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shotcrete testing

Tests on Shotcrete

 

Shotcrete is the unique method of placing concrete and is suited for unusual applications that require careful attention to detail from design through construction. The main goal of shotcrete testing is to confirm that the shotcrete placed meets the project requirements.
So in this article, we will cover the tests conducted on shotcrete.

  • Fibre Content

  • Shotcrete Thickness

  • Compressive Strength of Shotcrete

  • Shotcrete Penetrometer Test (Early Strength)

  • Flexural Toughness

Fibre Content:  

 

This test is conducted to measure the fibre quantity in the shotcrete after mixing. This test verifies the shotcrete’s uniformly mixing. Sometimes, It is also a cross-check by the Engineer whether the contractor is adding the exact quantity of fibre or not. Generally, the fibre content we use in shotcrete lies between 0.5-2% of the gross weight of concrete and is expressed in kg/m³.

 Fiber content is calculated by :

  • By Wash out method
  • In-situ core test (by crushing)




Wash-out:

 

To check the fiber content, we use a cylindrical vessel of known volume say 10L. After filling the shotcrete into the vessel, it is washed with water until the cement content is not flown away. With the help of the magnet, the fiber is taken out and weighed. The average value shall exceed the minimum value and no value should be less than 75% of the designed value. 

shotcrete test

Formula that we use in calculation is:

             Cf(kg/m³) =    mf*1000/Vc

Where, mf = mass of fibre extracted from shotcrete

               Vc = volume of sample in (m³)

               Cf = fibre content in(m³)

 

In-Situ Core Crushing:

 

After the extraction of core from the tunnel, it is trimmed in a specified dimension and calculate its cylindrical volume. The specimen (Core) is crushed into powder form to extract the fiber present in the shotcrete and weighed it. After this, the same procedure has to be followed as in the washout method.

shotcrete  test

 

Shotcrete Thickness:

 

After the extraction of the core, the thickness of the applied shotcrete is measured by measuring the length of the core. The average thickness of the four cores should not be less than the designed thickness. The test shall consist of 4 numbers drill holes on a 1m² pattern.

 

Compressive strength of Shotcrete Core:

 

After the extraction of the core by the instruction of the engineer, the same has to be taken to the laboratory for testing. The core has to be trimmed in both sides by cutting machines in a 1:2 ratio of Diameter/Length. The main points of the procedure will be given below taken from IS 516:1959.

 

  • The specimen has to be put into the water of temperature 27±2°c for 48 hours before testing.
  • Preferably three numbers of specimens and all should be taken from the side or a crown portion, required for the testing.
  • The end of all cylindrical specimen that is not within the limit of ±0.05mm shall be capped and capping is done in three ways:
    • Neat cement
    • Sulphur
    • Hard plaster
  • While the specimen is ready to test, it is taken to a compression testing machine (CTM) having a capacity of 2000 KN. The rate of loading needs to be fixed 140 kg/sq cm/minute.
  • Average of three values is taken as the compressive strength of the core and should be within ±15% of the variation.
  • The correction factor for height/diameter has to be applied as per the curve given below
shotcrete test
  • The equivalent cube strength of the concrete shall be resolved by multiplying the corrected cylinder strength by 5/4.

 

Shotcrete Penetrometer Test (Early Strength):

 

In underground works especially in NATM, the repetition of activities is fast therefore we need a quick setting of the shotcrete so that the blast can be taken after 2-3 hrs of final spraying. To ensure the early setting of the shotcrete, we perform this test. The code which we follow for this test is EN 14482 Edition 11/06.

 

General specification:

 

This method utilizing a needle (Æ 3 ± 0,1 mm), which is driven by means of a penetrometer into the young sprayed concrete. The force required to drive the needle 15 ± 2 mm in to the sprayed concrete is recorded.

Penetration Needle Method

 

In Penetration Needle Method, we can check the early strength of sprayed shotcrete from 0.2-1 N/mm². In this test, two graphs are very important to calculate the actual (early) strength of the shotcrete.

penetration needle method -shotcrete test

 

Calibration Curve:

 

In penetrometer, there is encircled marking given in Kilo Pounds which we need to note down while punching shotcrete, to convert into N/mm² through this curve. Time is also taken into consideration.

Calibration curve - Shotcrete method

 

J-Curve:

 

After getting values, we can compare it with the given graph to check the results showing the type of curve it is following with respect to the time.

J- curve

 

Flexural Toughness Test:

 

 The energy equivalent to the area under the load-deflection curve up to a specified deflection. Means, the energy absorbed by a shotcrete beam of size (350*100*100) mm under the three-point load given by load vs displacement graph. The code that we follow for this test is ASTM C1018 and ASTM C1609.

 

Few terms that we need to understand are:

 

First crack (£)-  First crack is the point on the load-deflection curve at which the form of the curve first becomes nonlinear.

Toughness Indices (I5): The number obtained by dividing the area up to a deflection of 3.0 times the first-crack deflection by the area up to first crack.  

 Toughness Indices (I10): The number obtained by dividing the area up to a deflection of 5.5 times the first-crack deflection by the area up to first crack.   

Residual Strength Factor (R5,10): Residual Strength factor is the number obtained by calculating the value of 20 (I10 − I5). 

 

Method of testing:

 

 Moulded or sawn beams of fiber-reinforced concrete are tested in flexure using the third-point loading arrangement specified in Test Method. After marking the beam in three equal spans, it is kept at a three-point loading system and the rate of loading is applied which should be 0.06mm per minute. Load and beam deflection are controlled continuously by means of an X-Y plotter, or incrementally by means of dial gauges read at sufficiently frequent intervals to ensure accurate reproduction of the load-deflection curve.

A point termed the first crack which corresponds approximately to the onset of cracking in the concrete matrix is identified on the load-deflection curve. The first-crack load and deflection are used to determine the first-crack flexural strength and to determine end-point deflections for toughness calculations. Computations of toughness and toughness indices are supported areas under the load-deflection curve up to the first-crack deflection and up to the required end-point deflection.

First crack-deflection for three-point loading is estimated assuming elastic behaviour up to first crack from the equation:

∂=23PL3/1296EI{1+216D2(1+µ)/115L2}


Mid Span deflection ∂, expressed as mm/min is as calculated by the above formula

∂ = 0.06mm/min b = 100 mm
P = 4.2 KN I = 7593750 mm4
L = 350 mm D = 100 mm
E = 14 Gpa µ = 0.2

After getting the value of first crack, we can easily calculate the value of I5, I10 & R5,10.

 

 

Editor :

Mr. Ankit Ranaut

Mr. Ankit is a Civil Engineer who is working as an Assistant mananger in larsen and toubro (L&T) He is an Author, Editor and Partner at constructionblogs.in