Analysis technique for autogenous shrinkage in high performance concrete with mineral and chemical admixtures

Construction and Building Materials 34 (2012) 1–10

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Analysis technique for autogenous shrinkage in high performance concrete with mineral and chemical admixtures Sung Won Yoo a, Seung-Jun Kwon b,⇑, Sang Hwa Jung c a

Woosuk University, Dept. of Civil Eng., Jeonbuk 565-701, South Korea Hannam University, Dept. of Civil Eng., Daejeon 306-791, South Korea c Korea Conformity Laboratories, 1465-4 Seocho-gu, Seoul 137-707, South Korea b

a r t i c l e

i n f o

Article history: Received 1 November 2011 Received in revised form 6 February 2012 Accepted 14 February 2012 Available online 16 March 2012 Keywords: AGS (Autogenous Shrinkage) HPC (High Performance Concrete) SRA (Shrinkage Reducing Agent) EA (Expansive Agent) Setting time

a b s t r a c t This paper presents an analysis technique for AGS (autogenous shrinkage) in high performance concrete (HPC) with mineral and chemical admixtures. In this paper, comprehensive tests are performed to evaluate the behavior of AGS in HPC. Six mix proportions for evaluating the effect of FA (fly ash), SF (silica fume), and their combinations are prepared. Additional six mix proportions are also prepared for evaluation of the effect of SRA (shrinkage reducing agent), EA (expansive agent), and their combinations. Considering initial setting time and amount of admixture, analysis technique is proposed for prediction of AGS in HPC with mineral and chemical admixture, which can handle the compressive strain due to SRA and EA addition. The results from the proposed technique are compared with test results and they showed a reasonable agreement with test results for verification. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The use of HPC (high performance concrete) is gradually increasing due to the supply for demands on various performances. In order to satisfy the required performances of HPC such as low permeability and diffusivity, low w/b (water to binder) ratio with large quantity of binder is essential. In the concrete with w/b below 0.5, most of mixed water remains bound to binder, which causes reduction of specific volume of water through chemical absorption during hydration reaction. This causes change in cement paste volume. When volumetric shrinkage is restrained without additional supply of water, small pores in cement paste gradually develop, and vapor pressure and relative humidity decrease accordingly. This mechanism known as self-desiccation induces shrinkage of concrete, so called AGS (autogenous shrinkage) [1,2]. Actually strain of AGS is relatively smaller than that of drying shrinkage in concrete with normal w/b ratio, which amounts to 2040  106 year1 to 5 years. That is not critical in the design of normal concrete member. Although entire shrinkage in concrete with low w/b ratio and large amount of binder decrease compared with that in normal strength concrete, contribution of AGS to the total shrinkage becomes greater than that of drying shrinkage as illustrated in Fig. 1 [3]. Under the process of large AGS, cracks easily occur and they

⇑ Corresponding author. Tel.: +82 42 629 8020; fax: +82 42 629 8366. E-mail address: [email protected] (S.-J. Kwon). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2012.02.005

can be main route of harmful substances like chloride ion and carbon dioxide [4–6]. AGS in concrete is reported to be much dependent on the mix condition such as w/b ratio, degree of hydration, and properties of aggregates than compressive strength. For prediction of AGS behavior, Tazawa [7,8] suggested a prediction model based on experimental results considering the effects of cement (binder) type, w/b ratio, and temperature on AGS. Japan Concrete Institute (JCI) adopted and modified the model for its effectiveness [9]. Jonasson and Hedlund [10] also derived a semi-empirical model for AGS based on w/b ratio. Besides the above models, RILEM [11] and CEB-FIP [3] adopted practical models for AGS considering compressive strength at 28 days since compressive strength was closely related with general behavior of concrete. These practical models are very effective and provide reasonable results for HPC, however modification is needed for concrete with chemical admixture since strain may be changed from compression to tension when chemical admixture is added at early age. Recently, several models considering behavior in early aged concrete like porosity and pore structure have been proposed [12–14]. In this paper, AGS in HPC with mineral admixture-FA (fly ash) and SF (slica fume) and chemical admixture-SRA (shrinkage reducing agent) and EA (expansion agent) are evaluated from 13 mix proportions. For evaluation of basic properties of HPC, setting time and compressive strength (7, 28, 91, and 180 days) are measured. Setting time in HPC is affected by many parameters like mix proportions, unit water content, and type of aggregate [1]. This is also

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S.W. Yoo et al. / Construction and Building Materials 34 (2012) 1–10

test results, the relationship between AGS and setting time is investigated and analysis technique is proposed for HPC considering various admixtures. AGS in HPC and the effect of various admixtures are discussed with the previous conventional techniques in this paper. 2. Test program 2.1. Mix proportions For evaluation of AGS in HPC, total 13 mix proportions are prepared. HPC with w/b of 0.3 is planned with flow of 60 cm and air content of 4.5%. For HPC with mineral admixture, FA (15% and 30%) and SF (7.5% and 15%) and their combinations are considered. For HPC with chemical admixtures, SR (0.5% and 1.0%) and EA (5% and 10%) are prepared two Types of combinations of SR and EA are prepared as well. For the properties of HPC, several tests of fresh concrete are performed including slump, flow, air content, surface tension, and setting time. The summary of test program is listed in

Fig. 1. Evolution of AGS and drying shrinkage of concrete with time [3].

much related with drying shrinkage since absorption of water and hydration of cement occur simultaneously [1,12,13]. Based on the

Table 1 Test program. Test factor

Test level

Mix proportion items

Test items

w/b (%) Target slump flow (cm) Target air content (%) Replacement ratio of mineral admixture (%)

1 1 1 6

Replacement ratio of chemical admixture (%)

6

Fresh concrete Hardened concrete

4 3

30.0 60.0 ± 5.0 4.5 ± 1.5 FA (15.0% and 30.0%) SF (7.5% and 15%) Combinations (FA: SF) ? (10.0%: 5.0% and 20.0%: 10.0%) SRA (0.5% and 1.0%) EA (5.0% and 10.0%) Combinations (SRA:EA) ? (0.25%: 2.50% and 0.5%:5.0%) Slump, slump flow, air content, setting time Compressive strength (7, 28, 91, 180 days) AGS (0.5–49 days) Surface tension (10 min, 6 h, and 24 h)

Table 2 Mix proportion of concrete (mineral admixture). w/b (%)

30.0

S/a (%)

45.0

Target slump flow (cm)

Target air content (%)

Replacement of admixture (%)

Weight per unit volume (kg/m3)

FA

SF

W

60 ± 5

4.5 ± 1.5

0.0 15.0 30.0 0.0 0.0 10.0 20.0

0.0 0.0 0.0 7.5 15.0 5.0 10.0

175

Binder OPC

FA

SF

583 496 408 540 496 496 408

0 88 175 0 0 58 117

0 0 0 44 88 29 58

S

G

715 701 687 708 700 701 686

847 831 814 839 830 831 814

HPWRA (Binder  wt.%)

AE (Binder  wt.%)

1.9 1.8 1.7 2.2 2.5 1.9 2.1

0.042 0.055 0.065 0.028 0.045 0.038 0.042

W: water, C: cement, S: sand, G: Gravel. S/a: ratio of sand/total aggregate, OPC: ordinary portland cement, HPWRA: high performance water reducing agent, AE: air entrainer.

Table 3 Mix proportion of concrete (shrinkage control admixture; SRA and EA). w/b (%)

S/a (%)

Target slump flow (cm)

Target air content (%)

30.0

45.0

60 ± 5

4.5 ± 1.5

Content of shrinkage reducing material (%)

Weight per unit volume (kg/m3)

EA

SRA

W

C

S

G

5.0 10.0 0.0 0.0 2.5 5.0

0.0 0.0 0.5 1.0 0.25 0.50

175

554 525 580 578 567 551

714 713 715 715 714 714

846 845 847 847 847 846

EA (kg/m3)

SRA (kg/m3)

HPWRA (C  wt.%)

AE (C  wt.%)

29.2 58.3 0.0 0.0 14.6 29.2

0.0 0.0 2.9 5.8 1.5 2.9

1.9 1.9 2.1 2.2 1.9 1.9

0.055 0.065 0.040 0.045 0.028 0.045

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S.W. Yoo et al. / Construction and Building Materials 34 (2012) 1–10 Table 4 Chemical properties of cement and mineral admixtures. Item/Type

Surface area (cm2/g)

Specific gravity (g/cm3)

Ig. loss (%)

Chemical composition (%) SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

OPC FA SF

3413 3850 240,000

3.15 2.13 2.10

1.40 3.82 1.50

21.01 65.3 96.00

6.40 16.6 0.25

3.12 5.58 0.12

61.33 – 0.38

3.02 0.82 0.10

2.14 0.51 –

Ig. loss: ignition loss.

Table 5 Physical properties of aggregates. Type of aggregates

Specific weight (g/cm3)

Fineness modulus (FM)

Absorption (%)

Volumetric weight (kg/m3)

Percent passing through sieve No.200 (%)

Fine aggregate Coarse aggregate

2.67 2.63

2.60 6.87

1.83 0.63

1422 1429

1.83 0.31

content test based on KS F 2421 [16] and setting time based on KS F 2436 [17] are conducted consecutively.

Table 6 Properties of SRA. Main constituent

Density (g/cm3)

Solid ratio (%)

Shape

Color

Solubility

Glycol-type

3.18

31.0

Powder

White

Soluble powder

Table 1. Tables 2 and 3 list mix proportions with mineral and chemical admixtures, respectively. 2.2. Test materials OPC (Ordinary Portland Cement) is used for binder and river sand from Yeongi district of Chungnam Province is used for fine aggregates. Crushed aggregate with maximum size of 25 mm from Oksan Mountain is used for gravel. Table 4 presents the chemical properties of cement and mineral admixtures. Table 5 lists the physical properties of aggregate. The properties of chemical admixtures are listed in Table 6 for SRA and Table 7 for EA, respectively. AE (air entrainer) agent is used with 0.042–0.065% of cement weight for target air content of 4.5 ± 1.5%. HPWRA (high performance water reducing agent) is added with 1.7–2.2% of binder weight for target fluidity of 60 cm. Table 8 lists the properties of HPWRA and AE agent. 2.3. Test procedures 2.3.1. Outline Normal pan mixer is used for concrete mixing and slump test is performed referred to KS F 2402 [15]. After measuring slump, flow test is carried out and mean value of measurement is evaluated. Air

2.3.2. AGS test AGS test for HPC is conducted referred to the method proposed by AGS committee of the Japan Concrete Institute [18,19]. The specimen illustrated in Fig. 2 is manufactured using steel beam mold of 100  100  400 mm. Gauge plugs are embedded at the depth of 30 ± 5 mm from each side so that the shrinkage strain can be monitored. Teflon sheet of 1 mm thickness is placed on the bottom of the mold for free movement of the specimen without restraint. Then the specimen is covered with polyester film on the top surface to avoid evaporation and absorption of outside moisture. Test setup for AGS measurement is shown in Fig. 2. Hydration heat in concrete accelerates cement hydration process and this can influence the behavior of AGS. Temperature variation (2–3 °C) measured at the center of the specimens is considered for calibration of lateral strain through thermal coefficient (1.0  106°C-1). Consequently, the effect of hydration heat on AGS is evaluated to be negligible. After removal of steel frame, concrete specimens with polyester film are exposed to control condition with temperature of 20 ± 2 °C and RH (relative humidity) of 65 ± 5%, then sealed hermetically with aluminum adhesive tape. Mean values measured from end gauges in three specimens are used for the results of AGS. The ratios of weight loss are smaller than 0.05% – JCI criteria [9] so that they are not considered in AGS evaluation. The ratios of weight loss during AGS measurement are plotted in Fig. 3. 2.3.3. Measurement of surface tension Surface tension in HPC with chemical admixtures is measured through Du Nouy tensiometer. In this test, aggregates are removed

Table 7 Properties of EA. Specific surface (cm2/g)

Density (g/cm3)

Ig. loss (%)

3100

2.93

1.4

Chemical composition (%) CaO

SiO2

Fe2O3

SO3

Al2O3

f-CaO

70.2

0.4

4.7

17.3

2.8

51

Table 8 Properties of HPWRA and AE agent. Type

Constituent

External appearance

State

Specific weight (20 °C)

High performance water reducing agent AE agent

Naphthalene Lignin sulfonate

Dark brown Colorless

Liquid Liquid

1.05 ± 0.02 1.02

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S.W. Yoo et al. / Construction and Building Materials 34 (2012) 1–10

Fig. 2. AGS specimen and test device.

from the mix in Table 3 and surface tension is monitored for 1 day. The extracted liquid from the cement paste solution though filtering is used. The device and the concept of surface tension measurement are shown in Fig. 4. The extracted liquid is put in the thin dish where platinum ring is set. The ring is gradually lifted and the liquid surface is also lifted along to the ring. When the liquid column reaches highest point, the membrane breaks. In this condition, applied force on the ring can be assumed as the summation of surface tension acting on the inner/outer radius of the ring and the weight of the lifted liquid columns (Q,Q0 ). Since the thickness of the ring is so thin that the weight of the liquid columns can be negligible, the surface tension (T) is obtained as equation.

T¼

W 2p ðr1 þ r2 Þ

ð1Þ

where, T is surface tension, W is pulling force to downward, r1 and r2 are radius of outer and inner ring as shown in Fig. 4. 3. Evaluation of AGS and the related test results 3.1. Setting time and compressive strength in HPC

(a) HPC with mineral admixture

(b) HPC with chemical admixture Fig. 3. Ratio of mass loss in HPC specimens.

As previously described, setting time is affected by water content. Since AGS is also initiated by pore pressure due to free water in pore [20], their relationship can be evaluated. Table 9 summarizes the characteristics of unhardened HPC. Setting time in HPC with FA is delayed and the reason is reported to be the release of SO2 present at the surface of FA 3 [1]. SF contributes to the progress of hydration of the amorphous silica in SF and this arises from the extreme fineness of SF particles which provides nucleation sites for calcium hydroxide. This also accelerates hydration process and accordingly reduces setting time [21]. HPC with SRA and EA shows reduced setting time but the effect of dosage amount is small. The changes in setting time in HPC are plotted in Fig. 5. Typical strength development with FA and SF is shown in Fig. 6. In HPC with FA, strength at the early age (28 days) is slightly lower than control case (OPC concrete), however strength results at 91 days are higher than those in OPC concrete through pozzolanic reaction [1]. Strength in SF HPC rapidly increases. The contribution of SF is reported to be improvement in packing action for interfacial zone of aggregate [22]. The effect of chemical admixtures (SRA and EA) on strength development seems to be small.

Fig. 4. Device and concept of surface tension measuring.

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S.W. Yoo et al. / Construction and Building Materials 34 (2012) 1–10 Table 9 Characteristics of unhardened concrete. Items/types

Slump (cm)

Slump flow (cm)

Air (%)

OPC FA15 FA30 SF7.5 SF15 FA10:SF5 FA20:SF10 SRA 0.5 SRA 1.0 EA 5 EA 10 SRA 0.25: EA 2.5 SRA 0.5: EA 5.0

26.0 26.3 27.0 26.3 25.3 25.7 25.6 26.0 25.4 26.3 26.5 27.0 26.0

62.5 64.5 69.0 60.0 56.5 60.5 58.5 62.0 60.5 65.5 68.0 68.0 65.5

4.5 4.0 4.3 3.7 3.5 5.4 3.7 5.0 5.0 4.6 4.7 4.5 4.2

Setting time Initial (hr:min)

Final (hr:min)

11:00 12:30 15:42 10:13 9:22 10:11 13:25 9:31 9:16 10:30 9:23 9:32 9:08

12:27 15:13 18:30 12:53 13:00 13:00 15:21 11:30 11:23 12:48 12:08 11:53 11:33

20 initial setting time final setting time

setting time (hour)

16

12

8

4

0% 5. A

2. 5% 0. A

0. SR

SR

A

+E

A +E % 25

20 FA

FA

5%

% 10

0%

5%

EA

EA

A

1.

5% 0. SR

SR

+S %

% 10

A

F1

0%

5%

%

F

15

+S

5% 7. SF

SF

% 30

%

FA

15 FA

O

PC

0

Fig. 5. Initial and final setting time in various HPC.

compressive strength (Mpa)

80 7 dyas

28 dyas

91 days

70 60 50 40

0% A

2. +E

SR A

0.

5%

+E

A

5.

5%

% 10 EA

SR A

0.

25 %

+S

F1 SR 0% A 0. 5% SR A 1. 0% EA 5%

5% 20 % FA

FA

10

%

+S

F

15 % SF

5% SF

7.

% 30 FA

% 15 FA

O

PC

30

Fig. 6. Compressive strengths in various HPC.

3.2. AGS evaluation 3.2.1. AGS in HPC with mineral admixtures The effects of the type and replacement ratio of mineral admixture on AGS are shown in Fig. 7. It is observed that AGS strain in

OPC concrete at 49 days is about 350  106 (mm/mm). The ASG in HPC with FA decreases compared with that in OPC concrete and it decreases continuously with larger FA replacement. The behavior of AGS in HPC with FA seems to be related with the hydration process and the delay of the initial hydration. Since the

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S.W. Yoo et al. / Construction and Building Materials 34 (2012) 1–10 Table 10 Measurement of surface tension. Mix design

OPC SR0.5 SR1.0 EA5 EA10 SR0.25:EA2.5 SR0.5:EA5

Surface tension (dyne/cm) 10 min

6h

24 h

66.7 55.3 47.2 67.1 68.2 62.1 56.8

69.3 53.1 45.2 69.0 69.6 59.3 53.5

71.5 51.6 43.2 71.2 71.7 58.6 52.4

Fig. 7. Variations of AGS in HPC with mineral admixtures.

Fig. 9. Surface tension in HPC with various admixtures.

Fig. 8. AGS in HPC with SRA and EA.

relative humidity in concrete with FA remains quasi-constant even if hydration proceeds, self-desiccation does not practically occur, and subsequently reduces AGS shrinkage. Another reason for decrease in AGS is the delay of the hydration reaction, which is related with poor occurrence of self-desiccation. As shown in Fig. 7, FA can be considered as admixture contributing to the reduction of AGS, however it also brings reduction of initial strength and acceleration of the carbonation [23,24]. AGS in HPC with SF increases compared to that in OPC concrete. This can be explained by the increase in capillary tension due to the development of micro pores inside the hardened cement paste and the consumption of the crystal lattices of calcium hydroxide (Ca(OH)2). This impedes the shrinkage deformation due to pozzolanic reaction [25]. AGS in HPC with combined FA and SF seems to be dependent on the amount of FA and SF. AGS in HPC containing FA 10% and SF 20% shows slightly larger AGS than that containing FA 5% and SF 10%. Use of both admixtures in adequate amount can provide decrease in AGS as well as improvement of the strength. For HPC with SF, appropriate use of SRA and EA can be helpful for the control of AGS at early age. The results in HPC with various mineral admixtures are presented in Fig. 7. 3.2.2. AGS in HPC with chemical admixtures SRA can reduce surface tension so that capillary stress can be linearly reduced based on the capillary pressure [26]. This also contributes to reduction of drying shrinkage. Another effect of SRA on reducing shrinkage is increased surface area of hydrating cement system [27]. According to the previous research, a morphology change occurs in cement pastes with SRA, and some long prismatic needles are present on the material surface [27]. These are though

to be crystals of ettingite or hydrated lime [27]. EA agent reacting with water slowly produces an expansion through the formation of the corresponding hydroxide. The transformation from calcium oxide to calcium hydroxide causes major increase in solid volume about 90% [28]. In spite of little change of surface tension, expansion mechanism in HPC due to EA addition can be explained as delayed calcium hydroxide formation. For use of both SRA and EA, synergetic interactions are previously studied [27]. Massive formation of elongated crystals causes initial expansion of cement system. SRA, organic hydrophobic molecule can reduce the water permittivity and this also lowers water solubility, which induces high expansion of cement mortar [27,28]. In Fig. 8, the behaviors of AGS in HPC with chemical admixtures are shown. As shown in Fig. 8, use of SRA is very effective for reducing AGS strain. Compared to control case (without chemical admixtures), SRA 0.5% addition reduces 18% of AGS and SRA 1.0% does 34% of AGS. The effect of EA appears more effective. 10% of EA use enables to reduce AGS strain of 68% (110  106) compared to control case. Consistent with previous research [27], synergistic interaction can be found in this test. AGS strain in HPC with SR 0.5% and EA 5.0% is dramatically reduced to 68  106 at 49 days, which presents 80% reduction compared to the control case. 3.3. Surface tension For capillary stress which causes drying shrinkage can be assumed as Eq. (2) [26,29].

P¼

2l r

ð2Þ

where, P is capillary pressure, l is surface tension, r is radius of meniscus in pore channel. If surface tension decreases, capillary pressure and related shrinkage are reduced. Table 10 summarizes the results of surface tension test in Section 2.3.3. It is observed that

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S.W. Yoo et al. / Construction and Building Materials 34 (2012) 1–10 Table 11 Summary of conventional techniques for AGS. Researcher

Technique

Parameter h   i 5 0:3  exp  t1

Jonasson and Hedlund [10,19]

eas ðtÞ ¼ ½0:65 þ 1:3ðW=BÞ  10

Tazawa [7,8]

ec(t) = c  ec0(W/B)  b(t) ec0 ðW=BÞ ¼ 3070 exp½7:2ðW=BÞð0:2  W=B  0:5Þ

3

ec0 ðW=BÞ ¼ 80 ð0:5 < W=BÞ b(t) = 1 - exp [ - a(t - t0)b]

RILEM [11]

For t < 28 days,

eas ðt; fc28 Þ ¼ 0

ðtÞ : ffcc28 < 0:1

  2:2f c ðtÞ eas ðt; fc28 Þ ¼ ðfc28  20Þ  0:2  106 : fc28 fc ðtÞ  0:1 fc28 For t P 28 days, t  Þ  106 eas ðt; fc28 Þ ¼ ðfc28  20Þð2:8  1:1 exp 96 CEB-FIP [3]

ecas(t) = ecas0(fcm)  bas()t,  fcm 2:5 ecas0 ðfcm Þ ¼ aas 6þfcm0fcm  106 f  cm0    0:5 bas ðtÞ ¼ 1  exp 0:2 tt1

eas(t): AGS t: age (day), W/B: w/b ratio. ec(t): AGS c: influence coefficient of the type of cement and admixture (1.0 for OPC) ec0(t)(W/B): limit-value of the AGS strain W/B: w/b ratio, b(t): function of AGS a, b:experimental constants t0: initial setting time (day) eas (t, f28): AGS strain from the setting time to t days, fc28: compressive strength at 28 days,

fc(t): compressive strength at t days, if unknown, fc(t) = {t/(1.40 + 0.95t)}f28 fcm is the mean compressive strength (MPa), fcm0 is 10 MPa,

aas is a coefficient dependent on the type of cement (700 for OPC), t1is 1 day.

Fig. 10. Comparison of previous techniques for AGS.

Fig. 11. Comparison with constant c and assumed constant.

OPC and EA concrete show little changes with elapsed time, but the cases of SRA and SRA plus EA show significant decrease in surface tension with time, which is consistent with the previous researches [27]. The results are shown in Fig. 9.

results. For the effectiveness in this paper, Tazawa’s model is employed and modified for considering effect of various admixtures. 4.2. Modification of the previous technique for AGS

4. Proposal of analysis technique for AGS in HPC with admixtures 4.1. Conventional techniques In this section, previous techniques for AGS in concrete are studied. These models have semi-empirical formulations with major parameters such as w/b and compressive strength. For selection of the most appropriate technique, the conventional techniques listed in Table 11 are compared with the experimental results. In Fig. 10, AGS from previous prediction models are plotted and compared with the experimental results of OPC concrete. It is shown that the models proposed by RILEM [11] and CEB-FIP [3] provide similar AGS behavior but show considerable small values compared to the experimental results. On the other hand, the results from the Tazawa model [7,8] are in a good agreement with test

4.2.1. HPC with mineral admixture Based on the test results, the previous technique [7,8] is modified through regression analysis. The experimental constants a and b in Table 11 are fixed as 0.375 and 0.5, respectively. The amount of shrinkage is governed by c constant. With fixing constants a and b, c is obtained through regression analysis for best fitting curve. The initial setting time is closely related with shrinkage, so that the relationship between constant c and setting time ratio to OPC concrete is assumed as Eq. (3)

c ¼ ðT set =T OPC Þn

ð3Þ

where, Tset is initial setting time of HPC with mineral admixture, TOPC is initial setting in HPC with OPC, n is assumed as 0.75. In Fig. 11, The comparison with obtained c from regression analysis and results from Eq. (3) is presented.

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S.W. Yoo et al. / Construction and Building Materials 34 (2012) 1–10

As shown in Fig. 11, assumed results from Eq. (3) show reasonable estimation. Considering Eq. (3), the modified Tazawa’s technique can be written as Eq. (4).

ec ðtÞ ¼ ðT set =T OPC Þ0:75  3070 exp½7:2ðW=BÞ n h io  1  exp 0:375ðt  t 0 Þ0:5

ð4Þ

The comparison of AGS between test and the proposed technique is shown in Fig. 12, which shows a reasonable agreement. 4.2.2. HPC with chemical admixtures (SRA plus EA) In HPC with chemical admixtures, complicated behavior can be measured including compressive strains since concrete with EA has self swelling due to delayed formation of Ca(OH)2 [20,21]. The technique of Tazawa cannot be directly applied to the compressive strain field. For considering compressive strains, Eq. (5) is proposed.

(a) HPC with FA

ec ðtÞ ¼ c  ec0 ðW=BÞ  bðtÞ þ

l ð lnnmx Þ2 e nx

ð5Þ

where, c is a parameter depending on the type of cement and admixture. Values of l, m, and n are the parameters for the amplitude, center, and width of the curve. The additional second term enables to handle the compressive strain with the amount of EA. Through multi-regression analysis, experiment constants (c, l, m, and n) are obtained for best fitting of test results. c is set to consider the initial setting time ratio (Tset/TOPC) like Eq. (4). l, m and n are assumed as function of SRA and EA amount. Through regression analysis for AGS in SRA and EA concrete, functions for l, m, and n are proposed, which reasonably satisfies the simulation of AGS behavior. In Table 12, the constants for proposed technique are listed with regression analysis results. The following comparisons for AGS in Fig. 13 show that the proposed technique reasonably predicts AGS in HPC with chemical admixtures admixture.

(b) HPC with SF

4.3. Verification of the proposed technique

(c) HPC with FA and SF Fig. 12. Comparison with test results and AGS from proposed technique.

For the verification of the proposed technique, five types of mix proportions are prepared. For mineral admixtures, replacement of FA 20% and SF 10% are considered. For SRA and EA, 0.75% and 7.5% additions are prepared. Their combination is considered as 0.375% and 3.75%. Based on the test procedure in Section 2.3, setting time and AGS are measured. Table 13 lists the mix proportions for HPC and Table 14 shows the setting time for five mix proportions. Through Eqs. (4) and (5), AGS are evaluated and they are compared with test results. The comparisons are presented in Fig. 14, which shows a good agreement with test results and the proposed technique.

Table 12 Functions for experimental constants (c, l, m and n). Type of binder

c

l

m

n

OPC SRA 0.5% SRA 1.0% Regression analysis

1 0.83 0.65 (c - 0.65) = 0.352  [1 - exp { - 31.058(x - 0.842)}] x = (Tset/TOPC)R2 = 0.99 0.62 0.30 (c - 1) = 5.077(x - 1) x = (Tset/TOPC), R2 = 0.90 0.47 0.20 (c - 1) = 4.899(x - 1) x = (Tset/TOPC) R2 = 0.98

0 0 0 0

0 0 0 0

0 0 0 0

1301.67 2103.56 l = 220.35  EA R2 = 0.90 561.07 1435.03 l = 93.445 + 429.254SRA + 209.138EA R2 = 0.89

6.63 8.53 m = 3.693EA0.364 R2 = 0.98 3.40 6.66 m = 2.435 + 0.978SRA + 0.655EA R2 = 0.92

2.09 2.33 n = 1.624EA0.157 R2 = 0.99 1.39 1.72 n = 0.176 + 0.344EA R2 = 0.88

EA 5.0% EA 10.0% Regression analysis SRA 0.25% + EA 2.5% SRA 0.5% + EA 5.0% Regression analysis

9

S.W. Yoo et al. / Construction and Building Materials 34 (2012) 1–10 Table 14 Setting for 5 mix proportions. Type of admixtures (%)

Setting time Initial (hr:min)

Ratio to OPC initial time

FA 20% SF 10% EA 7.5% SRA 0.75% EA 3.75% + SRA 0.375%

13:52 9:47 10:04 9:25 9:37

1.26 0.89 0.92 0.86 0.87

(a) AGS in HPC with SRA

(a) AGS in HPC with mineral admixtures

(b) AGS in HPC with EA

(b) AGS in HPC with chemical admixtures Fig. 14. Verification of the proposed technique.

5. Conclusions AGS strains in HPC have various behavior and they are dependent on the mineral/chemical admixtures. In HPC with chemical admixtures, AGS is significantly reduced and compressive strains are measured in HPC with EA at early age. In this paper, AGS in HPC with mineral (FA and SF) and chemical (SRA and EA) admix-

(c) AGS in HPC with SRA and EA Fig. 13. Comparison of ASG with test and predicted results.

Table 13 Mix proportion for verification. w/b (%)

S/a (%)

Target slump flow (cm)

Target air content (%)

Replacement of admixture (%)

Weight per unit volume (kg/m3)

FA

W

60 ± 5

4.5 ± 1.5

20.0 0.0 0.0 10.0 Addition of agent (%)

467 117 0 694 525 0 58 704 Weight per unit volume (kg/m3)

823 835

EA 7.50 0.00 3.75

W 175

G 846 847 847

SF

B OPC

30.0

45.0

SRA 0.00 0.75 0.375

S FA

EA 44 0 22

AE (B  wt.%)

1.75 2.38 SP (B  wt.%)

0.06 0.038 AE (B  wt.%)

1.90 2.15 1.90

0.006 0.043 0.037

SF

175

C 540 579 559

SP (B  wt.%) G

SR 0.0 4.4 2.2

S 713 418 714

10

S.W. Yoo et al. / Construction and Building Materials 34 (2012) 1–10

tures are measured including various tests like strength, surface tension, and setting time. Through modifying the previous technique with experimental parameters, simple technique which can consider various effects of mineral and chemical admixtures on AGS is proposed. The conclusions on this study are as follows. 1) For AGS in HPC with mineral admixtures, the most appropriate conventional technique was selected through comparison with test results. Based on the test results, experimental parameters for the selected prediction technique were obtained through regression analysis. The ratio of setting time to OPC concrete was observed to have very close relationship with AGS, so that it was employed as the parameter to control the magnitude of AGS. For HPC with chemical admixtures, new term was introduced for handling the compressive strain which was caused by EA addition. Several parameters considering the amount of chemical admixtures were introduced for reasonable prediction of AGS. 2) SRA was very effective for reducing AGS. AGS decreased by 18% in the case of SRA 0.5% and by 34% in the case of SRA 1.0% compared to control case. The addition of EA was evaluated to be more effective. 10% of EA enabled to reduce 68% of AGS. AGS in HPC with SR 0.5% and EA 5.0% was dramatically reduced to 68  106 (l strain) at 49 days, which showed 80% reduction to control case.

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