SHRINKAGE AND CRACKING BEHAVIOR OF HIGH PERFORMANCE CONCRETE CONTAINING A MGO-CAO COMPOSED EXPANSIVE AGENT

Transcription

1 SHRINKAGE AND CRACKING BEHAVIOR OF HIGH PERFORMANCE CONCRETE CONTAINING A MGO-CAO COMPOSED EXPANSIVE AGENT Changwen Miao (1), Qian Tian (1), Jiaping Liu (1), Fei Guo (1), Shouzhi Zhang (1) and Yujiang Wang (1) (1) Jiangsu Academy of Building Science Co., Ltd, Nanjing, China. Abstract This paper gives a comprehensive investigation on the effects of a new type of MgO-CaO composite expansive agent (MC) on the shrinkage and associated cracking behaviour of hardened high performance concrete, including length change under saturated condition, sealed condition and drying condition, and cracking under single-ring restraint, dual-ring restraint and deformation-controlled Temperature Stress Testing Machine restraint. Experimental results showed that the addition of 8% MC not only built effective expansion in HPC whatever under saturated condition or under sealed condition, but also substantially reduced its drying shrinkage. The measured shrinkage reduction was up to 65% at the age of 12d of drying. It was also found that the addition of MC could effectively improve the shrinkage cracking resistance of HPC even under drying condition. The experimental results of Temperature Stress Testing Machine showed that the expansion of MC could build effective pre-compression and partially compensate the tensile stress during the course of temperature descending, which lowered the cracking temperature of HPC more than 1 C. 1. Introduction The formation of expanding hydration in Portland cement concretes can be responsible for both deleterious and beneficial phenomena. To build moderate expansion in concrete within a safe rage has been proved to be an effective way to improve the shrinkage cracking problems of concrete. Ettringite expanding agent has been widely used in Chinese engineering to produce shrinkage compensating concrete. The swelling of the gel state expansive ingredients by water adsorption and the crystallization pressure of the ettringite growth can build effective expansion at the early age of hydration and compensate the shrinkage of concrete[1]. However, previous study found that the expansion effectiveness of ettringite expanding agent depends on the free access of outside water and presence of lime in the pore water[2], which cannot be fully ensured in the modern high performance concretes due to their very low permeability with low water to binder ratio, and little lime after hydration with high incorporation of mineral admixtures such as slag or

2 fly ash. It has been found that, under the condition where curing is difficult to be handled, the ettringite expansive agent may increase the danger of shrinkage cracking[3]. Moreover, ettringite has been found to be unstable around 8 C and may induce DEF in mass structures[4-5]. In order to improve the shrinkage cracking resistance of high performance concrete, a new type of MgO- CaO composite expansive agent(mc) has been newly developed in Jiangsu Bote New Materials Co. Ltd. It is mainly composed by three kinds of single expansion components namely highactive MgO, which is calcined at C and has a relatively higher hydration speed, lowactive MgO, whcih is calcined at C and has a slower hydration speed, and calcined CaO component calcined at C [6]. In this paper, the effects of this new type of expansive agent on the shrinkage and associated cracking behaviour of hardened high performance concrete, including length change under saturated condition, sealed condition and drying condition, and cracking under single-ring restraint, dual-ring restraint and uniaxial restrained Temperature Stress Testing Machine restraint was comprehensively investigated. 2. Materials and methods 2.1 Materials Binders: Type II Portland cement with 52.5 grade from Nanjing Jiangnan cement plant was used. Class I fly ash specified came from Nanjing Thermo Electrical Plant. A ground granulated blast furnace slag(sl) with Blaine fineness of 439 m 2 /kg was used here. The chemical and physical properties of the binders are shown in Table 1. Table 1: chemical composition and physical properties of the cementitious materials Chemical composition (weight-%) Specific Name density (symbol) (kg/m 3 area SiO 2 Al 2 O 3 CaO MgO Fe 2 O 3 SO 3 LOI ) (m 2 /kg) Cement(C ) Fly ash (Fa) / Slag(GGBS) Superplasticizer: Poly-naphthalene sulfonates superplasticizer(jm-b) was used. It came from Jiangsu Bote New Materials Co., ltd. Coarse Aggregate: 5-25mm continuous graded crushed basalt. Its density was 27 kg/m 3. Fine Aggregate: River sand with fineness modulus of 2.65 and density of 265 kg/m 3. Expansive agents: The expansive agent used is a new type of MgO-CaO expansive agent (named MC), which was newly developed in Jiangsu Bote New Materials Co.Ltd. It is a composite expansive agent mainly composed by three kinds of single expansion component namely high-active MgO, low-active MgO and calcined CaO[6]. The main chemical composition of MC was given in Table 2.

3 Table 2: chemical composition and physical properties of MC expansive agents Chemical composition (weight-%) Blaine fineness Al (m 2 2 O 3 CaO SO 3 MgO Fe 2 O 3 LOI Al 2 O 3 /kg) Mix proportions Table 3: Mixture proportion design of the experiments Name (symbol) Water to binder ratio Concrete mix proportion(kg/m 3 ) C Fa GGBS Sand Coarse aggregate water JM-B MC C45ref / C45MC C35ref / C35MC Cref / CMC The mix proportions of the concretes are shown in table 3. Six kinds of mixtures were designed. All of the mixtures incorporated high volume of mineral admixtures and superplasticizers. The slump of the fresh concrete mixtures was maintained in the range of 16-2cm. 2.3 Main testing methods Deformation behaviours Prism specimens were used for deformation behavior experiments. The size of the specimens was mm. All the concrete specimens were demoulded 24h±2h after casting. Then the specimens were divided into three batches and stored in three kinds of curing schemes respectively to examine the effects of MC on the length change of concrete. The first batch were saturated cured in 2±1 C water to measure the expansion at different ages; the second batch were sealed with a thin PVC film inside and a self-adhesive foil outside to measure autogenous length change at different stages; the third batch were first saturated cured in a standard curing chamber(98%rh and 2±2 C) for 2d and then put into a drying chamber(6%±5%rh and 2±2 C) to measure drying shrinkage at different ages Shrinkage cracking behaviours Three kinds of cracking experiments were applied to examine the effects of MC on the shrinkage cracking behaviours of concrete. The first was single ring test referenced to AASHTO suggested method[7], as denoted in Fig.1. The whole setup was made up by the inner steel ring with a diameter of 25mm and the outer mould with a diameter of 3mm. The net height of the mould is 15mm. The Mortar was sieved out from the fresh concrete with a 5mm sieve and was cast into the interspace Fig.1 Single-ring test setup between the inner ring and the outer mould. The outer mould

4 was removed 24h later. The rest was firstly cured in a standard curing chamber(98%rh and 2±2 C) for 2d and then put into a CO 2 -free drying chamber(6%±5%rh and 2±2 C) to measure the time of initial cracking and the crack width. The second was the dual-ring test, which was referenced to the newly developed set-up in Pur due university [8], as denoted in Figure 2. The inner ring had a inner radius (R1) of 52mm and outer radius(r2) of 6mm.The outer ring had a inner radius (R3) of 85mm and outer radius(r 4) of 95mm. Both rings had net heights of 25mm. Outer ring inner ring (a) size of the dual-ring (b) a picture of the real setup Fig. 2 Experimental setup of the dual-ring[8] Both the inner ring and the outer ring had four strain gages placed 9º from one another at the mid-height on the inner circumference. Each strain gage used, had a measuring resolution of ±.1 µm/m. The instrumented rings were placed at the centre of a non-absorptive base which was lined with plastic sheets. Fresh cement paste of the mixtures in Table3 was then cast between the two rings and vibrated for 3 seconds. After casting, all the specimens were put into a drying chamber (T=2 C±2 C, RH=6%±3%) directly to provide a severe curing condition of practice. The pressure that was exerted on the inner ring(p1) and the outer ring(p2) by the paste could be calculated using Equation 1: 2 2 R2 R1 P1 I* ES 2R (1) R4 R3 P2 O* ES 2 2R3 Where Es was the elastic modulus of the ring and ε I and ε o was the strain measured on the inner ring and the outer ring, respectively. The circumferential stress (σc) that develops in the cement paste ring specimen on the location of r could be calculated by equation (2): 2 2 R3 2 1 R C r 2 P1 r 2 P2 R R (2) 2 2 R3 R3 The third was the uniaxial restrained Temperature Stress Testing Machine[9-1], as denoted in Figure 3. The whole setup was closed-loop computer-controlled and under the deformation-controlled scheme, which could provide a restraint degree up to nearly 1%.

5 The size of the section of the specimen was 15mm 15mm 15mm. The fresh concrete mixture was directly cast into the mould of the TSTM and vibrated. A temperature sensor with resolution of.1 Cwas inserted into a preembedded cooper pipe in the middle of the specimen. Then put a plastic sheet on the concrete surface and add the upper cover. At Fig.3 TSTM test setup room temperature of 2±2 C and sealed condition, the concrete specimen hydrates and its temperature increases to the peak value due to cement hydration. Then keep the maximum temperature constant for 48h. After that, decrease the temperature of the specimen at a speed of 1 C/h by the function of the cycling media. A free specimen with the same mixture was cast and measured under the exact temperature history of the restrained specimen simultaneously to measure its free length change. 3. Results and discussion 3.1 Influence of MC on strength Table 4 presents effects of MC on the compressive strength and splitting tensile strength of concrete. Table 4: Effects of MC on the compressive strength and splitting tensile strength of concrete. C45ref C45MC C35ref C35MC Compressive strength/mpa Splitting tensile strength/mpa 7d d d d Table 4 shows a decrease of compressive when replacing 8% of the cementitious materials with equal mass of MC expansive agents. The compressive reduction is in the range of 1%. The negative influence of MC on compressive strength indicates a less cementitious property of the hydration products of MC than the normal hydration products of cement materials. Also, the formation of the expansive agent may also produce a coarser pore structure. However, compared with compressive strength, the splitting strength of the concrete is less influenced by the incorporation of MC. 3.2 Effects of MC on the deformation behavior of concrete The deformation behaviors of the concrete specimens under the three kinds of curing conditions are shown in Fig.4.

6 l engt h change/ C45r ef C45MC C35r ef C35MC l engt h change/ C45ref C45MC C35ref C35MC (a) expansion under saturated condition (b) autogenous deformation under sealed condition l engt h change/ C45ref C45MC C35ref C35MC (c) drying shrinkage under drying condition Fig. 4 Effect of MC on the length change of concrete under different curing conditions Figure 4(a) gives the expansion development of the two kinds of hardened concretes with and without MC in 2 C water. Both C45 and C35 reference specimens show apparent expansion cured in saturated condition. However, the expansion of C35 specimen was observed much lower than that of C45 concrete. There are several mechanisms corresponding to the swelling of hydrating cement concrete immersed in water. The hydration of cement reacted with the penetrated water from the outside is along with the absolute volume increase of solid, which causes the apparent swelling of concrete[1]. The continuous hydration products grows into the original pores and leads to a more and more densified pore structure, which precedes the further access of water outside. The growth of some crystalline hydration products such as ettringite and calcium hydroxide could also build internal expansion pressure. However, only a little of such kinds of products could be formed for the reference concrete. During the hardening process of cement, the expansion pressure is confined by the stiffness of the growing solid skeleton of the concrete. Both the stiffness and the density of the skeleton of the higher strength concrete develop faster than that of the lower strength concrete, which confines the expansion of concrete to some extent. Therefore, the observed expansion of the reference concrete is higher for C45 than for C35. When incorporated with MC expansive agent, both C45 and C35 concrete show substantial expansion comparing with reference

7 concrete cured in saturated condition. The expansion develops fast at the beginning of immersion and becomes gradually steady at later age. The ultimate expansion due to the function of MC at 8% dosage is around 2 microns at the end of measurement. The expansion results from the hydration products of MC, i.e. magnesium hydroxide and calcium hydroxide. Figure 4(b) shows the autogenous deformation of the two kinds of hardened concretes with and without MC. Under sealed condition, both C45 and C35 reference specimens show apparent autogenous shrinkage. However, a little amount of expansion can be observed at the initial age of hydration for C45 concrete. At the early age, the autogenous shrinkage of C35 concrete is much higher than that of C45 concrete due to its lower water to binder ratio. The incorporation of MC effectively builds substantial early age expansion under sealed condition and eliminates the autogenous shrinkage of both kinds of concretes. For both kinds of concrete, the expansion of the concrete specimens develops fast at early age and reaches a maximum around 5d-1d. After that a slighter trend of decline can be observed on the curves of development. However, this kind of decline trend becomes gradually steady in the long run, approximately 6d-12d later. At the end of testing age, no shrinkage was observed for both kinds of concrete, there is still substantial residual expansion in C45 concrete. Figure 4(c) illustrates the drying shrinkage of the two kinds of hardened concretes with and without MC. Under drying condition, i.e. under the condition of 6% RH and 2 C, whatever with and without MC, both C45 and C35 specimens show apparent drying shrinkage. However, the incorporation of MC expansive agent effectively reduces the drying shrinkage of both kinds of concrete. Especially for C35 concrete, at the dosage of 8%, the addition of the new kinds of expansive agents can reduce the drying shrinkage by nearly 2/3 at the end of testing age. For the drying shrinkage testing, all the specimens were put into the drying chamber after two days of fogging curing at standard curing chamber, i.e. RH is maintained above 98% and temperature is 2±2 C. Once faced to drying atmosphere, the internal water in the capillary network of the concrete will gradually transport into the surroundings under the driven force of RH difference. At the initial age of drying, no much difference was observed between the shrinkage development of the concretes with and without MC expansive agents. With the further progressing of drying, the difference increases and the effect of MC become more and more obvious. It is still unclear why this new kinds of MC expansive agents can effectively reduce the drying shrinkage of high performance concrete. The CaO component is majorly speculated to be responsible for this effect due to its less sensitivity to RH change. Further study is being conducted to explain the mechanism of the drying shrinkage inhibition of this new kind of expansive agent. Experimental results give a very promising foreground of this new kind of expansive agent in controlling the early-age volume stability as well as the long-term volume stability for high performance concrete. At a dosage of 8%, the introduction of MC expansive agent could build effective and moderate expansion in high performance concrete under saturated condition. While under sealed condition, the introduction of 8% MC can also build effective early-age expansion even for the high performance concrete with low water to binder ratio. Even under drying condition, the introduction of MC can effectively reduce the drying shrinkage of high performance concrete to a substantial extent. Such kinds of expanding effects give a

8 promising higher volume stability and lower cracking risk of HPC in the engineering, especially for those projects with poor curing conditions. 3.3 Effects of MC on the shrinkage cracking behavior of HPC Single-ring test Table 5: Effects of MC on the drying shrinkage cracking resistance of HPC matrix (Singlering test) Initial cracking time Initial crack width/mm C45ref 4d 21h.1 C45Mc 11d 3h.5 C35ref 4d 18h.3 C35Mc 7d 16h.1 Table 5 gives the experimental results of single-ring cracking test. Once the casted ring specimen is exposed to the drying condition, water in the internal capillary network will be gradually lost from the matrix and lead to drying shrinkage. Due to the restraint of the inner steel ring, tensile stress induces in the outer cement matrix and develops with the progressing of drying. When the induced tensile stress is higher than the tensile strength of the matrix, crack occurs in the matrix. The value of initial cracking time as well as the value of crack width represents the relative cracking resistance between different mixtures. Table 5 shows the difference of cracking resistance between the HPC matrix with and without MC expansive agent. It can be obviously concluded that the incorporation of MC effectively improves the cracking resistance of the HPC matrix for the both kinds of concrete. The initial cracking time of C35, i.e. 4d and 18h after exposure, is a little earlier than that of C45(4d 21h), which indicates a higher cracking sensitivity of high strength concrete. The incorporation of 8% MC delays the initial cracking time by 6d more and reduces the crack width by 5% for C45 matrix. For C35 matrix with 8% MC, the initial cracking time was also delayed by 6% and the crack width was only 1/3 of the reference mixture. These experimental results coincide well with the drying shrinkage results in Figure 3. The cracking resistance of the matrix was effectively improved due to a much lower drying shrinkage with the incorporation of this new kind of expansive agent Dual ring test The experimental results of the dual-ring test are given in Fig.5 and Fig.6. Fig. 5 gives the strain development of both the internal ring and the outer ring of the two kinds of concrete with and without MC expansive agent. Under the drying condition, both C45 and C35 reference specimens without MC shrink as expected, which induces compression upon the internal ring. So compression strain (minus as denoted in the figures) develops in the internal ring since the beginning of drying for the reference specimen. Because there is no expansion in the matrix of the reference concrete under the drying condition, the recorded strain in the outer ring maintains zero through the experiment. For both C45 and C35 concrete, tensile stress develops since the beginning of drying. The compression strain in the internal ring develops with the progressing of drying and the tensile stress increases continuously. Once the tensile stress exceeds the tensile strength of the matrix, cracks appear in the exposed face, which corresponds to the sudden break in the strain development curves. As observed in the

9 single ring test, the matrix of C35 concrete cracks earlier than that of C45 concrete. The matrix of C45 concrete cracks at 5.2 d and the corresponding calculated tensile stress is 2.2 MPa. While C35 matrix cracks at 4.5d and the corresponding calculated tensile stress is 3.3MPa. When exposed to the same drying condition, the tensile stress in high strength concrete develops faster than that of low strength concrete, which can partly explain the relative higher cracking sensitivity of high strength concrete. When incorporated with MC expansive agent, tensile strain was observed firstly in the outer ring while a minor compression strain was observed in the inner ring simultaneously. l engt h change/ out er r i ng internal ring l engt h change/ out er r i ng internal ring -4-4 (a) C45ref (b) C45MC l engt h change/ out er r i ng -2 internal ring out er r i ng internal ring -2 (c)c35ref (d) C35MC Fig. 5 Strain developments of both the inner ring and the outer ring of the specimen l engt h change/ C45r ef 2 C35r ef st r ess/ MPa C45MC st r ess/ MPa C35MC -4 (a) C45ref and C45MC (b) C35ref and C35MC Fig. 6 Effect of MC on the maximum stress development in the matrix calculated by Equation(1) and Equation(2) according to the measurement results of strain development. -4

10 According to equation (1), equation(2) and the tested strain value in Fig.5, the calculated stress is expansion compression value, as shown in Fig.6. After 1-2 days of development the expansion strain reaches a maximum and then begins to drop, accordingly the compression decrease. Further drying consumes the early-age built expansion gradually. After a certain age (around 5-6d), the expansion is consumed out and the compression stress drops to zero. After that the strain of the outer ring maintains constant and the inner ring begins to be under compression. The matrix is under tension due to drying accordingly. Once the tensile stress exceeds the tensile strength of the matrix, cracks appears. However, the cracking time of the matrix is substantially postponed with the incorporation with MC expansive agent. For C45 concrete, the cracking time of the matrix with MC delays by 5.5d than the reference specimen and is more than two times of the later. While for C35 concrete, no crack appears till the end of the experiment in the matrix with MC. At the end of the test, the tested tensile stress is about 1.2MPa, which is much less than the cracking stress in the reference matrix, even less than the residual stress. The stress develops very slowly and the stress increasing rate is only.3mpa/d at the end. It can be predicted that the stress develops very slow after that and the cracking risk of C35 matrix is very low. The testing results illustrate an even better effect of cracking resistance improvement of MC in high strength concrete under drying condition. These experimental results further confirm that this new kind of expansive agent can partly hydrate and build expansion in the matrix of concrete even under drying condition. Moreover, the effect of cracking resistance improvement of MC can be better displayed in dual-ring test because the outer ring can provide the restraint for the expansion and store part of the expansion energy. This part of expansion energy can be firstly consumed and compensate part of the shrinkage stress, which benefits the cracking resistance of concrete and gives a more reasonable way of modeling for the shrinkage-compensating concrete in practice Temperature stress testing The experimental results of the temperature-stress Testing Machine are illustrated in Table 6 and Figure 7. From Fig. 7 and Table 6 it can be seen that under the designed temperature history, the temperature development of the concrete with MC is similar to that of the reference concrete and their maximum is also very close at the stage of temperature rise. Under the free condition, after setting, the reference concrete expands due to the temperature rise and reaches a maximum expansion of The incorporation of MC introduces obviously additional expansion within the concrete due to the formation of the expanding products and increases the maximum expansion by 49%. During the stage of constant temperature, the reference concrete began to shrink due to the autogenous shrinkage resulted from the chemical shrinkage of further cement hydration. While the concrete incorporated with MC keep steady expansion in the function of the expanding agents. The expansion produced during the course of temperature rise and constant temperature can provide stress reserve to compensate part of the shrinkage during the course of temperature drop. Once the temperature drops, both the reference concrete and the concrete with MC begin to shrink. The deformation of the reference concrete specimen changes gradually from compressive strain to tensile strain. However, there is still residual expansion in the concrete specimen with MC even at the end of the temperature drop. The test has to be ended due to the limitation of operating temperature of the machine.

11 t emper at ur e/ 4 CMC 3 t i me / h Cr e f -1-2 st r ess/ MPa 1 CMC time/h Cr e f -3 l engt h change/ CMC time/h Cr ef Fig. 7 experimental results of the TSTM l engt h change/ CMC 2 1 time/h Cr e f Table 6: Compare of the major experimental parameters parameters unit CMC Cref Maximum compression stress MPa Time of Maximum compression stress h Maximum expansion(restrained) Maximum expansion(free) Time of Maximum expansion h Maximum temperature C Maximum temperature increase C The 2 nd -stress temperature C Time of 2 nd -stress temperature h Cracking stress MPa > Cracking time h > Cracking temperature C < Under the nearly fully restrained condition of TSTM, after setting, a minor compression stress develops in the reference concrete due to the temperature rise of cement hydration. A maximum compression stress is up to.29 MPa. With the addition of MC, the compression stress increases due to the additional expansion of expanding agent. The maximum compression stress is up to.78mpa, which is 2.7 times that of the reference concrete and provide additional compression stress reserve. During the stage of constant temperature, the

12 compression stress of the reference concrete begins to decrease due to the autogenous shrinkage of the concrete and almost consume out before the start of temperature drop. While the concrete with MC still have compression stress reserve at the end of constant temperature stage. Once the temperature began to drop, tensile stress soon arises in the reference specimen and develops very fast due to thermal contraction. When the tensile stress reaches a maximum of 2. MPa and exceeds the tensile strength of the reference concrete, crack appears. The recorded temperature at this moment is -7.9 C. While for the concrete incorporated with MC, the initial temperature drop need to consume the residual compression stress and delay the time of 2 nd -stress temperature (i.e. the moment when the stress changes from compression into tension) by 38h. After that, the tensile stress develops also in the concrete specimen with MC and increases with the further drop of the temperature. It should be noticed that when the tensile stress exceeds the cracking stress of the reference specimen, no crack arises in the concrete specimen with MC. Even at the end of the experiment, when the temperature is C and the tensile stress is up to 3.1 MPa, there is still no crack in the concrete specimen with MC. These experimental results indicated that the incorporation of MC expansive agent not only builds substantial expansion in HPC and effectively compensates part of the tension due to autogenous shrinkage and thermal contraction and, but also effectively increases the tensile strength of HPC. Thus the cracking resistance of the HPC can be improved to a large extent. Although the most important representative index - cracking temperature of the concrete with MC was not measured due to the operating limitation of the TSTM in this experiment, it could be deduced that the cracking temperature was more than 1 C. The experimental results are enough to prove the good efficiency of this new kind of expansive agent in improving the cracking resistance of HPC of autogenous shrinkage and thermal contraction as compared with the published results in the literatures [11]. 4. Conclusions The effect of the new type of MgO-CaO composite expansive agent(mc) on the shrinkage and associated cracking behaviour of hardened high performance concrete was systematically investigated with different kinds of curing and restraint conditions. Experimental results showed that at a dosage of 8%: The incorporation of MC not only built effective expansion in HPC whatever under saturated condition or under sealed condition, but also substantially reduced its drying shrinkage. Under drying condition, the cracking resistance of the matrix could be effectively improved. If the expansion was restrained, this kind of improving effect could be strengthened because expansion built in the matrix of HPC even under drying condition. Under the uniaxial restrained Temperature Stress Testing Machine, the incorporation of MC not only built substantial expansion in HPC and effectively compensated part of the tension during the temperature descending stage, but also effectively increased the tensile strength of HPC. Both of the effects benefit the cracking resistance of HPC for thermal contraction and autogenous shrinkage.

13 Experimental results indicated a promising foreground of this new kind of expansive agent in controlling the early-age volume stability as well as the long-term volume stability for high performance concrete, especially for those projects with poor curing conditions. Acknowledgement The study of this work is financially supported by National Basic Research Program of China(973 program) (Grant No.29CB6232), National Natural Science Foundation of China Grant No and Grant No References [1] Nagataki S.,Gomi H., 'Expansive Admixtures (Mainly Ettringite)'. Cem. Concr. Res., 2(1998) [2] Mehta P.K., 'Mechanism of expansion associated with ettringite formation'. Cem. Concr. Res., 3(1973) 1-6. [3] Zhou Q.,Lachowski E.E.,Glasser E.P.,'a Decomposition Product of Ettringite'. Cem. Concr. Res.,34(24) [4] Barbarulo R., Peycelona H., Prene S., Marchandc J., 'Delayed ettringite formation symptoms on mortars induced by high temperature due to cement heat of hydration or late thermal cycle'. Cem. Concr. Res., 35 (25) [5] Diamond S., 'Delayed Ettringite Formation Processes and Problems'. Cem. Concr. Res., 18(1996) [6] Qian T., 'Development and application of a MgO based composite expansive agent in high performance concrete'. Confidential Technical Report(29)(in Chinese), Jiangsu Bote New Materials Co., Ltd, Nanjing, China. [7] AASHTO Designation: PP (25), 'Estimating the Cracking Tendency of Concrete', American Association of State Highway and Transportation Officials.United States. [8] Sant, G., Rajabipour, F., Lura, P., and Weiss, W. J., 'examining Residual Stress Development' In 'Cementitious Materials Experiencing An Early-Age Expansion? ' ACI Special publication, Warsaw Poland [9] Springenschmid, R., Breitenbucher R. and Mangold M., Development of the cracking frame and the temperature-stress testing machine, in Thermal Cracking in Concrete at Early Ages, Proc. RILEM Symp., E&FN SPON, (1994) [1] Kovler K., Testing system for determining the mechanical behavior of early age concrete under restrained and free uniaxial shrinkage, Mater. Struct. 27 (17) (1994) [11] Burrows R. W. 'The visible and invisible cracking of concrete', ACI Monograph(1998), No. 11, American Concrete Institute, Farmington Hills, Michigan 78

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