Maawa World

Specific Gravity of Soil The specific gravity is the ratio between the weight per unit volume of the material and the weight per unit volume of water at a stated temperature, usually 20°C. There are three ways of determining and expressing specific gravity; specific gravity of the solids, the apparent specific gravity, or the bulk specific gravity.  The specific gravity of the solid substance of most inorganic soils varies between 2.60 and 2.80. Tropical iron-rich laterite as well as some lateritic soils will generally have a specific gravity of 3.0 or more. Sand particles composed of quartz have a specific gravity around 2.65. Clays generally range from 2.68 to 2.75, but can have values as high as 3.50. The solids of soil particles are composed of minerals. Generally these minerals will have a specific gravity greater than 2.60. Therefore, values of specific gravity smaller than 2.60 indicate the possible presence of organic matter.

Soil Soil is a heterogeneous aggregation of uncemented or weakly cemented mineral grains enclosing voids of varying sizes. These voids may contain air, water, organic matter, or different combinations of these materials. The engineer must therefore be concerned not only with the sizes of the particles, but also with the voids between them and particularly what these voids enclose (water, air, or organic materials). Rocks Geologists classify rocks into three basic groups: igneous, formed by cooling from a molten state; sedimentary, formed by accumulation and cementation of existing particles and remains of plants and animals; and metamorphic, formed from existing rocks subjected to heat and pressure. Exposed to the atmosphere, these rocks disintegrate and decompose either by mechanical action (wind, water, ice, and vegetation) or chemical action, or both. The resulting material may remain where it is formed; or it may be transported by water, glaciers, wind, or gravity and deposi

FERRO-CEMENT The term ferro-cement implies the combination of ferrous product with cement. Generally this combination is in the form of steel wires meshes embedded in a portland cement mortar. Wire mesh is usually of 0.8 to 1.00 m diameter steel wires at 5 mm to 50 mm spacing and the cement mortar is of cement sand ratio of 1:2 or 1:3. 6 mm diameter bars are also used at large spacing, preferably in the corners. Sand may be replaced by baby jelly. The water cement ratio used is between 0.4 to 0.45. Ferro-cement reinforcement is assembled into its final desired shape and plastered directly. There is no need for form work. Minimum two layers of reinforcing steel meshes are required. According to American Concrete Institute “Ferro cement is a thin walled reinforced concrete construction where usually a hydraulic cement is reinforced with layers of continuous and relatively small diameter mesh. The mesh used may be metallic or any other suitable material.” Ferro-cement is fas

CELLULAR CONCRETE It is a light weight concrete produced by introducing large voids in the concrete or mortar. Its density varies from 3 kN/m3 to 8 kN/m3 whereas plain concrete density is 24 kN/m3. It is also known as aerated, foamed or gas concrete. Properties of cellular concrete: It has the following properties: 1. It has low weight. 2. It has good fire resistance. 3. It has good thermal insulation property. 4. Thermal expansion is negligible. 5. Freezing and thawing problems are absent. 6. Sound absorption is good. 7. It has less tendency to spall. Uses of Cellular Concrete 1. It is used for the construction of partition walls. 2. It is used for partitions for heat insulation purposes. 3. It is used for the construction of hollow filled floors.

FIBRE-REINFORCED CONCRETE (FRC) Plain concrete possesses deficiencies like low tensile strength, limited ductility and low resistance to cracking. The cracks develop even before loading. After loading micro cracks widen and propagate, exposing concrete to atmospheric actions. If closely spaced and uniformly dispered fibres are provided while mixing concrete, cracks are arrested and static and dynamic properties are improved. Fibre reinforced concrete can be defined as a composite material of concrete or mortar with discontinuous and uniformly distributed fibres. Commonly used fibres are of steel, nylon, asbestos, coir, glass, carbon and polypropylene. The length to lateral dimension of fibres range from 30 to 150. The diameter of fibres vary from 0.25 to 0.75 mm Fibre reinforced concrete is having better tensile strength, ductility and resistance to cracking. Uses of FRC 1. For wearing coat of air fields, roads and refractory linings. 2. For manufacturing precast products lik

PRESTRESSED CONCRETE (PSC) Strength of concrete in tension is very low and hence it is ignored in R.C.C. design. Concrete in tension is acting as a cover to steel and helping to keep steel at desired distance. Thus in R.C.C. lot of concrete is not properly utilized. Prestressing the concrete is one of the method of utilizing entire concrete. The principle of prestressed concrete is to introduce calculated compressive stresses in the zones wherever tensile stresses are expected in the concrete structural elements. When such structural element is used stresses developed due to loading has to first nullify these compressive stresses before introducing tensile stress in concrete. Thus in prestressed concrete entire concrete is utilized to resist the load. Another important advantage of PSC is hair cracks are avoided in the concrete and hence durability is high. The fatigue strength of PSC is also more. The deflections of PSC beam is much less and hence can be used for longer spans also.

REINFORCED BRICK CONCRETE (RBC) It is the combination of reinforcement, brick and concrete. It is well known fact that concrete is very weak in tension. Hence in the slabs, lintels and beams the concrete in the portion below the neutral axis do not participate in resisting the load. It acts as a filler material only. Hence to achieve economy the concrete in tensile zone may be replaced by bricks or tiles. Dense cement mortar is used to embed the reinforcement. The reinforcement may be steel bars, expanded mesh etc.

REINFORCED CEMENT CONCRETE (R.C.C.) Concrete is good in resisting compression but is very weak in resisting tension. Hence reinforcement is provided in the concrete wherever tensile stress is expected. The best reinforcement is steel, since tensile strength of steel is quite high and the bond between steel and concrete is good. As the elastic modulus of steel is high, for the same extension the force resisted by steel is high compared to concrete. However in tensile zone, hair cracks in concrete are unavoidable. Reinforcements are usually in the form of mild steel or ribbed steel bars of 6 mm to 32 mm diameter. A cage of reinforcements is prepared as per the design requirements, kept in a form work and then green concrete is poured. After the concrete hardens, the form work is removed. The composite material of steel and concrete now called R.C.C. acts as a structural member and can resist tensile as well as compressive stresses very well. Properties of R.C.C./Requirement of Goo

Desirable Properties of Concrete Appropriate quality and quantity of cement, fine aggregate, coarse aggregate and water should be used so that the green concrete has the following properties: (a) Desired workability (b) No seggregation in transporting and placing (c) No bleeding and (d) No harshness. Hardened concrete should have (a) required characteristic strength (b) minimum dimensional changes (c) good durability (d) impermeable (e) good resistance to wear and tear. Uses of Concrete 1. As bed concrete below column footings, wall footings, on wall at supports to beams 2. As sill concrete 3. Over the parapet walls as coping concrete 4. For flagging the area around buildings 5. For pavements 6. For making building blocks. However major use of concrete is as a major ingradient of reinforced and prestressed concrete. Many structural elements like footings, columns, beams, chejjas, lintels, roofs are made with R.C.C. Cement concrete is used for making storage st

Tests on Concrete The following are some of the important tests conducted on concrete: 1. Slump test. 2. Compaction factor test. 3. Crushing strength test. 1. Slump Test: This test is conducted to determine the workability of concrete. It needs a slump cone for test. Slump cone is a vessel in the shape of a frustum of a cone with diameter at bottom 200 mm and 50 mm at top and 300 mm high. This cone is kept over a impervious platform and is filled with concrete in four layers. Each layer is tamped with a 16 mm pointed rod for 25 times. After filling completely the cone is gently pulled up. The decrease in the height of the concrete is called slump. Higher the slump, more workable is the concrete. 2. Compaction Factor Test: This is another test to identify the workability of concrete. This test is conducted in the laboratory. The test equipment consists of two hoppers and a cylinder fixed to a stand, the dimensions and the distances between the three vessels being standar

Properties of Concrete Concrete has completely different properties when it is the plastic stage and when hardened. Concrete in the plastic stage is also known as green concrete. The properties of green concrete include: 1. Workability 2. Segregation 3. Bleeding 4. Harshness. The properties of hardened concrete are: 1. Strength 2. Resistance to wear 3. Dimensional changes 4. Durability 5. Impermeability. Properties of Green Concrete 1. Workability: This is defined as the ease with which concrete can be compacted fully without seggregating and bleeding. It can also be defined as the amount of internal work required to fully compact the concrete to optimum density. The workability depends upon the quantity of water, grading, shape and the percentage of the aggregates present in the concrete. Workability is measured by (a) The slump observed when the frustum of the standard cone filled with concrete is lifted and removed. (b) The compaction factor determined

Curing of Concrete Curing may be defined as the process of maintaining satisfactory moisture and temperature conditions for freshly placed concrete for some specified time for proper hardening of concrete. Curing in the early ages of concrete is more important. Curing for 14 days is very important. Better to continue it for 7 to 14 days more. If curing is not done properly, the strength of concrete reduces. Cracks develop due shrinkage. The durability of concrete structure reduces. The following curing methods are employed: (a) Spraying of water (b) Covering the surface with wet gunny bags, straw etc. (c) Ponding (d) Steam curing and (e) Application of curing compounds. (a) Spraying of water: Walls, columns, plastered surfaces are cured by sprinkling water. (b) Wet covering the surface: Columns and other vertical surfaces may be cured by covering the surfaces with wet gunny bags or straw. (c) Ponding: The horizontal surfaces like slab and floors are cured by stagnatin

PLAIN CONCRETE Major ingredients of concrete are! 1. Binding material (like cement, lime, polymer) 2. Fine aggregate (sand) 3. Coarse aggregates (crushed stone, jelly) 4. Water. A small quantity of admixtures like air entraining agents, water proofing agents, workability agents etc. may also be added to impart special properties to the plain concrete mixture. Depending upon the proportion of ingredient, strength of concrete varies. It is possible to determine the proportion of the ingredients for a particular strength by mix design procedure. In the absence of mix design the ingredients are proportioned as 1:1:2,  1:1/1-2 :3,  1:2:4,  1:3:6 and 1:4:8, which is the ratio of weights of cement to sand to coarse aggregate. In proportioning of concrete it is kept in mind that voids in coarse aggregates are filled with sand and the voids in sand are filled with cement paste. Proportion of ingredients usually adopted for various works are shown in Table. Proportion o

Concrete! Plain concrete, commonly known as concrete, is an intimate mixture of binding material, fine aggregate, coarse aggregate and water. This can be easily moulded to desired shape and size before it looses plasticity and hardens. Plain concrete is strong in compression but very weak in tension. The tensile property is introduced in concrete by inducting different materials and this attempt has given rise to RCC, RBC, PSC, FRC, cellular concrete and Ferro cement. Proportioning, mixing, curing, properties, tests and uses of plain concrete is dealt in detail. The other improved versions of concrete are explained and their special properties and uses are pointed out.

The following tests are conducted on the prepared mortars to ensure their quality! 1. Crushing Test 2. Tensile Strength Test 3. Adhesive Test. 1. Crushing Test: This test is carried out on a brick work with the mortar. This brick work is crushed in a compression testing machine and the load is noted down. Then the crushing strength is obtained as load divided by cross-sectional area. 2. Tensile Strength Test: The mortar prepared is placed in a mould of bricket which has central cross-sectional area as 38 mm × 38 mm. After curing the briquette is pulled under the grips of tensile testing machine. The ultimate load noted. Then the tensile strength of mortar is load divided by the central cross-sectional area. 3. Adhesive Test: Two bricks are joined together with mortar to be tested. The upper brick is suspended from an overhead support. A board is hung from the lower brick. Then weights are added to the board till the bricks separate. The adhesive strength is the load di

The following are some of the special mortars! 1. Cement clay mortar 2. Gauged mortar 3. Decorative mortar. 1. Cement Clay Mortar: Quality of clay mortar can be improved by adding cement to the mix. Normal proportion of clay to cement is 1:1. It maintains the economy to some extent and there is sufficient improvements in the durability of mud-mortar. 2. Gauged Mortar: It is the mortar obtained by adding cement to lime mortar. The usual proportion of cement, lime and sand are 1:1:6, 1:2:9 and 1:3:12. This mortar is to be used within half an hour after mixing cement. Obviously, it is cheaper than cement mortar and its quality is between that of cement mortar and lime mortar. 3. Decorative Mortar: These mortars are obtained by using coloured cement. They are used to give pleasant appearance to outer walls.

Clay lumps are collected and are wetted with water and allowed to mature for 1 or 2 days. It is kneeded well until it attains required consistancy. Sometimes fibrous materials like gobber is added in the mix. It prevents cracks in the plaster. If plaster is to be used for outer walls, it is sprayed or painted with bitumen. It is cheap mortar. Its durability is less. It is normally used for the construction of temporary sheds and cheap houses in rural areas.