Impact of course aggregate on mix design of concrete

Coarse aggregate is considered as one of the vital elements of concrete and captures the major volume in the mix as well as impacts the concrete mix design significantly. Its different characteristics like strength, maximum size, shape, and water absorption affect water demand, the quantity of cement and fine aggregate in concrete mixture.

It is found that high maximum coarse aggregate size can reduce water demand in the mixture as such aggregate contains lower surface area with regards to small coarse aggregate size.

While considering shape, rounded type aggregate offers reasonable mix design for normal strength concrete.

For high strength concrete, angular coarse aggregate is essential. The scope of segregation is reduced when coarser aggregate is graded perfectly for concrete mix design.

While considering strength, greater aggregate strength generates higher concrete strength.

1. Maximum Aggregate Size: The maximum size of coarse aggregate is another vital factor that checks water demand to attain definite workability. It also determines the quantity of fine aggregate content required for developing cohesive mix.

For a specified weight, if the maximum size of aggregate is increased, the surface area of coarse aggregates is reduced and conversely. When the maximum size of coarse aggregate is decreased, the surface area of coarse aggregate is raised. If the surface area is large, the water demand will be increased to coat the particles and produce workability.

For smaller maximum size of coarse aggregate, larger fine aggregate content is required to coat particles and retain cohesiveness of concrete mix. Therefore, for similar workability, 40mm down aggregate contains lower water/cement ratio, thus strength is raised with regards to 20mm down aggregate. Due to its lower water demand, higher maximum size of coarse aggregate can reduce the cement consumption.

Maximum size of aggregate is confined with clear cover and minimum distance among the reinforcement bars. Maximum size of coarse aggregate is lower than clear cover or minimum distance among the reinforcement bars. As a result, the aggregates can get through the reinforcement in congested areas, to form dense and uniform concrete.

2. Grading of Coarse Aggregate: Grading means the establishment of the particle-size distribution for aggregate. It influences the amount of cement and water requirements, workability, pumpability, and stability of concrete. The grading of coarse aggregate is crucial to acquire cohesive and dense concrete. The voids due to larger coarse aggregate particles are filled with smaller coarse aggregate particles.

If the grading of coarse aggregate is perfect, the scope of segregation is reduced, specifically for higher workability and the compatibility of concrete in enhanced. The coarse aggregate grading limits are provided in ASTM C33/ C33M and IS 383 – 1970 – table 2, Clause 4.1 and 4.2 for single size aggregate as well as graded aggregate.

3. Shape of Coarse Aggregate: The shape of coarse aggregates range from round, angular, or irregular. Rounded aggregates contain lowest water demand because of lower surface area, and also contain lowest mortar paste requirement.

These properties facilitate to produce rounded aggregate to give in the most reasonable mixes for concrete grades up to M35. However, for concrete grades of M40 and above the scope of bond failure would slant the balance in support of angular aggregate containing larger surface area.

Flaky and elongated coarse aggregate particles raise the water demand as well as the susceptibility of segregation. The flexural strength of concrete is minimized due to flakiness and elongation. Specifications provided by Ministry of Surface Transport confine the combined flakiness and elongation to 30% by weight of coarse aggregates.

4. Strength of Coarse Aggregate: Material strength of coarse aggregate is determined by crushing strength of rock, aggregate crushing value, aggregate impact value, aggregate abrasion value. The IS limits for above tests range from Aggregate Crushing value, Aggregate Impact value, and Aggregate abrasion value.

5. Aggregate Absorption: The purpose of aggregate absorption is to employ a correction factor for aggregates in dry condition and find out water demand for concrete in saturated surface dry condition. Aggregate can consume water up to 2 % by weight when remains in bone dry state. But occasionally, the aggregate absorption remains as high as 5%.

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M Sand Vs River Sand - Which sand is better for Construction?

M Sand or Robo Sand is gaining popularity due to ecological factors and insufficient number of superior quality river sand

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Some useful tips to measure loads on column, beam and slab

In order to work out the total load on columns, Beam and Slab, there should be clear ideas on the types of loads enforcing on the column.

Different Loads operating on Column:

1) Column Self Weight X Number of floors
2) Beams Self Weight per running meter
3) Load of walls per running meter
4) Total load on Slab (Dead load + Live load + Self weight)

Apart from above loading, the columns are also susceptible to bending moments which should be taken into consideration in the final design.

For Colomn: The Self weight of Concrete remains approx 2400 kg/m3, that is similar to 240 kN and self weight of steel is approx 8000 kg/m3.

Therefore, if we consider a column size of 230 mm x 600 mm with 1% steel and 3 meters standard height, the self weight of column is approx 1000 kg per floor that is equivalent to 10 kN.

At the time of making calculation, self weight of columns is taken as 10 to 15 kN per floor.

For Beam: Similar method is also used for making calculations of beam. Suppose, each meter of beam contains dimensions of 230 mm x 450 mm without slab thickness. Therefore, the self weight should be approx 2.5 kN per running meter.

For Walls: The Density of bricks differs among 1500 to 2000 kg per cubic meter. For a brick wall with thickness 6 inch, height 3 meter a length 1 meter. The load / running meter should be equivalent to 0.150 x 1 x 3 x 2000 = 900 kg, that is identical to 9 kN/meter. This method is useful for working out the load of brick per running meter for any brick type.

For aerated concrete blocks and autoclaved concrete blocks similar to Aerocon or Siporex, the weight per cubic meter should remain 550 to 700 kg per cubic meter.

When these blocks are utilized for construction, the wall loads for each running meter should remain as low as 4 kN/meter, the cost of the project is decreased considerably with the use of this block.

For Slab: Suppose, the slab contains thickness of 125 mm.

Therefore, self weight of each square meter of slab should be = 0.125 x 1 x 2400 = 300 kg that is identical to 3 kN.

Now, If finishing load is taken to be 1 kN per meter and superimposed live load to be 2 kN per meter. Therefore, from above data, the load of slab can be calculated as 6 to 7 kN approximately per square meter.

Factor of Safety: At the end, once the total load on a column is computed, consider the factor of safety that is very crucial for any building design for safe and convenient performance of building during its design life cycle.

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Published By
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www.bimoutsourcing.com
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Details on Anti-termite Treatment in construction site

Use of Anti-termite treatment in construction site is very crucial. It should be performed prior to binding works. The following points should be taken into consideration for anti-termite treatment :-

1. Preconstruction Activities:

a. Detach all wooden pieces, concrete debris, plastic strips, tree stump roots, deleterious materials etc.
b. The soil should be leveled and compacted for bottom of the blinding prior to apply the treatment.
c. Before starting the treatment, all relevant tests should be conducted as per the approval of the site engineer.

2. Construction Activities:

a. Under PCC of Foundation
b. After getting the approval of engineering for further proceeding, the application should start instantly initially arranging and mixing the solutions of approved anti-termite material diluted with water at a rate of 1:49 Liter water according to the suggestions of manufacturer.

c. A solution of approved Anti-termite material diluted with water at a rate of 1:49 Liter should be sprinkled before casting of blinding of footing or any foundation type with the application rate of 4 to 5 liter per square meter.
d. After that, the treated area should be covered with polythene sheet, it should be opened free till 2 hours, and then casting should be conducted. Casting should be finished within 2 to 24 hours after the use of Anti-termite.

3. Trench Around the Foundation Wall

4. Backfill the sides up to the top level of the projected foundation (assuming that the height of the foundation will surpass over 1 meter ).

5. Create a 150mm wide and 150mm deep trench across the perimeter of the foundation.

6. A solution of Approved Anti-termite material diluted with water at a rate of 1:49 Liter should be sprinkled with an application rate of 5 -6 Liter per linear meter into the trench across the edge of the foundation.

7. The trench should be backfilled within 24 hours of treatment.

8. Under PCC of Tie Beam

9. A solution of Approved Anti-termite material diluted with water at a rate of 1:49 Liter should be sprinkled before starting casting of PCC of tie beams with the application rate of 4 to 5 Liter square meter.

10. The treated area should be instantly wrapped with polythene sheet, it will be opened for 2 hours prior to casting. And casting should be finished within 2 to 24 hours after using the Anti-termite.
11. Underground Slab and Along the Side of the Beams
12. The trench should be built up with a measure of 150mm by a 150mm depth across the border of the tie beams.
13. A solution of the Approved Anti-termite material diluted with water at a rate of 1:49 Liter should be sprinkled before casting ground slab with the application rate of 4 to 5 per square meter.
14. With the similar solution of Approved Anti-termite material, use 5 to 6 Liter per linear meter into the trench across the border of the tie beams. Backfill the trench after allowing the termiticide to be settled within 2 hours. The backfilling should be accomplished within 2 to 24 hours after using the anti-termite.
15. Along the Outside Perimeter of the Building.
16. Create a 150mm in wide and 150mm deep trench, across the outside periphery of the building at the similar level of the landscape.
17. A solution of Approved Anti-termite material diluted with water at a rate of 1:49 Liter should be sprinkled into the trench across the external perimeter of the building with an application rate of 5 to 6 Liter per linear meter.
18. Back fill the trench within 2 to 24 hours after the treatment.

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Causes of dampness and preventive measures

When the materials consume water, dampness in any building happens. Dampness is harmful for the residents of the building.

Damp Proof Course(DPC): The purpose of a course is to resist the penetration of the damp into the building and it is called as damp proof course(DPC). DPC should be provided when the construction of the building is started.

Reasons for Dampness: The dampness happens for the following reasons -

1. Ground Water Table: When the groundwater table is high, it significantly impact the foundation since the building material utilized for foundation consumes the water from the ground through capillary action.

2. Rain: When there is no proper protection for the external walls, then rainwater may harm the building.

3. When the building is situated in such an area that the water can’t be easily discharged, dampness will occur.

4. There is also possibility for dampness due to bad workmanship in construction.

5. When the walls are constructed afresh, dampness may occur for a short period.

6. If the slope of a roof remains very flat, the rainwater may enter and water is temporarily stockpiled on the roof.

7. There should be proper damp proofing course on the uncovered tops of the parapet walls and compound walls so that the damp can’t penetrate through these exposed tops.

Impacts of Dampness - The dampness may provide the following harmful effects:

1. The plaster will be soothed and may crumble. 2. Electric fitting will be affected. 3. Distempers or paints will be damaged. 4. Unhygienic conditions will be created for dwellers. 5. Growth of termites. 6. Steel utilized in building construction will be eroded.

7. Unattractive patches will develop on the wall surface and ceiling. 8. The material utilized as floor covering will be damaged. 9. Due to continuous existence of moisture in the walls, efflorescence will occur and it results in detaching of stone, bricks, tiles etc.

Methods of Damp Proofing - To get rid of dampness, the following techniques should be applied:

1. Application of Damp Proof Course: Damp proof course stands for the layer of materials like bituminous, cement, stones etc.

They are arranged in the building at all suitable positions from where water may penetrate the building. Normally, it is arranged in the building at plinth level for walls, over the concrete bed for flooring.

2. Surface Treatment: Under this method, a thin film of water repellent material is arranged over the surface which fills the holes of the materials of the building which are uncovered to the moisture.

3. Integral Damp Proofing Treatment: Under this method, water repelled compounds are blended to the concrete or mortar throughout the mixing process of concrete. These compounds function as barriers and resist the entry of moisture to the building.

4. Cavity Wall or Hollow Wall: It comprises of cavity or air drains into the wall to resist the rising of moisture from the ground to the wall.

5. Pressure Grouting or Cementation: Under this method, holes are drilled at the various sections of the building at selected points. Then thin cement paste is plunged into the holes by pressure to make the structure water-resistant.

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Published By
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www.constructioncost.co
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How to make concrete formwork system safe in the job site

If the safety plans for formwork system remain perfect, construction project will be completed according to approved construction practice and without unnecessary hazards like failure of formwork and consequent delay in construction and possible loss of life. Normally, safety will be commenced in the planning and management of a project.

The safety of workers and the public is mainly dependent on the construction work supervision, equipment deployment practices, and construction method used.

Formwork designers, project planners, and managers should abide by the requirements of relevant standards like OSHA regulation with the purpose of providing good safety planning.

1. Supervision and Inspection: Supervision work should be perfect throughout placing formwork, concrete pouring, and stripping of formworks for maintaining safety in the use of forms.

A supervisor has to take the responsibility for the construction of formwork based on the design and deploying a secure erection method to get rid of overloading of members.

Rectifications of the false work, forms, or erection method in construction site should be performed as per suggestions of the form designer. The application of unusual loads should be controlled when the formwork system is not designed for such loads.

2. Platforms and Access for Workers: There should be proper working platform for the works to be accomplished at elevated positions. Long ladder to the tops of false work should be rest or passing platform. In specific structures like bridges, special scaffolds should be utilized or any other appropriate equipment for the detachment of decks and piers forms.

There should be proper safety signs and barriers to restrict the entry of the unauthorized individuals to the working area throughout erection and striping formwork system. Platforms and access means for worker should be adhered to suitable codes like OSHA requirements.

3. Monitoring Concreting Practices: Controlling concrete pouring plays an important role in formulating the proper safety plan. The sequence and rate of concrete placement should take limitations into account which are provided in the formwork drawings.

At the time of placing concrete, unbalanced form loading should not occur. As for example, in beam and slab construction, initially fill the beams and then work outward uniformly on both sides while setting the slab. The durability of the formwork system is increased in a column-and-slab structure by concreting the columns minimum one day before placing the slab.

Hardened concrete in the columns contributes to extra lateral stiffness to the formwork structure all through the concreting of the slab. Repeatedly, Reinforced concrete column is built up and then erection of floor system is started.

4. Advancement of Soil Bearing and Bracing: Normally, the condition of soil is unidentified while designing the forms and scaffoldings; assumed bearing capacity is applied. Therefore, the contractor should verify whether the bearing strength of the soil at construction site is higher or smaller as compared to the one accepted during design phase. If soil bearing strength is dubious, then it’s stability can be improved by tamping or making a cover with layer of crushed stone. Formworks should be designed to resist gradual collapse caused by localized failures.

5. Shoring and Reshoring: Setting of out of plumb shores, bent jacks, and defective timber should not be done else these supporting elements would support only a small area of design loads. Throughout Concreting, forms should be checked repeatedly to detect problems and instantly rectify them.

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Variations among framed structures & load bearing structures

Structures are available with several models like solid, framed, shell, load bearing, membrane, composite, trusses, cables and arches, surface structure etc. They are categorized depending on the geometry so as to obtain strength and withstand various types of loads.

Under a framed structure system, a framework or ‘skeleton’ comprising beams and columns is applied to bear the structural loads down the building to the foundations. Generally, the framework is made of steel or reinforced concrete, but in very small (normally single-storey) structures, it is built with timber or aluminium.

Under a non-framed structure system, the wall itself becomes load-bearing. These load-bearing walls are normally constructed by masonry, but reinforced concrete is also used to build up them. Here, the loads are transferred to the foundations through walls.

Given below, the points of variations among framed structure and load bearing structure.

1. Framed Structure: A framed structure integrates different structural components like beam, column and slab which are attached together to defend the gravity and various lateral loads. The purpose of these structures is to control the large forces, moments caused by the applied loads.

Load Bearing Structure: In Load bearing structure, the loads of the roofs along with lateral loads are carried by walls, and through walls they are delivered to lower floor and finally to foundations.

2. Framed Structure: Framed structure contains beam, column, and slab.

Load Bearing Structure: Load bearing structure contains heavy masonry walls with brick or stone that provides support to the whole structure.

3. Framed Structure: In framed structure, vertical load transfer path directs from slab/floor to beam, beam to column and column to footing and then to soil.

Load Bearing Structure: In load bearing structure, vertical load transfer path directs from slab/floor to walls and walls to footing.

4. Framed Structure: Multi storey buildings with various heights are built up. These buildings are normally suitable for office, hotel, residential apartment and provide the vertical circulation in the form of stairs and lifts which engross up to 20% of the floor area.

Load Bearing Structure: Limited storey buildings are built up. For load-bearing construction, in several countries, even 14 storied buildings are constructed only with masonry.

5. Framed Structure: Framed structure has strong resistance capacity against Earthquake.

Load Bearing Structure: Load bearing structure is not very effective to withstand Earthquake due to its limitations. But for low rise buildings, it functions equally well.

6. Framed Structure: In framed structure all the walls are leaner.

Load Bearing Structure: In load bearing structure walls are denser.

7. Framed Structure: In these types of structures, there are lots of carpet areas and they are leaner.

Load Bearing Structure: In these types of structures less carpet area is available, as walls are thicker and hence carpet area efficiency of planning is less.

8. Framed Structure: Less excavation is required for this type of construction.

Load Bearing Structure: Higher excavation is required for this type of construction.

9. Framed Structure: It is less material intensive.

Load Bearing Structure: It is more material intensive and as a result the dead load is increased

10. Framed Structure: Thickness of wall is unchanged during the construction. Thickness of wall is not changed if the height is raised.

Load Bearing Structure: Thickness of wall remains inconsistent during the construction. The thickness of the wall is raised when the height is higher.

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