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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Published By
Rajib Dey
www.constructioncost.co
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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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Published By
Rajib Dey
www.constructioncost.co
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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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Published By
Rajib Dey
www.constructioncost.co
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