عمل آوری داخلی با سنگدانه های سبک وزن و تاثیر آن در خصوصیات مکانیکی بتن با سطح عملکرد بالا

عمل آوری داخلی بتن با سطح عملکرد بالا بوسیله سنگدانه های سبک وزن از پیش اشباع شده به عنوان یک روش مناسب برای کاهش و یا حذف احتمالی خود خشک شدگی و خود جمع شدگی شناخته می شود. اگرچه جایگزینی این مواد با مقداری از سنگدانه های معمولی، ممکن است به خصوصیات مکانیکی مانند مقاومت فشاری بتن لطمه وارد کند. خصوصیت اصلی ماده عمل آورنده داخلی، توانایی آن در نگهداری آب مورد نیاز بتن برای عمل آوری آن می باشد. با توجه به این خصوصیت مواد عمل آورنده معمولا مواد با تخلخل زیاد و دارای مشخصات مکانیکی ضعیف هستند که باید اثرات این مواد در خصوصیات مهم بتن با عملکرد بالا بررسی شود. این اثرات و ارتباط آن با مواد عمل آوری داخلی همچنان به عنوان یک موضوع تحقیقاتی مورد توجه قرار دارد. درمقاله حاضر، سعی می شود خصوصیات مکانیکی و پایداری بتن از جمله مقاومت در برابر نفوذ کلریدها، نفوذ پذیری هوا، جذب آب، خود جمع شدگی خشک شدگی و همچنین وزن بتن بررسی شود. نمونه ها با نسبت آب به سیمان مختلف ساخته گردید. اثرات عمل آوری داخلی بر روی خصوصیات مربوطه به بتن با سطح عملکرد بالا به صورت تابعی از نسبت آب به سیمان بیان شده است.

Effect of Internal Curing On Mechanical Properties andDurability of HighStrength Concretes

HPC has become popular due to its superior mechanical and durability properties.HPC significantly reduces maintenance costs and enhances service life. Internal curing of high performance concrete (HPC) by presaturated lightweight aggregates is a wellestablished method of counteracting selfdesiccation and autogenous shrinkage. However, by introducing the internal water reservoirs strength and durability properties can be injured. Concrete is a strong and durable material that has been utilized since the beginning of civilization. It is understood that many of the early structures are deteriorating or have already deteriorated away; however, with the knowledge of the material properties of concrete that is available, it is hard to imagine that concrete structures is prematurely failing before their intended service life. There are many factors involved in these failures, some of which are due to environmental conditions and others which have arisen from human errors or lack of knowledge. Early age properties of concrete are vital to its longterm performance. Many contractors and owners are interested only on the strength of concrete in 28 days or 90 days, that they overlook the importance of other early age issue especially when the mix design has water to cementitious material ratio (w/cm) lower than ~0.42. Internal curing with LWA has been successfully used recently in large construction projects of normal density concrete structures. For example, in January 2005, about 190 000 m of internallycured concrete was used in a large paving project in Hutchins, Texas. Although the benefits of internal curing for highperformance concrete structures have been evidenced in the laboratory and some field investigations (such as those previously mentioned), the literature does not provide any significant quantitative information regarding the additional extension of the service life that can be achieved by the use of internal curing in concrete structures. A great concern for design engineers and contractors is whether the concrete will achieve the specified compressive strength and all durability requirements in the structure during service. Cusson & Hoogeveen (2008) demonstrated that internal curing can reduce autogenous shrinkage considerably without affecting the strength and stiffness of highperformance concrete. This was achieved by reducing the amount of mix water in the concrete by an amount equal to that used in the LWA for internal curing, thus reducing the effective watercement ratio (w/c ) of the concrete when using higher quantities of presoaked LWA. Tests by the widely accepted methods of durability assessment, such as resistance to chloride penetration, air permeability, water absorption, autogenous and drying shrinkage and mass loss, were conducted on HPC mixes made at water to cement ratios in the range of 0.21–0.33. The effect of internal curing on the durability related properties of highperformance concretes as a function of water to cement ratio is reported.