Concrete- Midterm Exam 1
List 5 reasons to use chemical admixtures.
Adjust setting time or hardening, Reduce water demand, Increase workability, Intentionally entrain air, Adjust other fresh or hardened concrete properties.
List the materials used in concrete by absolute volume from large to small.
Aggregates: 60-75% Water: 14-21% Cement: 7-15% Air: up to 8%
What is the typical range of absorption rate of the recycled concrete?
3%-10%
Seawater containing up to ______ of dissolved salts is generally suitable as mixing water for concrete not containing reinforcing steel.
35,000 ppm
Match four primary cement phases with chemical formulae
3CaO• SiO2 = C3S = Tricalcium silicate 2CaO• SiO2 = C2S = Dicalcium silicate 3CaO•Al2O3 = C3A = Tricalcium aluminate 4CaO•Al2O3 •Fe2O3 = C4AF = Tetracalcium aluminoferrite
What are the typical amounts of aggregates in concrete by volume?
60% to 75% of the concrete volume
What is the approximate density of water in pcf (lb/ft3)?
62.4 pcf (lb/ft3) or 1000 kg/m3
What is the typical range of the bulk density of normal aggregates?
75 lb/ft3 to 110 lb/ft3
What is the simple requirement of water for concrete that does not need any testing or qualification?
Almost any potable water can be used as mixing water in concrete
What is the full name of "ACI"?
American Concrete Institute
What is the full name of "ASTM"?
American Society for Testing and Materials
What is the final product from the intense heating of raw materials in the kiln?
Clinker
Define Concrete
A mixture of aggregates, usually sand, and either gravel or crushed stone, held together by a binder of cementitious paste (cement, water, supplement cementitious materials, and chemicals)
Flash set or quick set
A rapid and early loss of workability in paste, mortar, or concrete Flash set cannot be dispelled, nor can the plasticity be regained by further mixing without the addition of water.
False Set
A significant loss of plasticity without the evolution of much heat shortly after mixing. Additional mixing without added water breaks up these crystals to restore workability
Which of these is not correct as a result of reduced water content and water-cement ratio in concrete?
The reduced water content and water-cement ratio can produce concretes with: (1) ultimate compressive strengths in excess of 70MPa (10,000 psi), (2) increased early strength gain, (3) reduced chloride-ion penetration, and (4) other beneficial properties associated with low water-cement ratio concrete.
What are two primary mechanisms that improves the resistance of concrete to sulfate or seawater attack?
This is done primarily by reducing permeability and by reducing the amount of reactive elements needed for expansive sulfate reactions.
Why are concrete pavements frequently textured?
To improve traction and reduce hydroplaning
What is the largest amount phase in the clinker?
Tricalcium silicate (Alite)
Why did Joseph Aspdin name his patented product as "portland cement"
When set, Aspdin's product resembled the color of the natural limestone quarried on the Isle of Portland in the English Channel.
What is the type of admixture that increases workability without increasing the water content of concrete?
a water-reducing admixture
How much water is required to complete hydration?
about 40%
How much water is saved when using normal (conventional) water reducers?
about 5% to 10%
what is the average particle size of portland cement?
around 15 microns
When does the cement hydration start?
as soon as the cement comes in contact with water.
What is the name of the shown problem in the fresh concrete mix?
bleeding
What is the primary cementitious binder in hardened portland cement concrete? Provide the full name.
calcium silicate hydrate (C-S-H)
What type of fly ash has both pozzolanic and cementitious properties?
class c
Why is seawater not suitable for use in production of concrete with steel reinforcement and prestressed concrete?
due to the risk of corrosion of the reinforcement
What is the name of a crystalline deposit that may develop on the surfaces of concrete/masonry because of moisture and salt, as shown in the picture?
efflorescence
SCMs generally require an increase in the amount of air-entraining admixture necessary due to the increase in ______of the cementitious materials content.
fineness
What is the most widely used SCM in concrete?
fly ash
Which SCM has the most beneficial effect on water demand?
fly ash (Class C and F)
The ______ are areas that have higher ambient air temperatures as compared to their surrounding areas.
heat islands
Recycled-concrete aggregate generally has a______and______than conventional aggregate.
higher absorption and lower specific gravity
Which of these is not expected when increasing entrained air content on concrete properties?
increase permeability
Provide a brief explanation of the given cement type. Type IT (P30)(S15)
indicates a ternary blended cement with 30% pozzolan and 15% slag.
Explain two different setting times
initial set: the time that elapses from the moment water is added until the paste ceases to be fluid and plastic final set: the time required for the paste to acquire a certain degree of hardness
The majority of supplementary cementing materials typically have a ______ heat of hydration than portland cement.
lower
Calculate the bulk density of the given condition in pcf. 60 lb of cement in 5 gallon container
mass/volume= 60lb/5gallon x 1gallon/0.1337= 89.8pcf
Where is recycled-concrete aggregate (RCA) primarily used?
pavement reconstruction
Slag cement is the glassy material formed from molten slag produced in blast furnaces as a byproduct from
production of iron used in steel making.
What is the single detrimental effect of an increase in air content in concrete?
results in a decrease in the strength of the concrete
The use of SCMs will generally______ the setting time of concrete.
retard/increase
What are two primary concerns with aggregates obtained from seabeds?
seashells, salt
What is the other term for relative density?
specific gravity
High-range water reducers, also known as ______produce flowing concrete with slumps greater than 6 in (150 mm)
superplasticizers
List 4 factors that the freeze-thaw resistance of concrete is dependent.
the air void system of the paste, the strength of the concrete, the water-to-cementitious materials ratio, and the quality of aggregate relative to its freeze-thaw resistance.
Fly ash is a byproduct of
the combustion of pulverized coal in electric power generating plants.
Silica fume is the ultrafine non-crystalline silica produced in electric-arc furnaces as an industrial byproduct of
the production of silicon metals and ferrosilicon alloys.
What is the performance requirements for questionable water sources in terms of setting time?
the setting time should not be accelerated by more than 60 minutes or retarded by more than 90 minutes as compared to the control batch
List 3 SCMs that are good for increasing workability
the use of fly ash, slag cement, and calcined clay and shale increase workability.
Almost any potable water can be used as mixing water in concrete
true
What is the appropriate type of portland cements when the dissolved sulfate level in water is about 1,000 ppm?
type II
If the lower early-age strength is observed with using some SCMs, what can be done to compensate?
using a lower w/cm mixture
What do "w/c" stand for?
water to cement ratio
What is the name of SCM for the given picture?
Fly ash
What are two SCMs often used in the production of high-strength concrete?
Fly ash, silica fume
What is hydration of cement?
Hydration is a chemical reaction between cement and water to form cementitious hydration products.
What is the main purpose of using air-entraining portland cements?
Improves resistance to freezing and thawing
What is the gap-graded aggregate?
In gap-graded aggregates certain particle sizes are intentionally omitted
Which of these statements is not correct regarding the relationship between SCMs and bleeding?
Increasing the SCM content typically increases bleeding
What is the typical range of compressive strength of general-use concrete? Provide the answer in psi unit.
Lower bound: 3000 psi. Upper bound: 6000 psi
Which of these is not a typical property of manufactured aggregates?
Non-uniform in size
What is the required amount of particles finer than No. 4 sieve in No. 57 stone per ASTM C33?
Per Table 6-4. Grading Requirements for Coarse Aggregates
Provide a brief explanation of the given cement type. Type MS
Performance based hydraulic cements with moderate sulfate resistance
List two common conditions that increase the reactivity of both hydraulic and pozzolanic reactions.
Pozzolanic reaction: the greater the surface area of the pozzolan, the faster it can react. Like most chemical reactions, the pozzolanic reaction increases with temperature Hydraulic reaction: reactivity increases with the fineness of the slag cement and the glass content of the slag, and with temperature.
Which of these is not the typical effect of the presence of chloride in concrete?
Reducing drying shrinkage
What are the chemist formula and its shorthand of the given name or mineral phase? silicon dioxide (silica)
SiO2, S
Which SCM has the smallest average particle?
Silica Fume
What is the full name of "SRI"?
Solar Reflectance Index
What is the full name of "SCMs"?
Supplementary Cementing Materials or Supplementary Cementitious Materials
How do SCMs improve the resistance to chemical and sulfate attack?
Supplementary cementitious materials often reduce chemical attack by reducing the permeability of concrete. Most supplementary cementing materials can improve the resistance of concrete to sulfate or seawater attack. This is done primarily by reducing permeability and by reducing the amount of reactive elements needed for expansive sulfate reactions.
What is the range of the density (unit weight) of freshly mixed normalweight concrete that is made of normalweight aggregates?
140 lb/ft3 to 150 lb/ft3
What is the typical dosage range of Class F fly ash?
15% to 25%
What can be expected in terms of capillary porosity, strength, and transport properties when water cement ratio (w/c) increases?
1. Increase capillary porosity, 2. Decrease strength, 3. Increase transport properties such as permeability and diffusivity
List 6 advantages of reducing water content in concrete.
1. Increased compressive and flexural strength, 2. Lower permeability and increased watertightness, 3. Increased durability and resistance to weathering, 4. Better bond between concrete and reinforcement, 5. Reduced drying shrinkage and cracking, 6. Less volume change from wetting and drying
What are two major distinctions between false set and flash set in terms of their development process and restoring workability?
1. Loss of plasticity without the evolution of much heat 2. Reversibility of restoring workability without water
What are two major anticipated problems with cement stored in contact with moisture?
1. Sets more slowly, 2. Has less strength than cement that is kept dry
When do hydration and strength gain virtually stop? List 2 conditions
1. When the relative humidity within the concrete drops to about 80%, 2. When the temperature of the concrete drops below freezing
What are required to continue hydration?
1. favorable moisture and temperature conditions exist 2. space for hydration products is available
Compressive strength is influenced by the cement type, specifically:
1. phase composition 2. sulfate content 3. fineness of the cement
What are two factors that are controlled during curing to develop the desired properties of concrete?
1. the maintenance of a satisfactory moisture content 2. temperature so that the desired properties may develop.
ASTM C 94, Standard Specification for Ready-Mixed Concrete (AASHTOM157) requires two conditions when transporting concrete. What are the requirements in terms of time and revolution limits?
1. within 90 minutes after the addition of water to the mixture, 2. within 300 revolutions of the mixing drum
What is the size of the majority of the entrained air voids in normal concrete?
10 μm and 100 μm in diameter.
What is the performance requirements for questionable water sources in terms of compressive strength?
At 7 days, the compressive strength needs to be 90% of the control, according to ASTM C1602 (Table 5.1)
What are the shorthand and name of the given chemist formula? 3CaO•Al2O3
C3A, Tricalcium aluminate
What are the shorthand and name of the given chemist formula? 3CaO•SiO2
C3S, Tricalcium silicate
List two major products that are produced when portland cement hydrates. Provide the full names.
Calcium silicate hydrate (C-S-H) Calcium hydroxide (CH)
What is the primary product that is responsible for the strength and other properties of hydrated cement?
Calcium silicate hydrate = C-S-H = 3CaO•2SiO2•8H2O
List five major components of portland cement.
Calcium, Silica, Iron, Alumina, Sulfate
What are two classes of of fly ash in accordance with ASTM C618?
Class F and Class C.
What is the appropriate type of portland cement per ASTM C150 for cold weather concreting?
Type III
What is the appropriate type of portland cement per ASTM C150 for rapid form removal for precast concrete?
Type III
