What is durability of concrete?

It is the ability to resist weathering action, chemical attack, abrasion, or any process of deterioration.

Durability and sustainability

Durability of concrete structure of paramount importance for the following reason

• Financial investment especially in infrastructure projects is very high. Due to this, building durable structures should be sustained and practiced by the construction industry.
• It is the responsibility of the designer, consultant, engineer to design and build the structure keeping the environmental condition, usage etc in mind. This ensures the long life of the building and adds to durability of concrete.
• Also the selection of right material and construction practices by the contractor are vital practices to be followed.

How do we ensure ‘durable concrete’?

The following is an important list of control factors that need attention in the production of durable concrete.

• Structural design of concrete including the cover to reinforcement.
• The concrete mix design and specification (Water to Cement ratio, maximum cement content, type of cement).
• Workability and cohesiveness.
• Batching/ mixing/ transportation/ placing and compacting.
• The curing process.
• Maintenance and usage during service life.

Reasons for deterioration

Deterioration of concrete can take place basically due to porosity.

Concrete has porosity of several types:

• Capillary pores: Depending on the water cement ratio, the number of capillary pores can increase. The table shows the age at which capillary pores get blocked with different water cement ratios. It clearly shows that if W/C ratio is more than 0.7, the capillary pores will never be blocked and remain for the entire lifetime of the structure. Meaning, the durability of the structure/ concrete is in equation.

Water Cement Ratio	Age at which capillary pores become blocked
0.40	3 days
0.45	7 days
0.50	14 days
0.55	14 days
0.60	3 months
0.70	Over 1 year
Over 0.70	Infinity

Coefficient of Permeability represents the level of permeability of concrete. The table below gives the coefficient of permeability of concrete made using same materials but with different W/C ratios:

Coefficient of Permeability for different W/C   

Sr. No.	W/C	Coefficient of Permeability (Valenta)
1	0.35	1.05 × 10-3
2	0.50	10.30 × 10-3
3	0.65	1000 × 10-3

When the w/c ratio is increased two-fold, the coefficient of permeability increases thousand-fold. This clearly explains how important w/c ratio is to durability and strength of concrete.

• Entrapped air: Proper compaction methods should be followed to ensure removal of all entrapped air within the concrete.
• Honey combs: Proper concrete mix and good concrete practice are the key tools to eliminate honeycombs in concrete.
• Cracks: Good concrete practice, right choice of materials, right design and maintenance – all contribute to minimize cracks in concrete.

Concrete in Aggressive Environments.

Deterioration of concrete takes place in stages. The speed of deterioration and damage to concrete will greatly depend on the following:

Physical Factors

• Continuous or discountinuous cracks.4weathering effects.
• Cyclic loading of impact loading.

Chemical Factors

• Environmental actions or aggressive chemicals, iron and their penetration.

Entry of Chemicals into Concrete and their impact

Trigger	Impact
Chloride	Enters through capillary pores, corrodes reinforcement, causes swelling of concrete
Water	Enters through capillary pores, corrodes reinforcement
Carbon Dioxide	Reacts with concrete and deteriorates it
Sulphates	Reacts with concrete and deteriorates it

For the design purpose of concrete, aggressive environment is defined depending on the severity of presence of chemicals stated above.

Preventive measures for corrosion of reinforcement

• Use minimum water cement ratio.
• Avoid entrapped air and segregation during construction.
• Avoid plastic and drying shrinkage cracking of concrete by taking adequate care in designing concrete mix and by proper construction practices especially curing.
• The concrete mix should have good workability and cohesiveness, so that it can be placed and compacted properly.
• Protective coating on steel.
• Another very important preventing measure is to use blended cement containing fly ash or slag.

Type of Cement	Chloride Diffusion Sq.cm / S × 108
OPC	4.47
Pozzolana Cement (70% OPC & 30% Fly ash)	1.47
Slag Cement (35% OPC & 65% slag)	0.41
Sulphate Resistant Cement	10.00

The table shows clearly that Pozzolana or Fly Ash cement Prevent chloride entering into concrete.

Read More

Water Cement ratio

Role of water in cement

When cement is mixed with water, hydrated cement paste is formed. It consists of 3 parts:

• Hydration products
• Anhydrous cement
• Capillary pores

As cement hydration progresses, the amount of capillary pores decree. However, when the hydration process is completed. Anhydrous cement will disappear and hydrous cement and capillary pores will remain. Therefore, higher the water content added to cement, higher the amount of capillary pores in the cement.

As the capillary pores in cement paste reduces, the strength increases and permeability of the concrete also decreases. Therefore, in order to achieve higher strength, and less permeability, it is very important the amount of water used in cement, i.e. water should be proportionate with cement content.

Water to cement ratio

Water-cement ratio of weight of water to the weight of cement used in a concrete mix. It has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability. The water-cement ratio is independent of the total cement content (and the total water content) of a concrete mix.

The outcome of not maintaining a proper water to cement ratio would be:

• Not achieving required compressive strength of concrete.
• Reduction of concrete durability due to higher permeability.
• Loss of fresh concrete properties.

The relationship between the strength and water cement ratio was established by Duff Abrams in 1918 as a result of extensive testing at the Lewis Institute, University of Illinois. Popularly known as Abram’s water/cement ratio rule, this inverse relation is represented as follows:

f_c = k₁/k₂^w/c

W/C represent the water/cement ratio of the concrete mixture and k1 and k2 are empirical constants.

Understand the role or Water Cement

The water cement ratio helps us determine the strength and durability of the concrete.

Relationship of water/cement ratio with other elements in concreting.

In design a concrete mix, there has to be a reasonable balance between workability, strength, durability and cost consideration. How do we achieve this?

• Workability of concrete can be expressed in terms of consistency and cohesiveness.
• Consistency of the mix is measured in terms or the slump of the mix (i.e. wetness of the mix).
• For given slump, the water requirement generally decreases when.
• The maximum size of a well graded aggregate in increased.
• The content of angular and rough textured particles in aggregates is reduced.
• The amount of entrained air in the concrete mixture is increased,

• Cohesiveness is a measure of compactability which is generally evaluated by trowelability and visual judgment of resistance to segregation.
• If the cohesiveness is poor, the sand/ coarse aggregate proportion or partial replacement of coarse sand with a finer and increase of cement/ aggregate ratio at the given water cement ratio may be done.
• However, past experience and visual judgment supports in deciding the correct water content in the mix.

The W/C ratio and durability

The W/C ratio affects the porosity and thereby the durability of the concrete, the higher the porosity, permeability to many external chemicals and substances is increased. This results in faster deterioration of concrete. 


SUMMARY

• W/C ratio is one of the most important factors in making concrete and plays an important role with cement in concrete. 
• When excess water to cement ratio is used it affects to the concrete strength and durability. 
• As a rule of thumb, the lower the water content, the better the strength and durability. Although water is required for the workability of concrete and for cement hydration reaction, it is should be noted that achieve-ment of workability by adding more water should be avoided as much as possible. Use of correct water to cement ratio will ensure enhanced performance of concrete. 

• In designing a concrete mix, there has to be a reasonable balance between workability, strength, durability and cost consideration. 

Read More

Manufactured Sand

What is M – Sand?

          M Sand or manufactured sand, is the substitute in place of natural sand which is being used in the production of concrete, worldwide. The extraction of natural sand and gravel has become a heavily taxes activity due to the emerging scarcity and tendency to introduce environmental hazards. However, with its various advantageous characteristics such as compliance with local regulations and product consistency, M sand has emerged as a product of high demand.

Why it’s better?

          Upon closer observations, it is found that the shape of M sand, effect of micro – fines on concrete, characteristics such as modulus of elasticity, shrinkage and creep are of vital importance.

However, when compared to quarry dust, crusher operated M sand is processed commodity obtained under well controlled production of aggregates with fine particles size distribution, improved size and surface texture.

          On the other hand, quarry dust is the byproduct formed during the production of crusher aggregates and it is often angular, sub angular and flaky in shape, unstable in surface texture and likely to be quite improve.

               Moreover, concrete produced with M sand bares higher flexural strength, improved abrasion resistance, higher unit weight and low permeability due to the fine fitting of pores in between concrete with micro fines.

Comparison between M Sand, Natural Sand and Quarry Dust

Properties	M Sand	Natural sand	Quarry Dust
Colour	Grey	Tan	Grey
Particle shapes	Rounded, oval, octagonal, cubical	Rounded, oval, cubical	Flaky
Product	Manufactured as per BS, ASTM etc. standards	Naturally occurring granular material composed of finely divided rocks and minerals	Elongated (shapeless) 1)      Fractured dust of jaw crusher 2)      Waste product of stone crushing
Manufactured process	A controlled crushing process with international technology and imported machines	Extracted from naturally occurring sources such as rivers etc.	No controlled manufacturing process since it is the by-product of a stone crusher
Clay content	0 – 0.75%	1 – 10%	0.5 – 6.0%
Flowability	Good	Good	Poor
Shape index	16.06	23	26.7
Gradation	Complies with BS, ASTM etc. standards	Complies with BS, ASTM etc. standards	Does not comply with any standards
Strength	Recommended for use in construction industry due to high compressive and tensile strength. Manufactured to requirement	Recommended for use in concrete and masonry work worldwide	Not recommended for use in industry since it has poor quality
Environmental impact	None	High	None

Read More

Structural Planning

When design a structural elements of a structure, main thing is soil type. Foundation type is depend on soil type. Simply there have many laboratory test has to done.

For example: One is Triaxial test. This test done for clay type soil. For sand direct shear test has to be done.

Borehole test.

For borehole test normally we do Standard Penetration Test (SPT).

For SPT used 150mm diameter tube. This tube is driven to the ground using hammer. The weight of this hammer is 5.9T. The drop height is 1.5m. After 0.5m penetration, take the sample out and then again place the tube in same place and do the test. Each 0.5m penetration take out the sample. Count how many drops needed to penetrate 0.5m depth.

Soil type classification.

Example graph of SPT

How to justify the soil type at site.

First take the soil sample by using hand. Then drop the soil sample freely. If soil particles are stick in palm, it is sand. If not stick soil type is silt or clay. For identify whether its clay or silt, there have another trick. Take that soil sample in to palm and drop water to it. If particles move towards hand, it is a silt. If not clay.

N = No of drops

            N>50 – Rock

            N, 30-50 – Very sense soil

            N, 10-30 – Loose soil

            N<10 – Not appropriate for construction

Soil has to be removed up to N is 30.

If it is a pad foundation N should be greater than 30 below to 1.5B (B is the width of the pad)

 A= Foot print

a= Total foot print

If a > 50%A, Rafter foundation is needed.

Strength of soil.

Safe Bearing Capacity of Different Soil
Non-Cohesive soil		Cohesive Soil
Type of Soil	Values in kN/m2	Type of Soil	Values in kN/m2
Gravel, Sand and Gravel Compact offering high resistance to penetration when excavated by tool	450	Soft shale, hard or stiff clay in deep bed, dry	450
Coarse Sand Compact and Dry	450	Medium clay readily indented with a thumb nail	250
Medium Sand Compact and Dry	250	Moist clay and sand clay mixture which can be indented with strong thumb pressure	150
Fine Sand, Silt (dry lumps) easily pulverized by the fingers	150	Black cotton soil or other shrinkable or expansive clay in dry condition (50% saturation)	150
Fine sand, loose and dry	100	Soft clay indented with moderate thumb pressure	100
		Very soft clay which can be penetrated several inches with the thumb	50

Structural elements planning.

Beam length for each type.

Beam Type	Cantilevers (meters)	Simply Supported (meters)	Fixed/Continuous (meters)
Rectangular	3	6	8
Flanged	5	10	12

Slab length of each type.

Support Condition	Cantilevers (meters)		Simply Supported (meters)		Fixed/Continuous (meters)
Slab Type	One way	Two way	One way	Two way	One way	Two way
Max. Span in meters	1.5	2.0	3.5	4.5	4.5	6.6

Read More