Concrete is one of the widely produced construction materials which is a combination of cement, water, and aggregates. Today, there is no construction without concrete. At least a small portion of the structure is constructed with concrete even if they are constructed with alternative materials.

History of Concrete

The history of this material extends up to the 6500BC as per the resources on the internet.

As per the article in the GIATEC it has initiated from UAE. They have created concrete structures. In 3000BC Egypt and China have used in construction.

Further, it can be observed in 600BC in Rome. By 200BC Rome has started using widely.

The following figure extracted from the GIATEC web site indicates the part of the development in this area.

How to Make the Concrete

Mainly, there are four additives. They are cement, water, fine aggregate and coarse aggregates. These materials are mixed together as per the mix proportions selected from the mix design and confirmed by a trail mix.

In addition to the above basic additives, other additives such as admixtures, etc are added to improve its performance.

Mixing can be done manually or by an automated method which is used mostly in the world. In this method, all the materials are transferred to the mixing bin as per their mixing proportions.

After mixing, it directly loaded to the truck mixture. Depending on the workability requirements and based on the required strength, the water-cement ratio is controlled. The indicated of the workability of concrete is measured using the slump test at the batching plant and on arrival to the site.

Cement

Let’s discuss the most important material that made all the things in the concrete.

The cement reacts with the water and creates the bond between the aggregates.

Cement is a complicated material that included the materials. Further, its reaction to create is bit complexed for the basic engineering design as it involves chemistry.

The following reactions can observe the process of cement making.

2CaO + SiO₂ = Ca₂SiO₄ (declaim silicate (C₂S))

3CaO + SiO₂ = Ca₃SiO₅ (tricalcium silicate (C₃S))

3CaO + Al₂O₃ = Ca₃Al₂O₆ (dicalcium aluminate (C₂A))

4CaO + Al₂O₃ + Fe₂O₃ = Ca₄Al₂Fe₂O₁₀ (tetracalcium aluminoferrite(C₄AF))

Out of these materials, C₃S and C₂S are the most important compounds that contribute to the strength. C₃S develops the strength initially and it is in the first four weeks. C₂S develops the strength mostly after the first four weeks as shown in the following figure.

Further reading on cement could be made from the article, cement and cement additives. It also states the different classification of cement. In addition, the classification of cement can be made based on its compressive strength. Cement shall be tested to verify the quality. The following tests are carried out usually.

1. Fineness
2. Compressive strength
3. Heat of hydration
4. Initial and final setting times
5. Soundness
6. Normal Consistency

The article, 6 Different Cement Tests, provides further information on the testing procedures and the relevant standards for the same. 

Hydration of Cement

The reaction of the cement with the water and making the concrete by creating necessary bonding with other additives is called hydration. This reaction creates a very had material that is very strong in compersion. However, it is very weak in tension.

As discussed above, the materials C₂S, C₃S, C₂A and C₄AF are included as basic materials in the cement. In addition, there are other materials such as Gypsum (CSH₂), etc. in the cement.

The reaction of these materials with each other and with water converts them into different forms as discussed below.

• Tricalcium aluminate + gypsum + water ® ettringite + heat : C₃A + 3CSH₂ + 26H ® C₆AS₃H₃₂, D H = 207 cal/g
• Tricalcium silicate + water ® calcium silicate hydrate + lime + heat : 2C₃S + 6H ® C₃S₂H₃ + 3CH, D H = 120 cal/g

• After all the gypsum reacted as per the fist equation, the ettringite becomes unstable and starts reacting with the remaining C3A. The reaction forms the monosulfate aluminate hydrate crystals 

• Tricalcium aluminate + ettringite + water ® monosulfate aluminate hydrate
  2C₃A + 3 C₆AS₃H₃₂ + 22H ® 3C₄ASH₁₈,
• Dicalcium silicates + water ® calcium silicate hydrate + lime 
• C₂S + 4H ® C₃S₂H₃ + CH, D H = 62 cal/g 

            The ferrite reactions with the gypsum

• First reaction : Ferrite + gypsum + water ® ettringite + ferric aluminum hydroxide + lime : C₄AF + 3CSH₂ + 3H ® C₆(A,F)S₃H₃₂ + (A,F)H₃ + CH
• Second Reaction : Ferrite + ettringite + lime + water ® garnets : C₄AF + C₆(A,F)S₃H₃₂ + 2CH +23H ® 3C₄(A,F)SH₁₈ + (A,F)H₃

Let’s discuss the Products of the Hydration

1. Calcium Silicate Hydrate: This is the main source of strength. This product is denoted as C-H-S.
2. Calcium Hydroxide: This is denoted as CH and is formed from the alite hydration.
3. Ettringite: They are rod-like crystals that form the early stage of reaction or some times later. Its reactivity causes cracks in the concrete especially when they react later.
4. Monosulfate: It forms 1-2 days after start the mixing.
5. Monocarbonate: The presence of fine limestone or limestone aggregate produces it. 

Properties of Concrete

Knowing the properties very important to designers to carrying out the structural design. There are very important parameters that directly affect structural performance.

Elasticity of Concrete

The modulus of elasticity of the concrete has been defied by different standards differently. As per most of the guidelines, it is related to the compressive strength (characteristic strength).

According to BS 8110 Part 2, there are equations that can be used to calculate the modulus of elasticity.

Further, there is a table in the BS 8110 Part 2, that can also be used to find the elastic modulus.

Eurocode 2 also has provided the values and formula to find the elastic modulus based on the cylinder strength.

This is Table 3 of Eurocode 2.

Poison ratio

Poisson’s ration represents the relationship between the longitudinal and transverse stresses. 

According to the BS 8110 part 01, the poison’s ratio is 0.2 for linear analysis.

According to Eurocode 2, it is 0.2 for uncracked concrete and 0 for cracked concrete.

Fire Resistance

Fire-resistance is a measure of the ability to withstand against a fire. It is specified in terms of hours. 

Generally, it is more fire-resistant than many construction materials.

According to the British Standards, fire resistance is specified in houres. 

Firstly, we decide how many hours it required to withstand the evacuation. Depending on that period, we select the cover for the reinforcement. 

Reinforcement is the main material in addition to the concrete, that carries the strength. 

Reinforcement is very sensitive to the heat and it expands rapidly with the rise of the temperature. If there is sufficient cover to the reinforcement, it protects the concrete up to a certain extent and minimizes the rise of the temperature of the steel also.

Concrete Temperature

We discuss the temperature on two occasions. They are the temperature at the time of pouring and the temperature that rises due to the hydration process.

According to BS 5328, the temperature has limited to 30⁰C when it pouring. However, it further states that this value could be vary depending on the specification of the project. 

The final target of limiting the temperature is to limit the rise of the temperature during the hydration process. 

If it rises by a significant amount without any control, it causes severe issues to strength and durability.

As per most of the literature, the rise of the temperature during the hydration process to the range 70-80⁰C is very critical. It leads to the formation of delayed ettringites which results in internal expansions due to the material created in the reaction.

This causes internal cracks reducing its strength and durability. 

The rise of the temperature could be minimized by reducing the pouring temperature and by using a different additive that reduces the heat of hydration. On such material that commonly uses is the fly ash.

Method of Limiting of Concrete temperature article discusses the available methods to limit the temperature and causes of it.

Further, the early age temperature of the concrete should also be controlled, avoid cracking of immature concrete. It could cause durability issues.

The most important aspect of it is the durability after construction. The article, Factors affecting durability of concrete provides more information on durability.

Compressive Strength

We use concrete because of its compressive strength. The hydration process creates a product that has strength like a rock.

The compressive strength is termed as the characteristic strength of the concrete in the design.

Most of the structural elements are subjected to axial forces, shear forces and bending moments. Concrete provides resistance to each of these loadings.

Concrete is very strong in compression and it is dominant in the axial compression elements. The whole section is effecting the compression loads. However, in cases like bending, part of the section is effective. 

As we know, in the beams that subjected to bending moment, we only consider the 0.45 times the depth to the neutral axis for the compression stress block as per the British standards. This value may be slightly changing with the other guidelines. However, the whole section is not effectively contributing the carry the loads.

As discussed above, the compressive strength is specified as the characteristic strength. This specific value is used in the structural designs and it is verified during the construction through the testings.

Commonly used method to specify it as C25/30. Here, the first number is the cylinder strength. The second number is the cube strength. 

 There are different factors affecting the compressive strength. The main factor is the water-cement ratio. In addition, there are other factors such as mix proportions, void ratio, curing time, compaction, etc.

Tensile Strength of Concrete

As we all know, the tensile strength is very low. Further, the tensile strength range is not widely used in structural designs except for a few occasions.

When we design the prestress concrete, we use the tensile strength depending on the design class selected.

There are different formulas to calculate the tensile strength.

Table 3 extracted from the Eurocode 2 shown above can be used to find the tensile strength. BS 8110 Part 1 also has given a table for prestressing concrete works.

The incapability to withstand tensile forces is the weakness of the concrete. Steel has both properties. Therefore, when there are tensile stresses, we used steel instead as an effective material.

Workability of Concrete

The workability is how much it is easy to handle after mixing until pouring is completed. There are different definitions of workability. 

However, in simple terms, it is the ability to workable during its handling.

The workability is measured in terms of slump test or floor test.

When the slump is low, it is measured from the slump test and a high slump is measured from the flow test.

Permeability of Concrete

The permeability is a measure of the rate of flowing fluids through a solid having voids.

Permeability is very important for structures that retain the fluids. It has to be controlled adequately.

The durability is severely affected by the permeability. Therefore it is required to control it adequately. 

Following factors are affecting the durability of concrete

1. Water/Cement Ratio: Low water-cement ratio reduces the permeability. When we increase the cement content, it improves the hydration process and more reactions occur. Thus, permeability reduces.
2. Curing: Curing is one of the most important factors to improve the condition in the surface zone. If there is adequate curing, it reduces the porosity by improving the reactivity of concrete. When there is adequate dampness in the concrete, it helps to cement to react adequately.
3. Use of admixtures: There are special admixtures called waterproofing admixtures. They reduce permeability. 
4. Compaction of Concrete: Adequate compaction affected not only the permeability but also to he strength. Poor compaction creates more voids and that leads to an increase in permeability.
5. Pore Structure: It is very important to have a well-connected pore structure for permeability. The more the connectivity of the pores, more the permeability.
6. Age of Concrete: Permeability is varying with the age or the degree of hydration.

There are two main methods that can be used to test the permeability. They are 

1. Constant Head Method
2. Falling Head Method

More information on these tests is discussed in the article Construction Material Testing Techniques. 

Impact Resistance of Concrete

Concrete is capable of carrying the impact loads on them. They are sudden loads that applied to structural elements in a form of point load or pressure loads or distributed loads.

There are several occasions that the impact loads are applied in the structural elements. Some of them are as follows.

• Blast loads
• Accident loads
• Progressive Collapse loads

It has a higher bearing capacity of such a sudden increase in the loadings. Further, the material strength also can be enhanced with a higher rate of strain. 

A multiplication factor can be applied to the characteristic strength when designs are done.

According to the book, Blast Effects on Buildings, in bending and compression, factors 1.25 and 1.15 can be applied respectively.

Further reading on the material strength enhancement could be studied from the article Design strength enhancements.

Abrasion Resistance of Concrete

Abrasion resistance is very important in the surfaces that were exposed to wearing. 

Especially in car park buildings that surface is exposed to wear, it is very important to improve the surface condition. The following factors could be affecting abrasion resistance.

• Strength of the concrete
• Further, increase the cement content and reduce the water content improves the abrasion resistance.
• Use of well-graded natural sand improves the abrasion
• Coarse aggregate should be free from soft sandstone or soft limestone

Different Types of Concrete

Depending on the application, they can be categorized as discussed below. 

Mass concrete

Use of pure concrete for the construction without using any kind of additives such as reinforcement, fiber, etc. to enhance its strength.

Mass concrete defined as follows in the ACI standards.

Any volume of concrete in which a combination of dimensions of the member being cast, the boundary conditions can lead to undesirable thermal stresses, cracking, deleterious chemical reactions, or reduction in the long-term strength as a result of elevated concrete temperature due to heat from hydration

Different grades are being used for construction. Mostly, low-grades are used for the construction as the heat generated in the hydration process is minimal. Since we are not proving reinforcement to minimize the cracking, limiting the temperature is vital to avoid the thermal cracking of concrete.

Further, large volumes are usually poured into the structures that cast from the mass concrete.

Gravity structures are constructed from mass concrete mostly. Since it does not carry the tensile stresses, it has to be in compression always. Designers should make sure all the surface of the structure is not subjecting to the tensile stresses when the loads are applied to them.

 As we know, these structures are subjected to lateral loads. They generate overturning moments creating tension in the face of the structure. These tensile forces should be balanced by the weight of the structure. When design, the face of the structure is kept in compression for all the load cases. 

You may refer to the article Mass Concrete for further information.

Reinforced concrete

Reinforced concrete is defined as structural concrete reinforced with no less than the minimum amount of prestressing tendons or nonprestressed reinforcement by ACI 318.

In simple terms, the structural concrete that having the reinforcement is called the reinforced concrete. This is mostly a known thing in the world.

The article reinforced concrete discussed further on this topic.

Roller compacted concrete (RCC)

RCC also the same as conventional concrete having cement, water, and aggregates. 

But the mixture is different from the conventional one.

And also, this concrete is compacted until the required density is achieved.

The following key aspects can be observed in the roller compacted concrete construction.

• As the name implies the RCC is subjected to some compaction. The moisture content of the mix shall be monitored properly. Water shall not be added throughout the compaction as the optimum moisture content provide better compaction.
• RCC concrete is compacted to achieve 98% modified proctor
• One of the most important factors in RCC construction is the treatment of the construction joints. Joints should be moist and fresh to adhere to the new layer.
• Adequate curing shall be done to achieve the expected strength and durability.

There are many advantages of roller-compaction. Some of them are as follows.

• It reduces the cement content
• It has good strength
• No reinforcements are required
• Less cost for reinforcements
• Less risk of cracking when it hardening
• RCC can use used in road construction, dam construction, etc.

The article roller compacted concrete could be useful if you need further information on this subject.

Self Compacting Concrete

The name itself provides an idea about it. It is flowable and does not require vibration to compact.

This type of concrete is not used usually. They are used on special occasions. Some of them are as follows.

• Heavily reinforced sections
• Concreting of Piles
• Column construction where the vibration is difficult or based on the type of the project
• Raft foundation construction
• Drill shaft construction
• Construction of earth retaining systems
• Retrofitting and repair work

There are advantages and disadvantages of the self-compacting concrete. Some of the useful advantages are as follows.

• Faster construction time
• Excellent durability
• Very high compaction
• Grater bond to reinforcements
• Reduces the permeability of the structure
• Flowing through the areas congested with reinforcements
• Easy to handle
• Less labor force is required during the pouring
• Finishing of the concreting work is good
• Reduce skilled labor

Some of the disadvantages of self-compacting are as follows.

• Formwork shall be design to withstand high pressure than usual
• More experience is required to produce it
• Selection of the material is strict
• Should be strict on the quality control 

There are many tests that involve self-compacting concrete to make sure its quality. Some of the test done during the construction is as follows.

Filling Ability Test	Passing Ability Test	Segregation Resistance Test
Slump Floor Test	J-Ring Test	V-funnel at T5 minutes
T50cm Slump Flow	L-Box Test	GTM Screen Stability Test
V-Funnel Test	U-Box Test
Orimet	Fill-Box Test

High-performance concrete (HPC)

ACI defines high-performance concrete as concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practices.

It is different from conventional concrete. It adds more benefits to the user than the normal concrete.

Further, HPC is on the occasions where we need high performance.

The following advantages could be achieved from the high-performance concrete.

• High strength
• High Early strength
• High Modulus of Elasticity
• High Abrasion Resistance
• High Durable
• High Capability to Withstand in Severe Environments
• Low Permeability
• High Resistance to Chemical Attacks
• High Impact Resistance
• Volume Stability
• Easy to Handle and Place
• Proper Compaction without segregation

Prestressed Concrete

Prestressed concrete is used widely in the construction industry due to the advantages added to the construction.

Prestress construction is very used full to construct and easy the handle when they are doing on a small scale. However, when the scale is expanding, heavy equipment is required for the construction. 

Prestressing is very useful to support the large spans where it is not effective in doing with normal construction. Further, floor slab construction can also be done with these systems.

The article was written as Bridge Design to BS 5400 provides information about the design of the Post tension beam.

Further, there are more advantages of the Prestressing as discussed in the article advantages of prestressed concrete.

Fiber Reinforced Concrete

According to the ACI, fiber reinforced concrete is defined as concrete containing dispersed, randomly oriented fibers.

Fibers are added to the concrete and they act as reinforcements. They improve the strength. Further, they act as the tensile reinforcement where there are tensile stresses.

There are different types of fibers.

• Glass fibers
• Polypropylene and nylon fibers
•  Steel fibers

Steel fibers are used for construction works widely where fiber reinforced concrete elements are cast. They can be used with or without reinforcements. Further, they improve structural strength.

Construction of industrial floors is done from fiber reinforced concrete. Mostly, reinforcements are not used in this type of construction. There are guidelines like Technical Report 34, Concrete Industrial Ground Floors, a guide to design and construction published by The Concrete Society.

Further, there are other standards that could be used for design and construction.

• EN 14889-1:2006
• EN 14845-1:2007 
• ASTM A820-16
• ASTM C1018-97

Concrete Grades 

Concrete Grade is an indicator of its characteristic strength. According to British standards, grades are specified as C20, C25, C30, C40, etc.

The characteristic strength is considered in the structural design is determined based on the probability of test result of a given grade. The definition is given in BS 5328 Part 1 and the article Characteristic strength of Concrete discusses more on this subject.

According to the Eurocode, there is a different definition is used. It uses the cylinder strength to represent the characteristic strength.

Strength relationship C25/30, C30/37, C35/40, etc are used.

Concrete Design

Variations of the stress and strains are considered in the structural designs. Based on most of the guidelines, the limiting strain of the concrete is 0.0035.

As per British standards, concrete fails when its strain reaches 0.0035. Therefore, it considered avoiding concrete reaching this strain during its ultimate limit stage loading.

Further, failure of the concrete earlier than the steel makes severe issues as it does not provide prior warnings. Therefore, it is considered to avoid failing concrete earlier than steel.

As indicated in the above figure, steel starts yielding at strain 0.002. Thus, the balance consideration of the strain diagram becomes when concrete and steel strains reach 0.0035 and 0.002 respectively.

The corresponding x/d ratio for the balance condition is 0.64.

British standards consider x/d = 0.5 which is less than that of the balanced condition. It could be due to various reasons such as sections reacting to the additional forces due to moment distributions etc. 

Further, consideration of less x/d ratio reduces the concrete stain at the ultimate limit state.

When x/d = 0.5, both the strains (steel and concrete ) become 0.002 which is good in terms of concrete failure.

In summary, the ultimate goal of limiting the x/d ratio is to make sure the ductile failures.

Creep of Concrete

Concrete subjected to creep deformation when it subjected to long-term load.

It causes the deformation of the structure such as long-term deflections. However, it is not causing structural failures generally.

The following are affecting the creep.

• Aggregates
• Mix proportions
• Agee

Additives to the Concrete

Different additives are used as fillers to replace the cement and as an additive to reduce the heat of hydration.

Further, they increase the surface area as the material like silica fume has a higher surface area.

Fly as used in construction widely to replace the cement or to reduce the cement content. Especially in thick concretes where it produces higher heat of hydration, fly ash added to reduce the heat.

Another material widely added is the ground granulated blast furnace slag. 

The article cement and cement additives discussed more on each of these additives in detail.

Admixtures

The majority of the concrete poured today uses the admixtures. In the future, there may not be concrete without admixtures.

The industry has developed widely and they add many advantages to conventional concrete. Different type of admixtures provides a variety of advantage to the industry.

The following advantages could be highlighted.

• Maintains the workability of the concrete until placing
• They can be used to extent the setting time or advance the setting time
• Cost of construction reduces

According to the BS EN 934-2-2001, the following type of admixtures can be identified.

• Water reducing/plasticizing admixtures
• High range water reducing/ superplasticizing admixtures
• Water retaining admixtures
• Air entraining admixtures
• Set acceleration admixtures
• Hardening accelerating admixtures
• Set retarding admixtures
• Water resisting admixtures
• Set retarding/water reducing/plasticizing admixtures
• Set retarding/high range water reducing/superplasticizing admixtures
• Set accelerating/water reducing/plasticizing admixtures.

The article Concrete Admixtures could be referred for more information and testing of admixtures.

Concrete Mix Design

Once all set to proceed with the construction, mix designs are done. The designer specifies the required strength of the concrete. It is the responsibility of the contractor to produce the same grade.

Based on the available material and resources, mix designs are carried out for each grade and they are submitted for the engineer’s approval.

Water cement ratio, mix proportions of cement, water, sand, and coarse aggregate are specified in the mix design.

In addition, if any admixtures are used, their dosages will also be indicated in the mix design.

After getting the approval of the engineer, verification of the mix design is done. Trial mixtures are done for each mix design to make sure the specified mix will achieve the target strength.

Target strength is specified in the mix design and it has to be achieved by concrete texted from trial mixtures.

Target Strength = f_ck + 1.65xσ

Where, 

f_ck – Characteristic Strength of Concrete

1.65 – a factor could be varied with different standards

σ – Standard deviation, could be selected based on the type of concrete

If the concrete achieved the target strength, it okay to proceed with the construction. 

You may refer to the related article factors affecting concrete mix design.

Concrete Pouring

Most people do not consider the importance of concrete pouring though they consider other quality control aspects.

Therefore, knowing the key factors related to concrete pouring is vital. The following key factors can be highlighted related to quality control to be considered when concrete is poured.

• Concrete Setting Time: Attention shall be made to the initial and final setting times. They shall be tested before starting the concrete pouring. It could be carried out when the trial mixtures are done. The setting time can be tested as mention in article 6 different cement tests. Further, the setting time can be modified by adding admixtures.  
• Formation of Cold Joints: The cold joints that form when the concrete is poured on concrete that has started the setting, shall be avoided. 
• Pouring Pattern: Or the construction sequence shall be planed before start the concrete pouring. It shall be planned as per the project and based on the resources available.
• Concrete Compaction: It is required to compact adequately to achieve the required quality and strength. 
• Free Fall Height: Generally, free fall height is limited to 3 to 5 feet to avoid segregation.
• Temperature Control: The controlling rise of the concrete shall be done to avoid cracking and to minimize its effects on the durability of concrete. Methods of limiting the rise of the temperature of concrete are discussed in a separate article.

Further, the article written on the subject concrete pouring could be referred to as further information.

 Compaction of Concrete 

It shall be made sure to compact the concrete adequately. Poor compaction results in many issues such as durability.

Further, poor compaction reduces the strength and increase the permeability.

The following issues could be rise due to poor compaction.

• Low strength
• Increase the porosity
• Formation of the honeycombs 
• Effect to the durability

The following method of compactions is widely used.

1. Manual compaction: Compact by hand. Rods or similar types of equipment could be used.
2. Mechanical Compaction by Vibration: Poker vibrator, formwork vibration, Table vibrator, Flatform vibrator, Vibratory roller, etc. methods are used for compaction.
3. Other methods such as pressure and jolting and spinning method are also used.

The following figure provides a clear idea of how compaction is important.

If more void due to the compaction, it directly affects the strength.  Therefore, much care shall be made during the concreting work.

Concrete Curing

One of the most important aspects that needs to be attended after concrete pouring is the curing. It provides greater benefits to the structure.

• It increases the strength
• Reduce the permeability
• Avoids plastic shrinkage cracks
• Increase the abrasion resistance
• Improve the durability

There are many factors affecting the curing. The article factors affecting curing time of concrete discuss these issues.

Depending on the type of work, a suitable curing method shall be selected. The methods used for thinner concrete are not applicable when it thicker.

There are many curing methods and out of those, 11 methods are shown below.

1. Water Curing
2. Wet Covering
3. Formwork Curing
4. Membrane Curing
5. Sheet Curing
6. Curing by Absorbing Heat
7. Hot mixing method
8. Electrical curing
9. Infra-Red Curing
10. Cover with Sand or Sawdust, Soil, etc
11. Natural Curing (Exposed concrete)

Each of the methods has discussed in detail in the article methods for curing of concrete.

 

Durability

All structures are constructed for a certain life span and accordingly, the classification of the structure is made. Different structural class is used in the design to specify its life span.

Designs are carried out based on the initially identified parameters. Failure to comply with durability requirements could lead to the following issues.

• Reinforcement corrosion
• Deterioration
• Structural failures
• Cracking and spalling of concrete
• Periodic maintenance and the cost 

Therefore, it is very important to attend to the durability at the starting structural design until the structures exist.

The article was written as Durability requirements in reinforced concrete design have more relevant technical information related to this subject.

The key factors affecting the durability which are listed in the article Factors affecting durability of concrete are shown below.

1. High humidity and Rain
2. Ultraviolet resistance
3. Chemical Resistance
4. Seawater exposure
5. Chloride resistance and steel corrosion
6. Sulfate resistance
7. Resistance to Alkali-silica reaction (ASR)
8. Carbonation
9. Abrasion Resistance
10. Moderate to severe exposure condition for concrete
11. Resistance to freezing and Thawing
12. Cement Content
13. Aggregate quality
14. Water Quality
15. Concrete compaction
16. Placing time after batching and cold joint formation
17. Curing period
18. Permeability
19. Temperature
20. Construction defects (honeycombs, cracks, etc.)

Each of the above is discussed in detail in the article factors affecting durability of the concrete.

Concrete Testing

Testing is done as a measure of quality control and to check whether it has achieved the required strength. Further, it is required to verify the values assumed in the design are achieved by the construction.

Further, when there are certain doubts about the test results or if it is required to verify the strength of the concrete identified as defective, testing is done. 

Testing for Quality Control

These tests are done at the initial stage. The prime objective of these testing is to make sure the poured concrete has achieved the required or the specified strength.

In addition to the strength verifications, workability, slump, temperature, etc also tested to make sure the quality of the concrete.

Slump Test and Flow Test

These tests are carried out to check the workability. 

Design slump is specified in the mix design shall be achieved at the time of receiving to the site. 

Before starting a concrete pouring slump test is carried out. Samplings that are taken from each truck mixture is tested.

There is an allowable range for the slump. For example, if the design slump is 150 and when the allowable rage is ±25, the slump at the site should be between 125 -175. It is not achieved by the concrete, the truck can be rejected.

The allowable rage will be varying depending on the slump classes etc. Therefore, the limits specified in the mix design shall be followed or relevant specifications shall be referred to.

Cube and Cylinder Test

The most widely and commonly used method to check the strength is the testing cubes or cylinders.

Depending on the type of the project or the referred standards, testing cubes or cylinders is done.

In British standards, they used cube strength for structural design wors. However, Eurocode 2 used the cylinder strength for structural design.

The test cubes cast at the time of pouring of concrete are submerged in the baths. A sampling of the test cubes shall be as per the relevant specification of the projects.

Usually, testing is done for 7 days and 28 days.

 Testing for Construction Defects

When there are issues in the construction, testing is done to make sure it has adequate strength. Further, these tests are specially required when there are no test results are available.

Further, failure of the concrete cubes to achieve the specified strength also leads to dong these tests.

Mainly there are two types of test categorizes based on the way they are done.

1. Non-destructive tests
2. Destructive tests

Non-Destructive Test

The name itself implies the testing method. In this test method, no damage is done to the concrete when testing is done.

Without making any harm to the concrete, these tests are carried out.

However, there is always a doubt about the test results. Believing these provide 100% accuracy is not practical. 

The destructive test provides accurate results as we test the actual samples. 

There are many non-destructive testing methods as discussed in the article Nondestructive testing of concrete. They are listed as follows.

1. Visual inspection
2. Half –cell electoral potential method
3. Rebound hammer test
4. Carbonation depth measurement test
5. Permeability test
6. Penetration resistance or Windsor probe test
7. Cover meter testing
8. Radiographic testing
9. Ultrasonic pulse velocity testing
10. Tomographic modeling
11. Impact eco testing
12. Ground penetrating radar or impulse radar testing
13. Infrared thermography

Destructive Tests

In this test method, concrete samples are taken or they testing at the site using that material.

The following tests are widely used in the construction industry as discussed in the article destructive testing of concrete.

1. Concrete core cutting
2. Pull-out Testing

A core sample is taken cutting and it is tested for strength. If the tested sample has achieved the required strength, it could be acceptable. Further, these test results are correlated with the non-destructive test results for better understanding.

Environmental Impact of Concrete

Concrete is not environmentally friendly material. It affects severy on the environment.

As a material, it has higher embodied energy. The emission of greenhouse gases is very high for the production of concrete.

There are several materials added to concrete. Out of those materials, cement production has a very high emission of greenhouse gases. The production of cement is very in the world and the rate of production is also increasing with the emerging economies.

The above figure taken from the internet provides a clear idea about the impact of the construction industry on the environment. Since this impact is not irrevocable, highest attention shall be made on this.

Use of the alternative materials, reduce the embodied energy of a structure, etc. shall be followed to protect the environment.

Currently, the world thinking to move toward green technologies that have less impact on the environment.

Recycling and Reusing

It is recycled as a measure to reduce its environmental impacts. There are a few steps to be followed in recycling.

• Firstly, large pieces are crushed with special industrial equipment.
• After that, the broken particles are screened to remove any dirt or contaminating particles. Additional processes and equipment such as water flotation, separators, and magnets are used to remove other impurities.
• Concrete is separated into large and small aggregate.

Further, concrete is resued when there is a place to pour or put. Especially at the end of the construction, there is a considerable quantity to be removed as debris. These materials can be used for filling other sites. A similar arrangement can be made of reused materials.