Concrete Sweating: What is it and How to Prevent it

Concrete is used for all kinds of construction works these days. It is the first choice for building supports and beams because of its excellent load-bearing capacity, tensile strength, and longevity. However, nothing is a hundred percent impervious to weather effects, and neither is concrete.

Concrete Sweating, or more technically known as SSS (Sweating Slab Syndrome) occurs when water droplets are accumulated on the surface of a concrete structure. The phenomena occur generally overnight and may look like the concrete structure is sweating, hence the name. This can be a potentially dangerous issue in some cases.

Reasons for Concrete Sweating

There can be two main reasons why a structure is showing the SSS - dew point and subsurface moisture. Let us discuss them below.

Dew Point

When you take a soda out of the fridge and keep it out for a while, the soda bottle or can starts sweating. The same thing happens with concrete as well, especially overnight. A concrete surface is rather cooler than the air in contact with it. When the concrete is cooler than the dew point (the temperature at which the water vapor in the air starts to condense into liquid), the moisture in air sticks to the concrete. This, in turn, starts forming the water droplets. Which, to uninformed eyes, looks like as if the concrete has started sweating.

Subsurface Moisture

A much rarer phenomenon, this occurs when the concrete itself retains some moisture inside it. When the concrete is formed, much water is used to set the cement. All that water gets absorbed to form the crystals that harden the concrete. But the issue is, this process can go on for a long time. Initially, most types of cement can set in a day, but they harden over a long time - over weeks, even months. While the composite is hardening, the hydrostatic pressure inside them rises a lot. This, in turn, pushes the water out of the concrete slabs or structures. In this case, you can say that the concrete really does sweat, in view that the water comes from inside, not outside.

There can be some secondary reasons for SSS. For example, if the concrete slab is adjacent to wetness (wet soil or water body), the capillaries in a porous concrete can suck up moisture from the wet surface to the dry surface. Also, if the concrete mix has salts in it then due to their hygroscopic nature the salts will attract water into the concrete. The material on top of the concrete can also be the culprit in attracting water, like dust or rubber.

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Some useful terms of construction materials

In civil engineering sectors, there are different types of terms associated with various construction materials. Given below, the details about them.

Aggregate: It belongs to all particles of sand, broken stone or gravel etc. which are applied to form Concrete.

Bulking: It means the surge in volume of sand or aggregate resulting from the absorption of Water.

Concrete: Concrete belongs to a building material that comprises of specified quantities of Cement, Sand, Aggregates and water. At the time of mixing, transmission and placing, the concrete sets and solidifies through the method of hydration as the water reacts with cement and binds other elements finally results in producing a material similar to stone.

Curing: The process of retaining the concrete damp once it is arranged in its position to finish the chemical combination of cement and water.

Final Setting: It happens when the concrete has been properly set but has not been solidified yet.

There should be adequate time for the framework to be detached. Final set happens in about 3 to 4 hours with ordinary cement and should not require in excess of 10 hours.

Hardening: It specifies the increase in strength of a mortar or concrete and occurs at the end of the initial set.

Initial Setting: The period passed between the time when water is added initially to time of cement to develop a paste and the time when that paste is stopped to be fluid and plastic to a certain degree under the specified conditions of test.

Lean mix: It belongs to a concrete mix with a low cement content.

Screeding: It means acquiring a level surface as the exact height with the use of a piece of wood or metal containing a straight edge.

Setting: It stands for the chemical action that starts when water is added to cement and causes the plastic nature of cement to go away gradually. There are initial set and final set of cement.

Segregation: It means the detachment of the particles with various sizes in a concrete mix. The bigger aggregates settle at the bottom. Segregation impacts the strength of the concrete significantly.

Striking: It means dismantling and elimination of form work or centering.

Workability: It is the property of the freshly mixed concrete(or mortar) that ascertains the ease or difficulty with which it can be treated in order to make full compaction.

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How to measure concrete mix design

Concrete mix design refers to the method utilized for finding out the exact ratios of cement, sand and aggregates for concrete so that the target strength of concrete can be obtained.

For concrete mix design, different types of laboratory testing and calculations should be accomplished to get exact mix ratios. This method is suitable for Building structures where superior grades of concrete are required like M 25 and over as well as large construction projects where quantity of concrete consumption is extreme.

The main objective of concrete mix design is to arrange the proper ratios of materials so that the application of concrete becomes cost-effective to maintain perfect strength of structural members. In a project, large quantity of concrete & construction work are required and saving in quantity of materials like cement makes the construction project cost-effective.

Concrete Mix design of M – 20, M – 25, M – 30 and superior grade of concrete are measured from following steps:

Concrete Mix Design:

Necessary data for Mix Design of Concrete:

• Concrete Mix Design Date:-

(a) Characteristic compressive strength of concrete necessary at end of 28 days = M 25
(b) Nominal maximum size of aggregate applied = 20 mm
(c) Shape of Coarse Aggregate = Angular
(d) Necessary workability at site = 50-75 mm (slump Value)

(e) Quality control is performed as per IS: 456
(f) Type of exposure condition of concrete (as specified in IS: 456) = Mild
(g) Type of cement applied = PSC as per IS: 456 – 2000
(h) Method of providing Concrete on Site = pumpable concrete

(ii) Material testing data (set in the laboratory):

(a) Specific gravity of cement = 3.15
(b) Specific gravity of FA = 2.64

(c) Specific gravity of CA = 2.84
(d) The surface of aggregates is supposed to be in dry condition.
(e) Fine aggregates are abided by Zone II of IS – 383

To get details on the method of M-25 grade concrete mix design, click on the following link. civiconcepts.com

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Calculation of the quantities cement, sand & water in mortar for any brickwork

In this civil engineering video tutorial, you will learn how to work out the quantities of cement, sand and water in the mortar of any brickwork construction. Also learn how to calculate wastage.

The volume of mortar in a brick wall can be calculated as follow :-

Volume of brick wall = total volume of all bricks + volume of mortar

So, volume of mortar = Volume of brick wall – total volume of all bricks except mortar

The calculation is done in the following ways :

Total volume of bricks with mortar is taken as 10 cubic meter

The thickness of mortar is considered as 10 mm

The proportion of cement and sand = 1:6

The size of each brick = 190 mm x 90 mm x 90 mm

The size of brick with mortar = 200 mm x 100 mm x 100 mm (after adding 10 mm mortar thickness)

Volume of each brick along with mortar = Length x Breadth x Height = 0.2 x 0.1 x 0.1 = 0.002 cum (here, each is divided with 1000)

So, total numbers of bricks in 10 cum = 10 / 0.002 = 5000

The volume of each brick except mortar = 0.19 x 0.09 x 0.09 = 0.001539 cum

The volume required for 5000 numbers of bricks = 0.001539 x 5000 = 7.695 cum

So, volume of mortar = Volume of total brick work with mortar – Volume of total bricks = 10 – 7.695 = 2.305 cum (wet volume)

Because of frog filling, brick joint filling as well as wastage, the quantity is raised by 15%.

So, the volume of wet mortar = 2.305 + (15% of 2.305) = 2.651 cum or cubic meter.

To covert wet mortar to dry mortar add 33% as follow :-

Volume of dry mortar = 2.651 + (33% of 2.651) = 3.526 cum

The quantities of cement, sand and water will be calculated as follow :-

Cement = Volume of dry mortar x ratio of cement / sum of ratio x density of cement = 3.526 x 1/7 x 1440 = 725.348 = 14.51 bags (sum of ratio = 1+6 = 7)

Sand = Volume of dry mortar x ratio of sand / sum of ratio x 35.3147 (1 cum = 35.3147 cft) = 3.526 x 6/7 x 35.3147 = 107 cft or cubic feet

Suppose, the ratio of water and cement = 0.45

So, water / cement = 0.45

Water = 0.45 x cement = 0.45 x 725.348 = 326 litres.

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Roof Slab Design Process

The design of a roof slab is based on the thickness of slab, size of bars, distances of bars, hooks, cranks and laps of bars, minimum cover of concrete etc. The concrete mix for the slab is normally provided with volume and it’s ratio is 1:2:4 ( cement, sand and aggregate).

Because of severe loading, the slab is prone to potential or sagging bonding movement on the bottom surface at the centre of the span and negative or hogging bending moment at top near supports. To counter these moments main reinforcement is arranged at bottom surface along the direction of span to get rid of positive bending moment.

Based on the span, an RCC slab should be designed as either one way reinforced slab or two way reinforced slab. In one way slab, beams provide support to the slabs on the two opposite sides. When the ratio among the length and width of the room over which the slab will be arranged remains in excess of 2, one way slab is suitable. One way slab comprises of main reinforcement distancing among the supports with dispersion of reinforcement at right angles to it. Two way slab stretches in two directions, when the length to breadth ratio remains under 2. Under such situation, main reinforcement is arranged in both directions in the form of a mesh.

The complete thickness of slab normally remains 10 cm though the minimum thickness increases up to 7.5 cm. The minimum area of reinforcement per unit width of a slab shall not remain under 0.15% concerning the gross cross sectional area of concrete. But because of other constraints, it is essential to arrange greater percentage of steel in the slab.

Spacing of bars of main reinforcement should not go beyond 3 times the effective depth of slab and spacing for the distribution bars should not remain in excess of five times the effective depth. Spacing in excess of 20 cm diameter for main bars and not in excess of 40 cm for distribution bars should be arranged.

Bars not more than 6 mm diameter or surpassing 18 mm should not be recommended for slabs. 10 mm or 12 mm plain M.S. bars or 8 mm or 10 mm tor-steel bars are normally utilized. They are arranged at the bottom of the slab with minimum cover of 25 mm of concrete.

Besides, the bars arranged at the bottom along the main direction, bars along the longer extent are also provided on top of the main bars and at right angles to them. These are known as ‘ distribution’ bars or ‘temperature reinforcement’ or ‘ transverse bars’. The purpose of the distribution bars is to control cracks caused by deviation of temperature and shrinkage stresses. They also help in dispersing loads. The area of such reinforcement normally remains 20 % of that of main reinforcement. Plain M.S. bars with 6 mm diameter are normally utilized for this purpose.

In all buildings, slabs cover the full width of the wall over which walls are uplifted. It produces partially fixed conditions for the slab. Under such condition, every alternate bar should be bent up. This arrangement is inexpensive in respect of arranging extra bars at the top. The main bottom bar is cranked at one-fifth of the span at either end and taken up.

Main and distribution bars are attached together through black mild steel wire containing 16 guage by affording a cross and twisting the ends. If wire with lower diameter is applied, double strands of wire should be arranged rather than single strand. To bind 1 tonne of reinforcing bars, around 5 kg. of binding wire is necessary.

As for example, consider a 2.5 M or 8’0” wide verandah where the thickness of slab should be 10 cm. Main bars should be 10 mm plain M.S. or 8 mm tor-steel and arranged at 20 cm centre at bottom of the slab along the short span with a cover of 25 mm. Each alternate bar is cranked up to 0.5 M from the edge of wall and at both ends 6 mm plain M.S. bars should be arranged as distribution bars and are provided at 30 cm centres at the top of the bottom bars. For the cranked bars adjacent to the supports at the top of the slab distribution bars with similar size and spacing are arranged under the top bars.



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Some vital short notes for civil engineering students

Least Cover for several RCC members

Slab = 20 mm
Beam = 25 mm 
Column = 40 mm
Footing = 50 mm

Exact unit weight of steel

The unit weight concerning steel should be 78.5 KN/m3

Proportion of cement, sand and aggregate for different types of concrete mix grades

M10 = 1:3:6
M15 = 1:2:4
M20 = 1:1.5:3
M25 = 1:1:2
M30 = 1:1:1

Exact unit weight of concrete

It is based on the type of aggregates as well as the amount of voids. With adherence to IS : 456-2000, the unit weight of PCC is 24KN/m3 and RCC is 25KN/m3.

Crank Length In Slab

The crank length should be 0.42D. Here, D denotes depth of slab. It is obtained by deducting bottom cover from top cover i.e. Top Cover – Bottom Cover.

Initial & final setting time of standard cement mix

Initial setting time for standard cement mix should remain approx 30 minutes for different types of cements. For masonry cement it should be 90 minutes. Final setting time of standard cement mix should be 10 hours maximum. For masonry cement, it should not go beyond 24 hours.

Compressive Strength Of Brick

For first class brick, the strength should be 105 kg/cm2
For second class brick, the strength should be 70 kg/cm2
For fire brick, the strength should be 125 kg/cm2

The weight of steel bar in reinforcement

The weight of bar should be kg/m = D2/162.2, here dia means diameter of bar in mm.

Steps for design mix concrete according to IS10262

Target denotes strength fm = fck+1.65 σ, σ = standard deviation
Grade of concrete = N/mm2
Standard deviation for M10, M15 = 3.5
Standard deviation for M20, M25 = 4
Standard deviation for M30 to M50 = 5

Strength of concrete

Flexural strength of concrete (modulus of rupture) fcr = .7√fck

Split tensile strength of concrete (fct) fct = 66fcr

Minimum size of column footing

Minimum size of footing for 9” x 9” column should not be under 1.5 x 1.5 feet

Minimum size of footing for 1’ x 1’ RCC column should not be under 2’ x 2’

Standard Height Of Building Floor

The height should be 11 feet but should not be under 10 feet.

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Tips to reduce bleeding in concrete

Concrete is formed by mixing cement, fine aggregates and course aggregates. If process for mixing, curing and placing is inaccurate, several issues may arise. Bleeding in concrete is one of the vital issues. Bleeding means a type of segregation, in which water emerges out of concrete.

Bleeding in concrete happens if the course aggregates is like to settle down and free water in the mix moves up to the surface and produces a paste of cement on the surface called “laitance”. This upward transition of water at the time of penetrating from bottom to top, creates continuous channels. If the applicable water cement ratio is 0.6 or more, the bleeding channels will be continuous.

Penetration in the structure occurs due to these continuous bleeding channels. While upward transition is going on, the water is stored underneath the aggregates and forms water voids and minimizes the bond among the aggregates and paste.

In the same way, the water that is stored underneath the reinforcing bars minimizes the bond among the reinforcement and concrete. When bleeding remains at normal rate, it is not harmful but high rate of bleeding can lead to damage of bond.

Reasons for bleeding: Bleeding in the concrete mix is caused by segregation. For segregation, heavy aggregate particles are settled down and owing to this water moves up to the surface and develops a layer. This upward transition of water also conveys the fine particles of cement with it. The top surface of slabs and pavements will not contain superior wearing quality.

Bleeding mostly operates on the surface of concrete, when water to cement ratio is greater. The rate of bleeding is based on the type of cement utilized, quantity of fine aggregate.

Impacts Of Bleeding:

1. Because of bleeding, the consistency of concrete is lost.
2. Due to bleeding, permeability in concrete occurs.
3. The water stored underneath the reinforcing bars, decreases the bond among the reinforcement and concrete.
4. While bleeding is going on, the storage of water develops a water voids and minimizes the bond among the aggregate and cement paste.
5. Because of bleeding, pumping ability of concrete is decreased.
6. Bleeding raises the water-cement ratio at the top.
7. The storage of water at the top leads to delayed surface finishing.

Some useful steps to minimize bleeding:

1) Put in minimum water content in the concrete mix, chemical admixtures should be applied to minimize the quantity of water to maintain desired workability.
2) Design the concrete mix correctly.
3) Apply fly ash or other additional cementitious materials.
4) Air entraining admixtures is useful for minimizing the bleeding.
5) Put in extra cement in the mix.
6) The amount of fine aggregate should be raised when sand is coarser (fineness modulus of 2.5 to 2.8 is recommended) in mix and lessen aggregate proportionately.

There should be perfect precautions throughout formation of mix and method of mixing to lessen bleeding as it leads to weak structure and an interruption in construction cycle.

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Concrete Mix Design & Concrete Calculator – A useful app for civil engineers

Concrete Calculator is a free calculator that can be used for the following purposes :-

1. Measure cement, sand and aggregate quantity in concrete.
2. Measure the number of premix bags necessary for your project.

3. There is option to settle your own size and rate of premix bags.
4. Measure the volume of concrete necessary for slabs, walls, footings and columns.
5. Work out the weight of materials essential for making the calculated volume of concrete.

The purpose of concrete mix design method is to make proportion of the materials of concrete (cement, sand, and aggregate) inexpensively to attain superior strength and stability on the basis of the materials obtainable at a construction site.

The nominal concrete mix proportions adhering to the code may contain a greater amount of cement with regard to the actual amount necessary when it is designed on the basis of actual design parameters, consequently the cement requirement may be low for the equivalent grade of concrete for a specified site.

The proportions originating from concrete mix design are examined for their strength through compressive strength test on concrete cubes and cylinders.

This concrete calculator is specifically designed for professional Civil Engineers, Concrete Technologists, Civil Engineering Students and DIY(Do It Yourself) enthusiasts similarly.

The user interface is very simple and results are provided specifying the amount of ingredients necessary in kilograms. The design steps are also provided in order that s the user will be able to simply validate the calculations.

To download, click on the following link play.google.com

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Specifications of cement concrete in detail

Cement, fine aggregates, coarse aggregates and water are mainly used to form cement concrete. While writing detailed specifications, explain The specifications of every single component should be narrated briefly at the time of formulating detail specifications.

CEMENT: Cement belongs to the elementary and most vital component of cement concrete. The quality of cement should be fresh while utilizing it for construction work and it should satisfy the standard specifications.

FINE AGGREGATE: Sand is utilized as fine aggregate in cement concrete. Sand particles should comprise of coarse, sharp and angular edges. Size of sand particles should be maintained in such a manner that it can move through 4.75mm sieve. Sand should be clean and does not contain dust and organic matters. Sea sand should not be applied for construction work due to extreme salt contents.

Stone dust is also applied as fine aggregate in cement concrete, but prior to application, check that it adheres to specifications.

COARSE AGGREGATE: Normally, pieces of feverous rocks are applied as coarse aggregates. These stones should be solid, strong, long-lasting and clean. The shapes of aggregates should be cubic or closed to cubic shape. The shape of coarse aggregates should not be laminated and elongated. It should be clean and does not contain from any irrelevant organic matters.

The size of coarse aggregates should satisfy the approved construction work requirements. It should not move through the sieve size of 5mm and coarse aggregates should be graded. Voids should not go beyond 42%.

PROPORTIONING: Based on their fixed ratios, cement, sand and coarse aggregates should be calculated. Prepare a standard measuring box as per the volume of one cement bag. Volume of one cement bag is 1.25 cubic foot.

When, it is required to calculate the ratio of sand, bulking of sand should be taken into consideration. Choose dry sand at the time of making calculation of proportioning. Work out the moisture content in sand and include extra volume of sand. Constantly work out moisture content throughout construction work and include extra volume of sand based on the amount of moisture. It is suggested not to compact coarse aggregates at the time of proportioning.

CONCRETE MIXING: Mixing machine is suitable for large scale construction works whereas hand mixing is chosen since it is cost-effective for smaller concrete works.

CONCRETE CONSISTENCY: Concrete consistency is based on water to cement ratio. Surplus amount of water reduces the strength of concrete and also concrete constituents can be easily detached. The quantity of water given below should be applied against 50kg cement bag.

Concrete Ratio - Amount of Water
1:3:6 - 34 liters
1:2:4 - 30 liters
1:2:3 - 27 liters
1:1:2 - 25 liters

If vibrator is utilized for concrete compaction, then the amount of water should be reduced as per suggestion of engineer in charge.

 
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Common methods for concrete construction

In this civil engineering article, you will get detail information on the most common methods for developing a concrete structure.

Designing the concrete mix: The most vital part in the method is to find out the components that will form the concrete and their ratios. Several types of variables should be taken into consideration at the time of designing which range from cement type, aggregate size and type, amount of water, and mineral and chemical admixtures.

A good mix design can also lead to improper or substandard quality concrete when it is accomplished inappropriately. Based on the type of project, it is decided who will take responsibility for designing the mix. For large, publicly funded projects, a licensed civil engineer should be liable for the final design.

For residential projects like foundations and driveways it is the private contractor, who will prepare the mix design. For do-it-yourself projects it is of course the homeowner who should take responsibility to design his own mix.

To produce an exact mix design, initially, detect the properties that the fresh and solidified concrete should have and then move backwards to opt for the most inexpensive mix design that provides these properties.

Here, focus should be given on the following factors :

Supported Loads: Concrete is formed with different types of strengths, so this is considered as the gateway of the mix design. As the cost of concrete scales rather narrowly with its strength, one does not like to make the concrete stronger than it should have been.

However, if the application provides support to considerably small loads, it is generally not a good idea to indicate weak concrete since weak concrete does not have good stability. For low load applications the quality of the concrete is settled with other factors like resistance to freezing or wear resistance.

Workability: The necessary workability is mainly based on how the concrete will be arranged. Concrete can be poured, pumped, and even sprayed into place, and it will impact the desired workability. Various other factors like the shape of the molds, the rebar spacing, and the accessible equipment at the site for solidifying the fresh concrete once it is placed should also be taken into consideration.

Workability is generally settled with the slump, the tendency for the fresh concrete is to expand under its own weight when it is arranged onto a flat surface.

Environmental conditions: When the concrete is uncovered to severe conditions, then this may perfectly ascertain the required concrete quality in spite of the applied loads. In cold-weather locations the concrete should have the strong resistance capacity against freezing. Besides, it must have the ability to resist the corrosive effects of salt. Underground applications should have the capability to withstand the penetration of moisture and aggressive species from the soil. For almost any type of conditions or mode of attack, the most effective way for defense is to retain the w/c low.

Surface wear: For some applications the physical loads can erode the concrete rather than breaking it. For roads, parking garages, driveways, and industrial floors, the longevity of the structure depends on the hardness and wear resistance of the top layer of concrete.



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Properties and benefits of self compacting concrete

Self-consolidating alias self-compacting concrete is an extremely flowable type of concrete that does not need vibration for setting and compaction.

It is compressed into each corner of a formwork perfectly with its self weight exclusive of any external vibrators. It is a fully engineered concrete with greater fluidity.

Self compacting concrete retains all the strength and characteristics of concrete satisfying desired performance requirements.

In some specific conditions the inclusion of superplasticizers and viscosity modifier with the mix, the bleeding and segregation is significantly reduced. The strength is decreased for the concrete that segregates and as a result honeycombed areas are produced next to the formwork. A properly designed SCC mix does not segregate and it retains extreme deformability and outstanding stability characteristics.

Advantages of SCC:

a. Curtailment in site manpower
b. Problems due to vibrators are reduced
c. Easy to set
d. Rapid construction
e. Superior surface finish
f. The strength is enhanced because of superior compaction and uniformity of concrete.

Characteristics Of Self-Compacting Concrete

Self-compacting concrete has strong resistance capacity against segregation with mineral fillers or fines as well as special admixtures. It should have the flexibility to be flown and filled special forms under its own weight, it should be flowable enough to move across highly reinforced areas, and should have the capability to get rid of aggregate segregation. This type of concrete should fulfill special project requirements regarding placement and flow.

Self-compacting concrete having a similar water cement or cement binder ratio will generally contain a marginally higher strength with regard to conventional vibrated concrete, without proper vibration, a superior interface among the aggregate and hardened paste will be created.

The concrete mix of SCC should be set at a considerably higher velocity as compared to traditional concrete. Self-compacting concrete should be set at heights greater than 5 meters devoid of aggregate segregation. It can also be applied in areas with normal and congested reinforcement, with aggregates as large as 2 inches.

Self-Compacting Concrete Uses - Self-compacting concrete is mostly found in bridges and even on pre-cast sections.

a. Conclusion
b. Self compacting concrete can save time, cost as well as improve strength.
c. SSC can be efficiently transformed into congested reinforced areas like columns, drilled shafts.

To get more details, go through the following link csengineermag.com



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Effects of too much water in concrete

If additional water is present in concrete mix, the concrete can be set easily but the strength & quality of the concrete is reduced significantly. Due to additional water, the damage to concrete structures occurs in the following was :-

1. Strength Reduction 2. Drying shrinkage 3. Loss of abrasive resistance 4. Rise in permeability 5. Dusting and scaling 6. Reduced durability

1. Strength Reduction: Compressive strength is a vital characteristic of solidified concrete and it is decreased due to redundant quantity of water in concrete. The additional water will not engage in the hydration process and contain in concrete even after being solidified.

When the water is vulnerable to atmosphere, it vaporizes and produces voids in the concrete and these voids will decrease the compressive strength of concrete.

2. Drying Shrinkage: If water-cement ratio is raised, the drying shrinkage is also raised and concrete become poor in tensile strength and consequently, cracks build up on the concrete surface.

3. Loss of Abrasive Resistance: Abrasive resistance of concrete is related to its strength directly. With the rise in additional water quantity in concrete, the strength of the concrete is reduced and as a result, the abrasive resistance is also decreased.

4. Permeability: The concrete gets permeable once the additional water in solidified concrete is evaporated. The voids created due to evaporation, will consume water and transform the concrete structure permeable.

5. Dusting: The additional water in concrete mix fetches the fine aggregate to the top, therefore, after getting solidified, a fine loose powder will be arranged on the top of the concrete surface. This method is known as dusting.

6. Scaling: Because of additional water content in concrete, scaling of concrete happens. Under this situation, the top layer of the solidified concrete surface is detached. It is caused by the reaction of water with freeze and thaw effects./p>

7. Reduced durability: The above-explained effects finally result in decreasing the stability of concrete. So, it is necessary to maintain the exact water cement ration for the development of long-lasting concrete. Low water-cement ratio facilitates to produce more long-lasting concrete. By including air entraining admixtures, the longevity is enhanced with low water content.



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Some useful tips to enhance the longevity of concrete piles

If concrete is mixed perfectly and compacted to a solid impervious, the longevity of all the construction materials is significantly increased in a non aggressive atmosphere.

The strength of concrete is influenced by sulphate and sulfuric acid that takes place normally in soils, erosive chemicals existent in industrial waste in fill materials and organic acids and carbon dioxide existent in ground water.

A solid, properly compacted concrete can efficiently safeguard the concrete piles, pile cap and ground beams against the attack by sulphates. The low penetrability of dense concrete resists or significantly controls the ingress of the sulphates into the pore spaces of the concrete.

That's why high strength precast concrete piles are mostly recommended for application. Although these are not acceptable for all the site conditions and bored cast in situ / driven cast in situ piles, so, at the time of application, these should be designed perfectly to attain necessary degree of impenetrability and defiance to aggressive action.

Both high alumina cement and super sulphated cement are not suitable for piling work. As an alternative, reliance is provided on the resistance of solid impervious concrete that is formed with a low water cement ratio. Coating of tar or bitumen on the surface, metal sheeting or glass fibre wrapping filled with bitumen may be chosen.

A layer of heavy gauge polythene sheeting provided on a sand carpet or on blinding concrete is arranged to safeguard pile caps and ground beams on the underside. The vertical sides are safeguarded once the formwork is eliminated with the use of hot bitumen spray coats, bituminous paint, trowelled on mastic asphalt or adhesive plastic sheeting.

Preventative measures against the aggressive action caused by sea water on concrete should only be taken into consideration with regard to precast concrete piles. Cast in situ concrete is utilized only as a centering to steel tubes or cylindrical precast concrete shell pills. The precast concrete piles for marine condition, a minimum ordinary portland cement content of 360 kg/m3 and a maximum water cement ratio of 0.45 by weight should be chosen.



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Types of plasticizers commonly used and their benefits

Plasticizers & super-plasticizers facilitate concrete to enhance it's workability.

Due to it’s strong fluidity, shotcrete is highly beneficial. When the homogeneous mix is applied pneumatically, it simply conforms with even rugged surfaces whereas maintaining the initial resistance to compression to offer good structural support from the beginning.

A family of chemical polymers like plasticizers and super-plasticizers are liable for the material’s fluidity. These are also called water reducers to minimize the total water-to-cement ratio, providing it a more ‘liquid’ consistency devoid of diluting the mix with water.

These additives are normally added all through the mixing process and allow the concrete mix to become yielding until application keeping its consistency unchanged.

Plasticizers and super-plasticizers provide a temporary dispersing effect, that produces a comprehensive hydration of each cement particle, making the fluidity of the mix better.

This mix is amalgamated with volcanic fly ash to form the type of hydraulic concrete, a fully hardy and weatherproof material along with extremely erosive salt water.

Plasticizers: first steps - First generation of plasticizers is known as Lignosulphonates. It belongs to a byproduct originated from wood processing that is frequently applied in recent times to form a workable mix with only basic raw materials.

These additives, known as Mid-Range Water Reducers [MRWR] affix themselves to the surface of a cement particle, that bears both positive and negative charges. Plasticizer polymers, which are negatively charged, neutralize the positive charges on the cement surface, transforming the entire surface completely negative.

It causes a physical effect that allows the negatively charged cement particles to keep away each other, providing a dispersing effect to develop better water infiltration. This mix can now function well devoid of adding more water, and allows for a cutback in the overall amount of water necessary, reducing the water-cement ratio by around 10%.

But these additives can defer the curing process, which may sequentially produce further complicacies. If curing is not done within a certain timeframe, a greater amount of hydrostatic pressure could amass in a formwork column over a long lasting period, causing the formwork to burst.

The second generation: Plasticizers 2.0: This type of plasticizer can significantly lessen the water-to-cement ratio of around 25%. Polysulfonates like naphthalene and melamine offer same type of working mechanism to the first generation of plasticizers, producing an electrical dispersing albeit of superior intensity.

These polymers conform to the cement particles, charging them negatively to form repulsion among related particles, allowing water to flow and hydrate the mix in an efficient manner.

This similar repulsion activity also activates major air occlusion, raising the workability of the mix but at the same time developing pockets of air that reduces its resistance and settle its structural integrity.

This type of polymer may also put other challenges, since it provides a very narrow window of ‘workability’: as soon as the cement is hydrated, it is likely to develop a crust-like byproduct that makes application complicated.

Super-plasticizers: the third generation - Super-plasticizers stands for the additives which bring huge benefits along with a water-to-cement ratio curtailment of around 40%.

Polycarboxylates alias High Range Water Reducers (HRWR) function on the base of sterical in spite of electrostatic repulsion. A major steric effect is steric hindrance for which a chemical reaction can’t occur. In this case it stops cement particles from agglomerating.

Polycarboxylates stands for complex co-polymers which are applied to satisfy several functions, and comprise of a negatively charged ‘backbone’ molecule with polymeric side chains.

Most of these additives can be amalgamated jointly, and blended with other types like air-entraining, accelerating and retarding additives
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Published By

Rajib Dey

www.constructioncost.co

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