required before the value of z4Ap is close to the value calculated using Eq. (7). This is a major advantage over using a trial-and-error process to produce SCC.

As a final confmation of the self-flowing ability of the concrete produced in this study, severai slump tests were per- fonned using compositions where the rheology of the paste was within the S E . Figure 22 shows a picture of a material with a compositian of 30 volume % silica fume and 70 volume % Cement (of paste solids), 54 volume % solids (of total paste), and 2.3 weight % high-range water-reducing admixture (of paste solids) once the slump cone was lifted. The slump height was approximately 29 cm (1 1 in.), and the diameter of the slumped material (that is, slump flow) was 60.9 cm (24 in.). As noted in the figure, there is no segregation of the coarse aggregates even at the periphery of the material. As a

J Fig. 2241ump of self-compacting concwte. Thew is no segregation of aggregate even at periphery of slumped material.

I

i'

t c

L

f

Fig. 2 3 4 m s s section of hardened selj-compacting concrete. There is no segregation of aggregates.

visual confirmation of the segregation resistance of the mate- . riai, Fig. 23 shows a cross section of the hardened concrete originally cast into a cylinder immediately after mixing. Once again, there is no segregation of the coarse aggregates.

CONCLUSIONS A new segregation-controlled design methodology was

introduced for SCC. The theory assumes that for a given aggregate particle size distribution and volume fraction, the rheology and density of the cement paste matrix dictates the fluidity and segregation resistance of concrete. Additionally, the idea is proposed that a minimum paste yield stress and viscosity must be exceeded to avoid segregation under both static (rest) and dynamic (flow) conditions, respectively.

The new SCC design approach suggests that concrete will have its greatest fluidity at the lowest paste yield stress and viscosity where segregation is still avoided. Conversely, segregation resistance is optimized at the highest yield stress and viscosity that still produces self-flowing materials. Thus, there is a critical range of yield stress and viscosity where segregation is minimized, yet the material is self-flowing. This segregation-resistant, high workability region is designated the S E .


Experimenis were conducted to test the validity of the static segregation control thmry. Glass spheres were gently placed on the surface of fresh cement paste and allowed to fa11 freely. There was good agreement between the measured final resting height of the spheres and the theoretical predictions.

The appiicability of the ihmry for designing SCC was tested by measuring the flow properties of concrete using a U-tube appmtus. The aggregate particle size distribution and volume fraction were held constant, and the rheology of the cement paste matrix was systematically altered. The yield stress, viscosity, and density of cement paste containing silica fume, high-range water-reducing admixture, and cellulose were determined in an initial set of experiments. Thus, any changes in the flow properties of the concrete were directly related to the rheology of the paste.

Although there is variation in the relative size of the SFZ for different paste compositions, the results suggest that SCC can be produced using the new design methodology. As an example, the slump height of concrete produced using the new approach was approximately 29 cm ( 1 1 in.), and the diameter of the slumped material (slump flow) was 60.9 cm (24 in.). There was no segregation of the coarse aggregate, even at the periphery of the slumped material.

ACKNOWLEDGMENTS The authors would like to thank D. Lynn Johnson for his helpful sugges-

tions and Y. Jennifer Su for her experimental assistance during this project. The authors would also like to acknowledge Harold S. Haller and Co. for supplying the siatistical software. Funding for this work was provided by the National Science Foundation Center for Advanced Cement-Based Materials (ACBM) under Grant CHE-912OOO2.

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ACI Materialc Journal/November-December 2001 439

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