Identification of a Flow State
Identification of a Flow State, or Identifying Plastic Deformation, during the Flow Table Test (FTT) procedure stated in Appendix 2 of the International Maritime Solid Bulk Cargoes Code (IMSBC Code) is considered to be one of the most complicated aspects of the test as it is based on the operators interpretation of when the sample is showing 'Plastic Deformation'. There are many techniques used by reputable laboratories to identify the flow state during the Flow Table Test, in order to determine the Flow Moisture Point (FMP) and therefore the Transportable Moisture Limit (TML), that are not mentioned in the procedure in the IMSBC Code.
IMSBC Code Method
- A flow state is considered to have been reached when the moisture content and compaction of the sample produce a level of saturation such that plastic deformation occurs (In certain conditions, the diameter of the cone may increase before the flow moisture point is reached, due to low friction between the grains rather than to plastic flow. This must not be mistaken for a flow state.). At this stage, the moulded sides of the sample may deform, giving a convex or concave profile.
- With repeated action of the flow table, the sample continues to slump and to flow outwards. In certain materials, cracks may also develop on the top surface. Cracking, with the appearance of free moisture, is not, however, an indication of development of a flow state. In most cases, measurement of the deformation is helpful in deciding whether or not plastic flow has occurred. A template which, for example, will indicate an increase in diameter of up to 3 mm in any part of the cone is a useful guide for this purpose. Some additional observations may be useful. For example: when the (increasing) moisture content is approaching the FMP, the sample cone begins to show a tendency to stick to the mould. Further, when the sample is pushed off the table, the sample may leave tracks (stripes) of moisture on the table. If such stripes are seen, the moisture content may be above the FMP: the absence of tracks (stripes) is not necessarily an indication of being below the FMP.
- Measuring the diameter of the cone, at the base or at half height, will always be useful. By addition of water in increments of 0.4% to 0.5% and applying 25 drops of the flow table, the first diameter increase will generally be between 1 and 5 mm and after a further increment of water the base diameter will have expanded by between 5 and 10 mm.
Identifying the flow state, or when the sample is above the FMP, can sometimes be difficult to see visually, especially if there is significant amounts of course particles. Measurements and visual interpretation of the sample, in combination, are used to help determine whether the sample has reached the flow state. As the sample gets closer to the FMP, it may be harder to determine whether it is showing plastic deformation. Because the flow table test is based on the operators interpretation of the state of the sample, it may be important for the operator to take additional measurements to indicate whether it is showing plastic deformation.
Some of the additional measurements that are commonly recorded are as follows:
|Height||Measurement from the top of the sample to the base.||If the sample is not breaking up (as described below), a reduction in height could indicate the sample is slumping and therefore could indicate plastic deformation.|
|Top Width||Measurement of the diameter of the top of the sample.||When plastic deformation of the sample occurs it is common to see a reduction in the diameter of the top of the sample. This measurement is sometimes hard to take if the sample contains coarse particles.|
|Middle Width||Measurement of the diameter of the middle of the sample.||Although not accurate if a datum is not available, when plastic deformation of the sample occurs the middle of the sample may expand or contract at a consistent rate.|
|Base Width||Measurement of the diameter of the base of the sample.||When plastic deformation of the sample occurs the base of the sample will expand at somewhat linear rate. This measurement is see to be the most important and therefore the average of four points around the base is commonly taken.|
Because the flow table test is based on the operators interpretation of the state of the sample, it may be important for the operator to make notes on visual observations so if they referring back to the results they can clearly identify the state the tested sample was in and to corroborate the final results.
Some observations, with relevant interpretations, that may be useful to note when performing the flow table test are as follows:
|No Measurable Edges, Breaking Up or Crumbling||When there are no edges because the sample has broken up (base, top, height, etc.)||This could indicate the sample is not showing plastic deformation, therefore it is under the flow moisture point. This is because the sample has most likely broken up because of insufficient friction between the particles. More moisture is most likely needed in order show plastic deformation. It also could indicate the particles within the sample are too coarse or uniform to show plastic deformation.|
|Moving||With each drop of the flow table the sample moves or jumps.||If after 50 blows the sample is still showing this characteristic, it is most likely indicating that the sample is under the flow moisture point, but if after a number of successive drops of the table this characteristic changes, it could indicate that the sample is over the flow moisture point. This is because as pore pressures within the sample alter the density and degree of saturation it can force moisture to the surface and this is when plastic deformation may begin to occur. When the sample is jumping or moving it indicates there is no cohesion of the sample to the flow table top and when there is no cohesion between the particles the sample usually does not show plastic deformation.|
|Bleed Water||When free moisture is being expelled from the sample during dropping the flow table.||This is a good indication that the same is very close to the flow moisture point. It does not indicate whether it is over or under the FMP, but with the addition of other observations and measurements it can be determined.|
|Pulsing||This is when with each drop of the flow table the sample appears to pulse slightly (expand and contract).||This is a good indication that there are excess pore pressures building up within the sample. Excess pore pressures can produce a reduction in the shear strength of the sample and may eventually cause it to show plastic deformation. Additional observations and measurements are needed to prove if this is the case.||No Example|
|Smearing||Smearing of moisture and fine particles from the sample on the flow table top.||Although it is a good note to take, this is not necessary a good indication of whether the sample is showing plastic deformation or over/under the flow moisture point. This is because depending on the sample and amount of fines, smearing may occur well below the flow moisture point.||Notice the smearing in the top image when the sample is not showing plastic deformation.|
|Seepage of Moisture from Mould||When moisture from the sample seeps from the base of the flow table mould while tamping/compacting the sample.||Although it is a good note to take, this also is an indication of whether a sample is is going to show plastic deformation or whether it is over/under the flow moisture point. This indicates that there free moisture within the sample. Free moisture indicates the sample is close to saturation where it is likely to more liable to show plastic deformation||No Example|
Stages of a Flow Table Test
- International Maritime Organisation, International Maritime Solid Bulk Cargoes Code, 2013 Edition, London: International Maritime Organization.
- Munro, M. and A. Mohajerani, Determination of Transportable Moisture Limit of Iron Ore Fines for the Prevention of Liquefaction in Bulk Carriers. Marine Structures, 2015. 40(1): p. 193-224.