Civil Engineering Basics

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➡️COMMON CONCRETE PROBLEMS AND THEIR PREVENTION ⏩There are many problems we might be facing during and after concreting. *To produce high quality concrete we must take some precautions to avoid those common problems during concreting. In this article, we will discuss on common concrete problems and how to prevent them. A) BLEEDING: Bleeding refers to as a tendency of water to appear on the top surface of concrete after finishing. Due to bleeding some measure of water (with sand particles and other cementing materials) appears at the surface of the concrete. *Following precautions should be taken to reduce bleeding in concrete. 1. Design the mix appropriately. 2. Include least water content in the mix. 3. Use greater amount of cement content. 4. Use greater amount of fine particles. 5. Utilize a little measure of air entraining admixture. B) SEGREGATION: Segregation means separation of coarse aggregates from the concrete surface due to poor compaction. It is generally seen in the plastic stage of concrete. As a result honeycomb, laitance, scaling, porous layer, bond failure etc. can be formed in concrete. Following precautions should be adopted to prevent segregation in concrete. 1. Design the mix appropriately. 2. Never use excessive water content. 3. Take care of handling, placing, and proper compaction of concrete. 4. Do not allow the concrete to be dropped from more heights. 5. Use air entraining admixture. 6. Keep the formwork to be watertight. C) LAITANCE: The appearance of cement-sand particles on the surface of freshly placed concrete is known as laitance. It is mainly occurred due to the bad effect of bleeding and segregation of concrete. The bond between subsequent layers of concrete becomes weaker and as a result, laitance is developed. Following precautions can be taken to stop the occurrence of laitance in concrete. 1. Clay, dust, silt content etc should be removed before mixing the concrete. 2. Water-cement ratio should be maintained properly. 3. Water should not be sprayed on the concrete surface during finishing work. 4. Use well graded fine aggregates in the mix. 5. Add little amount of water reducing admixture in the concrete mix. D) SCALING: Scaling is the physical deterioration of concrete in which the surface layer of concrete broke down, pitted or flaked away. Due to this effect concrete surface becomes worse. Scaling can be prevented by taking same precautions adopted for laitance. E) PLASTIC SHRINKAGE CRACKS: When the evaporation rate of water mixed in the concrete is greater than the bleed water of concrete, plastic shrinkage cracks are developed on the surface of the concrete. Basically, this type of cracks occurs in very hot climate. F) DUSTING: Dusting can be prevented by taking following precautions. 1. Maintain a suitable water/cement ratio in the concrete. 2. Utilize dust free aggregates in the mix. 3. Guarantee appropriate hydration of concrete. 4. Avoid early surface finishing of concrete.

Unit Weight Of Building Materials . *Water- 1000 Kg/ m3 *Bricks (broken) - 1420 Kg/ m3 *Bricks(common) -1600 Kg/ m3 *Cement(ordinary)- 1440 Kg/ m3 *Cement (rapid hardening)- 1250 Kg/ m3 *Cement Mortar - 2000 Kg/ m3 *Cement Concrete (Plain) - 2400 Kg/ m3 *Cement Concrete (Reinforced) -2500 Kg/ m3 *Glass - 2500 Kg/ m3 *Lime Concrete - 1900 Kg/ m3 *Cement Plaster - 2000 Kg/ m3 *Lime Plaster - 1700 Kg/ m3 *Stones (Ballast) - 1720 Kg/ m3 *Stones (Aggregates)- 1750 Kg/ m3 *Stones (Basalt)- 2850 Kg/ m3 *Stones (Granite) - 2450 Kg/ m3 *Stones (Marble)- 2650 Kg/ m3 *Timber (Oak, Sal) - 510 Kg/ m3 *Timber (Mango) - 650 Kg/ m3 *Timber (Teak)- 625 Kg/ m3 *Coal- 600 Kg/ m3 *Plastics- 1250 Kg/ m3 *Oils- 800 Kg/ m3 *Ashes- 650 Kg/ m3 *Clinker- 750 Kg/ m3 *Rubber- 1300 Kg/ m3 *Slag- 1500 Kg/ m3 *Clay Soil- 1900 Kg/ m3 *Sand (dry) - 1540 to 1600 Kg/ m3 *Sand (wet)- 1760 to 2000 Kg/ m3 *Steel- 7850 Kg/ m3 *Chalk- 2100 Kg/ m3 *Bitumen- 1040 Kg/ m3

👉BASICS OF CRACKING PROBLEMS: . ▶The subject “ CRACKS “ or “ CRACKING “ in concrete is indeed much debated. However, there is inadequate clarity as to the reason for cracking and what may be done to limit its occurrence or eliminate it altogether. The phenomenon of the appearance of cracks in concrete is often a subject that raises several questions. Either at the initial stage or with time, cracks may appear in a concrete structure. It should first of all be accepted that cracks of some type or other are inevitable in any concrete. They can be minimized to a large extent by proper practices. *Cracks may be big or small, structural or non- structural. All of them can be a nuisance by way of either marring the appearance or function as a passage for atmospheric polluting gases and moisture to either the body of concrete and causes distress. *Cracks in the concrete structure do not always mean that the structure is not usable. As cracking of concrete is inherent, what really matters is the type of structure and the nature of cracking. Cracks that are acceptable for building structures may not be acceptable for water relating structures. Considering that cracking can never be completely eliminated, it is necessary to be aware of the causes, evaluation and the methods of repair whoever required. CRACKS Cracks are generally of tow types : Structural and nonstructural *Cracks can occur in hardened or unhardened concrete. *STRUCTURAL CRACKS It is fundamental that hardened reinforced concrete cracks in the tensile zone when subjected to externally imposed structural loads. By means of appropriate design and detailing technique, these cracks can be limited to acceptable levels in terms of structural integrity and aesthetics. *NON_STRUCTURAL CRACKS **Concrete is also liable to crack in both the plastic and hardened states due to the nature of its constituent materials. These intrinsic cracks are as inevitable as structural cracks. It is often possible to predict when intrinsic cracks are likely and to limit them to acceptable levels by means of good design and/or proper construction techniques. *DORMANT AND LIVE CRACKS: Cracks caused by shrinkage, temperature gradient due to hydration and alkali silica reaction ( ASR ) are less dangerous the cracks caused by thermal expansion and loads. This is because shrinkage and hydration temperature gradient result in dormant cracks ( where the width does not vary with time) and other mechanisms result in live cracks (where the width increases with time Dormant cracks, especially narrow ones, are more likely to undergo auto genius healing and realkalisation. The crack may heal owing to following conditions: • carbon dioxide diffusing into the crack leading to carbonation of the calcium hydroxide and precipitation of calcium carbonate. • Cement hydration due to presence of moisture. • Hydration diffusing from the bulk concrete into the heaked zone thereby increasing the pH in that area. Live cracks are more likely than domant ones. They result in collection of debris at the base owing to movement of the sides on the cracks. This can impede temporarily the passage of depassivating substances to the reinforcements. However, autodeneous healing is unlikely with live cracks. ▶CAUSES OF CRACKING: Cracking will occur whenever the applied strain exceeds the tensile strain capacity of the concrete. *The tensile strain capacity of concrete varies with age and with Age at which cracks may occur. *Cracks can appear at any time. The age which some typical can occur are given in the Table. ▶TABLE – AGE AT WHICH CRACKS MAY APPEAR: *Sr. no. types of cracks Age 1. plastic shrinkage First few minutes or hours after concreting . 2. plastic settlement First few minutes or hours after concreting. 3. due to corrosion After one year or many years. 4. due to loads After loads are imposed and the time duration of loading.

translational movements, or relative displacement of the member-ends in all directions but cannot prevent any rotational movements . Its reaction forces are single linear forces of unknown direction or horizontal and vertical forces which are components of the single force of unknown direction. Pinned support is just like a human elbow. It can be extended and flexed (rotation), but you cannot move your forearm left to right (translation). One benefit of pinned supports is not having internal moment forces and only their axial force playing a big role in designing them. However, a single pinned support cannot completely restrain a structure. At least two supports are needed to resist the moment. Applying in trusses is one frequent way we can use this support. Fixed support Rigid or fixed supports maintain the angular relationship between the joined elements and provide both force and moment resistance. It exerts forces acting in any direction and prevents all translational movements (horizontal and vertical) as well as all rotational movement of a member. These supports’ reaction forces are horizontal and vertical components of a linear resultant ; a moment.It is a rigid type of support or connection. The application of the fixed support is beneficial when we can only use single support, and people most widely used this type as the only support for a cantilever . They are common in beam-to-column connections of moment-resisting steel frames and beam, column and slab connections in concrete frames. Hanger support Hanger support only exerts a force and prevents a member from acting or translating away in the direction of the hanger. However, this support cannot prevent translational movement in all directions and any rotational movement. This is one of the simplest structural forms in which the elements are in pure tension. Structures of this type range from simple guyed or stayed structures to large cable-supported bridge and roof systems. Simple support Simple support is basically where the structural member rests on an external structure as in two concrete blocks holding a resting plank of wood on their tops. This support is similar to roller support in a sense that restrains vertical forces but not horizontal forces. Therefore, it is not widely used in real life structures unless the engineer can be sure that the member will not translate.

Structural support ... ... Structural support is a part of a building or structure providing the necessary stiffness and strength in order to resist the internal forces (vertical forces of gravity and lateral forces due to wind and earthquakes) and guide them safely to the ground. External loads (actions of other bodies) that act on buildings cause internal forces (forces and couples by the rest of the structure) in building support structures. Supports can be either at the end or at any intermediate point along a structural member or a constituent part of a building and they are referred to as connections, joints or restraints. Building support structures, no matter the materials, have to give accurate and safe results. A structure depends less on the weight and stiffness of a material and more on its geometry for stability. Whatever the condition is, a specific rigidity is necessary for connection designs. The support connection type has effects on the load bearing capacity of each element, which makes up a structural system. Each support condition influences the behaviour of the elements and therefore, the system. Structures can be either Horizontal- span support systems (floor and roof structures) or Vertical building structure systems (walls, frames cores, etc.) Structure Structure is necessary for buildings but architecture, as an idea, does not require structure. Every building has both load-bearing structures and non-load bearing portions. Structural members form systems and transfer the loads that are acting upon the structural systems, through a series of elements to the ground. Building Structure Elements include Line (beams , columns, cables , frames or arches , space frames , surface elements (walls, slab or shells ) and Freeform. The structure's functional requirements will narrow the possible forms that one can consider. Other factors such as the availability of materials, foundation conditions, the aesthetic requirements and economic limitations also play important roles in establishing the structural form. Structural systems or all their members and parts are considered to be in equilibrium if the systems are initially at rest and remain at rest when a system of forces and couples acts on them. They are not aspects of a model that should be guessed. To be able to analyze a structure, it is necessary to be clear about the forces that can be quite complicated. There are two types of forces, External Forces which are the actions of other bodies on the structure under consideration and Internal Forces which the rest of the structure exert on a member or portion of the structure as forces and couples. A little deflection or play is required for a structure to protect other surrounding materials from those forces. Support structure There are five basic idealized support structure types, categorized by the types of deflection they constrain: roller , pinned , fixed , hanger and simple support. Roller supports Roller support allows thermal expansion and contraction of the span and prevents damage on other structural members such as a pinned support. The typical application of Roller supports is in large bridges. In civil engineering, roller supports can be seen at one end of a bridge. Roller support cannot prevent translational movements in horizontal or lateral directions and any rotational movement but prevents vertical translations. Its reaction force is a single linear force perpendicular to, and away from, the surface (upward or downward). This support type is assumed to be capable of resisting normal displacement. It can be rubber bearings , rocker or a set of gears allowing a limited amount of lateral movement. A structure on roller skates, for example, remains in place as long as it must only support itself. As soon as lateral load pushes on the structure, a structure on roller skates will roll away in response to the force. Pinned support Pinned support attaches the only web of a beam to a girder called a shear connection. The support can exert a force on a member acting in any direction and prevent

STAGES OF CONSTRUCTION. https://www.facebook.com/106570887781783/posts/173043624467842/?app=fbl

in the soil is just sufficient to fill the voids of soil. That is degree of saturation is of 100%. So, there is no change in volume of soil if we reduce the shrinkage limit. Proctor’s Compaction Test on Soil Proctor’s test is conducted to determine compaction characteristics of soil. Compaction of soil is nothing but reducing air voids in the soil by densification. The degree of Compaction is measured in terms of dry density of soil. In Proctor’s Compaction Test, given soil sample sieved through 20mm and 4.75 mm sieves. Percentage passing 4.75mm and percentage retained on 4.75mm are mixed with certain proportions. Add water to it and leave it in air tight container for 20hrs. Mix the soil and divide it into 6 – 8 parts. Position the mold and pour one part of soil into the mold as 3layers with 25 blows of ramming for each layer. Remove the base plate and Weight the soil along with mold. Remove the soil from mold and take the small portion of soil sample at different layers and conduct water content test. from the values find out the dry density of soil and water content and draw a graph between them and note down the maximum dry density and optimum water content of the compacted soil sample at highest point on the curve. FAQs What are test conducted in soil before construction? The important test conducted on soil before building construction are: 1. Moisture content test 2. Atterberg limits tests 3. Specific gravity of soil 4. Dry density of soil 5. Compaction test (Proctor’s test) What is dry density of soil? The weight of soil particles in a given volume of sample is termed as dry density of soil. Dry density of soil depends upon void ratio and specific gravity of soil. Based on values of dry density soil is classified into dense, medium dense and loose categories. Dry density of soil is calculated by core cutter method, sand replacement method and water-displacement method. How to determine moisture content of soil? Moisture content or water content in soil is an important parameter for building construction. It is determined by several methods and they are: 1. Oven drying method 2. Calcium carbide method 3. Torsion balance method 4. Pycnometer method 5. Sand bath method 6. Radiation method 7. Alcohol method Of all the above oven drying method is most common and accurate method. In this method the soil sample is taken and weighed and put it in oven and dried at 110 + 5 C. After 24 hours soil is taken out and weighed. The difference between the two weights is noted as weight of water or moisture content in the soil.

Types of Soil Tests for Building Construction Various tests on soil are conducted to decide the quality of soil for building construction. Some tests are conducted in laboratory and some are in the field. Here we will discuss about the importance of various soil tests for building construction. The tests on soil are as follows. 1. Moisture content test 2. Atterberg limits tests 3. Specific gravity of soil 4. Dry density of soil 5. Compaction test (Proctor’s test) Moisture Content Test on Soil Moisture content or water content in soil is an important parameter for building construction. It is determined by several methods and they are Oven drying method Calcium carbide method Torsion balance method Pycnometer method Sand bath method Radiation method Alcohol method Of all the above oven drying method is most common and accurate method. In this method the soil sample is taken and weighed and put it in oven and dried at 110 + 5 C. After 24 hours soil is taken out and weighed. The difference between the two weights is noted as weight of water or moisture content in the soil. Specific Gravity Test on Soil Specific gravity of soil is the ratio of the unit weight of soil solids to that of the water. It is determined by many methods and they are. Density bottle method Pycnometer method Gas jar method Shrinkage limit method Measuring flask method Density bottle method and Pycnometer method are simple and common methods. Dry Density Test on Soil The weight of soil particles in a given volume of sample is termed as dry density of soil. Dry density of soil depends upon void ratio and specific gravity of soil. Based on values of dry density soil is classified into dense, medium dense and loose categories. Dry density of soil is calculated by core cutter method, sand replacement method and water-displacement method. Core Cutter Method for Soil Dry Density Testing In this methods a cylindrical core cutter of standard dimensions is used to cut the soil in the ground and lift the cutter up with soil sample. The taken out sample is weighed and noted. Finally water content for that sample is determined and dry density is calculated from the below relation. Sand Replacement Method for Soil Dry Density Testing In this method also, a hole is created in the ground by excavating soil whose dry density is to be find. The hole is filled with uniform sand of known dry density. So by dividing the mass of sand poured into the hole with dry density of sand gives the volume of hole. So we can calculate the soil dry density from above formula. Atterberg Limits Test on Soil To measure the critical water content of a fine grained soil, Atterberg provided 3 limits which exhibits the properties of fine grained soil at different conditions. The limits are liquid limit, plastic limit and shrinkage limit. These limits are calculated by individual tests as follows. Liquid Limit Test on Soil In this test, Casagrande’s liquid limit device is used which consist a cup with moving up and down mechanism. The cup is filled with soil sample and groove is created in the middle of cup with proper tool. When the cup is moved up and down with the help of handle the groove becomes closed at some point. Note down the number of blows required to close the groove. After that water content of soil is determined. Repeat this procedure 3 times and draw a graph between log N and water content of soil. Water content corresponding to N=25 is the liquid limit of soil. Plastic Limit Test on Soil Take the soil sample and add some water to make it plastic enough to shape into small ball. Leave it for some time and after that put that ball in the glass plate and rolled it into threads of 3mm diameter. If the threads do not break when we roll it to below 3mm diameter, then water content is more than the plastic limit. In that case reduce water content and repeat the same procedure until crumbling occurs at 3mm diameter. Finally find out the water content of resultant soil which value is nothing but plastic limit. Shrinkage Limit Test on Soil In case of shrinkage limit, the water content