Reinforced Concrete I PDF (University of Halabja)

# Reinforced Concrete I ## University of Halabja/ Civil Engineering Department ### 3rd Stage Reinforced Concrete I ### Dr. Nasih H. Askandar ### 3rd YEAR CLASS (2024 - 2025) ## Chapter 1: Introduction and Overview ### Dr. Nasih Habeeb Askandar #### Ph.D. in Structural Engineering and Construc...

# Reinforced Concrete I ## University of Halabja/ Civil Engineering Department ### 3rd Stage Reinforced Concrete I ### Dr. Nasih H. Askandar ### 3rd YEAR CLASS (2024 - 2025) ## Chapter 1: Introduction and Overview ### Dr. Nasih Habeeb Askandar #### Ph.D. in Structural Engineering and Construction Materials ## Ch. 1/ Introduction and Overview: ### 1-1 Concrete and Reinforced Concrete Concrete is a mixture of sand, gravel, crushed rock, or other aggregates held together in a rocklike mass with a paste of cement and water. Sometimes one or more admixtures are added to change specific characteristics of the concrete. These admixtures influence workability, durability, and the time of hardening. Concrete has high compressive strength, but low tensile strength. Reinforced concrete combines concrete and steel. The steel reinforcement provides the tensile strength needed because concrete lacks it. Steel reinforcement resists compression forces and is commonly used in columns. ### 1-2 Historical Background: The real breakthrough for concrete occurred in 1824 when an English bricklayer named Joseph Aspdin obtained a patent for cement that he called Portland cement. Its color was similar to the stone quarried on the Isle of Portland off the English coast. Joseph Aspdin made his cement by taking certain quantities of clay and limestone, pulverizing them, burning them in a kitchen stove, and grinding the resulting clinker into a fine powder. During the early years, this cement was primarily used in stuccos. The earliest uses of concrete are not well documented. Many early developments are credited to French individuals, such as Francois Le Brun, Joseph Lambot, and Joseph Monier. - 1832: Le Brun built a concrete house and followed with a school and a church using the same material. - 1850: Lambot built a reinforced concrete boat. - Monier is credited with inventing reinforced concrete for use in basins or tubs and reservoirs. In 1867, received a patent for concrete construction with a mesh of iron wire. From 1867 to 1881, Monier received patents for reinforced concrete railroad ties, floor slabs, arches, footbridges, buildings. François Coignet developed basic methods of design for reinforced concrete structures and wrote a book in 1861 highlighting applications. Coignet recognized that adding too much water to the mix significantly reduced concrete strength. William E. Ward, building the first reinforced concrete building in the United States in Port Chester, New York, in 1875. In 1883, he presented a paper claiming that the concept came to him from watching English laborers attempt to remove hardened cement from iron tools. Thaddeus Hyatt analyzed the stresses in a reinforced concrete beam correctly. In 1877, Hyatt published a 28-page book "An Account of Some Experiments with Portland Cement Concrete, Combined with Iron as a Building Material." This book praised the use of reinforced concrete, stating that "rolled beams (steel) have to be taken largely on faith", but placed great emphasis on concrete's fire resistance. E. L. Ransome of San Francisco used reinforced concrete in the early 1870s. He is known as the originator of deformed (or twisted) bars. **In 1884, Ransome received a patent for deformed bars which were square in cross-section and cold-twisted with one complete turn in a length of not more than 12 times the bar diameter. The twisting improved bonding or adhesion between the concrete and steel.** In 1890, in San Francisco, Ransome built the Leland Stanford Jr. Museum, a reinforced concrete building measuring 95 m long, with 2 stories. In 1906, this building withstood an earthquake and resulting fire. The limited damage to this and other concrete structures led to widespread acceptance of reinforced concrete construction along the West Coast. Since the early 1900s, its development and use have expanded rapidly. ### 1-3 Advantages of Reinforced Concrete as a Structural Material The success of this universal construction material can be understood by its advantages. - High compressive strength per unit cost compared with other materials. - Resistant to the effects of fire and water. - Rigid structures. - Requires minimal ongoing maintenance. - Economical for footings, floor slabs, basement walls, piers, and similar applications. - Easy to cast into diverse shapes, such as slabs, beams, columns, arches, and shells. - Utilizes inexpensive local materials such as sand, gravel, and water. Cement and steel are relatively limited, making concrete a viable option in diverse regions. ### 1-4 Disadvantages of Reinforced Concrete as a Structural Material While concrete offers advantages, important disadvantages should be considered for all structural designs. - Concrete has a very low tensile strength, necessitating the use of tensile reinforcing. - Formwork is required to maintain the position of concrete until it sufficiently hardens. This shoring can become expensive due to the necessary falsework or shoring to support structures until they gain strength. - The low strength-to-weight ratio of concrete contributes to heavy members. For long-span structures, this becomes a major factor in design. - Concrete properties are highly variable due to: - Variations in proportioning. - Inconsistent mixing. - Placing, and curing procedures. - Shrinkage and creep are additional characteristics that can create issues in concrete. ### 1-5 Types of Portland Cement - **Type I- The common, Normal Portland Cement (NPC) all-purpose cement used for general construction work.** - **Type II-A modified cement that has a lower heat of hydration than does Type I cement and that can withstand some exposure to sulfate attack.** - **Type III-A high-early-strength cement This cement does have a much higher heat of hydration, enabling us to obtain desired strengths in 3 to 7 days rather than the normal 28 days. These cement are particularly useful for the fabrication of precast members and for emergency repairs of concrete.** - **Type IV-A low-heat cement that produces concrete that generates heat very slowly. It is used for very large concrete structures.** - **Type V-A cement used for concretes that are to be exposed to high concentrations of sulfate.** ### 1-6 Admixtures Admixtures enhance the performance of concrete and lower costs. - **Air-entraining admixtures** increase concrete's resistance to freezing and thawing. They provide better resistance to deicing salts. Air-entraining agents cause the mixing water to foam, creating air bubbles within the concrete. - **Accelerating admixtures**, like calcium chloride, accelerate early strength development. These are particularly useful in cold climates. - **Retarding admixtures** slow the setting process of concrete and manage temperature increases. They consist of various acids or sugars. Concrete truck drivers often carry sugar, using it if caught in traffic jams or delays to slow the setting process. - **Superplasticizers** reduce water content in concretes. They are organic sulfonates. Superplasticizers increase slump while allowing for less cement usage. These are frequently used to produce workable concretes with higher strength while using the same amount of cement. - **Waterproofing materials** are typically applied to hardened concrete surfaces. However, they may also be added to concrete mixes. These admixtures are often soap or petroleum products, sometimes asphalt emulsions. ### 1-7 Properties of Concrete Thorough knowledge of concrete's properties is required before designing any reinforced concrete structures. ### 1-7-1 Compressive Strength The compressive strength of concrete, $f_c$, is determined by testing it to failure. The test uses 28-day-old 150 mm diameter by 300mm concrete cylinders at a specific loading rate. The cylinders are usually kept underwater or in a controlled room with constant temperature and 100% humidity. While concretes can reach 28-day ultimate strengths of 17.5MPa (ACI 19.2.1) up to 70MPa to 140MPa, the range of 21MPa to 50MPa encompasses the majority of concretes used. The compressive strength of concrete is significantly influenced by the size and shape of the test units and the method used to apply the load. In many countries, the standardized test specimen is a cube measuring 150 mm on each side. For a given batch of concrete, the 150mm by 300mm cylinders will have compressive strengths roughly 80% of the values in MPa determined from cubes. ### 1-7-2 High-Strength Concretes Concretes with compressive strengths exceeding 70MPa are referred to as **high-strength concretes**. They are also commonly called **high-performance concretes** based on additional desirable characteristics. - **Durability:** Low permeability makes them resistant to physical and chemical agents which can cause deterioration. For average applications, 21MPa and 28MPa concretes are common. Prestressed construction typically utilizes 35MPa and 60MPa strengths. High-rise buildings often employ concretes with strengths reaching 63MPa or 70MPa. Readymix companies now commonly produce high-strength concretes, making them more readily available. Concretes with strengths exceeding 140MPa are increasingly produced, called **super-high-strength concretes** or **super-high-performance concretes**. ### 1-7-3 Static Modulus of Elasticity Concrete lacks a clear-cut modulus of elasticity. Its value changes based on several factors. - Concrete strength. - Age. - Type of loading. - Characteristics of the cement and aggregates. Several definitions of the modulus exist: - **Initial modulus**: The slope of the stress-strain diagram at the origin of the curve. - **Tangent modulus**: The slope of a tangent to the stress-strain curve at some point on the curve. For example, at 50% of the concrete's ultimate strength. - **Secant modulus**: The slope of the line drawn from the origin to a point on the stress-strain curve. This point is typically between 25% and 50% of the ultimate compressive strength. - **Apparent modulus** or **long-term modulus**: Determined from stresses and strains observed after a load has been applied for a set amount of time. The ACI Code in Section 19.2.2 provides a formula for calculating the modulus of elasticity of concrete: $E_c = w^{1.5} (0.043)\sqrt{f_c}$ Where: - $E_c$ is the modulus of elasticity in GPa - $w$ is the weight of the concrete in kg/m³. - $f_c$ is the concrete?s specified 28-day compressive strength in MPa. This formula is a secant modulus and represents the slope of a line from the origin to the point on the stress-strain curve corresponding to a compressive stress of 0.45* $f_c$. It's often used for calculating the stresses occuring during the estimated dead and live loads a structure must support. For normal-weight concrete (approximately 2400kg/m³), the ACI code offers a simplified formula: $E_c = 4700\sqrt{f_c}$ ### 1-7-4 Poisson's Ratio When a concrete cylinder experiences a compressive load, it not only shortens in length but also expands laterally. **Poisson's ratio** is the ratio of this lateral expansion to the longitudinal shortening. It varies from approximately 0.11 for high-strength concretes to a maximum of approximately 0.21 for weaker-grade concretes. The average is about 0.16. ### 1-7-5 Shrinkage When materials for concrete are mixed, the cement and water paste fills the spaces between the aggregates. This mixture must be workable enough to allow for the placement of reinforcing bars and ensure spread throughout the forms. Workability typically requires more water (sometimes twice) than necessary for the cement and water to chemically react (known as hydration). Concrete shrinkage occurs as the extra water used in the mix evaporates, leaving cracks on the surface. These can reduce the shear strength, deteriorate the structure?s appearance, and expose the reinforcing to conditions that promote corrosion. Approximately 90% of shrinkage occurs within the first year. The rate of shrinkage is proportional to the surface area of a member relative to its volume. Members with smaller cross-sections are more prone to shrinkage. To minimize shrinkage, consider: - Minimize the amount of mixing water. - Properly cure the concrete. - Place concrete in small sections, allowing the previous sections to partially shrink before proceeding. - Use construction joints to control the location and size of cracks. - Incorporate shrinkage reinforcement. - Select dense and nonporous aggregates. ### 1-7-6 Fatigue Some reinforced concrete members are subjected to numerous cycles of loading, like bridges. This repeat loading can lead to fatigue failure. **Stress ratio** is the ratio of the minimum applied stress to the maximum applied stress. A stress ratio of 0.1 indicates a significant difference between these values. At 1 million loading cycles, the fatigue strength of concrete is approximately 50% to 70% of its initial static strength. Realistic moisture and temperature conditions have limited effect on fatigue strength. ### 1-7-7 Creep When concrete is subjected to sustained compressive loads, it deforms over time. This deformation is called **creep** or **plastic flow**. - An initial elastic shortening occurs instantly when the load is applied. - If the load remains, the concrete will continue to shorten, over long periods. - The final deformation usually reaches two to three times the original elastic shortening. Approximately 75% of the total creep occurs within the first year after the load is applied. Removing the load allows recovery of elastic strain with a small recapture of creep strain. Reapplying the load will continue to generate both elastic and creep strain. Creep varies based on the amount of stress. It is directly proportional to stress as long as the sustained stress remains below half of the concrete's compressive strength ($f_c$). Above that level, creep rapidly increases. Additional factors affecting creep: - Higher-strength concretes have less creep than lower-strength concretes at the same stress levels. - Creep increases with higher temperatures. - The amount of free pore water that can escape from the concrete influences creep. Creep is greater at lower humidity levels. - The cement-water paste is primarily responsible for creep. - Concrete members with a larger volume-to-surface area ratio experience less creep because the free water has to travel a larger distance. ### 1-7-8 Tensile Strength The tensile strength of concrete is typically 8% to 15% of its compressive strength. The presence of microcracks within concrete accounts for this modest tensile strength. - These cracks have minimal impact when concrete experiences compressive loads, allowing for the transfer of compression forces. Closing the cracks under pressure negates their impact. - However, under tensile loading, the cracks remain open, undermining the concrete's strength. While tensile strength is typically disregarded in calculations, it directly affects the size and distribution of cracks and is a factor influencing concrete's deflection under load. Tensile strength has little information available as it's difficult to measure directly using an axial tension test. Two indirect tests commonly used: - **Modulus of rupture**: This method measures the flexural tensile strength. It involves loading a plain rectangular beam with concentrated forces applied at its one-third points. The beam fails when cracks form on the tension side. The equation for the modulus of rupture ($f_r$ ) is: $f_r = \frac{Mc}{I} = \frac{6M}{bh^2} = \frac{PL}{bh^2}$ Where: - b is the beam width. - h is the beam depth. - M is the maximum computed moment (PL/6). This approach assumes a linear distribution of stress, leading to inaccuracies. The code offers a simplified formula for the modulus of rupture ($f_r$ ) which is 0.622* $f_c$. This coefficient is adjusted for lightweight aggregates. - **Split-cylinder test**: This test uses a concrete cylinder placed horizontally and subjected to a compressive load along its entire length. The failure occurs when the cylinder splits along its axis. The split-cylinder strength ($f_{t}$) is calculated using the following formula: $f_t = \frac{2P}{\pi LD}$ Where: - P is the maximum compressive force. - L is the cylinder length. - D is the cylinder diameter. ### 1-8 Reinforcing Steel Reinforcement in concrete structures comes in three primary forms: - Bars, welded wire fabric, or strands. Bars are described as either **plain** or **deformed**. Deformed bars feature ribbed projections rolled onto their surface, enhancing bonding between the concrete and reinforcing steel. This is the standard form for most reinforced concrete structures. - **Reinforcing bar sizes** in the metric system are numbered 10, 12, 16, 20, 22, 25, 30, 32, 35, 43, and 57 mm. - **Longer bars** are available, generally up to 18m, but often require special ordering. - **Flexibility** can be a challenge with longer bars. - **Welded wire fabric** is also commonly used for reinforcing slabs, pavements, and shells. - **Spacing** in welded wire fabric is determined by cold-drawn wires in both directions, connected at their intersections using welding. - This reinforcement is preferred when there is limited space to place multiple reinforcing bars. ### 1-8-1 Grades of Reinforcing Steel The ACI Code uses SI units, converting customary values of concrete strengths ($f_c$) and steel yield strengths ($f_y$). However, the values are typically rounded off for clarity. - **Bar sizes in SI units:** 10, 12, 16, 20, 22, 25, 30, 32, 35, 43, and 57 mm. - **Steel reinforcing grades:** 280, 350, 420, 520, and 690MPa. These correspond to: Grade 40, 50, 60, 75, and 100ksi bars. - **Concrete strengths in SI units:** 17, 21, 24, 28, 35, 42,....MPa. These correspond to 2500, 3000, 3500, 4000, 5000, 6000....psi concretes. ### 1-8-2 Corrosive Environments When reinforced concrete is exposed to deicing salts, seawater, or spray from these substances, special corrosion protection for the reinforcing is critical. This is especially important in structures like bridge decks, parking garages, wastewater treatment plants, and coastal buildings. Reinforcement corrosion occurs when it's not adequately protected. Corrosion generates oxides that occupy more space than the original metal, creating pressure that can lead to cracking and spalling of the concrete. This reduces the protective cover for the steel, accelerates corrosion, and diminishes bonding between the steel and concrete. The result is a significant reduction in the overall lifespan of the structure. ### 1-8-3 Compatibility of Concrete and Steel The combination of concrete and steel is remarkably successful in reinforced concrete structures. Each material's strengths compensate for the other?s weaknesses. - **Modulus of elasticity:** Steel and concrete have similar moduli of elasticity. This ensures that the composite material experiences consistent thermal expansion and contraction. - **Bonding:** Steel and concrete bond well due to the material properties, the natural roughness of the steel bars, and the shape of deformations. - **Modulus of Elasticity:** Steel has a high modulus of elasticity. - **Cost:** Steel offers a low cost. ### 1-9 Introduction to Loads The most critical task for a structural designer is accurately estimating the loads a structure might face during its lifespan. This includes overlooking no reasonably foreseeable load and determining the worst possible combinations of these loads. Loads are categorized as: - Dead loads. - Live loads. - Environmental loads ### 1-9-1 Dead Loads Dead loads are consistent in magnitude and remain in a fixed position. These include the weight of the structure itself, plus fixtures permanently attached to it. For a reinforced concrete building, dead loads include: - Frames. - Walls. - Floors. - Ceilings. - Stairways. - Roofs. - Plumbing. The exact weights are determined during the structural analysis when specific building components are identified. These weights should be compared to initial estimates. If discrepancies are significant, the design should be reevaluated to ensure safety. ### 1-9-2 Live Loads Live loads vary in magnitude and location. Examples include: - Occupancy loads. - Warehouse materials. - Construction loads. - Overhead service cranes. - Operational equipment loads. Live loads are typically distributed uniformly over an entire floor, induced by gravity, and act downwards. The ASCE 7 Code permits live load reductions based on the principle that a structure is unlikely to simultaneously experience its full design live load over its entire floor area. ### 1-9-3 Environmental Loads Environmental loads result from: - **Rain:** While snow loads are often more severe, in warmer climates, rain on flat roofs can create problems if it accumulates faster than runoff occurs. This leads to ponding and deflection of the roof, with potential for collapse. - **Snow and Ice:** Snow and ice loads are significant in colder regions. One meter of snow creates a load equivalent to approximately 960N/mm². - **Wind:** A major contributor to structural failures, particularly in structures like bridges. The Tay Bridge collapse in Scotland in 1879, (resulting in 75 deaths), and the Tacoma Narrows Bridge collapse in 1940 are notable examples of structural failure due to wind. Wind loads are also implicated in building failures, such as the collapse of the Union Carbide Building in Toronto in 1958. - **Seismic loads:** In earthquake-prone regions, seismic loads are crucial design considerations. Many areas of the world are susceptible. The potential for damage is significant for all types of structures. - Numerous structures have failed during earthquakes throughout history. In Armenia in 1988, an earthquake caused around 50,000 fatalities. The 2008 earthquake in Sichuan Province, China, was responsible for 69,000 fatalities and 18,000 disappearances. ### 1-9-4 Design Codes Design codes enforce good practices for reinforced concrete design. Though not legally binding in themselves, governments incorporate them into local building codes through legal means. - The ACI Code (American Concrete Institute) and the IBC Code (International Building Code), both widely used in the United States, represent key examples. - The IBC code reflects the consolidation of three regional building codes: Building Officials and Code Administrators International, International Conference of Building Officials, and Southern Building Code Congress International. - The ACI Code 318 is referenced by the IBC for concrete design provisions with minor modifications. - The ACI Code 318 is a global influence on concrete codes in Canada, Mexico, and numerous other countries. The ACI revises its codes as new information on the behavior of reinforced concrete becomes available. Other notable sources for reinforced concrete specifications are: - The American Association of State Highway and Transportation Officials (AASHTO). - The American Railway Engineering Association (AREA). ### 1-9-5 Load Factors Load factors are numbers greater than 1.0, used to increase the estimated loads applied to structures. They account for uncertainty in load estimations. Load factors are applied to all types of members, not just beams and slabs. The goal is to approach the highest possible loads expected during the structure's lifespan, ensuring safety. - Load factors are commonly higher for live and environmental loads than for dead loads. This reflects the greater difficulty in accurately estimating these types of loads. Section 5.3 of the ACI Code sets the design strength ($U$) for reinforced concrete members. - For reinforced concrete design, the required strength $U$ must equal or exceed the maximum values derived from the load combinations defined by ACI Equations 5.3. - This includes aligning with provisions within the International Building Code (IBC) and ASCE/SEI 7-10.2. **The primary load in these equations is represented by these variables:** - **D**: dead load - **L**: Live load - **L_r**: Roof Live Load - **S**: Snow load - **R**: Rain load - **W**: Wind load - **E**: Seismic or earthquake load effects The magnitude of the load factors does not vary based on the potential consequence of failure. For example, hospitals or high-rise buildings are generally not treated with higher load factors than barns. However, the magnitude of wind and seismic loads do reflect the importance of the structure. For example, a hospital might experience a 50% higher seismic load than a comparable building with lesser risk. ### 1-10 Problems: 1. Describe the methods used to determine the tensile strength of concrete. 2. Explain the process of shrinkage in concrete, and how an engineer can minimize it. 3. Define high-strength concrete. Explain its advantages and visually demonstrate its stress-strain curve compared to lower-strength concrete. 4. Discuss the reasons why steel is preferred over other metals for reinforcing concrete. 5. Define the concept of modulus of elasticity. Outline the various methods for determining it, and illustrate how these methods apply to concrete and steel. 6. Explain the main reasons steel is preferred over other materials in reinforced concrete. 7. List the advantages of RC as a structural material. 8. List the disadvantages of RC as a structural material. 9. Explain the concept of creep. What factors influence creep? 10. Explain the process of shrinkage in concrete and how an engineer can minimize it. 11. Identify who patented cement and when. Describe the process for making cement. 12. Draw stress-strain curves for different concrete strengths. Begin with 31 MPa and extend to 120 MPa, highlighting similarities and differences between them. 13. Identify who patented cement and when. Describe the process for making cement. 14. Draw stress-strain curves for different steel bars used for reinforcing concrete. Start with 420 MPa and go up to 1860 MPa, highlighting similarities and differences between the curves. 15. Explain how cement was invented, the approximate year, the method used to make early cement, and the events that contributed to its widespread adoption. 16. Describe the primary differences between normal-strength concrete and high-strength concrete. 17. Discuss the various methods for testing the tensile strength of concrete, providing a brief explanation of each one and a visual representation.