CGN3501C Practice Test

Test 1 Complete Study Guide

What are the SI units for length, mass, and force?

meter (m), kilogram (kg), newton (N)

autoclave expansion test

Test for Soundness of Portland Cement (the ability to retain its volume after setting without undue expansion) Limits the maximum autoclave expansion to 0.80%. High percentage means it has expanded too much

What is creep?

continued yielding under sustained stress. May cause reduction of prestress in prestressed concrete structures

consolidation definition

The process of molding concrete in order to eliminate

pockets of entrapped air.

How do we ensure uniformity in batching for a concrete mix?

Batching by weight

How long should we mix a concrete mixture?

Required plant mixing time: 1 to 3 minutes. Lab: 5-10 minutes

different ways concrete can be mixed

Central mixed concrete: mixed completely in a stationary mixer. Shrink-mixed concrete: mixed partially in a stationary mixer; mixing completed in a truck mixer. Truck Mixed: mixed completely in a truck mixer.

different types of transportation/conveying around job site

Mixer Truck, Special Dump Truck, Bucket, Buggy, Belt Conveyor, Pump, Chute

How do we consolidate the fresh concrete that is placed?

(1) Hand rodding or tamping (2) Internal vibrator (3) External vibrator (a) Vibrating Screed (b) Form vibrator - clamped to the outside of the forms.

What are the basic steps in finishing a concrete slab?

Strikeoff or Screeding - Striking off excess concrete to bring the top surface to proper grade

Bullfloating - Leveling to eliminate high and low spots

Floating - to firmly embed the aggregate, to compact the surface and to remove any surface imperfections. To be performed when the concrete is able to sustain foot pressure with only slight indentation

Troweling - Finishing to obtain a smooth, dense surface

Texturing - Finishing to obtain desired surface texture (ex: brooming to obtain slip-resistant surface)

isolation joint

separates a concrete slab from another structural element like a wall or column, allowing independent movement between the two without causing damage

construction joint

a gap created where concrete is poured in stages, allowing for controlled placement and preventing random cracking

control/contraction joint

a weakened plane cut into a concrete slab to direct cracks to specific locations, primarily to manage shrinkage during curing and permit thermal shortening

Materials finer than No. 200 sieve

Desirable to have low percentage of material passing No. 200 sieve

Typical maximum allowable: 1% for coarse aggregate, 3-5% for fine aggregate

Florida limestone characteristics

lighter color, lower density, more moisture, not as strong

Why is it important that concrete be properly cured? What is the basic principle to follow to ensure proper curing of concrete?

To keep the concrete moist and to control the temperature of the concrete. General approaches include preventing loss of moisture, supplying additional moisture, accelerating strength gain (with steam or heating coil).

effects of hot weather on concreting

Increased rate of setting, Slump loss earlier, Increased possibility of plastic cracking, Increased loss of moisture during curing, Difficulties in controlling entrained-air content

measures to take in hot weather concreting

Cool the aggregate and the mixing water. Ice can be used as mixing water. Reduce the time between placement and start of curing. Cover the concrete during curing to reduce evaporation of water. Sprinkle additional water to the concrete during curing. Use tepid water (rather than icy water) to avoid thermal shocks

effects of cold weather on concrete

Lower rate of hydration at low temperature. Freezing of water will damage the concrete

measures to take in cold weather concreting

Use insulated forms. Use Type III cement or an accelerating admixture. Use heaters to keep concrete warm. Use air-entrainment in concrete. Allow longer curing time before stripping forms. Use the Maturity Method to relate curing history of the concrete cylinder to that of the in-place concrete.

is florida limestone a good aggregate?

Ok for normal concrete (ex: housing, pavement, low-rise buildings), but have to adjust for absorption. Good for compressive strengths up to 6000 psi.

Can NOT be used for high-rise buildings, long-spanning bridges

list and explain curing methods

Sprinkling - Continuous sprinkling to keep the surface wet

Wet covering - Covering with moisture retaining fabrics (such as burlap or straw) and intermittent sprinkling

Ponding - Retaining a pond of water by means of earth dikes

Waterproof covers - covering with waterproof paper or plastic sheets, or spraying surface with a curing compound which acts as a water barrier

Steam curing

false set

the stiffening of concrete shortly after mixing, with no generation of heat (due to improper composition of cement, or pre-hydration of cement.) Plasticity can be restored by rigorous remixing. Can be prevented by longer mixing time.

Functions of aggregate in concrete

Economy (used as space filler since it’s less expensive than portland cement). Strength. Reduction in shrinkage and expansion

density of water

1 g/cm3 = 1 Mg/m3 = 62.4 lbm/ft3

What is the bulk unit weight of blast-furnace slag?

70 to 85 pcf

What is blast-furnace slag used for?

Making precast concrete products, such as masonry blocks, where high strength is not required.

what problem might sulfur content in blast-furnace slag cause?

Durability problem. FDOT spec. limits sulfur content to a maximum of 1.5%

1 in-lbf is how many joules?

0.113 J

1 psi is how many kPa?

6.898 kPa

1 inch is what in cm?

2.54 cm

1 lbf is what in N?

4.45 N

Consistency, Slump(in), Type of Work

Dry, 0-1

Stiff, 0.5-2.5, Dams, Large Foundations

Medium, 2-5.5, Pavement, Footings

Wet, 5-8, Thin slabs, Columns

Sloppy, 7-10

nominal maximum size

The smallest sieve which the major portion of the aggregate must pass. It may retain 5% to 15% of the aggregate, depending on the size number of the aggregate. One size larger than the first sieve to retain more than 10%

well-graded or dense-graded

Well distributed in various sizes, resulting in low air voids and high density when compacted

gap-graded

missing a few sizes

Both uniform and gap-graded aggregates are

open-graded (high air voids and low density)

apparent density

Mass of aggregate / (volume of solids + impermeable voids)

true density

Mass of aggregate / volume of solids

ball penetration test purpose

Measures consistency of fresh concrete

Bleeding of fresh concrete

the appearance of water on the surface after a concrete has been placed and compacted

Methods to reduce bleeding

Use air entrainment & reduce water content. Increase the proportion of sand and/or cement in the concrete mix. Use mineral admixtures such as fly ash or ground blast furnace slag

factors affecting strength

Water-cement ratio: Lower ratio = higher strength

Age (or curing time): Strength increases with curing time

Curing Condition: Longer moist curing & higher temperature = higher strength

Type of cement & admixture: affects rate of strength gain & ultimate strength

Strength of aggregate

Moisture content: higher moisture = lower strength

factors affecting workability

Water Content: most important factor affecting consistency. Higher water content, higher slump.

Cement Content: decreasing the cement content = harsher mixture = harder to finish. Increasing = gives better cohesiveness but higher stickiness

Aggregate Size: increasing the maximum aggregate size increases the slump of the fresh concrete

Aggregate angularity and roughness: higher angularity and roughness produce fresh concrete with lower slump

Admixture: water reducing, air-entraining and fly ash admixtures increase the slump of the fresh concrete

Low permeability of concrete is needed to prevent

Disintegration caused by freezing of saturated porous concrete. Dissolving of slowly soluble components in concrete. Chloride intrusion causes corrosion of rebars.

Tensile Strength =

10% of compressive strength

Flexural strength =

15% - 20% of compressive strength

density of normal concrete

2.4 Mg/m3 (150 pcf)

density of concrete made with Florida limestone

2.24 Mg/m3 (140 pcf)

uniform gradation

mostly one size

aggregate soundness test purpose

to measure resistance of an aggregate to weathering through cycles of soaking in sodium or magnesium sulfate and oven drying

soundness definition

durability, resistance to weathering

Test for Potential Alkali Reactivity (Mortar Bar Method) purpose

to determine the potential alkali reactivity of an aggregate. If expansion is greater than 0.05% at 3 months or 0.10% in 6 months, the aggregate is considered to be alkali reactive.

Organic Impurities Test (for sand) purpose

to determine the presence of injurious organic compounds in natural sands. Results in pass/fail

standard sieves

6”, 3”, 1.5”, 3/4”, 3/8”, #4, #8, #16, #30, #50, #100, #200

Loss on Ignition Test

Purpose: To measure the level of pre-hydration of a cement

Limits the maximum at 3.0% for Types I, II, III and V and 2.5% for Type IV Portland cement

Tests for fineness

Units of m2/kg. The greater the fineness, the more rapid the rate of hydration (will react with water faster)

Two commonly used methods are (1) Wagner Turbidimeter test and (2) Blaine Air Permeability test

Type I Portland Cement

For general use. No limits are placed on any of the four principal compounds.

1 lbm is what in grams?

1 lbm = 454 g

How is pressure measured in SI units?

Pascals (Pa), where 1 Pa = 1 N/m².

What is the density of water in SI units?

1 g/cm³ = 1 Mg/m³ = 1000 kg/m³

What are the three types of rocks based on their formation?

Igneous, Sedimentary, Metamorphic

What is the purpose of the L.A. Abrasion Test?

To determine the hardness or resistance to abrasion of an aggregate.

What does a lower percentage indicate in the L.A. Abrasion Test?

tougher, more durable aggregate

What is the maximum ASTM threshold for the Soundness Test of aggregates?

Loss should be less than 12%.

What is the definition of coarse aggregate?

Particles retained on the No. 4 sieve (approximately 4.75 mm or 3/16 in in size)

What type of aggregate passes through the No. 4 sieve?

Fine aggregate.

What is the typical range for the fineness modulus (FM) of fine aggregates?

FM typically ranges from about 2.3 (very fine) to 3.1 (very coarse).

What is the purpose of the Specific Gravity Test?

To determine the specific gravity (density relative to water) of the aggregate.

What does the term 'maximum aggregate size' referrer to?

The smallest sieve through which 100% of the aggregate passes.

What is the main purpose of a control or contraction joint in concrete?

To allow concrete to crack at the joint rather than randomly.

What is the density range for normal natural aggregates?

Approximately 95 to 105 pounds per cubic foot (pcf).

What limit should the loss in the L.A Abrasion Test not exceed according to FDOT thresholds?

Loss should be less than 45%.

What does a significant drop in mortar strength indicate regarding organic impurities?

It indicates that the level of organic impurity is injurious to the strength.

How is bulk density defined?

Mass of aggregate / (volume of solids + voids)

What happens to the workability of concrete when aggregates are rougher and more angular?

produce fresh concrete with lower slump. requires more water for the same workability.

What is a typical application for heavyweight aggregates?

Used for specialized applications like radiation shielding.

What is the definition of fine aggregate?

Particles that pass through the No. 4 sieve (less than 4.75 mm or 3/16 in in size).

What is the purpose of the moisture content test in aggregates?

To determine the amount of water contained in the aggregate so it can be accounted for in mix designs.

What does the term 'maximum density' refer to in concrete mix design?

The highest weight or compactness achieved by a concrete mix under specified conditions.

What is alkali-silica reaction (ASR) in concrete?

A deleterious reaction between alkalis in cement and reactive silica in aggregates, causing expansion and cracking.

What is the definition of slump in concrete testing?

Measure of the consistency or workability of fresh concrete, determined by how far a concrete mix settles in a cone.

What is the effect of excessive moisture in coarse aggregates on concrete?

inaccurate water-cement ratio, affecting concrete strength and workability

What is the purpose of the water absorption test for aggregates?

To measure the porosity of aggregates and provide guidance for mix design adjustments.

What are the typical environmental conditions required for curing concrete?

moist conditions, ideally at temperatures above 50°F (10°C) for at least 7 days.

What is the role of air-entraining agents in concrete?

Create tiny air bubbles in concrete, improving freeze-thaw resistance and workability

How does the grading of aggregates affect concrete?

Proper grading helps achieve optimal packing of aggregates, improving strength and reducing voids.

What is the standard curing time for concrete?

at least 28 days for optimal strength development

What is the significance of using recycled aggregates in concrete?

Can reduce environmental impact and promote sustainable building practices.

How does temperature affect the setting time of concrete?

Higher temperatures accelerate setting time, while lower temperatures can delay it.

What is the difference between hydraulic and non-hydraulic cements?

Hydraulic cement hardens by reacting with water to form a water-resistant product. The presence of air is not required for the hardening process. ex: Portland cement. Nonhydraulic cement reacts with water to form a product which is not stable in water. The hydration product may then react with air to form a water-resistant product. ex: quick lime

What is the effect of using too much water in concrete mix?

Can weaken the concrete by increasing porosity and decreasing strength.

What is the function of reinforcement in concrete?

Provides tensile strength to concrete, which is weak in tension.

What does the term 'curing' refer to in concrete construction?

process of maintaining adequate moisture, temperature, and time to allow the concrete to hydrate properly.

What factors can affect the strength of concrete?

Water Cement Ratio: Lower water/cement ➔ higher strength

Age (or curing time): Strength generally increases with curing time

Curing Condition: Longer moist curing & higher temperature ➔higher strength

Type of cement & admixture: Affects rate of strength gain & ultimate strength

Strength of aggregate: Strength of concrete is limited by the strength of aggregate

Moisture Content of Concrete: higher moisture ➔ lower strength

What is the significance of using fly ash in concrete?

Can enhance workability, reduce required cement content, and improve long-term strength. Early strength is reduced.

What does 'workability' refer to in concrete?

The ease with which concrete can be mixed, placed, and finished.

What is segregation in concrete?

The separation of coarse aggregate from the sand and cement slurry in the mix, leading to inconsistent strength.

What effect does curing temperature have on early strength development?

Higher curing temperatures can accelerate early strength gain, but may also cause rapid evaporation and cracking.

What is the role of pozzolans in concrete?

improve strength and durability, causes lower early strength & lower heat of hydration as compared with normal concrete, permeability is reduced, alkali-aggregate expansion is reduced, ultimate strength can be higher than that of normal concrete. Portland blast-furnace slag cement has a more rapid strength gain than Portland pozzolan cement, water resistance is increased since calcium hydroxide is consumed