Concrete Exam 1 Latest 2024 with complete solution

Concrete Exam 1 Latest 2024 with complete solution What is the significance of the microstructure of a material? How do you define microstructure? - Answer ️️ -· Microstructure is the type, amount, size, shape, and distribution of phases present in a solid. Microstructure of a material is significant because it is now understood that the overall properties of a material come from the microstructure. Properties of a material can be altered by changing the microstructure. Describe some of the unique features of the concrete microstructure that make it difficult to predict the behavior of the material from its microstructure? - Answer ️️ -The two phases of the microstructure are neither homogeneously distributed with respect to each other, nor are they themselves homogeneous. Concrete has many features that make its microstructure, but they are very unpredictable and nonuniform through the structure. Composed of three phases, the aggregates, cement paste, and interfacial transition zone. C-S-H - Answer ️️ -makes up 50-60% of the volumes of solids and it provides the majority of the long-term strength and durability of Portland Cement. It can range from poorly crystalline fibers to reticular network. Has a layer structure with high surface area. Its strength comes from Van Der Waals forces. calcium sulfoaluminates - Answer ️️ -15-20% of the solid volume. Favors the formation of ettringite, early in the hydration process. Ettringite then transforms into monosulfate hydrate. Calcium Hydroxide - Answer ️️ -· 20-25% of the volume of solids. Forms large crystals with a distinctive hexagonal-prism morphology. Can be nondescript to stacks of large plates. Has a lower surface area than C-S-H. How many types of voids are present in a hydrated cement paste? What are their typical dimensions? Discuss the significance of the C-S-H interlayer space with respect to properties of the hydrated cement paste. - Answer ️️ -1. Interlayer Space in the C-S-H- the space can be anywhere from 5 to 25 A which is too small to have an adverse effect of strength and permeability. Leads to creepage 2. Capillary Voids- 10-50 μm in high water cement ratio pastes or 3-5 μm in early hydration. Macropores are voids 50nm and micropores are voids 50nm. Shrinkage and creepage 3. Air voids- spherical can range from 50-200 μm. Adversely affect strength How many types of water are associated with a saturated cement paste? Discuss the significance of each. Why is it desirable to distinguish between the free water in large capillaries and the water held in small capillaries? - Answer ️️ -1. Capillary water- Made up of large voids called free water because its removal does not cause any volume change, while water removal in the small voids causes shrinkage in the system. 2. Adsorbed water- the loss of absorbed water is responsible for the shrinkage of the hydrated cement paste. 3. Interlayer water- A monomolecular water layer between the layer of C-S-H is strongly held by hydrogen bonding. This water is only lost on strong drying. 4. Chemically combined water- Integral part of the microstructure of various hydration products. It is evolved when the hydrates decompose on heating. It is important to distinguish between free water in small and large capillaries because the removal of water in the large voids does not cause any change in volume and the removal of water in small capillaries causes shrinkage. When a saturated cement paste is dried, the loss of water is not directly proportional to the drying shrinkage. Explain why. - Answer ️️ -· There is no shrinkage until the free capillary water is removed. In a hydrating cement paste the relationship between porosity and impermeability is exponential. Explain why. - Answer ️️ -· When hydration begins, the voids in the cement particles begin to fill up with hydration products. When a cement is hydrated the connections between pores become nonexistent. The permeability of a fully hydrated cement paste becomes 10^6 times less than that of a young paste. 9. Draw a typical sketch showing how the microstructure of hydration products in the aggregate- cement paste interfacial transition zone (ITZ) is different from the bulk cement paste. - Answer ️️-· The structure of hydrated cement paste is comprised of regions of dense hydrated cement paste and pores · The microstructure of hydrated cement paste in the vicinity of aggregate particles is different from the bulk cement paste · The interfacial transition zone (ITZ) between coarse aggregate particles and the bulk cement paste is more porous Discuss why the strength of the I ITZ is generally lower than the strength of the bulk hydrated cement paste. Explain why concrete fails in brittle manner in tension but not in compression. - Answer ️️ -A Water film forms around the large aggregate particles, increasing w/c ratio Ettringite and Calcium hydroxide form large crystals due to the increased w/c ratio As hydration continues the crystals grow to fill the voids and increase strength. In tension, the micro-cracking within the ITZ expands until cracks reach the cement matrix and the concrete fails. In compression, strain hardening prevents micro cracks from expanding until tensile stresses occur. Everything else remaining the same, the strength and impermeability of a mortar will decrease as coarse aggregate of increasing size is introduced. Explain why. - Answer ️️ -· With an increase in aggregate size, there is an increase of water-cement ratio in the ITZ, this makes the concrete more permeable and weaker. With a larger aggregate size there a thicker film of water and the ITZ formed under these conditions is more susceptible to cracking when subjected to the influence of tensile stresses induced by differential movements