Introduction
Blending material gradations to obtain consistent
characteristics and performance can be a tremendous challenge, especially when
faced with sources that can change over time.
Without tools to show you both mathematically and graphically what is
happening, means that any adjustments to the blend are like a shot in the
dark. Not only can this become an
endless cycle of trial and error, it can be costly.
But blending materials is not just about maintaining
consistency, it’s also about optimizing blends to meet specific
requirements. That requirement may be to
produce a concrete sand blend from different sand classifier stations; it may
be the optimum combination of materials needed to produce the lowest cost blend
that still meets the specification; a gradation that meets a specific job-mix
formula (JMF) and uses a fixed amount of RAP; it may be the densest blend of
materials needed for an asphalt mix; or it could be the closest blend of
materials needed to make a good pumpable concrete mix. A blending tool needs to be robust enough to
allow for multiple methods to adjust, evaluate, and optimize material
blends.
Stonemont Software blending tools not
only meet those requirements, our automated features give the user time to
really focus on the results rather than the process.
Blending
Stonemont Software includes blending tools for aggregate,
asphalt, and concrete blending. Many of
the tools are the same but there are a few specific blending tools specific to
concrete mixes. All the blending tools
within Stonemont Software have a similar and familiar layout. They allow the user to hand-enter or query in
component materials, and enter or load specifications and targets.
Historically, blending was performed by manually changing
the percent contribution of each component material until a satisfactory blend
was achieved. This manual blending
technique is supported in Stonemont Software and is still useful if there are
fixed percentages that need to be met or specific ratios of materials that are
required. However, manual blending can
be very frustrating and time-consuming and typically those solutions would not
be considered optimal. Therefore, Stonemont
Software offers the unsurpassed benefits of manual, mathematical, and visual
blending.
Mathematical Blending
With the wide use of spreadsheets came mathematical
blending, which allowed users to save significant time by reducing the
iterations necessary by manual blending.
Mathematical blending allows more complexity to be introduced to the
blending process by adding an objective function and constraints to the problem. For example, if you are trying to produce the
lowest cost blend that meets a certain specification or target, then lowest
cost is the objective function and the specification or target values are
mathematical constraints on the blending process. The objective function doesn’t have to be in
terms of price, it can be thought of as percentage of material. For example, if you have low inventory on a
particular material that is used in an existing blend, you can set the
objective function value for that material to be lower than all other materials
in the blend and maximize the blend objective rather than minimize the
objective.
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Figure 1. |
The most common constraint is to include specifications or
target values (Figure 1). Specifications are
typically an external constraint prescribed by an agency or customer. Targets are typically an in internal
constraint used to improve process control and provide a warning band prior to
exceeding specifications.
Specifications and targets are usually a band including an upper and
lower value. A target or JMF is
typically a single constraint from which a specification or target band can be
developed. Blending to a target or JMF
is useful when attempting to meet a prescribed blend or recreate a blend,
possibly using different materials, that was particularly suitable for a given
purpose. As previously mentioned,
blending to specifications is useful when trying to minimize or maximize the
objective function of the blend.
However, you must be careful to not use actual prescribed specifications
since mathematical blending may cause the blended result of at least one sieve
or other quality parameter to be on or near a specification limit. Therefore, it is recommended to adjust your
specifications to provide a margin of safety.
The
MixRisk tool that was discussed previously can help ensure that your
blend specifications are appropriate and help you better understand the
potential future performance of the blend.
Sometimes, cost is not the overriding factor but rather to
produce a blend that will reduce the risk of future performance issues. That ideal or least risk blend would be one
that is centered within the specifications.
Stonemont Software provides a tool to quickly blend to the middle of the
specifications, which reduces the guess work and manual iterations necessary to
achieve such a blend. This tool is often
used to get the blend as close to ideal as possible before further optimizing
the blend for a particular need. It also
is a quick way to see how different blends of materials can be modified to meet
different specifications. Sometimes the
priority is to create a densely graded asphalt or concrete mix. Stonemont Software provides the ability to
quickly blend to the maximum density line on a .45 power chart. It is used often as a toggle to see the
difference between blending to meet a specification and blending to
max-density.
A common constraint includes specifying a fixed percentage
of component material to use in the blend.
In an aggregate blend you may need to maintain a specific percentage of
a material to balance the plant, in an asphalt mix you may want to fix the
percentage of RAP in the blend, for a concrete mix you will likely choose to
lock the cement in the mix while optimizing the rest of the blend. Another constraint is to specify minimum
and/or maximum percent contributions for each component material in the
blend. This constraint can help prevent
one of the unwelcome artifacts of mathematical blending, which is setting
material contributions to unpractical values or not setting them at all since
the blend constraints can be met without contribution from some materials. For example, a fine sand bin may plug up if
the percentage of material is too small to allow the gates to open up wide
enough for the material to flow. When
batching materials, too much of one material may limit how much can be batched
at one time. Too little material can
create excessively long batches while the plant attempts to jog the small
amount or worse, drops too much because it cannot batch such a small
amount. These are but a few of the
reasons why it may be practical to put constraints on the minimum and maximum
component percentages in a blend.
As more constraints are added or the tighter the constraints
are on the blending process the more control they exert on the solution. For example, if specifications are entered
for all sieves to be 0 to 100 then the objective function to minimize cost will
control the solution. As we tighten the
specifications they will limit the solution to an acceptable range. An important advantage of mathematical blending
over manual blending is that you will be notified immediately if no solution is
possible given the current constraints, which save a lot of time manually
iterating material percentages before realizing no solution is possible.
Visual Bending
There are times when you may want to modify a manual or
mathematically produced blend. One
example may be to move the resulting gradation further from a specification
boundary. Sometimes it is easier to do
this visually rather than in tabular form, so Stonemont Software offers a very
powerful suite of visual blending tools.
These tools allow users to point-and-click or drag-and-drop points on
charts to specify new target blending values.
Sieve Charts
With the sieve chart visual blending tool, it is simply a
matter of dragging-and-dropping target gradation values and then recalculating
the blend to see how close the materials can actually get to that blend. This is a great tool for quickly achieving a
reasonable compromise with less than optimum materials or for fine tuning a
blend that was started using some of the other tools. In the example shown in Figure 2, targets
(green triangles) were dragged-and-dropped from the original locations (open
circles) with the intention to fine up the blend on the coarser sieves and
coarsen up the blend on the finer sieves. The results shown in Figure 3 are a close
match to the target gradation. If
satisfied with the results, they can be posted to the material blend or targets
can be re dragged and another attempt can be made to further modify the blend.
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Figure 2. |
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Figure 3. |
Power Charts
The power chart visual blending tool functions similarly to
the sieve chart blending tool but provides the .45 power curve as the canvas
for drag-and-drop functionality. This
tool will calculate the max density line based on the current gradation. If changes to the gradation are significant
enough to affect the max density line then it will automatically shift
according to the updated gradation. This
tool is very useful for fine tuning a blend of materials. For this example, the goal was to move the
blend closer to the maximum density line (but not exactly to the maximum
density line or we would use the blend to maximum density line
functionality!). Figure 4 shows how the targets (green triangles) were moved closer to the
maximum density line from their original locations (black triangles). Figure 5 shows the results of calculating the
blend using the new visually created targets.
As you can see the fit is not perfect to the target locations. When working with material blends, including
several component materials can really help with fine tuning and improves the
chances that a given blend can be met. What
works best may surprise you.
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Figure 4. |
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Figure 5. |
Individual Retained
Charts
What works best for percent passing specifications does not
always work out best when considering individual retained specifications. The individual retained chart blending tool
is available for those times when meeting an individual retained specification
is necessary. Primarily found in
concrete specifications, the individual retained values for a material blend
can give a better understanding of how the material is distributed throughout
the gradation. Spikes in material
retained on any one sieve are quite apparent on an individual retained chart
which can often go unnoticed on a percent passing chart.
In this example, the original
material blend (blue diamonds) has a significant portion retained on the #100
sieve (Figure 6). Since this is a fine
aggregate material blend from the combination of classifier stations, the target
gradation (green triangles) has been dragged out to fit a haystack
configuration. Figure 7 demonstrates how
well blending can be fine-tuned when several materials are considered as part
of the blend.
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Figure 6. |
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Figure 7. |
Coarseness Workability
Charts
Unique to concrete mix designs, the coarseness workability
visual blending tool is a good starting point when adjusting a concrete
mix. When combined with the other visual
blending and evaluation tools, it can give you a well-rounded view of how a
concrete mix may potentially perform. It
is important to note that this tool is only used to modify the aggregate
proportion of the concrete blend so the adjusted workability value is not shown
on this chart. The adjusted workability
value can be viewed on our standard coarseness workability charts for concrete
mix design development that are not used for visual blending.
In this example, the goal was
to slightly improve the workability of the mix.
The blue dot in Figure 8 represents the blended material in the original
concrete mix. To adjust the mix and
improve the workability according to the coarseness workability chart, simply
click an alternative target location as shown on Figure 9. If the new aggregate blend is possible, the
adjustment can be posted back to the mix and all material amounts and
percentages will be updated accordingly.
The program will provide immediate feedback if it is not possible to
achieve the attempted adjustment. See a short video of
visual blending using the coarseness workability chart in Stonemont Software. Other
concrete specific blending tools include the ability to blend to a specific
unit weight or density.
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Figure 8. |
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Figure 9. |
Summary
Although spreadsheets allow for mathematical blending, their
usefulness is limited because they are typically not an integrated part of a
quality control and mix design software package. Stonemont Software offers the unsurpassed benefits
of manual, mathematical, and visual blending.
Stonemont Software also offers the ability to manage all blends
company-wide and the ability to easily query for changes in component materials
that makes for a very robust set of blending tools.
Material blending is a critical need for the aggregate,
asphalt, and concrete material industries.
If you are wasting time exporting material gradations to a spreadsheet
to perform material blending or your mix design software does not provide
adequate blending tools then it is time to step up to the powerful blending
tools that are an integral part of the Stonemont quality control and mix design
software.