ELECTIRC CONE PENETRATION TEST ON SOIL

One of the advanced techniques in Soil Investigation is Electric Cone Penetration Test (ECPT). It is a widely used geotechnical investigation technique that involves measuring the resistance of soil to a penetrating cone. ECPT test is essential in determining the soil’s strength and consistency, which are critical factors in designing foundation structures, embankments, and retaining walls. Here, we will discuss the concept of ECPT, its procedure, and its applications in geotechnical engineering.

CONCEPT:-

ECPT involves inserting a cone-shaped probe, known as a penetrometer, into the soil at a constant rate. The penetrometer measures the electrical resistance of the soil as it penetrates. The resistance is directly proportional to the soil's strength, which is an excellent indicator of soil properties.

The electric cone penetrometer is an electrical device that calculates soil resistance and pore pressure, which is the pressure of fluids in the soil. The probe consists of a pointed tip, followed by a cylindrical section, and then a conical section. The conical section has a base area of 10 cm2, and the apex angle is 60 degrees. The cylindrical section is used to measure pore pressure and has small holes for fluid entry.

The information gathered during an ECPT can be extremely helpful in determining the density, strength, and other geotechnical characteristics of the soil or substance being examined. It aids in assessing the load-bearing capacity, soil behavior, and potential risks related to slope stability, foundation design, and other geotechnical projects by engineers and geotechnical specialists.

CONCLUSION:-

To gain a thorough understanding of the subsurface conditions at a specific site, the ECPT is frequently utilized in conjunction with other geotechnical experiments and investigations.

WATER CONTENT OF SOIL BY CALCIUM CARBIDE METHOD

Water Content of soil is the quantity of soil contained in a sample of soil. Generally this is expressed in ratio.

Here : s-soil (dry), v-void (pores filled with water or air), w-water, a-air. V is volume, M is mass.

Volumetric Water Content is defined by

where V_w is the volume of water and V_T = V_s + V_v = V_s + V_w + V_a is the total volume (that is soil volume + water volume + air space).

Gravimetric water content is expressed by mass (weight) as follows:

where m_w is the mass of water and m_t is the bulk mass. The bulk mass is taken as the total mass, except for geotechnical and soil science applications where oven-dried soil (m_s, see the diagram) is conventionally used as m_t

   We can determine the water content in soil by calcium carbide method as per IS: 2720 (Part II) - 1973.

PRINCIPLE
     It is a method for rapid determination of water content from the gas pressure developed by the reaction of calcium carbide with the free water of the soil. From the calibrated scale of the pressure gauge the percentage of water on total mass of wet soil is obtained and the same is converted to water content on dry mass of soil.

APPARATUS

     i) Metallic pressure vessel, with a clamp for sealing the cup, alongwith a gauge calibrated in percentage water content.
     ii) Counterpoised balance, for weighing the sample
     iii) Scoop, for measuring the absorbent (Calcium Carbide)
     iv) Steel balls - 3 steel balls of about 12.5mm dia. and 1 steel ball of 25mm dia.
     v) One bottle of the absorbent (Calcium Carbide)

PREPARATION OF SAMPLE
     Sand - No special preparation. Coarse powders may be ground and pulverized.
     Cohesive and plastic soil - Soil is tested with addition of steel ball in the pressure vessels.
     The test requires about 6g of sample.

PROCEDURE
     i) Set up the balance, place the sample in the pan till the mark on the balance arm matches with the index mark.
     ii) Check that the cup and the body are clean.
     iii) Hold the body horizontally and gently deposit the levelled, scoop-full of the absorbent (Calcium Carbide) inside the chamber.
     iv) Transfer the weighed soil from the pan to the cup.
     v) Hold cup and chamber horizontally, bringing them together without disturbing the sample and the absorbent.
     vi) Clamp the cup tightly into place. If the sample is bulky, reverse the above placement, that is, put the sample in the chamber and the absorbent in the cup.
     vii) In case of clayey soils, place all the 4 steel balls (3 smaller and 1 bigger) in the body alongwith the absorbent.
     viii) Shake the unit up and down vigorously in this position for about 15 seconds.
     ix) Hold the unit horizontally, rotating it for 10 seconds, so that the balls roll around the inner circumference of the body. 
     x) Rest for 20 seconds.
     xi) Repeat the above cycle until the pressure gauge reading is constant and note the reading. Usually it takes 4 to 8 minutes to achieve constant reading. This is the water content (m) obtained on wet mass basis. 
     xii) Finally, release the pressure slowly by opening the clamp screw and taking the cup out, empty the contents and clean the instrument with a brush.

REPORTING OF RESULTS
     The water content on dry mass basis,

MAXIMUM DRY DENSITY AND OPTIMUM MOISTURE CONTENT OF SOIL

     The peak dry unit weight is called the "maximum dry density” and the Optimum Water Content, is the water content at the soil’s maximum dry density.
     Compaction is the process of increasing the bulk density of a soil or aggregate by driving out air. For any soil, at a given compactive effort, the density obtained depends on the moisture content. For any soil, an “optimum water content” exists at which it will achieve it’s maximum density.
      This Test determines the optimum water content and maximum dry density of for a soil as per IS: 2720 (Part 8) - 1983.A required range for moisture is often specified isIe, 3% below and 2% above optimum.For example, if optimum water content is 16%, the acceptable range would be from 13% to 18%.

APPARATUS

     i) Cylindrical metal mould - it should be either of 100mm dia. and 1000cc volume or 150mm dia. and 2250cc volume and should conform to IS: 10074 – 1982.
     ii) Balances - one of 10kg capacity, sensitive to 1g and the other of 200g capacity, sensitive to 0.01g. 
    iii) Oven - thermostatically controlled with an interior of non corroding material to maintain temperature between 105 and 110^oC.
     iv) Steel straightedge - 30cm long.
     v) IS Sieves of sizes - 4.75mm, 19mm and 37.5mm.

PREPARATION OF SAMPLE
     A representative portion of air-dried soil material, large enough to provide about 6kg of material passing through a 19mm IS Sieve (for soils not susceptible to crushing during compaction) or about 15kg of material passing through a 19mm IS Sieve (for soils susceptible to crushing during compaction), should be taken.This portion should be sieved through a 19mm IS Sieve and the coarse fraction rejected after its proportion of the total sample has been recorded.
     Aggregations of particles should be broken down so that if the sample was sieved through a 4.75mm IS Sieve, only separated individual particles would be retained.

PROCEDURE
      A) Soil not susceptible to crushing during compaction –
      i) A 5kg sample of air-dried soil passing through the 19mm IS Sieve should be taken. The sample should be mixed thoroughly with a suitable amount of water depending on the soil type (for sandy and gravelly soil - 3 to 5% and for cohesive soil - 12 to 16% below the plastic limit). The soil sample should be stored in a sealed container for a minimum period of 16hrs. 
      ii) The mould of 1000cc capacity with base plate attached,should be weighed to the nearest 1g (W₁ ). The mould should be placed on a solid base, such as a concrete floor or plinth and the moist soil should be compacted into the mould, with the extension attached, in five layers of approximately equal mass, each layer being given 25 blows from the 4.9kg rammer dropped from a height of 450mm above the soil. The blows should be distributed uniformly over the surface of each layer. The amount of soil used should be sufficient to fill the mould, leaving not more than about 6mm to be struck off when the extension is removed.The extension should be removed and the compacted soil should be levelled off carefully to the top of the mould by means of the straight edge. The mould and soil should then be weighed to the nearest gram (W₂).
     iii) The compacted soil specimen should be removed from the mould and placed onto the mixing tray. The water content (w) of a representative sample of the specimen should be determined as in Para 5.1.
     iv) The remaining soil specimen should be broken up, rubbed through 19mm IS Sieve and then mixed with the remaining original sample. Suitable increments of water should be added successively and mixed into the sample, and the above operations i.e. Para ii) to iv) should be repeated for each increment of water added. The total number of determinations made should be at least five and the moisture contents should be such that the optimum moisture content at which the maximum dry density occurs,lies within that range.
     B) Soil susceptible to crushing during compaction –
or more 2.5kg samples of air-dried soil passing through the 19mm IS Sieve, should be taken. The samples should each be mixed thoroughly with different amounts of water and stored in a sealed container as mentioned in Para A) i), above. Follow the operations given in Para A) ii) to iv), above.
     C) Compaction in large size mould – For compacting soil containing coarse material upto 37.5mm size, the 225⁰cc mould should be used. A sample weighing about 30kg and passing through the 37.5mm IS Sieve is used for the test. Soil is compacted in five layers, each layer being given 55 blows of the 4.9kg rammer. The rest of the procedure is the same as in Para A) or B), above.

REPORTING OF RESULTS
      Bulk density γ in g/cc of each compacted specimen should be calculated from the equation,

 

where, V = volume in cc of the mould.
The dry density γ_d in g/cc is calculated from the equation,

      The dry densities, γ_d obtained in a series of determinations should be plotted against the corresponding moisture contents, w. A smooth curve should be drawn through the resulting points and the position of the maximum on the curve should be determined. A sample graph is shown below:

      The dry density in g/cc corresponding to the maximum point on the moisture content/dry density curve should be reported as the maximum dry density to the nearest 0.01.
The percentage moisture content corresponding to the maximum dry density on the moisture content/dry density curve should be reported as the optimum moisture content and quoted to the nearest 0.2 for values below 5%, to the nearest 0.5 for values from 5 to 10% and to the nearest whole number for values exceeding 10%.

Free Swell Index Determination Test

DEFINITION
      Free Swell Index is the increase in volume of a soil, without any external constraints,on submergence in water

It is determined by the following way as per IS: 2720 (Part XL) - 1977.

PRINCIPLE
     Free swell or differential free swell, also termed as free swell index, is the increase in volume of soil without any external constraint when subjected to submergence in water.

APPARATUS
i) IS Sieve of size 425μm
ii) Oven
iii) Balance, with an accuracy of 0.01g
iv) Graduated glass cylinder- 2 nos., each of 100ml capacity

PROCEDURE

i) Take two specimens of 10g each of pulverised soil passing through 425μm IS Sieve and oven-dry.
ii) Pour each soil specimen into a graduated glass cylinder of 100ml capacity.
iii) Pour distilled water in one and kerosene oil in the other cylinder upto 100ml mark.
iv) Remove entrapped air by gently shaking or stirring with a glass rod.
v) Allow the suspension to attain the state of equilibrium (for not less than 24hrs.).
vi) Final volume of soil in each of the cylinder should be read out.

REPORTING OF RESULTS

where, V_d = volume of soil specimen read from the graduated cylinder containing distilled water.
            V_k = volume of soil specimen read from the graduated cylinder containing kerosene.

Water Content of Soil by Oven Drying Method

 Water Content of soil is the quantity of soil contained in a sample of soil. Generally this is expressed in ratio.

Here : s-soil (dry), v-void (pores filled with water or air), w-water, a-air. V is volume, M is mass.

Volumetric Water Content is defined by

where V_w is the volume of water and V_T = V_s + V_v = V_s + V_w + V_a is the total volume (that is soil volume + water volume + air space).

Gravimetric water content is expressed by mass (weight) as follows:

where m_w is the mass of water and m_t is the bulk mass. The bulk mass is taken as the total mass, except for geotechnical and soil science applications where oven-dried soil (m_s, see the diagram) is conventionally used as m_t

By Oven Drying Method  we can determine the Gravimetric water content in soil  as per IS: 2720 (Part II) - 1973.

PRINCIPLE
The water content (w) of a soil sample is equal to the mass of water divided by the mass of solids.

APPARATUS

i) Thermostatically controlled oven maintained at a temperature of 110 ± 5^oC
ii) Weighing balance, with an accuracy of 0.04% of the weight of the soil taken
iii) Air-tight container made of non-corrodible material with lid
iv) Tongs

SAMPLE
The soil specimen should be representative of the soil mass. The quantity of the specimen taken would depend upon the gradation and the maximum size of particles as under:

Size of particles 90 percent passing through IS Sieve	Minimum quantity of soil specimen to be taken for test (g)
425μm 2.0mm 4.75mm 9.50mm 19mm 37.5mm	25 50 200 300 500 1000

PROCEDURE
i) Clean the container, dry it and weigh it with the lid (Weight 'W₁').
ii) Take the required quantity of the wet soil specimen in the container and weigh it with the lid (Weight 'W₂').
iii) Place the container, with its lid removed, in the oven till its weight becomes constant (Normally for 24hrs.).
iv) When the soil has dried, remove the container from the oven, using tongs.
v) Find the weight 'W₃' of the container with the lid and the dry soil sample.

REPORTING OF RESULTS
The water content,  

 

An average of three determinations should be taken.
A sample proforma for the record of the test results is given below

In-Situ Dry Density by Sand Replacement Method

        This test method sets out the procedure for the determination for the insitu dry density of compacted soils, gravels and crushed rock materials in earth works and pavement layers by the sand replacement method using a sand pouring cone. This test is generally limited to materials with a maximum particle size of 5cm

AIM
To determine the in-situ dry density of soil by sand replacement method as per IS: 2720 (Part XXVIII) – 1974.

APPARATUS

i) Sand-pouring cylinder conforming to IS: 2720 (Part XXVIII) –1974
ii) Cylindrical calibrating container conforming to IS: 2720 (PartXXVIII) - 1974
iii) Soil cutting and excavating tools such as a scraper tool,bent spoon
iv) Glass plate - 450mm square and 9mm thick or larger
v) Metal containers to collect excavated soil
vi) Metal tray - 300mm square and 40mm deep with a 100mm hole in the centre
vii) Balance, with an accuracy of 1gm

PROCEDURE
A. Calibration of apparatus
a) The method given below should be followed for the determination of the weight of sand in the cone of the pouring cylinder:
i) The pouring cylinder should be filled so that the level of the sand in the cylinder is within about 10mm of the top. Its total initial weight (W₁) should be maintained constant throughout the tests for which the calibration is used. A volume of sand equivalent to that of the excavated hole in the soil (or equal to that of the calibrating container) should be allowed to runout of the cylinder under gravity. The shutter of the pouring cylinder should then be closed and the cylinder placed on a plain surface, such as a glass plate.
ii) The shutter of the pouring cylinder should be opened and sand allowed to runout. When no further movement of sand takes place in the cylinder, the shutter should be closed and the cylinder removed carefully.
iii) The sand that had filled the cone of the pouring cylinder (that is, the sand that is left on the plain surface) should be collected and weighed to the nearest gram.
iv) These measurements should be repeated at least thrice and the mean weight (W₂) taken.

b) The method described below should be followed for the determination of the bulk density of the sand ( γ_s )
i) The internal volume (V) in ml of the calibrating container should be determined from the weight of water contained in the container when filled to the brim. The volume may also be calculated from the measured internal dimensions of the container.
ii) The pouring cylinder should be placed concentrically on the top of the calibrating container after being filled to the constant weight (W1) as in Para a) i), above. The shutter of the pouring cylinder should be closed during the operation. The shutter should be opened and sand allowed to runout. When no further movement of sand takes place in the cylinder, the shutter should be closed. The pouring cylinder should be removed and weighed to the nearest gram.
iii) These measurements should be repeated at least thrice and the mean weight (W₃) taken.
B. Measurement of soil density
The following method should be followed for the measurement of soil density:
i) A flat area, approximately 450sq.mm of the soil to be tested should be exposed and trimmed down to a level surface, preferably with the aid of the scraper tool.
ii) The metal tray with a central hole should be laid on the prepared surface of the soil with the hole over the portion of the soil to be tested. The hole in the soil should then be excavated using the hole in the tray as a pattern, to the depth of the layer to be tested upto a maximum of 150mm. The excavated soil should be carefully collected, leaving no loose material in the hole and weighed to the nearest gram(Ww). The metal tray should be removed before the pouring cylinder is placed in position over the excavated hole.
iii) The water content (w) of the excavated soil should be determined by the method specified in Para 5.1. Alternatively, the whole of the excavated soil should be dried and weighed (W_d).
iv) The pouring cylinder, filled to the constant weight (W₁) as above, should be so placed that the base of the cylinder covers the hole concentrically. The shutter shoul then be opened and sand allowed to runout into the hole. The pouring cylinder and the surrounding area should not be vibrated during this period. When no further movement of sand takes place, the shutter should be closed. The cylinder should be removed and weighed to the nearest gram (W₄).

CALCULATIONS
i) The weight of sand (W_a) in gram, required to fill the calibrating container should be calculated from the formula:
W_a = W₁ – W₃ – W₂
ii) The bulk density of the sand (γ_s) in kg/m3 should be calculated from the formula:
γ_s = V/W_a×1000
ii) The weight of sand (W_b) in gram, required to fill the excavated hole should be calculated from the formula:
W_b = W₁ – W₄ – W₂
iv) The bulk density (γ_b), that is, the weight of the wet soil per cubic meter should be calculated from the formula:
γ_d=W_w/W_b×γ_s Kg/m³
v) The dry density (γ_d), that is, the weight of dry soil per cubic meter should be calculated from the formula:
γ_d=100γ_b/(100+w) Kg/m³
γ_d=W_d/W_b×γ_s Kg/m³

REPORTING OF RESULTS
The following values should be reported:
i) dry density of soil in kg/m3 to the nearest whole number; also to be calculated and reported in g/cc correct to the second place of decimal
ii) water content of the soil in percent reported to two significant figures.
A sample proforma for the record of the test results is given below..

IN-SITU DRY DENSITY BY CORE CUTTER METHOD

AIM
To determine the in-situ dry density of soil by core cutter method as per IS: 2720 (Part XXIX) - 1975.

APPARATUS

i) Cylindrical core cutter
ii) Steel dolley
iii) Steel rammer
iv) Balance, with an accuracy of 1g
v) Straightedge
vi) Square metal tray - 300mm x 300mm x 40mm
vii) Trowel

PROCEDURE
i) The internal volume (V) of the core cutter in cc should be  calculated from its dimensions which should be measured to  the nearest 0.25mm.
ii) The core cutter should be weighed to the nearest gram (W₁).
iii) A small area, approximately 30cm square of the soil layer to  be tested should be exposed and levelled. The steel dolly  should be placed on top of the cutter and the latter should be rammed down vertically into the soil layer until only about 15mm of the dolly protrudes above the surface, care being  taken not to rock the cutter. The cutter should then be dug  out of the surrounding soil, care being taken to allow some soil to project from the lower end of the cutter. The ends of  the soil core should then be trimmed flat in level with the  ends of the cutter by means of the straightedge.
iv) The cutter containing the soil core should be weighed to the  nearest gram (W₂).
v) The soil core should be removed from the cutter and a  representative sample should be placed in an air-tight container and its water content (w) determined as in Para 

REPORTING OF RESULTS

Bulk density of the soil  γ =(W₂ -W₁)/V g/cc

Dry density of the soil γ_d = 100γ/(100+w) g/cc

Average of at least three determinations should be reported to the second place of decimal in g/cc.
A sample proforma for the record of the test results is given below..

PLASTIC LIMIT TEST

Plastic limit is the water content below which the soil stops behaving as a plastic material.The plastic limit is determined by rolling a part of a soil into thread,when the thread begins to crumble at a diameter of 3.18mm or 1/8", the water content at this stage is the plastic limit.

AIM
To determine the plastic limit of soil as per IS: 2720 (Part 5)- 1985.

PRINCIPLE
The plastic limit of fine-grained soil is the water content of the soil below which it ceases to be plastic. It begins to crumble when rolled into threads of 3mm dia.

APPARATUS
i) Porcelain evaporating dish about 120mm dia.
ii) Spatula
iii) Container to determine moisture content
iv) Balance, with an accuracy of 0.01g
v) Oven
vi) Ground glass plate - 20cm x 15cm
vii) Rod - 3mm dia. and about 10cm long

PREPARATION OF SAMPLE
Take out 30g of air-dried soil from a thoroughly mixed sample of the soil passing through 425μm IS Sieve. Mix the soil with distilled water in an evaporating dish and leave the soil mass for naturing. This period may be upto 24hrs.

PROCEDURE
i) Take about 8g of the soil and roll it with fingers on a glass plate. The rate of rolling should be between 80 to 90 strokes per minute to form a 3mm dia.
ii) If the dia. of the threads can be reduced to less than 3mm, without any cracks appearing, it means that the water content is more than its plastic limit. Knead the soil to reduce the water content and roll it into a thread again.
iii) Repeat the process of alternate rolling and kneading until the thread crumbles.
iv) Collect and keep the pieces of crumbled soil thread in the container used to determine the moisture content.
v) Repeat the process at least twice more with fresh samples of plastic soil each time.

REPORTING OF RESULTS
The plastic limit should be determined for at least three portions of the soil passing through 425μm IS Sieve. The average water content to the nearest whole number should be reported.

LIQUID LIMIT TEST

The lowest water content at which the soil is in a liquid state is called the liquid limit.At liquid limit, the clay is practically like a liquid but possesses a small strength.It is primarily used by civil and geotechnical engineers as a physical property of a soil.  The liquid limit allows engineers to classify soils into their applications.For determining Liquid Limit the most popular test is Casagrande,s Liquid Limit Test.

AIM
To determine the liquid limit of soil as per IS: 2720 (Part 5)- 1985.

PRINCIPLE
The liquid limit of fine-grained soil is the water content at which soil behaves practically like a liquid, but has small shear strength. It's flow closes the groove in just 25 blows in Casagrande’s liquid limit device.

APPARATUS

i) Casagrande’s liquid limit device
ii) Grooving tools of both standard and ASTM types
iii) Oven
iv) Evaporating dish
v) Spatula
vi) IS Sieve of size 425μm
vii) Weighing balance, with 0.01g accuracy
viii) Wash bottle
ix) Air-tight and non-corrodible container for determination of moisture content

PREPARATION OF SAMPLE
i) Air-dry the soil sample and break the clods. Remove the organic matter like tree roots, pieces of bark, etc.
ii) About 100g of the specimen passing through 425μm IS Sieve is mixed thoroughly with distilled water in the evaporating dish and left for 24hrs. for soaking.

PROCEDURE
i) Place a portion of the paste in the cup of the liquid limit device.
ii) Level the mix so as to have a maximum depth of 1cm.
iii) Draw the grooving tool through the sample along the symmetrical axis of the cup, holding the tool perpendicular to the cup.
iv) For normal fine grained soil: The Casagrande's tool is used to cut a groove 2mm wide at the bottom, 11mm wide at the top and 8mm deep.
v) For sandy soil: The ASTM tool is used to cut a groove 2mm wide at the bottom, 13.6mm wide at the top and 10mm deep.
vi) After the soil pat has been cut by a proper grooving tool, the handle is rotated at the rate of about 2 revolutions per second and the no. of blows counted, till the two parts of the soil sample come into contact for about 10mm length.
vii) Take about 10g of soil near the closed groove and determine its water content.
viii) The soil of the cup is transferred to the dish containing the soil paste and mixed thoroughly after adding a little more water. Repeat the test.
ix) By altering the water content of the soil and repeating the foregoing operations, obtain at least 5 readings in the range of 15 to 35 blows. Don’t mix dry soil to change its consistency.
x) Liquid limit is determined by plotting a ‘flow curve’ on a semi-log graph, with no. of blows as abscissa (log scale) and the water content as ordinate and drawing the best straight line through the plotted points.
xi) Water content corresponding to 25 blows, is the value of the liquid limit.

REPORTING OF RESULTS
Report the water content corresponding to 25 blows, read from the 'flow curve' as the liquid limit. A sample ‘flow curve’ is given below.

Direct Shear Test of Soil

     The direct shear test used for soil (Powers 1968) can be performed with fresh concrete to assess the cohesive strength of a concrete mixture. The results of the test are given in terms of soil mechanics parameters, not in terms of yield stress and plastic viscosity.
      The device, as described by Powers (1968), consists of a ring shaped container filled with compacted concrete. The lower half of the device is held in a fixed position while the upper half of the device is rotated slowly, resulting in a maximum shear stress on the plane between the two halves of the container. A vertical load can be applied to the concrete during the test. The test measures the angle of rotation of the upper container and the corresponding torque required to turn the container.
       A typical plot of torque versus relative displacement shows an initial linear increase in torque up to a maximum value and then a decline followed by a gradual leveling off of the curve. The maximum stress is considered the “static friction” and the stress after the plot has leveled off is considered the “sliding friction.” The linear relationship between static friction and normal stress allows the calculation of the angle of internal friction.
Advantages: • The test essentially determines the yield stress of the concrete.
• The test provides additional information, namely the angle of internal friction, not available from most conventional tests.
Disadvantages: • The results of the test are not described in terms of shear stress and shear rate. The use of the direct shear test predates the establishment of concrete as a Bingham material. The additional information provided by the test is not necessarily useful.
• The test does not provide a measure of plastic viscosity.
• The test is strictly a laboratory device.

PERMEABILITY TEST

       Permeability of concrete is important when dealing with durability of concrete particularly in concrete used for water retaining structures or watertight sub-structures.Structures exposed to harsh environmental conditions also require low porosity as well as permeability. Such adverse elements can result in degradation of reinforced concrete, for example, corrosion of steel leading to an increase in the volume of the steel, cracking and eventual spalling of the concrete. Permeability tests measure the ease with which liquids, ions and gases can penetrate into the concrete. In situ tests are available for assessing the ease with which water, gas and deleterious matter such as chloride ions can penetrate into the concrete.

PROCEDURE
     A comprehensive review of the wide range of test methods is given in the Concrete Society Technical Report No. 31. Two of the most widely established methods are the initial surface absorption test (ISAT) and the modified Figg air permeability test. The former measures the ease of water penetration into the surface layer of the concrete while the latter can be used to determine the rate of water as well as air penetration into the surface layer of the concrete which is also called the covercrete. Another newly developed technique uses modification of the laboratory test to determine chloride ion permeability. All the site tests emphasize the measurement of permeability of the outer layer of concrete as this layer is viewed as most important for the durability of concrete.

EQUIPMENTS

1. Initial surface absorption test
      Details of the ISAT is given in BS 1881:Part 5 which measures the surface water absorption. In this method, a cup with a minimum surface area of 5000 mm2 is sealed to the concrete surface and filled with water. The rate at which water is absorbed into the concrete under a pressure head of 200 mm is measured by movement along a capillary tube attached to the cup. When water comes into contact with dry concrete it is absorbed by capillary action initially at a high rate but at a decreasing rate as the water filled length of the capillary increases. This is the basis of initial surface absorption, which is defined as the rate of water flow into concrete per unit area at a stated interval from the start of test at a constant applied head at room temperature.

2. Modified Figg permeability Test
      The modified Figg permeability test can be used to determine the air or wate permeability of the surface layer of the concrete. In both the air and water permeability test a hole of 10 mm diameter is drilled 40 mm deep normal to the concrete surface. A plug is inserted into this hole to form an airtight cavity in the concrete. In the air permeability test, the pressure in the cavity is reduced to –55 kPa using a hand operated vacuum pump and the pump is isolated. The time for the air to permeate through the concrete to increase the cavity pressure to –50 kPa is noted and taken as the measure of the air permeability of the concrete. Water permeability is measured at a head of 100 mm with a very fine canula passing through a hypodermic needle to touch the base of the cavity. A two-way connector is used to connect this to a syringe and to a horizontal capillary tube set 100 mm above the base of the cavity. Water is injected through the syringe to replace all the air and after one minute the syringe isolated with a water meniscus in a suitable position. The time for the meniscus to move 50 mm is taken as a measure of the water permeability of the concrete.

3. In situ rapid chloride ion permeability test
     This method was originally designed for laboratory application but has been modified for in situ use. The procedure for the laboratory test is given in AASHTO T277 and ASTM C1202. The technique is based on the principle that charged ions, such as chloride (Cl- ), will accelerate in an electric field towards the pole of opposite charge. The ions will reach terminal velocity when the frictional resistance of the surrounding media reaches equilibrium with the accelerating force. This is the basis of “electrophoresis”, which is utilized in many chemical and biological studies.
      A DC power supply is used to apply a constant voltage between the copper screen and the steel reinforcement. The total current flowing between the mesh and the reinforcing bar over a period of six hours is then measured. The total electric charge (in coulombs) is computed and can be related to the chloride ion permeability of the concrete.

APPLICATIONS
       The methods described do not measure permeability directly but produce a ‘permeability index’, which is related closely to the method of measurement. In general, the test method used should be selected as appropriate for the permeation mechanism relevant to the performance requirements of the concrete being studied. Various permeation mechanisms exist depending on the permeation medium, which include absorption and capillary effects, pressure differential permeability and ionic and gas diffusion.
        Most of these methods measure the permeability or porosity of the surface layer of concrete and not the intrinsic permeability of the core of the concrete.  The covercrete has been known to significantly affect the concrete durability since deterioration such as carbonation and leaching starts from the concrete surface. This layer thus provides the first defense against any degradation.

RANGE AND LIMITATIONS
      For the ISAT, tests on oven dried specimens give reasonably consistent results but in other cases results are less reliable. This may prove to be a problem with in situ concrete.
      Particular difficulties have also been encountered with in situ use in achieving a watertight fixing. The test has been found to be very sensitive to changes in quality and to correlate with observed weathering behaviour. The main application is as a quality control test for precast units but application to durability assessment of in situ concrete is growing.