Construction and Building Materials 24 (2010) 1855–1860

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Construction and Building Materials

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The relationship between autogenous shrinkage and pore structure of cementpaste with mineral admixtures

Yue Li a,*, Junling Bao a, Yilin Guo b

a Key Lab Urban Security and Disaster Engineering, MOE, Beijing University of Technology, Beijing 100124, PR Chinab Research Institute of Highway, Ministry of Communication, Beijing 100088, PR China

a r t i c l e i n f o

Article history:Received 7 November 2009Received in revised form 24 March 2010Accepted 1 April 2010Available online 22 April 2010

Keywords:Mineral admixturesCement pasteAutogenous shrinkagePore structure

0950-0618/$ - see front matter Crown Copyright � 2doi:10.1016/j.conbuildmat.2010.04.018

* Corresponding author. Tel.: +86 13381307466.E-mail addresses: liyue@bjut.edu.cn, liuyueguang1

a b s t r a c t

In this paper, the combination of fly ash and silica fume, or fly ash and blast furnace slag were used as thecomposite mineral admixtures in cement paste. The autogenous shrinkage and the pore structure of thehardened cement paste with mineral admixtures were tested, and the relationship of the autogenousshrinkage and pore structure also was discussed. The results indicate that fly ash can reduce the autog-enous shrinkage, and silica fume can increase the autogenous shrinkage, and the effect of blast furnaceslag is between the two above; although both silica fume and blast furnace slag can weaken the porosityand the mean diameter of cement paste, and increase the volumetric percentage of pores whose diameteris between 5 and 50 nm and pore specific surface, silica fume is better than blast furnace slag in changingthe pore structure. The relationship between the autogenous shrinkage and volumetric percentage ofpores whose diameter is between 5 and 50 nm is obviously proportional.

Crown Copyright � 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction

As the concrete sixth component, the mineral admixtures havebeen widely used. But the engineering application of mineraladmixture shows that, some kinds of admixtures will lead to an in-crease of the concrete shrinkage and the restrained concrete isprone to have greater self-tensile stress and easy to crack [1,2]. Itwas found that the autogenous shrinkage in concrete with lowwater–cement ratio is the major factor to generate larger shrinkageand early cracking [3]. Autogenous shrinkage is a phenomenon inwhich cementitious materials shrink at a constant temperaturewithout any change in weight, i.e., the macroscopic volume changeoccurring with little moisture transferred to the exterior environ-ment [4]. Autogenous shrinkage is affected by several factors, suchas water-binder ratio, mineral admixture type and adding dosage,etc. Tazawa [4,5] found that the autogenous shrinkage would in-crease with the water–cement ratio decreased. Fly ash (FA) couldeffectively reduce the autogenous shrinkage of concrete if the FAdosage was large [6]. The concrete with blast furnace slag (BFS)or silica fume (SF) exhibited greater autogenous shrinkage than or-dinary concrete, and higher percentage of substitution leaded to alarger increase in autogenous shrinkage at later ages [4,7,8].

As the most important component in concrete microstructure,pore structure will strongly influence the macroscopic propertiesof cement concrete. The test results of Linhua and Yugang [9]

010 Published by Elsevier Ltd. All r

63@163.com (Y. Li).

showed that: FA could effectively improve pore structure of hard-ened cement paste, the big pore volume reduced and the small porevolume increased with FA increasing. Kdurekovic [10] reported thatthe majority of pore in the cement paste with SF was smaller than15 nm even after hydration 1 day, and an increase of median poresize was observed in some pastes with SF after 250 days of watercuring. Malami et al. [11] revealed that natural pozzolan, or FA, orBFS did not affect the porosity of cement paste when less than15% was used. However, a significant increase in the porosity wasobserved in the cement paste when the additions of these admix-tures were higher than 15%. Li and Chen [12] studied influence ofground mineral admixtures on pore structure of cement paste withsuperplasticizer. The results showed that pore structure of cementpaste was worse at early ages by singly and compositely adding anykind of mineral admixtures. At later ages, both BFS and steel slag(SS) obviously reduced porosity of cement paste and both BFS andFA obviously reduced median pore diameter and improved porediameter size distribution. At later ages, sequence of decreasingporosity capacity was BFS > SS > FA, while sequence of reducingmedian pore diameter and improving pore size distribution capac-ity was BFS > FA > SS. Pore structure of cement paste at later agescould be improved by compositely adding mineral admixtures.

It is well known that the mineral admixture will change thepore structure, meanwhile, the change of micro-pore structure willlead to the macro volume change of cement paste, and the autog-enous shrinkage is one of the macro volume changes. Therefore,the pore structure and the autogenous shrinkage should benaturally closely linked. In fact, the autogenous shrinkage micro-

ights reserved.

Fig. 1. The layout of measuring equipment of composite cement paste autogenousshrinkage (1) seal membrane (2) cement paste specimen (3) gauge head (4) dialindicator (5) magnetic support (6) smooth plate.

1856 Y. Li et al. / Construction and Building Materials 24 (2010) 1855–1860

scopic model [13] also quantitatively demonstrated that the autog-enous shrinkage of hardening cement paste is caused by the capil-lary force due to the liquid phase in capillary consumption with thehydration. Since the capillary is a kind of important micro-pore, itis necessary to research the relationship between the autogenousshrinkage and the pore structure. However, the quantitative rela-tionship is not adequately studied up to now. Therefore, this papercarried out the above research.

2. Experimental

2.1. Raw materials and testing methods

The P.I 42.5 Portland cement was used, and the 3 and 28 days compressivestrengths were 32.3 MPa and 59.1 MPa, respectively. Superplasticizer was thenaphthalene water reducer with the water reducing ratio 26% and the addition dos-age was 1% of cementitious material mass. The chemical and physical properties ofcement and mineral admixtures are given in Table 1.

The measuring device of autogenous shrinkage was shown in Fig. 1. The dimen-sion of specimens for autogenous shrinkage measuring is 40 mm � 40 mm �160 mm. The preparation steps of testing specimen were shown as follows: first,1 mm thick layer of polytetrafluoroethylene (PTFE) bedding layer was putted inthe mold bottom, and a couple of probes were installed at the ends of mold, andthen placed a layer of 0.1 mm thick polyester film in the mold. According the Chi-nese standard GB175-2007, the cement paste was fully mixed and cast in mode andvibrated on the vibration table. At last, the surfaces of specimens were covered bythe 0.1 mm thick polyester film to prevent evaporation.

Measurement method was as follows: the dial gauge was used to be measuringinstrument. Each end surface of the mold has one hole. The gauge body was embed-ded in the cement paste and the head penetrated the hole to contact with the dialindicator. The autogenous shrinkage measure started from the initial setting time to24 h with the specimens in the mold. After 24 h, the specimens were remolded andthe dial gauges were re-adjusted. During the whole measuring process, the speci-mens were sealed by plastic film. The whole measuring period is 28 days fromthe initial setting time and the length changes were recorded continuously within 28 days. The specimens were always cured at (20 ± 2) �C and relative humidity(90 ± 5)%.

Mercury intrusion porosimetry (MIP) was used to test the pore structure ofharden specimens. The mercury porosimeter (Auto Pore IV 9510) is capable of gen-erating 0–414 MPa pressure and measuring 3 nm–360 lm pore diameter. The spec-imens for MIP test were broken into 3–5 mm pieces and stored in ethanol solutionfor carbonation prohibition and hydration termination. Before MIP test, the speci-mens were dried by putting them into an over at 60 �C for 24 h and stored in sealedcontainers.

2.2. Mix design

The mix design of testing specimens was divided into A, B groups according tothe different combination of mineral admixtures. In group A, SF and FA wereblended with at different proportions; in group B, BFS and FA were blended. The de-tailed mix design was shown in Table 2.

3. Results and discussion

3.1. The results and discussion of autogenous shrinkage

The autogenous shrinkage of group A cement paste with SF andFA is presented in Fig. 2. From the slope of the curves in Fig. 2, itcan be concluded that autogenous shrinkage occurs mainly in the14 hydration days, which can reach about 80% of the total autoge-nous shrinkage at 28 hydration days, i.e., autogenous shrinkage in-creases fast at earlier ages while slowly at later ages. Thisphenomena is agree with Tazawa’s research result that the 14 days

Table 1Chemical and physical properties of raw materials.

Materials CaO (%) SiO2 (%) Al2O3 (%) Fe2O3 (%) MgO

C 62.6 21.3 4.67 3.31 3.05FA 4.77 54.88 26.89 6.49 1.31SF 1.72 92 0.78 0.79 2.71BFS 34.54 28.15 16 1.1 6

C: cement, FA: fly ash, SF: silica fume, BFS: ground blast furnace slag, D: density, BS: Bl

autogenous shrinkage will be about 67% of that in 500 days if thew/c ratio of cement paste is as low as 0.3 [14]. With the same min-eral admixture mass, the larger the mass ratio of SF/FA is, the fasterthe autogenous shrinkage at earlier ages will be, and the larger thetotal autogenous shrinkage is. It can be concluded from the resultsthat SF and FA can affect the performance of autogenous shrinkage.If the mass ratio of SF/FA is less, combined mineral admixtures willshow the FA’s limiting effect on autogenous shrinkage [6]. But ifthe SF/FA ratio is increased to a certain degree, combined mineraladmixtures show the SF’s enhancement effect on autogenousshrinkage [4,5,8].

Fig. 3 gives the autogenous shrinkage results of group B withBFS and FA. The autogenous shrinkage trend of group B is similarto group A, about 85% of autogenous shrinkage is occurred in theearly 14 days. When the mass ratio of the BFS/FA is small, theFA’s limiting effect on autogenous shrinkage is obvious [6]. Whenthe value is greater than 1, autogenous shrinkage is significantlyincreased. On the other hand, autogenous shrinkage of cementpaste with higher replacement ratio of BFS is increased comparedwith that of ordinary cement paste. This result is in agreementwith those reported by previous researches [7,15,16].

Compared with A3 and B4, the effect of SF and BFS on autoge-nous shrinkage can be made clearly. The mass of FA in A3 and B4is same. The cement and SF dosages in A3 are 162 kg/m3 and163 kg/m3 more than that in B4, respectively. However, the BFSdosage in B4 is 326 kg/m3 more than that in A3. The sum of162 kg/m3 (cement) and 163 kg/m3 (SF) is almost equal to326 kg/m3 (BFS). Previous researches [14] showed that the autog-enous shrinkage would increase with higher replacement ratio ofBFS, which is to say the impact of BFS on the autogenous shrinkageis greater than that of cement. In this case, the effect of cement onshrinkage is supposed to be same as that of BSF (in fact it is smal-ler). We deduct the BFS 162 kg/m3 that is equal to the cement fromBFS 326 kg/m3, and then get the surplus BFS 164 kg/m3 that is al-most equal to SF 163 kg/m3. Under the same dosages of SF and sur-plus BFS, the autogenous shrinkage of A3 is much greater than thatof B4, so the influence of SF on autogenous shrinkage is greaterthan that of BFS.

The main component of FA is aluminum silicate glass cheno-sphere, its smooth surface and smaller surface area lead to poorabsorption of water, these series of physical characteristics resultin different degrees of ‘‘water-reducing” effect. Earlier findingshave shown that the internal humidity of concrete with more addi-tion of FA decline in a sealed curing conditions, indicating FA re-

(%) SO3 (%) R2O (%) Cl (%) D (kg/m3) BS (m2/kg)

2.11 0.75 0.007 3200 3811.16 1.93 0.001 2600 4541.16 – 0.018 2200 222050.32 0.91 0.005 2900 416

aine surface, R2O: K2O + Na2O.

Table 2Mix design of composite cement paste.

Material dosage of unit volume /(kg/m3) Mix mass parameters

Samples C W FA SF BFS S W/B SF/FA BFS/FA

A1 1139 488 407 81 0 17 0.3 0.2 0A2 1139 488 361 127 0 17 0.3 0.35 0A3 1139 488 326 163 0 17 0.3 0.5 0A4 1139 488 279 209 0 17 0.3 0.75 0A5 1139 488 244 244 0 17 0.3 1.00 0A6 1139 488 217 271 0 17 0.3 1.25 0B1 977 488 562 0 90 20 0.3 0 0.16B2 977 488 488 0 163 20 0.3 0 0.33B3 977 488 393 0 259 20 0.3 0 0.66B4 977 488 326 0 326 20 0.3 0 1B5 977 488 260 0 391 20 0.3 0 1.5B6 977 488 163 0 488 20 0.3 0 3

*C: cement, W: water, FA: fly ash, SF: silica fume, BFS: ground blast furnace slag, S: Superplasticizer, W/B: water/blender.

Fig. 2. Autogenous shrinkage of samples with SF and FA at 28 days.

Fig. 3. Autogenous shrinkage of samples with BFS and FA at 28 days.

Y. Li et al. / Construction and Building Materials 24 (2010) 1855–1860 1857

duce cement hydration, thus weakening autogenous shrinkage[17]. The activity and hydration level of BFS are greater than thatof FA, accelerate water consumption rate and internal drying pro-cess in the sealed conditions, so that the critical radius of the cap-illary reduce rapidly and capillary force grow fast, thereby

increasing self-desiccation and autogenous shrinkage caused bythe self-drying cementitious system. SF is a kind of spherical ul-tra-fine powder with higher mineral activity and larger surfacearea. Compared with FA and BFS, pozzolanic activity of SF is largest,leading to increase the self-desiccation and autogenous shrinkagerapidly. From autogenous shrinkage point of view, SF dosageshould be controlled to avoid concrete cracking due to the exces-sive shrinkage.

3.2. Prediction equations for autogenous shrinkage

In order to predict how the autogenous shrinkage changesdepending on the hydration time, the prediction equations ofautogenous shrinkage should be established, which also is the fo-cus and hot point in the autogenous shrinkage research field atpresent. At present, the main equations for autogenous shrinkagepredication in concrete were shown as follows [18]

easðtÞ ¼ c � e28 ðw=cmÞ � bðtÞ ð1Þ

where,

e28 ¼ 2080 � exp½�7:4 ðw=cmÞ� ð2Þ

bðtÞ ¼ exp a 1� 28� t1500

t � t1500

� �b" #( )

ð3Þ

where, easðtÞ is autogenous shrinkage strain (�10�6), e28 is autoge-nous shrinkage strain at 28 days (�10�6), c is coefficient to describeto the effect of BFS, t1500 is time at which the ultrasonic pulse veloc-ity reaches 1500 m/s (day), a, b are constants that depend on thereplacement level of BFS, t is age of concrete (day).

However, Eqs. (1)–(3) are not suitable for the autogenousshrinkage prediction of cement paste due to the following reasons:firstly, it is proposed for the concrete instead of the cement pastewithout aggregate, a large error will occur even if the equationswere amended. Secondly, c and a, b are only depended on the effectand replacement ratio of BFS, not including other mineral mix-tures. At last, easðtÞ and t1500 are necessary to get the calculation re-sults, however, they are complicated to be measured.

In this paper, the early autogenous shrinkage prediction equa-tion of cement paste with mineral admixture was established byadopting the polynomial form, which can be expressed as:

esðtÞ ¼ at3 þ bt3 þ ct ð4Þ

where, esðtÞ is the autogenous shrinkage of cement paste when thetest time is t, t is age of cement paste (day), a, b, c are the constantsof experiment for adjusting the prediction accuracy. Based on theEq. (4), the autogenous shrinkage regression equations of A, B

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groups are shown in Table 3. The correlation coefficients of eachequation are more than 0.93, therefore, the level of agreement be-tween the theoretical prediction values and the experimental datais higher, which shows that the prediction Eq. (4) is able to betterreflect the autogenous shrinkage trends of cement paste with min-eral admixtures.

3.3. The results and discussion of pore structure

From Table 4, it is clear that the specific surface and volumetricpercentage of pores whose diameter is between 5 and 50 nm areincreased with SF increasing at the ages of 14 days and 28 days,respectively. But the porosity and mean diameter have a reducingtrend. Ref. [10] reported that the majority of pore in the cementpaste with SF was 15 nm smaller than that of ordinary cementpaste without SF even after hydration 1 day, which is also ap-proved by our results in Table 4. Group B has the same trend withgroup A on the specific surface and 5 and 50 nm volumetric per-centage. At the same time, it can be seen that BFS can reduceporosity and median pore diameter, which is also agree with thereference [12]. The mean diameter and porosity at 28 days are low-er than that of 14 days, since the continual hydration of cementpaste and more hydration products fill in the pores. In addition,the larger pores will be divided into smaller pores which lead tothe increment of 5 and 50 nm volumetric percentages.

Different mineral mixtures show different effect on the changeof pore structure. The reasons can be attributed to their differentphysical and hydration activities. SF and BFS are the mineral finepowder with high activity and specific surface, their grain diame-ters are smaller than that of FA and cement, so that they can fillin the gaps that are formed by the cement and FA grains accumu-late at first, and then they hydrate fast to generate hydration prod-ucts which can fill in the pores and split the original large pore into

Table 3The fitted polynomials of autogenous shrinkage with time of composite cement paste.

Samples Fitted polynomials Correlation coefficients: R2

A1 es (t) = 0.039 t3 � 2.299 t2 + 50.62 t 0.982A2 es (t) = 0.043 t3 � 2.563 t2 + 53.40 t 0.973A3 es (t) = 0.046 t3 � 2.810 t2 + 57.34 t 0.967A4 es (t) = 0.056 t3 � 3.190 t2 + 63.59 t 0.969A5 es (t) = 0.066 t3 � 3.664 t2 + 70.20 t 0.954A6 es (t) = 0.071 t3 � 4.108 t2 + 75.90 t 0.936B1 es (t) = 0.018 t3 � 1.036 t2 + 23.59 t 0.978B2 es (t) = 0.021 t3 � 1.217 t2 + 25.94 t 0.978B3 es (t) = 0.018 t3 � 1.177 t2 + 26.22 t 0.973B4 es (t) = 0.025 t3 � 1.215 t2 + 30.39 t 0.989B5 es (t) = 0.020 t3 � 1.237 t2 + 37.20 t 0.988B6 es (t) = 0.038 t3 � 2.110 t2 + 48.25 t 0.979

Table 4Pore parameters of group A and B at the ages of 14 days and 28 days.

Sample SS (m2/g) MD (nm)

14 days 28 days 14 days 28 day

A1 4.3 3.6 63.3 58.2A2 5.2 4.4 61.9 56.9A3 7.4 6.3 59.4 54.7A4 10.1 8.6 56.8 52.2A5 14.0 11.8 49.7 45.6A6 14.9 12.6 40.0 36.8B1 2.9 2.5 114.2 105.0B2 3.0 2.6 113.9 104.5B3 3.6 3.1 112.3 103.3B4 4.2 3.5 109.0 100.2B5 4.8 4.1 104.6 96.2B6 5.5 4.6 98.3 90.5

*SS: specific surface, MD: mean diameter, P: porosity; VP: volumetric percentage of por

many smaller pores, and each pore is not connected. Therefore, thelarger pore volume is reduced and the density and pore structure isimproved.

3.4. The relationship between autogenous shrinkage and porestructure

Based on the research results of the autogenous shrinkage andthe pore structure of cement pastes with mineral admixtures, itindicates that the effect of BFS and SF on autogenous shrinkage issimilar to that on pore structure. With SF and BFS content in-creases, the autogenous shrinkage has an increasing trend, andporosity and mean diameter have a downward trend, while thepore specific surface is an upward trend. The volumetric percent-age of pores whose diameter is between 5 and 50 nm of hardenedcement paste are significant positive correlation with autogenousshrinkage.

Capillary theory put forward that the self-desiccation duringthe hydration will lead to the change of liquid surface tension,which is the main reason of autogenous shrinkage development.With the water surface dropping within the capillary, the curva-ture of curved surface becomes larger, resulting in the larger sur-face tension. Therefore, the curvature-surface radius distributionof the pore represents the pore size distribution of hardened ce-ment paste. The material’s pore size distribution can be combinedto analyze the autogenous shrinkage. Because 5–50 nm pores arethe main area where capillary stress is easy to occur and developfast with the self-desiccation, the pore is divided into two catego-ries: 5–50 nm and 50–100 nm according to the diameter. Here, wefocus on the relationship between the percentage of 5–50 nmpores to the total pore volume and the autogenous shrinkage ofthe cement paste. Based on the autogenous shrinkage and the porestructure test results, a numerical regression formula with loga-rithmic function format is established to indicate the relationshipbetween the autogenous shrinkage and volumetric percentage ofpores whose diameter is between 5 and 50 nm at hydration14 days and 28 days. The formula is shown in Eq. (5) as following

lnðes � aÞ ¼ bpþ c ð5Þ

where, es is the autogenous shrinkage at the ages of 14 days and28 days (10�6), p is volumetric percentage of pores whose diameteris between 5 and 50 nm (%), a, b, c are the constants of experimentfor adjusting the prediction accuracy. The relationship between theautogenous shrinkage and 5–50 nm pores percentage is describedby the formula (5). The pore structure dates in Table 4 and theautogenous shrinkage dates in Figs. 2 and 3 are put into the Eq.(5). The Eqs. (6)–(9) are got and their corresponding numericalregression curves are shown in the Figs. 4–7

P (%) VP (%)

s 14 days 28 days 14 days 28 days

15.5 12.0 56.6 65.614.9 11.7 62.1 70.514.4 11.2 65.8 74.814.1 11.0 68.0 76.413.8 10.8 70.5 78.213.5 10.7 72.9 79.715.0 11.8 45.3 50.915.0 11.7 46.6 52.314.8 11.6 47.5 53.114.6 11.5 49.1 55.214.5 11.4 50.0 56.114.4 11.3 52.22 58.6

es whose diameter is between 5 and 50 nm.

Fig. 4. Regression curve of group A at the age of 14 days.

Fig. 5. Regression curve of group B at the age of 14 days.

Fig. 6. Regression curve of group A at the age of 28 days.

Fig. 7. Regression curve of group B at the age of 28 days.

Y. Li et al. / Construction and Building Materials 24 (2010) 1855–1860 1859

lnðes�330Þ¼0:825 p�0:022 Correlation coefficient : R2¼0:933ð6Þ

lnðes�150Þ¼0:782 pþ0:817 Correlation coefficient : R2¼0:972ð7Þ

lnðes�410Þ¼0:715 pþ0:491 Correlation coefficient : R2¼0:958ð8Þ

lnðes�190Þ¼0:830 pþ0:735 Correlation coefficient : R2¼0:971ð9Þ

Above results in Figs. 2 and 3 and Table 4 show that the largerdosage of SF and BFS will cause the 5 and 50 nm volumetric per-centages to increase, at the same time, the autogenous shrinkagealso is increased. This illustrate that the macro volume change(autogenous shrinkage) is the appearance of the change on mi-cro-pore structure, and Figs. 4–7 precisely reflect this relationship.There is a well linear correlation between the natural logarithm ofautogenous shrinkage and 5 and 50 nm volumetric percentage athydration14 days, which also is very well at 28 days. The correla-tion coefficient R2 of Eqs. (6)–(9) were greater than 0.93, and thecalculated values and the actual measured values match each othervery well. Therefore, a conclusion can be drew that the Eq. (5) canreflect the relationship between autogenous shrinkage and porestructure of different hardened cement paste with different min-eral mixtures. The Eq. (5) also proves that autogenous shrinkageis mainly controlled by the 5 and 50 nm volumetric percentage,which quantitatively demonstrate the relationship between autog-enous shrinkage and pore structure of cement paste.

4. Conclusions

(1) FA, SF and BFS have different influences on the autogenousshrinkage of cement paste with mineral admixtures. FA willweaken the autogenous shrinkage, and the SF will increasethe autogenous shrinkage, and the effect of BFS is betweenthe two mentioned above. When the mass ratios of SF/FAor BFS/FA are constant, the autogenous shrinkage dependingon the hydration time can be synthesized to the form ofpolynomials.

(2) Both SF and BFS can refine the micro-pore structure ofcement-based materials, i.e., weaken the porosity and thepore mean diameter, and increase specific surface and the

1860 Y. Li et al. / Construction and Building Materials 24 (2010) 1855–1860

volumetric percentage of pores whose diameter is between5 and 50 nm. Compared with the BFS, the enhancing effectof SF is more obvious.

(3) The 5–50 nm pore volumetric percentage is one of the mainfactors influencing the autogenous shrinkage. The larger thevolumetric percentage is, the more obvious the capillaryeffect will be, and so is the larger autogenous shrinkage.There is significant positive correlation between the per-centage of 5–50 nm pores and the natural logarithm ofautogenous shrinkage, and a numerical regression formulawith logarithmic function format was established todescribe the relationship. The results show that the calcu-lated values and the actual measured values match eachother very well.

Acknowledgements

The authors would like to acknowledge the financial supportprovided by China 973 project ‘‘the basic research of environ-ment-friendly in modern concrete (No. 2009CB23201)”, and theexcellent talent project of Beijing (No. 2009A005015000006), andBeijing Natural Science Foundation (No. 8100001).

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