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سال پنجم شماره 1 (بهار و تابستان 1392)

Poly Ethylene Terephthalate is a kind of polymer which is used in manufacturing polyester fibers, bottle resin and engineering polyester in most of the countries around the world. Vast usage of this polymer in food packing industries and long-term decomposition of this kind of waste in nature encouraged many researchers around the world to find new ways to recycle and reuse them. In this article, with a more comprehensive approach than previous studies, effects of 5, 10 and 15 percent of PET particle usage instead of sand were investigated. For this purpose, cubic and cylindrical samples with different water to cement ratios were made and the physical properties were obtained with the corresponding tests. Moreover, the samples which were cured in a standard condition were used for mechanical properties testing and the factors were extracted. Results show that fresh concrete with PET particles has less workability. Mechanical properties tests show that the elasticity modulus decreases for these concrete samples. The compressive and flexural strength increases at first but they decrease after a while. Ultrasonic wave tests show that concrete samples with PET particles have porous structure.

Studies show that addition of different fibers does not fundamentally change the behavior of the concrete prior to its maximum stress while it greatly improves the concrete post-cracking behavior. This method positively influences other properties of the concrete including toughness, fracture energy and flexural strength. On the other hand, Using Nano-silica particles as a product of pozolanic reaction, can strongly improve the permeability of concrete by increasing transition layer of fiber and cement matrix. Thus concrete having the properties of both self compacting concrete and fiber reinforced concrete with strengthened micro matrices can improve the fabrication of durable structures with high performance level. In this research, the combined effect of nano-silica particles and fibers type (steel, polypropylene and glass) on toughness, fracture energy and flexural strength, rheological behavior (L-box, slump flow and T50) of Self-Compacting concrete were evaluated.For this purpose, forty mixtures in A, B, C and D series representing 0, 2, 4 and 6 percent of Nano-silica particles replacing cement content were cast. Each series involved three different fiber type and content. 0.2, 0.3 and 0.5% volume for steel fiber, 0.1, 0.15 and 0.2% of volume for polypropylene fiber and finally 0.15, 0.2 and 0.3% of volume for glass fiber. The results show that the combined usage of optimum percent of fiber and Nano-silica particles will improve the toughness, fracture energy and flexural strength of self-compacting concrete.

Pyrogenic nanosilicas and nanosilica sols are two types of nanosilicas, which have been investigated in numerous studies of cement-based materials. Different production process of these materials leads to different characteristics of them. For instance, nanosilica sols are monodispersed particles in water; while, in the production process of pyrogenic nanosilicas, particles fuse together and form primary aggregates with sizes up to 100 nanometers. The primary aggregates also bind together and form agglomerates with sizes from a few micrometers up to a few hundred micrometers. This article presents the results of an investigation on the rate of lime consumption of the product Aerosil 200 (a pyrogenic nanosilica with surface area of 200 m2/g) and Levasil 200/30 (a nanosilica sol with surface area of 200 m2/g and concentration of 30%). The results show that despite the agglomeration state of the pyrgenic nanosilica, this material has faster pozzolanic reactivity than the nanosilica sol in lime and cement pastes. The results also indicate that the nanosilicas have accelerating influence on the hydration degree of cement at early ages. However, by progress of hydration and from 7 days lower hydration degree of cement compared to the plain paste was observed. It appears that considerable water absorption of nanosilicas is responsible for the lower available water for hydration of cement and consequently lower hydration degree of cement.

Plastic shrinkage and settlement occurs when concrete is in the fresh state. In reinforced concrete structures, the presence of rebar is effective on the shrinkage and settlement. In this study, four types of self-consolidating concrete(SCC) that are evaluated including: simple self-consolidating concrete (S), SCC containing silica fume (SS), SCC containing silica fume and styrene butadiene rubber latex (SSL) SCC containing silica fume and styrene butadiene rubber latex and fiber (SSLF). Then effect of changing in rebar placing on plastic shrinkage and settlement of SCC has been evaluated. The results show that reducing the height of the rebar placing in concrete (reducing in concrete cover), cause to change cracking due from plastic shrinkage to plastic settlement. Therefore cracking pattern has changed from Angle to the vertical. SCC containing fibers and latex shows the lowest plastic shrinkage and settlement and cracking area. Also cracking has not been observed in SCC containing fibers (SSLF).

Nowadays, many old buildings are destroyed by explosive material and replaced by new structures. During explosion, some of the concrete structures experience kind of unexpected loads which lead to localized destruction on one or more structural members and then finally the whole structure will be modified under the new distribution loading. This study will examine the behavior of reinforced concrete members when some of its members will be suffered local damage under different loading and will predict the primary performance of structure under these loading conditions. In this regard, the equivalent section method is used for the modeling of reinforced concrete elements under static and dynamic loads. In this research, finite element software “Abaqus will be used. Analysis results show that the method of equivalent section will indicate the behavior of structures with high accuracy and will reduce the analysis time due to declining of members and constraints in the modeling. This modeling approach also can be related to the mechanical failure such as crack growth and behavior of matter in the formation of cracks.

Reactive powder concrete is a new type of high-strength concrete, which due to extra fine materials and pozzolans in addition to high hydraulicly active materials is refered to as Reactive Powder Concrete (RPC). The objective of this paper is to make feasibility study on construction and production of RPC using locally available materials in Assalooyeh region. In this research study, some of the properties of RPC are investigated. Six mixes, using 0%, 10%, 15%, 20%, 30% and 40% silica fume, based on required cement content,water to cement ratio and optimized super-plasticizer are designed. Experimental program included reology of fresh mixes, compressive and flexural strengths, three types of curing condition and micro-structure of the concretes. Test results and analysis indicate that mixes with 15% silica fume provide the highest compressive and flexural strengths for the three types of curing regimes.

Since, self-compacting concrete and mortar are self-levelling and do not need any compaction during its application, are receiving more popularity as repair materials in concrete structures repair industry. This paper presents the results obtained using the in-situ twist off method for assessing the in-situ strength- that it has an important role on bond strength between this layer and concrete substrate- of self-compacting concrete and mortars applied to different concrete substrate. The effects of five different substrate conditions on the in-situ strength of repair layer were studied and the results were also compared with the predicted values obtained from the fuzzy logic and neural networks.