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.

PROPER METHODS FOR CONCRETE                                 PLACEMENT

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.

Aggregates

Aggregates

Aggregates

In Civil Engineering Aggregates plays vital role. Therefore we have to have good knowledge of aggregates. Making it most simple to understand, we will first see why and what for we need Aggregates.

1) Metal  :- It take Strength in the concrete. 

2) Sand :-   Only to fill gaps 

Metal :-

 Cement is only binding agent. So  it binds Metals.  And make a big pavement. which is as per design can take loads. For proper binding and interlocking we need metal a bit angular  and hard. Aggregates which pass through sieve of opening of size 80 mm and are entirely retained on 4.75 mm  sieve, are known as coarse aggregates.
            The size of the Particles to be used for concrete depends on the type of work and Reinforcement. Coarse aggregates consist of aggregates such as stone gravel.

Acceptance :-

•     Should not be porous, as porous material corrodes the reinforcement.
•     Elongates and laminated particles are good in shear.
•     Must be clean and free from clay lumps, vegetable and other organic material. Clay/ dirt in the           aggregates slows down the setting and hardening of cement and reduces the strength.
•     Angular and roughly cubical particles are ideal.
•    25 mm  and 12.5 mm size coarse aggregates are normally used in R.C.C. works.
•     For mass concrete upto 40 mm size coarse aggregates can be used.
•     The maximum size of coarse aggregates should be within the limits specified.

Sand:- 

Sand is use to fill gaps. so it all depends of fineness modulus [F.M.], F.M. may range between 2.6 % to 3.6% for concrete up to 1.6% for plastering , and up to 3% for masonry as per IS 386-1963. sand passes through 4.75 mm I.S. Sieve.
            Sand should consist of sharp angular grains of various sizes. Recent studies show that rounded grains too interlock sufficient, to produce a strong concrete.

Acceptance :-

•     Ensure that the sand is river wet or artificially we  [ to increase the bulk-age.]
•     Check if the fineness [ type ] of the sand supplied is as per the purchase order.
•     Sand should be free from slit, clay, slat, mica and organic material. Sand is generally found     to contain some percentage of slit and clay. maximum 7%  of silt  and clay be allowed in sand at construction sites.

Aggregates test:-

Metal :-
                             
1) Water Absorption test :- For metal to be used in concrete work, water absorption should not be more than 4%.                 
Sand :-

1) Test for bulking of Sand:- This bulking of sand should be known for proper correction to be applied when calculating dry and requirement.
2) Silt content of sand :- Total silt content should not be more than 7% for good quality sand.
3) Organic Impurities :- This should not be more than 1% of the total sand received.