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Published on 6/21/2018

Published by marketing@concretebroker.com.au

What are the most frequently asked questions about concrete?

Lets ask the experts for the answers to everything you need to know about concrete


Cover photo courtesy of : catholicexchange.com

 

Source : quora.com

 

What are the most frequently asked questions about concrete? In other words, what do you need to know about concrete? Or more importantly, what didn't you know?

At concretebroker we provide individuals and the concrete industry with the tools, information and resources required to purchase, pour and place concrete. 

So when it comes to answering the most frequently answered questions, we turn to the experts, at quora.com one of our favourite online portals.....

 

 

1. What is the workability of concrete?

Kinjal Mistry

Kinjal Mistry, B.E. from Dharmsinh Desai University, Nadiad (2016)

Answered Dec 7, 2017

Workability is the property of freshly mixed concrete. The workability of concretesimply means the ability to work with concrete. Fresh concrete is said to be workable if it can be easily transported, placed, compacted, and finished without any segregation.

Concrete must be workable so that it achieves maximum density with a reasonable amount of compaction effort. If concrete is not workable or relatively less workable, it will not be compacted to its desired density resulting in less strength and porosity ultimately.

For more details: Workability of Fresh Concrete

 

2. In M20 concrete, what is the meaning of M?

Jay Hind Chaurasia

Jay Hind Chaurasia, former Student at Bansal Institute of Science and Technology (2013-2017)

Answered Jun 28, 2017

As concrete have various grades which defines its strength and durability and selection of grade of concrete depends upon the exposer conditions and desirable strength.

M20 is one of the grade of concrete having proportion 1:1.5:3 ( where, 1- part cement, 1.5-part sand and 3-part aggregate).

Here, prefix ‘M’ in M20 stands for mix and the numeric ‘20’ stands for characteristic strength of concrete in N/mm^2.

Hope it will help you..

 

 

3. What does the 20 of the M20 concrete stands for?

Abhishek Rawat

Abhishek Rawat, B.tech. Civil Engineering, B. S. Anangpuria Institute of Technology & Management (2017)

Answered Oct 14, 2017

20 is the characteristic compressive strength of concrete.

It means concrete has compressive strength 20MPa.

i.e. it can take maximum of 20N of load in 1 square millimeter of area.

& This strength is achieved after 28 days of proper curing.

Also it is called as characteristic compressive strength not compressive strength because we assume that 95% of samples of concrete have this strength, 5% of samples may fail below this strength.

 

 

4. What is the difference between plain concrete and reinforced concrete?

Vivek Singh

Vivek Singh, Founder - Director at BRICKS TO WOOD

Answered Dec 14, 2017

Concrete is the basis of much of Civilization’s Infrastructure and much of its physical development. It is almost twice as much concrete is used throughout the world than all other building materials combined use.

Difference between PCC and RCC:

PLAIN CEMENT CONCRETE (PCC): - It is also called Cement Concrete (CC) or Binding Concrete.

This is a construction material which consists of Cement, Sand, Aggregate (Coarse and Fine) , Water and Admixtures.

· It is used for providing a rigid, level space and impervious bed to RCC in foundation.

· It is also used under flooring (stone, tile, wooden flooring etc., ).

· It is good at resisting compression.

· It can be used over Brick flat soiling or without Brick flat soiling.

· It is also used as filler like lump concrete which is mixer of Boulder and PCC.

Thickness: PCC is normally between 50 to 75 mm

Concrete Ratio: Normally used proportions are 1:2:4, 1:3:6, 1:4:8

Unit: Cubic Meter (Cum)

REINFORCED CEMENT CONCRETE (RCC) :-

Concrete is material with a mix of Cement, Sand, Aggregate and Water, that is good for Compression not in Tension….

To resolve this critical issue by reinforcing steel, by casting wet concrete around strong steel bars. When the concrete set and hardens around the bars, we get a new composite material, REINFORCED CONCRETE (RCC). So it is a good material which works good either in Compression or Tension.

Concrete resists squeezing due to compressive strength and Steel resists bending and stretching due to tensile strength. This enhance ductility, reduce long term deflections or increase the flexural capacity for beams.

Reinforced concrete acts as a composite material inside another where steel bars provide the reinforcement to the concrete.

Usually TMT Steel bars are used which are firmly anchored inside the concretewithout the risk of slipping. We use steel because it prevents excessive cracking resulting from shrinkage or temperature variation (hot and cold temperature) . Lateral reinforcement as ties and stirrups are used to provide resistance to principal tensile stresses resulting from shear resistance.

Thickness: RCC thickness depends as per design requirements.

Concrete Ratio: It can be nominal mix or design mix, depends on site requirements.

Unit: Cubic Meter (Cum)

 

5. What is green concrete & where it is used?

Krishna Chouhan

Krishna Chouhan, Management Trainee at Bharat Petroleum Corporation Limited (2018-present)

Updated Jan 31

WHAT IS GREEN CONCRETE?

An environmentally friendly concrete, which uses less energy in its production and produces less CO₂ than the normal concrete is called green concrete.

Utilization of fly ash as part of replacement of cement or as mineral admixture in concrete saves on cement and hence the emission of carbon dioxide.

SUITABILITY OF GREEN CONCRETE IN STRUCTURES

Several factors which enhances the suitability of green concrete in structures includes:

  1. Reduce the dead load of the structure and reduce the crane age load; allow handling, lifting flexibility with lighter weight.
  2. Reduction of emission of CO₂ by 30%.
  3. Increased concrete industries use of waste products by 20%.
  4. Good thermal and fire resistance, sound insulation than the traditional concrete.
  5. Improve damping resistance of the building.
  6. No environmental pollution and sustainable development.
  7. It requires less maintenance and repairs.
  8. Compressive strength behaviour of the concrete with water cement ratio is more than that of conventional concrete.
  9. Flexural strength of the green concrete is almost same as conventional concrete.

ADVANTAGES OF GREEN CONCRETE

  1. Optimized mix design mean easier handling, better consistency and easier finishing.
  2. Reduction in shrinkage and creep
  3. Green concrete uses local and recycled materials.
  4. The heat of hydration is technically lower than the traditional concrete.

This results in a lower temperature rise in large concrete pours which is distinct advantage of green concrete.

APPLICATION OF GREEN CONCRETE

Following are the major application of green concrete:-

  • It is used in construction of bridges.
  • It is used in construction of building.
  • It can be used in road construction.

Source : GREEN CONCRETE

 

We Civil Engineers

 

6. What is prestressed concrete?

Bharath Aadithya

Bharath Aadithya, We plan, We design, We build

Answered Aug 11, 2016

Originally Answered: What is prestressed concrete? How is it used?

What is prestressed concrete? How is it used?

Before getting into prestressed concrete, let us go back to the basics first.

We all know that concrete is strong in compression and weak in tension. This is reason for providing reinforcement (in the form of steel bars) to resist tension/tensile force acting on beams/columns/slabs etcetera.

RC structures under service load undergoes deflection causing the bottom of the beam (tensile zone) to elongate, causing cracks. Generally, steel bars are provided to limit the crack widths and resist the tensile force which the concrete lacks.

Here, the rebar acts as ‘passive reinforcement’. Rebars (steel reinforcement) provided at the bottom of the bar, does not carry any forces until the concrete has already deflected enough to crack.

How it is used?

This is where prestressing comes into action. The principle behind prestressed concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service.

Simply, Permanent pre-compression is produced in the areas subjected to tension using high tensile strength steel wires or alloys. Now, a portion of tensile stress is counteracted, thereby reducing the cross-sectional area of steel reinforcement.

As as result, the concrete does not crack because the pre-stressing has reduced the tensile stress in the section below cracking stress. hence concrete is treated as a elastic material.

Now, the concrete is said to have two compressive force:

  1. Internal prestressing force
  2. External forces (Dead load, Live load etc.)

These two forces must counteract each other.

When loads are applied, reinforcing steel takes on more stress and the compressive force in the concrete is reduced greatly, so that it doesn’t become a tensile force. As a result the concrete is less prone to cracks or failures because it is always under compression.

Definition: Prestressed concrete is concrete that has had internal stresses introduced to counteract, to the degree desired, the tensile stresses that will be imposed in service. The stress is usually imposed by tendons of individual hard-drawn wires, cables of hard-drawn wires, or bars of high strength alloy steel. Prestressing may be achieved either by pretensioning or by post-tensioning.

Definition Source: Prestressed, Pretensioned and Post-Tensioned Concrete

Prestressed concrete structures can be classified in a number of ways depending upon the feature of designs and constructions.

1. Pre-tensioning: As the name says, the steel wires or tendons are tensioned first and the concrete is poured later. Tendons are temporarily anchored and tensioned and the prestress is transferred to the concrete after it is hardened.

Then the tendon tries to shrink back to the original length, but resisted by the bond between the concrete thereby inducing compressive force in it.

2. Post-tensioning: Post- tensioning similar to pre-tensioning, where concrete is poured first, allowed to harden and the tendons is tensioned later. Tendons are placed in sheathing at suitable places in the member before casting and later after hardening of concrete.

Uses:

  • Longer spans
  • Unique designs: irregular shapes
  • Shorter construction cycles
  • Cost reduction
  • Shorter floor-to-floor heights
  • Superior structural performance

 

7. What is the creep of concrete?

Mrinmay Santra

Mrinmay Santra, Founder & CEO at Civil Daily

Answered Jan 6, 2017

Creep Of Concrete:

Creep can be defined as the elastic and long-term deformation of concreteunder a continuous load. Generally, a long term pressure changes the shape of concrete structure and the deformation occurs along the direction of the applied load. When the continuous load is removed, the strain is decreased immediately. The amount of the decreased strain is equal to the elastic strain at the given age. This quick recovery is then followed by a continuous decrease in strain, known as creep recovery that is a part of total creep strain suffered by the concrete.

Creep Coefficient:

The ratio of the ultimate creep strain to the elastic strain at the age of loading is termed as creep coefficient.

Photo Source: Theconstructor

Factors Affecting Creep Of Concrete:

The factors that affect creep of concrete are similar to the factors affecting shrinkage, which are as following:

1. Water-cement Ratio:

The rate of creep is increased with increasing water cement ratio.

2. Humidity:

It is influenced by humidity and drying condition of the atmosphere.

3. Age Of Concrete:

The rate of creep rapidly decreases with time. The time taken by a concretestructure to attained creep is 5 years.

4. Aggregate:

Aggregates with moisture movement and low elastic modulus cause a large amount of creep. The rate of creep generally decreases with the increase of the size of aggregates.

5. Admixtures:

Some admixtures (mainly accelerators) are also responsible for causing creep in concrete.

Other Factors:

  • Types of cement.
  • Entrained air.
  • Concrete strength.
  • Improper curing etc.

Source: Creep Of Concrete - Dailycivil .com

 

8. What is the grading of concrete?

Aakarsh Chaturvedi

Aakarsh Chaturvedi, worked at Trainee at Larsen & Toubro

Answered Oct 20, 2017

 

Grading of Concrete.

Grading of concrete can be defined as the designation of concrete based upon its strength and mix characteristics for its use. In this designation concrete is graded as M10, M15, M20, M25 and so on, where M specifies the mix and the number corresponds to the strength of concrete after 28 days in N/mm^2. These grade also represent the ratio of aggregates to be used with respect to the cement. Some of these mix ratios are as follows

M10-(1:3:6)

M15-(1:2:4)

M20-(1:1.5:3)

M25-(1:1.2.2.4)

For gaining concrete strengths above M25 design mix has to be prepared as per the standards.

 

9. What is the difference between reinforced concrete and prestressed concrete?

Ankur Kumar

Ankur Kumar, former Internship at National Institute Of Hydrology (2015)

Answered Feb 4, 2017

REINFORCED CONCRETE:

  • Reinforced Concrete (RC) is a composite material in which concrete'srelatively low tensile strength and ductility are counteracted by the introduction of reinforcement having higher tensile strength and ductility.
  • The reinforcement is usually, though not necessarily, steel reinforcing bars (rebar) and is usually embedded passively in the concrete before the concrete sets. Modern reinforced concrete can contain varied reinforcing materials made of steel, polymers or alternate composite material in conjunction with rebar or not.
  • Reinforcing schemes are generally designed to resist tensile stresses in particular regions of the concrete that might cause unacceptable cracking or structural failure.

PRESTRESSED CONCRETE :

  • P.S.C. is a Structural concrete in which internal stresses have been introduced to reduce potential tensile stresses in the concrete resulting from loads.
  • This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete volume, and is done to improve the performance of the concrete in service.
  • Tendons may consist of single wire, multi-wire strands or threaded bars, and are most commonly made from high-tensile steels, carbon fibre or a ramid fibre.

ADVANTAGES OF P.S.C OVER R.C.C

  1. P.S.C. is more durable since there are no tensile cracks, whereas in R.C.C tensile cracks are unavoidable, hence there is greater danger of adverse environmental effects.
  2. As high strength concrete is used and also since dead load moments can be neutralized P.S.C., dead weight of P.S.C. member is much less compared to what is required for R.C.C. member. The dead weight of P.S.C member is about one-third of equivalent R.C. members.
  3. The material cost in P.S.C. is much less than that in equivalent R.C. members.
  4. The deformation of P.S.C. members is much less compared to that for an equivalent R.C. member. In case of beams deflection of P.S.C. beams is about one - Fourth of that of equivalent R.C. beams.
  5. P.S.C has high resilience that is., a considerable capacity for recovering completely from the effect of occasional overloadings, without suffering any serious harm. In case of P.S.C. cracks in such situation which develop temporarily will close up completely.
  6. The fatigue strength of P.S.C. is very good.

DISADVANTAGES OF P.S.C. OVER R.C.C

  1. It requires skilled labour and good quality control.
  2. It needs special technique to apply prestressing forces and Anchorage the wires.

 

 

10. Why are houses in the USA made from wood and brick, whereas houses in other developed countries are made from concrete or tin? Which is more durable and less costly? Can plumbing be hidden inside the walls in a concretehouse, as in a wood house?

Cédric Ballet

Cédric Ballet, Master Finance & Economics, Pantheon-Sorbonne University (2005)

Answered May 31, 2017

Pretty much everywhere in the world houses and buildings are built with CMU blocks, because they are inexpensive and easy to work with.

My understanding is that the US builders keep building with wood because it is the most inexpensive local construction material and US consumers are obviously not worried about durability. They also probably lack the manpower and knowledge basis for this type of work so they stick with what they know better.

In Europe, people usually expect their house to last several generations, whereas the oldest homes in the US must be 120 years old; maybe there is something about that.

Houses in the US are tore down and rebuilt at a much higher pace than in old Europe.

In the long run, concrete homes are very likely more economical; they last much longer, they do not have mold or termite issues, concrete blocks do not crack or twist like wood does. They hold off much better against elements.

Still, they offer great flexibility; you can easily delete and move the walls as long as they are not bearing the roof.

On top of that, CMU walls have higher fire and weather insulation properties than a typical wood frame wall. A hollow CMU block has a fire rating of 2 hours upgraded to 4 hours if the cells are grouted or filled with other filling materials(which is the case most of the time)

A typical wood frame wall made of 2x6 with 2x 1/2 drywall board and fiberglass wool insulation would likely fail in 30 minutes, unless built with modern 5/8 commercial grade X type gypsum boards; but still the wall would only hold 1 hour against 4 hours with a CMU wall.

The vast majority of houses in the mediterranean do not have air conditionning even though the temperatures often reach over 100 degrees. Walls are typically made of stucco over hollow or grouted (filled) CMU blocks and clay tiles roofs.

People usually keep the windows and doors closed during the hottest hours of the day and open at night so the house remain cold without using ventilation or AC, which proves the effectiveness of such constructions against extreme temperatures

 

 

11. What is the meaning of M in M15, M20 etc, grade of concrete?

Nikhil Saurabh

Nikhil Saurabh, B. Tech Civil Engineering, GLA University, Mathura

Answered Jul 2, 2017 · Upvoted by Shanil Anshuman, M.S. Civil Engineering, Missouri University of Science & Technology (2010)

IS 456–2000 has designated concrete mix into a number of grades like M10, M15, M20, M25 etc.

Here M refer to Mix Ratio

For M10 it is (1:3:6)

For M15 (1:2:4)

For M20 (1:1.5:3)

For M25 (1:1:2)

Note- M25 concrete mix will attain a characteristic compressive strength of 25 N/sq.mm after 28 days which when tested on standard cube of 150x150x150 mm

 

 

12. What is lean concrete?

Krishna Chouhan

Krishna Chouhan, Management Trainee at Bharat Petroleum Corporation Limited (2018-present)

Answered Apr 2

Thank you Koustav Mukherjee A2A.

Lean Concrete is ‘not Rich’, in other words the ratio of cement to water and of course to aggregate would be low.

the higher the Aggregate/cement ratio, the leaner the concrete. In lean concrete, less quantity of paste is available for providing lubrication, per unit surface of aggregate and hence the mobility of aggregate is restricted .

Example:-

M5 GRADE OF CONCRETE → 1:5;10 (CEMENT:SAND:AGGREGATES)

M7.5 GRADE OF CONCRETE→1:4:8

M10 GRADE OF CONCRETE→1:3:6

Advantages:-

  1. Main function of the lean concrete is to provide the uniform surface to the foundation concrete and to prevent the direct contact of foundation concrete from the soil.
  2. Lean concrete is used under the foundations.
  3. Lean concrete is used to provide a level surface , where main foundation (raft, isolated or any other type) can be placed.
  4. Another purpose is protection of main foundation from soil below, as moisture or other chemicals in soil like sulphates may attack concreteand can weaken it.

If you need a quality foundation and the entire structure as a whole, then use lean concrete.

Source:- What is lean concrete & where it is used

Hope it helps

We Civil Engineers

 

13. What is the reason behind the curing of concrete?

Bharath Aadithya

Bharath Aadithya, We plan, We design, We build

Answered Feb 28, 2016

Originally Answered: Why do we cure concrete?

Curing is necessary because :

  • Concrete strength depends on the growth of crystals within the matrix of the concrete. These crystals grow from a reaction between Portland cement and water—a reaction known as hydration. If there isn't enough water, the crystals can't grow and the concrete doesn't develop the strength it should. If there is enough water, the crystals grow out like tiny rock-hard fingers wrapping around the sand and gravel in the mix and intertwining with one another
  • When most people think of curing, they think only of maintaining moisture on the surface of the concrete. But curing is more than that—it is giving the concrete what it needs to gain strength properly.
  • The other important aspect of curing is temperature—the concrete can't be too cold or too hot. As fresh concrete gets cooler, the hydration reaction slows down. The temperature of the concrete is what's important here, not necessarily the air temperature. Below about 50 F, hydration slows down a lot; below about 40 F, it virtually stops.
  • Hot concrete has the opposite problem: the reaction goes too fast, and since the reaction is exothermic (produces heat), it can quickly cause temperature differentials within the concrete that can lead to cracking. And cement that reacts too quickly doesn't have time for the crystals to grow properly so it doesn't develop as much strength as it should.

WHEN IS THE RIGHT TIME TO CURE CONCRETE?

So the objective is to keep our young and impressionable concrete damp and at the right temperature (ideally between 50 and 85 F). The most frequently overlooked curing aspect is keeping exposed concrete surfaces moist while they are hydrating.

There are three phases of curing and the length of time each lasts depends on the concrete and the environmental conditions :

  1. When concrete is first placed for a slab, bleed water rises as the concretemixture settles. During this period (initial set), if the bleed water is evaporating from the surface faster than it is rising out of the concretethen you need to do some initial curing or else you are likely to end up with plastic shrinkage cracks. 
  2. Between initial set and final set, intermediate curing would be needed if the finishing (or stamping) is complete prior to final set.
  3. After final set, you need to do final curing.

The problem is that if the exposed surfaces dry out then the concrete can't hydrate and our young concrete ends up with very sensitive skin—easily scratched and sometimes actually dusty.

TL; DR

  • We cure concrete to maximize its strength. i.e. increase structural integrity and reduce the likelihood of cracks.
  • Put simply, concrete is a mixture of sand, cement and aggregate. It is the chemical reaction between water and the cement that causes the concrete to set.
  • Water is a critical factor in the strength of concrete.
  • Think of cement particles like pool balls, round and smooth. Once water is added they start to grow out capillary type arms that intertwine with the arms of surrounding cement particles.
  • This is the process that makes concrete go stiff as the particles join up.
  • Too much water and the particles are too far apart to bond strongly.
  • Too little water and the capillary arms do not grow long enough to bond strongly.
  • The trick is to have the correct amount of water in the mix. The concretecompanies are on to this.
  • The longer the water remains in the concrete, the longer this chemical reaction will occur, meaning that the concrete will continue to grow in strength.

 

14. What is the difference between normal concrete and green concrete?

Dana M Dietz

Dana M Dietz, Over 26+ years of roadway design and construction in public and private sector

Answered Jun 18, 2017

I am used to the term “green” concrete to mean concrete that has set, but not hardened. People and vehicles can get on it, but it still has to cure. It tends to have a greenish color to it.

There are things that have to be done when the concrete is green.

For example, while the concrete is green, we saw cut control joints into concretepavement.

This allows us to somewhat control the cracking of the concrete. This needs to be done while the concrete is green, or we will miss the opportunity to control the cracking. Plus, it is more difficult to saw cut when it is cured out a little more.

There is another term for green concrete. It is the production of concrete using as many recycled materials as possible, and leaving the smallest carbon footprint as possible.

Under normal circumstances, the process of making concrete does produce CO2 emissions. It is produced when crushing the coarse and fine aggregates. It is produced when making cement. It is produced when batching the concrete, and when placing the concrete.

There are a number of alternative environmental requirements with which green concrete structures must comply:

  • CO2 emissions shall be reduced by at least 30 %.
  • At least 20 % of the concrete shall be residual products used as aggregate.
  • Use of concrete industries own residual products.
  • Use of new types of residual products, previously land filled or disposed of in other ways.
  • CO2-neutral, waste-derived fuels shall substitute fossil fuels in the cement production by at least 10 %. (Source: GREEN CONCRETE)

This is a new technology as far as I know. There are still limitations to making and using Green Concrete. One of them is that they don’t seem to have the design life that normal concrete has.

 

15. What is soundness of concrete?

Creative Concrete

Creative Concrete, works at Brisbane, Queensland, Australia

Answered Jan 5, 2017

Soundness of concrete is very important property. Soundness refers to the ability of cement paste to retain its volume after it has get hardened.

The unsoundness in cement is due to the presence of excess of lime than that could be combined with acidic oxide at the kiln. This is also due to inadequate burning or insufficiency in fineness of grinding or thorough mixing of raw materials. It is also likely that too high a proportion of magnesium content or calcium sulphate content may cause unsoundness in concrete. For this reason the magnesia content allowed in cement is limited to 6 per cent.

 

16. What is the difference between mass concrete and reinforced concrete?

Sohail

Sohail, Senior Engineer at Nespak

Answered Jul 21, 2017

Sorry to say but most of the answers provided here are technically incorrect. Mass concrete is defined by ACI 116R as:

“any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimize cracking.”

and reinforced concrete is defined by ACI 116R as:

“structural concrete reinforced with no less than the minimum amount of prestressing tendons or nonprestressed reinforcement as specified by ACI 318.”

Concrete is basically a good insulator. It doesn’t allow heat to pass through it easily. Hydration of cement is exothermic reaction. When the dimensions of concrete are large enough, the heat generated during hydration of cement increases the temperature of core of concrete, whereas the surface temperature of concrete being in contact with the atmosphere cools down due to loss of heat to the environment (depending upon the environmental conditions).

This difference in temperature between core of concrete and its surface causes tensile stresses on surface of concrete (as core expands due to rise in temperature) which when exceed tensile strain capacity of concrete causes it to crack. Such cracks are normally called thermal cracks. Thermal cracking mainly depends on cement content (per cubic meter of concrete), volume of pour (dimensions of concrete to be casted monolithically), placement temperature of concrete and environmental conditions. 
Mass concrete is normally placed in dams, bridge foundations, bridge piers, mat foundations, pile caps, thick walls, and tunnel linings. Mass concrete may or may not be reinforced depending upon the intended purpose of the structure. Most of the mass concrete is generally placed in gravity dams which doesn’t require reinforcements but even in these gravity dams, piers for spillway gates and spillway chutes are also heavily reinforced mass concrete.

Volumetric shrinkage is another phenomenon which can cause cracking in mass concrete due to thermal changes, autogenous or drying shrinkage.

The measures which are taken to reduce the cracks in mass concrete are by limiting the cement content (either by replacing cement with slag/fly ash or increasing the size of coarse aggregates), limiting placement temperature, cooling system, limiting size of pour and proper curing arrangements.

References:

  1. ACI 116R: http://dl.mycivil.ir/dozanani/AC...
  2. Evaluating Cracking in Concrete: Procedures
     

 

17. What is the difference between lean concrete and PCC concrete?

Santosh Kulkarni

Santosh Kulkarni, Enjoying 41st year in Construction

Good question.

These two terms, Lean concrete and PCC can be used as synonyms for each other.

There is a third term which also means the same. ( Cement Concrete or CC)

Technically, Lean concrete and Rich concrete are the terms used to denote concrete prepared using lesser cement and more cement respectively.

When we say PCC, it means a concrete which does not have any reinforcement in it.

As per our standard IS 456:2000,concrete of grade M15 and below needs to be used for the Plain Cement Concrete.

In terms of nominal mix, the proportions like

  1. 1:5:10
  2. 1:4:8
  3. 1:3:6

Are all examples of the lean concrete and are also used for the PCC.

For Reinforced cement concrete, grade of M20 and above must be used.

PCC is used for :

  1. Leveling course below footings
  2. Bed concrete below the plinth
  3. Coping on top of brick work
  4. Plinth protection

Hope this gives you the required clarity.

Thanks for the A2A

Happy concreting

 

18. What is the density of concrete?

Ramachandra Vinayak Sakhadeo

Ramachandra Vinayak Sakhadeo, Occasional Consultant at L&T Ramboll Consulting Engineers Limited

Answered Oct 21, 2017

Plain concrete is 2200 kg. / cubic meter. R.C.C. is 2400 kg. / cubic meter. These densities are for estimate purpose. Actual density could vary slightly depending on the density of coarse aggregates.

 

 

19. What is the diference between ready mix concrete and normal concrete?

Krishna Chouhan

Krishna Chouhan, Management Trainee at Bharat Petroleum Corporation Limited (2018-present)

 

Normal concrete:-

Concrete is a composite material composed of coarse aggregate, fine aggregate bonded together with a fluid cement that hardens over time.

It can be mixed by machines such as Non tilting mixtures, Tilting Mixtures, Single drum, Double drum, Pan mixers.

Machine mixing is obtained for large quantities, Supply of concrete is continuous and it requires low water/cement ratio.

Mixing of normal concrete in machine at site[/caption]

It also can mixed with hands if the quantity of concrete is small, Supply is intermittent & it requires high water/cement ratio.

Hand mixing of concrete[/caption]

Ready mixed concrete:-

Instead of batched and mixed on the site, concrete is delivered ready for placing from central mixing plant is called ready mixed concrete.

In other words "Ready mix concrete is the concrete which is designed ,mixed in the factory (Batching plants) and transported by agitating trucks to the site of work . Its is one type of controlled concrete having high work efficiency. By this process also we achieve a good quality assurance because proper batching ,mixing and designing is being done".

Manufacturing process of ready mixed concrete[/caption]

Merits:-

  1. Suitable for sites located in/near congested areas.
  2. Very good quality control, as it is manufactured in controlled conditions.
  3. Suitable when small quantities at different time is required.
  4. Suitable for very large projects.
  5. For adverse site/climate conditions.

Demerits:-

  • Transportation cost is added.
  • Chances of segregation, if proper care is not taken.

Source:- Difference between Ready mixed concrete and Normal concrete

Also learn,

Different Grades of OPC cement

What is CLEAR COVER? MINIMUM CLEAR COVER FOR SLAB, BEAM, COLUMN AND FOUNDATION

Hope it helps

 

20. What is the difference between cement and concrete?

Prathyaksh Shetty

Prathyaksh Shetty, MTech from IIT Madras

Updated Dec 19, 2015

Similar to the difference between:

  • Coffee Powder and Coffee
  • Flour and Cake
  • Screenplay and Movie
  • Leader and Team
  • Armored Suit and Iron Man
  • Sun and Solar System
  • Love and Marriage
  • Air and Lays Chips

Non-smartass answer: 

Cement is a fine powder manufactured using mainly limestone which when added to a mixture of stone aggregates and sand along with water, binds the constituents to produce a cohesive, strong and durable product called concrete.

 

21. How can we repair cracks in concrete roofs?

Rakshita Nagayach

Rakshita Nagayach, Civil Engineer at Municipal Corporation Indore, SGSITS Batch 2015

Answered Jun 11, 2015 · Upvoted by Rakesh Ravula, B.Tech Civil Engineering, Indian Institute of Technology, Bhubaneswar (2016)

Cracks in concrete must be repaired as soon as possible. Now that monsoon is approaching, it's better to get the job done in no time. I believe you can go for any of these methods according to your convenience: 
 

  • Waterproofing: It will solve the water-dripping problem immediately, but it is not a permanent solution. If the rains have already begun, get the roof waterproofed and later get the proper work done.
  • Epoxy Grouting: I personally suggest you to go for this. It takes less time and is a proper permanent and suitable method. The method includes: 
  1. Cleaning of the crack surface and removing any type of oily dirt or contaminants.
  2. Widen the crack just a little to allow the grouting to occur at a faster pace.
  3. Fill the epoxy grouting (from Epoxy Injection) and seal the surfaces immediately to prevent epoxy gel from falling off. If Epoxy grouting cannot be done, you can do the same using Plaster of Paris with water, ONLY if the crack is shallow and not widening. When the gel dries up properly, paint the roof to give it a proper finish and to protect it from dust and water.
  • Stitching Method: Holes are drilled at both the ends of the crack and 'stapled' using short-legged u-shaped reinforcement unit. The holes are then filled by epoxy gel or non-shrink grout. 

Hope this helps.
Thanks for the A2A Manish Gupta.

 

22. What is the meaning of M15 M20 M10 concrete?

Amit Ratan

Amit Ratan, Pursuing B.Tech in Civil Engineering

Updated Jul 28, 2016

M10 M15 M20 etc. are all grades of concrete in the Indian Standard issued by Bureau of Indian Standards.

The 'M' denotes 'Mix' followed by a number representing the compressive strength of that mix in N/mm^2.

A mix is a specific ratio in which cement, fine(sand) and coarse aggregate(gravel) are mixed. The mix is assigned a minimum compressive strength by casting a cube of side 150mm and then after 28 days of curing testing it with a compression testing machine.

e.g. The ratio of cement and aggregates in an M20 mix is 1 : 1.5 : 3 which should have a compressive strength of 20 N/mm^2 or more.

 

23. Why do we put steel in concrete for construction?

Varun Chowdary

Varun Chowdary, Structural design engineer

Answered Jan 22, 2016

Steel, which has high tensile strength, is used with concrete in order to counteract the concrete's low tensile strength and ductility. The main purpose of inclusion of steel is resist tensile stress in particular regions of the concrete that may cause structural failure or cracking.

Question may arise why only steel has to be used when there are many materials, which have more tensile strength than steel, available. Well that's because of the compatibility and understanding between the steel and concrete. In what way? Mainly because of the following reasons:

1) The coefficient of thermal expansion of concrete is almost similar to that of steel, eliminating large internal stresses due to differences in thermal expansion or contraction.

2) Cement paste in concrete conforms to the surface of steel after it gets hardened, permitting any stress to be transmitted efficiently between the two materials. Moreover, steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel.

3) The alkali chemical reaction in the hardened cement paste forms a passivating layer on the steel surface to make it much resistant to corrosion.

However, to give optimum results, reinforcement needs to have the following properties at least:

• Bond with concrete

• High relative strength 

• Ductility 

• Resistance against corrosion.

Hope this helps. For further assistance regarding this topic you can contact me. You can also receive all the information on the subject and can also ask questions from experts by visiting Super Brand in Rebar Category (TATA Tiscon)

To read more about RCC construction : RCC Construction

 

24. What is the difference between plum concrete and plain concrete?

David Wilmshurst

David Wilmshurst, Technical Manager - Materials Consultancy and Lab (1985-present)

Answered Jan 6

Plums are just big bits of aggregate. They may he hand placed into mass concrete, or machine placed “dumped”, but in either case, usually after the concrete is placed.

So. There is little difference. It is just that the plums displace concrete, and in some cases make the concrete cheaper. And improve drying shrinkage and raise elastic modulus. If using good rock

 

 

 

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