جستجوی مقالات مرتبط با کلیدواژه "نسبت آب به سیمان" در نشریات گروه "عمران"

Recycling of concrete has been the focus of researchers owing to the large volume of concrete waste and the limited resources of valuable materials. Recycled aggregate concrete (RAC) contains aggregates of various types of concrete and building materials that have been destroyed due to their shelf life or as a result of war and natural disasters. In order to use different types of recycled aggregates (RA), it is necessary to conduct more detailed and sensitive research on the behavior of RAC in order to reach the highest possible level of quality and cost in the construction process. On the other hand, RA are made of concrete with different properties and its aggregates have been subjected to different form of loading, hence it is important to carefully examine their behavior and properties. Therefore, in this study, the strength and the durability of RAC after freeze and thaw cycles were compared with natural concrete while different water/cement ratios are used.
In this study, RA were extracted from demolished waste concrete specimens and used to produce three mixtures named RC, RN and NC while 100, 50 and 0 percent of RA were replaced respectively. Water to cement ratio were kept as 45 and 55 percent of cement. A total number of 57 samples for flexural, compressive, tensile strength and ultrasonic tests were made. 24 samples were placed in the refrigerator and freeze and thaw cycles were applied on. 18 and 15 specimens were prepared for testing at 28 and 120 days of age, respectively. According to ASTM C666, the relative dynamic modulus of the specimens was tested using ultrasonic test after each 40 cycle’s period. After the test was finished, compressive strength, single point bending, indirect tensile tests and elastic modulus were determined for each specimens. Finally, the behavior of recycled and natural concrete was compared.
The results showed that the freeze and thaw cycles reduced the resistance of both RAC and NAC. Recycled aggregates have 2.5 times more water absorption than Natural aggregates. RAC has lost 41%, 18%, 10% and 28% of its compressive, flexural, tensile strength and modulus of elasticity, after the freeze and thaw cycle, respectively. The 100% replacement of RA reduced even more the mechanical behavior of samples. The relative dynamic modulus at the end of the cycles for RAC was 2% higher than that of NAC, indicating its better durability properties against freeze and thaw cycles. Concrete with 50% replacement of recycled aggregate has almost the same durability and strength as NAC, hence is recommended to be use in similar condition

Relationship between the compressive strength of concrete in terms of the age of concrete is set out in the American Concrete Institute Regulations for Type I and Type III Cement. In this study, the relationship between compressive strength in terms of the age of concrete containing Portland Cement Type II has been experimentally investigated. The relationship has also been explored for crumb rubberized concrete. For this purpose, three cement contents: 350, 400 and 450 kg / m³, as well as two water-to-cement ratios of 0.5 and 0.65, and crumb rubber with ratios of 5 to 20% by the Volume of sand have been used. In the relations obtained, α and β values for the ordinary concretes containing cement type II are 3.13 and 0.91, respectively, while for the crumb rubberized concrete are 3.32 and 0.9, respectively. In this study, the relationship between compressive strength in terms of the age of concrete containing Portland Cement Type II has been experimentally investigated. The relationship has also been explored for crumb rubberized concrete. For this purpose, three cement contents: 350, 400 and 450 kg / m³, as well as two water-to-cement ratios of 0.5 and 0.65, and crumb rubber with ratios of 5 to 20% by the Volume of sand have been used. In the relations obtained, α and β values for the ordinary concretes containing cement type II are 3.13 and 0.91, respectively, while for the crumb rubberized concrete are 3.32 and 0.9, respectively.

The high volume of carbon dioxide produced in cement plants and the feasibility of reusing waste materials from factories in recent years has become one of the main concerns of research centers and environmental associations. Therefore, the main purpose of this research is to evaluate the feasibility of reusing waste materials with a focus on waste ceramic powder (WCP), as a semi-active aluminosilicate material that can be replaced as a percentage of cement used in concrete. WCP, in addition to activating the potential of using a waste material in concrete, can also reduce cement consumption. Therefore, in this research, cement replacement percentages between 0 and 50% in three water to cement ratios of 0.3, 0.4 and 0.5 in 24 concrete mixtures have been used to perform compressive strength tests. In order to provide a usable computational model, the gene expression programming (GEP) method has been used to predict the compressive strength of the samples. The results of experimental research indicate that in the ratio of water to cement 0.3 at the age of 90 days, the sample containing 20% of WCP has reached a compressive strength of 72.57 MPa. This result is almost equal to the control sample and therefore the use of this percentage is recommended for this ratio of water to cementitious materials. Finally, the results indicate the very good performance of the GEP method by increasing the number of chromosomes and increasing the correlation coefficient between experimental and numerical data up to 98%. Therefore, the GEP method has a significant advantage over other methods by providing an analytical relationship and high accuracy.

Increasing study and research on the components of concrete structures has led to the availability of materials, the use of which in turn has been able to have a tremendous impact on concrete production. Concrete is undoubtedly the most important building material, the production and consumption of which is increasing day by day and it is used in almost all structures. The performance of cement as one of the main components of concrete products has always been considered and with the increasing development of Iran's cement industry in terms of quantity in the production of cement, cement quality control by examining physical and chemical properties is very important. In this study, we tried to investigate the factors affecting the strength of hydrated cement used in concrete structures in Shiraz and the results indicate a decrease in concrete strength due to increasing the ratio of water to cement and also C3S and C2S are the two main compounds causing cement strength are hydrated.

Permeability is one of the most effective parameters on concrete durability. Therefore, in this paper penetration of water into concrete is studied. Although most of the researchers have considered the coefficient of permeability obtained from one dimensional Darcy’s equation, in the present paper due to movement of water in all directions, two-dimensional diffusion equation which defines penetration of fluid into a porous material has been used for first time. For this purpose, cubic concrete specimens with different W/C ratios were prepared and their permeability was measured using “Cylindrical chamber” method. In this method, applied pressures and durations of water penetration were varied. The two-dimensional equation considered, was solved using Laplace and Henkel transformations and the obtained results were compared with the “Cylindrical chamber” results. Comparison of the theoretical and experimental results showed that the average respective percentage errors calculated for the estimation of wet curve, maximum penetration depth, average penetration depth and wet surface were 23.07, 13.64, 21.41 and 1.66. The coefficients of determination between pressure magnititude and duration of water penetration, considering the variables of maximum penetration depth, average penetration depth, wet surface, penetrated volume and optimum diffusion coefficients were seen to be higher than 0.95. Furthermore, no reliable correlation was observed between the optimum diffusion coefficients and the mentioned variables.

Ultrasonic pulse waves velocities as a non-destructive test method has been utilized for assessing concrete properties. Most studies in this context were performed on specimens with no externally applied load and only were expressed the pulse wave velocity in terms of other concrete characteristics. Reinhardt and Grosse study showed that pulse wave velocity changes with different water to cement ratios for different cement types after 24 hours (Reinhardt & Grosse et al., 2004). The objective of this research was to study the pulse wave velocity of structural concrete under real conditions subjected to different loading and different intervals of compressive stresses and strains.  This can help to have a better understanding of concrete behavior and lead to an independent model from load- displacement parameter for health monitoring of concrete structures. To this aim, concrete cube specimens were prepared for cement content of 400 kg/m3, water to cement ratio from 0.3 to 0.5 with varying increment of 0.02 and equal ratio of fine to coarse aggregate.

Today, concrete structures have a crucial role in the infrastructure of society. The conditions and performance of these structures are important. In recent years, due to corrosion phenomenon, a number of concrete structures in Southern areas of Iran have suffered damage or premature failure. Concrete damage in the chloride environments is the most common failure of reinforced concrete structures and an important problem for civil engineers which today they are faced with the maintenance of them. Recent societal shift toward sustainable consumption and growth applied to civil infrastructure systems requires the construction materials to be designed and used with utmost attention to their durability and long term response. Pozzolanic materials including silica fume, fly ash, slag, and metakaolin have been used in recent decades for developing high performance concrete with improved workability, strength and durability. Metakaolin has been used as a pozzolan for high performance concrete applications. This material is a thermally activated alumino-silicate which is mostly manufactured by calcinations of kaolin clay at the temperature range of 500-850°C [1].

Due to the high usage of concrete in construction, civil engineers try to decrease the defects of concrete and increase the benefits of it by using some methods. One of these methods is adding additive materials in order to achieve mentioned target. Undoubtedly in the usage of these materials engineers should pay attention to economic issues, decreasing costs of project and their availability. Most of these additive materials used in concrete, are chemicals and albeit they improve some properties of concrete they have some negative effects on other features of it. In this investigation an herbal additive material is used in concrete, this herbal material is the powder of the leaves of a plant named Mahur or Khargushak)Verbascum Thapsus) that grows in the mountains naturally. The experimental results of specimens in this investigation indicate that adding this herbal material during concrete mixing increases compressive strength up to 12% and tensile strength 20% to 25%. Due to the significant increase in tensile strength of concrete by adding the powder of Mahur additive, the surficial cracks of concrete decrease. By the way the amount of used cement in order to reach to a specific strength decreases.

Marin environments such as Qheshm Island, are the most destructive environments for reinforced concrete structures. Concrete quality is one of the important factors in corrosion. Nowadays use of pozzolanic materials is one of the methods to improve the concrete qualities. In this study, by placing specimens with different water to cement ratios and different amounts of Metakaolin in splash zone and measuring the rate of corrosion and half-cell potential using GalvaPulse method, the effect of these factors on corrosion of reinforcements during 32 month is evaluated. The test results indicate that the specimen with lower water to cement ratio has lower likelihood of corrosion. Increasing the water to cement ratio from 0.35 to 0.45, decreases the time-to-corrosion to 50%. Using Metakaolin also reduces the rate of corrosion.

Concrete properties are dependent on many parameters such as ingredients، production and technology، construction methods and curing conditions that have made concrete as a complex and unpredictable material. But these are virtually ignorable against its precious advantageous and precise studies on above parameters may help us to produce concretes with our desirable properties. Ingredients are of those important parameters that play an essential role. Therefore it is seriously important to have enough knowledge about ingredients، their combined effects and interactions and also effects of their properties on concrete properties. Pozzolan materials can help us to achieve desirable characteristics of concrete and develop its mechanical and physical parameters، hence they are used widely. Improvement of mechanical parameters، permeability reduction، and durability increase are among their fabulous benefits of application. They are deemed as essential ingredients of high durable and strength concretes. Pozzolan weighted ratio is major parameters which develop a homogenous and united medium and improve the physical and mechanical parameters of concrete. The effects of pozzolan on physical and mechanical properties of concrete، moreover it’s specifications، depend on the parameters of other ingredients، such as type of aggregates، particle size distribution of aggregates، fineness modulus of fine aggregates، water-cement ratio، cement type and ect. Then study of combined effects of pozzolan specifications and parameters of other ingredients is inevitable for achieving to a concrete with ideal parameters. As we know، microsilica is the most popular pozzolan material which is used widely and it’s parameters and other ingredients parameters (such as type of aggregates، particle size distribution of aggregates، fineness modulus of fine aggregates، water-cement ratio، cement type and ect) have combined effects on the variation of concrete physical and mechanical parameters. So، in this study we selected microsilica as a pozzolan to determine the combined effects of its weighted ratio content، fineness modulus of fine aggregates and water-cement ratio on physical and mechanical properties of concrete. In this study، the combined effects of microsilica weighted ratio content، fineness modulus and water-cement ratio on physical and mechanical properties of concrete were investigated. For determination the effects of microsilica weighted ratio content on concrete parameters in different water-cement ratios and particle size distributions، 5 different microsilica weighted ratio contents، 3 water-cement ratios and 3 particle size distributions، were selected and totally 45 mix designs were prepared and subjected to slump test and compressive strength، tensile strength and modulus of elasticity tests. Obtained results show a direct relationship between microsilica weighted ratio content and physical and mechanical properties of concrete. Increase in microsilica content from 0% to 10 % in all water-cement ratios and fineness modulus، leads to slump decrease and mechanical parameters increase، while from 10 % to 20 % of microsilica content، mechanical properties fall down، but the reduction trend of slump continues. Also it can be seen that the effect of different weighted of microsilica، on physical and mechanical properties of concrete، reduces by increasing in water-cement ratio and intensifies with increment of fineness modulus of fine aggregates.

Many attempts have been done to increase the service life of the concrete structures. In this regard, the concrete properties should be evaluated by non destructive testing methods (NDT) periodically to estimate how the structures may behave in the future. It is so important to select a proper and simple method to examine the concrete properties during the service life. In the present paper, try has been done to evaluate the water cement ratio of the hardened concrete via non destructive measurement of the electrical resistance the concrete during one freeze thaw cycle. Five mix proportions were prepared. The electrical resistance of the cured specimens within one slow freeze thaw cycle was monitored within 24 hours at 1 minute interval. The results declared that as the temperature dropped to zero or minus 15 °C, the electrical resistance variation of concrete is highly affected by water- cement ratio. The characteristics of the curve such as slope, upper and lower limits of electrical resistance curve are dependent on the water-cement ratio of the mixture. The results of the present work can relieve engineers´ anxiety about the proper water-cement ratio which was used in the concrete mixture.

1. IntroductionConcrete structures in aggressive environments greatly suffer from corrosion which causes a premature failure in their life span. Persian Gulf region is one of the most aggressive environments in which concrete structures need further attention in order to increase their durability and serviceability as well. In the present investigation, the half-cell potential and corrosion resistance of rebar embedded in concrete with different water to cement ratios (0.35, 0.40, 0.45 and 0.50) exposed to splash zones in Qeshm Island for 18 month ware studied (Fig 1). The results demonstrated that concretes with lower water to cement ratio perform better in terms of corrosion protection and process retardation.2. Experimental Study2.1. MaterialsType II Portland cement was used throughout the experiment. Total aggregate mass of about 1850 kg/m3 with a largest size of 19 mm were used. Also, to achieve desired workability of 7-10cm in different mixture designs, the use of poly carbixilate-based superplasticiser was necessary.2.2. Mixture proportionsFour mix proportions with different w/c ratios were used for casting the concrete specimens. The details of mix proportion are shown in Table1.2.3. Concrete specimensReinforced concrete specimens of size 550X300X200 mm were prepared each having three rebars (anode) at top and six rebars (cathode) at bottom embedded at 30 mm cover from both sides (Fig. 2). Concrete prisms of size 150X150X600 mm were made for determination of chloride penetration after nine months of exposure. Prism samples were also moist cured for 2 days and were sealed at all sides with polyurethane-based coating and only the top side of prisms was left uncoated to be exposed to chloride ions.2.4. Methods of measurementThe macro cell and micro cell corrosion were investigated in this study. For this reason, various methods of corrosion measurements such as macro cell, half cell and corrosion rate methods were used throughout the investigation.3. Results and discussion3.1. MacrocellThe variation of macrocell current with time was almost negligible for specimen C1. In specimen C2, there was no indication of corrosion initiation while the current in specimen C3 reached the threshold value of 10μA after 13 months. The anodic current of specimen C4 passed the threshold value after about 6 months of exposure to chloride ion. The unusual behavior of cell 1 in specimen C3 was due to the fact that the steel rebars might be slightly rusted at the time they were embedded in concrete.3.2. Halfcell Specimens C1 and C2 did not reach the threshold value, while the specimens C3 and C4 attained a more negative potential of -230 after 14 and 5 months of exposure, respectively. Results obtained from macrocell and halfcell potential measurements indicate that the rebars in specimens C1 and C2 have steel remained unaffected by corrosion whilst specimen C3 and C4 have reached the threshold value after 14 and 6 months of exposure, respectively. 3.3. Corrosion rateAt splash zone, even after 18 months of exposure, the corrosion rate of rebar in specimen C1 did not pass the threshold value required for the initiation of corrosion. In specimen C2, although the corrosion rate passed the threshold value but it followed a continues trend and there was no sudden changes in the rate of corrosion concluding that the corrosion had not started yet. But in specimen C3, sudden changes in corrosion rate confirmed the initiation of corrosion after 13 month of exposure. Specimen C4 experienced the corrosion process in month 9-11. It is interesting to note that after 12 month of exposure, specimen C4 showed a marginal increase of corrosion rate and the measurement of corrosion rate was not possible after that. 3.4. Critical chloride measurementThe chloride amount penetrated into concrete specimens was measured after 9 months of exposure and thus the bases for comparison of chloride content and variations in macrocell current, halfcell potential and corrosion rate for critical chloride determination were taken as 9 month’s measurements. The results are presented in Table 2. According to these results, the macrocell current and the halfcell potential methods are environmentally dependant, and therefore, these two methods have more accuracy in corrosion activity estimation at splash zone. The data from these studies clearly indicate that macrocell current and halfcell potential methods are practical when the corrosion activity has passed the threshold value otherwise they are not more accurate for the estimation of corrosion initiation time. However, comparing the chloride content and corrosion rate of specimens with different w/c ratio, it can be inferred that critical chloride content in concrete specimens is higher at splash zone with the estimated value of about 0.11 % of concrete weight at splash zone.4. Conclusion[1] The critical chloride content required for corrosion to initiate was found to be 0.11% of concrete weight at splash zone under environmental condition of Persian Gulf region.[2] Considering the test results, splash zone is found to be the most sever zone in terms of chloride ion penetration and corrosion activity.[3] According to the test results, the macrocell current and halfcell potential have acceptable accuracy in the determination of corrosion initiation time.[4] The use of high w/c ratio concretes in reinforced concrete structures in Persian Gulf region causes an accelerated corrosion and failure in those structures; and as mentioned earlier, the corrosion in concrete specimens with w/c ratio of above 0.4 and the cover of 30 mm is evident. Therefore, the modification of concrete structures by using pozzolans as cement replacement is necessary.

Concrete structures in aggressive environments greatly suffer from corrosion which causes a premature failure in their life span. Persian Gulf region is one of the most aggressive environments in which concrete structures need further attention in order to increase their durability and serviceability as well. In the present investigation the half-cell potential and corrosion resistance of rebar embedded in concrete with different water to cement ratios exposed to splash zones in Qeshm Island for 18 month was studied. The results demonstrated that concretes with lower water to cement ratio perform better in terms of corrosion protection and process retardation.