Reinforced Concrete Principles
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Questions and Answers

What is the main reason for reinforcing concrete?

  • To enhance its compressive strength
  • To increase its strength in tension (correct)
  • To improve its ductility
  • To prevent microcracking

    • What characterizes the initial response of concrete under uniaxial compression loading?

  • Immediate failure due to low tensile strength
  • Instantaneous formation of mortar cracks
  • Linear stress-strain behavior
  • Development of bond cracks due to shrinkage (correct)

    • At what load stress level do localized mortar cracks begin to develop between bond cracks?

  • 75-80% of compressive strength
  • 30-40% of compressive strength
  • 60-70% of compressive strength
  • 50-60% of compressive strength (correct)

    • Which standard is NOT mentioned as a guideline for concrete sample preparation?

    <p>ASTM D75</p> Signup and view all the answers

    What is the standard concrete age considered for compressive strength for design purposes?

    <p>28 days</p> Signup and view all the answers

    What is the primary purpose of structural analysis in the design process?

    <p>To determine the forces and deformations within the structure</p> Signup and view all the answers

    Which of the following best describes plain concrete?

    <p>Concrete with no reinforcement or minimal reinforcement compared to reinforced concrete</p> Signup and view all the answers

    What is an advantage of using structural concrete in construction?

    <p>Low labor skill requirements for assembly and construction</p> Signup and view all the answers

    During the evaluation phase of structural design, what is being checked?

    <p>Compliance of members with strength and serviceability requirements</p> Signup and view all the answers

    Which option is NOT a characteristic of reinforced concrete?

    <p>Has no structural function in a load-bearing system</p> Signup and view all the answers

    What is a disadvantage of structural concrete?

    <p>Requirement for complex formwork</p> Signup and view all the answers

    What was a significant milestone in the historical development of concrete?

    <p>The use of lime mortar mixed with volcanic ash by the Romans</p> Signup and view all the answers

    What is the final step of the structural design process?

    <p>Final decision on the design's optimum status</p> Signup and view all the answers

    What does the strength reduction factor (Φ) symbolize in structural concrete design?

    <p>The reliability of a structural member's performance</p> Signup and view all the answers

    Which is NOT a reason for setting load factors and strength reduction factors in structural design?

    <p>Minimizing construction costs</p> Signup and view all the answers

    Which of the following codes specifically addresses minimum design loads?

    <p>2015 National Structural Code of the Philippines Vol. 1</p> Signup and view all the answers

    What is a key consideration in designing for economy within structural concrete?

    <p>Maximizing formwork reuse</p> Signup and view all the answers

    In terms of construction costs, which of these is typically the largest component in cast-in-place buildings?

    <p>Floor and roof systems</p> Signup and view all the answers

    What design approach can lead to increased overall construction costs?

    <p>Haunched beams and deep spandrel beams for aesthetic value</p> Signup and view all the answers

    Which of the following best defines 'Qu' in the context of structural design?

    <p>The total factored load</p> Signup and view all the answers

    Why should standard column sizes be used throughout a building's design?

    <p>To simplify formwork</p> Signup and view all the answers

    How does the water-cement (w/c) ratio affect concrete compressive strength?

    <p>Lower w/c leads to higher compressive strength</p> Signup and view all the answers

    Which type of cement is used for sites with moderate sulfate exposure?

    <p>Type II (modified)</p> Signup and view all the answers

    What effect do supplementary cementitious materials have on concrete?

    <p>Improve workability in some cases</p> Signup and view all the answers

    Which condition is NOT a factor affecting the compressive strength of concrete?

    <p>Color of the concrete mix</p> Signup and view all the answers

    What is the primary effect of curing conditions on concrete strength?

    <p>It affects moisture and temperature conditions</p> Signup and view all the answers

    What is the significance of the first seven days in the aging of concrete?

    <p>Concrete strength increases significantly during this period</p> Signup and view all the answers

    Which method is used to determine the tensile strength of concrete?

    <p>Both ASTM C78 and ASTM C496</p> Signup and view all the answers

    Which of the following factors can cause lower recorded strength during concrete testing?

    <p>Low strain rates</p> Signup and view all the answers

    What is the typical range of the coefficient of thermal expansion for siliceous aggregates?

    <p>5-7 (10-6)/°F</p> Signup and view all the answers

    What is essential to prevent the corrosion of steel in concrete?

    <p>Maintaining a pH above 11-12</p> Signup and view all the answers

    How does air entrainment assist in concrete structures regarding freeze-thaw cycles?

    <p>It creates microscopic voids to relieve pressures.</p> Signup and view all the answers

    What effect does surface rust on steel reinforcement have before concrete pouring?

    <p>It improves the bond between concrete and steel.</p> Signup and view all the answers

    Which of the following methods can help mitigate against chemical attacks on concrete?

    <p>Utilizing appropriate types of cement</p> Signup and view all the answers

    What may cause the breakdown of concrete during freezing conditions?

    <p>Pressure in water-filled pores</p> Signup and view all the answers

    Which condition can exacerbate the coefficient of thermal expansion in concrete?

    <p>Increased temperature</p> Signup and view all the answers

    Which of the following structures is particularly vulnerable to chemical attack?

    <p>Pavements</p> Signup and view all the answers

    What happens to the modulus of elasticity of concrete as the temperature increases?

    <p>It decreases.</p> Signup and view all the answers

    Which color indicates that concrete has been severely damaged when heated?

    <p>Gray</p> Signup and view all the answers

    What effect do subfreezing temperatures have on moist concrete?

    <p>They significantly increase compressive strength.</p> Signup and view all the answers

    Which grade of hot-rolled deformed bars is the most commonly used?

    <p>Grade 40/300</p> Signup and view all the answers

    What type of reinforcement is defined as 'bars, wires, strands, fibers, or other slender elements' in concrete?

    <p>Steel reinforcement</p> Signup and view all the answers

    Which ASTM specification is associated with low-alloy steel deformed bars for concrete reinforcement?

    <p>ASTM A706</p> Signup and view all the answers

    What is the primary benefit of using hot-rolled deformed bars in concrete?

    <p>Improved bond and anchorage into concrete.</p> Signup and view all the answers

    How does dry concrete react to low temperatures compared to moist concrete?

    <p>It shows less significant impact than moist concrete.</p> Signup and view all the answers

    Study Notes

    Introduction to Structural Concrete Design

    • Course title: CEPRCD30 (Principles of Reinforced Concrete)
    • Instructor: Jerome Z. Tadiosa, CE, MSc
    • Institution: National University – Manila

    Intended Learning Outcomes

    • Describe the structural design process and considerations.
    • Explain the description, development, and classification of structural concrete.
    • Enumerate and describe the design philosophies in structural concrete design, and the relevant codes and standards used.
    • Enumerate and describe the materials used in structural concrete construction.

    Reading Guide

    • Chapters 1-3, Wight (2016)
    • Chapter 1, McCormac & Brown (2016)
    • Sections 1.1-1.2, Chapter 1, Salmon et.al. (2009)

    Lecture Outline

    • Introduction to structural design process
    • Introduction to structural concrete design
    • Materials in structural concrete design

    Structural Design

    • Defined as a mixture of art and science, combining the experienced engineer's intuitive feeling for the behavior of structures, with a sound knowledge of basic engineering principles, to produce a safe, economical structure that will serve its intended purpose.
    • A properly-designed structure should satisfy four criteria:
      • Appropriateness: functionality and aesthetics
      • Economy: optimal benefit-cost ratio, preferably minimum cost
      • Structural adequacy: strength and serviceability requirements
      • Maintainability: minimum maintenance cost and time

    General Design Process

    • Major phases include:
      • Definition of client's needs
      • Development of project concept
      • Design of individual systems (structural analysis and design, utilities and other systems)
    • Structural design is sequential and iterative in nature
    • It follows a series of steps without skipping
    • It involves previous steps, as repeated decisions may result.

    Structural Design Process (Steps)

    1. Planning: setting and finalizing project details
    2. Preliminary structural configuration: initial arrangement of structural members
    3. Establishment of loads: applying loads on structure model based on material, function, and site conditions
    4. Preliminary member selection: initial sizing of structural members
    5. Structural analysis: modeling and analyzing structure to determine forces and deformations
    6. Evaluation: checking individual members against strength and serviceability requirements, as well as client specifications
    7. Redesign: repetition of previous steps depending on the results of the evaluation
    8. Final decision: determining whether the latest iteration of design is optimum

    Structural Concrete

    • Defined as "plain or reinforced concrete in a member that is part of a structural system required to transfer loads along a load path to the ground".
    • Concrete is a mixture of hydraulic cement, aggregates, and water, with or without admixtures, fibers, or other cementitious materials.
    • Plain concrete is a "structural concrete with no reinforcement or with less reinforcement than the minimum amount specified for reinforced concrete in the applicable building code".
    • Reinforced concrete is a "structural concrete reinforced with no less than the minimum amount of prestressing steel or non-prestressed reinforcement as specified in the applicable building code".
    • Reinforced concrete may be classified as either steel-reinforced concrete (using rebars) or prestressed concrete (using tendons).

    Advantages of Structural Concrete

    • High compressive strength
    • Great resistance to fire and water
    • Rigidity
    • Low maintenance
    • Long service life
    • Most economical material for substructures and floor slabs
    • Moldability
    • Cheap cost of components
    • Lower required labor skill requirement

    Disadvantages of Structural Concrete

    • Low tensile strength
    • Formworks requirement
    • Low strength-to-weight ratio
    • Low strength-to-volume ratio
    • Variation of properties due to proportioning and mixing (quality consistency issue)

    Historical Background of Concrete

    • Lime mortar was first used in the Minoan civilization.
    • Romans mixed lime mortar with volcanic ash to create stronger, water-resistant mortar.
    • John Smeaton experimented with mixes of limestone and clay to create water-resistant cement.
    • Joseph Aspdin created Portland cement by heating ground limestone and clay.

    Historical Background of Concrete (Key Figures)

    • W. B. Wilkinson: Patented a reinforced concrete floor system using hollow plaster domes and reinforcement
    • Joseph Lambot: Built a reinforced concrete rowboat and patented his concept, showing reinforced beams and columns with iron bars
    • Thaddeus Hyatt: Experimented with reinforced concrete beams, though his work remained unknown until 1877
    • Joseph Monier: Patented reinforced concrete for tubs, pipes, tanks, plates, bridges, and stairs
    • W. E. Ward: Built the first reinforced concrete house in the United States
    • E. L. Ransome: Patented a twisted steel reinforcing bar and built significant structures using reinforced concrete
    • Coignet and de Tedeskko: Extended Koenen's theories to develop the working-stress design method for reinforced concrete
    • E. Freyssinet: Pioneered the use of high-strength steel wire for prestressing

    Limit States

    • Limit states are the conditions of a structure or a structural member when it becomes unfit for its intended use.
    • Limit states are classified into three basic groups: strength, serviceability, and special limit states.

    Limit States (Specific Types)

    • Strength limit states: involve structural failure or collapse (loss of equilibrium, failure, progressive collapse, formation of plastic mechanism, instability, fatigue)
    • Serviceability limit states: involve disruption of functional use (excessive deformations, excessive cracking, undesirable vibrations)
    • Special limit states: involve damage or failure due to abnormal conditions (extreme calamities, fire, corrosion effects)

    Limit States Design

    • Limit states design involves:
      • Identification of all possible failure modes
      • Determination of acceptable levels of safety
      • Structural design consideration of significant limit states
      • For structural concrete, limit states design is done by using the ultimate strength design method (USD).
      • USD is a design method that requires service loads to be multiplied by load factors and nominal strengths to be multiplied by strength reduction factors.

    Strength Design Methods

    • The basic criterion for strength requirement is that the capacity (or resistance) of a member should be greater than or equal to the demand (or load effects) placed on the said member (i.e., capacity ≥ demand).
    • For structural concrete design using USD: Rn ≥ Qu

    USD Load Combinations

    • Factor load combination using Strength Design or Load and Resistance Factor Design

    Service Load Combinations

    • Basic load combinations using Allowable Stress or Allowable Strength Design

    Structural Safety

    • Reasons for setting load factors and strength reduction factors include:
      • Variability in strength
      • Variability in loadings
      • Consequences of failure

    Codes and Standards for Structural Concrete

    • 2015 National Structural Code of the Philippines (NSCP)
    • Chapter 2: Minimum Design Loads
    • Chapter 4: Structural Concrete
    • ACI 318M-14 (Building Code Requirements for Structural Concrete)
    • ACI 318R-14 (Commentary on Building Code Requirements for Structural Concrete)

    Design for Economy

    • Economy is a major goal in structural design.
    • In cast-in-place building, floor and roof systems make up about 90% of the total cost.
    • Material costs increase with larger column spacing, but formwork reuse reduces costs.
    • Overcomplicating design can increase forming costs and complexity..

    Design for Sustainability

    • Durability and longevity are key for sustainability.
    • Reinforced concrete is valued for its aesthetics, versatility, initial and life-cycle economic benefits, and thermal properties.
    • Sustainable/green construction is a compromise between economic, social, and environmental factors.

    Materials for Structural Concrete Construction

    • Concrete is a composite material (aggregates, cement, water, admixtures)
    • The aggregates make up the bulk of the weight and volume
    • The cement and water mixture binds the aggregates.
    • Admixtures improve concrete properties (strength and workability)
    • Concrete is strong in compression but weak in tension, which requires reinforcement.

    Concrete (Stress-Strain Relationship)

    • The stress-strain relationship of concrete is nonlinear and appears to be somewhat ductile
    • This is due to the development of microcracking
    • Microcracks are internal cracks between 1/8" and 1/2"
    • Classified as bond cracks or mortar cracks
    • Concrete mix design is usually performed using traditional proportions (e.g., DPWH-modified proportions, or ACI 211.1-91 proportions)

    Mechanism of Concrete Failure in Compression

    • Four stages leading to failure under uniaxial compression:
      • Development of no-load bond cracks due to shrinkage
      • Development of bond cracks due to stresses
      • Development of localized mortar cracks
      • Increase in mortar cracks

    Compressive Strength of Concrete

    • Concrete sample preparation and testing are performed according to standards (ASTM C31, ASTM C39)
    • Test cylinders are prepared in two sizes (H/D= 2.0)
    • 6-inch (150 mm) diameter by 12-inch (300 mm) height
    • 4-inch (100 mm) diameter by 8-inch (200 mm) height
    • Standard concrete age is 28 days

    Factors Affecting Concrete Compressive Strength

    • Water-cement ratio (lower w/c generally leads to higher strength)
    • Type of cement (Type I, II, III, IV, V)
    • Supplementary cementitious materials (pozzolans, fly ash)
    • Aggregates (strength, grading, quality, toughness)
    • Mixing water (potable water)
    • Curing conditions (moisture, temperature, duration)

    Tensile Strength of Concrete (Modulus of Rupture)

    • Tensile strength is determined by testing in accordance with ASTM C78 (flexural) or ASTM C496 (splitting)

    Modulus of Rupture

    • Calculation of modulus of rupture (fr) for concrete can vary depending on the type of concrete (lightweight, normal-weight)
    • Calculated by fr = 0.622√f'c (with various values for A depending on the concrete composition based on Table 419.2.4.2.)

    Factors Affecting Tensile Strength

    • Same factors affect compressive and tensile strength.
    • Concrete made with crushed rock has higher tensile strength than concrete made with rounded gravel.
    • Tensile strength develops more quickly than compressive strength.

    Stress-Strain Curve of Concrete in Compression

    • Relationship of stress and strain varies by stress levels
    • Tangent modulus varies
    • Secant modulus varies

    Modulus of Elasticity and Poisson's Ratio of Concrete

    • Modulus of elasticity (Ec) varies based on the concrete composition.
    • Formula values of Ec include: Ec = wc^1.50.043f'c Ec = 4700√f'c
    • Poisson's ratio varies (0.11 to 0.21)
    • Recommended values:
      • 0.20 (compression)
      • 0.18 (tension)
      • 0.18-0.20 (tension and compression)

    Time-Dependent Volume Changes

    • Shrinkage (decrease in volume during hardening and drying under constant temperature):

      • Drying shrinkage (loss of adsorbed water)
      • Autogenous shrinkage (occurs without moisture loss)
      • Carbonation shrinkage (in environments with CO2)

    • Creep (permanent deformation due to sustained loads):

      • Creep strains increase over time
      • Creep slows over time as bonds are formed
      • Creep strains are greater than initial elastic strain

    • Thermal Expansion: depends on composition, moisture, content, and age.

      • Coefficient of thermal expansion varies by aggregate type (siliceous, limestone, lightweight) also increases with temperature.

    Durability Issues in Concrete Structures

    • Corrosion of steel

      • Oxidation process requires oxygen and moisture
      • Starting pH of fresh concrete (high alkaline) helps prevent corrosion
      • Rust expansion can crack concrete
      • Mitigation involves using minimum w/c ratios, minimum clear concrete cover, and limiting chloride

    • Freezing and Thawing

      • Pressure development in water within pores as concrete freezes
      • Air entrainment reduces these pressures, preventing damage
      • Mitigation involves air entrainment, minimum w/c ratio, concrete strength, and proper drainage

    • Chemical attack

      • Sulfate attack, alkali-silica reaction
      • Mitigation involves using appropriate cement types, checking aggregate source

    • Extreme Temperatures

      • High temperatures can cause cracks and spalling; modulus of elasticity and strength decrease with temperature increase
      • Early-age concrete is affected by fire, particularly with relation to its tensile strength
      • Coloring during high temperatures, pink, gray

    Steel Reinforcement

    • Steel reinforcement is "bars, wires, strands, fibers, or other slender elements embedded in a matrix"
    • Non-prestressed reinforcement types (hot-rolled deformed bars, welded wire fabric)
    • Prestressed concrete reinforcement (steel tendons)

    Hot-Rolled Deformed Bars

    • Steel bars with lugs/deformations rolled into surface to improve bond.
    • Classified based on ASTM specifications (e.g. ASTM A615, ASTM A706, ASTM A996)

    Hot-Rolled Deformed Bars (Properties)

    • Different strength grades (e.g., Grade 33/230, Grade 40/300, Grade 60/420, Grade 75/520)
    • Available in various diameters/sizes

    Hot-Rolled Deformed Bars (Additional Properties)

    • Fatigue Strength

    • Stress-Strain Relationship at Elevated Temperatures: decreases as temp decreases above 850F.

    Compatibility of Concrete and Steel

    • Concrete and steel work together because concrete resists compressive stress and steel resists tensile stress, with adequate bond between them.
    • Concrete protects steel against corrosion and fire, and they respond to thermal expansion similarly.

    References

    • American Concrete Institute (ACI)
    • Association of Structural Engineers of the Philippines (ASEP)
    • McCormac & Brown
    • Salmon et.al.

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    Introduction to Structural Concrete Design PDF

    Description

    This quiz focuses on the key concepts of reinforced concrete, its properties, and structural analysis. Test your knowledge on topics such as material behavior under load, standards for sample preparation, and the advantages and disadvantages of structural concrete. Perfect for students and professionals in civil engineering.

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