Structural Concrete Design Course Introduction

Introduction to Structural Concrete Design

• The presentation is about an introductory course on Structural Concrete Design (CEPRCD30).
• The instructor is Jerome Z. Tadiosa, CE, MSc, an Assistant Professor 2 in Civil Engineering at National University-Manila.
• The intended learning outcomes include describing the structural design process, explaining the development and classification of structural concrete, enumerating and describing design philosophies and codes/standards, and listing materials used in construction.

Reading Guide

• Students are asked to read designated chapters from specific textbooks for extra clarity on the course material.
• The noted references are Chapter 1-3 by Wight (2016), Chapter 1 by McCormac & Brown (2016), and sections 1.1-1.2 of Chapter 1 by Salmon et.al. (2009).

Lecture Outline

• The course will cover the introduction to structural design process, introduction to structural concrete design, and materials in structural concrete design.

Structural Design

• Structural design is a combination of art and science.
• It involves the engineer's intuitive understanding of structure behavior and applying basic engineering principles to create a safe, economic structure that serves its purpose.
• A properly designed structure should meet appropriateness (functionality and aesthetics), economy (optimal benefit-cost ratio), structural adequacy (strength and serviceability), and maintainability (minimum maintenance cost and time).

General Design Process

• 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, with steps and potential repetition from previous decisions.

Structural Design Process

• The structural design process details planning (setting and finalizing project details); preliminary structural configuration (initial arrangement of structural members); establishing loads (applying loading on structure models based on material, functions, and site conditions ); preliminary member selection (initial sizing of structural members); structural analysis (modeling and analyzing structure for forces and deformations); evaluation (checking individual members for strength and serviceability requirements, including client specifications); redesign (repeating previous steps based on evaluation results); and final decision (determining if the design is optimal).

Structural Concrete

• Defined as plain or reinforced concrete within a member of a structural system responsible for transferring loads to the ground.
• Concrete is the mixture of hydraulic cement, aggregates, and water with or without admixtures, fibers, or other cementitious materials.
• Plain concrete has no reinforcement or less than the minimum amount specified by building codes.
• Reinforced concrete contains a minimum amount of prestressing steel or reinforcement to meet building codes.

Advantages of Structural Concrete

• High compressive strength.
• Resists fire and water.
• Rigid.
• Low maintenance.
• Long service life.
• Most economical for substructures and floor slabs.
• Moldable.
• Inexpensive components.
• Low skill labor requirements.

Disadvantages of Structural Concrete

• Low tensile strength.
• Requires formwork.
• Low strength-to-weight ratio.
• Low strength-to-volume ratio.
• Quality consistency issues due to variation in properties because of proportioning and mixing.

Historical Background of Concrete

• Lime mortar was first used in Minoan civilization in Crete (around 2000 BC).
• Romans blended lime mortar with volcanic ash (pozzolana) to create water-resistant mortar in the 3rd century BC for increased strength.
• John Smeaton experimented with mixes of limestone and clay for water-resistance in cement, creating a design for The Eddystone Lighthouse.
• Joseph Aspdin created Portland cement through heating limestone and clay in 1824.
• Overheating of cement mixtures created clinker, improving cement strength discovered by I.C. Johnson in 1845.
• W..B. Wilkinson, Joseph Lambot, Thaddeus Hyatt, Joseph Monier, W.E. Ward, and E.L. Ransome were pivotal in the development of structural concrete design.

Limit States

• Limit states define conditions when a structure or member becomes unusable.
• Limit states categorize into strength (structural failure/collapse, loss of equilibrium, fatigue, etc.), serviceability (disruption of function, excessive deformation, and vibration), and special (failure due to abnormal conditions, e.g. fire).
• Limit states design entails identifying failure modes/limit states, determining safety levels for each state, and performing structural designs that consider significant limit states.
• For structural concrete, limit states design is executed using the ultimate strength design (USD) method.

Strength Design Method

• USD design requires member capacity to exceed the demand.
• The formula is: Rn ≥ Qu
• The symbols have the following meanings:
  • Ф: strength reduction factor
  • R: nominal member strength
  • Q: total factored load
  • Y¡, Qi: load factor and ith loading type

USD Load Combinations

• Articulates the crucial load combinations for USD using the 2015 NSCP.
• Provides specific load combinations for factored loads (example: 1.4(D + F), 1.2(D+F+T)+1.6(L+H)).

Service Load Combinations

• Presents combinations of loads for allowable stress/strength design procedures in the 2015 NSCP.
• Examples of load combinations are given (for example: D+F, D+H+F+L+T).

Structural Safety

• Setting load factors and strength reduction factors is to account for inconsistencies in strength, loadings, and the potential consequences of failure.
• Load factors and strength reduction factors compensate for inconsistencies in concrete material properties, variations in loading conditions on the structure, and considering the possibility of structural failure.

Codes and Standards for Structural Concrete

• The 2015 National Structural Code of the Philippines (NSCP) Vol 1, 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), and various ACI codes/standards.

Design for Economy

• Economy involves minimizing construction and financing costs by optimizing building design.
• Floor and roof systems typically constitute nearly 90% of construction costs in cast-in-place buildings.
• Optimizing formwork reuse, appropriate material costs, and simpler designs can boost economy in concrete building designs.

Design for Sustainability

• Durability and longevity are key factors for sustainable construction for reinforced concrete.
• Aesthetic qualities, versatility, and economic benefits of reinforced concrete make it an excellent construction choice for sustainability.
• Sustainable construction values a balance between economic, social, and environmental aspects.

Materials for Structural Concrete Construction

• Concrete is composed of aggregates (coarse and fine), cement, water, and admixtures.
• Cement and water form a paste that binds the aggregates.
• Admixtures improve concrete properties like strength and workability.
• Concrete has high compressive strength but is brittle in tension, thus requiring reinforcement.

Concrete

• The stress-strain relationship for concrete is nonlinear and appears as somewhat ductile.
• Microcracks develop in the concrete when subjected to load/stress, ranging from 1/8" to 1/2".
• Concrete mix design for structural purposes often incorporates traditional proportions, such as those from DPWH or the ACI 211.1-91 standard.

Mechanism of Concrete Failure in Compression

• Four stages identified in the microcrack development of concrete during uniaxial compression loading.
• Stage 1: Shrinkage of cement paste develops no-load bond cracks.
• Stage 2: Bond cracks due to stresses exceeding aggregate strength in the 30-40% concrete compressive stress phase.
• Stage 3: Localized mortar cracks occur when stress reaches between 50-60% of concrete compressive strength.
• Stage 4: Increased mortar crack development happens when load stress reaches 75-80% of concrete compressive strength.

Compressive Strength of Concrete

• Concrete sample preparation and testing adhere to ASTM C31 (Standard Practice for Making and Curing Concrete Test Specimens in the Field) and ASTM C39 (Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens).
• The standard specimen sizes are 6 in/150 mm diameter and 12 in/300 mm height, and 4 in/100 mm diameter and 8 in/200 mm height.
• The standard age for compressive strength is calculated using 28 days.

Factors Affecting Concrete Compressive Strength

• Lower water-cement ratios yield higher compressive strength.
• Cement type influences strength.
• Cement Type I (normal) for general construction and Type II (modified) for moderate sulfate exposure
• Type III (high early strength)
• Type IV (low heat)
• Type V (sulfate resisting)
• Supplementary cementitious materials minimize heat of hydration and improve workability.
• Aggregate strength, grading, quality, and toughness influence concrete strength.
• Potable water is used in concrete mixing, and salt water is generally avoided.

Factors Affecting Concrete Compressive Strength (Curing Conditions)

• Moisture and temperature conditions and curing duration affect concrete strength.
• Concrete strength increases with age, especially in the first seven days under optimal curing conditions.
• Low temperatures (-10°C to –12°C) limit young-age concrete strength gains; high strain rates show higher recorded strength compared to low strain rates.

Tensile Strength of Concrete (Modulus of Rupture)

• Tensile strength calculation methods adhere to ASTM C78 (Flexural Strength using Simple Beam with Third-Point Loading) and ASTM C496 (Splitting Tensile Strength of Cylindrical Concrete Specimens).

Modulus of Rupture

• Modulus of rupture fr for concrete is calculated using a formula that depends on A.

Factors Affecting Tensile Strength of Concrete

• Factors affecting tensile strength are similar to compressive strength. Crushing rock aggregate shows roughly 20% higher tensile strength than rounded gravel. Tensile strength gains faster than compressive strength.

Stress-Strain Curve of Concrete in Compression

• The stress-strain relationship for concrete in compression is nonlinear but appears somewhat ductile.
• A stress-strain curve demonstrates the relationship between stress (psi) and strain (in/in) in concrete under compression at various stress levels (e.g., 16,000 psi, 12,000 psi).

Modulus of Elasticity and Poisson's Ratio of Concrete

• Modulus of elasticity is calculated using appropriate formulas, considering values of water-cement (w/c) ratio and concrete strength.
• Poisson's ratio varies between 0.11 and 0.21 or between 0.15 and 0.20.
• Recommended values include 0.20 (compression), 0.18 (tension), and 0.18–0.20 (tension + compression).

Time-Dependent Volume Changes (Shrinkage)

• Shrinkage is a decrease in concrete volume during hardening and drying under constant temperature.
• Types include drying shrinkage (loss of adsorbed water linked to relative humidity) and autogenous shrinkage (loss associated with hydration reactions).
• Carbonation shrinkage happens in carbon dioxide-rich environments.
• Shrinkage is less significant in larger members because of a higher volume-to-surface-area ratio.

Time-Dependent Volume Changes (Creep)

• Creep is a permanent deformation in a material due to sustained load or elevated temperature.
• Creep occurs due to thinning of adsorbed water layers between gel particles.
• Creep develops over time and may be up to three times the elastic strain, leading to stress redistribution in the concrete and reducing prestressing effects.
• Creep influences factors include sustained stress ratio, concrete age, humidity, member size and concrete composition, temperature and water-cement ratio.

Time-Dependent Volume Changes (Thermal Expansion)

• Thermal expansion in concrete depends on composition, moisture content, and age; varies based on aggregate type.
• Coefficients of thermal expansion values differ by aggregate type: Siliceous (5-7(10-6)/°F); Limestone or calcareous (3.5-5.0(10-6)/°F); and Lightweight (3.6-6.2(10-6)/°F).
• General coefficient is ~5.5(10-6)/°F.
• Thermal expansion may also increase with temperature, especially at high temperatures.

Durability Issues in Concrete Structures

• Corrosion of steel in concrete involves oxidation processes requiring oxygen and moisture while fresh concrete has a pH around 13 to prevent corrosion
• Surface rust on steel can expand and lead to spalling and cracking, which can be mitigated by minimum concrete strength, minimum w/c ratio, and limiting chloride content in concrete mixtures.
• Breakdown due to freezing and thawing involves water-filled pores under pressure that can crack concrete. Air entrainment of small voids in concrete can limit freeze-thaw damage, while minimum w/c ratio, concrete strength and providing proper drainage are crucial.
• Chemical attack such as sulfate attack and alkali-silica reaction can undermine concrete durability, which can be mitigated by using appropriate cement types and checking aggregate sources.

Extreme Temperature Behavior of Concrete (High Temperature and Fire)

• Concrete generally performs well under fire conditions for a certain duration, but temperature gradients and sudden cooling can lead to surface cracking, spalling, and decreased strength and modulus of elasticity for the concrete.
• The aggregate type influences temperature-dependent behavior.
• Concrete color changes when heated, with color transitions (pink, gray) indicating damage severity, and severely damaged concrete should be replaced.

Extreme Temperature Behavior of Concrete (Very Cold Temperatures)

• Concrete strength increases with decrease in temperature, particularly in moist concrete without frozen water.
• Subzero temperatures significantly increase the strength, tensile strength, and modulus of elasticity of moist concrete.
• Dry concrete is less affected by low temperatures.

Steel Reinforcement

• Steel reinforcement are bars, wires, strands, fibers, or slender elements embedded in a matrix to resist forces.
• Common non-prestressed options are hot-rolled deformed bars and welded wire fabric, while steel tendons are used in prestressed concrete.
• Recent developments involve other reinforcement types, mainly fiber reinforcement.

Hot-Rolled Deformed Bars

• Steel bars, with surface lugs or deformations, are rolled into the concrete surface to improve bond and anchorage.
• Standards from ASTM A615, ASTM A706, and ASTM A996, classify hot-rolled steel bars.

Hot-Rolled Deformed Bars (Properties)

• Properties like minimum tensile and yield strength, elongation in a gauge length, and pin diameter for bend test vary by bar type and size according to the ASTM specifications.

Hot-Rolled Deformed Bars (Fatigue Strength)

• Fatigue strength describes the bar's ability to withstand repeated loading without failure, evaluated by the relationship between stress range (S) and cycles to failure(N).

Hot-Rolled Deformed Bars (Strength at High Temperatures)

• Yield and ultimate tensile strength of hot-rolled deformed bars decrease with increasing temperature.

Compatibility of Concrete and Steel

• Concrete and steel reinforcement work together because concrete resists compressive forces while steel handles tensile forces, with adequate bonding between them.
• Concrete protects reinforcing steel from corrosion and fire. They also respond to thermal expansion similarly.

References (List of Sources)

• Several books and organizations are listed.

This quiz covers the fundamentals of Structural Concrete Design from the introductory course CEPRCD30. It includes learning outcomes related to design processes, classifications of structural concrete, and relevant codes and standards. Students will also explore the essential materials used in construction related to concrete structures.