CGN3501C Practice Test

Below is a set of answers to the review questions that uses the information provided in your Test 1 Study Guide. (Note that many of these topics can be discussed in several acceptable ways. The answers below summarize key points from your guide.)

UNITS AND PREFIXES

SI and American Units
- SI (International System) Units:
  - Length: meter (m)
  - Mass: kilogram (kg)
  - Force: newton (N)
    (1 N is the force needed to accelerate 1 kg at 1 m/s²)
  - Pressure: pascal (Pa)
    (1 Pa = 1 N/m²)
  - Energy: joule (J)
    (1 J = 1 N·m)
- American (Old English or U.S. Customary) Units:
  - Length: inch (in) and foot (ft)
    (1 in. = 2.54 cm)
  - Mass: pound mass (lbm)
    (1 lbm ≈ 454 g)
  - Force: pound force (lbf)
    (1 lbf ≈ 4.45 N)
  - Pressure: pounds per square inch (psi)
  - Energy: foot–pound (ft–lb)
Standardized Metric Prefixes
Exponent
Factor
Prefix
Symbol
10⁹
1,000,000,000
giga
G
10⁶
1,000,000
mega
M
10³
1,000
kilo
k
10⁻²
0.01
centi
c
10⁻³
0.001
milli
m
10⁻⁶
0.000001
micro
µ
10⁻⁹
0.000000001
nano
n
Conversions
- Mass:
  1 lbm = 454 g
- Length:
  1 in. = 2.54 cm
- Force:
  1 lbf = 4.45 N
Density of Water
- In SI units:
  1 g/cm³ = 1 Mg/m³ = 1000 kg/m³
- In American units:
  Approximately 62.4 lbm/ft³
Converting American Units to SI (and vice versa)
While your guide lists key conversion factors, here are examples:
- Density:
  Since water is 62.4 lbm/ft³ and also 1000 kg/m³, you can use these as base values. (Remember that 1 lbm ≈ 0.454 kg and 1 ft ≈ 0.3048 m for other conversions.)
- Pressure:
  1 psi is known to equal about 6895 Pa.
- Energy:
  1 ft–lb is approximately 1.36 J.
Use these factors as needed for converting any given values between systems.

Exponent	Factor	Prefix	Symbol
10⁹	1,000,000,000	giga	G
10⁶	1,000,000	mega	M
10³	1,000	kilo	k
10⁻²	0.01	centi	c
10⁻³	0.001	milli	m
10⁻⁶	0.000001	micro	µ
10⁻⁹	0.000000001	nano	n

AGGREGATES FOR PORTLAND CEMENT CONCRETE

Types of Rock
- Igneous Rocks:
  - Formation: Formed from the cooling and solidification of magma.
  - Intrusive Igneous:
    Formed by slow cooling beneath the Earth’s surface, resulting in coarse, crystalline minerals (e.g., granite or trap rock).
  - Extrusive Igneous:
    Formed by rapid cooling at or near the surface, yielding a finer grain or even glassy structure (e.g., basalt or perlite).
- Sedimentary Rocks:
  - Formation: Formed by the weathering, disintegration, and deposition of existing rock material; later compacted into rock (e.g., limestone, sandstone, shale).
    For example, Florida limestone is a sedimentary rock that is lighter in color, lower in density, and has higher moisture content.
- Metamorphic Rocks:
  - Formation: Formed when igneous or sedimentary rocks are transformed by heat and pressure; they tend to be harder and denser (e.g., marble, slate).
Examples of Artificial (Synthetic) Aggregates
- Blast Furnace Slag: A waste by–product from iron and steel production.
- Expanded Clay or Expanded Shale: Made by heating clay or shale until it softens, then cooling rapidly to “trap” air and create lightweight particles.
Definitions of Coarse and Fine Aggregates
- Coarse Aggregate:
  Particles retained on the No. 4 sieve (approximately 4.75 mm or 3/16 in in size).
- Fine Aggregate:
  Particles that pass the No. 4 sieve (typically natural sand).
Functions of Aggregate in Concrete
- Economy:
  Aggregates are less expensive than Portland cement and serve as a space filler.
- Strength:
  They contribute significantly to the strength of the hardened concrete.
- Dimensional Stability:
  Help to reduce shrinkage and expansion by providing a stable, interlocking structure.
- Workability:
  Proper gradation (size distribution) enhances the packing of particles, which in turn reduces voids and the amount of cement paste required.
Aggregate Tests – Purpose, Reporting, Meaning, and Typical Thresholds
Here are brief descriptions of the tests noted in your study guide:
1. L.A. Abrasion Test:
  - Purpose: To determine the hardness or resistance to abrasion of an aggregate.
  - Reporting: As percent weight loss after a specified number of drum rotations.
  - Meaning: A lower percentage indicates a tougher, more durable aggregate.
  - FDOT Threshold: Loss should be less than 45%.
2. Soundness Test:
  - Purpose: To measure resistance to weathering (using cycles of soaking in a sulfate solution and oven drying).
  - Reporting: As percent weight loss after testing cycles.
  - Meaning: Lower loss means higher durability.
  - FDOT Threshold: Loss should be less than 12%.
3. Sieve Analysis:
  - Purpose: To determine the gradation (particle-size distribution) of aggregates.
  - Reporting: As cumulative percent passing through a series of standard sieves.
  - Meaning: Helps determine whether the aggregate is well–graded, gap–graded, or uniform.
4. Potential Alkali Reactivity (Mortar Bar Test):
  - Purpose: To assess the potential of aggregates (especially reactive silica) to react with alkalis in cement.
  - Reporting: As expansion (usually in percent) of mortar bars over time.
  - Meaning: Excessive expansion indicates potential for deleterious alkali–aggregate reactions.
5. Organic Impurity Test (for Sand):
  - Purpose: To check for harmful organic materials that can interfere with cement bonding.
  - Reporting: Often as a “strength ratio” after testing; a ratio greater than 95% is desired.
6. Effect of Organic Impurity of Fine Aggregate on Strength of Mortar:
  - Purpose: To determine how organic impurities affect the strength of mortar.
  - Reporting/Meaning: A significant drop in mortar strength indicates that the level of organic impurity is injurious.
7. Materials Finer than No. 200 Sieve:
  - Purpose: To quantify the very fine particles (fines) present in the aggregate.
  - Reporting: As a percentage passing the No. 200 sieve (75 micrometers).
  - Typical Limits: Usually a maximum of 1% for coarse aggregates and 3–5% for fine aggregates.
8. Clay Lumps and Friable Particles in Aggregate:
  - Purpose: To identify the presence of clay lumps or friable (easily broken) particles that can weaken concrete.
  - Reporting: As a percentage by weight.
  - Typical Limits: About 3% for fine aggregate and between 2–10% for coarse aggregate.
9. Specific Gravity Test:
  - Purpose: To determine the specific gravity (density relative to water) of the aggregate.
  - Reporting: As a unitless numerical value. Typical values for natural aggregates are about 2.6–2.8.
10. Absorption and Surface Moisture Test:
  - Purpose: To measure how much water an aggregate absorbs, which affects mix water content.
  - Reporting: As the percent increase in weight from dry to saturated–surface–dry (SSD) condition.
11. Bulk Unit Weight Test:
  - Purpose: To determine the weight per unit volume (including voids) of aggregate.
  - Reporting: In lb/ft³ (or pcf) in the U.S. system. Typical natural aggregates have a unit weight of about 95–105 pcf.
Standard Sieve Sizes
The study guide lists the following standard sieve openings (approximately):
- 6″, 3″, 1.5″, 3/4″, 3/8″, then No. 4, No. 8, No. 16, No. 30, No. 50, No. 100, and finally No. 200.
(Note: Each successive sieve is about half the size of the preceding standard sieve.)
Fineness Modulus (FM) of Fine Aggregate
- Calculation:
  FM is defined as the sum of the cumulative percentages (by weight) retained on the following standard sieves: 6″, 3″, 1.5″, 3/4″, 3/8″, No. 4, No. 8, No. 16, No. 30, No. 50, and No. 100; then divided by 100.
  (Nonstandard sieves and those larger than No. 100 are excluded.)
- Interpretation:
  - A smaller FM indicates a finer aggregate.
  - A larger FM indicates a coarser aggregate.
- Typical Range:
  For fine aggregates used in concrete, FM typically ranges roughly from about 2.3 (very fine) to 3.1 (very coarse).
Maximum Density Curves
- Fuller’s Maximum Density Curve:
  Given by the equation
  P=100(dD)0.5P = 100\left(\frac{d}{D}\right)^{0.5}
  where PP is the percent passing at sieve size dd and DD is the maximum aggregate size. This curve represents an ideal dense (well–graded) aggregate gradation.
- FHWA Maximum Density Curve:
  Similar in concept but uses an exponent of 0.45 instead of 0.5:
  P=100(dD)0.45P = 100\left(\frac{d}{D}\right)^{0.45}
  When the sieve sizes are plotted raised to the 0.45 power, the curve appears as a straight line, which can simplify comparison with actual gradations.
Aggregate Size Terminology
- (A) Maximum Aggregate Size:
  The smallest sieve through which 100% of the aggregate passes.
- (B) Nominal Maximum Aggregate Size:
  The smallest sieve that the major portion of the aggregate (typically between 85% and 95%) passes. In many specifications (e.g., Superpave), it is defined as one size larger than the first sieve that retains more than 10% of the aggregate.
Gradation Types
- Uniform Gradation:
  Aggregates that are almost all the same size. On a gradation chart, the curve shows a sharp spike.
- Gap-Graded:
  Aggregates in which certain intermediate sizes are missing. The gradation curve has “gaps” or sudden jumps.
- Well-Graded (Dense-Graded):
  A continuous and well–distributed range of sizes that leads to low air voids and high density when compacted.
- Open-Graded Aggregates:
  Typically, both uniform and gap-graded aggregates are considered open-graded because they do not pack as densely (resulting in high air voids), whereas well-graded aggregates are dense.
Gradation Chart with Sieve Sizes Raised to 0.45
When sieve sizes are plotted on the horizontal axis after being raised to the 0.45 power, the FHWA maximum density gradation (which follows P=100(d/D)0.45P = 100(d/D)^{0.45}) appears as a straight line. This linear presentation can help in comparing an aggregate’s gradation against the ideal.
Computations for Specific Gravity and Absorption
- (A) Dry Bulk Specific Gravity:
  For impermeable aggregates, it is calculated by measuring the dry weight in air and the volume (via water displacement). For permeable aggregates, the test is adjusted.
- (B) SSD (Saturated Surface–Dry) Bulk Specific Gravity:
  Measured by saturating the aggregate (so the pores are filled) but removing excess surface water, then using the weight difference between air and water.
- (C) Absorption:
  Computed as
  % Absorption=(SSD Weight)−(Dry Weight)(Dry Weight)×100%.\%\,\text{Absorption} = \frac{\text{(SSD Weight)} - \text{(Dry Weight)}}{\text{(Dry Weight)}} \times 100\%.
Bulk Density vs. Bulk Unit Weight
- Bulk Density:
  The mass per unit volume of the aggregate (including the voids between particles); typically expressed in kg/m³.
- Bulk Unit Weight:
  Essentially the weight per unit volume, often expressed in pounds per cubic foot (pcf). (Remember that weight is mass times the acceleration due to gravity.)
In practice, these terms are closely related but may be distinguished based on whether mass or force units are used.
Reducing Segregation of Aggregate
To reduce segregation during handling and storage:
- Minimize Drop Heights:
  Use chutes, conveyors, or other methods that reduce free-fall.
- Control Moisture Content:
  Proper moisture can help particles stick together.
- Use Appropriate Storage Equipment:
  Hoppers and bins designed for gentle flow can help maintain a uniform mix.
- Avoid Excessively Wide Particle-Size Ranges:
  Too wide a range can lead to separation of fine and coarse particles.
Desirable Aggregate Properties and Associated Tests
Desirable Properties:
- Hard, strong, and durable.
- Free of injurious organic impurities.
- Low alkali reactivity with cement.
- Proper gradation for good workability and optimum packing.
- Low water absorption (or well characterized so mix designs can be adjusted).
Tests to Ensure Quality:
- L.A. Abrasion and Soundness Tests: For durability.
- Sieve Analysis (including Fineness Modulus): To verify gradation.
- Organic Impurity Test: To check for harmful organics in sand.
- Specific Gravity and Absorption Tests: To determine density and water demand.
- Tests for Clay Lumps/Friable Particles and for Lightweight Particles: To ensure the aggregate’s integrity and suitability.
Use of Florida Limestone as Aggregate
Florida’s indigenous limestone is more porous and has higher water absorption (3–10%). This means:
- Mix Adjustments:
  The mix design must account for extra water absorbed by the aggregate.
- Strength Limitations:
  Generally acceptable for normal concrete applications (e.g., housing, pavements, low-rise buildings) with compressive strengths up to about 6000 psi.
- Limitations:
  It is not recommended for high-rise buildings or long–span bridges where higher strength and lower absorption are critical.
Effects of Particle Shape and Surface Texture
- Fresh Concrete:
  - Rounded Aggregates: Provide higher workability (greater slump) but may reduce the bond with cement paste.
  - Angular or Rough Aggregates: Tend to reduce workability (lower slump) but improve cohesiveness.
- Hardened Concrete:
  - Angular, rough aggregates create a better mechanical interlock with cement paste, contributing to higher strength and durability.
  - Excessively rounded particles might result in a weaker interfacial bond.
Typical Bulk Unit Weights
- (A) Normal Natural Aggregates: Approximately 95 to 105 pounds per cubic foot (pcf).
- (B) Lightweight Aggregates: Generally less than 70 pcf. (Examples include pumice, expanded clays, or expanded shale.)
- (C) Heavyweight Aggregates: Typically over 130 pcf. (Used for specialized applications like radiation shielding.)
- (D) Blast-Furnace Slag: Usually in the range of 70 to 85 pcf.
Recycled Concrete as Aggregate
- Properties of Recycled Concrete Aggregates:
  They usually have a higher water absorption and lower specific gravity because of residual cement paste, and may also be more angular.
- Properties of Concrete Made with Recycled Aggregates:
  The resulting concrete often shows lower strength and durability compared to concrete made with natural aggregates unless mix designs are carefully adjusted.