Clery Report 2012 - McDaniel College

Hossain, Islam, and Copeland 1

INFLUENCE OF ULTRAFINE FLY ASH ON THE SHRINKAGE AND CRACKING

TENDENCY OF CONCRETE AND THE IMPLICATIONS FOR BRIDGE DECKS

Akhter B. Hossain (Corresponding Author)Assistant Professor

Department of Civil EngineeringUniversity of South Alabama

Mobile, AL 36688Email: [email protected]

Phone: 251-460-7438Fax: 251-461-1400

Samantha IslamProject Engineer

Volkert & Associates, Inc. Mobile, AL

Email: [email protected]: 251-342-1070Fax: 251-461-1400

Kevin D. CopelandManager, Technical ServiceBoral Material Technologies

San Antonio, TX 78216Email: [email protected]

Phone: 210-349-4069 Fax: 210-979-6110

Number of Words: Abstract 241, Body 4491, 2 Tables, 9 Figures = Total 7482

Submitted to the Transportation Research Board for possible presentation and publication

Date of Submission: November 17, 2006

TRB 2007 Annual Meeting CD-ROM Paper revised from original submittal.


INFLUENCE OF ULTRAFINE FLY ASH ON THE SHRINKAGE AND CRACKING

TENDENCY OF CONCRETE AND THE IMPLICATIONS FOR BRIDGE DECKS

Akhter B. Hossain, Samantha Islam, and Kevin D. Copeland

ABSTRACTEarly age shrinkage cracking of concrete bridge decks is a major concern to the transportation industry, including construction contractors and state DOTs. The repair of concrete bridge decks is expensive and difficult. The most cost effective solution is to prevent cracking from occurring in the first place by carefully selecting proper ingredients for bridge deck concrete. Selection of the material for a bridge deck requires proper understanding of how the material influences the shrinkage cracking potential of concrete. This paper describes an experimental study that was performed in order to understand the influence of ultrafine fly ash (UFFA) on the shrinkage cracking of concrete mixtures. The paper provides a relative comparison of the performance of concrete containing UFFA with that of concrete containing silica fume (SF). Several mortar mixtures were prepared with varying UFFA and SF content. Free shrinkage strain and splitting tensile strength measurements were performed using standard test methods. In addition, the restrained ring test was used to determine the residual tensile stress development in concrete ring specimens. The cracking potential of the mixtures was assessed by computing simple ratios of the residual tensile stress in the rings and the splitting tensile strength of the mixtures. The results of this study favored the use of UFFA in bridge deck concrete because concrete containing UFFA demonstrated significant reduction in both shrinkage and shrinkage cracking potential. Conversely, the concrete containing SF demonstrated higher level of shrinkage and higher cracking potential.



INTRODUCTIONIn recent years, high-performance concrete (HPC) has become widely used in highway bridges where strength and durability are two important considerations. In HPC, a part of portland cement is replaced by pozzolanic materials. These pozzolans improve the strength and durability of concrete (1,2,3). They improve the strength of concrete by chemically reacting with the lime present in concrete to form additional cementing compounds. In addition, they improve the durability of concrete structures by reducing the permeability and enhancing the chemical resistance of concrete.

Fly ash, a byproduct of the coal burning process in electric power plants, is one of the most widely used pozzolanic materials. The proper type and quantity of fly ash in concrete improve concrete rheology, increase long-term compressive strength, and decrease permeability. However, it has been found that the typical fly ash (Class F) often results in a decrease in the rate of strength gain. For this reason, the use of high volume of fly ash is often avoided in situations, such as a bridge in need of an overlay, where a delay in construction may cause significant inconvenience to the public. Researchers (4,5) have found that the rate of strength gain of concrete containing fly ash can be improved by decreasing the fly ash particle size. It was also found that a fly ash with finer particle size can provide the durability and strength benefits to concrete at a much earlier age than a typical fly ash.

Ultrafine fly ash (UFFA) with a mean particle diameter of 1-5 micron is a relatively new pozzolan. UFFA is carefully processed by mechanically separating the ultra fine fraction from the parent fly ash. UFFA at a relatively low replacement level (5-15% of the cement mass) has been found to contribute more to strength gain and permeability reduction in concrete than typical fly ash. Performance of UFFA in concrete is found to be comparable to other highly reactive pozzolans such as silica fume (6,7,8). Addition of UFFA in concrete has been found to enhance the long term performance of concrete in terms of chloride penetration, alkali-silica reactivity and sulfate attack (7,8). Due to these advantages UFFA seems to be ideal for use in highway bridge decks. However, high-performance bridge decks have a tendency to develop early-age cracking due to excessive shrinkage (7,9,10,11). If concrete cracks prematurely, reducing the permeability with a pozzolan is ineffective because the cracks provide direct pathways for the aggressive agents into concrete. Hence, in order to evaluate the performance of UFFA in concrete mixtures for bridge decks, it is vital to study the restrained shrinkage cracking of these concrete mixtures.

Limited information exists in the literature regarding the influence of UFFA on the restrained shrinkage behavior of concrete. In a recent study, Subramaniam et al. (7) found that concrete containing UFFA demonstrated an increase in the age of cracking in the restrained ring test when compared with plain concrete. A significant reduction in shrinkage played a major role in delaying the age of cracking in UFFA concrete. This study, however, did not provide any quantitative information regarding tensile stress development in the material, which is a major factor that influences the shrinkage cracking potential of concrete(7,10,11). To understand the restrained shrinkage cracking mechanism of concrete containing UFFA, it is vital to consider shrinkage stress development in addition to shrinkage strains. This paper describes a study in which an attempt was made to investigate the influence of ultrafine fly ash on restrained shrinkage cracking potential of concrete. A stress solution was used to determine the tensile stresses that develop in the restrained concrete ring specimens. The tensile stresses in the ring specimens due to restrained shrinkage were combined with concrete’s time-dependent tensile strength to assess the cracking potential of the mixtures.



RESEARCH SIGNIFICANCE Many bridge decks in the United States suffer from early age transverse cracking as a result of excessive shrinkage of the bridge deck concrete under restraint. The repair of concrete bridge decks is difficult and expensive. It is accepted that the most effective way of dealing with this problem is to select concrete mixtures for bridge decks that have lower tendencies to crack. The selection of concrete ingredients for bridge decks, the selection of a pozzolan for example, should not be based on their contribution to strength and permeability properties alone. Their influence on shrinkage and resulting cracking should be considered as well. This paper describes an experimental study in which the influence of ultrafine fly ash on the shrinkage and cracking tendency of mortar mixtures was investigated. This paper also provides a critical comparison of the relative performances of concrete mixtures made with ultrafine fly ash and silica fume at various addition rates. Silica fume has been chosen in this paper because it is another ultrafine pozzolan which is widely used in concretes for bridge decks.

EXPERIMENTAL PROCEDUREIn this study, a series of experiments was performed to investigate the influence of ultrafine fly ash (UFFA) on the early age stress development and cracking in restrained low w/cm ratio mortar. The mortar mixtures were made with Type I cement, ultrafine fly ash (UFFA) and silica fume (SF). The UFFA used in this study met the requirements of Class F fly ash according to ASTM C-618 (12) and the SF met the requirements of ASTM C-1240 (12). UFFA and SF were added to the mixtures by replacing a portion of the cement by mass. The mortar mixtures had w/cm ratio of 0.3 and sand volume of 50%. Table 1 shows the mixture proportions used in this study. The Plain mixture was used as the base line for comparison with other mixtures. Mixtures UFFA-8 and UFFA-12 were obtained by replacing 8% and 12% of cement by mass in the mixtures respectively with ultrafine fly ash. SF-6 and SF-9 mixtures were prepared by replacing 6% and 9% of cement by mass respectively with silica fume. A high-range water reducing admixture was added to the mixtures to maintain 75 mm – 100 mm slump. After mixing, a portion of the mixture was used to determine the setting times according to ASTM C-403. The rest of the mixture was placed in the forms, rodded, finished with a steel trowel, placed under plastic sheets to prevent moisture loss, and maintained at 21°C for 24 hours. The specimens were demolded at an age of 24 hours and placed in a 21°C, 50% relative humidity (RH) environment for the remainder of the experiment.

The results of the following tests were used in this study to better understand the influence of UFFA on the stress development and the resulting cracking in restrained concrete:

• setting time test,• free shrinkage test, • restrained ring test, and• splitting tensile strength test.

These test methods are briefly described in the following sections.



TABLE 1 Mixture ProportionsMix Sand

(kg/m3)Cement(kg/m3)

UFFA(kg/m3)

SF(kg/m3)

Water(kg/m3)

Water/ Cement Material

Aggregate Volume Fraction

Plain 1307 763 - - 234 0.3 0.5UFFA-8 1307 689 59 - 229 0.3 0.5UFFA-12 1307 654 89 - 227 0.3 0.5SF-6 1307 706 - 45 231 0.3 0.5SF-9 1307 679 - 67 229 0.3 0.5

Setting Time TestIn this study, setting times of the mortar mixtures were needed to develop the time-dependent material property functions discussed later in this paper. ASTM C-403 (12) method was used to determine the setting time of each mixture. According to this method, the stiffening of a mortar mixture is indicated by the increasing pressure required to cause a standard penetrometer needle to penetrate 25 mm into the mixture over a period of 10 seconds. To measure setting time, a portion of the fresh mortar mixture was collected in a cylindrical container with 250 mm diameter. The mortar depth in the container was 150 mm. The resistance of mortar to 25 mm penetration by the standard needle was recorded at suitable intervals. The penetration resistance and elapsed time were used to determine the setting times. During the setting time tests, the surrounding temperature was maintained at 210C.

Free Shrinkage TestIn this study, the following two methods of free shrinkage measurement were performed:

• shrinkage measurement of the mixtures during the first 24 hrs after casting, and • shrinkage measurement of mortar prisms beginning 24 hours after casting.In the first method, the shrinkage of the mortar mixtures was assessed while the

mixtures were still in the form. The standard method of free shrinkage measurement of concrete according to ASTM C-157 allows measurement to begin 24 hours after casting. Recent studies have shown that concrete mixtures may demonstrate substantial autogenous shrinkage during the first 24 hours of material development (10). Therefore, ASTM C-157 may neglect a significant amount of shrinkage in the overall measurement. To overcome this problem, in a recent study, Nassif et al. (13) used vibrating wire embedment strain gages to measure the early age shrinkage of concrete. To assess the shrinkage that the mortar mixtures may experience during the first 24 hours, vibrating wire strain gages were used in this study. Fresh mortar mixture was collected in a container with 250 mm x 75 mm x 75 mm internal dimension. A vibrating wire strain gage was embedded into the mixture in the center of the container. The strain measurements began when final setting times of the mortar mixtures were reached.

In the second method, mortar prisms were prepared for free shrinkage strain measurements. The prisms had 75 mm (3 in.) square cross sections and 250 mm (10 in.) gage length. The method was very similar to ASTM C-157 which uses the length change of concrete prisms to determine the free shrinkage strain. However, unlike the ASTM C-157 procedure, this study used two different specimen boundary conditions: (a) partially sealed prisms with two ends and two sides sealed with aluminum tape, and (b) completely sealed



prisms with both ends and all sides sealed with aluminum tape. The partially sealed specimens had two drying sides with drying condition identical to the mortar in the restrained rings described later in this paper. This was done to ensure that free shrinkage from the prisms can be compared directly to the ring specimens without the need for any geometric corrections. While partially sealed specimens allowed measurement of overall shrinkage in mortar specimens, it was assumed in this study that the sealed specimens allowed measurement of autogenous shrinkage only. The changes in length of the mortar prisms were measured using a digital comparator.

Restrained Ring TestThe restrained ring test is a widely used method for simulating the restrained shrinkage of concrete. In the restrained ring test, a concrete annulus is cast around a steel ring as shown in Figure 1(a). The specimen is then allowed to dry in a controlled environment. If unrestrained, the concrete would normally shrink, however the steel ring prevents (restrains) this shrinkageresulting in the development of tensile stresses. The tensile stress may be high enough to cause cracking in the ring specimen. This method is very popular because it is economical and very simple to perform. American Association of State Highway and Transportation Officials (AASHTO) has recommended the ring test as a standard method for evaluating the cracking susceptibility of concrete mixtures (14). In this study, the ring test was used to investigate the influence of UFFA on restrained shrinkage behavior of the mortar mixtures. However, in this study, the ring specimens were slightly different from the AASHTO version of the rings. Slight modifications in the ring’s dimension and in the boundary condition enabled a simple stress solution to be used to determine the residual stresses in the ringspecimens due to restrained shrinkage. The stress solution is discussed later in this paper.

(a) Restrained ring specimen (b) Ring test setup

FIGURE 1 Restrained ring test.

Several mortar ring specimens were prepared with different UFFA and SF content. The mortar in the rings had an inner diameter of 319 mm (12.75 in.) and an outer diameter of 450 mm (18 in.). The wall thicknesses of the steel rings were 9.5 mm (3/8 in.). The height of the ring specimens used in this study was 75 mm (3 in.) which was smaller than the rings specified in AASHTO PP-34 (14). This was done to increase the shrinkage rate and to enable a direct comparison to results of test prisms with dimensions that are similar to those

Mortar ring

Aluminum tape

Steel ring

Strain gage

Data acquisition box



described in ASTM C-157 (75 mm square cross-section with a 250 mm gage length). Each steel ring used in this study had four strain gages attached at the mid-height of the innersurface of the steel ring. The strain gages were connected to a data acquisition system for continuous monitoring. Strain data were collected at 10 minute intervals startingapproximately 30 minutes after water came in contact with cement in the mixing process. The specimens were sealed for the first 24 hours. At the time of demolding the circumference of each specimen was sealed using two layers of aluminum tape. The specimen was then placed in a constant RH (50%) and temperature (21oC) room to allow drying from top and bottom surface of the rings. By permitting drying from only the top and bottom surface of the ring moisture is lost uniformly along the radial dimension thereby simplifying modeling and providing uniform moisture loss along the radial dimension of the specimen (10,11).

Splitting Tensile Strength TestTensile strength of concrete is an important factor that influences cracking. When the tensile stress in concrete due to restrained shrinkage exceeds concrete’s tensile strength, a crack develops. Since measurement of true tensile strength of concrete is very difficult, splitting tensile strength of mortar cylinders was measured in this study. The cylinders used for splitting tensile strength determination had a length that was equal to the ring height 75 mm (3 in.) with a 100 mm (4 in.) diameter, however the remainder of the testing procedure was consistent with that specified by ASTM-C-496 (12). Splitting tensile strength was measured at 1, 3,7,14 and 28 day ages.

EXPERIMENTAL RESULTSThe results of the experiments performed in this study are discussed in the following sections:

Setting Time TestThe results of setting time test are presented in Figure 2. The figure shows the plots of penetration resistance of the mortar mixtures versus elapsed time. According to ASTM C-403, initial and final setting times are defined as the times at which the penetration resistance reaches vales of 3.5MPa (500 psi) and 27.6 MPa (4000psi) respectively. To determine the setting times of the mortar mixtures, the following power function was fitted to the penetration test data as recommended by ASTM C-403 (13):

P = ctd (1)

where P is the penetration resistance in MPa, t is the elapsed time in minutes and c and d are regression constants. The regression fits of Equation 1 for the mortar mixtures used in this study are shown in Figure 2. Table 2 shows the initial and final setting times of the mixtures.



0 100 200 300 400 500 600 700

Time (minutes)

0

5

10

15

20

25

30

35

40

45

Pen

tera

tion

Res

ista

nce

0

1000

2000

3000

4000

5000

6000PLAIN

UFFA-8

UFFA-12

SF-6

SF-9

Final Setting

Initial Setting

(MPa) (psi)

FIGURE 2 Determination of setting times.

TABLE 2 Initial and Final Setting TimesMix Initial Setting

Time Final Setting Time

Plain 500 min (8.33 hrs) 625 min (10.42 hrs)UFFA-8 355 min (5.92 hrs) 442 min (7.37 hrs)UFFA-12 282 min (4.70 hrs) 383 min (6.38 hrs)SF-6 435 min (7.25 hrs) 565 min (9.42 hrs)SF-9 540 min (9.00 hrs) 675 min (11.25 hrs)

Free Shrinkage TestFigure 3 shows the results of free shrinkage tests. Figure 3(a) shows the overall shrinkage of the mortar mixtures used in this study. It is evident from the figure that the mortar specimens containing UFFA demonstrated significantly lower overall shrinkage than the plain specimen while the specimens containing SF demonstrated higher overall shrinkage than the plain specimen. It is interesting to note that the overall shrinkage decreased with increasing UFFA content while it increased with increasing SF content. Figure 3(b) shows the autogenous shrinkage of the mortar mixtures. The figure demonstrates a similar trend in the autogenous shrinkage as in the overall shrinkage: the autogenous shrinkage decreased when UFFA content increased and increased when SF content increased



0 10 20 30Time (days)

-1000

-800

-600

-400

-200

0

Tot

alS

hrin

kage

Str

ain

(mic

rost

rain

)

Plain

UFFA-8

UFFA-12

SF-6

SF-9

0 10 20 30Time (days)

-1000

-800

-600

-400

-200

0

Sea

led

Shr

inka

geS

trai

n(m

icro

stra

in)

Plain

UFFA-8

UFFA-12

SF-6

SF-9

(a) Overall shrinkage (b) Autogenous shrinkage

FIGURE 3 Free shrinkage measurements.

Restrained Ring TestFigure 4 shows the average steel strains in the restrained mortar ring specimens used in this study. It can be noticed in the figure that the ring specimens with UFFA demonstrated lower strain levels than the Plain specimen. Also, it is evident from the figure that the strain level decreased with increasing UFFA content. For example, at an age of 5 days ring with Plain mixture experienced 81 microstrain whereas rings with UFFA-6 and UFFA-12 mixtures experienced 75 and 68 microstrain respectively. The figure also shows that the addition of silica fume increased the strain level in the restrained ring specimens. For example, at same age of 5 days ring specimens with SF-6 and SF-9 mixtures demonstrated 87 and 93 microstrain respectively. In addition, it can be seen that an abrupt change of strain was observed in the restrained specimens, which coincided with the age a visible crack was observed.

0 4 8 12 16Time (days)

-120

-80

-40

0

Ave

rage

Ste

elS

trai

n(m

icro

stra

in)

Plain

UFFA-8

UFFA-12

SF-6

SF-9

Cracking

FIGURE 4 Average steel strains.



Figure 5 shows the age of cracking in the restrained ring specimens prepared with the mortar mixtures used in this study. It can be seen in the figure that the addition of UFFA improved the cracking resistance of concrete. While the plain mortar mixture cracked at an age of 9.8 days, the UFFA-8 mixture cracked at 11.8 days and the UFFA-12 mixture cracked at an age of 13.7 days. While the addition of UFFA increased the age of cracking in the mortar mixtures used in this study, the addition of silica fume decreased the age of crackingas shown in Figure 5. The figure shows that the addition of silica fume up to 6% (SF-9) didnot cause a significant decrease in the restrained shrinkage cracking of concrete, but an increase in silica fume content to 9% showed a significant decrease in the age of cracking. While the SF-6 mixture cracked at 9.6 days, the SF-9 mixture cracked at 7.7 days.

0 2 4 6 8 10 12 14

SF-9

SF-6

Plain

UFFA-8

UFFA-12

Age of Cracking (Days)

FIGURE 5 Age of cracking.

0 10 20 30Time (days)

0

2

4

6

Spl

ittin

gT

ensi

leS

tren

gth

0

200

400

600

800

(psi)

Plain

UFFA-8

UFFA-12

SF-6

SF-9

(MPa)

FIGURE 6 Splitting tensile strength development.

Splitting Tensile Strength TestFigure 6 demonstrates the results of splitting tensile strength test. It can be seen in Figure 6 that at earlier ages (7 days and earlier) the splitting tensile strengths of the five mixtures were comparable while at later ages (7 days and later) the splitting tensile strengths of UFFA and SF mortar mixtures were higher than the plain mortar mixture. It should be noted that the



strengths reported in this paper were obtained from specimens that were cured at 50 percent relative humidity (RH), the same RH at which the restrained ring specimens were cured. The mixtures would show higher strength if they were moist cured. This improvement would be more pronounced in mixtures containing the pozzolans as reported in the literature (15).

ANALYSIS OF RESULTS

Determination of Actual Residual Stresses in the Restrained Ring SpecimensThe measured average steel strains from the restrained ring tests were used to compute the maximum residual tensile stress that develops in the ring specimen. Hossain and Weiss(10,11) showed that if a restrained concrete ring specimen experiences uniform shrinkage in the radial direction (if allowed to dry from top and bottom surfaces) then the maximum tensile stress develops at the interface between concrete and steel as shown in Figure 7(a). This maximum tensile stress at age t can be calculated using Equation 2.

2R1RSsteelmaxactual CCE(t)ε(t)σ ⋅⋅⋅−=− (2)

where, steelε is the average strain measured in the steel ring, Es is the elastic modulus of steel,

and C1R and C2R are two constants that depend on the geometry of the ring and the steel material properties. Due to space limitations, details on this stress solution are provided elsewhere (10). The maximum residual stresses in the five mortar mixtures were calculated according to this approach and presented in Figure 7.

Figure 7 shows the actual maximum residual tensile stresses that were computed for the mixtures tested in this study using Equation 2. It is evident from Figure 7 that the restrained mortar specimens containing ultrafine fly ash demonstrated a lower level of residual tensile stress than the plain specimens. It is evident that the stress level decreased with increasing UFFA content. Figure 7 also shows the influence of silica fume on residual stress development. It can be seen that addition of silica fume increased the stress level in the restrained mortar specimens.

0 4 8 12 16Time (days)

0

1

2

3

4

Max

imum

Ten

sile

Str

ess

0

200

400

600

100

300

500

(psi)

Plain

UFFA-8

UFFA-12

SF-6

SF-12

Maximum TensileStress

(a) Residual stress distribution (b) Maximum residual stress development

(MPa)

FIGURE 7 Residual stress developments.



Development of Time-Dependent Material Property FunctionSince the splitting tensile strength test provided material property at specific ages, time-dependent functions of splitting tensile strength were developed for the mortar mixtures to obtain tensile strength at any other age. Statistical regression analysis was used for this purpose. The time-dependent splitting tensile strength functions were needed in this study to determine the risk of cracking at different ages.

The time dependent functions for the splitting tensile strength were obtained using an approach used by Hossain and Weiss (10). According to this approach, the time- dependent function of splitting tensile strength is represented by Equation 3.

)tK(t1

)tK(t

f

(t)f

0

0

infsp

sp

−+−

=−

(3)

In this equation (t)fsp is the tensile strength at any age t, infspf − is the splitting tensile strength

of concrete at an infinite age, t0 is the age at final setting, and K is rate constant. The splitting tensile strength at infinite age was obtained using an approach similar to that recommended by Malhotra and Carino (16). According to this approach, the value of a material property at infinite age can be obtained from the intercept of the material property versus inverse of time graph. The time dependent fits of the expression to the splitting tensile strength data are shown in Figure 8. Also, Figure 8 provides the values of fsp-inf and K for the mortar mixtures used in this study .

0 10 20 30

Time (days)

0

0.2

0.4

0.6

0.8

1 UFFA-8

0 10 20 30

Time (days)

0

0.2

0.4

0.6

0.8

1 UFFA-12

0 10 20 30

Time (days)

0

0.2

0.4

0.6

0.8

1

Ten

sile

Str

engt

hR

atio

(fsp

/f sp-

inf)

SF-6

0 10 20 30

Time (days)

0

0.2

0.4

0.6

0.8

1 SF-9

0 10 20 30

Time (days)

0

0.2

0.4

0.6

0.8

1

Ten

sile

Str

engt

hR

atio

(fsp

/f sp-

inf)

Plain

fsp-inf =4.6 MPaK =1.8

fsp-inf =5.1MPaK =1.52




FIGURE 8 Regression fits of time-dependent splitting tensile strength function.

Assessing the Risk of CrackingOnce the time-dependent maximum residual stress in concrete ring and the time-dependent splitting tensile strength of concrete are available, the cracking potential of concrete at age t



can be calculated based on the simple ratio of the actual maximum residual stress ( max-actualσ )

and the splitting tensile strength (fSP) as shown in Equation 4.

)t(f

)t()t(

sp

maxactualCR

−σ=Θ (4)

Cracking potential ( CRΘ ) gives a simple measure of how close the specimen may be to

failure at different ages. The cracking potentials for the mixtures tested in this study werecomputed and presented in Figure 9. Although theoretically failure is expected to occur when the cracking potential reaches 1.0, experimental evidence typically shows that cracking takes place at lower values (10,17). From Figure 9 it is evident that all the specimens cracked when the cracking potential (Θ) ratio reached values slightly lower than 1.0 as expected. It is evident from Figure 9 that in general specimens containing UFFA demonstrate lower cracking potential than the plain specimen, whereas specimens containing silica fume demonstrated higher cracking potential as shown in Figure 9.

0 4 8 12 16Time (days)

0

0.2

0.4

0.6

0.8

1

Cra

ckin

gP

oten

tial

Plain

UFFA-8

UFFA-12

SF-6

SF-9

FIGURE 9 Cracking potential of different mixtures.

SUMMARY AND CONCLUSIONSThis paper has provided illustrations of how ultrafine fly ash influences the shrinkage stress development and cracking in restrained concrete. In summary:

• Free shrinkage of mortar containing various percentages of UFFA and SF by mass of cement was measured. It was found that the addition of UFFA resulted in significant reduction in shrinkage while addition of SF resulted in increase in shrinkage. It was interesting to note that shrinkage level decreased with increasing UFFA content and increased with increasing SF content.

• A simple stress solution was used to obtain the residual tensile stresses in the restrained rings due to shrinkage. It was found that the addition of UFFA reduced the tensile



stress level in the restrained mortar rings, whereas addition of silica fume increased the stress level in the restrained mortar rings.

• The cracking susceptibility (or cracking potential) of the mixtures was assessed by computing simple ratios of the residual tensile stress in the rings and the splitting tensile strength of the mixtures. It was found that the addition of UFFA reduced the cracking potential of the mixtures while addition of SF increased the cracking potential. As a result, it was found that the specimens containing UFFA cracked at later ages than the plain specimen while specimens containing SF cracked at earlier ages.

Based on the results of this study, it appears that UFFA has a beneficial use for bridge decks because it reduces the shrinkage level and cracking potential of concrete mixtures. On the other hand, the results of this study suggest that the use of silica fume in concretes for bridge decks requires extreme care because SF increases the shrinkage cracking potential of concrete.

ACKNOWLEDGEMENTSThe research described in this paper was sponsored by Boral Material Technologies Inc. (BMTI). The financial support and the technical assistance of BMTI staffs as well as the assistance of students Satiar Shirajee and Brian Reid are gratefully acknowledged.

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14. AASHTO. Standard Practice for Estimating the Crack Tendency of Concrete. AASHTO Designation PP-34-89, 1989, pp. 179-182.

15. Ozer, B., and H. Ozkul. The Influence of Initial Water Curing on the Strength Development of Ordinary Portland and Pozzolanic Cement Concretes. Cement and Concrete Research, 2004 (in press).

16. Malhotra, V. M., and N.J. Carino. CRC Handbook on Nondestructive Testing of Concrete. CRC Press Florida, 1991, pp. 101-146.

17. van Breugel, K., and S.J. Lokhorst. Stress-Based Crack Criterion as a Basis for the Prevention of Through Cracks in Concrete Structures at Early-Ages. RILEM International Conference on Early-Age Cracking in Cementitous Systems, Editors: K., Kovler, and A. Bentur, Haifa Israel, 2001, pp. 145-158.


Clery Report 2012 - McDaniel College

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