Primary Requirements
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NRMCA Plant certification is valid for 2 years from the date of
inspection.
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Certification of delivery vehicles is valid for 14 months from the
date of inspection.
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Delivery vehicles can be inspected in-house by company personnel and
their procedures have to be audited by the inspector when the plant is
being inspected. This is identified as Option A in the Check List. See
Certification of Delivery Vehicles
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The company hires the inspector and the fees established are between
the company and the inspector. NRMCA maintains a listing of approved
inspectors on its website. A licensed engineer working for the company
and approved by NRMCA is permitted to perform the inspections.
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NRMCA communicates with the inspector when corrective action is needed
on the submitted checklist.
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The certificate of conformance is sent to the inspector who needs to
sign and seal it before giving it to the company. The company should
contact the inspector on the status of certification before contacting NRMCA.
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It can take 2 to 4 weeks for processing a certification depending on
the number being processed at NRMCA. An expedited process is available
when contingencies arise. Plan ahead to avoid requesting certification
just prior to the start of a project.
Plant Inspector Guide
This Guide provides detailed discussion, images and numerical examples
for each item in the Check List. It addresses the intent of each
provision in the certification to ensure uniformity of inspection. The
Guide is used as a basis for approving inspectors and assistants for the
NRMCA Plant Certification Program.
Companies interested in certification or who are currently certified are
advised to obtain a copy of the Guide to understand the requirements and
to maintain the status of production facilities in
conformance with the
certification. Copies of the Plant Inspector Guide can be ordered from NRMCA by contacting
Jacques Jenkins,
by email or
at 1-888-846-7622, x1165 or from the
NRMCA website.
Company Agreement
A responsible company official completes and signs an agreement for the
duration of the certification. The company person signing this agreement
should have financial and operational responsibility over the management
of the production facility; for planning and directing the plant
personnel; and taking corrective action when necessary.
The agreement establishes responsibility for the individual and the
company for the duration of the certification and encompasses the
following:
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Accuracy checks of measuring devices (scales, water meters, admixture
dispensers and moisture probes) will be performed at least once every 6
months. State DOTs may have a requirement for these to be performed more
frequently. The requirement with the greater frequency governs.
Documentation of these verifications should be maintained and made
available to the inspector during the plant inspection.
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Certification of delivery vehicles operating will be current during
the period of the plant certification.
The person signing the agreement bears the responsibility for
maintaining the plant in conformance during the certification period.
Not performing the items stated in the agreement can result in refusal
of certification at the next request. The inspector is asked to verify
that these items have been performed during the prior certification
period.
Scale accuracy verification
There are some differences in what the scale companies are accustomed to
doing and what is required by the NRMCA plant certification and ASTM
C94. The following should be communicated with the scale company before
they do scale checks:
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Minimum quantity of test weights should be at 10% of scale capacity.
Aggregate scale capacities will generally govern the minimum amount of
test weights required – about 4000 lbs is typically needed. (There may
be situations where the plant configuration does not permit the minimum
test weights to be used and that has to be verified by the plant
inspector).
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Test weights should be certified to be accurate to 0.01% of their
indicated load within the last two years. This is typically not a
problem with commercial scale companies.
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Scale checks should be done through range of use of the scales. Scale
companies may only verify it through 50% of the scale capacity.
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Up through 50% of the scale capacity, scale checks should be done
using a build-up test load using a combination of product and test
weights in a process called a substitute test. Scale increments should
not be skipped. In substitute testing, product in the scale should only
be to the load previously verified – as close as possible.
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Over 50% of the scale capacity, strain test loading is permitted. An
unknown quantity of product is charged and the incremental weight
indication with the test weights is verified. At least two points should
be tested in this portion of the scale – through typical range of use.
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Scale accuracy requirements (ASTM C94) is the greater of
● ±0.15% of scale capacity (governs at the lower end)
● ±0.4% of applied test load
● If it's not accurate the scale has to be adjusted.
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A copy of the scale verification data sheets should be obtained to
indicate details of the test loads used, test load increments, load
indications and load error. A certificate just indicating a scale is OK
is not acceptable.
Definitions of load testing, discussions and
numerical examples of the scale accuracy verification are available in
the NRMCA Plant Inspector Guide.
Scale accuracy should be verified anytime the plant is moved (portable
plants), maintenance activities on the plant impact the weighing
systems, or when there is a concern on scale accuracy determined from
the batchman operating the batching process or the quality of concrete.
Water meter accuracy verification
Accuracy of water meters should be verified at least once every six
months. Maintain documentation of these verifications for review by the
inspector.
Accuracy of water meters is the same as that required for batching
accuracy - ±1.5%.
Measure accuracy using a large enough volume of water – at least 50
gallons.
Methods include:
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Discharge water into a 55-gallon drum and weigh the quantity – can be
suspended from the cement scale.
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Perform this at least 3 times to obtain an average.
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Some discharge 200+ gallons into a truck mixer if truck scales are
available. After weighing the truck, water is discharged and the truck
is weighed empty to determine water in the drum.
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Discharge water into a calibrated container of at least 50 gallon
capacity. Calibrations on the container should be accurate enough to
measure to within the required accuracy tolerance.
When water is measured by weight, accuracy requirements for scales
apply.
More details and numerical examples of water meter accuracy verification
are in the NRMCA Plant Inspector Guide.
Admixture dispenser accuracy verification
In a signed agreement, the Company official agrees to check the batching
accuracy of dispensers of liquid admixtures at least every 6 months.
Admixtures should be dispensed and measured to within ± 3 percent of the
desired amount. Admixture dispensers should be accurate to within ±3% of
the required quantity or ±the minimum dosage rate per 100 lbs of cement,
whichever is greater.
This dispenser accuracy check is usually performed by collecting the
chemical admixture into a calibrated container. It is acceptable to use
a dispenser measuring unit as the calibrated container.
The process for checking accuracy using a calibrated dispenser measuring
unit is as follows:
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A dosage quantity check should be performed for each dispenser.
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Determine the target quantity based on a typical dosage rate within
the recommended dosage range.
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Dose target quantity of admixture into the dispenser measuring unit
and hold for visual measurement.
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Calculate and confirm accuracy.
When the admixture is measured by a meter and batched through a direct
feed admixture system (with no dispenser measuring unit), the accuracy
check is conducted using a calibrated flask/container. The process for
checking accuracy using graduated flask or container:
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A dosage quantity check should be performed for each dispenser.
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Determine the target quantity based on a typical dosage rate within
the recommended dosage range, but usually limited to under 5 gallons.
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Dose target quantity of admixture into calibrated flask/container for
measurement.
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Calculate and confirm accuracy.
The company can perform their own dispenser accuracy checks instead of
calling the dispenser representatives. If the meter is not accurate, the
admixture dispenser representatives should be called in to calibrate the
dispensing system.
Documentation of admixture dispenser accuracy verification should be
retained for review by the inspector.
Some producers collect admixture at the point of discharge into the
mixer to verify the accuracy of the dispensing system. This process
checks the integrity of the lines through which the admixture flows in
addition to the dispenser accuracy. This is a desirable check but not
required for the purpose of verifying the accuracy of dispenser
measuring systems.
Definitions, further explanation and numerical examples of verifying
admixture dispenser measuring accuracy are provided in the NRMCA Plant
Inspector Guide.
Moisture Probe accuracy verification
Follow manufacturer’s directions for verifying accuracy.
The accuracy of moisture probes should be relative to an aggregate
moisture measurement by drying out an aggregate sample on a hot plate or
microwave oven (ASTM C566). The aggregate sample tested should be obtained from as close to the
location of the moisture probe measuring the aggregate moisture.
There is no stated accuracy (tolerance) for moisture probes in the NRMCA
Check List. Using the required accuracy (±1.5%) for water meters can be
considered as a basis for requiring corrections to the device
(calibration).
Maintain documentation of these checks and provide for review to the
inspector.
