RC Chap 1-3 PDF

Introduction to Concrete ……. ​ Segment 1: Describe the Components of Concrete ​ Objective 1: Describe the Components of Concrete.​ Cement and Concrete ​ Many people use the terms cement and concrete interchangeably, but there are significant differences between the t...

Introduction to Concrete ……. ​ Segment 1: Describe the Components of Concrete ​ Objective 1: Describe the Components of Concrete.​ Cement and Concrete ​ Many people use the terms cement and concrete interchangeably, but there are significant differences between the two substances. ​ Many people use the terms cement and concrete interchangeably, but there are significant differences between the two substances. Here are examples of cement and concrete before they have been mixed with water. The Composition of Cement ​ Cement consists of a mixture of calcium, silicon, aluminum, iron and small amounts of other ingredients that are baked and ground into a very fine powder. Cement reacts with water to form a hard, solid material. The reaction that takes place is called hydration. Mortar ​ Cement mixed with sand and water can be used as mortar between bricks or stones to build a solid structure. But cement by itself is not very strong. In fact, the mortar between bricks is the weak link in a brick structure.​ The Composition of Concrete ​ Concrete is a mixture of cement with water and aggregate which is essentially sand and gravel. This mixture forms a substance that is very strong in compression when it is cured. ​ The typical ratio of ingredients by volume is 3:2:1 of coarse aggregate to sand to cement. The amount of water to be added is expressed as a weight ratio of the water to the cement. ​ Typically this weight ratio is in a range of 0.3:1 to 0.7:1 of the weight of the cement to be added. ​ When concrete is placed and formed the coarse aggregate is pushed beneath the surface, so the exposed surface of the concrete is cement and sand. ​ This is the reason that the surface of cured concrete looks similar to the surface of cured cement. The cross- section shows how coarse aggregate is pushed below the surface of hardened concrete. ​ Segment 1: Describe the Components of Concrete ​ Objective 2: Describe Six Applications for Concrete​ ​ Six Applications for Concrete ​ One of the major advantages of using concrete in construction is its versatility. Concrete can be formed into almost any shape and has very good compressive strength. ​ When steel or fibers are added to concrete, its tensile strength is improved along with its compressive strength, which allows it to be used in even more applications. Foundations and Slabs ​ Beams and Columns - * Concrete is used to construct beams and columns in applications that will experience compressive forces. * Concrete columns are prevalent in the exterior of buildings of Greek and Roman architecture for decoration as well as structural support. They are also used on the insides of buildings to provide support while keeping the interior of the building open.  ​ Walls - *Concrete can be used to form walls in " structures above the ground or below ground level, as in a basement. Concrete walls provide fire resistance and good compressive strength. They also provide a smoother surface and more uniform strength than masonry walls. ​ Roads - *Concrete can be used to pave roads that require a rigid pavement. *Concrete slabs for road paving are usually 6 to 14 inches thick depending on the expected weight load. The concrete is reinforced with steel rods to add tensile strength. *Asphalt is used instead of concrete to pave roads in applications that require a flexible pavement. The type of pavement that is used is based on the condition and the stability of the base soil beneath the road as well as climatic conditions. ​ Roofs - *Roofs can be made of concrete, but the lack of tensile strength of concrete requires that roofs with large spans be arched or domed. *Vertical loads on arches and domes result in compressive forces. Vertical loads on flat 100fs result in bending moments and tensile forces. *If a flat roof is to be made of concrete. numerous vertical supports must be provided to keep the span of the roof limited. ​ Statues and Ornaments - *The ability to form concrete into almost any desired shape makes it ideal for the manufacturing of statues or ornamental features on buildings. *A form is first created, and the concrete is placed into the form and allowed to cure. The form is opened and the concrete object is removed. This process can be repealed to provide multiple objects of the same shape. *Many intricate shapes can be made from concrete. Segment 1: Describe the Components of Concrete ​ Objective 3: Describe the Function of Aggregate and Admixtures in Concrete Physical Properties of Concrete ​ The physical properties of concrete often need to be modified for a specific job. These modifications may include increasing the volume, improving the stability, or improving the resistance to wear, erosion, or acids. ​ Due to climatic conditions, the concrete may need to cure more quickly or more slowly. In some cases, the form may be a difficult shape and the plasticity or workability of the concrete may need to be improved without adding extra water. ​ Adding aggregate or admixtures to the concrete can modify these physical properties. The Use of Aggregate ​ Aggregate is the sand, gravel, or crushed stone that is added to concrete to increase its tenth, volume, or stability. Aggregates make up 60% lo 75% of the fall volume of the concrete. ​ For this reason, it should consist of particles with adequate strength and resistance to environmental conditions and it should not contain materials that will cause breakdown of the concrete. Aggregate is classified as either fine or coarse. Fine Aggregate ​ Fine aggregate is natural or manufactured sand with particles up to 3/8 inch. Fine aggregate is used alone when making thin concrete slabs or when a smooth surface is desired.  ​ Fine aggregate is also used in conjunction with coarse aggregate in concrete mixtures. The small particles of fine aggregate will fill in around the coarse aggregate particles, adding to the strength of the concrete. Coarse Aggregate ​ Coarse aggregate particles can range in size from pea-size to six inches in length. They can have rounded or sharp edges. Larger aggregate reduces the cost of the concrete mixture because it reduces the amount of cement that needs to be used. ​ If the aggregate is too large, it creates large concentrations of cement between the aggregate particles. It is important that the aggregate size be as uniform as possible The Ratio and Sizes of Ingredients ​ The ratio of ingredients and the size of the aggregate in the concrete can greatly affect the properties of the concrete before and after it hardens. ​ In critical applications, trial sections of concrete must be created and tested to make sure that the mixture meets the strength requirements. The results of the tests lead to adjustments and more trials until the correct mixture is achieved. ​ Here are some examples of various sizes and shapes of coarse aggregate. The aggregate used in concrete mixtures is usually & material that is readily available locally because of its high weight and associated high cost of shipping. The Use of Admixtures ​ Certain chemicals can be added to the concrete during the mixing process to affect the properties of the concrete. These are called admixtures. ​ Admixtures can accelerate or decelerate the setting time, reduce the water and cement requirements in the mixture, improve durability in challenging environments, increase strength, or change other properties of the concrete. Admixtures can also make the concrete more resistant to chemicals or radiation. ​ Admixtures can be added to the concrete mixture in liquid or dry form and can account for 1% to 30% of the total weight of the concrete mixture. Air-Entraining Agents ​ Air-entraining agents are added to concrete either through the use of air-entrained cement or as an admixture that is added during the concrete mixing process. ​ Intentional air-entrapment is desired in concrete because it increases the concrete's ability to withstand freezing and thawing cycles. The water that remains in the concrete after it cures | expands and contracts when it freezes and thaws. ​ The air bubbles trapped in the concrete as a result of the air-entraining agents give freezing water space to expand into. Large pockets of entrained ai that form due to poor compacting of the concrete weaken the concrete and should be avoided Segment 1: Describe the Components of Concrete ​ Objective 4: Define Coarse Aggregate Grades Coarse Aggregate Grades Defined ​ Coarse aggregate is defined as aggregate that has particles greater than 3/16 inch. Coarse aggregate grades are determined by the size of the individual aggregate particles. ​ A representative sample of the aggregate is placed through a collection of different-sized sieves, and the grade is determined by the percentage of material that passes through each of the sieves. ​ For example, a 1-1/2 inch aggregate will have the majority of its particles pass through a 1-1/2 inch sieve, but the 1-inch sieve will catch most of the smaller particles. Segment 1: Describe the Components of Concrete ​ Objective 5: Describe Six Classifications of Concrete and Give an Example of Each ​ ​ Six Types of Concrete ​ Concretes can be classified by their physical properties, mechanical properties, composition, and how they are placed. There are six types of concrete. ​ General-Use Concretes - *General-use concrete can be used for just about everything except foundations and exposed paving. The mix proportions for this concrete are 3:2:1 (coarse aggregate to fine aggregate 1o cement). *The design compressive strength of general-use concrete is between 3,000 and 5,000 psi. They reach 80% of their final strength after 7 days of curing, and they achieve 100% of their strength after 28 days of curing. *General-use concrete has a dry unit weight of 130 to 155 lbs/ft^3. Other concrete is classified by how their properties relate to the properties of general-use concrete. ​ Lightweight Concretes - *The lower density is achieved by using lightweight aggregate in the concrete mix. Lightweight concrete is used in structural applications in which high strength is not required and the lower weight and density of the concrete lessens the load on the structure supporting it. *This lightweight aggregate can be composed of shale, clay. slate, pumice, LB |4 perlite, or vermiculite. The typical design of compressive strength of lightweight concrete ranges from 3,000 to 5,000 psi. High-strength concrete can be made with lightweight aggregates as well. *Lightweight concrete has a unit weight of 85 10 115 Lbs/ft’. ​ Heavyweight Concretes - *Heavyweight concrete uses aggregates that are very dense, such as hematite, magnalite, steel punchings, or steel shot.or steel shot. *They are used mainly for radiation shielding, but can also be used in applications such as anchors and counterweights in which high weight is needed. ​ *With the exception of density, the physical properties of heavyweight concrete are similar to those of general-use concrete. *Heavyweight concrete has a unit weight of up to 400 lbs/ft^3. ​ High-Strength Concretes - * High-strength concrete is defined as concrete having a compressive strength of greater than 6000 psi. Some buildings contain concrete with strengths up 0 20,000 psi. *The high strength is achieved through the use of high-strength cement, admixtures such as fly ash or silica fume, and carefully sized aggregates. *Since low water content is usually used in high- strength concrete, special attention must be paid to compacting the concrete into the forms. ​ High-Early-Strength Concretes - *High-strength concrete is used wherever expected loads exceed the strength of normal concrete. *High-Early-Strength (HES) concrete is concrete thal achieves s targeted strength quicker than normal concrete, generally from a few hours (or several days. The quicker curing can be achieved by using admixtures, HES cement, high cement content, | low water-cement ratio, special curing methods, or a combination of these methods. *HES concrete is used in situations or applications where time or conditions do not allow adequate time for normal impending cold weather, and fasttrack building projects.  ​ Fiber-Reinforced Concretes - *A fiber-reinforced concrete is a normal concrete that has had fibers added to it in order to increase its tensile strength and resistance to cracking. The fibers can be made of glass, steel, FRP, or various other materials. The fibers consist of up to 5% of the volume of the concrete mixture. *Fiber-reinforced concrete is used where the application of concrete_is going to be thin and subject to cracking, such as slabs on grades and pavement overlays. While these various types of concrete perform differently, they all look alike. Segment 1: Describe the Components of Concrete ​ Objective 6: Describe How Water Affects Concrete Strength ​ ​ Effects of Water-Cement Ratio ​ The ratio of water to cement is key to the strength of concrete. Generally speaking, the lower the waler-to-cement ratio, the stronger the concrete will be. However, enough water must be added to the concrete to lubricate the mixture for easy workability. The table shows the effect of the water-cement ratio on the strength of the concrete mixture. Excessive Amount of Water in Cement ​ Too much water in a concrete mixture can have other harmful effects on the concrete besides decreasing its strength. ​ Excess water increases shrinkage during curing that can cause concrete to crack. Advantages of Reducing the Water Content of Concrete ​ There are many advantages to reducing the waler content of the concrete. ​ Increased compressive and flexural strength ​ Increased water tightness and lower absorption ​ Increased weather resistance ​ Better bond between successive layers and between concrete and reinforcement ​ Less volume change from wetting and drying Water-Cement Weight Ratio ​ As opposed 1o the dry ingredients in a concrete mixture that are measured as a ratio of volumes to one another, the amount of water that is used in a concrete mixture is expressed as a weight ratio of waler lo cement. ​ Because the density of cement can vary, it is common to weigh the cement before it is added a mixture of ensure that the correct weight ratio of water is then added to the mixture. ​ One way to determine if the water content of a concrete mixture is correct is to perform a slump test. Segment 2: Concrete Mixing and Placement ​ Objective 7: Describe the Steps Used to Mix Concrete Mix Concrete Evenly ​ When mixing concrete, the ingredients should be thoroughly mixed to ensure that the concrete reaches its design strength. Any unmixed portions of concrete will result in weak spots. ​ The appearance should be uniform and the ingredients evenly distributed. If a batch of concrete has been properly mixed, samples taken from different portions of the batch will have essentially the same weight, air content, slump, and coarse aggregate content. Hand Mixing Concrete ​ The tools for mixing concrete depend on the size of the batch. Small batches, such as what is needed to install a mailbox post or a basketball goal at home, are easily mixed in a small container like a wheelbarrow. ​ A hoe or a shovel can be used to combine the ingredients. Mixing concrete by hand is difficult work, but if it is done carefully, it can be just as strong as premixed concrete delivered from a central mixing plant. Steps to Hand Mixing Concrete ​ Step 1. Measure the Fine and Coarse Aggregate - Measure the fine and coarse aggregate into a compact pile on a hard, flat surface, such as a sheet of plywood, or into a fairy flat container. ​ Step 2. Add the Cement - Form a crater in the top of the pile and pour the measured cement into it. ​ Step 3. Mix the Ingredients - Turn the pile methodically until it is uniform in color and texture ​ Step 4. Add Water and Dry Material - Form a crater in the pile and add some of the measured water. Bring the dry material to the water and mix, adding water as necessary, until the uniform in color and consistency. ​ Step 5. Test the Mixture - Test the mixture by flattening the surface with the back of the shovel. The surface should be close-knit and moist but not showing too much water. Concrete Mixers ​ For large jobs, like the construction of a new building, the concrete can be mixed off-site and delivered to the job, or the ingredients can be delivered to the jobsite and mixed there by either a mobile batcher mixer or a stationary mixer. ​ There are four steps which are general guidelines for mixing concrete in a miser. ​ Step 1. Add Water - Place up to 10% of the water in the drum before adding solid materials. ​ Step 2. Start Mixing in Solid Materials - Add the water uniformly with all other materials, leaving about 10% to be added after all other materials are in the mixing drum. ​ Step 3. Add Admixtures If Needed - When admixtures are used, they should be added one at a time to prevent any interaction that may affect the concrete. ​ Step 4. Mix Concrete - The mixing period starts from the time all cement and aggregate are in the mixer drum. General specifications require 1 minute of mixing for up to 1 cubic yard of mix, with an increase of 15 seconds for each additional cubic yard. Variation of Mix Process. ​ The process used to mix large batches of concrete can vary based on the type of cement used, any addition of different admixtures, if heated water is used, and how the concrete is to be delivered to the jobsite. Segment 2: Concrete Mixing and Placement ​ Objective 7: Describe How Concrete Is Measured The Concrete Unit of Measurement ​ Before concrete is mixed, or premixed concrete is ordered, the amount of concrete needed to complete the job must be calculated. The unit used to describe the amount of concrete to be used in a job is called a yard. A yard of concrete is short for a ‘cubic yard, or 27 cubic feet. Guidelines for Measuring Concrete ​ There are general guidelines to use when estimating the amount of concrete needed. ​ Measure the length, width, and thickness of the concrete in feet. Convert any inch measurement to fractions of a fool. ​ Multiply the length, width, and thickness together and then divide the answer by 27 (the number of cubic feet in a cubic yard). This is the number of cubic yards of concrete you will need. ​ Add on 15% to 20% more than required by the calculation. This makes up for any unforeseen variables so there will be enough to finish the job. Segment 2: Concrete Mixing and Placement ​ Objective 9: Describe Four Methods of Delivering Concrete to the Jobsite On-Site Concrete Mixing ​ Large jobs, such as roads, bridges, or skyscrapers, require a lot of concrete. Some contractors may elect to have a stationary mixer on-site concrete as needed. Stationary mixers come in sizes from 2 cubic feet up to 12 cubic yards. Four Types of Concrete Trucks ​ Mobile Batcher Mixer - used for intermittent _production of concrete at the jobsite. This truck transports the materials (to the site and is used to batch and mix the materials for a quick and precise proportioning of the specified concrete. This is a one-man operation. ​ Non-Agitating Truck - The non-agitating truck does not mix the concrete but instead is used to transport premixed concrete short distances over smooth roadways. When this method is used, the concrete slump should be limited, and there is a possibility of segregation of the concrete, which is separation of the aggregate from the cement. ​ Truck Agitator - Truck agitators are used to transport premixed concrete for pavements, structures and buildings. The concrete must be discharged from the truck within 90 minutes of mixing. These trucks operate from central mixing plants where quality concrete is produced under controlled conditions. This concrete is uniform and homogenous on discharge. The construction crew must be ready onsite to handle the concrete upon arrival. ​ Truck Mixer - Truck mixers are used to transport concrete for pavements, structures, and buildings. The truck mixer operates out of a batching facility where the materials are added into the mixer and the truck mixes the concrete Time from mixing to discharge is the same as that for the truck agitator. The discharge is uniform and homogenous, also the same as for the truck agitator. However, quality control is not as good with this method as itis with central mixing and truck agitators Segment 2: Concrete Mixing and Placement ​ Objective 10: Describe Nine Methods of Transporting and Placing Concrete Nine Methods of Transporting and Placing Concrete ​ Once the concrete reaches the jobsite, the problem arises as to how to transport it to the forms. There are nine methods used to transport concrete to the desired location on the jobsite. ​ Belt Conveyors - *A belt conveyor is typically a reinforced, rubberized belt that moves continuously, transporting material from one- location to another. Belt conveyors can be used to transport the concrete to lower or higher locations, if necessary. *Typically they are used between the main discharge point (the location of the tuck or mixer) and a secondary discharge point. The secondary discharge point is an area closer to the forms. From there, the construction crew will obtain the concrete needed to fill those forms. *Belt conveyors have an adjustable reach, a movable diverter, and variable forward and reverse speed. They can place large amounts of concrete quickly in a limited access area. *Belt conveyors may be mounted on the truck mixers to speed delivery. Care must be taken to prevent segregation at the end of the conveyor and remove all mortar from the return belt. ​ Buckets - A bucket is a large funnel-shaped container that can hold from 1/2 to 12 cubic yards of concrete. Buckets transport concrete from the central discharge point either for a secondary discharge point or directly to the forms. They are used with cranes, cableways and helicopters for construction of buildings, dams, and bridges. Buckets allow for clean discharge and a wide range of capacities. ​ Chutes - *A chute is an inclined trough through which concrete is transported from the truck to the desired location. Chutes are used to transport concrete to a lower level than the central discharge point on all types of concrete construction. *They are low cost, easy to maneuver, and require no power, as gravity does most of the work *Chutes must be adequately supported in all positions. Care must be taken to prevent segregation at the end of the chute. ​ Drop Chutes- A drop chute is used to place concrete in vertical forms and resembles a funnel with a long extension on the output end. Some drop chutes are one piece and some are assembled from loosely connected segments. They carry the concrete directly to the bottom of the form without segregation or spillage on the form sides. ​ Pneumatic Guns - Pneumatic guns spray concrete, or shotcrete, at a high velocity. Pneumatic guns are used when concrete is to be placed in difficult locations and where thin sections and large areas are needed. This method is ideal for use with free-form shapes, for repairing and strengthening buildings, for protective coatings, and for thin linings. The quality of the work depends on the skill of the operator. ​ Pumps - *Pumps are used 1o transport concrete directly from a central discharge point of the forms or a secondary discharge point. *The pipelines for the pump take up little space and can be easily extended. The concrete is delivered in §§ a continuous stream either vertically or horizontally. *Stationary pump booms can provide continuous concrete for tall building construction. This method requires a constant supply of freshly mixed concrete with average consistency and no tendency to segregate. ​ Screw Spreaders - platform mounted, screw-shaped spreader. It is used to spread concrete over flat areas, such as a roadway. A batch of concrete is discharged from a bucket or a truck and the sorrow spreader quickly spreads the concrete over a wide area at a uniform depth. This concrete will have uniform compaction before vibration is used for the final compaction. ​ Tremies - A tremie is a long, watertight pipe used to pour concrete into difficult-to-access areas, such as underwater or below ground. ​ Wheelbarrows and Power Buggies - Wheelbarrows and power buggies, which are like motorized wheelbarrows, are used for short distances on flat surfaces, especially where accessibility to the area is restricted. This method is extremely versatile, particularly inside of a structure and at sites where placing conditions are constantly changing. However, it is also very slow and labor intensive. Segment 2: Concrete Mixing and Placement ​ Objective 11: Describe the Six Steps Involved in Placing and Finishing Concrete Placing and Finishing Concrete ​ Placing concrete involves many more steps than just constructing forms and placing and smoothing the concrete. ​ If all of the steps are not performed when concrete is placed, the finished concrete will likely have internal air voids and weak spots and will eventually crack. ​ There are six steps involved in placing and finishing concrete. ​ Preparation - *The ground beneath the concrete, often called the subgrade, must be compacted before the concrete is placed to prevent any settling during or after the curing of the concrete. *Setting of the subgrade could lead to avoid under the concrete and potential for the concrete to fail or settle due to lack of support. *‘The forms for the desired shape of the concrete must be constructed. Wood forms need to prevent them from distorting after the concrete is placed. *Oil, or release agents, should be applied to the forms to allow them to be easily removed after the concrete has set up. If needed, reinforcement steel should be placed inside the forms. *Before the concrete is placed, wood forms and the subgrade should be wetted in order to prevent the forms and ground from pulling moisture out of the concrete. ​ Placement - Concrete should be deposited continuously as near as worked too much, there is a greater chance for the concrete to be segregated. Placement of the concrete should begin at the far point of the form and work its way back towards the source of the concrete. ​ Consolidation - *Consolidation involves the compacting of the freshly placed concrete in order to get the concrete to pour around steel reinforcement and to eliminate unwanted entrapped air. *Consolidation can be performed by hand through insertion of a rod repeatedly into the concrete. This method, called tamping,is not recommended for large jobs. *Mechanical consolidation can be accomplished with vibrators that are inserted into the concrete, placed on the outside of the forms, or placed on the surface of the concrete. ​ Screeding - Screeding is the process of striking off the excess concrete on the top surface of the form with a straight edge. A long board can be used to do this on a small surface or specially designed vibratory screeds can be used on large surfaces that will consolidate the | concrete while removing the excess. ​ Bull Floating or Darbying - * Bull floating or darbying, is the process of pressing down on the surface of the concrete with a flat bottomed hand tool to push 1 aggregate below the surface of the concrete and eliminate high and low spots on the surface. This should be done immediately after screeding. *After bull floating, there is a good chance that bleed water will form on the surface of the concrete. This is excess water that rises to the surface of the form. It is important 1o let the bleed water evaporate or to remove the water before proceeding to the final smoothing steps. *If your foot will only sink about 1/4 inch into the concrete, it is ready for finishing ​ Finishing - *After the bleeding process of the concrete is completed, edging must be performed around the edges of the forms.  *Edging consists of running a trowel at a depth of about 1 inch along the edge of the forms, and then running the edger flat along the surface of the concrete at the edges of the forms. This process compacts the concrete at the edges of the forms. *After edging the concrete, the concrete needs 1o be floated. Floating is the final preparation for smoothing of the concrete with a wood or metal hand float or a finishing machine using float blades. *The float should be moved across the surface of the concrete in a sawing motion to fill voids and embed aggregate. *Floating provides a relatively smooth finish but for an especially smooth, hard surface, floating should be followed by troweling.The trowel's flat steel blade is similar to a float. *As the surface of the concrete stiffens, the trowel should be used concrete. The final smoothing of the concrete with a trowel should make a ringing sound as it is moved across the surface of the concrete. Segment 2: Concrete Mixing and Placement ​ Objective 12: Describe the Concrete Curing Process Hydration ​ Concrete cures through a process called hydration. The water in concrete undergoes a chemical reaction with the cement, turning them into a solid mass This chemical reaction is exothermic, which means it puts off heat. ​ In certain conditions, especially in the summertime or when large masses of concrete are constructed that have large surface areas, precautions must be taken to ensure that the heat of the reaction does not cause the water to evaporate before all of the chemical reaction takes place. ​ If the water does evaporate prematurely, unhydrated cement remains in the concrete. There are several ways to prevent this from happening. Curing ​ The concrete can be covered with plastic or wet burlap to maintain the heat of hydration and keep the moisture with the concrete while it is curing. ​ Water can be sprayed on the concrete periodically or continuously while the concrete is curing ​ Sometimes using cold water in the concrete mixture is sufficient to prevent the water from evaporating. The Hoover Dam ​ An example of the amount of heat produced by concrete curing occurred during the construction of the Hoover Dam on the Colorado River. ​ The dam was constructed entirely of concrete, but it was placed in individual blocks. Pipes were installed in each of the blocks with refrigerated water pumped through them to cool the concrete. ​ It has been estimated that the concrete in Hoover Dam, which was constructed in 1935, would still be cooling today had it been constructed in just one pour. In Addition, after it was cured, the strength of the concrete would not have been adequate because the evaporation of the water would have caused incomplete curing of the concerete. Cure Rate vs. Compressive Strength ​ Concrete without curing rate admixtures will be set up in as little as 24 hours, but it will take about 7 days for the concrete to reach 80 percent of its final strength. Concrete that is cured will reach its final strength after 28 days. The final strength is much greater for concrete that is moistened than for concrete that is exposed to air. Cure Time ​ The amount of time that the concrete is kept moist and allowed to cure is dependent on the required strength and time restrictions on the project. This shows how the strength of concrete is affected by its curing rate. Segment 3: Concrete Testing ​ Objective 13:Describe Three Tests for Concrete Three Common Strength Tests Performed on Concrete ​ The many variables that affect the strength of concrete require that tests be performed on samples of the concrete mixtures in applications where the strength is critical While there are many tests that can be performed on concrete, the following three tests are among the more common ​ Slump Test - *The slump of a concrete mixture is a property that describes how fluid the mixture is. The slump test involves placing three layers of a fresh mixture of concrete into a conical form. with each layer being rodded (worked into the cone with a rod) 25 times. *Once the final layer is rodded, the top is leveled off and the cone is slowly removed. The concrete will quickly settle to a new height. The difference in height is the amount of slump. *The test must begin within 5 minutes from the time the sample was obtained and it should be completed within 2-1/2 minutes, since concrete loses slump with time. Whether a given camplo's slump is acceptable or not depends upon the specific application for the concrete. ​ Compression Test - A compression test involves using a testing apparatus to test a cured concrete sample's compressive strength The test setup consists of a cylinder of cured concrete that is placed in a hydraulic vise. The amount of force being placed on the cylinder by the vise is shown on the gauge next Lo the vise. The force on the vise is increased until the concrete sample is crushed. ​ Rupture Test - A rupture test is a method of measuring the strength of a cured concrete sample that will have bending forces placed on it The sample is placed across a span, and a vertical force is applied to the sample until it breaks. Segment 3: Concrete Testing ​ Objective 14: Describe How Concrete Is Reinforced How is Concrete Reinforced ​ Concrete has a tensile strength that is about 10 percent of its compressive strength. Therefore, reinforcement is needed to assist the concrete in withstanding tensile forces. ​ One way of reinforcing the concrete is to insert bars, wires, or welded wire fabric into the concrete. ​ Thé steel the concrete will bear the tensile load that is applied to the | concrete and will also assist the concrete in resisting the compressive loads. ​ The reinforcements are placed inside the concrete forms before the concrete is placed, and then the concrete is placed around the reinforcements.  REINFORCED CONCRETE DESIGN INTRODUCTION ……. ​ 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 certain characteristics of the concrete such as its workability, durability, and time of hardening. Reinforced concrete is a combination of concrete and steel wherein the steel reinforcement provides the tensile strength lacking in the concrete. Advantages of Using Reinforced Concrete as a Structural Material 1.​ It has considerable compressive strength per unit cost compared with most other materials. 2.​ Reinforced concrete has great resistance to the actions of fire and water and, in fact, is the best structural material available for situations where water is present. 3.​ Reinforced concrete structures are very rigid. 4.​ It is a low-maintenance material. 5.​ As compared with other materials, it has a very long service life. 6.​ It is usually the only economical material available for footings, floor slabs, basement walls, piers, and similar applications. 7.​ A special feature of concrete is its ability to be cast into an extraordinary variety of shapes, from simple slabs, beams, and columns to great arches and shells. 8.​ In most areas, concrete takes advantage of inexpensive local materials (sand, gravel, and water). 9.​ A lower grade of skilled labor is required for erection as compared with other materials such as structural steel. Disadvantages of Using Reinforced Concrete as a Structural Material 1.​ Concrete has a very low tensile strength, requiring the use of tensile reinforcing. 2.​ Forms (which are expensive) are required to hold the concrete in place until it hardens sufficiently. In addition, falsework or shoring may be necessary to keep the forms in place for roofs, walls, floors, and similar structures until the concrete members gain sufficient strength to support themselves. 3.​ The low strength per unit of weight of concrete leads to heavy members. This becomes an increasingly important matter for long-span structures, where concrete’s large deadweight has a great effect on bending moments. 4.​ Similarly, the low strength per unit of volume of concrete means members will be relatively large, an important consideration for tall buildings and long-span structures. 5.​ The properties of concrete vary widely because of variations in its proportioning and mixing. Furthermore, the placing and curing of concrete is not as carefully controlled as is the production of other materials, such as structural steel and laminated wood. Concrete ​ Concrete Cement and Water ​ Used in binding aggregates (sand and gravel) ​ Water/cement ratio greatly affects the strength of concrete ​ Curing of Concrete ​ Curing is performed by submerging the specimen underwater. This is done in order to prevent moisture loss. Rapid moisture loss leads to cracking and loss of strength of the concrete specimen. ​ Note: Ideally, the maximum strength of concrete is attained at the 28th day of curing. STRESS – STRAIN RELATIONSHIP OF CONCRETE ​ Proportionality Limit - Stress is proportional to strain. ​ Elastic Limit - The material returns to its original shape when the load is removed. ​ Ultimate Compressive Strength - The highest stress on the stress-strain curve. ​ Break Point / Rupture - Failure occurs. The concrete cracks in tension. ​ fr = 0.62λ √fc’ ​ Hooke’s Law - The stress is directly proportional to strain up to the proportionality limit ​ σ = Eϵ where E is the Modulus of Elasticity Design Codes ​ Design codes provide detailed technical standards and are used to establish the requirements for the actual structural design. It should be realized, however, that codes provide only a general guide for design. ​ “The ultimate responsibility for the design lies with the structural engineer.”​ — National Structural Code of the Philippines 2015 DESIGN ANALYSIS OF REINFORCED CONCRETE ​ WORKING STRESS DESIGN (WSD) METHOD ​ The behavior of concrete is LINEAR ELASTIC. ​ The consideration is up to the proportionality limit. ​ ULTIMATE STRESS DESIGN (USD) METHOD ​ The behavior of concrete is NON-LINEAR ELASTIC. ​ The consideration is up to the ultimate strength. CONCRETE : SHRINKAGE AND CREEP ​ SHRINKAGE - Contracting of a hardened concrete mixture due to the loss of water/moisture. Shrinkage temperature bars are used. ​ CREEP - Additional deformation because of the load applied for a very long time.  REINFORCED CONCRETE DESIGN STRUCTURAL ELEMENTS ……. ​ STRUCTURAL ELEMENTS ​ A structure refers to a system of connected parts used to support a load. Important examples related to civil engineering include buildings, bridges, and towers; ​ When designing a structure to serve a specified function for public use, the engineer must account for its safety, aesthetics, and serviceability, while taking into consideration economic and environmental constraints. Often this requires several independent studies of different solutions before final judgment can be made as to which structural form is most appropriate. This design process is both creative and technical and requires a fundamental knowledge of material properties and the laws of mechanics which govern material response. Once a preliminary design of a structure is proposed, the structure must then be analyzed to ensure that it has its required stiffness and strength. STRUCTURAL ELEMENTS : SLABS ​ Slabs are flat horizontal panels that support the floor. It can be supported by beams/girders on edges or directly by columns. They carry gravity loads and transfer them to the vertical components (columns and/or walls), and also act as horizontal diaphragms by transferring the lateral load to the vertical components of a structure. ​ TYPES 1. One – way Floor System - One-way floor system is a slab or deck that is supported such that it delivers its load to the supporting members by one-way action. It is often referred to as a one-way slab. s/l < 0.50 * ”s” is for shorter span and “l” is for longer span *One-way slab bends in only one direction along the short span ​ 2. Two – way Floor System - If the support ratio is s / l >= 0.50 , the load is assumed to be delivered to the supporting beams and girders in two directions. When this is the case the slab is referred to as a two-way slab. STRUCTURAL ELEMENTS : BEAMS and GIRDERS ​ Beams. Beams are usually straight horizontal members used primarily to carry vertical loads. Quite often they are classified according to the way they are supported, as indicated in the figure. ​ Beams are primarily designed to resist bending moment; however, if they are short and carry large loads, the internal shear force may become quite large and this force may govern their design. ​ For bending and deflections, if the deformations disappear and the structure regains its original shape when the actions causing the deformations are removed, the deformations are termed elastic deformations. ​ The permanent deformations of structures are referred to as inelastic, or plastic, deformations. ​ A positive moment tends to bend a beam or horizontal member concave upward. ​ Likewise, a negative moment tends to bend the beam or member concave downward  REINFORCED CONCRETE DESIGN LOADS ON STRUCTURES ……. LOADS ON STRUCTURES ​ Once the structural form has been determined, the actual design begins with those elements that are subjected to the primary loads the structure is intended to carry, and proceeds in sequence to the various supporting members until the foundation is reached. In order to design a structure, it is therefore necessary to first specify the loads that act on it. ​ Thus, a building floor slab would be designed first, followed by the supporting beams, columns, and last, the foundation footings. DESIGN CODES ​ The NATIONAL BUILDING CODE, also known as Presidential Decree No. 1096, is a government policy covering technical requirements in constructing or renovating buildings and structures in the Philippines to secure the life, health, property and welfare of the Filipinos. LOADS ON STRUCTURES GRAVITY LOADS ​ The vertical loads, due mainly to the occupancy, self-weight and snow or rain, are commonly referred to as gravity loads. ​ Dead loads consist of the weights of the various structural members and the weights of any objects that are permanently attached to the structure. The values for dead loads are shown in NSCP Section 204, Tables 204-1 and 204-2 for common material densities and minimum design dead loads for common components. ​ Live Loads can vary both in their magnitude and location. They may be caused by the weights of objects temporarily placed on a structure, moving vehicles, or natural forces. NSCP Section 204, Table 205-1 provides recommended design live loads depending on the use of the space ​ Snow and Rain Loads. In some parts of the country, roof loading due to snow or rain can be quite severe, and therefore protection against possible failure is of primary concern. ​ Hydrostatic and Soil Pressure. When structures are used to retain water, soil, or granular materials, the pressure developed by these loadings becomes an important criterion for their design. ​ Impact Loads. When live loads are applied rapidly to a structure, they cause larger stresses than those that would be produced if the same loads would have been applied gradually. The dynamic effect of the load that causes this increase in stress in the structure is referred to as impact. LOADS ON STRUCTURES LATERAL LOADS ​ The horizontal loads, induced mainly by wind and earthquake are called lateral loads. ​ Wind Loads. When structures block the flow of wind, the wind’s kinetic energy is converted into potential energy of pressure, which causes a wind loading. The effect of wind on a structure depends upon the density and velocity of the air, the angle of incidence of the wind, the shape and stiffness of the structure, and the roughness of its surface. ​ Earthquake Loads. Earthquakes produce loadings on a structure through its interaction with the ground and its response characteristics. These loadings result from the structure’s distortion caused by the ground’s motion and the lateral resistance of the structure.