CONTAMINATED CONSTRUCTION SITE INVESTIGATION

Contaminated construction sites are those which possess risk to human health and environment. With the increase in demand for infrastructure development around the world and shortage of land available for the same, contaminated sites may be used for new construction projects.

The Contaminated construction site can be the result of:

Any industry in the past on current site

Wastes being dumped at site

Contamination through chemicals used for agricultural purposes

Contamination in site filling materials

Soil contamination through demolition of existing structure.

The presence of chemical contamination of soil or ground about to subside creates risk of health hazards for construction workmen.

Following are the objectives for contaminated construction site investigation:

1. To identify the types of hazards, their extent and importance for assessment of potential risks to human and environment.

2. To identify suitable remedial measures for the existing contamination hazards.

The hazards that may occur at contaminated sites are:

1. Settlement problems of ground such as ground subsidence due to decomposition, weathering and natural compaction soil, leaching and sudden collapse.

2. Obstructions from existing remains of old foundation, buried walls, pile foundations etc.

3. Radioactive substances, biological contaminations, toxic powders, asbestos, fibres, liquids, explosives etc. which can attack construction workers.

4. Fire, smoke, gases, volcanic areas, microbial reaction of organic matter, explosions from combustible materials etc. posses greater risks for construction personnel.

5. Contamination affecting the construction materials with chemical reactions, contaminated ground and ground water can affect the health of humans.

6. Polluted streams of water, aquifers, wind action on contaminated dusts etc. which may affect the health.

The site investigation for contaminated soil is not sufficient just by observation of ground surface conditions and the investigation needs to be carried out for more details on the ground conditions below the site. The extent and intensity of this investigation depends on the type and project and its magnitude, conditions of construction site and its variations.

The code of practices for contaminated soil investigation for construction should be followed as per the local applicable standards. The investigation should be properly planned and executed sufficiently to get all the data pertaining to ground conditions to minimize the risk of health hazards to workmen and damage to the environment.

Too little investigation of contaminated sites may not reveal potential hazards of construction site and extra expenditure will be required for safety, while in-depth site investigation more than required may prove to be uneconomical for construction project.

REPAIR OF REINFORCEMENT IN CONCRETE

Repair of Reinforcement in Concrete

The reinforcement repair techniques are different for mild steel and prestressing steel.

1. Mild reinforcing steel

The damaged bars may either be replaced or supplemented by additional reinforcement based on engineering judgment, the purpose of the reinforcement and the required structural strength of the member.

a) Replacement: In case it is decided to replace the bars, splicing of reinforcement with the remaining steel must be done. The lap length must be according to the provision of ACI 318 and the welding (if used) must satisfy ACI 318 and American Welding Society (AWS) D1.4 (or the codal provisions of the respective country). Butt welding is usually avoided due to the high degree of skill required to perform a full penetration weld because the back side of a bar is not usually accessible. Welding of bars larger than 25 mm may cause problems because the embedded bars may get hot enough to expand and crack the surrounding concrete. Mechanical connectors may also be used according to the code requirements.

b) Supplemental reinforcement: This alternative is selected when the reinforcement has lost cross section, the original reinforcement was inadequate, or the existing member needs to be strengthened. The allowable loss of cross-sectional area of the existing reinforcing steel and the decision to add supplemental reinforcement must be evaluated on a case-by-case basis and is the responsibility of the engineer. The damaged reinforcing bar must be cleaned and extra space is to be created by removing concrete to allow placement of the supplemental bar beside the old bar. The length of the supplemental bar must be equal to the length of the deteriorated segment of the existing bar plus a lap-splice length for smaller diameter bar on each end.

Reinforcing bars, having corrosion of their original deformations, give less bond and this factor must be considered while designing the repair of the reinforcement.

c) Coating of reinforcement: New and existing bars that have been cleaned may be coated with epoxy, polymer cement slurry, or a zinc-rich coating for protection against corrosion. The coating must have a thickness less than 0.3 mm to minimize loss of bond development at the deformations.

2. Prestressing steel

Deterioration or damage to the strands or bars can result from impact, design error, overload, corrosion, or fire. Fire may anneal cold-worked, high-strength prestressing steel. The unbonded high-strength strands may need to be detensioned before repair and retensioned after repair to restore the initial structural integrity of the member.

a) Bonded strands: Because the prestressed strand is bonded, only the exposed and damaged section is restressed following repairs. The repair procedure requires replacing the damaged section with the new section of strand connected to the existing ends of the undamaged strands. The new strand section and the exposed lengths of the existing strand must be post-tensioned to match the stress level of the bonded strand.

b) Unbonded tendons: The strands are protected against corrosion by the sheathing, corrosion-inhibiting material (commonly grease), or both. Corrosion of the end connections and the strand has been the primary cause of failure of unbonded tendons. A deteriorated portion of a strand can be exposed by excavating the concrete and cutting the sheathing. Unbonded tendons can be tested to verify their ability to carry the design load. This can be done by attaching a chuck and coupler to the exposed end of the strand and performing a lift-off test. This usually requires at least 20 mm of free strand beyond the bulkhead. If there is excessive corrosion in the strand, failure occurs and the strand must be replaced or spliced. Shoring of the span being repaired and adjacent spans up to several bays away may be required before removing or retensioning unbonded prestressed strands.

The strand is cut on both sides of the deterioration and the removed portion of the strand is replaced with a new section. The new strand is spliced to the existing strand at the location of the cuts. The repaired strand is then prestressed. Carbon fiber or equivalent systems are available to supplement the reinforcement in prestressed, post-tensioned, and mild steel reinforced structures. This system is normally glued onto the exterior surface. Unless the component being reinforced is unloaded, the strengthening system only provides reinforcement for future loadings. Fiber wrapping is commonly used for reinforcing columns, especially in earthquake zones. There are systems available that recover the dried and damaged protective barrier within the sheathing.

POOR CONSTRUCTION METHODS AND WORKMANSHIP TO AVOID

Poor construction methods and workmanship is responsible for the failure of buildings and structure. The poor construction methods and workmanship is caused due to negligence and inadequate quality control at construction site. The effects of some of the poor construction methods are discussed below:

(a) Incorrect placement of steel

Incorrect placement of steel can result in insufficient cover, leading to corrosion of the reinforcement. If the bars are placed grossly out of position or in  the wrong position, collapse can occur when the element is fully loaded.

(b) Inadequate cover to reinforcement

Inadequate cover to reinforcement permits ingress of moisture, gases and other substances and leads to corrosion of the reinforcement and cracking and spalling of the concrete.

(c) Incorrectly made construction joints

The main faults in construction joints are lack of preparation and  poor compaction. The old concrete should be washed and a layer of rich concrete laid before pouring is continued. Poor joints allow ingress of moisture and staining of the concrete face.

(d) Grout leakage

Grout leakage occurs where formwork joints do not fit together properly. The result is a porous area of concrete that has little or no cement and fine aggregate. All formwork joints should be properly sealed.

(e) Poor compaction

If concrete is not properly compacted by ramming or vibration the result is a  portion of porous honeycomb concrete. This part must be hacked out and recast. Complete compaction is essential to give a dense, impermeable concrete.

(f) Segregation

Segregation occurs when the mix ingredients become separated. It is the result of

1. dropping the mix through too great a height in placing (chutes or pipes should be used in such cases)

2. using a harsh mix with high coarse aggregate content

3. large aggregate sinking due to over-vibration or use of too much plasticizer

Fig: Seggregation of concrete

Segregation results in uneven concrete texture, or porous concrete in some cases.

(g) Poor curing

A poor curing procedure can result in loss of water through evaporation. This can cause a reduction in strength if there is not sufficient water for complete hydration of the cement. Loss of water can cause shrinkage cracking. During curing the  concrete should be kept damp and covered.

(h) Too high a water content

Excess water increases workability but decreases the strength and increases the porosity and permeability of the hardened concrete,which can lead to corrosion of the reinforcement. The correct water-to-cement ratio for the mix should be strictly enforced.

PLASTIC SHRINKAGE CRACKS & ITS PREVENTION IN CONCRETE

Plastic Shrinkage Cracks and Its Prevention in Concrete

Cracking caused by plastic shrinkage in concrete occurs most commonly on the exposed surfaces of freshly placed floors and slabs or other elements with large surface areas when they are subjected to a very rapid loss of moisture caused by low humidity and wind or high temperature or both.

Plastic shrinkage usually occurs prior to final finishing, before curing starts. When moisture evaporates from the surface of freshly placed concrete faster than it is placed by curing water, the surface concrete shrinks. Due to the restraint provided by the concrete on the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks that are usually not short and run in all directions. In most cases, these cracks are wide at the surface. They range from a few millimeters to many meters in length and are spaced from a few centimeters to as much as 3 m apart.

Preventing Plastic Shrinkage Cracks in Concrete

Plastic shrinkage cracks may extend the full depth of elevated structural slabs. Since cracking because of plastic shrinkage is due to a differential volume change in the plastic concrete, successful control measures require a reduction in the relative volume change between the surface and other portions of the concrete. There are many methods and techniques to prevent this type of crack in case of rapid loss of moisture due to hot weather and dry winds. These methods include the use of fog nozzles to saturate the air above the surface and using plastic sheeting to cover the surface between the final finishing operations. In many cases, during construction it is preferable to use wind breakers to reduce the wind velocity; sunshades to reduce the surface temperature are also helpful. Additionally, it is good practice to schedule flat work after the walls have been erected.

FACTORS AFFECTING CONCRETE MIX DESIGN STRENGTH

Factors that affects the concrete mix design strengths are:

Variables in Mix Design

A. Water/cement ratio

B. Cement content

C. Relative proportion of fine & coarse aggregates

D. Use of admixtures

A. Water/cement ratio

Water to cement ratio (W/C ratio) is the single most important factor governing the strength and durability of concrete. Strength of concretedepends upon W/C ratio rather than the cement content. Abram’s law states that higher the water/cement ratio, lower is the strength of concrete. As a thumb rule every 1% increase in quantity of water added, reduces the strength of concrete by 5%. A water/cement ratio of only 0.38 is required for complete hydration of cement. (Although this is the theoretical limit, water cement ratio lower than 0.38 will also increase the strength, since all the cement that is added, does not hydrate) Water added for workability over and above this water/cement ratio of 0.38, evaporates leaving cavities in the concrete. These cavities are in the form of thin capillaries. They reduce the strength and durability of concrete. Hence, it is very important to control the water/cement ratio on site. Every extra liter of water will approx. reduce the strength of concrete by 2 to 3 N/mm2and increase the workability by 25 mm. As stated earlier, the water/cement ratio strongly influences the permeability of concrete and durability of concrete. Revised IS 456-2000 has restricted the maximum water/cement ratios for durability considerations by clause 8.2.4.1, table 5.

B. Cement content

Cement is the core material in concrete, which acts as a binding agent and imparts strength to the concrete. From durability considerations cement content should not be reduced below 300Kg/m3 for RCC. IS 456 –2000 recommends higher cement contents for more severe conditions of exposure of weathering agents to the concrete. It is not necessary that higher cement content would result in higher strength. In fact latest findings show that for the same water/cement ratio, a leaner mix will give better strength. However, this does not mean that we can achieve higher grades of concrete by just lowering the water/cement ratio. This is because lower water/cement ratios will mean lower water contents and result in lower workability. In fact for achieving a given workability, a certain quantity of water will be required. If lower water/cement ratio is to be achieved without disturbing the workability,cement content will have to be increased. Higher cement content helps us in getting the desired workability at a lower water/cement ratio. In most of the mix design methods, the water contents to achieve different workability levels are given in form of empirical relations.

Water/cement ratios required to achieve target mean strengths are interpolated from graphs given in IS 10262 Clause 3.1 and 3.2 fig 2. The cement content is found as follows: –

Thus, we see that higher the workability of concrete, greater is cement content required and vice versa. Also, greater the water/cement ratio, lower is the cement content required and vice versa.

C. Relative proportion of fine, coarse aggregates gradation of aggregates

Aggregates are of two types as below:

a. Coarse aggregate (Metal): These are particles retained on standard IS 4.75mm sieve.

b. Fine aggregate(Sand): These are particles passing standard IS 4.75mm sieve.

Proportion of fine aggregates to coarse aggregate depends on following:

i. Fineness of sand: Generally, when the sand is fine, smaller proportion of it is enough to get a cohesive mix; while coarser the sand, greater has to be its proportion with respect to coarse aggregate.

ii. Size& shape of coarse aggregates: Greater the size of coarse aggregate lesser is the surface area and lesser is the proportion of fine aggregate required and vice versa. Flaky aggregates have more surface area and require greater proportion of fine aggregates to get cohesive mix. Similarly, rounded aggregate have lesser surface area and require lesser proportion of fine aggregate to get a cohesive mix.

iii. Cement content: Leaner mixes require more proportion of fine aggregates than richer mixes. This is because cement particles also contribute to the fines in concrete.

CONSTRUCTION MATERIALS MANAGEMENT

Construction Materials management can be defined as "the function responsible for the coordination of planning, sourcing, purchasing, moving, storing and controlling materials in an optimum manner so as a pre-decided service can be provided at a minimum cost". By another definition, "materials management can be said to be that process of management which coordinates, supervises and executes the tasks associated with the flow of materials to, through, and out of an organization in an integrated fashion".

Lee and Dobler define materials management as, "a confederacy of traditional materials activities bound by common idea – the idea of an integrated management approach to planning, acquisition, conversion, flow and distribution of production materials from the raw material state to the finished product state."

From the above definitions, it is clear that the scope of materials management is vast. It has, directly or indirectly, impact on the activities of many related departments in the organization. Broadly, following can be identified as its main functions:

Based on the sales forecast and production plans, the materials planning and control is done. This involves estimating the individual requirements of parts, preparing materials budget, forecasting the levels of inventories, scheduling the orders and monitoring the performance in relation to production and sales.

Purchasing

This includes selection of sources of supply, finalization of terms of purchase, placement of purchase orders, follow-up maintenance of smooth relations with suppliers, approval of payments to suppliers, evaluating and rating suppliers.

Stores and Inventory Control 

This involves physical control of materials, preservation of stores, minimization of obsolescence and damage through timely disposal and efficient handling, maintenance of stores records, proper location and stocking. Stores is also responsible for the physical verification of stocks and reconciling them with book figures. The inventory control covers aspects such as setting inventory levels, ABC analysis, fixing economical ordering quantities, setting safety stock levels, lead time analysis and reporting.

                                    Fig: Construction Materials Management

IMPORTANCE OF CONSTRUCTION MATERIALS MANAGEMENT

The fast developing Indian economy has placed before the materials manager a tremendous challenge and responsibility. In many organizations, materials form the largest single expenditure item. An analysis of the financial statements of a large number of private and public sector organizations indicate that materials account for nearly 60% of the total expenditure. The information on the average materials expenditure for different industry groups is shown in Table 1.

                 Table 1 : Average Material Cost as Percent of Total Cost

Percentage of Total Cost	Industry Groups
Above 75	Construction, fabrication, electrodes, tea etc.
65 – 75	Wool, sugar, jute, cotton, yarn, commercial vehicles, earth moving equipment, scooters, furniture etc.
55 – 65	Cotton textile, bread, ship building, cables, electricity generators, refrigerators, heavy machinery etc.
45 – 55	Chemicals, cement, pharmaceuticals, electronics, paper, engineering, non-ferrous type machine tools, explosives etc.
35 – 45	Fertiliser, steel, cigarettes, transportation, asbestos, news print, newspapers, ferrow alloys, aircraft manufacturing.

Thus, the importance of materials management lies in the fact that any significant contribution made by the materials manager in reducing materials cost will go a long way in improving the profitability and the rate of return on investment. Such increase in profitability, no doubt, can be affected by increasing sales. But with the increased competition in the market, this alternative is not very easy to achieve.

Besides, some increase in the profitability can be achieved by concentrating on the materials cost which is typically a major rupee item for most organizations. In fact, as market pressure intensifies, organizations will be forced to cut down the costs and here, the materials management steps in to play its role.

Since materials form major part of total cost, these offer a very good scope for reduction of total cost. A small percent in materials cost can result in large percent increase in profitability.

Consider, for example, a small company has total sales of Rs. 1000. Total cost is Rs. 900. Thus, the profit is Rs. 100 which amount to 10% of the sales. Suppose, out of total cost of Rs. 900, materials cost is Rs. 600. Now if one percent saving in materials cost can be achieved, then the resultant saving is Rs. 6 (1 percent of 600) which directly adds to the profit, thus, profit becomes Rs. 106.

Therefore, in this case, we can see that 1 % saving in materials cost results into 6% increase in profit.

REPAIR OF SMALL AND LARGE CRACKS IN CONCRETE

Repair of small, medium and large cracks in concrete and repair of crushed concrete is required to enhance the strength and durability of damaged concrete members.

Repair of small and medium cracks in concrete:

Small and medium cracks in reinforced concrete and masonry structures reduce their strength considerably to bear the design loads. Thus repair of such cracks is necessary to restore the designed strength of members.

The repair of small and medium cracks is done by first marking out the critical damaged zones in concrete members. Then these cracks can be repaired by injecting cement grout or chemical grouts or by providing jacketing. The smaller cracks less than 0.75 mm width can be effectively repair by using pressure injection of epoxy.

The surface of the member near cracks is thoroughly cleaned. Loose materials are removed and plastic injection ports are placed along the length of crack at an interval equal to the thickness of the structural member. These ports are placed on both sides of the member and secured in placed with the help of epoxy seal.

When the epoxy seal has hardened, the low viscosity resin is injected into one port at a time starting from the port at lowest level and moving upwards. The injection through port is continued till the resin flows out from the adjacent port or from the other side of the member. Then the current injection port is closed and epoxy injection is continued from the adjacent port.

This process is carried out in sequence till all the ports and cracks are filled with the grout. This method can be used for all types of structural members such are beams, columns, walls and slabs. This method can also to repair of small cracks in individual masonry blocks or for filling large continuous cracks.

Repair of Large Cracks and Crushed Concrete:

Repair of large cracks (cracks wider than 5mm) and crushed concrete and masonry structure cannot be done using pressure injection or grouting. For repair of large cracks and crushed concrete, following procedure can be adopted:

1. The surface of cracks or crushed concrete is cleaned and all the loose materials are removed. These are then filled with quick setting cement mortar grouts.

2. If the cracks are large, then these cracks are dressed to have a V groove at both sides of the member for easy placement of grouts.

Fig: Filling of cement mortar and stone chips in large cracks in masonry walls.

3. For cracks which are very large, filler materials such as stone chips can be used.

4. Additional reinforcement and shear reinforcements can be used for heavily damaged concrete members or wherever necessary based on requirements.

These additional reinforcement should be protected from corrosion by using polymer mortar or epoxy coatings.

5. For damaged walls and roofs, additional reinforcement in the form of mesh is used on one side or both sides of the members. These mesh should sufficiently tied with existing members.

Fig: Reinforcement meshes in repair of roof slabs and walls. 1. Wire mesh on front face, 2. Clamps, 3. Wire mesh on back face, 4. Cement plaster, 5. Crack in member.

6. Stitching of cracks are done to prevent the widening of the existing cracks. In this case, holes of 6 to 10mm are drilled on both sides of the crack. Then these drilled holes are cleaned, legs of stitching dogs are anchored with short legs. The stitching of cracks is not a method of crack repair or to gain the lost strength, this method is used to prevent the cracks from propagating and widening.

CONCRETE REPAIR QUALITY CONTROL

Quality control in concrete repair works essential to regain lost strength in concrete due to cracks or other damages. Concrete repairs are required when structural members get damaged or cracks. The reason for cracks or damages can be many. It can be due to over-stressing, poor construction practices, environmental exposures, chemical attacks or with age of concrete member etc.

The concrete repair involves replacing, restoring or renewing of old or damaged concrete from existing structural member. The need for repair can vary from time to time depending on structural requirement or type of damages in the structural member.

The concrete repair procedure involves following steps:

1. Determining the cause of damage

2. Evaluation of damage to identify need and method of concrete repair

3. Preparation of damaged structural member

4. Application of repair method selected

5. Curing of repaired concrete member.

As listed above, the damages in concrete structures need to be carefully evaluated and repaired, each step involved need to be carefully performed. Inadequate workmanship, procedures or materials in concrete repairs may result in poor repair and may fail during occupancy involving significant cost.

Workmanship in Concrete Repairs:

Quality and durability of concrete repairs depends on the workmanship during repair process. The aim of repair is to provide strength and durability of structural member comparable to its original or designed strength.

For this, the workmen involved in repair process should have sufficient knowledge, skills and training to perform the concrete repair work. Work carried out should be done in a way that the repaired concrete is well bonded with existing concrete and durability requirement is met.

All the process should be carefully supervised by experienced personnel. Well trained, competent workmen are particularly essential when epoxy, polyurethane, or other resinous materials are used in repair of concrete.

Concrete Repair Procedures:

Selection of right procedures for concrete repair is essential to ensure repair quality control and techniques are carefully performed. Wrong or poor repair procedure and workmanship may lead to ineffective concrete repairs.

Repairs can be on old concrete surface or new concrete just after stripping of formwork. For new concrete surface, the repairs are easy and bond between repair concrete existing concrete surfaces will be same as the original construction work.

Materials for Concrete Repairs:

The selection of repair materials for concrete should be of high quality and as per the specifications requirements as per the need and type of repair method selected. Testing of repair materials should be done to ensure its quality and suitability for the given repair method.

Any materials procured from vendors should have its manufacturers test certificate and should be used only as per manufacturer’s specifications and approved methods. Suitability of materials for type of damage should be ensured as this may lead to high cost and failure of repair, if the materials used are unsuitable.

Care should be taken during mixing, proportioning, handling and placement of repair materials to ensure good concrete repair quality control.

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.

ll 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.

REQUIREMENTS FOR CONCRETE MIX DESIGN

Requirements of concrete mix design should be known before calculations for concrete mix. Mix design is done in the laboratory and samples from each mix designed is tested for confirmation of result. But before the mix design process is started, the information about available materials, strength of concrete required, workability, site conditions etc. are required to be known.

Following are the information required for concrete mix design:

1. Characteristic strength of concrete required: Characteristic strength is the strength of concrete below which not more than 5% of test results of samples are expected to fall. This can also be called as the grade of concrete required for mix design. For example, for M30 grade concrete, the required concrete compressive strength is 30 N/mm2 and characteristic strength is also the same.

2. Workability requirement of concrete: The workability of concrete is commonly measured by slump test. The slump value or workability requirement of concrete is based on the type of concrete construction.

Fig: Workability of Concrete – Slump Test

For example, reinforced concrete construction with high percentage of steel reinforcement, it will be difficult to compact the concrete with vibrators or other equipment. In this case, the workability of concrete should be such that the concrete flows to each and every part of the member. For concrete member, where it is easy to compact the concrete, low workability concrete can also be used.

It is also known that with increase in workability of concrete, the strength of concrete reduces. Thus, based on type of structure or structural member, the workability requirement of concrete should be assumed and considered in the mix design.

For pumped concrete, it is essential to have high workability to transfer concrete to greater heights with ease. This case also should be considered in the mix design.

3. Quality control at site: The strength and durability of concrete depends on the degree of quality control during construction operation at site. Nominal mixes of concrete assumes the worst quality control at site based on past experiences.

Thus, for design mix concrete, it is essential to understand the quality control capability of contractor and workmen at construction site in mixing, transporting, placing, compacting and curing of concrete. Each step in concrete construction process affects the strength and durability of concrete.

The availability of workmen also affects quality control of concrete. The more skilled workmen and supervision helps to maintain good quality construction.

4. Weather conditions: Weather impacts the setting time of concrete. In hot climate, the concrete tends to set early due to loss in moisture, and in this case, the concrete need to have higher water cement ratio or special admixtures to delay initial setting of concrete. Recommendations for concrete cooling agents also required to be mentioned in the mix design for very hot weather conditions.

In cold climates, the initial setting time of concrete increases as the moisture loss rate is very low. Due to this, water cement ratio is considered appropriately. Admixtures should also be recommended to prevent freezing of concrete in case of very cold climate.

5. Exposure conditions of concrete: Exposure conditions play an important role in the mix design of concrete. The exposure conditions such as chemical actions, coastal areas etc. needs to be considered for the given site. Generally exposure conditions as per code of practices are mild, moderate, severe, very severe and extreme exposure conditions for concrete constructions.

The grade of concrete and durability requirements of concrete changes with exposure conditions. For extreme exposure conditions some standard codes mention minimum strength of concrete as M35.

6. Batching and mixing methods: There are two types of batching method, i.e. volumetric batching and batching by weight. These two conditions should be known for concrete mix design calculations.

Fig: Batching and Mixing Methods for Concrete

Mixing methods include manual mixing, machine mixing, ready mix concrete etc. The quality control of concrete varies with each type of mixing method.

7. Quality of materials: Each construction material should have been tested in laboratory before it is considered for mix design calculations. The type of material, their moisture content, suitability for construction, and their chemical and physical properties affects the mix design of concrete. Type of cement to be used for construction, coarse and fine aggregates sources, their size and shape should be considered.

Fig: Quality of Materials for Concrete Construction

8. Special Requirements of concrete: Special requirement of concrete such as setting times, early strength, flexural strength,

CONCRETE MIX DESIGN AND ITS ADVANTAGES

Concrete mix design is of two types:

1. Nominal concrete mix

2. Designed concrete mix

Nominal concrete mixes are those specified by standard codes for common construction works. These mix takes into consideration the margin for quality control, material quality and workmanship in concrete construction.

M10, M15, M20 are the commonly used nominal mixes used in construction. For higher grade of concrete, i.e. M25 and above, it is advised to have designed mix concrete.

Designed mix concrete suggests proportions of cement, sand, aggregates and water (and sometimes admixtures) based on actual material quality, degree of quality control, quality of materials and their moisture content for given concrete compressive strength required for the project. Designed mix concrete are carried out in laboratory and based on various tests and revisions in mix designs, the final mix proportions are suggested.

The concrete mix can be designed from M10 to various grades of concrete such as M50, M80, M100 etc for various workability requirements from no slump to 150mm slump values. These grades of concrete can be achieved by variations in the mix proportions and laboratory tests to ascertain it.

Sometimes admixtures are also required to enhance some properties of concrete such as workability, setting time etc. These admixtures also need to be considered during concrete mix design calculations for its optimum use. Their overdose can affect the properties of concrete and can cause harm to strength and durability.

Concrete mix design is the method of proportioning of ingredients of concrete to enhance its properties during plastic stage as well as during hardened stage, as well as to find economical mix proportions.

Properties desired from concrete in plastic stage: –

• Workability – Suitable workability for proper placement of concrete in structural member.

• Cohesiveness – better cohesiveness between cement and aggregates to prevent segregation of concrete.

• Initial set retardation – to control the initial setting time of concrete based on requirements.

Properties desired from concrete in hardened stage:-

• Strength – Strength of concrete is the main objective of the concrete mix design.

• Imperviousness – Better mix proportions to improve imperviousness for protection of reinforcement form corrosion and enhanced durability of concrete.

• Durability – To increase the durability of concrete.

Advantages of Concrete Mix Design:

Concrete mix design is economically proportioning of concrete ingredients for better strength and durability based on construction site. While the nominal concrete mix may have higher amount of cement, when it is designed mix, the cement requirement may be low for the same grade of concrete for a given site. The proportions resulting from concrete mix design are tested for their strength with the help of compressive strength test on concrete cubes and cylinders.

The concrete mix design proves to provide better quality economically.

Following are the advantages of concrete mix designs:

1. Good quality concrete as per requirements – this means the concrete will have required strength, workability, impermeability, durability, density and homogeneity.

2. Nominal mix concrete may suggest more cement than other materials, and concrete mix designs gives the accurate quantity of cement consumption. Thus it is an economical solution for large projects.

It is possible to save up to 15% of cement for M20 grade of concrete with the help of concrete mix design. In fact higher the grade of concrete more are the savings. Lower cement content also results in lower heat of hydration and hence reduces shrinkage cracks.

3. Best use of available materials:

The nominal mix of concrete does not consider the quality of local construction materials. The concrete mix design is based on the quality of available materials locally. Thus it is also an economical solution to reduce the transportation cost of materials from long distance.

4. Desired Concrete Properties:

The designed mix concrete will have desired concrete properties based on project or construction requirements. Requirements such as durability, strength, setting times, workability etc. can be controlled with the type of construction with concrete mix design.

Other requirements such as early de-shuttering, pumpability, flexural strength, lightweight concrete can also be controlled.