The Plate Load Test: its Procedure and Equation for Calculation

A Plate Load Test is a valuable method to determine the strength of the soil beneath the surface. It's akin to gently pushing the ground to gauge its capacity to withstand the weight. In this post, we'll walk you through the straightforward procedure of this test:

Step 1: At a specified depth (D), create a pit measuring 5 times the size of the plate (Bp) in both length and width. The plate load test is conducted by drilling a hole of dimensions Bp x Bp within the pit, as depicted in the figure below.

Step 2: Insert a sizable flat plate into the hole. The plate is connected to a machine capable of applying weight. Begin by applying and releasing an initial load of 7 kN/m².

Step 3: Slowly increment the weight on the plate. The plate can be loaded using Kentledge or through a truss/beam reaction.

Step 4: Apply loads in increments of 20% of the safe load or 1/10th of the ultimate load.

Step 5: Record the settlement at intervals of 1, 5, 10, 20, and 60 minutes, and subsequently at 2-hour intervals.

Step 6: Increase Load When the settlement rate reaches 2 mm per minute, apply a higher load.

Step 7: Repeat the test until failure occurs or a 20 mm settlement is reached.

Analyzing the Results: Pay close attention to the results. Consider the amount of weight added before the soil's compression. This weight offers insights into the soil's strength. If the soil compresses significantly, it may not be very strong. Conversely, minimal compression indicates greater strength.

Bearing Capacity: 

Bearing capacity mirrors ground strength. It signifies the weight the soil can bear before sinking or collapsing.

Calculation of Bearing Capacity from Plate Load Test:

The load-settlement curve is drawn after collecting field data. It is a logarithmic graph with the applied load on the X-axis and the settlement on the Y-axis. The ultimate load for the plate is obtained from the graph, which is the corresponding load for one-fifth of the plate width settlement.

The curve is broken at one point when the points are plotted on the graph. The ultimate load on the plate is the appropriate load to that breakpoint. The ultimate bearing capacity can be estimated using the plate's ultimate load. To estimate the safe bearing capacity of soil from the foundation, the ultimate bearing capacity is divided by a suitable factor of safety.

Bearing Capacity for Clay: For clayey soil, calculate the ultimate bearing capacity using the equation:

Ultimate Bearing Capacity = Ultimate Load for the Plate

Bearing Capacity for Sand: For sandy soil, the equation is slightly differs: 

Ultimate Bearing Capacity = Ultimate Load for the Plate * (Width of pit/Size of pit)

Safe Bearing Capacity = Ultimate Bearing Capacity/Factor of Safety

Typically, the range for the factor of safety varies from 2 to 3

You can calculate the ultimate bearing capacity by inputting values for cohesion, unit weight, plate width, and soil factors. This reveals how much weight the soil can support per unit area. The Plate Load Test and understanding bearing capacity are fundamental tools in engineering, ensuring that the ground beneath us is sturdy and reliable.

COMPRESSION TESTING MACHINE

Compression Strength:-

It means the maximum compressive stress that under gradually applied load a given solid material will sustain without fracture.

Compression tests:-

Well these are used to determine how a product or material reacts when it is compressed, squashed, crushed or flattened by measuring fundamental parameters that determine the specimen behaviour under a compressive load. These include the elastic limit, which for "Hooke's" materials is approximately equal to the proportional limit, and also known as yield point or yield strength, Young's Modulus (these, although mostly associated with tensile testing, may have compressive analogs) and compressive strength.

Procedure to Conduct test as follows:-

1) Preparation : Check all the things you need are ready. Check concrete compression machine is in working order.

2) Safety : Wear hand gloves and safety goggles.

3) Taking measurement : Take the measurement of concrete specimens (which are sent to laboratory for testing). Calculate the cross sectional area (unit should be on mm2) and put down on paper. Do the same for each specimen.

4) Start machine : Turn on the machine. Place one concrete specimen in the centre of loading area.

5) Lowering piston : Lower the piston against the top of concrete specimen by pushing the lever. Don't apply load just now. Just place the piston on top of concrete specimen so that it's touching that.

6) Applying load: Now the piston is on top of specimen. It is the time to apply load. Pull the lever into holding position. Start the compression test by Pressing the zero button on the display board.

7) Increasing pressure : By turning pressure increasing valve counter-clockwise, adjust the pressure on piston so that it matches concrete compression strength value. Apply the load gradually without shock.

8) Test is complete : Observe the concrete specimen. When it begins to break stop applying load.

9) Recording : Record the ultimate load on paper displaying on machine's display screen.

10) Clean the machine: When the piston is back into its position, clean the creaked concrete from the machine.

11) Turning off machine: Match your record once again with the result on display screen. The result should still be on display screen. And then turn off the machine.

12) Calculate concrete compressive strength : The result we got from testing machine is the ultimate load to break the concrete specimen.

*)The load unit is generally in lb. We have to convert it in newton (N). Our purpose is, to know the concrete compressive strength.

*)We know, compressive strength is equal to ultimate load divided by cross sectional area of concrete specimen. We took the concrete specimen's measurement before starting the testing and calculated cross sectional area.

*)Now we got the ultimate load. So we can now calculate the concrete compressive strength.

Compressive strength = Ultimate load (N) / cross sectional area (mm2).

The unit of compressive strength will be N/mm2.

Concrete Slump Test

Slump test is a method used to determine the consistency of concrete. The consistency, or stiffness, indicates how much water has been used in the mix. The stiffness of the concrete mix should be matched to the requirements for the finished product quality Concrete Slump Test.

The concrete slump test is used for the measurement of a property of fresh concrete. The test is an empirical test that measures the workability of fresh concrete. More specifically, it measures consistency between batches. The test is popular due to the simplicity of apparatus used and simple procedure.

Procedure To conduct the Slump Test as follows:-

1) To obtain a representative sample, take samples from two or more regular intervals throughout the discharge of the mixer or truck. DO NOT take samples at the beginning or the end of the discharge.

2) Dampen inside of cone and place it on a smooth, moist, non-absorbent, level surface large enough to accommodate both the slumped concrete and the slump cone. Stand or, foot pieces throughout the test procedure to hold the cone firmly in place.

3) Fill cone 1/3 full by volume and rod 25 times with 5/8-inchdiameter x 24-inch-long hemispherical tip steel tamping rod. (This is a specification requirement which will produce nonstandard results unless followed exactly.) Distribute rodding evenly over the entire cross section of the sample

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4) Fill cone 2/3 full by volume. Rod this layer 25 times with rod penetrating into, but not through first layer. Distribute rodding evenly over the entire cross section of the layer.

5) Fill cone to overflowing. Rod this layer 25 times with rod penetrating into but not through, second layer. Distribute rodding evenly over the entire cross section of this layer

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6) Remove the excess concrete from the top of the cone, using tamping rod as a screed. Clean overflow from base of cone.

7) Immediately lift cone vertically with slow, even motion. Do not jar the concrete or tilt the cone during this process. Invert the withdrawn cone, and place next to, but not touching the slumped concrete. (Perform in 5-10 seconds with no lateral or torsional motion)

8) Lay a straight edge across the top of the slump cone. Measure the amount of slump in inches from the bottom of the straight edge to the top of the slumped concrete at a point over the original center of the base. The slump operation shall be completed in a maximum elapsed time of 2 1/2 minutes. Discard concrete. DO NOT use in any other tests.

Types of Slump:-

1) Collapse Slump:-

In a collapse slump the concrete collapses completely. A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which slump test is not appropriate.

2) Shear Slump:-

In a shear slump the top portion of the concrete shears off and slips sideways.

If a shear or collapse slump is achieved, a fresh sample should be taken and the test is repeated.

If the shear slump persists, as may the case with harsh mixes, this is an indication of lack of cohesion of the mix.

3) True Slump:-

In a true slump the concrete simply subsides, keeping more or less to shape.

This is the only slump which is used in various tests. Mixes of stiff consistence have a Zero slump, so that in the rather dry range no variation can be detected between mixes of different workability.

DETERMINATION OF LIQUID LIMIT OF SOIL

This is best method for determination of liquid limit in the laboratory with the aid of the standard mechanical liquid limit device, designed by Arthur Casagrande and adopted by the ISI, as given in IS:2720 (Part V)–1985. The apparatus required are the mechanical liquid limit device, grooving tool, porcelain evaporating dish, flat glass plate, spatula, palette knives, balance, oven wash bottle with distilled water and containers. The soil sample should pass 425–μ IS Sieve. A sample of about 1.20 N should be taken. Two types of grooving tools—Type A (Casagrande type) and Type B (ASTM type)—are used depending upon the nature of the soil.

The cam raises the brass cup to a specified height of 1 cm from where the cup drops upon the block exerting a blow on the latter. The cranking is to be performed at a specified rate of two rotations per second. The grooving tool is meant to cut a standard groove in the soil sample just prior to giving blows.

Air-dried soil sample of 1.20 N passing 425–μ I.S. Sieve is taken and is mixed with water and kneaded for achieving uniformity. The mixing time is specified as 5 to 10 min. by some authorities. The soil paste is placed in the liquid limit cup, and levelled off with the help of the spatula. A clean and sharp groove is cut in the middle by means of a grooving tool. The crank is rotated at about 2 revolutions per second and the number of blows required to make the halves of the soil pat separated by the groove meet for a length of about 12 mm is counted. The soil cake before and after the test are shown in below figThe water content is determined from a small quantity of the soil paste.

This operation is repeated a few more times at different consistencies or moisture contents. The soil samples should be prepared at such consistencies that the number of blows or shocks required to close the groove will be less and more than 25. The relationship between the number of blows and corresponding moisture contents thus obtained are plotted on semi-logarithmic graph paper, with the logarithm of the number of blows on the x-axis, and the moisture contents on the y-axis. The graph thus obtained, i.e., the best fit straight line, is referred to as the "Flow-graph’ or ‘Flow curve’.

Casagrande apparatus

The moisture content corresponding to 25 blows from the flow curve is taken as the liquid limit of the soil. This is the practical definition of this limit with specific reference to the liquid limit apparatus and the standard procedure recommended. Experience indicates that such as curve is actually a straight line.

The equation to this straight line will be

(w2 – w1) = If log10(N1/N2)

where w1 and w2 are the water contents corresponding to the number of blows N1 and N2 and If is the slope of the flow curve, called the ‘flow index’.

If = (w2 – w1)/log10 (N1/N2)

If the flow curve is extended such that N1 and N2 correspond to one log-cycle difference, If will be merely the difference of the corresponding water contents.

One-point Method Attempts have been made to simplify the trial and error procedure of the determination of liquid limit described above. One such is the ‘One-point method’ which aims at determining the liquid limit with just one reading of the number of the blows and the corresponding moisture content. The trial moisture content should be as near the liquid limit as possible. This can be done with a bit of experience with the concerned soils. For soils with liquid limit between 50 and 120%, the accepted range shall require 20 to 30 drops to close the groove. For soils with liquid limit less than 50%, a range of 15 to 35 drops is acceptable. At least two consistent consecutive closures shall be observed before taking the moisture content sample for calculation of the liquid limit. The test shall always proceed from the drier to the water condition of the soil. (IS: 2720, Part V-1970).

The water content wN of the soil of the accepted trial shall be calculated. The liquid limit

wL of the soil shall be calculated by the following relationship.

wL = wN(N/25)x .

where

N = number of drops required to close the groove at the moisture content wN. Preliminary work indicates that x = 0.092 for soils with liquid limit less than 50% and x = 0.120 for soils with liquid limit more than 50%.

Standard Consistency Test and Setting time test for Concrete

Standard Consistency Test

For finding out initial setting time, final setting time and soundness of cement, and strength a parameter known as standard consistency has to be used. It is pertinent at this stage to describe the procedure of conducting standard consistency test. The standard consistency of a cement paste is defined as that consistency which will permit a Vicat plunger having 10 mm diameter and 50 mm length to penetrate to a depth of 33-35 mm from the top of the mould shown in below fig. The appartus is called Vicat Appartus. This appartus is used to find out the percentage of water required to produce a cement paste of standard consistency.

The standard consistency of the cement paste is some time called normal consistency (CPNC). The following procedures is adopted to find out standard consistency. Take about 500 gms of cement and prepare a paste with a weighed quantity of water (say 24 per cent by weight of cement) for the first trial. The paste must be prepared in a standard manner and filled into the Vicat mould within 3-5 minutes. After completely filling the mould, shake the mould to expel air. A standard plunger, 10 mm diameter, 50 mm long is attached and brought down to touch the surface of the paste in the test block and quickly released allowing it to sink into the paste by its own weight. Take the reading by noting the depth of penetration of the plunger. Conduct a 2nd trial (say with 25 per cent of water) and find out the depth of penetration of plunger. Similarly, conduct trials with higher and higher water/cement ratios till such time the plunger penetrates for a depth of 33-35 mm from the top. That particular percentage of water which allows the plunger to penetrate only to a depth of 33-35 mm from the top is known as the percentage of water required to produce a cement paste of standard consistency. This percentage is usually denoted as ‘P’. The test is required to be conducted in a constant temperature (27° + 2°C) and constant humidity (90%).

Setting time of Concrete

Setting Time:

Initial setting time and final setting time are the two important physical properties of cement. Initial setting time is the time taken by the cement from adding of water to the starting of losing its plasticity. Final setting time is the time lapsed from adding of the water to complete loss of plasticity. Vicat apparatus is used for finding the setting times. Vicat apparatus consists of a movable rod to which any one of the three needles shown in figure can be attached. An indicator is attached to the movable rod. A vicat mould is associated with this apparatus which is in the form of split cylinder.

Before finding initial and final setting time it is necessary to determine water to be added to get standard consistency. For this 300 gms of cement is mixed with about 30% water and cement paste prepared is filled in the mould which rests on non porous plate. The plunger is attached to the movable rod of vicat apparatus and gently lowered to touch the paste in the mould. Then the plunger is allowed to move freely. If the penetration is 5 mm to 7 mm from the bottom of the mould, then cement is having standard consistency. If not, experiment is repeated with different proportion of water fill water required for standard consistency is found. Then the tests for initial and final setting times can be carried out as explained below:

Initial Setting Time:

300 gms of cement is thoroughly mixed with 0.85 times the water for standard consistency and vicat mould is completely filled and top surface is levelled. 1 mm square needle is fixed to the rod and gently placed over the paste. Then it is freely allowed to penetrate. In the beginning the needle penetrates the paste completely. As time lapses the paste start losing its plasticity and offers resistance to penetration. When needle can penetrate up to 5 to 7 mm above bottom of the paste experiment is stopped and time lapsed between the addition of water and end if the experiment is noted as initial setting time.

Final Setting Time.

The square needle is replaced with annular collar. Experiment is continued by allowing this needle to freely move after gently touching the surface of the paste. Time lapsed between the addition of water and the mark of needle but not of annular ring is found on the paste. This time is noted as final setting time.

Tests For Aggregates (Elongation and Flakiness Tests)

Tests For Aggregates

1) Determination Of Flakiness Index

The flakiness index of aggregate is the percentage by weight of particles in it whose least dimension (thickness) is less than three-fifths of their mean dimension. The test is not applicable to sizes smaller than 6.3 mm.

This test is conducted by using a metal thickness gauge, of the description shown in below fig A sufficient quantity of aggregate is taken such that a minimum number of 200 pieces of any fraction can be tested. Each fraction is gauged in turn for thickness on the metal gauge. The total amount passing in the guage is weighed to an accuracy of 0.1 per cent of the weight of the samples taken. The flakiness index is taken as the total weight of the material passing the various thickness gauges expressed as a percentage of the total weight of the sample taken. Table 3.18 shows the standard dimensions of thickness and length gauges.

2) Determination Of Elongation Index

The elongation index on an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than 1.8 times their mean dimension. The elongation index is not applicable to sizes smaller than 6.3 mm.

This test is conducted by using metal length gauge of the description shown in below fig.

A sufficient quantity of aggregate is taken to provide a minimum number of 200 pieces of any fraction to be tested. Each fraction shall be gauged individually for length on the metal guage.

The guage length used shall be that specified in column of 4 of Table 3.18 for the appropriate size of material. The total amount retained by the guage length shall be weighed to an accuracy of at least 0.1 per cent of the weight of the test samples taken. The elongation index is the total weight of the material retained on the various length gauges expressed as a percentage of the total weight of the sample gauged. The presence of elongated particles in excess of 10 to 15 per cent is generally considered undesirable, but no recoganised limits are laid down.