MOBILE MIXER INSPECTION

GENERAL

The following instruction is to be used when inspecting the operation of a mobile mixer paving plant for bridge deck overlays, patching, and miscellaneous concrete items.  Mobile mixers also may be used for dowel bar retrofit projects.

GENERAL EQUIPMENT

Equipment shall meet the requirements of Article 2001.20, paragraph D. 

EQUIPMENT USED FOR DECK OVERLAYS

Mobile mixers used for deck overlays as described in Article 2413.  The following is a general policy regarding the calibration of units and other requirements for bridge deck overlays:

1. The contractor will perform the calibration. The contractor shall notify the District Materials Engineer at least 48 hours in advance of calibration, so that witnessing can be provided at the option of the District Materials Engineer. It is recommended that the contractor initially calibrate the unit prior to notification. A unit will be completely calibrated at the beginning of the construction season and no more than 6 months thereafter, prior to placement of any concrete. Copies of all calibration forms shall be kept with the unit and be available for inspection at any time.  These should be retained by the company for a minimum of two (2) years.

2. It is recommended that all units be transported empty from job site to job site.

3. The contractor shall notify the District Materials Engineer at least 48 hours prior to use on subsequent projects. Partial calibrations may be performed at the discretion of the District Materials Engineer. Partial calibrations may be performed for a change in source of any of the materials or when a problem or question relative to the proportioning of materials exists.

4. The dilution of any admixture is to be witnessed and documented by the inspector. Documentation should include the date, dilution rate, admixture identification, etc.

5. Tolerance in proportioning shall be in accordance with ASTM C685.

a. Cement	0 to +4%
b. Fine Aggregate	+2%
c. Coarse Aggregate	+2%
d. Admixtures	+3%
e. Water	+1%

6. Reporting Forms #E115, # E120, #821297, and #820020.

EQUIPMENT USED FOR MISCELLANEOUS CONCRETE PLACEMENTS

In accordance with Article 2001.20, paragraph D, mobile mixers may be used on miscellaneous concrete placements less than 50 cubic yards per day for the following items only.  The Engineer may waive the 50 cubic yard limit on these items.

Article	Work Type
2426	Structural Repair
2503	Utility Access Intakes, Catch basins
2505	Guard Rail Anchorages

The placement limit does not apply to the following items:

Article	Work Type
2304	Temporary Pavement
2416	Pipe Collars
2503	Abandonment, collars
2511	Sidewalk and Trails
2512	Curb and Gutter
2515	Driveways
2517	Railroad Approach Sections
2519	Fence Posts
2529	Patching (Full depth), rumble strip
2530	Patching (Partial depth)
2554	Thrust Blocks

Calibration of equipment used for miscellaneous concrete placements shall include the requirements for deck overlays with the following additions:

1. Partial cement calibrations may be performed when the mix design changes. The DME may allow a concrete yield check or partial cement calibrations for Class C, M, or DBR mix designs. DME may also obtain cylinders to verify strength.

2. The unit will be calibrated for C-4 mix design for all miscellaneous concrete placements.  The unit will be calibrated for M-4 mix design for pavement patching.

3. Unit shall be equipped to deliver a daily batch ticket upon request. Aggregate moistures shall be included in the total water to cement ratio.

Storage & Handling of Materials

Aggregates

· Certified compliance

· Separation of materials

· Storage area floor shall be firm & clean or a minimum of 18” of similar material

Cementitious Material

· Approved certified sources

· No intermingling of products or sources

· Stored in suitable weatherproof enclosures

Water

· Sample when required

Admixtures

· Verify acceptance of lot

· Circulate 5 min. per 100 gal. of solution

· Proper storage to prevent freezing

GENERAL CALIBRATION PROCEDURE – EQUIPMENT WITH DIAL SETTING

Calibration of a concrete mobile mixer as required by specification is performed to accurately proportion, by volume, materials for an intended concrete mix design. Proportioning of the rock and sand is based on the cement meter count determined for each concrete mobile mixer. Once the counts (revolutions) per each 94-pound bag of cement has been determined then delivery gates will be calibrated to proportion the correct amount of rock and sand.

Calibration form #820020 (Appendix A) is a step-by-step process to determine the correct settings for each material delivered for a specific concrete mix design.

• Complete the header information including each material source, specific gravities, and weights as determined for the applicable concrete mix design.

• Cement meter count is determined by averaging five different timed delivery weights to determine the pounds of cement representing each meter count.

• Because the contractor normally uses 94# bags of cement, the counts are determined per each bag of cement delivered.

• The rock and sand weight delivered for each bag of cement is determined by dividing the wet weight of each aggregate by the number of 94# bags of cement in each cubic yard of that concrete mix design.

• 8.78 represents a mix design (M-4 & O-4WR Mix) requiring 825 pounds of Type I cement.
• 6.64 represent a mix design (C-4 Mix) requiring 624 pounds of Type I cement.
• 7.00 represent a minimum of 658 pounds of rapid set (RS) patch material used for dowel bar retrofit. Adjust for mixes greater than the minimum.

• The aggregate weight per each bag of cement then needs to be divided by the cement meter counts per bag. The result will determine the target pounds per count of each aggregate.

•  A couple trial runs may be necessary to get the gate set for delivery within the 2% tolerance. Once the gate is set, run a couple check runs and average to determine compliance.

• Discharge water for calibrated time and determine the weight of water. Weight should be within required tolerance.

• Admixture is introduced into the mix by measuring the flow time per bag. The time per bag is determined by using data from the cement meter count calibration.

• Divide the total seconds of the five runs by the total counts of the five runs.
• Multiply the result by the count per bag to determine the seconds per bag.
• Seconds per bag is the time needed to introduce the specified admixture for 94 pounds of cement.
• Each specific admixture will give a specified quantity, usually ounces, per 100 pounds of cement.
• Verify that the correct amount is delivered for each 94 pounds of cement within the determined time in seconds per bag.

Appendix A contains a blank form #820020 and a working example.

CALIBRATION PROCEDURE – EQUIPMENT WITH AUTOMATED GATE SETTING

Calibration of a concrete mobile mixer as required by specification is performed to accurately proportion, by volume, materials for an intended concrete mix design for equipment with automated gate settings.

For cement, sand and rock, begin each with the following steps.

· Place a deflector to ensure all cement is discharged into container

· Set mobile mixer to run at proper operating speed

· Record weight of container or tare on scale

· Place container under the mixer

Cement Calibration

· Run a set count of cement approximately 100 counts

· Weigh the cement on the scale to the nearest 0.5 lb.

· Enter the weight of the cement into the calibration form on the mixer

· Repeat for a total of five trials at approximately the same number of counts

Sand and Rock Calibration

· Run a set count to yield approximately 50 to 60 lbs.

· Weigh the cement on the scale to the nearest 0.5 lb.

· Enter the weight of the cement into the calibration form on the mixer

· Using approximately same number of counts, repeat for a total of five trials at both high setting and low setting

Water Calibration

· Ensure all water is discharged into container

· Run a set count to yield approximately 1 gallon of water

· Weigh the water on the scale to the nearest 0.01 lb.

· Repeat for a total of five trials

Admixture Calibration

· Ensure all admixture is discharged into calibrated cup with milliliters

· Place cup under the admixture line

· Run for one minute

· Determine the milliliters of admixture from the cup to the nearest 1 ml

Use calibration form #820020A to input printouts from calibration. (Example Appendix B)

CONCRETE YIELD CHECK

The DME may allow either of the following concrete yield test methods to validate the concrete mix batched.

MOBILE MIXER CONCRETE YIELD CHECK – ¼ Cubic Yard

1. The initial yield check should be made with the proportioning controls set as determined

during the calibration procedure. Slump and air content should be within the

specification limits prior to performing the yield test. Record the air content.

2. Determine the cement meter count to produce 1/4 of a cubic yard of concrete mix being batched. Do this by dividing the cement factor by 4 and multiply this figure times the meter count for one bag of cement.

3. Run without extension chutes so material does not stick to chutes and can be deposited directly into yield box. Place the 36” x 36” x 9” deep yield box directly under the discharge end of the mixer.

4. Take a reading on the cement meter register.

5. Engage the main conveyor and the mix conveyor simultaneously. Allow the main conveyor to run until the cement meter register has advanced by the number of counts required to discharge 1/4 cubic yard of concrete and then shut it off quickly. The mix should be consolidated as the box is filling with a shovel or other satisfactory tool, especially in the corners.

6. The concrete should fill the yield box to within plus or minus one quarter inch of level full. If the yield is such to not fill the box within plus or minus one quarter inch level full, the DME may require a partial or complete calibration.

MOBILE MIXER QUICK YIELD CHECK

1. The quick yield check should be made with the proportioning controls set as determined

during the calibration procedure. Slump and air content should be within the

specification limits prior to performing the yield test. Determine the unit weight of the concrete mix batched. Record air content and unit weight.

2. Determine the cement meter count to produce 1 to 2 cubic feet of concrete mix being batched. Do this by multiplying the cement factor by 0.04 to 0.07 and multiply this figure times the meter count for one bag of cement.

3. Run without extension chutes so material does not stick to chutes and can be deposited directly into yield box. Tare scale with container. Place container directly under the discharge end of the mixer.

4. Take a reading on the cement meter register.

5. Engage the main conveyor and the mix conveyor simultaneously. Allow the main conveyor to run until the cement meter register has advanced by the number of counts required to discharge 1 to 2 cubic feet of concrete and then shut it off quickly. Record actual count for batch discharged.

6. Determine the theoretical quantity batched based on the cement count for the quantity batched.

7. The weight batched divided by the unit weight tested should be within ±2% of the theoretical quantity batched. If unit weight is not within ±2% of the theoretical quantity batched, the DME may require a partial or complete calibration.

PROPORTIONS

For deck overlays, an O-4WR mix design in accordance with Materials IM 529 shall be used.  Aggregate moisture content will be assumed at 0.5% for coarse aggregate and 3.0% for fine aggregate.

For miscellaneous concrete placements, a C-4 mix design in accordance with Materials IM 529 shall be used, except pavement patching where M-4 shall be used.  If aggregates are stored inside, moisture content will be assumed at 0.5% for coarse aggregate and 3.0% for fine aggregate.  If aggregates are stored outside, the District Materials Engineer may require periodic moisture testing.

For dowel bar retrofit, the mix design will be determined by the contractor to meet the specification requirements. Use aggregate moistures as described above.

For all mixes, determine amount of water contained in the coarse and fine aggregate and subtract from the water requirements.

Aggregate specific gravities are found in Table T203 which is revised annually, and care must be taken to use the current table.

SAMPLING AND TESTING

Sampling and testing for deck overlays shall be in accordance with Materials IM 204.  Miscellaneous concrete placements shall be in accordance with Materials IM 528 and IM 204 with the following additions:

1. Air content shall be tested once per placement by the RCE.  Slump shall be tested at least once per day.

REPORTS & REPORTING

1. Mobile mixer batch tickets submitted to the Resident Construction Engineer (RCE) and District Materials Engineer (DME) weekly.

2. Use forms #E115 for air and slump testing.  Use form # E120 for documentation of mix data.

NOTIFICATION

The producer shall provide notice to the RCE and DME within seven days of the anticipated miscellaneous placements.