Introduction to Structural Concrete Design

Introduction to Structural Concrete Design

• CEPRCD30 course is about principles of reinforced concrete
• Jerome Z. Tadiosa, CE, MSc is the Assistant Professor teaching the course
• The course is at the National University – Manila

Intended Learning Outcomes

• Students will describe the structural design process and considerations
• Students will explain the description, development, and classification of structural concrete
• Students will enumerate and describe the design philosophies in structural concrete design, and the relevant codes and standards used
• Students will enumerate and describe the materials used in structural concrete construction

Reading Guide

• Required readings include chapters 1-3, Wight (2016)
• Chapter 1 for McCormac & Brown (2016)
• Sections 1.1-1.2, Chapter 1 for Salmon et.al. (2009)

Lecture Outline

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

Structural Design

• Structural design combines art and science, using intuitive feeling for structure behavior, and basic engineering principles to make safe and economical structures.
• A properly designed structure satisfies four criteria:
  • Appropriateness (functionality and aesthetics)
  • Economy (optimal benefit-cost ratio, minimum cost)
  • Structural adequacy (strength and serviceability requirements)
  • Maintainability (minimum maintenance cost and time)

General Design Process

• Major phases include:
  • Defining client needs and priorities (function, aesthetics, budget)
  • Developing project concept (schematics, preliminary framework, materials)
  • Designing individual systems (structural analysis and design, utilities and other systems)
• Structural design is sequential and iterative, following steps without skipping and possibly repeating steps

Structural Design Process (Steps)

• Planning: Setting and finalizing project details
• Preliminary structural configuration: Initial arrangement of structural members
• Establishing loads: Applying loads, depending on material, function, and site conditions
• Preliminary member selection: Initial sizing of structural members
• Structural analysis: Modeling and analyzing, determining forces and deformations
• Evaluation: Checking individual members against strength and serviceability requirements, accommodating client specifications
• Redesign: Repeating previous steps based on evaluation results
• Final decision: Determining if the latest design iteration is optimal

Structural Concrete

• Defined as "plain or reinforced concrete part of a structural system required to transfer loads along a load path to the ground" (ACI, 2013)
• Concrete is a mixture of hydraulic cement, aggregates, and water, optionally with admixtures, fibers or cementitious materials (ACI, 2013)
• Plain concrete is structural concrete without reinforcement or less reinforcement compared to what the relevant code requires (ACI, 2013)
• Reinforced concrete is a structural concrete reinforced with at least the minimum amount of prestressing steel or non-prestressed reinforcement as specified in the applicable building code (ACI, 2013) and may be steel-reinforced (using rebars) or prestressed (using tendons)
• This course focuses on steel-reinforced concrete with a brief overview of prestressed concrete

Advantages of Structural Concrete

• High compressive strength
• Resistance to fire and water
• Rigidity
• Low maintenance
• Long service life
• Economical material for substructures and floor slabs
• Moldable
• 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
• Variations in properties due to proportioning and mixing

Historical Background of Concrete

• Lime mortar was first used by Minoan civilization (~2000 BC)
• Romans mixed lime mortar with volcanic ash (pozzolana) to create stronger, water-resistant mortar (~3rd century BC)
• John Smeaton (pre-1800) designed the Eddystone Lighthouse with a mix of limestone and clay to create water-resistant cement
• Joseph Aspdin (1824) created Portland cement by heating limestone and clay
• Accidental overheating of cement mixture (clinker) produced stronger cement (I.C. Johnson, 1845)
• W. B. Wilkinson (1854) patented reinforced concrete floor system
• Joseph Lambot (France, 1848) Built reinforced concrete rowboat and patented his concept (1855), showing reinforced beams and columns with iron bars
• Thaddeus Hyatt (US, ~1850s) Experimented with reinforced concrete beams though his work remained unknown (until 1877)
• Joseph Monier (France, 1867) followed by patents for pipes, tanks, plates, bridges, and stairs for 1868 to 1875
• W. E. Ward (USA, 1875): Built the first reinforced concrete house in the USA
• E. L. Ransome (California, ~1870s-1880s): Designed and built structures
• Coignet and de Tedeskko (1894): Extended Koenen's theories about reinforced concrete, widely used for 1900 to 1950
• E. Freyssinet (1928): High strength steel wire for prestressing

Limit States

• Limit states are conditions when a structure or member is unfit for its intended use.
• Classified into three groups:
  • Strength limit states: structural failure or collapse
  • Serviceability limit states: disruption of functional structure use
  • Special limit states: damage or failure due to abnormal conditions (like extreme calamities, fire, corrosion)
• Limit state design includes:
  • Identifying failure modes and limit states
  • Determining acceptable levels of safety for each limit state
  • Structural design considering significant limit states
• Structural concrete design uses ultimate strength design method (USD) to multiply service loads to strength and strength reduction factor

Strength Design Method

• The capacity (resistance) of a member should be greater than or equal to the demand (load effects).
• For structural concrete using USD:
  • \phi Rn ≥ Qu
  • Rn: nominal member strength
  • Qu: total factored load (load factor Yi x load Qi)
  • \phi: strength reduction factor

USD Load Combinations

• Specific load combinations for strength design or Load and Resistance Factor (LRFD) design

Service Load Combinations

• Basic load combinations for allowable stress or allowable strength design

Structural Safety

• Variability in strength (materials, dimensions, design assumptions)
• Variability in loadings (material densities, actual load intensities)
• Consequences of failure (higher potential losses)

Codes and Standards for Structural Concrete

• 2015 National Structural Code of the Philippines (NSCP) Vol. 1 (Chapter 2 for Minimum Design Loads, Chapter 4 for Structural Concrete)
• ACI 318M-14 (Building Code Requirements for Structural Concrete)
• ACI 318R-14 (Commentary on Building Code Requirements for Structural Concrete)
• Other ACI codes and standards

Design for Economy

• Economy is a major goal influenced by both construction costs and financing tied to speed of construction (especially in cast-in-place buildings)
• Material costs increase with larger column spacing, but formwork reuse can reduce costs.
• Choose beam, slab, and column sizes (to maximize form reuse)
• Simplifying designs reduces errors, saves time, results in more economical structures, avoids haunched beams and deep spandrel beams which complicate form movement
• Standard column sizes help simplify formwork
• Using high-strength concrete is economic for columns

Design for Sustainability

• Durability and longevity are key factors
• Reinforced concrete valued for aesthetic qualities, versatility, initial and life-cycle economical benefits, and thermal properties reducing energy cost
• Sustainable construction balances economic, social, and environmental values

Materials for Structural Concrete Construction

• Concrete is a composite of aggregates, cement, water, and admixtures.
• Aggregates make up the bulk, cement and water form a binding agent
• Admixtures improve properties (strength and workability)
• Concrete is strong in compression but weak in tension
• Steel reinforcement is required to compensate for the tension weakness

Concrete

• Stress-strain relationship is nonlinear, but appears somewhat ductile due to microcracking
• Microcracks range between 1/8" and 1/2", classified as bond or mortar cracks
• Concrete mix design for general use is commonly performed using traditional proportions (DPWH-modified) or ACI specifications (ACI 211.1-91)

Mechanism of Concrete Failure in Compression

• Four stages in microcrack development under uniaxial compression;
• No-load bond cracks from shrinkage during hydration
• Bond cracks from aggregate stresses exceeding their strength (~30-40% of compressive strength)
• Localized mortar cracks from load stresses between bond cracks (~50-60% of compressive strength)
• Mortar crack increase with load stress (~75-80% of compressive strength)

Compressive Strength of Concrete

• Sample preparations and testing are based on ASTM C31 and C39.
• Test cylinders are 6-in (150 mm) diameter by 12-in (300 mm) height and 4-in (100 mm) diameter by 8-in (200 mm) height
• Standard age for compressive strength is 28 days.

Factors Affecting Concrete Compressive Strength

• Water-cement (w/c) ratio (lower w/c generally leads to higher strength)
• Cement type (Type I for general use, Type II with lower heat hydration for sulfate exposure, Type III for high early strength, Type IV for lower heat, Type V for sulfate resistance)
• Use of supplementary cementitious materials (pozzolans)
• Aggregates (strength, grading, quality, toughness)
• Mixing water (potable)

Factors Affecting Concrete Compressive Strength (Curing condition and Age of concrete)

• Moisture and temperature conditions during curing affect the development of concrete strength
• Concrete strength increases with age, especially in the first 7 days of curing if optimal conditions are maintained.
• Young-age concrete strength improves as long as the temperature is between -10°C and -12°C
• Low strain rate results to lower recorded strength, higher strain rate results to higher recorded strength.

Tensile Strength of Concrete (Modulus of Rupture)

• Determined using ASTM C78 (Flexural Strength; Simple Beam with Third-Point Loading) or ASTM C496 (Splitting Tensile Strength of Cylindrical Concrete Specimens)
• Factors affecting tensile strength are similar to those affecting compressive strength
• Concrete made with crushed rock has better tensile strength than concrete made with rounded gravel.
• Tensile strength develops more quickly than compressive strength

Modulus of Rupture

• Calculating modulus of rupture using appropriate values for 'A' based on concrete composition

Factors Affecting Tensile Strength of Concrete

• Similar factors influence both compressiv and tensile strength
• Crushed rock concrete has higher tensile strength
• Tensile strength develops faster than compressive strength

Stress-Strain Curve of Concrete in Compression

• Provides stress-strain curves for various stress levels

Modulus of Elasticity and Poisson's Ratio of Concrete

• Modulus of elasticity, Ec, is calculated in two ways (using wc or f'c)
• Poisson's ratio varies between 0.11 and 0.21 (maybe also within 0.15 and 0.20)
• Recommended values are 0.20 (compression) and 0.18 (tension) or 0.18 to 0.20

Time-Dependent Volume Changes (Shrinkage)

• Shrinkage is a decrease in concrete volume during hardening and drying at constant temperature.
• Drying shrinkage is loss of adsorbed water affected by relative humidity (especially below 40% RH)
• Autogenous shrinkage is associated with hydration reactions and more significant in high-performance concrete
• Carbonation shrinkage happens in carbon dioxide-rich environments and contributes significantly to total shrinkage.

Time-Dependent Volume Changes (Creep)

• Creep is permanent deformation due to sustained loads and temperature
• In concrete, thinning of water layers between gel particles contributes to the decrease and possible forming of bonds.
• Creep strains develop over 2 to 5 years and can be 1-3 times the initial elastic strain
• Influenced by sustained stress ratio, concrete age, humidity, member size, concrete composition and water-cement ratio

Time-Dependent Volume Changes (Thermal Expansion)

• Concrete expansion depends on composition, moisture content, and age
• Coefficient of thermal expansion varies based on aggregate type (and examples of values)
• Coefficient of thermal expansion may increase with temperature, especially at high temperatures

Durability Issues in Concrete Structures (Corrosion of steel)

• Corrosion involves oxidation requiring oxygen and moisture, and starts when concrete pH drops below 11-12
• Concrete surface rust causes bond improvement, however expansion can lead to spalling and cracking
• Controlling corrosion involves minimum concrete strength, w/c ratio, clear concrete cover, limiting chloride content
• Epoxy-coated reinforcement can help control corrosion

Durability Issues in Concrete Structures (Breakdown due to freezing/thawing)

• Freezing pressures develop in water-filled pores, breaking down the concrete structure.
• Air entrainment helps resist freeze-thaw damage (microscopic voids that relieve pressure)

Durability Issues in Concrete Structures (Breakdown due to chemical attacks)

• Chemical presence and reactions in site environment like sulfate attacks, alkali-silica reaction may hinder concrete durability
• Can be mitigated by suitable cement types and checking aggregate source.
• Structures vulnerable to chemical attacks include pavements, bridge decks, parking garages, water tanks, and foundations

Extreme Temperature Behavior of Concrete (High Temperature and Fire)

• Concrete performs well within a certain time frame, but temperature gradients, leading to surface cracks and spalling
• Spalling gets worse if surface is cooled suddenly
• Modulus of elasticity and strength decrease with increased temperature
• Aggregate type in concrete mix may influence temperature-dependent behavior
• Early-age concrete is susceptible to fire impacts, especially tensile strength
• Different colors indicate different damage levels

Extreme Temperature Behavior of Concrete (Very Cold Temperatures)

• Concrete strength increases with decreasing temperature, especially in moist concrete without frozen water
• Subfreezing temperatures boost compressive strength, tensile strength, and modulus of elasticity for moist concrete
• Dry concrete is less susceptible to low temperatures

Steel Reinforcement

• Defined as bars, wires, strands, fibers, or other slender elements
• They are embedded in a matrix to resist forces.
• Common non-prestressed reinforcement types include hot-rolled deformed bars and welded wire fabric.
• Recent developments include other types of reinforcement, especially fibers.

Hot-Rolled Deformed Bars

• Steel bars with lugs or deformations into the surface to improve bond and anchorage
• Classified by governing ASTM specification.
• Specific grades (ASTM A615, A706, A996) and their properties (e.g., minimum tensile strength)

Hot-Rolled Deformed Bars (Fatigue Strength)

• Fatigue strength considers stress range, S (ksi), and cycles to failure, N (millions), showing tolerance limits.

Hot-Rolled Deformed Bars (Strength at High Temperatures)

• Both yield and ultimate strength decrease with increasing temperature starting around 850°F.

Compatibility of Concrete and Steel

• Concrete and steel work together since concrete withstands compressive stress, and steel withstands tensile stress
• Adequate bond is crucial for compatibility
• Concrete protects reinforcements from corrosion and fire
• Both similarly respond to thermal expansion

References

• Various books and organizations' publications are cited as references

This quiz covers the fundamental principles of structural concrete design as outlined in the course CEPRCD30. It addresses the design process, material classifications, and relevant standards in the field. Prepare to explore the key concepts from the recommended readings by Wight and McCormac & Brown.