早期养护方式对混凝土力学性能和耐久性的影响
摘要
合理充分的养护可以显著提高混凝土耐久性,特别是混凝土的早期养护更是决定混凝土后期性能的关键。在实际施工过程中,混凝土养护并未受到足够重视,并且现有的养护研究缺少对混凝土终凝硬化前以及实际施工恶劣环境下的养护研究。针对现有研究的不足,本论文把养护时间和养护环境结合起来,养护时间固定为硬化拆模前的短暂养护,养护方式模拟与实际工程相近的恶劣环境,包括风吹养护、高温养护、风吹灯照养护等与室内环境下的养护进行对比,研究了早期不同养护方式对混凝土力学性能及耐久性等的影响,同时为养护规范的更新提供有益参考。
研究表明,对水泥砂浆而言,硬化拆模前的不同养护对砂浆28d的抗压强度影响显著。拆模前养护对掺合料砂浆影响显著,拆模前覆膜养护的掺10%粉煤灰砂浆28d抗压强度能达到42MPa以上,略大于无掺合料砂浆的抗压强度;随着磨细矿渣和粉煤灰掺量的增加,拆模前覆膜养护对抗压强度的影响更加显著,掺50%粉煤灰砂浆28d抗压强度比3d的大200%以上。
对混凝土而言,无论是力学性能还是耐久性,拆模前的养护方式对拆模后室内养护的混凝土影响显著,对拆模后水养护的混凝土影响不显著;拆模前的养护方式对C30混凝土影响显著,对C60混凝土影响不显著,并且拆模前覆膜养护最有利于混凝土1h表面吸水率的降低、混凝土抗压强度的增大和耐久性的提高。
随着水灰比的增大,细石混凝土收缩增大,掺粉煤灰和磨细矿渣能减少细石混凝土的收缩;环境温度越低收缩越小,环境湿度越大,收缩越小,并且湿度对收缩的影响比温度显著。
混凝土的表面性能取决于表面孔隙与表面裂缝的综合作用。C30混凝土表面孔隙率大于表面裂缝率,C60混凝土表面裂缝率大于表面孔隙率。拆模前覆膜养护能显著改善混凝土表面裂缝在内部的扩展。拆模前的养护方式显著影响混凝土表层与内部水泥水化程度:拆模前覆膜养护最利于混凝土表层与内部水泥水化发展,拆模前风吹养护不利于混凝土表层水泥水化的发展,拆模前高温养护不利于混凝土内部水泥水化的发展。
Appropriate curing can improve concrete durability significantly, especially the early age curing of concrete is the key to the later properties of concrete. Actually in the practical construction, the curing of concrete is paid no more attention. There are some deficiencies for the current study on the concrete curing, because most of the studies have targeted the curing after concrete hardening, lacking of study of practical construction under harsh environment. For the lack of existing research, this paper has combined the curing time and curing condition together. Curing time is fixed to be the duration of hardening and the curing condition is set to be familiar with the practical construction under the harsh environment, such as wind curing, high temperature curing and light with wind curing. Comparing with the natural curing inside room, the effects of the curing methods under harsh condition on the properties of concrete have been studied in this paper. By the study of early curing time and curing methods, it is useful to provide information for the curing specification’s update.
As for mortar, it is indicated that curing methods before hardening are significant for compressive strength of 28d, and coated curing before hardening is the best among these three curing methods, thus the light with wind curing before hardening is the worst. Coated curing before hardening is significant for mortar with mineral admixture. The 28d compressive strength of mortar with 10% fly ash with coated curing before hardening is up to 42.0MPa which is slightly larger than that of mortar with no mineral admixture. The sensibility of coated curing before hardening to compressive strength increases as the amount of mineral admixture increases. The 28d compressive strength of mortar with 50% fly ash increases by 200% compared to the 3d’s. The water absorption of coated curing before hardening is the least.
As for the mechanical properties and durability of concrete, curing methods before hardening are not significant for concrete curing in water after the form removal, but it is very significant for that of concrete curing inside room after the form removal. Curing methods before hardening are more significant for C30 concrete than C60 concrete. The water absorption of concrete with wind curing before hardening is the largest among these three curing methods, and the water absorption of concrete with coated curing before hardening is the least. The compressive strength and the durability of concrete increase with the sequence of the curing methods before hardening: wind curing, inside room curing and coated curing.
The shrinkage of concrete increases as the ratio of“water to binder”increases. Adding mineral admixture can reduce the shrinkage especially can play a role in balancing the later shrinkage. The shrinkage increases as the temperature increases and the relative humidity decreases. The effect of relative humidity on shrinkage is more sensible than that of temperature.
The surface properties of concrete depend on the combined effects of surface pores and cracks. The surface porosity of C30 concrete is more than surface cracks, as for C60 concrete, the result is totally opposite. The coated curing before the form removal can extraordinarily reduce the inside extension of surface cracks in the concrete. The curing methods before hardening can affect the hydration degree of cement on the surface and inside. The coated curing before the form removal is benefit for hydration degree both of the surface and inside. The wind curing before the form removal can prevent the hydration degree of the surface concrete, thus the high temperature curing before the form removal can prevent the hydration degree of the inside concrete.
研究表明,对水泥砂浆而言,硬化拆模前的不同养护对砂浆28d的抗压强度影响显著。拆模前养护对掺合料砂浆影响显著,拆模前覆膜养护的掺10%粉煤灰砂浆28d抗压强度能达到42MPa以上,略大于无掺合料砂浆的抗压强度;随着磨细矿渣和粉煤灰掺量的增加,拆模前覆膜养护对抗压强度的影响更加显著,掺50%粉煤灰砂浆28d抗压强度比3d的大200%以上。
对混凝土而言,无论是力学性能还是耐久性,拆模前的养护方式对拆模后室内养护的混凝土影响显著,对拆模后水养护的混凝土影响不显著;拆模前的养护方式对C30混凝土影响显著,对C60混凝土影响不显著,并且拆模前覆膜养护最有利于混凝土1h表面吸水率的降低、混凝土抗压强度的增大和耐久性的提高。
随着水灰比的增大,细石混凝土收缩增大,掺粉煤灰和磨细矿渣能减少细石混凝土的收缩;环境温度越低收缩越小,环境湿度越大,收缩越小,并且湿度对收缩的影响比温度显著。
混凝土的表面性能取决于表面孔隙与表面裂缝的综合作用。C30混凝土表面孔隙率大于表面裂缝率,C60混凝土表面裂缝率大于表面孔隙率。拆模前覆膜养护能显著改善混凝土表面裂缝在内部的扩展。拆模前的养护方式显著影响混凝土表层与内部水泥水化程度:拆模前覆膜养护最利于混凝土表层与内部水泥水化发展,拆模前风吹养护不利于混凝土表层水泥水化的发展,拆模前高温养护不利于混凝土内部水泥水化的发展。
Appropriate curing can improve concrete durability significantly, especially the early age curing of concrete is the key to the later properties of concrete. Actually in the practical construction, the curing of concrete is paid no more attention. There are some deficiencies for the current study on the concrete curing, because most of the studies have targeted the curing after concrete hardening, lacking of study of practical construction under harsh environment. For the lack of existing research, this paper has combined the curing time and curing condition together. Curing time is fixed to be the duration of hardening and the curing condition is set to be familiar with the practical construction under the harsh environment, such as wind curing, high temperature curing and light with wind curing. Comparing with the natural curing inside room, the effects of the curing methods under harsh condition on the properties of concrete have been studied in this paper. By the study of early curing time and curing methods, it is useful to provide information for the curing specification’s update.
As for mortar, it is indicated that curing methods before hardening are significant for compressive strength of 28d, and coated curing before hardening is the best among these three curing methods, thus the light with wind curing before hardening is the worst. Coated curing before hardening is significant for mortar with mineral admixture. The 28d compressive strength of mortar with 10% fly ash with coated curing before hardening is up to 42.0MPa which is slightly larger than that of mortar with no mineral admixture. The sensibility of coated curing before hardening to compressive strength increases as the amount of mineral admixture increases. The 28d compressive strength of mortar with 50% fly ash increases by 200% compared to the 3d’s. The water absorption of coated curing before hardening is the least.
As for the mechanical properties and durability of concrete, curing methods before hardening are not significant for concrete curing in water after the form removal, but it is very significant for that of concrete curing inside room after the form removal. Curing methods before hardening are more significant for C30 concrete than C60 concrete. The water absorption of concrete with wind curing before hardening is the largest among these three curing methods, and the water absorption of concrete with coated curing before hardening is the least. The compressive strength and the durability of concrete increase with the sequence of the curing methods before hardening: wind curing, inside room curing and coated curing.
The shrinkage of concrete increases as the ratio of“water to binder”increases. Adding mineral admixture can reduce the shrinkage especially can play a role in balancing the later shrinkage. The shrinkage increases as the temperature increases and the relative humidity decreases. The effect of relative humidity on shrinkage is more sensible than that of temperature.
The surface properties of concrete depend on the combined effects of surface pores and cracks. The surface porosity of C30 concrete is more than surface cracks, as for C60 concrete, the result is totally opposite. The coated curing before the form removal can extraordinarily reduce the inside extension of surface cracks in the concrete. The curing methods before hardening can affect the hydration degree of cement on the surface and inside. The coated curing before the form removal is benefit for hydration degree both of the surface and inside. The wind curing before the form removal can prevent the hydration degree of the surface concrete, thus the high temperature curing before the form removal can prevent the hydration degree of the inside concrete.
引文
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[2] Mario Collepardi.混凝土新技术[M].刘数华,冷发光,李丽华译.北京:中国建材工业出版社, 2008: 197.
[3] Alizadeh R, Ghods P. Chini M. Effect of Curing Conditions on the Service Life Design of RC Structures in the Persian Gulf Region[J]. Journal of Materials in Civil Engineering, 2008, Vol. 20,No. 1: 2-8.
[4] Austin S A,Robins P J.Influence of early curing on the sub-surface permeability and strength of silica fume concrete[J]. Magazine of Concrete Research, 1997, 49(178): 23-34.
[5] Jensen O M, Hansen P F. Water-entrained cement-based materialsⅠ. Principles and theoretical background[J]. Cement and Concrete Research, 2001, 31(4): 647-654.
[6] Wang K, Cable J K, Zhi G. Evaluation of Pavement Curing Effectiveness and Curing Effects on Concrete Properties [J]. Journal of Materials in Civil Engineering 2006, 18(3): 377-389.
[7] Basheer P A M, Harmon N, Long A E,et al. AUTOCLAM permeability system for measuring absorption and permeability of concrete in the laboratory and on site[C]. Proceeding of the 2006 International Workshop on Concrete Durability and Testing Techniques,Chongqing: China, 2006,9: 54-84.
[8]建设部、质量监督局.混凝土结构验收规范(GB50204-2002).中国建筑工业出版社.2002:3
[9]黄煜镔,钱觉时.龄期和养护方式对高强混凝土力学性能的影响[J].硅酸盐通报,2007,26(3):427-430.
[10] Conroy-Jones G A, Barr B I G. Effect of curing on the tensile strength of medium to high strength concrete[J]. Magazine of Concrete Research, 2004, 56(3): 151-158.
[11]范伟丽.养护条件对混凝土强度及耐久性的影响[J].河南水利, 2006, 50(5):34-36
[12]谭克锋.早期高温养护对混凝土抗压强度的影响[J].建筑材料学报,2006(8):20-22.
[13] Almusallam A A. Effect of environmental conditions on the properties of fresh and harden concrete[J].Cement &Concrete Composites,2001,23:353-361.
[14]刘竞,丁德华,刘赞群.养护措施和湿养时间对掺与不掺矿渣混凝土性能的影响[J].硅酸盐学报,2008,36(5):901-911.
[15] Al-Gahtani A S. Effect of curing methods on the properties of plain and blended cement concretes[J]. Construction and Building Materials, 2010, 24(3): 308-314.
[16] Pierre Mounanaga, Abdelhafid Khelidj, Ahmed Loukili, et al. Predicting Ca(OH)2 content andchemical shrinkage of hydrating cement pastes using analytical approach, Cement and Concrete Research[J].2004(34):255-265.
[17] R.G. Patel, D.C. Killoh, L.J. Parrott and W.A. Gutteridge, Influence of curing at different relative humidities upon compound reactions and porosity in Portland cement paste[J]. Materials and Structures, 1988(21):192.
[18] Shattaf N R, Alshamsi A M, Swamy R N. Curing/Environment Effect on Pore Structure of Blended Cement[J]. Journal of Materials in Civil Engineering, 2001, 13(5): 380-388.
[19] Hanson J A. Effects of Curing and Drying Environments on Splitting Tensile Strength of Concrete[EB/OL]. Development Department Bulletin DX141, Portland Cement Association,1968[2008-12-5].http://www.portcement. org/pdf_files/DX141.pdf.
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