Guidance to Concrete Producers seeking and maintaining plant certification
- NRMCA Plant certification is valid for 2 years from the date of inspection.
- Certification of delivery vehicles is valid for 12 months from the date of inspection. Vehicles inspected in the last 60 days of the certification period will be certified for a period of 12 months from the previous expiration date.
- 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
- The company hires the inspector and the fees established are between the company and the inspector. NRMCA maintains a listing of approved inspectors. A licensed engineer working for the company and approved by NRMCA is permitted to perform the inspections.
- NRMCA communicates with the inspector when corrective action is needed on the submitted checklist.
- 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.
- 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 703-706-4865 or from the NRMCA e-store.
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:
- 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.
- 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. When the inspector indicates that these items were not accomplished during the previous certification, the company official’s supervisor will be notified and asked to ensure that the agreement is complied with. This is an important responsibility transferred to the company.
Scale accuracy verification
There are some differences in what the scale companies are accustomed to doing in accordance with NIST Handbook 44 and what is required by the NRMCA plant certification and ASTM C94. Some of these differences are described in the Plant Inspector’s Guide. The following should be communicated with the scale company before they do scale checks:
- 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).
- 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.
- Scale checks should be done through range of use of the scales. Scale companies may only verify it through 50% of the scale capacity.
- 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 substitution loading. Scale increments should not be skipped. In substitution loading, product in the scale should only be to the load previously verified – as close as possible.
- 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.
- 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.
- Maintenance tolerances in accordance with Handbook 44 are stated on the basis of scale divisions (min grad) but are generally more restrictive than those in C94 that state tolerances based on applied load or scale capacity.
- 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.
- Discharge water into a 55-gallon drum and weigh the quantity – can be suspended from the cement scale.
- Perform this at least 3 times to obtain an average.
- 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.
- 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:
- A dosage quantity check should be performed for each dispenser.
- Determine the target quantity based on a typical dosage rate within the recommended dosage range.
- Dose target quantity of admixture into the dispenser measuring unit and hold for visual measurement.
- 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:
- A dosage quantity check should be performed for each dispenser.
- Determine the target quantity based on a typical dosage rate within the recommended dosage range, but usually limited to under 5 gallons.
- Dose target quantity of admixture into calibrated flask/container for measurement.
- 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.