Smart Concrete – The newest upcoming technology to detects cracks in concrete earlier

The concrete is extensively used as a composite material for several types of structures but the concrete lacks the strength to resist tension and as a result it can be cracked easily. Therefore, the concrete should be examined thoroughly for cracks and proper and timely repairs should be undertaken so that the quality of the concrete is not deteriorated.

In this way, the safety and stability of different structures are maintained. Another process for determining cracks is to attach sensors to the structure but the installation charge is costly.

Smart concrete technology is the newest concept for examining the health of reinforced concrete structures. The type of concrete is reinforced with carbon fiber comprising of 0.2% to 0.5% of the volume. It can determine the pressure on the concrete structure well in advance. Smart concrete technology has witnessed broad laboratory testing, but has not launched yet.

It becomes functional with the addition of a small amount of short carbon fiber to the concrete with a traditional concrete mixer to change the electrical resistance of the concrete with regards to tension or pressure.

Accordingly, contact among the fiber and the cement matrix is impacted while the concrete is deformed or depressed, thus affecting the electrical resistivity of the concrete volume. Tension is then exposed by calculating the level of electrical resistance.

Smart concrete has the ability to recognize very small structural weaknesses before it gets significant and therefore becomes applicable in checking the internal conditions of the structure, especially after an earthquake.

Besides, identifying small cracks, smart concrete also allows stopping the development of cracks in traditional concrete and reinforce it to make it stronger. Besides, it takes a lot of strength to bend smart concrete, and consume more energy before breaking.

Smart concrete can also be applied in the construction of highways since they can easily find out the weight of the vehicle. Roads constructed with smart concrete can detect the position, weight, and speed of the vehicle.



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Some useful methods for enhancing ground condition

The civil engineers should be well aware with the ground improvement methods since the stability of the structure mainly depends on the good quality of soil.

The following methods are undertaken for the enhancement of the ground :-

Replacement of soil, Precompression of soil, Densification of soil, Reinforcement of Techniques, Grouting techniques, Stabilization using admixture, Geo textiles, Electro osmosis method.

Replacement of soil: Under this method, the unacceptable soil is uncovered and they are replaced with compacted fill. It is a standard method to rectify the issues with compressible soils. Evacuation and substitution is normally judicious just over the ground water table.

Pre-compression of soil: It is particularly suitable in soft clay and sandy or mud soils since the static weight of the fill motivates them to consolidate, thus making both settlement and strength properties superior.

Pre-compression offers the following benefits:

1. The method provides a long track record of success.
2. Any grading contractor will be able to execute the work.
3. The cost is moderately low, provided that soil for preloading is instantly available.

Densification of soil: The stability and firmness of the soil remains greater when the particles are stuffed in a dense configuration. It is very suitable for making the characteristics of the soil better.

It can be done in the following manners : VIBRO TECHNIQUES and DYNAMIC COMPACTION

Reinforcement of Techniques: The shear strength under the structures is raised with the utilization of stone columns. The method comprises of developing vertical holes in ground which are stuffed with compacted crushed stone, gravel and sand or a mixture.

Grouting Methods

PERMEATION GROUTING - Stopping leaks in below-grade structures, Stopping leaks in below-grade utilities, Excavating support of non-corrosive soils

JET GROUTING: Jet grouting applies a special pipe that is provided with horizontal jets to inject grout into the soil at high pressure.

Stabilization with admixture: SOIL STABILIZATION: It is the method of engineering characteristics of soil with the inclusion of some binding agents to join the soil particles. Soil stabilization is categorized as follow :-

1. Mechanical stabilization
2. Chemical stabilization

Geo textiles: Geotextiles stand for permeable fabrics manufactured products and there are other materials like polyester, poly-ethylene, polypropylene and poly vinyl chloride, nylon, fiber glass and different mixtures of these.

Electro osmosis method: The electro osmosis method is utilized to improve the shear strength and decrease the compressibility of soft clay and sandy soils underneath foundation, electro osmosis with or without electrochemical hardening is taken into consideration only for special occasions where the substitute of piling cannot be embraced.



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How impact value of coarse aggregate is determined

Impact value of coarse aggregate plays an important role to find out the moisture content of coarse aggregate.

For this purpose, the following equipments are required :-

1) Impact testing machine (metal base)
2) A cylindrical steel cup of internal dimension: 102mm & depth 50mm.
3) IS sieves of sizes 12.5, 10 & 2.36mm.
4) A straight metal tamping rod having circular cross section 16mm in dia. And 600mm long rounded at one end.
5) Weight balance of 01 gm.
6) A relevant oven thermostatically controlled to retain a temperature of 100°C to 110°C Samples:

1. i) Coarse Aggregate

The following methods should be applied :-

1. The impact machine stands devoid of wedging or packing upon the level plate. 
2. The cup should be attached rigidly in exact location on the base of the machine and entire test sample should arranged in it depending on fifteen blows of the metal hammer of weight 14 kg downfallen from a height of 38 cm.
3. The quantity of fine material that goes through 2.36mm resultantly from pounding will point out the strength of the sample of aggregate. The proportion of the weight of the fines (finer than 2.36mm size) developed to be taken as weight of the total sample selected is stated as a percentage. It is called as aggregate impact value as per IS 283-1970

Result:

Aggregate impact value does not surpass 45 percent by weight for aggregate utilized for concrete apart from wearing surface and 30 percent by weight for concrete for wearing surface like run way, roads and pavements.

To get more detail, go through the following link wrengineers.in



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Benefits of compressive strength of concrete & factors impacting compressive strength

Concrete becomes leading usable material as soon as water is added to it. The strength of concrete is mainly dependent on aggregates, while cement and sand are responsible for the characteristic of binding and workability together with flowability to concrete.

Compressive strength means the capability of the material or structure to provide resistance against compression. The Compressive strength of a material is based on the capability of the material to withstand failure that appears as cracks and fissure.

With compressive test, the push force enforced on the both faces of concrete specimen and the maximum compression borne by concrete devoid of failure is recorded.

The purpose of concrete testing is to deal with the Compressive strength of concrete as it facilitates to measure the concrete strength to protect gainst Compressive stresses among structures while other stresses like axial stresses and tensile stresses are developed with reinforcement and other means.

In technical term, Compressive Strength of concrete is explained as the Characteristic strength of 150mm size concrete cubes @28 days.

Compressive strength of Concrete and its advantages:-

Concrete is formed by mixing sand, cement, and aggregate. The strength of the concrete is based on several factors which range from individual compressive strength of its materials (Cement, Sand, aggregate), quality of materials applied, air entrainment mix proportions, water-cement ratio, curing methods and temperature effects.

With Compressive strength, the entire strength of the concrete as well as information on the above mentioned factors can be obtained. By initiating this test, it is possible to analyze the concrete strength psi and quality of concrete formed.

The compressive strength of concrete is influenced by the factors given below:-

Coarse aggregate: Concrete becomes uniform by blending aggregates, cement, sand, water and different other admixtures. In spite of perfect mixing, some micro cracks may occur because of variations in thermal and mechanical properties of coarse aggregates and cement matrix, which results in collapsing of concrete structures.

In concrete, the size of aggregate is a key factor for compressive strength. When the size of aggregate is raised, then the compressive strength will also be increased.

But later on, it is discovered that bigger size of aggregates can raise strength in preliminary stage but the strength is decreased greatly. It happens because of the abridged surface area for bond strength among cement matrix and aggregates and lower transition zone.

Air-entrainment: Air entrainment in concrete is a useful feature to avoid damages because of freezing and thawing. Later on, multidimensional benefits of air entrainment lead to improve the workability of concrete at lower water/cement ratio.

When the required workability is attained at lower water content, the concrete contains superior compressive strength which consecutively results in creating light concrete with superior compressive strength.

Water/Cement ratio: Excessive water is injurious to the strength of concrete. Cement is considered as the major binding material in concrete that requires water for hydration process, but the water quantity is constrained to about (0.20 to 0.25) % of cement content. The excess water is useful in case of workability and finishing of concrete.

When the water in the concrete matrix gets dried, it departs large interstitial spaces between aggregate and cement grains. This interstitial space in turn creates primary cracks throughout compressive strength testing of concrete.

Go through the following article, to get more details civilread.com



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How the strength of concrete is influenced by the temperature

The concrete strength performance in initial phase is significantly affected by the temperature control. In cold weather, the temperatures of concrete should be managed with powerblanket™ concrete curing blankets so that proper compression strengths are attained as soon as possible. Once, the concrete attains the perfect strength, the construction process is started.

The following four phases of works are required for building up RCC column :-
Column layout work
Column reinforcement work
Column formwork
Concrete Pouring into column

Given below, the detail segregation of strength gains after three days at different temperatures:

Temperature – Compressive Strength (psi)

70°F – 2,700
60°F – 2,150
50°F – 1,600
40°F – 1,200

30°F – 850
20°F – 400

In order to keep most favorable concrete strength, the following activities should be undertaken while placing concrete in cold weather:

Utilize a heated or warm concrete mix.

Keep an eye so that the concrete can’t be freezed – Apply powerblanket™ concrete curing blankets to manage temperature.

Ensure that the concrete should be arranged on a frozen sub-grade – Apply powerblanket™ concrete curing blankets to manage temperature.

Safeguard concrete against extreme drying.

Include accelerators to retain strength and normal set time.

Stay away from quick changes in concrete temperature with powerblanket™ concrete curing blankets.

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How to check the workability of concrete with flow table test

Workability belongs to a vital factor of concrete that directly affects the strength, quality and look of concrete as well as finds out how easily freshly produced concrete is blended, set, compacted and finished by retaining its consistency as it is.

To examine the functionality of the freshly mixed concrete, the following tests are generally conducted on field and lab.

Slump test, Kelly ball test, K slump test, Vee bee consistometer test, Flow table test

In this article, detail information is given on flow table test of concrete.

Flow table test of concrete: Under this test method, the functionality of concrete is established by checking the flowing property of concrete.

With Flow table test of concrete, the quality of concrete is found based on the uniformity, cohesiveness and the susceptibility to segregation.

This flow table test is adhered to BS 1881 part 105 of 1984 and DIN 1048 part I.

The following equipments are required for Flow Table Test:

Flow table is built up with metal with thickness 1.5mm and dimensions 750mmx 750mm, tamping rod constructed with hardwood, Scoop, Centimeter Scale, Metal Cone or mould (Lower Dia = 20cm, upper Dia = 13 cm, Height of Cone = 20cm). The central part of flow table is pointed with a concentric circle of dia 200mm to arrange a metal cone on it.

Flow table test is conducted with the following methods :-

Get concrete ready according to mix design and set the flow table on a horizontal surface.

Cleanse the dust or other gritty material on flow table and spray a hand of water on it.

Now, set the metal cone at the central part of the flow table and rest on it.

Pour the freshly mixed concrete in the mould containing two layers; each layer is tamped with tamping rod for 25times. Once the last layer is tamped, the overflowed concrete on the cone is jammed with a trowel.

Gradually, raise the mould upright & allow concrete to rest on its own devoid of any support.

The flow table is elevated at the height of 12.5mm and dropped. The same procedure is reiterated for 15times in 15secs.

Calculate the spread of concrete in diameter with centimetre scale horizontally and vertically. The arithmetic mean of the two diameters should be the calculation of flow in millimetres.



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Impact of impurities in water on concrete properties

If the water applied for concrete preparation contain impurities, the properties of concrete is influenced in the following manner:

a) The strength & longevity of concrete is decreased because of the existence of the impurities in the mixing water. It is found that water with extreme amount of dissolved salts weakens the compressive strength by 10 to 30% as compared to potable water.
b) The setting time of cement is modified. Existence of Zinc chlorides delays the setting of concrete to significantly so that no strength test can be done at 2 and 3 days.

Contrary, the existence of calcium chloride speeds up the setting and hardening.
c) Existence of extreme chlorides leads to dampness, surface efflorescence & raise the corrosion of reinforcing steel.
d) Existence of algae in water decreases the bond among aggregate & comet paste & as a result the strength of concrete is reduced significantly.

e) Existence of sugar up to 0.15% by weight of cement delays the setting of cement and the early strength is decreased. When the quantity of sugar is raised to 0.2% by weight of cement ,setting is expedited.
f) The existence of vegetable oils provides detrimental effect on the concrete strength, specifically at later ages.

Name of Impurities - Allowable limit
1.Organic matter - 200 mg/lit
2.Inorganic matter - 3000 mg/lit
3.Sulfates ( as SO2 ) - 400 mg/lit
4.Chlorides (as Cl)
a) For plain concrete - 2000 mg/lit
b) For R. C. C. - 500 mg/lit
5.Suspended matter - 2000 mg/lit

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Pundit Lab - Ultrasonic Pulse Velocity Tester

To check the quality and strength of in-situ concrete in structural members, ultrasonic pulse velocity test is most recommended.

It is applied for the following purposes :-

The uniformity of a material

The existence of voids, cracks or other inner flaws or defects.

Changes in the concrete in due course because of the cement hydration, damage from fire, frost or chemical attach.

The durability or modulus of a material.

The quality of the concrete with regard to specified standard requirements.

It comprises of a pair of transducers (probes) with various frequencies, electrical pulse generator, electrical timing devices and cables.

It is supported with the through-transmission method. It comes with size 180 x 110 x 160 mm. It’s weight is 3 Kg.

Digital display with precision +_0.1 microseconds.

The following testing method is used with the machine :-

Direct transmission

Pulse velocity is estimated in concrete with the arrangement of transducers across the member perfectly opposite to each other.

This method is considered as most perfect & authentic to determine the quality & strength of concrete. If there does not exist any other method, then this method will be suitable.

Semi Direct Transmission

Pulse velocity is estimated in concrete with the arrangement of transducers in planes perpendicular to each other.

This method is not so authentic to determine the quality/strength of concrete.

Process for estimation of pulse velocity :-

Velocity = (Distance travelled/path length)/Time taken

Pulse velocity in concrete will be demonstrated with Km/sec.

Proper correction factors should be used on the basis of site condition & factors which impact the velocity of pulse.



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Benefits and drawbacks of air entrained concrete

Air entrained concrete is formed by providing little air bubbles into concrete is called air entrainment. Air entrainment is performed with the use of air entraining Portland cement or air entraining agents like admixture.

The amount of air in such concrete is normally kept among four to seven percent of the volume of concrete.

Various types of methods like galvanometric method, volumetric method and pressure method are used to work out the concrete volume. The air bubbles mitigate internal pressure on the concrete by arranging chambers for water to be extended at the time of getting freezed.

Method - Air in concrete in integrated in the following ways:

 By utilizing gas forming materials like aluminium powder, zinc powder and hydrogen peroxide.  With the application of surface active agents that minimizes surface tension. These may contain natural wood resins and their soaps, animal or vegetable fats or oils, alkali salts of sulfonated or sulphated organic compounds.  With the help of cement dispersing agents.

Benefits - Given below, some of the benefits of air entrained concrete:

1. Workability of concrete is raised significantly.
2. The impact of freezing and thawing is reduced with air entraining agent.
3. Bleeding, segregation and laitance in concrete are decreased.
4. Entrained air makes the sulphate repelling strength of concrete.
5. Minimizes the chance of shrinkage and crack development in the concrete surface.

Drawbacks - Given below, some of the drawbacks of air entrained concrete:

a. The strength of concrete is reduced.
b. Air entraining agent can raise the porosity of concrete and consequently the unit weight is decreased.
c. Air-entrainment in concrete should not be performed if the site control is not superior because the air entrained in a concrete differs with the alteration in sand grading, errors in proportioning and workability of the mix and temperatures.



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Maturity Index is the latest method to find out concrete maturity & in-place strength during curing

The Maturity Index offers a simple process to estimate the in-place strength of concrete throughout curing. It is developed on the basis of the principle that the strength is can be obtained depending on the temperature-time history throughout the curing process.

By applying a thermocouple data logger with simple thermocouple temperature sensors implanted in the concrete, the data can be recorded easily. This data can then be instantly evaluated with excel spreadsheet to work out the maturity index.

Concrete maturity belongs to a measuring system of how much a batch of freshly poured concrete has cured. It is a quick indicator of the in-place strength at that point in time corresponding to the ultimate strength when the concrete is completely cured. Usually, the concrete strength is calculated on the basis of compression tests. The maturity method works out an index on the basis of time-temperature data that offer a relative indicator of strength.

This method is very useful since it can be performed instantly on-site with a temperature data logger and thermocouple sensors implanted in the pour. Besides, the method can save significant time and enhance safety by facilitating form removal, post-tensioning, joint sawing or putting structures in service ensuring that necessary strength has been achieved.

What is the Maturity Index?

The maturity is created on the perception that strength of concrete is dependent on the hydration of cement in the mix. This hydration stands for an exothermic reaction that emits heat. By recording the temperature profile at regular time gaps throughout the curing process, the strength of concrete can be computed on-site in real-time.

The American Society for Testing Methods (ASTM) developed a standard method for measuring the concrete strength with the maturing method, ASTM standard C 1074. This method is based on initial calibration process that needs assessment of the maturity index in line with more traditional compressive strength test to create the maturity index vs. strength curve. Once this curve is set it is supposed to be applicable for any pours which employ the identical mix recipe.

The maturity index is computed from the raw time temperature data with the following 2 expressions:

The temperature-time factor:
M(t)= ∑▒〖(Ta-To)∆t〗

Where:

M(t)= Maturity temperature-time factor
Ta = Average temperature during time interval ∆t
To = Datum temperature below which no hydration occurs
Δt = time interval between measurements

Equivalent age using an Arrhenius function:
t_e= ∑▒e^(-Q(1/Ta- 1/Ts)) ∆t

Where:
te = Equivalent age
Q = Activation energy divided by gas constant K
Ta = Average temperature during time interval ∆t
Ts = Datum temperature below which no hydration occurs

Δt = time interval between measurements

To get more details go through www.DataLoggerInc.com

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Different types of grades of concrete as per 456 : 2000 standard

This construction video tutorial from learning technology demonstrates the grades of concrete on the basis of IS 456 : 2000 standard. As per this standard the grades are categorized in three groups like ordinary concrete, standard concrete and high strength concrete. Here grades range from M10 to M80.

M10, M15 and M20 belong to Ordinary Concrete. M25, M30, M35, M40, M45, M50, M55 belong to Standard Concrete. M60, M65, M70, M75 and M80 belong to High Strength Concrete (HSC).

Here m denotes mix and number denotes characteristic compressive strength of 150 mm cube at 28 days in N/mm2 (Newton per millimeter square). It means if you take compressive test of a cube having dimensions 150 x 150 x 150 mm for 28 days, the value obtained is 10 N/mm2.

All these concrete grades are suitably for PCC work i.e. plain cement concrete work where rebar is not used.

Standard Concrete is generally used in 10 storied and higher buildings and super structures.

High strength concrete is mostly used for designing heavy structures.

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The role of water in concrete

Water Cement Ratio signifies the ratio among the weight of water to the weight of cement applied in concrete mix.

Generally, water cement ratio remains under 0.4 to 0.6 with adherence to IS Code 10262 (2009) for nominal mix (M10, M15 …. M25)

The strength of concrete is directly impacted by the water cement ratio. It enhances the strength if employed in perfect ratio and if the ratio is improper, the strength will be reduced.

The importance of Water in Concrete

Concrete refers to a macro content. It comprises of micro constituents like cement, sand, fine aggregate & Coarse aggregate. With the purpose of obtaining high strength concrete to resist the desired compressive strength, it is required to set exact ratio of admixture to unite these materials.

The role of water is important here to accelerate this chemical process by adding 23%-25% of the cement volume. It produces 15% of water cement paste also called gel to fill the voids in the concrete.

Impact of too much water in concrete: If additional water is added more than the permissible limit of 23%, the strength of concrete will be significantly affected.

If the task of adding water is continued to improve the workability then the concrete contains lots of fluid materials where the aggregates will settle down. As soon as the water is evaporated it puts down lots of voids in concrete which influences the concrete strength.

But if the guidelines are followed to retain the strength of the concrete then it will change the concrete workability and makes it difficult to manage and place them.

Workability signifies the capacity of concrete to manage, convey and place devoid of any segregation. The concrete becomes perfectly workable if it can be easily dealt with, placed and transported devoid of any segregation at the time of being placed in construction site.

For this purpose, plasticisers & superplasticizers are utilized to enhance the workability by keeping the W/C Ratio unchanged.

In order to know how to work out water cement ratio, go through the following construction article www.civilology.com

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Testing of Concrete masonry Blocks

Concrete masonry bricks are very useful in the rural areas as it is cheap, easily found and can form less strong buildings, the compressive strength of the bricks should be tested.

And in urban areas these kinds of buildings may present a combination of both office and houses or grocery stores and houses or offices and retail stores.

Concrete can be changed into formed masonry units like Hollow and Solid normal and light weight concrete blocks in right size to use for load and non-load bearing units for fencings or wallings. These concrete blocks are used more in the regions where soil bricks are costly, have less strength and not available all the time. As per the structural needs of masonry unit, the concrete mixes can be arranged using available ingredients or if not be suitable then with in the most economical distance.

Hollow concrete blocks are used for normal masonry when reinforced is used and it should not be leaner than 1 part cement of 8 parts room dry sand by volume. Compressive strength of concrete blocks or concrete masonry units is important to know the fitness of these in construction works for different reasons. Concrete masonry blocks are normally made of cement, amount and water which are usually comes in rectangular shape used in construction of masonry structure.

Patterns

20 full sized units shall be counted for length, wide and height and the center units shall be calculated for least thickness of face, shells and webs.

The minimal dimensions of concrete masonry block differ in three things such as:

• Length: 400/500/600mm
• Width: 200/100mm
• Height: 50, 75, 100, 150, 200 or 300mm.

Tests on Concrete Masonry Block Units

various tests are done on concrete masonry unit to check all the requirements, but among them three most applied tests are discussed in this article. The blocks of same mix shall be taken and classified to these following tests:

• Dimension measurement for all types of blocks

In this step all the blocks should be verified and calculated by the length, width and height with steel scale. After checking if the block found hollow, then the thickness of the web and face shell are calculated with caliper ruler, next a report need to be prepared included average length, width and height of block and with least face shell and web thickness using recorded dimensions.

• Density of the concrete masonry blocks (3 blocks)

At first the block has to be heated in the oven to 100°c, then the heated block need to be cooled in the room temperature. Next measure the dimensions of block to find out the volume and weight of it, the density of the block is calculated form a calculation and the standard density of 3 blocks will be the final block density. The formula is: mass/volume (kg/m3 )

• Compressive Strength tests

the average compressive strength of concrete masonry block is calculated by taking 8 blocks and all of them should be tested with in 3days after gathered in lab where the age of the blocks should be 28 days. The compressive strength is calculated in the Compressive

Strength Testing machine which has tow steel bearing blocks, one is in fixed position on which the masonry unit is kept and the other movable one can transfer the load to the masonry unit while applying. When the masonry unit’s bearing area is more than the steel blocks’ bearing area some different steel plates will be used. Those plates are organized on steel blocks in a way that the centre of masonry unit agreed with the center of thrust of blocks, bearing area units are completed with the Sulphur and granular materials coating. Then the unit in testing machine is placed and one-half of the expected highest loads are followed at a minimum rate while the rest is applied in not less than 2 minutes. Te load of masonry unit fails and the highest load is divided by basic sectional area of unit will give the compressive strength of block. By applying the same method, find out the rest 7 blocks’ compressive strength and then calculate the average strength of the 8 blocks which will be the final compressive strength of concrete masonry unit.

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