COMPRESSIVE STRENGTH OF CEMENT

Compressive Strength of Cement is the capacity of taking compressive loads of cement.

It is determined by compressive strength test on mortar cubes compacted by means of a standard vibration machine. Standard sand (IS:650) is used for the preparation of cement mortar. The specimen is in the form of cubes 70.6mm*70.6mm*70.6mm.

APPRATUS:-

1. Cube Mould of size 70.6mmX70.6mmX70.6mm (As per IS:10080)

2. Vibration Machine (As per IS: 10080)

3. Compression Testing Machine

4. Measuring Cylinder

5. Balance

PROCEDURE:-

1. Environmental condition for cement testing should be of temperature 27± 2⁰ C.

2. Take 200 g of cement and 600 g of standard sand and mix them dry thoroughly.

3. Add (P/4+3)X8 [ P is % of water required for preparing paste of standard consistency] to the dry mix of cement and sand and for a time of 3 to 4  minutes mix them properly si that they form a mix of uniform colour. If the mix is not homogeneous after 4 minutes reject the mix and do the procedure again.

4. Clean the cube moulds of cement thoroughly and place them  the vibrating machine. It should be hold properly in position by clamps provided on the machine..

5. Fill the mould with entire quantity of mortar using a suitable hopper attached to the top of the mould for facility of filling and vibrate it for 2 minutes at a specified speed of 12000±400 per minute to achieve full compaction.

6. Remove the mould from the machine and keep it in a place with temp of 27±2⁰C and relative humidity of 90% for 24 hours.

7. After 24 hrs remove the cube from the mould and immediately submerge in fresh clean water in temperature controlled cube curing tank where its temp. is kept in 27±2⁰C. They should be submerged in waster till testing.

8. Prepare at least 9 cubes in the manner explained .

9. Place the test cube on the platform of a compressive testing machine so that the cubes central line and central line of platform coincides. No packing materials to be used between the cubes and loading plates.

10. At the rate of 35 N/mm²/minute apply the load steadily and uniformly.

11. 3 Cubes to be tested on completion of 3,7 and 28 days of testing and to be reported as shown below.

CALCULATIONS:-

Compressive Strength = P/A

[ P=Maximum load applied to the cube. (N)

A=Cross sectional area (Calculated from the mean dimensions) (mm²) ]

• Compressive strength is reported to the nearest 0.5 N/mm²
• Specimens that are manifestly faulty, or that give strengths differing by more than 10% from the average value of all the test specimen should not be considered.
• Test three cubes for compressive strength for each period of curing.

REPORTING OF RESULTS:-

Test result of Cement Cube Testing is reported as following..

CONSISTENCY OF CEMENT

Consistency is defined as the amount of water that can be added to cement to produce a plastic paste of cement and the ingredients of of concrete.

The standard consistency of a cement paste is defined as that consistency which will permit the Vicat Plunger to penetrate to a point 5 to 7 mm from the bottom of the Vicat mould.

APPARATUS:-

1. Vicat apparatus conforming to IS: 5513 – 1976
2. Balance, whose permissible variation at a load of 1000g should be +1.0g
3. Gauging trowel conforming to IS: 10086 – 1982

PROCEDURE:-

1. Prepare a paste of weighed quantity of Cement with a weighed quantity of potable or distilled water, taking care that the time of gauging is not less than 3 minutes, nor more than 5 min, and the gauging shall be completed before any sign of setting occurs. The gauging time shall be counted from the time of adding water to the dry cement until commencing to fill the mould. Fill the Vicat mould E with this paste, the mould resting upon a non-porous plate. After completely filling the mould,smoothen the surface of the paste, making it level with the top of the mould. The mould may be slightly shaken to expel the air.

2. Place the test block in the mould, together with the non-porous resting plate, under the rod bearing the plunger; lower the plunger gently to touch the surface of the test block, and quickly release, allowing it to sink into the paste.This operation shah be carried out immediately after filling the mould.

3. Prepare trial pastes with varying percentages of water and test as described above until the amount of water necessary for making up the standard consistency as defined above is found.

CALCULATION:-

Prepare trial pastes with varying percentages of water and test as described above until the amount of water necessary for making up the standard consistency i.e. the vicat apparatus shows a reading of 5 to 7mm.

Sample table for calculation and representation of results for standard consistency of cement is shown below.

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,

AGGREGATE IMPACT VALUE TEST

    With Respect to concrete aggregates,toughness is usually considered the resistance of the material to failure by impact.Several attempts to develop a method of test for aggregates impact value have been made.The most successful and known test is described below..

APPARATUS
i) Impact testing machine conforming to IS: 2386 (Part IV) - 1963
ii) IS Sieves of sizes - 12.5mm, 10mm and 2.36mm
iii) A cylindrical metal measure of 75mm dia. and 50mm depth
iv) A tamping rod of 10mm circular cross section and 230mm length, rounded at one end
v) Oven

PREPARATION OF SAMPLE
i) The test sample should conform to the following grading:
- Passing through 12.5mm IS Sieve 100%
- Retention on 10mm IS Sieve 100%
ii) The sample should be oven-dried for 4hrs. at a temperature of 100 to 110^oC and cooled.
iii) The measure should be about one-third full with the prepared aggregates and tamped with 25 strokes of the tamping rod.A further similar quantity of aggregates should be added and a further tamping of 25 strokes given. The measure should finally be filled to overflow, tamped 25 times and the surplus aggregates struck off, using a tamping rod as a straight edge. The net weight of the aggregates in the measure should be determined to the nearest gram (Weight 'A').

PROCEDURE
i) The cup of the impact testing machine should be fixed firmly in position on the base of the machine and the whole of the test sample placed in it and compacted by 25 strokes of the tamping rod.
ii) The hammer should be raised to 380mm above the upper surface of the aggregates in the cup and allowed to fall freely onto the aggregates. The test sample should be subjected to a total of 15 such blows, each being delivered at an interval of not less than one second.

REPORTING OF RESULTS
i) The sample should be removed and sieved through a 2.36mm IS Sieve. The fraction passing through should be weighed (Weight 'B'). The fraction retained on the sieve should also be weighed (Weight 'C') and if the total weight (B+C) is less than the initial weight (A) by more than one gram, the result should be discarded and a fresh test done.
ii) The ratio of the weight of the fines formed to the total sample weight should be expressed as a percentage.

Aggregate impact value = B/A x 100%

iii) Two such tests should be carried out and the mean of the results should be reported.

A sample proforma for the record of the test results is given below..

    As per IS 283-1970 aggregate  impact value shall no exceed 45% by weight for aggregates used for concrete other than wearing surface and 30% for concrete of wearing surfaces (Run ways,Roads & Pavements)

Soundness Test of Cement

      It is very important that the cement after setting shall not undergo any appreciable change of volume. Certain cements have been found to undergo a large expansion after setting causing disruption of the set and hardened mass.This will cause serious difficulties for the durability of structures when such cement is used.This test is to ensure that cement does not show any appreciable subsequent expansion is of prime importance.

     The unsoundness in cement is due to the presence of excess of lime than that could be combined with acidic oxide at the kiln.This is also due to inadequate burning of insufficiency in fineness of grinding or through mixing of raw materials.It is also likely that too high a proportion of magnesium content or calcium sulphate content may cause unsoundness in cement. For this reason the magnesia content allowed in cement is limited to 6%.

APPARATUS
i) The apparatus for conducting the Le-Chatelier test should conform to IS: 5514 - 1969
ii) Balance, whose permissible variation at a load of 1000g should be +1.0g
iii) Water bath

PROCEDURE
i) The mould is placed on a glass sheet and it is filled with the cement paste formed by gauging cement with 0.78 times the water required to give a paste of standard consistency.
ii) Then the mould is covered with another piece of glass sheet, a small weight is placed on its covering glass sheet and immediately the whole assembly is submerged in water at a temperature of 27 ± 2^oC and kept it there for 24hrs.
iii) The distance separating the indicator points to the nearest 0.5mm is measured (say d _l ).
iv) The mould is submerged again in water at the temperature prescribed above.The water is bought to boiling point in 25 to 30 minutes and it is boiled for 3hrs.
v) Then  the mould is removed from the water, allowed to cool.The distance between the indicator points is measured (say d ₂ ).
vi) (d ₂ – d _l) represents the expansion of cement.

REPORTING OF RESULTS
    The mean of the two values to the nearest 0.5mm to represents the expansion of cement.

Bitumen Content Test

AIM
To determine the bitumen content as per ASTM 2172.

APPARATUS

i) Centrifuge extractor
ii) Miscellaneous - bowl, filter paper, balance and commercial benzene

SAMPLE
Take 500g sample

PROCEDURE
i) If the mixture is not soft enough to separate with a trowel, place 1000g of it in a large pan and warm upto 100^oC to separate the particles of the mixture uniformly.
ii) Place the sample (Weight ‘A’) in the centrifuge extractor. Cover the sample with benzene, put the filter paper on it with the cover plate tightly fitted on the bowl.
iii) Start the centrifuge extractor, revolving slowly and gradually increase the speed until the solvent ceases to flow from the outlet.
iv) Allow the centrifuge extractor to stop. Add 200ml benzene and repeat the procedure.
v) Repeat the procedure at least thrice, so that the extract is clear and not darker than the light straw colour and record the volume of total extract in the graduated vessel.
vi) Remove the filter paper from the bowl and dry in the oven at 110 + 5^oC. After 24hrs., take the weight of the extracted sample (Weight ‘B’).

REPORTING OF RESULTS

Repeat the test thrice and average the results.

Testing of Cement

Testing of Cement can be brought under two categories.
      (a) Field Testing        (b) Laboratory Testing

Field Testing
       It is sufficient to subject the cement to field tests when it is trusted for minor works. The following are the field tests.
(a) Open and the bag and take a good look at the cement. There should not be any visible lumps. The colour of the cement should normally be greening grey.
(b) Thrust your hand into the cement bag. It must give a cool feeling. There should not be any lump inside.
(c) Take a pinch of cement and feel between fingers. It should give a smooth and not a gritty feeling.
(d) Take a handful of cement and through it in to a bucket full of water, the particles should float for some time before they sink.
(e) Take about 100 grams of cement and a small quantity of water and make a stiff paste. From the stiff paste , pat a cake with sharp edges. Put it on a glass plate and slowly take it under water with a bucket.See the shape of the cake is not disturbed while taking it down the bottom of the bucket. After 24 hours the cake should retain its original shape and at the same time it should also set and attain some strength. 
      If a sample of cement satisfies the above field tests it may be concluded that the cement is not bad.The above tests does not really indicates that the cement is really good for important works. For using cement in important and major works it is incumbent on the part of the user to test the cement in the laboratory to confirm the requirements of the Standard Specifications with respect to its physical and chemical properties.The tests which are usually conducted in the Lab to confirm those properties are …
      1.Fineness Test   
      2.Initial and Final Setting Time Test
      3. Consistency of Cement 
      4. Compressive Strength of Cement
      5. Shear Test    
      6. Soundness Test    
      7. Heat of Hydration Test    
      8. Chemical Composition Test

INITIAL AND FINAL SETTING TIME OF CEMENT

Initial and Final Setting Time are two very important properties of  cement which are required  in estimating free time for transporting, placing, compaction and shaping of cement paste.

Initial setting time is the time from mixing dry cement with water till the beginning of interlocking of the gel.
Final setting time is the time from mixing dry cement with water till the end of interlocking of the gel

AIM
To determine the initial and the final setting time of cement as per IS: 4031 (Part 5) - 1988.

APPARATUS

i) VICAT Apparatus.
ii) Digital weighing scale, used to measure the weight of dry cement.
iii) Glass graduates, used to measure the volume of water.
iv) Trowel.
v) Mixing bowl.
vi) Stop-watch.
vii) Portland Pozzolna Cement.
viii) Water.

PROCEDURE
i) Prepare a cement paste by gauging the cement with 0.85 times the water required to give a paste of standard consistency.
ii) Start a stop-watch, the moment water is added to the cement.
iii) Fill the Vicat mould completely with the cement paste gauged as above, the mould resting on a non-porous plate and smooth off the surface of the paste making it level with the top of the mould. The cement block thus prepared in the mould is the test block.

A) INITIAL SETTING TIME
Place the test block under the rod bearing the needle.Lower the needle gently in order to make contact with the surface of the cement paste and release quickly, allowing it to penetrate the test block. Repeat the procedure till the needle fails to pierce the test block to a point 5.0 ± 0.5mm measured from the bottom of the mould.The time period elapsing between the time, water is added to the cement and the time, the needle fails to pierce the test block by 5.0 ± 0.5mm measured from the bottom of the mould, is the initial setting time.

B) FINAL SETTING TIME
Replace the above needle by the one with an annular attachment. The cement should be considered as finally set when, upon applying the needle gently to the surface of the test block, the needle makes an impression therein, while the attachment fails to do so. The period elapsing between the time, water is added to the cement and the time, the needle makes an impression on the surface of the test block, while the attachment fails to do so, is the final setting time.

REPORTING OF RESULTS
The results of the initial and the final setting time should be reported to the nearest five minutes.

CONCLUSION:
To measure the setting times of cement, we have to do our tests on cement of standard consistency. Normal consistency of standard cement can be gained by using the W \ C ratio and depending on 26%- 33%. The higher rate of water the more initial setting time needed.

WATER ABSORPTION TESTS OF AGGREAGATES

      Water Absorption is the percent of water necessary to add to the aggregate to obtain saturated surface dry (SSD) condition.

This test determines the water absorption of coarse aggregates as per IS: 2386 (Part III) - 1963.
APPARATUS
i) Wire basket - perforated, electroplated or plastic coated with wire hangers for suspending it from the balance
ii) Water-tight container for suspending the basket
iii) Dry soft absorbent cloth - 75cm x 45cm (2 nos.)
iv) Shallow tray of minimum 650 sq.cm area
v) Air-tight container of a capacity similar to the basket
vi) Oven
SAMPLE
A sample not less than 2000g should be used.

PROCEDURE
i) The sample should be thoroughly washed to remove finer particles and dust, drained and then placed in the wire basket and immersed in distilled water at a temperature between 22 and 32^oC.
ii) After immersion, the entrapped air should be removed by lifting the basket and allowing it to drop 25 times in 25 seconds. The basket and sample should remain immersed for a period of 24 + ½ hrs. afterwards.
iii) The basket and aggregates should then be removed from the water, allowed to drain for a few minutes, after which the aggregates should be gently emptied from the basket on to one of the dry clothes and gently surface-dried with the cloth, transferring it to a second dry cloth when the first would remove no further moisture. The aggregates should be spread on the second cloth and exposed to the atmosphere away from direct sunlight till it appears to be completely surface-dry. The aggregates should be weighed (Weight 'A').
iv) The aggregates should then be placed in an oven at a temperature of 100 to 110^oC for 24hrs. It should then be removed from the oven, cooled and weighed (Weight 'B').

REPORTING OF RESULTS
Water absorption = (A – B)/B x 100%
Two such tests should be done and the individual and mean results should be reported.

Flow Trough Test

    The flow trough test (Bartos, Sonebi, and Tamimi 2002) is used to measure the workability of highly flowable concretes. It was originally developed for measuring repair concretes.
    The test apparatus consists of a 230 mm wide, 1000 mm long horizontal steel trough. Approximately 6 liters of concrete is placed in a conical hopper and allowed to fall from the hopper onto one end of the trough. The time required for concrete to flow a certain distance (typically 750 mm) down the trough is recorded. The test is conducted three times immediately after mixing and another three times thirty minutes after mixing. The set of tests is conducted at thirty minutes in order to characterize the workability of the concrete at the time of placement. The concrete is agitated every five minutes in the thirty minutes between the initial and final sets of tests.
Advantages:
• The test method is simple and inexpensive.
• The results are a function of the time required for the concrete to flow both out of the cone and down the trough.
Disadvantages:
• The test is only appropriate for highly flowable concrete mixtures.
• The test is not standardized and not widely used.

Flow Table Test (DIN Flow Table)

      The flow table test (Tattersall 1991; Bartos 1992; Wong et al. 2000; Bartos, Sonebi, and Tamimi 2002) measures the horizontal spread of a concrete cone specimen after being subjected to jolting. Multiple versions of the test have been proposed since its original introduction in Germany in the 1930s. The test was added to the British Standards in 1983 in response to the increase use of highly fluid concretes. The test is sometime referred to as the DIN flow table, in
reference to its inclusion in German standard DIN 1048. The test is currently standardized in the Europe as EN 12350-5.
       The apparatus consists of a 700 mm square wooden top plate lined with a thin metal sheet, as shown in Figure. The plate is hinged on one end to a base, while on the other end, clips allow the plate to be lifted a vertical distance of 40 mm. Etched into the metal sheet are two perpendicular lines that cross in the center of the plate and a 200 mm circle concentric with the center of the plate. The frustum of a cone used to mold the concrete is shorter than the slump cone, with a top diameter of 130 mm and with a bottom diameter and height of 200 mm.

      To perform the test, the cone mold is placed in the center of the plate and filled in two layers, each of which is compacted with a tamping rod. The plate is lifted with the attached handle a distance of 40 mm and then dropped a total of 15 times. The horizontal spread of the concrete is measured. Resistance to segregation can be assessed qualitatively: in concrete mixes that are susceptible to segregation, the paste will tend separate from the coarse aggregate around the perimeter of the concrete mass.
     The test is applicable to a wide range of concrete workability, and is especially appropriate for highly fluid mixes that exhibit a collapsed slump. The results of the test can be correlated to slump, although it has been suggested that the initial horizontal spread, prior to jolting, correlates better to slump (Juvas 1994). Despite its simplicity, the test apparatus is large and must be placed on firm, level ground. The jolting of the concrete does not accurately simulate field  practices and cannot easily be treated analytically. In fact, the further the concrete spreads, the thinner the layer of concrete becomes and the less this thin layer represents the bulk properties of the concrete. Research has suggested that spread measurements for different concrete mixtures  converge with an increasing number of drops of the top plate (Tattersall 1991).

Advantages:
• The test is simple and can be used in the field.
• The test quickly provides a direct result.
• The test is dynamic, making it especially appropriate for highly thixotropic concrete mixtures.
Disadvantages:
• The test procedure does not represent actual placement conditions—concrete is typically  vibrated, not jolted.
• The test results tend to converge as the number of drops is increased. Near the end of the  test, the properties of the thin layer of concrete do reflect the bulk properties of the  concrete.
• The results are not given in terms of fundamental units. An analytical treatment of the  test would be difficult.

U-Box Test

       Like L-box test, U-box test measures the filling ability of self-compacting concrete. The U-box test originally developed in Japan and is sometimes referred to as the box-shaped test. Like other workability tests for self-compacting concrete, the U-box test is also applicable to highly flowable concretes and underwater concretes.

APPARATUS
        As shown in Figure the apparatus consists of a  U-shaped box. Concrete is placed in the left side of the box. An alternative version of the apparatus features a flat bottom instead of a curved bottom. Ideally, the box should be made of clear plastic to permit the observation of the concrete in the box. To start the test, the door dividing the two halves of the box is opened and concrete is allowed to flow from the left half of the box into the right half. Reinforcing bars are placed at the location of the door. Although the spacing of the bars is adjustable, the most common arrangement is 13 mm diameter bars with a clear spacing of 35 mm. The time from the opening of the door until the concrete ceases to flow is recorded. The height of the concrete in each side of the box is measured. A truly self-leveling fluid will rise to the same height on each side of the box. Concrete with good filling ability should reach a height of at least 30 cm on the right side of the box. In some versions of the test, a surcharge load is applied to the concrete on the left side of the box. This surcharge load is unnecessary for self-compacting concrete and is generally not used.
       With both the L-box and U-box tests, it is unknown what significance the effect of friction between the concrete and the walls has on the test results.

L-Box Test

      The L-box test is used to  measure the filling and passing ability of self-compacting concrete.This test was originally developed in Japan for underwater concrete, the test is also applicable for highly flowable concrete. 
        The apparatus consists of an L-shaped box, shown in Figure below. Concrete is initially placed in the vertical portion of the box, which measures 600 mm in height and 100 mm by 200 mm in section. A door between the vertical or horizontal portions of the box is opened and the concrete is allowed to flow through a line of vertical reinforcing bars and into the 700 mm long, 200 mm wide, and 150 mm tall horizontal portion of the box. In the most common arrangement of reinforcing bars, three 12 mm bars are spaced with a clear spacing of 35 mm. Generally, the spacing of the reinforcing bars should be three times the maximum aggregate size. It should be noted that various dimensions for the L-box have been used and no one set of dimensions is considered official; however, the dimensions described above seem to be the most common.

     The time for concrete to reach points 20 cm (T₂₀) and 40 cm (T₄₀) down the horizontal portion of the box is recorded. After the concrete comes to rest in the apparatus, the heights of the concrete at the end of the horizontal portion, H2, and in the vertical section, H1, are measured. The blocking ratio, H₂/H₁, for most tests should be 0.80 to 0.85. If the concrete being tested is truly self-leveling, like water, the value of the blocking ratio will be unity. Segregation resistance can be evaluated visually. A concrete sample with coarse aggregate particles that reach the far end of the horizontal part of the box exhibits good resistance to segregation. The L-box can be disassembled after the concrete has hardened. By cutting out samples of the hardened concrete, additional information about the concrete’s resistance to segregation can be determined, as shown by Tanaka et al. (1993).
        While the test does give valuable information about filling and passing ability, and to a lesser extent, segregation resistance, the test is not as simple as the slump flow test. Since there are no standardized dimensions, results from different test apparatuses cannot be compared directly.

 

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.

Powers Remolding Test

      The Powers remolding test (Powers 1968; Scanlon 1994; Wong et al. 2000) is similar to the Vebe consistometer. The test was develop by Powers and first presented in 1932. The test has been standardized by the US Army Corp of Engineers as CRD C6-74.
       The test apparatus consists of a 12 inch diameter cylindrical mold mounted on a standard drop table, described in ASTM C124 (which was withdrawn in 1973). A separate 8 ¼ inch diameter ring is attached at the top of the cylinder, as shown in Figure below. The concrete sample is compacted in the standard slump cone inside of the inner ring. Like the Vebe consistometer, a clear plate attached to a vertical stem rests on top of the concrete. The number of drops required to remold the concrete to the shape of the outer cylinder is a measurement of the “remolding effort.” The ring attached to the outer cylinder restricts the movement of the concrete and allows for the determination of the plastic shear capacity of the concrete mix. A mix with high shear capacity easily passes under the ring, whereas mixes with low shear capacity tend to clog and result in greater required remolding effort. It is possible that two mixes that require the same remolding effort when the ring is removed require different remolding efforts when the ring is in place. Research has shown that the Powers remolding test is more sensitive to changes in workability than the slump test (Scanlon 1994).

Advantages:
• The Powers remolding test is a dynamic test and is suitable for low slump concretes.
• The results of the test are obtained directly.
Disadvantages:
• The drop table must be mounted on an object of sufficient mass to absorb vibrations created by the drop table. Accordingly, the device is likely to be too large and bulky for field use.
• The test method is only suitable for low slump concretes.
• No analytical treatment or experimental testing of the test method has been performed to relate the test results to yield stress and/or plastic viscosity

Delivery-Chute Torque Meter

       The delivery-chute torque meter (US patent 4,332,158; Wong et al. 2000) is designed to measure the consistency of concrete as it exits a concrete mixing truck. The intent of the device is to measure slump accurately without having to wait for the conventional slump test to be performed.
      The hand-held device, which is shown in Figure 7, is inserted in flowing concrete in the delivery chute of a concrete mixing truck. The two curved sensing blades are attached to a vertical member that measures torque. The device is inserted in the delivery chute such that the sensing blades are orthogonal to the flow of concrete. The flowing concrete applies approximately
equivalent forces to each of the two sensing blades. These forces create opposing moments on the inner vertical member. Since the length of the moment arm for the right sensing blade is approximately twice that of the moment arm for the left sensing blade, a net torque is applied to the inner vertical member. The operator manually applies an opposing torque to the outer housing to keep the blades orthogonal to the flow of concrete. The magnitude of this applied torque is indicated on the flat circular plate located just above the two sensing blades. The torque measured with the device is correlated to slump, with the appropriate correlation marked on the circular plate. For concretes with different viscosities, different calibrations must be obtained. The geometry of the device allows the device to adjust automatically to changes in flow velocity and height.

Advantages:
• The device measures the workability of the concrete as it exits the mixer before it is placed.
• The torque (and associated slump) is read directly from the device. No computer or other sensing devices are required to determine slump.
Disadvantages:
• The torque meter is a single-point test that gives no indication of plastic viscosity. Readings are made at only one shear rate.
• The device must be calibrated for each concrete mixture.

J-Ring Test

       The J-ring test (EFNARC 2002; Bartos, Sonebi, and Tamimi 2002) extends common filling ability test methods to also characterize passing ability.
       The J-ring test device can be used with the slump flow test, the orimet test, or the V-funnel test. The J-ring, as shown in Figure 25, is a rectangular section (30 mm by 25 mm) open steel ring with a 300 mm diameter. Vertical holes drilled in the ring allow standard reinforcing bars to be attached to the ring. Each reinforcing bar is 100 mm long. The spacing of the bars is adjustable,
although 3 times the maximum aggregate size is typically recommended. For fiber-reinforced concrete, the bars should be placed 1 to 3 times the maximum fiber length.

     To conduct the J-ring test in conjunction with the slump flow test, the slump cone is placed in the center of the J-ring and filled with concrete. The slump cone is lifted and concrete is allowed to spread horizontally through the gaps between the bars. Alternatively, the orimet device or the Vfunnel can be positioned above center of the J-ring. Instead of measuring just the time for concrete to exit the orimet or the V-funnel, the concrete is also allowed to spread horizontally through the J-ring.
     Various interpretations of the test results have been suggested. The measures of passing ability and filling ability are not independent. To characterize filling ability and passing ability, the horizontal spread of the concrete sample is measured after the concrete passes through the gaps in the bars of the J-ring and comes to rest. Also, the difference in height of the concrete just inside the bars and just outside the bars is measured at four locations. The smaller this difference in heights is, the greater the passing ability of the concrete will be. Alternatively, the horizontal spread with and without the J-ring can be compared as a measure of passing ability.

Angles Flow Box Test

       The Angles flow box test (Scanlon 1994; Wong et al. 2000) attempts to simulate typical concrete construction in order to characterize the ease with which concrete can be placed. The test measures the ability of concrete to flow under vibration and to pass obstructions.
        The device consists of a rectangular box mounted on a vibrating table. Two adjacent vertical partitions are placed in the middle of the box to divide the box in half. The first partition consists of a screen of circular bars that are spaced so that the openings between the bars are the size of the maximum aggregate. The second partition is a solid, removable plate that initially holds concrete on one side of the box prior to the beginning of the test. After concrete has been loaded on one side of the box, the solid partition is removed and the vibrating table is started. The time for the concrete to pass through the screen and form a level surface throughout the box is recorded. The amount of bleeding and segregation that occurs during vibration can be observed visually. 
         Very little data is available on the validity of the test and on interpretation of the test results. The test method would not be appropriate for very low slump mixes. For highly flowable concrete mixtures, vibration may be unnecessary. A similar concept is used to test the workability of selfcompacting concrete.
Advantages:
• The test method represents actual field conditions. It is a dynamic test that subjects  concrete to vibration.
• The ability of concrete to pass obstructions and resist segregation is assessed.
Disadvantages:
• The test is bulky and would probably not be appropriate for field use.
• The test result is likely a function of both yield stress and plastic viscosity, although these  values are not directly recorded.

CEMAGREF-IMG

      The CEMAGREF-IMG (Ferraris and Brower 2001) is a large coaxial-cylinders rheometer originally developed to measure mud-flow rheology, but which has also been used to measure concrete rheology. Only one prototype of the device exists.
       Since the CEMAGREF-IMG was not initially intended to measure the rheology of concrete, it is significantly larger than other rheometers. In fact, the large size of the device makes it impractical for measuring concrete. The outer cylinder is 120 cm in diameter and 90 cm tall, while the inner cylinder is 76 cm in diameter. The rheometer holds 500 liters of concrete and is mounted on a trailer. The inner cylinder rotates and measures torque while the outer cylinder
remains stationary. Blades on the outer cylinder and a metallic grid on the inner cylinder reduce concrete slippage. Since the inner cylinder is mounted within the outer cylinder from the bottom instead of from the top, a rubber seal is provided at the base of the inner cylinder to ensure that all concrete remains within the gap between the cylinders. The torque on the inner cylinder at various rotation speeds is logged and used to calculate yield stress and plastic viscosity.
        Although the large dimensions of the CEMAGREF-IMG allow the testing of concrete mixtures with large maximum aggregate sizes, the ratio of the outer radius to the inner radius is too large. As a result, plug flow occurs as the concrete near the inner cylinder is sheared while the shear stress applied to the concrete near the outer cylinder is insufficient to overcome the yield stress
of the concrete. The large size of the CEMAGREF-IMG also makes the device impractical to transport.
Advantages:
• The device measures yield stress and plastic viscosity.
• The size of the device accommodates large maximum aggregate sizes.
Disadvantages:
• The device was not originally designed to measure concrete and is too large for common field use.
• The geometry of the device should be improved to more accurately measure concrete rheology.
• The seals at the bottom of the inner cylinder must be replaced periodically and must be accounted for in the device’s calibration.

Vibrating Slope Apparatus (VSA)

      Originally developed in the 1960s, the vibrating slope apparatus (Wong et al. 2000) was recently modified by the US Army Engineering Research and Development Center (ERDC) for the US Federal Highway Administration (FHWA). The device measures the workability of low slump concretes subjected to vibration at two different shear rates in order to determine a “workability
index” that is related to plastic viscosity and a “yield offset” that is related to yield stress. Researchers at the ERDC selected the vibrating slope apparatus over twenty other workability test devices as a superior choice to measure the workability of low slump concretes in the field.
       The vibrating slope apparatus as modified by the US Army Engineering Research and Development Center is shown in Figure 15. Concrete to be tested is placed in the chute, which can be set at a predefined angle. Three load cells continuously measure the mass of concrete in the chute during the test. Small transverse metal strips reduce slip between the concrete and the
bottom of the chute. A vibrator is mounted to the bottom of the chute. Eight vibration dampers ensure that the vibration is applied to the concrete and that the entire apparatus does not excessively vibrate and interfere with load cell measurements. Readings from the load cells are transmitted to a laptop computer, where the workability index and yield offset are calculated. The entire apparatus is designed to be rugged and easily portable.

      To operate the device, concrete is placed in the chute, which is set at a predefined angle (typically 10-15 degrees). The gate is opened and the vibrator is started, allowing concrete to fall from the chute into a bucket. The data from the load cells is used to calculate the discharge rate. Since the discharge rate generally decreases as concrete flows out of the chute, the maximum discharge rate is recorded. The test procedure is repeated a second time for a different incline angle. The results of the test are plotted as a graph of maximum discharge rate versus discharge angle. The straight line connecting the two data points is defined by Equation [1]:

                                                  R = WA + C                                                     [1]

where R = maximum discharge rate, W = workability index, A = discharge angle, and C = calculated yield offset.

        The intent of the research conducted by the ERDC for the FHWA was simply to determine if the vibrating slope apparatus would operate properly, not whether the device could accurately measure concrete rheology. The results of the preliminary ERDC laboratory testing were compared only to the slump and air content of each concrete mixture. Further, no analytical treatment of the test has been presented. Wong et al. (2000) claims that the y-intercept of the discharge rate versus discharge angle plot is the yield stress and that the slope of this plot is the dynamic viscosity; however, no effort is made to relate these parameters to fundamental units or confirm the validity of the test results. Since the yield stress of vibrated concrete is lower than the yield stress of unvibrated concrete, the yield stress recorded by the vibrating slope apparatus is not equivalent to the yield stress of the unvibrated concrete and is only applicable for the specific vibration applied by the vibrating slope apparatus. Before the vibrating slope apparatus can be used on a wider basis, the validity of the test results must be verified.
        The ERDC researchers encountered multiple problems in developing the vibrating slope apparatus prototype. Many of the problems were trivial and easily corrected. Other problems will require further work to resolve. The test device is large, bulky, and weighs 350 pounds. The ERDC researchers give no cost information in their report and do not compare the cost effectiveness of the vibrating slope apparatus to other test methods.

Advantages:
• Unlike many rheometers, the device measures the workability of low slump concretes.
• The results of the device are given in terms of parameters related to yield stress and
plastic viscosity.
• The device is designed to be rugged for field use.
Disadvantages:
• The results of the device have not been verified analytically or experimentally.
• The device is large, bulky, and heavy.
• Although the researchers have proposed using an embedded electronic device to record test data, the vibrating slope apparatus at this point still requires a notebook computer.
• The results of the test are only applicable for conditions with the same vibration as the vibration applied by the device.
• The shear rate is non-uniform throughout the test. The shear rate decreases as the mass of concrete in the chute decreases.