外文翻译低热硅酸盐水泥混凝土的抗裂性能

外文翻译Anti-Crack Performance of Low-HeatPortland Cement ConcreteAbstract: The properties of low-heat Portland cement concrete(LHC) were studied in detail. The experimental results show that the LHC concrete has characteristics of a higher physical mechanical behavior, deformation and durability. Compared with moderate-heat Portland cement(MHC), the average hydration heat of LHC concrete is reduced by about 17.5%. Under same mixing proportion, the adiabatic temperature rise of LHC concrete was reduced by 2 ℃-3℃,and the limits tension of LHC concrete was increased by 10×10-6-15×10-6than that of MHC. Moreover, it is indicated that LHC concrete has a better anti-crack behavior than MHC concrete. Key words: low-heat portland cement; mass concrete; high crack resistance; moderate-heat portland cement1 IntroductionThe investigation on crack of mass concrete is a hot problem to which attention has been paid for a long time. The cracks of the concrete are formed by multi-factors, but they are mainly caused by thermal displacements in mass concrete[1-3]. So the key technology on mass concrete is how to reduce thermal displacements and enhance the crack resistance of concrete.As well known, the hydration heat of bonding materials is the main reason that results in the temperature difference between outside and inside of mass concrete[4,5]. In order to reduce the inner temperature of hydroelectric concrete, several methods have been proposed in mix proportion design. These include using moderate-heat portland cement (MHC), reducing the content of cement, and increasing the Portland cement (OPC), MHC has advantages such as low heat of hydration, high growth rate of long-term strength, etc[6,7]. So it is more reasonable to use MHC in application of mass concrete.Low-heat portland cement (LHC), namely highbelite cement is currently attracting a great deal of interest worldwide. This is largely due to its lower energy consumption and CO2 emission in manufacture than conventional Portland cements.LHC has a lot of noticeable properties, such as low heat of hydration excellent durability,etc, so the further study continues to be important[8-10]. The long-term strength of C2S can approach to or even exceed that of C3S[11]. In addition, C2S has a series of characteristics superior to C3S. These include the low content of CaO, low hydration heat, good toughness, compact hydration products, excellent resistances to chemical corrosion, little dry shrinkage, etc[12,13].For hydroelectric concrete , the design requirements have some characteristics, such as long design age, low design strength, low hydration temperature rise, and low temperature gradient[14]. All these requirements agree with the characteristics of LHC. Furthermore, LHC has a high hydration activity at later ages, the effect of which can improve the inner micro-crack. Based on above-mentioned analyses, the properties of low-heat Portland cement concrete were studied in detail in this paper. Compared with the moderate-heat Portland cement (MHC) concrete, the anti-crack behavior of LHC concrete was analyzed.2 ExperimentalMHC was produced in Gezhouba Holding Company Cement Plant, China; and LHC was produced in Hunan Shimen Special Cement Co. Ltd., China. The chemical compositions and mineral compositions of cement are listed in Table 1 and Table 2 respectively, and the physical and mechanical properties of cement are listed in Table 3.In spite of a little difference in chemical compositions, there is an obvious dissimilarity between the mineral component of LHC and that of MHC because of the different burning schedule. The C3S (Alite) content of MHC is higher than that of LHC, and the C2S (Belite) content of LHC is higher than that of MHC. Alite is formed at temperatures of about 1 450 ℃, while Belite is formed at around 1 200 ℃. Therefore, LHC can be manufactured at lower kiln temperatures than MHC. And the amount of energy theoretically required to manufacture LHC is lower than that of MHC.Belite hydrates comparatively slowly, and the early compressive strengths of pastes, mortars, and concretes containing LHC are generally lower as a result. The long-term strength and durability of concrete made from LHC can potentially exceed those of MHC. The results from Table 3 show that the early strength of LHC pastes is lower than that of MHC pastes, and that the strength growth rate of LHC is higher than that of MHC.The hydration heat of bonding materials was tested. Class I fly ash of bonding materials came from Shandong Zhouxian Power Plant, China. The experimental results shown in Table 4 indicate that the hydration heat of LHC is much lower than that of MHC. The 1-day, 3-day and 7-day hydration heat of LHC without fly ash is 143 kJ/kg, 205 kJ/kg, 227 kJ/kg, respectively. The 1-day, 3-day and 7-day hydration heat of MHC without fly ash is 179 kJ/kg, 239 kJ/kg, 278 kJ/kg, respectively. Compared with MHC, the average hydration heat of LHC concrete is reduced by about 17.5%. Obviously, low hydration is of advantage to abate the pressure to temperature control, and to reduce the crack probability due to the temperature gradients. The adiabatic temperature of LHC concrete and MHC concrete was tested. As a result, the adiabatic temperature rise of LHC concrete is lower than that of MHC concrete and the different value ranges from 2 ℃to 3 ℃in general.After adding fly ash, all specimens show a lower hydration heat, and it decreases with increasing fly ash content. For MHC with 30% fly ash, the 1 d, 3 d, 7d accumulative hydration heat is reduced by 14.5%, 20.5%, 21.9%, respectively; and for LHC with 30% fly ash, the 1 d, 3 d, 7 d accumulative hydration heat is reduced by 21.7%, 26.3%, 23.3%, respectively. Obviously, the effect of fly ash on the hydration heat of LHC is more than that of MHC. It is well known that the fly ash activation could be activated by Ca(OH)2. LHC has a lower content of C3S and a higher content of C2S than MHC, so the Ca(OH)2, namely the exciter content in hydration products of LHC pastes is lower. Decreasing the hydration activation of fly ash reduces the hydration heat of bonding materials.3 Results and DiscussionIn this experiment, ZB-1A type retarding superplasticizer and DH9 air-entraining agent were used. The dosage of ZB-1 was 0.7% by the weight of the blending, and the dosage of DH9 was adjusted to give an air-containing of 4.5% to 6.0%. The parameters that affected the dosage included the composition and the fineness of thecement used, and whether the fly ash was used. Four gradations of aggregate were used,120 mm-80 mm: 80 mm-40 mm: 40 mm-20 mm: 20 mm-5 mm=30:30:20:20.The term water-to-cementitious was used instead of water-to-cement, and the water-to-cementitious ratio was maintained at 0.50 for all the blending. The slump of concrete was maintained at about 40 mm, and the air content was maintained at about 5.0% in the experimental. After being demoulded, all the specimens were in a standard curing chamber. The mix proportion parameter of concrete is listed in Table 5.3.1 Physical and mechanical propertiesThe physical and mechanical properties include strength, elastic modulus, limits tension, and so on. The results of strength shown in Table 6 indicate the early strength (7 d curing ages) of LHC (odd samples) concrete increases slowly. The ratio between 7 d compressive strength and 28 d compressive strength of LHC concrete is about 0.4, while for MHC concrete the ratio is about 0.6. Compared with MHC concrete, the growth rate of strength of LHC concrete becomes faster after 7 d curing ages. The compressive strength for 28 d, 90 d, 180 d curing ages of LHC concrete containing 20% of fly ash is 30.2 MPa, 43.8 MPa, 48.5 MPa, respectively, while that of MHC concrete containing 20% of fly ash is 28.3 MPa, 35.6 MPa, 39.8 MPa, respectively. The content of C2S in LHC is higher than that in MHC, which results in the above-mentioned difference.Table 6 shows that the strength growth rate of concrete made with fly ash blended cements is higher than that of blank specimens; the more the dosage of fly ash, the higher the growth rate. Fly ash has a glassy nature, which can react with Ca(OH)2. Since Ca(OH)2 is a hydration product of cement, the reaction between fly ash and Ca(OH)2, called “secondary hydration”, will happen at latish ages. The magnitude of Ca(OH)2 is affected by some factors, such as the water-to-cementitious, the dosage of cement.The elastic modulus and the limits tension of concrete are given in Table 7. Under same mixing proportion, the elastic modulus of LHC concrete is approximately equal to that of MHC; the 28-day limits tension of LHC concrete is increased by 10×10-6 to 15 ×10-6 than that of MHC, and the 90-day limits tension of LHC concrete is increased by 12×10-6 than that of MHC concrete. The above results show that the use of LHC improves the limits tension of concrete. Increasing the limits tension of concrete will be benefit to the crack resistance of concrete.3.2 Deformation characteristicsDeformation characteristics of concrete include drying shrinkage, autogenous deformation, creep, etc. The drying shrinkage of concrete is shown in Fig.1. The drying shrinkage increases with age. At early ages a up to 90 days, all the LHC concrete specimens show a lower drying shrinkage; and it decreases with increasingthe fly ash content. When containing 30% of fly ash, the drying shrinkage of LHC concrete is 363 ×10-6 at 90 days, while for MHC concrete the value is 408×10-6. As a result, the volume stability of LHC concrete is better than that of MHC concrete in drying environment.Experiment results of autogenous deformation of concrete are given in Fig.2. There is an obvious difference between the development of autogenous deformation of LHC concrete and that of MHC concrete. The autogenous deformation of LHC concrete has an expansive tendency. At early ages up to 14 days, the autogenous deformation of pure LHC samples increases with age, and the 14-day value reaches a peak of 20×10-6. The autogenous deformation of pure LHC samples decreases with age at 14 days to 90 days, and the 90-day value is 10×10-6. After adding 30% of fly ash, the autogenous deformation of LHC concrete increases with age, and the 90-day value is 61×10-6. The autogenous deformation of MHC concrete has a tendency to shrink, especially without fly ash.3.3. DurabilityThe durability of concrete is evaluated by antipenetrability grade and frost-resistant level. Under the pressure of 1.2 MPa, the permeability height of pure LHC samples is 3.1 cm, while that of pure MHC samples is 2.0 cm. The test data indicate that the LHC concrete has an excellent performance in anti-penetrability, as well as MHC concrete. The permeability of concrete increases somewhat with addition of fly ash. At the end of the 250 freezing and thawing cycling, there is a little difference in both mass and resonant frequency. Both LHC concrete and MHC concrete show an excellent frost-resistant behavior. The results of this work confirmthat LHC concrete systems have an adequate anti-penetrability and frost-resistance to adapting design requirement.3.4 Analysis of crack resistanceIn order to control the crack phenomena, it is important to accurately evaluate the anti-crack behavior.As well known, concrete is a kind of typical brittle materials, and its brittleness is associated with the anti-crack behavior[15]. The brittleness is measured by the ratio of tension strength to compressive strength. With the increase of the ratio, concrete has a less brittleness, better crack resistance and toughness. It is indicated from the experiment results shown in Table 6 that the ratio of LHC concrete at all stages of hydration is higher than that of MHC concrete, which shows that LHC concrete has a better anti-crack behavior.In the crack control and design of hydroelectric mass concrete, the original evaluation of crack resistance behavior of concrete is using the utmost tensile strength which is shown in the following expression of Eq.1.σ=εP E (1)where, εP is the limits tension of concrete, and E is the elastic modulus of tension, which is assumed to be equal to the elastic modulus of compression[16].It is indicated from the calculation results shown in Table 8 that the utmost tensile strength of LHC concrete at all stages of hydration is higher than that of MHC cncrete.The research on materials crack resistance which is the basis for esign, constr uction and the choice of raw materials, has been popular in today’s world. Through a great deal of research, it is widely thought that concrete with a better crack resistance has a higher tension strength and limits tension, lower elastic odulus and adiabatic temperature rise and better volume stability[17,18].Based on above-mentioned results, the LHC concrete has a higher tension strength and limits tension, lower elastic modulus and adiabatic temperature rise, and lower drying shrinkage than MHC concrete. Compared with MHC concrete, the autogenous deformation of LHC concrete has an expansive tendency. Although the early strength of LHC concrete is lower than that of MHC concrete, its later strength has approached to or even exceed that of MHC concrete.4 Conclusionsa) The early compressive strength (7 d curing ages) of LHC is lower, but its later strength (28 d, 90 d curing ages) has approached to or even exceed that of MHC.b) Compared with MHC, the average hydration heat of LHC concrete is reduced by about 17.5%.c) Under the same mixing proportion, the elastic modulus of LHC concrete is approximately equal to that of MHC, and the limits tension of LHC concrete is increased by 10×10-6-15×10-6 than that of MHC.d) The drying shrinkage of LHC concrete is obviously smaller than that of MHC concrete, and the autogenous deformation of LHC concrete has a tendency to expand.e ) The LHC concrete has a better anti-penetrability and frost resistance, as well as the MHC concrete.f) At all stages of hydration, the anti-crack strength of LHC concrete is higher than that of MHC concrete, and the former has a higher ratio of tension strength to compressive strength.References[1] C X Yu, Z Kong. 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