Reinforced Concrete Dr S.Sreenath Phd, MSc (Engg), MSc (ICM), MBA
Concrete Concrete is mainly composed of three materials: Cement Water Aggregate Admixture (Sometimes added)
Concrete: Advantages Good compressive strength Protection of embedded steel Flexibility of application
Concrete: Disadvantages Low tensile strength Rather high density Prone to chemical attack
Cement Powder form Mix with water, a chemical reaction takes place. Produces a very hard and strong binding medium for the aggregate particles.
Hydration of cement When cement comes in contact with water, the water molecules react with individual compounds. By time produce a firm mass which give cement its bonding properties, strength and hardness. This process is termed hydration.
Types of cement Ordinary Portland Cement (OPC) Rapid hardening Portland Cement (RHPC) Low-heat Portland Cement (LHPC) Sulfate resisting Portland Cement (SRPC) Portland Blastfurnance Cement (PBFC) Waterproof and Water Repellant Cement
Cement Storage and Preparation Kept dry at all times. Shed should have water proofed roof and walls. Raised floor. By elevating on bricks or blocks. Concrete batching plants, cement is stored in silos.
Aggregates Aggregates are inert particles bound together by the cement paste to form mortar. The performance of concrete depends largely on the physical, thermal and chemical properties of its aggregates. Also called as filler materials as 60 to 80% of the volume of concrete is made up of aggregates.
Classification of Aggregates Heavyweight aggregate Normal aggregate Lightweight aggregate
Functions of Aggregates Reduce costs Modify properties of concrete Reduce dry shrinkage
Water in Concrete React with cement and cause it to set and harden. It facilitates mixing, placing and compacting of the fresh concrete. Used for washing the aggregates. For curing purposes. Amount of water content is very important to decide the properties of fresh and hardened concrete. Tap water is acceptable for mixing water.
Admixtures To improve the properties of fresh or hardened concrete. Make changes in the properties of hydration, liberation of heat, formation of pores and the development of the gel structure in concrete. To increase workability without increasing water content or to decrease the water content at the same workability. To retard or accelerate both initial and final setting times.
Concrete Admixtures usuage To improve the workability of the fresh concrete. To reduce water content for a given workability thereby increasing the strength. To increase durability of hardened concrete. To retard setting or to increase it. To impart colour to concrete. To maintain volume stability by reducing or offsetting shrinkage during concreting. To increase concrete resistance to freezing and thawing.
Different types of Admixtures Air-entraining agents Accelerating agents Retarders Water reducers or plasticizers Super plasticizers Bonding admixtures Water repelling agents
Composite Action of Reinforced Concrete Concrete does not resist tensile forces. Reinforcement is designed to carry these tensile forces, which are transferred by bond between the interface of the two materials. For adequate bonding, some reinforcement bars are ribbed or twisted. Proper concrete mixes provide adequate impermeability of the concrete against water intrusion and bar corrosion. Close values of thermal efficient coefficient of steel and concrete.
Stress Strain Relations Load applied, strain increases linearly with stress. Concrete behaves as an elastic material. After further loading, the curve is no longer linear and the concrete behaves like plastic material. In the plastic range, if the load is removed, the deformation would not recover.
Stress Strain Curve for Concrete in compression
Compressive strength of Concrete Grade of Concrete Minimum compressive strength N/mm 2 at 7 days Specified characteristic compressive strength (N/mm 2 ) at 28 days M15 10 15 M20 13.5 20 M25 17 25 M30 20 30 M35 23.5 35 M40 27 40 M45 30 45
Compressive strength of Concrete Age Strength percent 1 day 16% 3 days 40% 7 days 65% 14 days 90% 28 days 99%
Compressive strength of concrete
Applications of Reinforced Concrete An important construction material. Versatility, adaptability, resistance to fire and corrosion result in negligence maintenance costs. Better quality cement resulted in stronger and more durable concrete. Ideal for column, beam, slab construction of residential and commercial structures. The durability of concrete is influenced by the exposure conditions, the concrete quality, the cover to the reinforcement and the width of cracks.
Revision - Hydration The reaction with water is termed "hydration". This involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass. 23 of 20
Revision - Hydration The main product involved in the binding of cement and water is heat, which is released when the concrete is being hardened. This process is known as the heat of hydration. Heat of hydration can be managed while designing and pouring concrete, during the process of curing and hardening. However, if designers do not allow for the heat, it can cause serious issues with cracking and can also disturb the structural integrity of the concrete.
Temperature rise of mass concrete under adiabatic condition (Adapted from ACI 2007.2R-07)
Setting time of Cement Initial setting time is that time period between the time water is added to cement and time at which 1 mm square section needle fails to penetrate the cement paste, placed in the Vicat s mould 5 mm to 7 mm from the bottom of the mould. Final setting time is that time period between the time water is added to cement and the time at which 1 mm needle makes an impression on the paste in the mould but 5 mm attachment does not make any impression.
Vicat Apparatus
Setting Time
Workability Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures increases concrete workability.
Concrete Slump Test The test measures consistency of concrete in that specific batch. It is performed to check consistency of freshly made concrete. Consistency refers to the ease with which concrete flows. It is used to indicate degree of wetness. Consistency affects workability of concrete.
Concrete Slump Test Metal mould, in the shape of the frustrum of a cone, open at both ends, and provided with the handle, top internal diameter 4 in (100 mm), and bottom internal diameter 8 in (200 mm) with a height of 1 ft (300 mm). A 2 ft (600 mm) long bullet nosed metal rod, 5/8 in (16 mm) in diameter. The test is carried out using a mould known as a slump cone. The cone is placed on a hard non-absorbent surface. This cone is filled with fresh concrete in three stages. Each time, each layer is tamped 25 times with a rod of standard dimensions. At the end of the third stage, concrete is struck off flush to the top of the mould. The mould is carefully lifted vertically upwards with twisting motion, so as not to disturb the concrete cone.
Specification of concrete Concrete strength is identified by grade. Grade 25 has a characteristic cube crushing strength of 25 N/sq mm after 28 days of curing. Normal concrete grades are 30,35 and 40. The concrete mix is classified as designed or prescribed. Designed mix Contractor responsible for the mix proportion to achieve the required strength and workability. Prescribed mix The C&S Consultant specifies the mix proportions.
Design Mix
Batching and Mixing of Concrete Batching refers to the process of measuring the correct amounts of the constituent materials. Done either by weight or volume. But weight recommended to be more accurate. Measurement equipment regularly calibrated.
Batching of cement Cement is batched either by bag or from a silo. Bulk cement is cheaper than bagged cement. Bulk cement, changes in mix proportions are made more easily.
Advantages of using bulk cement Bulk cement is cheaper than bagged cement. A gang of men are not needed to off-load the cement. The trouble of ensuring adequate protection of the cement during storage is avoided. The mixer can always be used to capacity without the risk of errors which may result from splitting bags. Changes in mix proportions are made more easily.
Batching of Aggregate Weigh buckets with balanced mechanical lever and load cell systems are widely used. Allowance to be made when the aggregate is moist.
Batching of Water Water can be measured by weight or volume. Allowance are made at the time of batching by deducting the water in the aggregate.
Batching of Admixtures Follow the recommendation of the manufacturer. Dosage to finalize based on design mix. Use of reliable and accurate dispersing equipment is important.
Mixing of Concrete The objective of mixing is to coat the surfaces of all aggregates with cement paste and to blend all the ingredients of concrete to give a uniform consistency. Mixing may be performed by machine or by hand. Machine mixing is generally more economical and convenient.
Mixing Time Mixing should continue until the concrete is uniform in colour and consistency. Concrete are discharged from hopper into the mixer truck. Mixing time depends on manufacturer s specification and the plant s performance.
Transporting of Concrete Transport must be rapid to ensure concrete does not dry out or lose its workability during the time it elapses between mixing and placing. Segregation must be reduced to a minimum in order to avoid non-uniform concrete. The transport should be organized. No additional water should be added to the concrete.
Time Limit In the case of ready mixed concrete, the usual specified requirement is that concrete shall be discharged from a truck mixer within two hours after the time of loading.
Methods of Transportation Truck mixers Wheel barrows Dumpers Steel skips and buckets Cranes Concrete pumps and pipe lines
Placing of Concrete Concrete shall be placed in a proper manner to yield optimum result.
Reinforcement and Formwork Is the formwork erected properly? Is the formwork correctly aligned and levelled? Can the formwork be struck without damaging the concrete? Has the release agent be applied? Is it the right one? Has the reinforcement the right cover? Are there enough spacers? Are the forms clean and free from rubbish or odd bits of timber or metal?
Concreting Time The interval between adding the cement to the aggregate and the completion of the placing of the concrete should not exceed 30 minutes. When the mix is transported by purpose made agitators, operating continuously, such interval may be extended to a maximum of 2 hours and the placing of concrete should not exceed 20 minutes. When retrading admixtures are used, such intervals may be further extended.
Earth Foundation Before placing concrete in the foundation, all loose earth must be removed from the bed. Any roots of trees passing through the foundation should be cut to prevent further growth which may pierce the concrete at a later date. The surface of the earth, if dry, should be made moist, so that earth does not absorb water from the concrete. However foundation is wet and rain soaked, the water and slush should be removed completely.
Placing Careless placing of concrete may cause movement and damage of the formwork. Concrete to be deposited as near as practicable to its final position. Concrete should not be deposited in a large quantity at any point. Concrete should be deposited in horizontal layers. Each layer should be compacted throughly before the succeeding layer is placed. Concrete should be carried out continuously.
Compaction of Concrete The objective of compaction is to remove the unwanted entrapped air. The amount of entrapped air is related to the workability. Effective way of compacting is by use of vibration. Vibration by internal or external vibrator. Over vibration will cause segregation.
Finishing of Concrete Common methods: 1. Screeding and levelling. 2. Brooming, Floating and Steel trowelling Screeding and levelling when the concrete is still in its plastic stage. Screeding is the process of cutting off excess concrete to bring the top surface of a slab to proper grade. Levelling is to estimate high and low spots and to embed large aggregate particles to produce desired finish.
Finishing concrete Floating can be done with wood or metal hand float or with a finishing machine using float blades. Floating is done to embed aggregates particles just beneath the surface. Floating is done to remove slight imperfections, humps and voids. Floating is done to compact the mortar at the surface.
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