חומרי בניין 1 - מאגר שאלות ממעבדות

The correct answer is 1.5 because it is the smallest number among the given options.

The characteristic that affects thermal resistance is porosity.

The correct answer is 7.5. This is because the question asks for the largest aggregate with a grain size, and 7.5 is the largest number among the options provided.

The question asks which material will have the highest strength. The passage states that all four materials have the same amount of cement as the main component. Therefore, the material with the highest strength would be the one that is most effective at utilizing the cement to create a strong structure. Since concrete is a type of material that is known for its strength and durability, it is reasonable to assume that it would have the highest strength among the four options.

The correct answer is "אינדיקציה לעבידות של הבטון" which translates to "indication of the durability of concrete". This suggests that measuring the slump in the concrete test is a way to assess its workability and durability. The slump test is commonly used in construction to determine the consistency and quality of fresh concrete before it sets. By measuring the slump, construction professionals can ensure that the concrete has the desired workability and will perform well in its intended application.

The strength of the wood in compression to its width is smaller than twice its strength in compression to its length.

The water requirement of concrete prepared with 25mm aggregate size will be lower than that of concrete prepared with 14mm aggregate size, assuming all other material proportions and properties are the same. This is because larger aggregate size requires less water to achieve the desired workability and strength of the concrete mixture.

The correct answer is "0.5 מ''ק של הבטון שווה 500 ליטר". This is because the question is asking for the correct statement about the volume of concrete. The statement "0.5 מ''ק של הבטון שווה 500 ליטר" states that 0.5 cubic meters of concrete is equal to 500 liters, which is a correct conversion.

The correct answer is "גדול יותר מפי שתיים" which means "greater than twice". This suggests that the strength of the wood in compression along its fibers is more than twice its strength in compression across its fibers.

When cement is burned to death, it undergoes a late hydration process that causes an increase in volume and cracking of the hardened mortar. This is because the burning process affects the chemical composition of the cement, leading to the formation of unstable compounds that expand and cause cracks in the hardened mortar. As a result, the burned cement is not suitable for use as it weakens the structure and can lead to further damage.

The correct answer is "אליט". אליט (alite) is one of the main components of Portland cement. It is a calcium silicate compound that contributes to the strength and durability of the cement.

The correct answer is "עד 5%". This means that the amount of gypsum in cement is limited to a maximum of 5%. This limitation is likely in place to ensure that the cement maintains its desired properties and performance. Exceeding this limit may negatively impact the strength, durability, and other characteristics of the cement.

During cold weather conditions, the use of an air entraining admixture with an additional 5% is recommended. This is because the air entraining admixture helps to improve the workability and freeze-thaw resistance of the concrete mixture. The added air entrainment creates tiny air bubbles in the concrete, which allows for expansion and contraction without causing cracks or damage to the structure. Therefore, using an air entraining admixture with an additional 5% helps to ensure the durability and performance of the concrete in cold temperatures.

The correct answer is 70X70 because it is the only option with a size smaller than the maximum grain size of 14.0 mm.

The addition of an air entrainer with a 5% dosage is expected to result in the lowest compressive strength. Air entrainers are additives that create tiny air bubbles in the concrete mix, which improves workability and freeze-thaw resistance but reduces the overall strength of the concrete. Therefore, adding an air entrainer with a 5% dosage would likely result in the lowest compressive strength compared to the other options.

The material with the highest volumetric weight among the given options is steel.

The aggregate with the highest values in all categories is option ג.

The correct answer is "משך הזמן שבו ניתן לצקת את הבטון." This means the duration of time in which the concrete can be poured. This is an important factor to consider in construction projects as it determines the time frame in which the concrete needs to be poured before it becomes unusable. It is crucial to complete the pouring process within this time frame to ensure the quality and strength of the concrete.

The consumption of water for concrete prepared from aggregate with a size of 9.5 mm will be higher than that of concrete prepared from aggregate with a size of 14 mm, assuming they have identical properties and strength.

The 150X150 pattern is the correct choice for casting concrete cubes because it has a larger size compared to the maximum grain size of the concrete mixture, which is 37.5 mm. This ensures that the cubes will have enough space for the concrete to flow and settle properly during the casting process.

The correct answer is "משך הזמן שבו ניתן לצקת את הבטון" (the duration in which the concrete can be poured). This answer is supported by the context of the question, which is discussing different measurements related to concrete. Out of the given options, the duration in which the concrete can be poured is the most relevant measurement in this context.

The correct answer is "מזרז התקשרות" because it is expected to provide the highest compressive strength at 1 day. A concrete admixture with high early strength development properties, such as a set accelerator, can enhance the early strength of the concrete mixture, allowing it to reach higher compressive strength at an early age. This is particularly useful in situations where early strength is required, such as in construction projects with tight schedules or when quick formwork removal is desired.

The aggregate with the highest absorption percentage, highest compaction percentage, highest length percentage, highest powder content percentage, and highest clay ball percentage is option ג.

The specific gravity of an aggregate is the ratio of its density to the density of water. In this question, the aggregate weighs 5.600 grams in dry condition, 3.115 grams in water, and 4.725 grams in dry condition in an oven. To find the specific gravity, we need to calculate the volume of the aggregate. The volume of the aggregate in dry condition is (5.600 - 4.725) grams, and the volume of water displaced is (5.600 - 3.115) grams. Dividing the volume of the aggregate by the volume of water displaced gives us a specific gravity of 2.25 grams/cubic centimeter (g/cm³).

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The given information states that both concrete mixtures have the same cement and water content. However, it also mentions that concrete B contains an air entrainment additive. Air entrainment additives are commonly used in concrete to improve its durability and resistance to freeze-thaw cycles. Therefore, it can be inferred that concrete B, which contains the air entrainment additive, is expected to be stronger than concrete A.

The correct answer is "גורמת להצטמקות גדולה יותר של החלק העליון יחסית לתחתון" (causes greater shrinkage of the upper part relative to the lower part). This is because the sentence states that the concrete undergoes shrinkage during the industrial foam casting process. Shrinkage refers to the decrease in volume or size of a material, and in this case, it specifically refers to the upper part of the concrete. The word "יחסית" (relative) indicates that the shrinkage of the upper part is greater compared to the lower part.

The material that is most suitable for withstanding pressure is steel.

The correct answer is "התפרקות אבן גיר" which translates to "Decomposition of limestone." Limestone is a common material used in cement production, and at temperatures of 1000-1200 degrees Celsius, it undergoes a process called calcination. During calcination, limestone decomposes into lime (calcium oxide) and carbon dioxide. This lime is then further processed to produce cement. Therefore, the decomposition of limestone is a crucial step in the production of cement at high temperatures.

The correct answer is "Adding water and converting molecules of CaO to molecules of Ca(OH)2." This is because the process of extinguishing lime involves adding water to the substance to maintain it in a moist state for proper hydration. This hydration process converts the molecules of CaO (calcium oxide) into molecules of Ca(OH)2 (calcium hydroxide).

The specific gravity of a substance is the ratio of its density to the density of a reference substance, usually water. In this case, the substance weighs 5.750 grams in dry condition and 3.215 grams in water. The weight in the oven is 4.735 grams. To find the specific gravity, we divide the weight in dry condition by the weight in water, which gives us 5.750/3.215 = 1.785. Since the weight in the oven is less than the weight in dry condition, the specific gravity is greater than 1. Therefore, the correct answer is 2.27 grams/cm³, which is the only option greater than 1.

The correct answer is "אבן גיר וחרסית" (limestone and clay). These materials are commonly used to produce clinker, which is a key ingredient in the production of cement. Limestone provides the necessary calcium, while clay provides the necessary silica, alumina, and iron oxide. These materials are mixed together and heated in a kiln to produce clinker, which is then ground into a fine powder to make cement.

The correct answer is "הזמן שבו ניתן לשנות את צורתו של הצמנט ללא יצרית סדקים" which translates to "the time in which the cement can be shaped without creating cracks". This answer refers to the setting time of the cement, which is the time it takes for the cement to harden and become solid. During this time, the cement can be molded and shaped without causing any cracks or damage to its structure.

All of the answers are correct. The factors that contribute to concrete segregation include prolonged compaction of the concrete, excess water in the fresh concrete mixture, and incorrect proportioning of the components in the concrete mixture.

Increasing the fineness of grinding (טחינה דקה יותר) can shorten the setting time of cement. When cement is finely ground, it increases the surface area available for chemical reactions to occur, resulting in faster hydration and setting. This means that the cement will harden and set more quickly, reducing the overall time required for the cement to fully cure and become solid.

The correct answer is "התאמת התכונות לתנאי עבודה" which translates to "matching the properties to the working conditions". This explanation suggests that עבידות refers to the process of adjusting or adapting the properties of the concrete to suit the specific working conditions or requirements.

The correct answer is "מכשיר בליין" (line device). This device is used to determine a specific area of cement. It is commonly used in construction and masonry work to ensure accuracy and precision in measuring and marking specific areas for cement application. The line device helps in achieving straight and level lines, which are crucial for the proper installation and alignment of cement.

The given material has a specific weight of 2300 kg/m³. Among the given options, concrete is the most suitable material because it has a relatively high density compared to wood, asphalt, steel, and gypsum. Concrete is commonly used in construction due to its strength and durability, making it a suitable choice for applications where weight is a factor.

The properties of fresh concrete are important because they determine its suitability for casting. Only when the concrete is fresh can its properties be measured, which will affect its strength. The properties of fresh concrete are the most important parameter in terms of the price of concrete. The properties of fresh concrete also determine the properties of hardened concrete.

The correct answer explains the process of determining the content of the powder by weighing the aggregate in a dry state, washing it over a sieve with a mesh size of 75 microns, drying it, and weighing it again. The difference in weight between the initial and final weighings represents the content of the powder.

The correct answer is "משך הזמן שלוקח לבטון להצטופף בריטוט" (the time it takes for concrete to set). This is because in the given context of VB testing for concrete, measuring the time it takes for concrete to set is an important factor. This information helps determine the strength and durability of the concrete, as well as its suitability for different applications.

The correct answer is ד because it has the lowest values for all the criteria mentioned. It has the lowest version rate (23%), lowest absorption rate (0.5%), lowest compaction rate (10%), lowest dust content (20%), and lowest clay balls content (0.2%). Therefore, ד is the best aggregate option based on the given criteria.

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The correct answer is "מחט לה-שטליה" because a needle for the hydration test is used to check for the presence of free lime in cement. The needle is inserted into the cement sample, and if it encounters resistance, it indicates the presence of free lime. The other options mentioned, such as a bottle for hydration or a device like Vickers or Blaine, are not suitable for indicating the presence of free lime in cement.

The given question asks for the pattern to be used for casting fresh concrete cubes. The maximum grain size of the concrete mixture is 25 mm. The answer "100X100" is the most suitable pattern size as it allows for the proper filling and compacting of the concrete mixture within the mold, ensuring the desired strength and integrity of the cubes. The other pattern sizes mentioned may not provide enough space for the concrete to properly flow and settle, leading to potential issues with the quality of the cubes.

The correct answer is 70X70 because the maximum grain size of the concrete mixture is 19 mm. Therefore, the mold size should be smaller than the maximum grain size to ensure that the concrete mixture can be properly poured and compacted into the mold. The 70X70 mold size is smaller than the maximum grain size, making it suitable for casting fresh concrete cubes.

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The possible cause for the concrete mixture to be very thick (low slump) compared to the other mixture with the same water/cement ratio and aggregates is a high percentage of dust.

The given correct answer, 346.54 KN, is the maximum load in the standard elastic modulus test. This is determined by the compressive strength of 65.4MPa at the age of 28 days.