Category Archives: CONCRETE

CONCRETE, Stone

Stones || Quality tests on stones ||Types and procedures

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Stones are the form of rocks from the earth’s crust. They find their application in the construction of residential and public buildings, dams, harbours, face-work of structures, road metal, and railway ballast. Besides, stones have good strength and durability.

Quality tests on stones are crucial for construction projects. Testing stone ensures that it meets the necessary standards for strength and durability. There are various tests for stones that assess different properties. These tests for stones include crushing strength, water absorption, and abrasion resistance. By performing these tests, engineers can decide the suitability of the stone for specific uses. Properly testing stone helps prevent structural failures and ensures long-lasting constructions. In this blog, we will explore the different types of stones and the procedures for each quality test. This information is essential for anyone involved in construction and material choice.

  1. Types of Stones Used in Construction
  2. Acid test
  3. Attrition test on stones
  4. Crushing test on stones
  5. Stone Crystalline test
  6. Freezing and Thawing Test
  7. Hardness Test
  8. Impact test
  9. Water absorption test
  10. Microscopic tests on stones
  11. Smiths tests

Types of Stones Used in Construction

Granite is a hard, durable stone, ideal for high-stress applications. Tests on stones, like crushing strength tests, highlight its robustness. Limestone is versatile but porous, requiring water absorption tests. Marble is prized for aesthetics, with abrasion resistance tests ensuring durability. Sandstone, used in paving and walls, needs strength and porosity tests. Slate, known for durability, is tested for impact resistance and is used in roofing and flooring. Proper testing stone ensures suitability for construction.

Qualities of Good Stone

We consider good-quality stones for the construction of important engineering structures. The next are the prime requirements of good-quality stones.

Also read : 9 lab tests for flexible pavements

  • The heavy stones have less porosity and high compactness. Thus the specific gravity of stones should be high.
  • Uniform and appealing colour stones are employed for decorative works.
  • Should have a homogeneous composition and should have less water absorption.
  • It should have the ability to get good polish.
  • Moreover, it should be free from iron oxides and calcium carbonate to resist fire.
  • The dense compaction of rocks can withstand the consequences of external agencies.
  • A good stone must be free from quarry sap.
Stones : Quality tests
Stones - Quality tests

Test on stones

Testing stone is essential to make sure durability and strength in construction. These tests for stones assess various properties. Proper tests on stones prevent structural issues.To conclude the strength, durability and other engineering properties of the stone, the following tests are performed. 

  • Acid test
  • Attrition test
  • Crushing test
  • Crystalline test
  • Freezing and thawing test
  • Hardness Test
  • Impact test
  • Water absorption test
  • Microscopic Test
  • Smith’s Test

Acid test

The acid test is a crucial procedure in testing stone for quality. Tests on stones help to determine the durability and suitability. Proper tests for stones ensure reliable construction materials.The acid tests determine the presence of calcium carbonate in rocks. The test method is as follows,

  • Take some 50 to 100g specimen randomly.
  • Then place them in the solution of sulphuric acid and hydrochloric acid having 1% strength for about one week.
  • Frequently mix the solution and immerse the specimen fully.
  • Subsequently, observe the specimen. 
  • Specimen with high lime content causes efflorescence due to the presence of an acid solution.
Quality tests on stones -Acid test

Attrition test on stones

The attrition test indicates the rate of wear of the stone under the sudden impact of loads. Another name of the attrition test is the abrasion test. The apparatus for the attrition test is Devel’s testing machine.

  • Take some sample specimen and break them into small pieces having 60mm size.
  • Now take 50N of stones and place them in the cylinder of the testing machine. 
  • Then close the cylinder and rotate them for 5 hours at a rate of 30 rpm. 
  • After 5 hours, take the samples outside and sieve them through a 1.5 mm mesh.
  • Weigh the amount of material retained in the sieve and calculate the percentage of wear using the following formula.

Percentage of wear = ( Loss in weight / Initial weight ) x 100

Thus, we get the percentage of wear.

Attrition test on stones
Stones

Crushing test on stones

The crushing test gives the strength of the stones. These tests are performed for stones to be used at the bottom of heavy structures.

  • For this test, cut the stone specimens into 40mm x40mmx 40mm and dress the sides.
  • Minimum of three specimens are needed for this test.
  • Before starting, place the specimen in water for 72 hours.
  • Then cover the load-bearing surface of the specimen with a plywood layer.
  • Now place the specimen in the testing machine.
  • Simultaneously, apply load axially at a rate of 13.7 N/mm2 per minute.
  • Note down the load at which the stone breaks. Calculate the strength using the following formula.

Crushing strength = Maximum load at which stone breaks / Loaded area 

However even weak stone possess high compression strength. For example, the crushing strength of stone for ordinary building works should not exceed 1N/mm^2. 

Stone Crystalline test

This test defines the weathering nature of stones. To conduct this test we need at least 4 cubes with 40mm size. The crystallisation of Calcium sulphate causes eroding of stones

  • To start with first, immerse the samples in a solution of sodium sulphate at normal room temperature.
  • After that dry them at 100 degree Celsius and repeat these steps 5 times.
  • Then note down their difference in weight in the percentage of the original weight.
  • The difference in weight shows the weathering quality of the stones. 

Freezing and Thawing Test

As stones in the construction work are exposed to sunlight, wind, rain etc. This test is necessary to carefully study the behaviour of stone. The test procedure is as follows.

  • Take the specimen and immerse it in water for 24 hours.
  • Then place it in a freezing mixture at 12 degree Celsius for 24 hours.
  • Frequently repeat the above two steps and observe the stone quality. 

Besides, perform this test only in the shade to prevent the consequences of rain, sunlight, etc.

Hardness Test

The hardness of the stone is its ability to resist scratch or rebound.

  • For this test, we use a penknife. This can not make a scratch on pard stones like granite.
  • Moh’s scale value determines the hardness of the specimen.
  • For example, Moh’s scale value is 1. Since it is easily scratchable.
  • Likewise for Quartz, Moh’s scale value is 7. Since it cannot be scratched with a knife.

Impact test

The impact test determines the toughness of the stone. The impact testing machine is the apparatus used for this test. The test procedure is as follows.

  • Take a specimen in a cylindrical shape with 25mm diameter and 25mm height. 
  • Then place it on the cast iron anvil of the machine
  • After that, allow a steel hammer of 20N to fall vertically over the specimen
  • The first blow height is at 1cm. For the second blow height, it is 2cm and so on. 
  • Gradually increase the height of the blow. Finally, note down the height at which the specimen breaks. 
  • The height at which the specimen breaks is the toughness index.

Water absorption test

Through this test, we can determine the porosity as well as moisture content. The water absorption test is as follows

  • Prepare a stone sample and record its weight asW1
  • Then immerse the cube in distilled water for 24 hours. 
  • After that wipe the water with a damp piece of cloth. Again weigh the sample as W2.
  • Now suspend the cube freely in water and record its weight as W3.
  • Subsequently, place the cube in boiling water for five hours. Again weigh the cube and record its weight as W4.
  • From the above data, we can also calculate the percentage absorption of water and saturation coefficient using the formula.

Percentage absorption by weight after 24 hours = (W2- W1)/ W * 100

Percentage absorption by volume after 24 hours = (W2 – W1)/( W2 – W1) * 100

Saturation coefficient = Water absorption / Total porosity = (W2- W1) – ( W4- W1)

Microscopic tests on stones

This test helps to study the geology of the stone. The sample is placed for microscopic examination to analyse the below properties.

  • Mineral components
  • Texture and nature of stones
  • Presence of malicious substance
  • Determining defects and pores
  • Size calculation, etc.

Smiths tests

Smith’s test calculates indicates the presence of earth matter in stones. 

  • In this test firstly, break the specimen into small pieces.
  • Then take a test tube with clear water and place these pieces in it.
  • Vigorously shake the test tube. The muddy colour of the water shows the presence of earthy matter.

Key Takeaways

Quality tests on stones are essential for ensuring their suitability in construction projects. Testing stone involves various tests, including crushing strength, water absorption, and abrasion resistance. Each test stone procedure is designed to assess specific properties, such as strength, durability, and weather resistance. Granite, limestone, marble, sandstone, and slate are commonly used stones that undergo these tests for stones. Procedures like the acid test, attrition test, and freezing and thawing test help determine the stone’s durability and resistance to environmental factors. Proper tests on stones prevent structural issues and ensure long-lasting constructions. Understanding these tests for stones is crucial for anyone involved in construction and material selection.

Conclusion

In conclusion, performing quality tests on stones is a fundamental step in construction to ensure the materials’ strength and durability. Testing stone through various procedures, such as crushing strength, water absorption, and impact tests, provides critical information about the stone’s properties. These tests for stones help in selecting the right material for different construction purposes, ensuring safety and longevity. Types of stones like granite, limestone, marble, sandstone, and slate each require specific testing methods. Therefore, incorporating these tests on stones into the construction process is vital for achieving reliable and robust structures. Properly testing stone materials guarantees their performance and enhances the overall quality of construction projects.

CONCRETE, quality tests on concrete

Compaction factor test – Workability of concrete-Test procedure

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Compaction factor test is a practical and reliable method used to assess the workability of concrete, especially when the mix is stiff and does not show clear results in a slump test. The compaction factor test for workability of concrete evaluates how easily concrete flows under gravity. It also assesses how it compacts by comparing partially compacted concrete with fully compacted concrete. In the workability of concrete compaction factor test, engineers gain a more accurate understanding of concrete consistency for low to medium workability mixes. Following the compaction factor test procedure helps achieve consistent results during laboratory testing. The compaction factor test of concrete is commonly applied in mix design and quality control and remains one of the dependable tests for workability of concrete used in practice. Compaction factor test is a lab test and is generally conducted on concrete samples having very low workability.

This article explains the compaction factor test, covering its principle, procedure, significance, advantages, limitations, and role in accurately evaluating the workability of concrete.

  1. Significance of Quality tests on concrete
  2. Production stage quality tests on fresh concrete
  3. Significance of compaction factor test
  4. Compaction factor test for workability of concrete
    1. Compaction Factor Test – Relevant codes
    2. Apparatus used
    3. Compaction factor test procedure
  5. Key Takeaways
  6. Conclusion

Significance of Quality tests on concrete

Quality tests on concrete are essential for ensuring strength, durability, and performance in construction. These tests help engineers assess the workability of fresh concrete and the quality of hardened concrete. Common quality tests on concrete include checks during the production stage (on fresh concrete before placing), tests on hardened concrete specimens, and evaluations performed directly on finished structures. Including tests like the flow table test for concrete ensures reliable results and consistent quality throughout a project.

A construction worker wearing a hard hat and safety vest examines concrete samples on a workbench, including a concrete compression testing machine and molds.
A civil engineer in a safety helmet and reflective vest analyzing concrete test specimens in a lab, showcasing the importance of quality control in concrete workability tests.

Following are the stages of quality tests

  • Production stage quality tests ( On fresh concrete before placing)
  • Hardened stage quality tests ( hardened concrete specimens)
  • On structures ( tests done on the structures )

Production stage quality tests on fresh concrete

Production stage quality tests on fresh concrete are crucial for ensuring the right workability and consistency before placement. These tests help engineers select the best mix for construction and maintain quality control. Key tests include the slump test, compaction factor test, Vee-Bee test, Kelly ball test, and the flow table test for concrete.

This article focuses on the Compaction factor test, explaining its procedure, importance, and how it compares with other workability tests like the slump test and other tests

Significance of compaction factor test

The compaction factor test is significant because it accurately measures the workability of concrete. This is especially true for stiff and low-slump mixes. This test provides more precise results than the slump test. It supports effective mix design. It also ensures consistent concrete quality through reliable laboratory-based workability assessment.

Compaction factor test for workability of concrete

The compaction factor test is used to determine the workability of concrete. Workability refers to the ease and uniformity with which freshly mixed concrete can be mixed, placed, compacted, and finished. Workability also represents the internal effort required to achieve full compaction. The compaction factor test of concrete was developed by the Road Research Laboratory, UK. It provides more precise and accurate results than the slump test. This test is especially suitable for stiff concrete mixes that do not slump and require vibration for compaction. The compaction factor test measures the degree of compaction under controlled conditions. It offers a reliable method for assessing concrete consistency in laboratory and quality control applications.

Also read : Bitumen – 9 lab tests on bitumen for flexible pavements.

Also read : Timber – Five important quality tests

Compaction Factor Test – Relevant codes

  • IS 1199 (Part 2): 2018 – Methods of Sampling and Analysis of Concrete – Workability
  • BS 1881 (Part 103) – Testing Concrete – Method for Determination of Compaction Factor
  • ASTM C1170 / C1170M – Workability of Fresh Concrete by Compaction Factor Method (Reference use)
  • DIN 1048 – Testing Methods for Fresh Concrete

These codes provide standardized procedures for conducting the Compaction Factor Test and ensure reliable evaluation of concrete workability.

The Compaction Factor Test method followed in IS 1199 (Part 2) is fundamentally the same as those in British standards. It is also similar to other international standards. All codes measure workability by comparing partially compacted concrete with fully compacted concrete.

  • Principle: Same across all standards
  • Apparatus: Same (two hoppers + cylinder)
  • Calculation: Same compaction factor ratio
  • Differences: Minor variations in dimensions, tolerances, and wording

Apparatus used

A female laboratory technician in a white lab coat and protective goggles is pouring a substance from a can into a blue funnel system, while a male colleague works in the background in a modern laboratory setting.
Laboratory setup for conducting the compaction factor test on concrete, demonstrating the apparatus used to measure workability.
  • Compacting factor apparatus
  • Cylindrical mould 
  • Hand scoop 
  • Trowel
  • Weighing machine
A blue civil engineering apparatus featuring two hoppers and a collecting bucket, designed for material testing.
Apparatus used for the Compaction Factor Test, designed to accurately measure concrete workability.

The figure shows the dimension of the hopper and the mould. The inside area of the hopper should be smooth. The apparatus is made of bronze or cast brass. At the lower ends of the hopper, there is a trap door with a 3mm thickness. A rigid frame attaches the hoppers and cylinder. After the completion of mixing, the test is conducted at a constant time interval. 

Compaction factor test procedure

  • Place the sample in the upper hopper of the apparatus using a hand scoop.
  • Fill the hopper with its brim level and open the trap door.
  • Then allow the concrete to fall into the lower hopper.
  • Immediately after the concrete rests, uncover the cylinder and open the lower trap door.
  • Cut off the excess amount of concrete above the cylinder using the trowel.
  • Then weigh the concrete in the cylinder nearest to 10g. This is the weight of partially compacted concrete.
  • Refill the cylinder with the same sample in the layers and the layer is heavily rammed or vibrated for full compaction.
  • Then weigh the fully compacted concrete and the weight of the empty cylinder.
  • Finally, Compute the compaction factor using the formula.

The compaction factor is the ratio of partially compacted concrete to fully compacted concrete.

Compaction factor = (W1 – W) / (W2 – W)

The compaction factor value varies from 0.7 to 0.95.

Key Takeaways

  • The Compaction Factor Test is used to evaluate the workability of concrete, especially stiff mixes.
  • It is more sensitive and accurate than the slump test for low-workability concrete.
  • The compaction factor test for workability of concrete measures ease of compaction under gravity.
  • It is best suited for laboratory-controlled conditions.
  • The workability of concrete compaction factor test helps compare different mix designs.
  • The test uses two hoppers and a cylindrical mould.
  • Following the compaction factor test procedure ensures consistent results.
  • The compaction factor test of concrete minimizes subjective judgment.
  • It supports quality control and mix optimization.
  • It is one of the reliable tests for workability of concrete.

Conclusion

The Compaction Factor Test remains an important method for assessing the workability of concrete. This is particularly true for mixes that show little or no slump. By measuring the ratio of partially compacted to fully compacted concrete, the compaction factor test for workability of concrete provides a clear and quantitative indication of consistency. When engineers follow the standard compaction factor test procedure, the results help in evaluating mix performance and maintaining uniform quality. The workability of concrete compaction factor test is especially useful in laboratory testing and mix design studies. As one of the established tests for workability of concrete, the compaction factor test of concrete continues to support accurate quality control and better construction outcomes.

CONCRETE

Reactive Powder Concrete- 6 Components and All Properties Explained

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Reactive powder concrete is an ultra-high strength and highly ductile composite material with advanced mechanical properties, hence has high durability. In this blog, I will walk you through the components, properties, advantages and limitations of RPC.

Let’s start from the definition.

What is reactive powder concrete?

Reactive Powder Concrete (RPC) is a type of Ultra High Performance Concrete (UHPC). The main constituents are cement, sand, silica fume, steel fiber and quartz powder with minimal water to binder ratio, without coarse aggregate.

Next, let me show you the development of RPC that presents different steps of formation.

Development of RPC

In RPC, water- binder ratio is very low, CA is absent- reduce the heterogeneity of concrete mix and optimize microstructure. Precise gradation of particles yields compact, well-arranged and high strength hydrates of maximum density.

Development of RPC is based on the following principles.

  • Elimination of CA- Homogeneity enhanced
  • Utilization of pozzolanic properties of silica fume
  • Granular mixture optimization- to enhance the compacted density
  • Optimal usage of superplasticizer- improves workability, reduce w/c
  • Apply pressure (before and during setting)- improves compaction
  • Post set heat treatment- to enhance microstructure
  • Addition of small-sized steel fibres- improves ductility

Let’s move on to the components of reactive powder concrete now.

Components of RPC

  COMPONENT      SELECTION PARAMETER  FUNCTION  PARTICLE SIZE
SandGood hardness, readily available and low costGives strength150-600 µm
CementC3S: 60% C2S: 22% C3A: 3.8% C4AF: 7.4%Binding material for production of primary hydrates1-100 µm
Quartz powderFinenessMax reactivity during heat-treating5-25µm
Silica fumeVery less quantity of impuritiesFilling the voids,
enhance rheology,
production of secondary hydrates		0.1-1µm
Steel fibersGood aspect ratio, straight in shapeImprove ductilityLength 13-25 mm   Dia. 0.15-0.2 mm
Super plasticizerLess retarding characteristicsReduce w/cPoly acrylate based
Components of RPC

Here comes the properties of RPC in the next section.

Properties of RPC

  • RPC possess better strength (compressive and flexural) and lower permeability compared to HPC
  • Fracture toughness is higher for RPC- higher ductility
  • Ultra-dense microstructure- waterproofing and durability characteristics
  • Higher corrosion resistance- can be used in chemically aggressive environments
  • Workability of RPC mixtures (with and without fibres) measured by mortar flow table test- (120-140 mm)
  • Workability of HPC mixtures (with and without fibres) measured by slump test- (120-150 mm)
  • Density of fresh RPC and HPC mixtures- 2500-2650 kg/m3 
  • Compressive Strength of RPC and HPC- linked to durability, found that compressive strength of RPC is suitable for nuclear waste containment

How about the factors affecting the strength of RPC?

Also read: Self healing concrete|Bacterial concrete-Preparations& Advantages

Factors affecting strength of RPC

  1. Silica fume percentage- Compressive strength decreases as dosage increases, fluctuates in the range of 25-35% silica fume 

2. Quartz Powder- improves filler effect, 20% increase in compressive strength under accelerated curing condition

3. Curing Regime- compressive strength increased by 10% when subjected to hot water curing

4. Temperature- 

  • Compressive strength

– decreases at 100°C

– increases from temperatures 200-500°C 

– decreases at temperatures above 600°C

-increases as fibre content increases

-decreases as fibre content increases above 300°C

  • Tensile strength

– decreases below 200°C

 -increases from temperatures 200-300°C

 -decreases above 300°C

5. Pre-Setting pressure- Compressive strength of RPC increased by two times

Time to see the advantages of reactive powder concrete.

Advantages of reactive powder concrete

Reactive powder concrete
Reactive powder concrete
  • High ductility
  • Low porosity and permeability
  • Increased resistance to corrosion and chemical attack
  • Significant dead load reduction is possible
  • Eliminates the need for supplemental shear and auxiliary reinforcing steel
  • Improved seismic performance
  • Useful for containment of nuclear wastes- as there is minimum penetration of liquid/gas through RPC

We will find out the applications of reactive powder concrete now.

Also read: Reinforced cement concrete- Working stress and limit state method

Applications of RPC

  • Pre-stressed applications
  • Light weight structures- roof of stadiums, long span bridges, space structures, high pressure pipes, and blast resistance structures
  • Containment of nuclear wastes
  • Pre-cast structures 

Before we wrap up, let’s take a quick stroll through the limitations.

Limitations of reactive powder concrete

  • More expensive compared to conventional concrete
  • Technological and economical difficulties in applying pressure to mix and heat treatment in field
  • Since RPC technology is in its infancy, the long term are still not known
  • Fine sand becomes equivalent to CA of conventional concrete, hence acting as a weak link
  • Portland cement plays the role of fine aggregate and silica fume of the cement of conventional concrete

So, in nutshell,

Conclusions

  • RPC is a recent advancement in the field of construction
  • It has superior performance characteristics compared to HPC
  • Based on the principle of optimization of microstructure to produce hydrates of the highest strength and durability
  • Usage of superior materials has increased the cost of RPC considerably
  • Ambiguity in long term properties, technological difficulties
  • Intense research in this field guarantees that its limitations shall be resolved and it will be tapped to its full potential in the near future

That’s it about reactive powder concrete. Let me know your feedback on the article and any other topic suggestions in comments.

Happy learning!

CONCRETE, Shotcrete

Shotcrete – A total overview||Shotcrete vs Gunite

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Shotcrete is a method of placing concrete where concrete/mortar is sprayed at high velocity to an overhead or vertical surface. Concrete is conventionally placed on the ground or to the formwork and compacted using appropriate vibration methods.

What is shotcrete? 

Shotcrete is a wet or dry mix of mortar/concrete sprayed to a vertical or overhead surface with high velocity. The mortar/concrete mix is conveyed through a hose and sprayed with a nozzle with high velocity. The force imparted by this application process consolidates the concrete/mortar resulting in an excellent bonding with almost all substrates. Shotcrete is also called sprayed concrete. The pressure will be maintained to around 20-30 N/cm2.

When mentioning about shotcrete another term which is commonly used is gunite or guniting.

Shotcrete process
Shotcrete process

What is gunite or guniting ?

Guniting and shotcrete are more or less the same. The difference between shotcrete and gunite is basically in the method of spraying and mixing ingredients.

Shotcrete spraying is accomplished through either of the following process.

a) Dry mix process

b) Wet mix process

The dry mix process is called gunite or guniting

In Gunite / dry mix process the dry materials are premixed and placed in a hopper. With the help of compressed air, the dry materials are driven to the nozzle. Water gets mixed with the ingredients from a separate source just before going out of the nozzle. The mixed material hit the surface to be applied at a super high velocity through the nozzle.

In the wet-mix process/Shotcrete, the ingredients are premixed with water like conventional concrete and placed in a hopper. Just like the dry-mix process, the wet-mix process uses compressed air to shoot the concrete through a hose. This is also called sprayed concrete were fully mixed concrete or mortar is sprayed through a nozzle.

Shotcrete of walls
Shotcrete of walls

Both these process are used depending on the area of application and site conditions.

Shotcrete vs Gunite – A comparison

Both of these methods have there pros and cons. The uses are distinguished depending on the area of application and nature of the job.

GUNITESHOTCRETE
In the gunite process, we can stop and start the job after some interval, and still, the material will stick on to the old surface due to the high velocity at which the material is applied.Shotcrete material is premixed and to be consumed within a specified time or else the material strength and qualities will be affected.
The applicators get much more work time since the cement mixing is done at the site. With gunite, you can stop and start the application without creating any cold joints. They got super high strength accompanied by low shrinkage.The joints formed in shotcrete due to any stoppage of work leads to a cold joint and the next layer applied after a certain period behaves as a different layer. A cold joint may develop and cracks may happen.
A super-skilled operator is required for gunite. The total quality, consistency, and strength of the gunite depend on the skill of the operator. And the ratio of water, cement sand mix. The nozzle operator controls the application, adjusting water levels as needed to get the right mix consistency.Highly skilled workers are not needed for a shotcrete application. The crew doesn’t have to be as skilled because the concrete comes already mixed and no need for any mixing or proportioning at the site. It’s easier to add specialised admixtures to the concrete
The high water content will ruin the mix and less water content will clog the hose and nozzle. Therefore care should be taken to maintain optimum water content.You don’t need to add water as it comes premixed with water. Sometimes water is added to improve the consistency of the mix in transit mixtures at site. This reduces the strength of the mix and cause shrinkage cracks.
Guniting gives a strong and uniform surfaceShotcrete gives a strong and consistent surface
Gunite produces a lot of oversprays called Rebound wastage. These rebound wastage are not to be reused and generate huge wastes. Disposing and housekeeping the area requires extra manpower.Wastage is less compared to Gunite in case of shotcrete.
Guniting is less expensive than shotcrete. Large-sized aggregates are not used in concrete, and mostly it will a cement-sand mix.Shot Crete is more expensive than gunite and uses aggregate up to 20 mm or as per design mix recommendations.
Completion time is slow compared to shotcreteThe shotcrete process is faster than gunite and got higher production rates.
Gunite is easier to clean up than shotcrete.
The hose is lighter and easier to use
Because you add water at the nozzle, the concrete is very fresh when you spray it.		If the hose gets clogged, it can be tough to clear which could be dangerous for the crew and property

The bottom line is that both gunite and shotcrete have their advantages and disadvantages. The truth is both will produce a high-quality, watertight, durable surface if you do the processes correctly. A super-skilled operator for doing the works improves the quality of the process.

Advantages of shotcrete

Shotcrete-sprayed concrete is increasingly becoming popular because of its extremely economical and flexible application process.

  • Shotcrete has got several advantages when compared to conventional concrete. For swimming pools, water retaining structures shotcrete is an ideal choice. They are easier to spray and admixtures can also be included to enhance mix qualities.
  • Shotcrete is durable and it imparts more strength to structures.
  • The process of application of shotcrete makes the concrete dense and less porous than conventional concrete.
  • They help in reducing shrinkage cracks, cold joints and produce an even, nonporous and durable concrete.
  • They help in reducing the construction time and reduce valuable time and money incurred as formwork costs and makes the work far more easier.
  • Shotcrete/gunite can save money in terms of shuttering, labour costs and can shorten the project delivery schedule.
  • Cold joints and areas where shuttering possibility is minimum can also be done using shotcrete methods.
  • It is possible to create any shape using shotcrete which is not possible using conventional concrete.
Gunite work
Gunite work

Application of Shotcrete/guniting

Shotcrete is extensively used in a wide range of applications due to there ease and flexibility in placing and capability of producing a dense and non porous concrete surface. They are used for..

  • Shotcrete is used for the construction of thin, lightly reinforced sections such as curtain walls.
  • Used for shell or folded plate roofs.
  • Used for underground structures and tunnels, lining works. For pre-stressed tanks, swimming pools, water retaining structures for treatment plants, etc.
  • Shotcrete/gunite is used for repairing deteriorated concrete and retrofitting works.
  • Used for structural steel encasing works.
  • Shotcrete is used for the formation of canal lining, reservoirs, and repairing of linings.
  • They are best suited for applications in underground structures, basement waterproofing works, water retaining structures, deep excavation slope protection, etc.

Disadvantages of Shotcrete/Gunite

  1. The success of shot crete depends on the skill of the operator.
  2. Shotcrete cost is higher than conventional concrete.
  3. Chances of concrete loosing its quality due to delay in spraying or pouring water to increase workability.
  4. Getting a perfect bond with some surfaces is very difficult.

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CONCRETE

Self Healing Concrete ||Bacterial Concrete -Preparation and Advantages

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Self healing concrete is a mind-blowing innovation in civil engineering. It can potentially contribute to ensure a longer lifespan of a structure. In this article, we will find out the details about bacterial concrete. A bacterial concrete is most effective in this category.

Let’s start from scratch.

What is self healing concrete?

The type of concrete that uses the process of self-filling up of cracks by the help of bacterial reaction in the concrete, after hardening is known as Self-healing concrete.

Why self healing concrete is important?

  • Unrepaired cracks lead to a reduction in the service life of the structure. Therefore, it is worth giving attention to.
  • Epoxy resins and other synthetic mixtures are alternatives. (But they are not good for human health)
  • Self healing concrete does not require human intervention

Let’s dive in deep now.

Bacterial Concrete

Concrete which is made by adding bacteria that precipitate calcite is called bacterial concrete.

  • It is microbiologically induced calcite precipitation.
  • Bacterial concrete heals cracks around 0.5 mm thickness.
  • It has improved Compressive & flexure strength than ordinary concrete.
  • Bacterial concrete is well suited for Small & Medium-Sized building
  • It is used on a limited scale & not commercially wide-spread.

We learnt the general details. Let’s be a bit technical now?

Bacteria Used in the self healing concrete

Bacteria used in bacterial concrete
Source : alchetron.com
  • “Bacillus pasteurii ”  is a common soil bacterium which is used in bacterial concrete.
  • Precipitates impermeable calcite layer over the surface of concrete.
  • Acid producing bacteria
  • Remains dormant & be viable for over 200 years under dry conditions

In the next section, let me quickly walk you through the mechanism of the self healing concrete.

Mechanism of Bacterial Concrete

  • Bacillus pasteurii is used along with Calcium Lactate. 
  • Both are added in the wet concrete when the mixing is done.
  • Water seeps in the cracks (or exposure to moisture)
  • Spores of the bacteria germinate & feeds on calcium lactate ,consuming oxygen.
  • The soluble calcium lactate is converted to insoluble limestone.
  • limestone starts to harden, filling the crack automatically
  • The advantage is that oxygen consumed helps in the prevention of corrosion of steel.

That’s it. Now, we will move on to the chemical process in bacterial concrete.

Chemical process of this self healing concrete

The steps in the process are as follows.

  • Water comes in contact with the unhydrated calcium.
  • Calcium hydroxide is produced by the help of bacteria, it acts as a catalyst.
  • This calcium hydroxide reacts with CO2 to form limestone and water.
  • This extra water molecule keeps the reaction going.
  • The limestone then hardens and seals the cracks.

Figure below shows the self healing concrete.

Source: sciencedirect.com

self-healing-concrete
bacterial-concrete

What about us study about the preparation of bacterial concrete now?

Preparation of Bacterial Concrete

There are mainly two methods.

  • By Direct application
  • By Encapsulation

Without delay, let’s meet each of them.

Direct Application

  • Bacterial spores and calcium lactate is added directly when mixing of concrete is done.
  • This doesn’t change the most properties of concrete.
  • When water comes in contact with this bacteria
  • They germinate & feeds on calcium lactate and produces limestone.

Encapsulation Method

  • The bacteria and its food (calcium lactate), are placed inside treated clay pellets.
  • Concrete is prepared.
  • Clay pellets break when crack occurs.
  • Bacteria germinate and eat down the calcium lactate & produce limestone.

Time to chill. Sit back and enjoy the advantages and disadvantages of what we have made through the process.

Advantages of bacterial concrete

  • Increase in compressive and flexural strength compared to normal concrete.
  • Self-repairing
  • Reduction in permeability of concrete
  • Resistance towards freeze-thaw attacks.
  • Low maintenance
  • Improves the durability of steel reinforcements.
  • Bacillus bacteria are harmless to human life.

Disadvantages of bacterial concrete

  • High Cost
  • Growth of bacteria is not suitable in some environment.
  • The clay pellets comprise 20% of the volume of the concrete. This may become a shear zone.
  • Design of mix concrete with bacteria  is not available in any IS code

We have reached the shore. Let’s wind this up with the conclusion.

Conclusion

  • Self healing concrete appears to be much more efficient
  • It has more advantages than disadvantages.
  • So far, Bacterial Concrete is the best approach in the field of self healing concrete

Hope the article served its purpose to quench your thirst to know about self healing concrete. Share your thoughts on this in comments.

CONCRETE, Water Cement Ratio

Water to cement ratio – An Important Strength Factor.

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Water to cement ratio is the ratio between the weight of water to the cement weight used in a concrete design mix. The water to cement ratio can significantly impact the workability and strength of the concrete.

Water to cement ratio – Significance

Concrete is manufactured by mixing cement, aggregates, and water in designed proportions. The process of proportioning is done as a volumetric (nominal mix) or by weight (design mix). Cement is the main ingredient of the concrete and acts as a binding material. In combination with water, cement undergoes a chemical reaction leading to the formation of thick and sticky gels responsible for the adhesion between the ingredients. The chemical process is called the heat of hydration. Cement and water are the most important ingredients of concrete and their ratios have a huge impact on the quality and strength of concrete. So for a design mix of required strength, the ratio of water to cement has to be maintained in the right proportions.

workability
CONCRETE POURING

Water-cement ratio – Calculation

Cement requires 23% water for initiating the chemical reaction and another 15% for the formation of gel to fill the voids.38% percent by weight of water to cement ratio is essential for the complete hydration process.

Table 5 - IS 456
Table 5- IS 456

As per IS 10262, the water-cement ratio varies from 0.4 to 0.6 based on exposure conditions. For the calculation of water to cement ratio, we need the cement content of the design mix. The minimum cement content is to be confirmed from IS 10253 for different grades.

For Mix 1:2:4 Moderate exposure condition water requirement would be-

Plain concrete(moderate) Table 5 IS 10252

The Minimum cement content for M15=240 kg/ m3

and Min water cement ratio =water/cement =0.60

=water/240=0.60

Min water requirement for mix=240*.60=120 liters

For Design mix, the W/C Ratio will depend upon the workability, strength requirements.

Fig 1 - IS 10262
Fig 1 IS 10262

Water-cement ratio effect on the workability of concrete

The water cement ratio has a huge impact on the workability and strength of the concrete. The workability of concrete means the ability of concrete to perform a concreting process involving mixing, transporting, placing, compacting with ease, and without any segregation. In other words, the workability of concrete indicates the ability of concrete to work easily. W/C ratio is also an important consideration for the workability of concrete. A high w/c ratio leads to higher workability because inter particle lubrication is increased. Similarly, a lower w/c ratio leads to lower workability because there is a lack of inter particle lubrication. However the w/c ratio is not the only parameter that decides the strength of concrete. The strength of concrete depends on a lot of factors like aggregate quality, cement proportions, etc.

Also Read : QUALITY TESTS TO BE DONE ON CONCRETE- SLUMP TEST

Effect of high w/c ratio

A high w/c ratio leads to higher workability because inter particle lubrication is increased. However, the overall concrete strength is reduced with the increase in the w/c ratio. The addition of more water gives dilute paste that has more pores at the micro-level and segregation of concrete.

HOW TO FIND OUT WATER CEMENT RATIO FOR DESIGN MIX AND NOMINAL MIX

The aggregates and cement particles take the excess water that is present in concrete. This consumption is uncontrollable if a large excess of water is present in the concrete. Hence, separate water channels are created resulting in bleeding on the surface. This creates weak zones in concrete that are susceptible to cracking under service loads. Concrete with a higher w/c ratio is also more susceptible to cracking and shrinkage. Shrinkage leads to micro-cracks, which are zones of weakness.

Once the fresh concrete is placed, excess water is squeezed out of the paste. When there is a large excess of water, that water bleeds out onto the surface. The micro-channels and passages that were created inside the concrete to allow that water to flow become weak zones. These weak zones are responsible for shrinkage cracks and micro-cracks when applied with service loads.

Concrete placing
Concrete placing

Effect of low w/c ratio

Lower water to cement ratio can contribute to high strength and high-quality concrete. For higher-strength concrete, lower ratios are used, along with a plasticizer to increase flowability. But the w/c ratio alone cannot give good concrete. A good mix proportion and quality aggregates and binding materials contribute to a good mix design. A low w/c ratio is hence one of the factors influencing good mix design.

Using low w/c ratio is the usual way to achieve high strength and high-quality concrete, but it does not guarantee that the resulting concrete should possess sufficient strength. Unless the aggregate gradation and proportion are balanced with the correct amount of cement paste, excessive shrinkage, cracking and curling can result. Good concrete results from good mix design and low w/c ratio is just a part of a perfect design mix.