Concrete Exam 1 Latest 2023 with complete solution
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Concrete
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Concrete
Concrete Exam 1 Latest 2023 with complete solution
What is the significance of the microstructure of a material? How do you define microstructure?
· Microstructure is the type, amount, size, shape, and distribution of phases present in a solid. Microstructure of a material is significant becau...
concrete exam 1 latest 2023 with complete solution what is the significance of the microstructure of a material how do you define microstructure · microstructure is the type
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Concrete Exam 1 Latest 2023 with complete solution
What is the significance of the microstructure of a material? How do you define
microstructure?
· 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?
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
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
15-20% of the solid volume. Favors the formation of ettringite, early in the hydration
process. Ettringite then transforms into monosulfate hydrate.
Calcium Hydroxide
· 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.
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?
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.
· 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.
· 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.
· 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.
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.
· 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 between the aggregates and hydrated cement paste.
When concrete is exposed to fire, why the elastic modulus shows a relatively higher
drop than the compressive strength
· Microcracking which develops in the ITZ reduces the transfer of stresses between the
mortar matrix and aggregate resulting in a larger decrease in elasticity than
compressive strength.
1. Why is strength the property most valued in concrete by designers and quality
control engineers?
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