QUALITY CONTROL MEASURES AT SITE

QUALITY CONTROL

MEASURES AT SITE

Study duties responsibilities, Tender specification, standards, codes of practice and work instruction.

Evolve effective acceptance/rejection procedures for construction materials in coordination with the project purchase department.

Do proper sampling and testing of steel, cement, concrete, aggregates, water, etc., and verify test results in view of standards and work specifications prior to their use in construction. Also control quality of electrodes to their use in welding.

Set procedures to control quality at the points of storage for raw materials, mixing and placing of concrete.

Follow the prescribed curing and deshuttering schedules.

Observe procedures to control quality of welded joints of structural steel members.

Evolve a system to check quality of workmanship in all construction activities.

Keep all revised Indian Standards and codes of practice available in QC laboratory and have them handy during discussion with client/consultant.

Maintain sequence of construction required under any activity.

Discuss QA/QC issues as a separate agenda during site review meetings with staff.

Observe regular schedule for maintenance, repairs and calibration of plants and equipments.

Keep spare parts/materials for laboratory equipments weigh batchers, batching plant, etc., always keep spare vibrators ready at site.

Carry work instruction cards in pocket while supervising/inspecting works.

Regularly maintain the formats prescribed under ISO 9002 Quality assurance system

Practice sound house keeping methods to achieve saving, safety and quality.

How to Calculate Number of Bricks Per Square Foot?

How to Calculate Number of

Bricks Per Square Foot?

No matter what the nature of a brickwork project, calculating the number of bricks per square foot helps determine how many bricks are needed for the project as a whole. You also need to know the square footage of the area where the bricks are needed, such as a wall or a patio. Once you've calculated these figures, you can estimate the amount of bricks needed. Include 5 to 10 percent of overage as well, in case of breakage or damaged bricks.(Figure-1)

Step 1:

Measure the length and width of one brick with a tape measure. Multiply the length and width, such as 4 inches wide by 8 inches long, to get the total square inches. Note: This is for bricks that will install with their broad faces up, as with patio pavers. If the bricks will install with their front faces exposed, as with standard wall brick, measure the length and the height, not the width..(Figure-2)

Step 2:

Divide the total square inches into 144, which is the total number of inches in a square foot. If the brick is 32 square inches, for instance, the result is 4.5 bricks per square foot (144 divided by 32)..(Figure-3)

Step 3:

Measure the length and width of the project space, such as a patio or wall 10 feet high, 12 feet wide. The resulting number -- in this case, 120 -- is the square footage of the project space..(Figure-4)

Step 4:

Determine the amount of bricks needed for the job by multiplying the square footage -- 120 square feet -- by the number of bricks in a square foot -- 4.5. For this instance, 540 bricks are needed. Keep in mind some bricks may be damaged or unusable, so purchase more than necessary for the job. A project such as a patio or walkway may not require mortar between bricks as a wall does. A mortarless installation requires more bricks than a project involving mortar..(Figure-5)

PROPER METHODS FOR CONCRETE PLACEMENT

PROPER METHODS FOR 

CONCRETE PLACEMENT 

PROCEDURE FOR PLACING 

OF CONCRETE:

Before any concrete is placed the entire placing programme consisting of equipment, layout, proposed procedures and methods is planned and no concrete is placed until formwork is inspected and found suitable for placement. Equipment for conveying concrete should be of such size and design as to ensure a practically continuous flow of concrete during depositing without segregation of materials considering the size of the job and placement location.

Concrete is placed in its final position before the cement reaches its initial set and concrete is compacted in its final position within 30 minutes of leaving the mixer and once compacted it should not be disturbed.

In all cases the concrete is deposited as nearly as practicable directly in its final position and should not be re-handled or caused to flow in a manner which may cause segregation, loss of materials, displacement of reinforcement, shuttering or embedded inserts or impair its strength. For locations where direct placement is not possible and in narrow forms suitable drop and Elephant Trunks to confine the movement of concrete is provided. Special care is taken where concrete is dropped from a height especially if reinforcement is in the way particularly in columns and thin walls.

Concrete should be placed in the shuttering by shovels or other methods and should not be dropped from a height more than one metre or handle in a manner which will cause segregation.

Concrete placed in restricted forms by borrows, buggies, cars, sort chutes or hand shoveling should be subjected to the requirement for vertical delivery of limited height to avoid segregation and should be deposited as nearly as practicable in it’s final position.

Concreting once started should be continuous until the pour is completed. Concrete should be placed in successive horizontal layers of uniform thickness ranging from 150 mm to 900 mm. These should be placed as rapidly as practicable to prevent the formation of cold joints or planes of weakness between each succeeding layers within the pour.

The thickness of each layer should be such that it can be deposited before the previous layer has stiffened. The bucket loads or other units of deposit should be spotted progressively along the face of the layer with such overlap as will facilitate spreading the layer to uniform depth and texture with a minimum of shoveling. Any tendency to segregation should be corrected by shoveling stones into mortar rather than mortar onto stones. Such a condition should be corrected by redesign of mix or other suitable means.

The top surface of each pour and bedding planes should be approximately horizontal unless otherwise specified in drawings.

WRITING SPECIFICATIONS CONSTRUCTION CONTRACTS

WRITING SPECIFICATIONS FOR

CONSTRUCTION CONTRACTS

In writing specifications for construction contracts, care must be taken to ensure consistency of requirements throughout and conformity with what is written in other documents. This consistency can be promoted if one person drafts all the documents or, if parts are written by others, one person carefully reads through the whole finished set of documents. An inconsistency in the documents can give rise to a major dispute under the contract, having a serious effect on its financial outcome.

Some principle guidelines for writing specifications are as follows.

• The layout and grouping of subjects should be logical. These need planning out beforehand.

• Requirements for each subject should be stated clearly, in logical order, and checked to see all aspects are covered.

• Language and punctuation should be checked to see they cannot give rise to ambiguity.

• Legal terms and phrases should not be used.

• To define obligations the words ‘shall’ or ‘must’ (not ‘should’ or ‘is to’, etc.) should be used.

• Quality must be precisely defined, not described as ‘best’, etc.

• Brevity should be sought by keeping to essential matters.

It is not easy to achieve an error-free specification. It is of considerable assistance to copy model clauses that, by use and modification over many previous contracts, have proved satisfactory in their wording. Such model clauses can be held on computer files so they are easy to reproduce and modify to make relevant to the particular project in hand. Copying whole texts from a previous specification which can result in contradictory requirements should not be adopted. Entirely new material is quite difficult to write and will almost certainly require more than one attempt to get it satisfactory.

The specification has to tell the contractor precisely:

• The extent of the work to be carried out;

• The quality and type of materials and workmanship required;

• Where necessary, the methods he is required to use, or may not use, to construct the works.

Under the first an informative description is given of what the contractor is to provide and all special factors, limitations, etc. applied. Under the second the detailed requirements are set out. The extent of detail adopted should relate to the quantity and importance of any particular type of work in relation to the works required. Thus the specification for concrete quality may be very extensive where much structural concrete is to be placed; but it may be quite short if concrete is only required as bedding or thrust blocks to a pipeline. A ‘tailormade’ specification appropriate to the nature of the work in the contract should be the aim.

Repetition of requirements should be avoided. If requirements appear in two places, ambiguity or conflict can be caused by differences of wording. Also there is a danger that a late alteration alters one statement but fails to alter its repetition elsewhere.

The third of the items noted above needs careful consideration, as there may be dangers and liabilities involved in telling the contractor how to go about his work. Some methods may need to be specified, such as the requirements concerning the handling and placing of concrete, but these and similar matters should be specified under workmanship and materials clauses. Other directions on method should be given only if essential for the design. For instance, if it is necessary to under-pin or shore up an existing structure, the exact method used should not be specified for, if the contractor follows the method and damage ensues, the liability for damage may lie on the designer. Usually there is no need to specify a particular method, but there may be a need to rule out certain methods; for example, that the contractor is not to use explosives.

It is important to avoid vague phraseology such as requiring the contractor to provide ‘matters, things and requisites of any kind’, or ‘materials of any sort or description’, etc. d or reasonably to be inferred from the contract.’ Similarly the phrase ‘excavation in all materials’ is ineffectual. The drafter might think it covers any rock encountered but it does not if the geological data supplied with the contract or reasonably available to the contractor provides no evidence of the existence of rock. Definitions such as those used in the Civil Engineering Standard Method of Measurement should be followed.

Repair Of Concrete Columns

Repair Of Concrete Columns

Before starting the repair of a column, the axial dead load, axial live load, horizontal load and its associated moments must be known. Repairs to concrete columns can be divided into two categories. Surface or cosmetic repair only covers local deterioration and structural repair restores or strengthens the affected columns. If the deterioration does not significantly reduce the cross section, the conventional concrete repair can successfully be employed.

Columns may be repaired by using one or more of the following methods:Encasement or enlargement of the column cross section (jacketing). Cathodic protection to stop reinforcing steel corrosion.Realkalization of the reinforcing steel to stop corrosion.Chloride extraction to retard the reinforcing steel corrosion.Confinement using steel plate, carbon, or glass fiber materials.Addition of shear collars to increase the shear capacity of intermediate floors. Addition of a steel plate assembly to increase moment capacity.Supplemental columns.The application of a protection system to prevent future corrosion. Following parameters are important for the design and the execution of the column repair:

Unloading columns

In those cases where the column deterioration is significant, unloading the column is usually required so that the entire cross section of the repaired column is capable of carrying the reintroduced design load. Without this unloading, the new repair will hardly carry any load. Drying shrinkage of new material may further reduce this share of load. Unfortunately, it can be difficult and expensive to unload columns, especially in high-rise buildings. If the existing load on a column is not removed before the repair, the jacket will only provide confinement to the existing column. The percentage of direct load taken by jacket will be very small (less than 25 percent of the jacket strength). If it is not possible to remove the load from the column, then a supplemental column system can provide an alternative method of support in combination with the repair of the existing column.

Redistribution of the load

In case of corrosion of reinforcement and significant concrete deterioration, the load is redistributed in the structure before repair to a new pattern which must be considered while designing the repair. Even the adjoining members may have been affected by this redistribution.

Supplemental reinforcing steel

The column ties can not usually be disturbed during the repair as it may cause buckling of the longitudinal bars. Hence, the supplemental vertical bars may be placed outside the original cage with extra ties. When the supplemental bars are placed outside the tie bars, the column dimensions should be increased to provide adequate cover. Hairpin ties, usually of stainless steel, are used to laterally support the supplemental bars.

Concrete removal

The removal of concrete within a column cage must only be done if the column is unloaded. Otherwise, the longitudinal bars may buckle and compression failure of column may take place.

Corroded reinforcing steel

It is not necessary to remove the corroded reinforcing bar with reduced cross-sectional area if the loss is supplemented with additional reinforcing bars. The lap length of such a splice must be provided corresponding to the area lost by corrosion to either side of the corroded portion of the reinforcing bar that is supplemented. The partially corroded reinforcing bars that are left in place must be thoroughly cleaned by sandblasting to obtain bare metal. The bars with excessive corrosion must be replaced with fresh reinforcement having full laps on both sides.

Corroded ties

The corroded ties can be replaced by adding stainless steel hairpin ties that are anchored into the concrete. It is often necessary to deposit extra material around columns to provide adequate cover over the supplemental ties.

Low-strength concrete

Where the concrete strength is low, resulting in insufficient load-carrying capacity, several alternatives are available:

Shore the column and remove and replace the in-place concrete.Shore the column and increase the size of the column to reduce the bending stresses, and to increase the confinement on already placed weak concrete. Wrap the column with carbon- or glass-reinforced plastic.Install a supplemental column.

Concrete without cement - A Green alternative:

Concrete without cement - A Green alternative:

Concrete without cement is possible with the use of flyash as an alternate for cement. Concrete is the most common material used for construction due to its properties such as strength, durability and easy availability. But cement is commonly used in preparation of concrete.

Cement has excellent binding property but its production requires large amount of energy which contributes for pollution and global warming. The process of cement production starts from mining for raw materials, crushing, blending and heating these materials at high temperature of 15000C and finally creating cement from heated materials.

All the process involved in manufacturing of cement requires large amount of energy, it involves huge costs, contributes to increase in CO2 emissions and other greenhouse gases. The production of cement contributes to 7% of the emissions of greenhouse gases and it is likely to double by the year 2014.

As the demand for more and more infrastructures is increasing day by day, the quantity of cement requirements is also increasing. With this, the control the emissions of greenhouse gases cannot be reduced to prevent global warming.

The green alternative to cement is the use of flyash, which has almost same property as cement, both physically and chemically. Flyash is a byproduct from the thermal power plants. It is a waste product and has no other use in power plants. The use of flyash also reduces the energy demand of cement plants as well as reduces the space required for its dumping thus reducing the environmental impact of both cement concrete construction and thermal power plants.

Flyash has been used in the production of cement known as Pozzolanic Portland Cement (PPC) due to its cementitious properties. Generally 25% of flyash is used in OPC to produce PPC.

The property of flyash produced depends on type of coal being used in power plants, nature of combustion process. And the flyash properties suitable for use in cement can be used for concrete construction.

Research at various places in the world has found that concrete in which cement was replaced with flyash, the concrete without cement offered exceptional performance in short term and long term strength of concrete and its workability relative to use of ordinary Portland cement concrete.

PHYSICAL LOAD TEST ON STRUCTURES

PHYSICAL LOAD TEST ON STRUCTURES

The procedure of load test presented here depends on ACI -2008 Chapter 20. In case there is doubt about the safety requirements of a structure, licensed design professional or building official can ask for a strength evaluation.

In the start, methods simpler than the load test are considered and load test can be avoided if all involved parties are satisfied with the result of such evaluation. A load test is required to determine the serviceability of the structure when the presence / effect of the strength deficiency and its remedial measures are not fully known or when the required dimensions and material properties for analysis are not available.

A load test is usually not made until the portion of the structure to be subjected to load is at least 56 days old. The test can only be performed at an earlier age if the owner of the structure, the contractor, and all involved parties agree. Load test is more suitable to clarify the doubts about the shear or bond strength but it can also be used to check deficiencies related with flexure or axial capacity. It is preferable to compare the results of the load test with the results of the analysis.

If a load test is decided as a means of the strength evaluation process for a particular project, the first step is that all the involved parties decide and agree upon the region to be loaded, the magnitude of the load, the load test procedure, and acceptance criteria. For a structure with significant deterioration, periodic reevaluations are recommended to be conducted even if the structure passes a load test. If the doubt about safety of a part or all of a structure involves deterioration, and if the observed response during the load test satisfies the acceptance criteria, the structure or part of the structure is permitted to remain in service for a specified time period. Periodic reevaluations are usually conducted at the end of each specified period. Another option for maintaining the structure in service is to limit the live load to a level determined to be appropriate.

The time period between successive inspections is based on the nature of the problem, environmental effects, nature of loading, and service history of the structure, repair and maintenance program and scope and extent of the inspection. After each evaluation, the building is declared serviceable for a specified period only.

Sometimes a concrete structure believed to be deficient passes a load test. This confusion or misunderstanding is due to the conservative design of concrete structures, extra reinforcing steel to control shrinkage, cracking and thermal effects, conservative design theories, over-estimation of the loads, extra concrete strength and multi-directional sharing of the loads not considered in ordinary designs.

1. Load arrangement

a) The spans and panels having more doubt during survey are considered.

b) The number and arrangement of spans or panels loaded are selected to maximize the deflection and stresses in the critical regions of the structural elements to be tested.

c) More than one test load arrangement is used if a single arrangement does not simultaneously produce maximum values of the force effects required to be studied for the adequacy of the structure.

d) The load is applied at locations where its effect on the suspected defect is a maximum. However, it is better to apply same type of load (point load or uniformly distributed load) as is expected on the structure under examination.

e) The pattern loading expected for the structure must also be considered in deciding the loading to produce maximum load effect in the area of the structure being tested. This includes use of checkerboard or similar type pattern loads.

2. Load intensity

The total test load is taken larger of the following three values:

(a) 1.15D + 1.5L + 0.4(Lr or S or R) – De

(b) 1.15D + 0.9L + 1.5(Lr or S or R) – De

(c) 1.3D – De

Where, D is the total dead load, L is the live load on floors,Lr is the roof live load, S is the snow load, R is the rain load and De is the dead load already in place. The live load Lcan be reduced as allowed by the building code. The load factor on the live load L in (b) is allowed to be reduced to 0.45 except for garages, areas occupied as places of public assembly, and all areas where L is greater than 4.8 kN/m².

Factors affecting construction cost for estimate

Factors affecting construction cost for estimate:

Preparation of a construction cost estimate for any project is a very complex process. Process of construction cost estimation contains many variable factors. Every variable has to be correctly estimated based on proper study, past experience and research to calculate total project cost of construction.

There are many factors which affect the construction cost estimate and have significant impact on project cost and they are as following:

1) Similar Construction Projects:

For the construction estimate, the best reference will be similar construction projects. The final cost of those similar projects can give the idea for the new construction project cost calculation. The final cost of past project needs to be factored with current construction cost indices.

2) Construction Material Costs:

Construction material cost consists of material cost, shipping charges and taxes applicable if any. So, it is important consider all these variations while calculating construction material cost.

3) Labor Wage Rates:

Labor wages varies place to place. So, local wage rate should be considered in calculation. If the project has to be started after several months of estimating the project cost, the probable variation in wage rates has to be considered in the calculation.

4) Construction Site Conditions:

Project site conditions can increase construction costs. Site conditions such as poor soil conditions, wetlands, contaminated materials, conflicting utilities (buried pipe, cables, overhead lines, etc.), environmentally sensitivity area, ground water, river or stream crossings, heavy traffic, buried storage tanks, archaeological sites, endangered species habitat and similar existing conditions etc. can increase the project cost during construction phase if these variations are not considered during estimation.

5) Inflation Factor:

A construction project can continue for years before completion. During the construction period, the cost of materials, tools, labors, equipments etc. may vary from time to time. These variation in the prices should be considered during cost estimation process.

6) Project Schedule:

Duration of construction project is affects the cost. Increase in project duration can increase the construction project cost due to increase in indirect costs, while reduction in construction cost also increases the project cost due to increase in direct costs. Therefore, construction project schedules also need to be considered during project cost estimation.

7) Quality of Plans & Specifications:

A good quality construction plans and specifications reduces the construction time by proper execution at site without delay. Any vague wording or poorly drawn plan not only causes confusion, but places doubt in the contractor’s mind which generally results in a higher construction cost.

8) Reputation of Engineer:

Smooth running of construction is vital for project to complete in time. The cost of projects will be higher with sound construction professional reputation. If a contractor is comfortable working with a particular engineer, or engineering firm, the project runs smoother and therefore is more cost-effective.

9) Regulatory Requirements:

Approvals from regulatory agencies can sometimes be costly. These costs also need to be considered during cost estimate.

10) Insurance Requirements:

Cost estimation for construction projects should also need to consider costs of insurance for various tools, equipments, construction workers etc. General insurance requirements, such as performance bond, payment bond and contractors general liability are normal costs of construction projects. In some special projects, there can be additional requirements which may have additional costs.

11) Size and Type of Construction Project:

For a large construction project, there can be high demand for workforce. For such a requirements, local workmen may not be sufficient and workmen from different regions need be called. These may incur extra costs such projects and also for the type of construction project where specialized workforce is required.

12) Location of Construction:

When a location of construction project is far away from available resources, it increases the project cost. Cost of transportation for workmen, equipments, materials, tools etc. increases with distance and adds to the project cost.

13) Engineering Review:

Sometimes it is necessary to carry out technical review of construction project to make sure the project will serve the required purpose with optimum operational and maintenance cost. This review cost shall also be added to the project cost.

14) Contingency:

It is always advisable to add at least 10% contingency towards the total project costs for unforeseen costs and inflation.

There are three types of tendering methods in construction

There are three types of tendering methods in construction 

by 1) open tendering, 2) selective tendering, 3) by negotiation.

Open Tendering Methods in Construction:

Under open tendering the employer advertises his proposed project, and permits as many contractors as are interested to apply for tender documents. Sometimes he calls for a deposit from applicants, the deposit being returned ‘on receipt of a bona fide tender’.

However, this method can be said to be wasteful of contractors’ resources since many may spend time preparing tenders to no effect. Also, knowing their chances of gaining the contract are small, contractors may not study the contract in detail to work out their minimum price, but simply quote a price that will be certain to bring them a profit if they win the contract.

Thus the employer may be offered only ‘a lottery of prices’ and not necessarily the lowest price for which his project could be constructed. If he chooses the lowest tender he runs the risk the tenderer has not studied the contract sufficiently to appraise the risks involved; or the tenderer might not have the technical or financial resources to undertake the work successfully.

It is true that the employer can check the resources and experience of the lowest bidder and reject his tender if the enquiry proves unsatisfactory; but several bids may be below the estimated cost of the job and, if such tenderers appear satisfactory and their bids are not far apart in value, it is difficult for the employer to choose other than the lowest. The engineer advising the employer may think there is a risk that all such low bids could prove unsatisfactory, but he cannot advise the employer what other bid to accept because he has no certainty of information.

Selective Tendering Methods in Construction:

Under selective tendering the employer advertises his project and invites contractors to apply to be placed on a selected list of contractors who will be invited to bid for the project. Contractors applying are given a list of information they should supply about themselves in order to ‘pre-qualify’.

The advantage to the employer is that he can select only those contractors, who have adequate experience, are financially sound, and have the resources and skills to do the work. Also, since only half a dozen or so contractors are selected, each contractor knows he has a reasonable chance of gaining the contract and therefore has an incentive to study the tender documents thoroughly and put forward his keenest price.

However, since contractors have all pre-qualified it is difficult to reject the lowest bid, even if it appears dubiously low – unless that is due to some obvious mistake.

A problem with both open and selective tendering is that a contractor’s circumstances can change after he has submitted his tender. He can make losses on other contracts which affect his financial stability; or may be so successful at tendering that he does not have enough skilled staff or men to deal with all the work he wins. Neither method of tendering nor any other means of procuring works can therefore guarantee avoidance of troubles.

Negotiated Tendering Methods in Construction:

Negotiated tenders are obtained by the employer inviting a contractor of his choice to submit prices for a project. Usually this is for specialized work or when particular equipment is needed as an extension of existing works, or for further work following a previous contract.

Sometimes negotiated tenders can be used when there is a very tight deadline, or emergency works are necessary. A negotiated tender has a good chance of being satisfactory because, more often than not, it is based on previous satisfactory working together by the employer and the contractor.

When invited to tender the contractor submits his prices, and if there are any queries these are discussed and usually settled without difficulty. Thus mistakes in pricing can be reduced, so that both the engineer advising the employer and the contractor are confident that the job should be completed to budget if no unforeseen troubles arise.

However, negotiated tenders for public works are rare because the standing rules of public authorities do not normally permit them. But a private employer or company not subject to restraints such as those mentioned in the next section can always negotiate a contract, and many do so, particularly for small jobs. Even when a negotiated tender is adopted it is usual to prepare full contract documents so that the contract is on a sound basis. Production of the documents also means they are available for open or selective tendering should a negotiated tender fail, or should the chosen contractor be unable to undertake the work.

What is soil stabilization ?

Soil stabilization is a method of improving soil properties by blending and mixing other materials.

Following are the various soil stabilization methods and materials:

1. Soil Stabilization with Cement:

The soil stabilized with cement is known as soil cement. The cementing action is believed to be the result of chemical reactions of cement with siliceous soil during hydration reaction. The important factors affecting the soil-cement are nature of soil content, conditions of mixing, compaction, curing and admixtures used.

The appropriate amounts of cement needed for different types of soils may be as follows:

Gravels – 5 to 10%Sands – 7 to 12%Silts – 12 to 15%, andClays – 12 – 20%

The quantity of cement for a compressive strength of 25 to 30 kg/cm² should normally be sufficient for tropical climate for soil stabilization.

If the layer of soil having surface area of A (m²), thickness H (cm) and dry density rd (tonnes/m3), has to be stabilized with p percentage of cement by weight on the basis of dry soil, cement mixture will be and, the amount of cement required for soil stabilization is given by Amount of cement required, in tonnes =

Lime, calcium chloride, sodium carbonate, sodium sulphate and fly ash are some of the additives commonly used with cement for cement stabilization of soil.

2. Soil Stabilization using Lime:

Slaked lime is very effective in treating heavy plastic clayey soils. Lime may be used alone or in combination with cement, bitumen or fly ash. Sandy soils can also be stabilized with these combinations. Lime has been mainly used for stabilizing the road bases and the subgrade.

Lime changes the nature of the adsorbed layer and provides pozzolanic action. Plasticity index of highly plastic soils are reduced by the addition of lime with soil. There is an increase in the optimum water content and a decrease in the maximum compacted density and he strength and durability of soil increases.

Normally 2 to 8% of lime may be required for coarse grained soils and 5 to 8% of lime may be required for plastic soils. The amount of fly ash as admixture may vary from 8 to 20% of the weight of the soil.

3. Soil Stabilization with Bitumen:

Asphalts and tars are bituminous materials which are used for stabilization of soil, generally for pavement construction. Bituminous materials when added to a soil, it imparts both cohesion and reduced water absorption. Depending upon the above actions and the nature of soils, bitumen stabilization is classified in following four types:

Sand bitumen stabilizationSoil Bitumen stabilizationWater proofed mechanical stabilization, andOiled earth.

4. Chemical Stabilization of Soil:

Calcium chloride being hygroscopic and deliquescent is used as a water retentive additive in mechanically stabilized soil bases and surfacing. The vapor pressure gets lowered, surface tension increases and rate of evaporation decreases. The freezing point of pure water gets lowered and it results in prevention or reduction of frost heave.

The depressing the electric double layer, the salt reduces the water pick up and thus the loss of strength of fine grained soils. Calcium chloride acts as a soil flocculent and facilitates compaction. Frequent application of calcium chloride may be necessary to make up for the loss of chemical by leaching action. For the salt to be effective, the relative humidity of the atmosphere should be above 30%.

Sodium chloride is the other chemical that can be used for this purpose with a stabilizing action similar to that of calcium chloride.

Sodium silicate is yet another chemical used for this purpose in combination with other chemicals such as calcium chloride, polymers, chrome lignin, alkyl chlorosilanes, siliconites, amines and quarternary ammonium salts, sodium hexametaphosphate, phosphoric acid combined with a wetting agent.

5. Electrical Stabilization of Clayey Soils:

Electrical stabilization of clayey soils is done by method known as electro-osmosis. This is an expensive method of soil stabilization and is mainly used for drainage of cohesive soils.

6. Soil Stabilization by Grouting:

In this method, stabilizers are introduced by injection into the soil. This method is not useful for clayey soils because of their low permeability. This is a costly method for soil stabilization.

This method is suitable for stabilizing buried zones of relatively limited extent. The grouting techniques can be classified as following:

Clay groutingChemical groutingChrome lignin groutingPolymer grouting, andBituminous grouting

7. Soil Stabilization by Geotextiles and Fabrics:

Geotextiles are porous fabrics made of synthetic materials such as polyethylene, polyester, nylons and polyvinyl chloride. Woven, non-woven and grid form varieties of geotextiles are available. Geotextiles have a high strength. When properly embedded in soil, it contributes to its stability. It is used in the construction of unpaved roads over soft soils.

Reinforcing the soil for stabilization by metallic strips into it and providing an anchor or tie back to restrain a facing skin element.

What is Analysis of Rates or Rate Analysis of Civil Works?

What is Analysis of Rates or Rate Analysis of Civil Works?

Every construction project is divided into number of activities. Each activity consists of different types of civil or construction works. For example, the in the construction of a building, the activities can be excavation or earthwork, Concrete work, masonry work, Wood work such as doors and windows, plumbing, flooring, waterproofing, finishing work such as plastering, painting or distempering.

The Activity earthwork can be divided into many types based on depth and type of soil. For example, an excavation of 1.5m deep in soft soil, an excavation of 3m deep in hard soil. Likewise, concrete work can be divided into many types based on its mix proportions and its placement. For example, M25 reinforced concrete work in foundation, M30 reinforced concrete work in columns, slabs etc. Likewise, there can be many small civil works in every construction project.

The cost of any construction project is calculated based on each works associated with every construction activity. Thus it is essential to calculate cost of each small works.

Rate analysis of Civil Works or Building Works is the determination of cost of each construction work per unit quantity. This cost includes the cost of materials, labours, machinery, contractors profit and other miscellaneous petty expenses required for the particular work to be complete in unit quantity. For example, cost of 1 cubic meter of M20 RCC work in slab, Cost of 1 cubic meter of excavation in soft soil of 1.5m depth, cost of 1 square meter of plastering of 20mm, cost of 1 square meter of painting work with specified paint in 2 layers or 3 layers as required.

The cost of materials in rate analysis is calculated as combination of cost of material at origin, its transportation costs, taxes. The rate of labour is based on skill of the labour, such as skilled labour, semi-skilled and unskilled labour. The cost of materials and labours vary from place to place. Thus, the cost of each construction work varies from place to place.

What are the factors affecting Analysis of Rates of Civil Works?

Factors which affect the rate analysis of civil works are:

Specification of the civil work and materials such as quality of materials, proportion of mortar or concrete, thickness of plastering, number of coats of painting, depth of excavation, type of soil etc.Location of the construction site – Distance of construction site from source of materials, availability of labours, availability of water, machinery etc. influence the rate analysis of construction work.Quantity of materials, number of different types of labours and rates of materials and labours influence the rate analysis.Profit of the contractor, miscellaneous expenses and other overheads also influence the rate analysis.

What are the elements of Rate Analysis of Civil Works?

Elements which constitute the rate analysis are:

a) Material cost inclusive of wastage

b) Labour cost

c) Plant & machinery owning and operating charges

d) Water charges

e) Taxes

f) Insurance/ risk coverage charges

g) Contractor’s overheads and profit

Why Analysis of rates is required in construction projects?

The rate analysis may be required in construction projects for following purposes:

For the purpose of tendering. In the case of tendering, the contractor may calculate cost of unit work involved in each construction activity for justified quoting of rates. The client may also require rate analysis to calculate the cost of construction project.To assess the requirements of quantities of labours, materials, machineries and capital to complete the project.To optimise the use of labour, materials and machineries and to know the alternatives to optimize the resources.To assess the rate of unit work from time to time for payment increase in material or labour costs or any deviations in work specifications, extra items of work to the contractor.To compare the cost of project with the sanctioned capital of the project to take necessary action or regularization of excess or less cost.To workout the budget of the construction project and control the cash flows at various stages of construction work.To find out the irrational rates quoted by the contractors during tendering process.To serve as the basic data in case of dispute among project owner and contractor.

10 Things to remember during Concrete Mix Design.

10 Things to remember during 

Concrete Mix Design.

Good quality concrete starts with the quality of materials, cost effective designs is actually a by-product of selecting the best quality material and good construction practices. Following are 10 Things to remember during Concrete Mix Design and Concrete Trials.

1. ACI and other standards only serves as a guide, initial designs must be confirmed by laboratory trial and plant trial, adjustments on the design shall be done during trial mixes. Initial design “on paper” is never the final design.

2. Always carry out trial mixes using the materials for actual use.

3. Carry out 2 or 3 design variations for every design target.

4. Consider always the factor of safety, (1.125, 1.2, 1.25, 1.3 X target strength)

5. Before proceeding to plant trials, always confirm the source of materials to be the same as the one used in the laboratory trials.

6. Check calibration of batching plant.

7. Carry out full tests of fresh concrete at the batching plant, specially the air content and yield which is very important in commercial batching plants.

8. Correct quality control procedures at the plant will prevent future concrete problems.

9. Follow admixture recommendations from your supplier (of course Sika)

10. Check and verify strength development, most critical stage is the 3 and 7 days strength.

Important note:

Technical knowledge is an advantage for batching plant staff, even if you have good concrete design but uncommon or wrong procedures are practiced it will eventually result to failures.

Types of foundation failure

Types of foundation failure :

Types of foundation failure depend on the load it is subjected to. A foundation can fail in three different ways under loads and they are:

1) Punching shear failure of foundation 

2) One-way shear failure of foundation

3) Flexure failure of foundation.

The above three modes of foundation failure should be checked during design stage of concrete foundation for the given load. Guidelines provided by standard codes of practice should be followed so that foundation does not fail in any of the failure types as mentioned under any possible load combinations when structure is occupied.

Types of foundation failure are discussed in detail below:

1) Punching shear failure of foundation:

Punching shear failure is also known as diagonal tension failure of foundation. In this mode of failure, foundation fails due to formation of inclined cracks around the perimeter of the column.

The critical section for punching shear failure is taken at d/2 from the face of the column, where d is the effective depth of footing.

To avoid punching shear failure, the ultimate upward shear force at this section in the foundation should be less than the shear resistance of concrete for the given percentage of concrete. Additional reinforcement should be provided to resist punching shear in case of shear resistance of concrete with reinforcement provided is not sufficient.

The failure of foundation in this mode appears as truncated cone or pyramid around the column, stanchion or pier as shown in figure below:

2) One Way shear failure of foundation:

Foundations in one-way shear failure fails in inclined cracks across full width of the footing that intercept the bottom of the footing slab at a distance d from the face of the column (called critical section), where d is the effective depth of footing slab.

In case where steel base plate is used under column directly on the footing slab, the distance d is measured from a line halfway between the face of column and the edge of the base plate.

To avoid one-way shear failure of foundations, the shear stress at the critical section of footing should be less than the shear strength of concrete with given percentage of reinforcement used.

One way shear failure of footing is shown in figure below:

3) Flexure failure of foundations:

During design of footing, Mu/bd2 is calculated to get the percentage of reinforcement for the moment the foundation is exposed to. Mu is the ultimate or factored moment; b is the width of footing. The critical section for flexure is considered at distance d from the face of footing. The standard codes takes care of flexure failure during design by providing percentage of reinforcement required to resist the bending moment. But in case, when bending moment increases in footings, then footing will fails as shown in figure below: