The Table of Contents for this section is available.
The work covered by this section consists of the production of portland cement concrete and its delivery to where it is incorporated into the work. Type IP blended cement or Type IS blended cement may be used in lieu of portland cement, and fly ash or ground granulated blast furnace slag may be substituted for a portion of the portland cement. This section covers all portland cement concrete to be used for portland cement concrete pavement, concrete structures other than prestressed members, and incidental construction.
Portland cement concrete shall be composed of portland cement, fine and coarse aggregates, and water. In addition, an air entraining agent or other chemical admixtures shall be added if required or may be added if permitted by these specifications. The portland cement concrete shall be of the class called for by the plans or specifications and shall be proportioned, mixed, and delivered in accordance with the requirements contained herein. The mix proportions for all portland cement concrete covered by this section shall be designed by the Contractor.
When concrete being placed in any one pour is furnished by more than one concrete plant, all concrete shall be of the same mix design, including sources of cement and all admixtures.
All materials shall meet the requirements of Division 10 shown below:
Coarse aggregate Article 1014-2
Fine aggregate Article 1014-1
Portland cement Article 1024-1
Type IP blended cement Article 1024-1
Fly ash Article 1024-5
Type IS blended cement Article 1024-1
Ground granulated blast furnace slag Article 1024-6
Water Article 1024-4
Air entraining agent Article 1024-3
Chemical admixtures Article 1024-3
1000-3 PORTLAND CEMENT CONCRETE FOR CONCRETE PAVEMENT.
(A) Composition and Design:
The Contractor shall submit a concrete paving mix design to the Engineer for approval a minimum of 30 days prior to proposed use.
The mix shall contain a minimum of 526 pounds of cement per cubic yard, with a maximum water cement ratio of 0.559, air content in the range of 4.5 to 5.5 percent, a slump of 2 to 3 inches, and a minimum flexural strength of 550 psi at 14 days.
The cement content of the mix design may be reduced by up to 20% and replaced with fly ash at a minimum rate of 1.2 pounds of fly ash to each pound of cement replaced. The maximum water cement ratio based on total cementitious materials shall be 0.538.
The cement content of the mix design may be reduced up to 50% and replaced with a blast furnace slag on a pound for pound basis.
The mix design shall include the source of aggregates, cement, fly ash, slag, and admixtures; the gradation and specific gravity of the aggregates; and the fineness modulus (F.M.) of the fine aggregate. The Contractor shall submit test results showing that the mix design conforms to the criteria, including the 14 day flexural strength of a minimum of 6 beams made and tested in accordance with AASHTO T126 and AASHTO T97. The mix design shall produce an average flexural strength sufficient to indicate that a minimum strength of 550 psi will be achieved in the field.
The Contractor, with the approval of the Engineer, may adjust the proportions of the fine and coarse aggregates and water as desired provided the cement content is not decreased, the water cement ratio is not increased, and such adjustments produce concrete in accordance with these specifications.
Where it is necessary to provide concrete with a higher slump for hand methods of placing and finishing, the mix proportions shall be adjusted by the Contractor, with the approval of the Engineer, to provide the necessary slump and to maintain the water cement ratio established by the original mix design.
(B) Air Entrainment:
The concrete shall be air entrained by the use of an approved air entraining agent which shall be dispensed along with the mixing water, unless otherwise permitted by the Engineer.
The air content of the freshly mixed concrete shall be 5.0 percent plus or minus 1.5 percent. The air content will be determined in accordance with AASHTO T152, T121, or T196. At the option of the Engineer, the air content may be measured by the Chace indicator, AASHTO T199, in which case sufficient tests will be made in accordance with AASHTO T152, T121, or T196 to establish correlation with the Chace indicator. Concrete will not be rejected based on tests made in accordance with AASHTO T199.
(C) Slump:
The concrete shall have a maximum slump of 1 1/2 inches where placed by a fully mechanized paving train. Where the concrete is placed by hand methods, the slump shall not exceed 3 inches.
The sample taken for determination of slump will be obtained immediately after the concrete has been discharged onto the road.
(D) Set Retarding Admixture:
By permission of the Engineer, the Contractor may use an approved set retarding admixture to facilitate placing and finishing.
The quantity of set retarding admixture per 100 lb. of cement shall be within the range recommended on the current list of approved admixtures issued by the Materials and Tests Unit.
(E) Water Reducing Admixtures:
By permission of the Engineer, the Contractor may use an approved water reducing admixture to facilitate placing and finishing.
The quantity of water reducing admixture per 100 lb. of cement shall be within the range recommended on the current list of approved admixtures issued by the Materials and Tests Unit.
(F) Contractor's Responsibility for Process Control:
The Contractor shall be responsible for controlling the materials and operations to produce uniform pavement meeting specification requirements. The Contractor shall submit a plan detailing his process control procedures and the type and frequency of testing and inspection deemed necessary to produce concrete that meets the requirements of the specifications. This plan shall be submitted to the Engineer at the preconstruction conference. The Contractor shall be responsible for performing all sampling, testing, and inspection necessary to provide adequate process control. He shall have present during all batching and delivery operations a certified concrete technician whose sole duty shall be to supervise the production and control of the concrete. This duty includes the following:
1. Tests and inspections necessary to maintain the stockpiles of aggregates in an unsegregated and uncontaminated condition.
2. Calibration of admixture dispensing system, weighing systems, and water gages.
3. Tests and adjustments of mix proportions for moisture content of aggregates.
4. Mixer performance tests prior to reducing mixing time of central mix plant to less than 90 seconds and at other times when deemed necessary by the Engineer.
5. Verifying the actual mixing time of the concrete after all materials are introduced into the mixer at the beginning of paving operations and at least once each month.
6. Testing all vibrators.
7. Tests necessary to document the slump and air content of the mix produced. Air content shall be determined at least twice each day.
8. Tests for depth of the pavement in the plastic state.
9. Furnishing data verifying that the approved theoretical cement content has been met at intervals not to exceed 50,000 square yards of pavement.
10. Signing all plant reports, batch tickets, or delivery tickets.
The Division of Highways certifies technicians who satisfactorily complete examinations prepared and administered by the Division of Highways.
All test procedures shall comply with the appropriate articles of Section 1000.
Tests may be witnessed by the Engineer. The results of all tests and inspections shall be documented and a copy shall be made available to the Engineer upon request. The Contractor shall take prompt action to correct conditions which have resulted in or could result in the submission of materials, products, or completed construction which do not conform to the requirements of the specifications.
(G) Contractor Not Relieved of Responsibility for End Result:
The Contractor shall not be relieved of his obligation to produce a uniform pavement meeting specifications by reason of:
1. The acceptance or approval by the Engineer of the concrete mix design or any adjustments;
2. Compliance with the concrete mix design and compliance with the testing requirements and other process control requirements by the Contractor; nor
3. The failure of the Engineer to perform any tests in the process control, nor the performance of any tests in the process control which indicate compliance with the specifications.
1000-4 PORTLAND CEMENT CONCRETE FOR STRUCTURES AND INCIDENTAL CONSTRUCTION.
(A) Composition and Design:
Concrete shall be of the class called for by the plans or specifications.
The Contractor shall submit to the Engineer proposed concrete mix designs for each class of concrete to be used in the work. Mix proportions shall be determined by a testing laboratory which has been approved by the N.C. Division of Highways and shall be based on laboratory trial batches meeting the requirements of Table 10001 and other applicable sections of these specifications. Quantities of fine and coarse aggregate necessary to provide concrete meeting the requirements of the specifications shall be determined by the method described in ACI211, "Recommended Practice for Selecting Proportions for Normal Weight Concrete", using the absolute volume basis.
Mix designs stated in terms of saturated surface dry weights shall be submitted on M&T Form 312 at least 35 days prior to using the proposed mix. If the concrete is to be placed by pumping, it shall be so noted on Form 312. Batch proportions shall be adjusted to compensate for surface moisture contained in the aggregates at the time of batching. Changes in the saturated surface dry mix proportions will not be permitted unless revised mix designs have been submitted to the Engineer and have been determined to be acceptable for use.
Form 312 shall be accompanied by a listing of laboratory test results of aggregate gradation, air content, slump, and compressive strength. Compressive strength of at least three 6" x 12" or 4" x 8" cylinders at the age of 7 and 28 days shall be listed. Laboratory tests shall be performed in accordance with the following test procedures:
Aggregate Gradation -- AASHTO T27
Air Content -- AASHTO T152
Slump -- AASHTO T119
Compressive Strength -- AASHTO T23 and T22
When the combination of materials is such that the required strength and/or a workable slump cannot be obtained at the minimum specified cement content with the maximum allowable water-cement ratio, the cement content shall be increased at no cost to the Department by whatever amount is required to produce the required strength and/or slump without exceeding the allowable water-cement ratio.
The Engineer will review the mix design only to ascertain general compliance with specification requirements. After this review has been completed, the Engineer will notify the Contractor in writing that the mix design is either acceptable or unacceptable. A mix shall not be used until written notice has been received. Acceptance of the mix design shall not relieve the Contractor of his responsibility to furnish an end-product meeting specification requirements. Upon request from the Contractor, a mix design accepted and used satisfactorily on any Department of Transportation project may be accepted for use on other projects.
(B) Air Entrainment:
All classes of concrete except Class S shall be air entrained, unless otherwise indicated on the plans or in the special provisions. Air entrainment shall be obtained by adding an air entraining agent at the time of mixing. The air content of the freshly mixed concrete, when tested at the job site, shall be 6.0 percent plus or minus 1.5 percent. The air content will be determined in accordance with AASHTO T152, T196, or T121. Measurement of air content may also be performed by the Chace indicator.
(C) Strength of Concrete:
The compressive strength of the concrete will be considered to be the average compressive strength test results of two 6" x 12" cylinders, or two 4" x 8" cylinders if the aggregate size is not larger than size 57 or 57M; cylinders shall be made from the concrete as delivered to the work and tested in accordance with AASHTO T22. Cylinders will be made at such frequencies as the Engineer may determine and cured in accordance with AASHTO T23 as modified by the Department. Copies of these modified test procedures are available upon request from the Materials and Tests Unit.
When the average compressive strength of the concrete test cylinders is less than the minimum strength specified in Table 10001 and the Engineer determines it is within reasonably close conformity with strength requirements, concrete strength will be considered acceptable. When the Engineer determines average cylinder strength is not within reasonably close conformity with specified strength, tests will be made on the in-place concrete and based on these test results, the concrete will either be accepted with no reduction in payment, or will be accepted at a reduced unit price or rejected as set forth in Article 1053.
(D) Temperature Requirements:
The concrete temperature at the time of placing in the forms shall be not less than 50F, nor more than 95F, except where other temperatures are required by Articles 4208, 4209 and 42016.
No concrete shall be placed when the air temperature, measured at the location of the concreting operation in the shade away from artificial heat, is below 35°F, without permission of the Engineer. When such permission is granted, the aggregates and/or water shall be uniformly heated to a temperature not higher than 150°F. The temperature of the heated concrete shall not be less than 55°F and not more than 80°F at the time it is placed in the forms.
For Class S concrete, the concrete temperature at the time of placing shall be not less than 40°F nor more than 75°F. Class S concrete shall not be deposited under water when the water temperature at the surface is less than 35°F.
(E) Elapsed Time for Placing Concrete:
Concrete shall be delivered to any monolithic unit of a structure at a rate
which will permit proper handling, placing, and finishing of the concrete; and shall be so regulated that the maximum interval between the placing of batches at the work site shall not exceed 20 minutes.
Concrete shall be placed before the elapsed time between adding the mixing water to the mix and placing the concrete in the forms exceeds that set forth in Table 10002.
(F) Use of Set Retarding Admixtures:
Class S concrete shall contain an approved set retarding admixture.
All concrete placed in the superstructure of bridges shall contain an approved set retarding admixture such that the concrete will remain workable until the entire operation of placing and finishing, including corrective measures if necessary, has been completed. The use of set retarding admixture may be waived by the Engineer when conditions clearly indicate that it is not needed.
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* Applicable to Class AA, A, S, and M concrete.
** Applicable to Class B concrete.
Concrete not placed in the superstructure of bridges may contain an approved set retarding admixture when permitted by the Engineer.
The quantity of set retarding admixture per 100 lb. of cement shall be within the range recommended on the current list of approved admixtures issued by the Materials and Tests Unit.
(G) Use of Water Reducing Admixtures:
By permission of the Engineer, the Contractor may use an approved water reducing admixture to facilitate placing and finishing.
The quantity of water reducing admixture per 100 lb. of cement shall be within the range recommended on the current list of approved admixtures issued by the Materials and Tests Unit.
(H) Use of Calcium Chloride:
Calcium chloride may be used as a set accelerating agent where so permitted by the Engineer. The amount of calcium chloride used shall be 1 lb. per 100 lb. of cement except where lesser amounts are directed by the Engineer. Calcium chloride shall not be used where steel reinforcement, metal conduit, or other metals will be in contact with the concrete. No calcium chloride shall be used in concrete which has a temperature higher than 70F, or when the air temperature is greater than 70F. Cold weather protection shall be provided for concrete containing calcium chloride in the same manner as is provided for concrete without calcium chloride.
Calcium chloride shall be in a liquid form when added to the concrete. The solution shall not be stronger than 1 pound of calcium chloride added to 1 quart of water, and shall be well mixed. To avoid incompatibility with other additives, the calcium chloride shall be added to the batch after all other ingredients have been put into the mixer.
(I) Use of Fly Ash:
Fly ash may be substituted for a portion of portland cement in all classes of concrete. In all classes except Class S, fly ash may be substituted for portland cement up to 20% by weight of the required cement noted in Table 10001. Substitution shall be at the rate of at least 1.2 pounds of fly ash per pound of cement. Fly ash shall not be substituted for a portion of Type IP or IS cement or for portland cement in high early strength concrete.
The maximum water cement ratio based on total cementitious materials shall be as follows for each class of concrete, except that fly ash shall be used in Class S in the proportions required by Table 10001.
(J) Use of Ground Granulated Blast Furnace Slag:
Granulated blast furnace slag may be substituted for a portion of portland cement in all classes of concrete except Class S. Slag may be substituted for up to 50% of the required cement on a pound for pound basis. Water cement ratio shall remain as shown in Table 10001. Slag shall not be substituted for a portion of Type IP or IS cement or for portland cement in high early strength concrete.
1000-5 CLASS M CONCRETE FOR REINFORCED BRICK MASONRY.
The provisions of Article 10004 will be applicable to Class M concrete, except that the mix proportions by volume shall be 1 part cement, 2 1/2 parts fine aggregate, and 1 1/2 parts coarse aggregate.
1000-6 CONCRETE FOR MACHINE PLACED CURB, CURB AND GUTTER, AND PAVED DITCH.
Concrete for machine placed curb, curb and gutter, and paved ditch shall be Class B concrete modified for machine placement.
1000-7 HIGH EARLY STRENGTH PORTLAND CEMENT CONCRETE.
High early strength portland cement concrete shall be used when required by the plans or special provisions. When not required, it may be used at the Contractor's option with the approval of the Engineer.
For all classes of concrete except Class AA, high early strength concrete may be produced by using Type III portland cement or a mix designed with the minimum specified amount of regular cement increased by 20 percent, but not to exceed 677 pounds per cubic yard of concrete. For Class AA concrete, high early strength concrete shall be produced by using Type III portland cement.
(A) Weighing Cement:
Cement shall be measured by weight on scales separate from those used for other materials, and in a hopper entirely free and independent of the hoppers used for weighing the aggregates. When the quantity of cement in a batch exceeds 30 percent of the full capacity of the scale, the quantity of cement as indicated by the scale shall be within plus or minus 1 percent of the required weight. For smaller batches, the quantity of cement as indicated by the scale shall be not less than the required amount nor more than 4 percent in excess. All beam type scales shall be equipped with a tare beam.
(B) Weighing Aggregates:
Aggregates shall be measured by weight. Batch weights shall be based on saturated surface dry materials and shall be the required weights plus the total weight of surface moisture contained in the aggregates. The individual aggregates, as weighed, shall be within plus or minus 2 percent of the required weights.
(C) Water:
Water shall be measured by volume or by weight. The quantity of water measured shall be within plus or minus 1 percent of the required amount.
(D) Admixture Dispensing Systems:
A separate dispensing system with separate fill and discharge lines shall be provided for each type of admixture to be used, except that admixtures may be measured and introduced into the mix manually if approval has been obtained from the Engineer. Each system shall be capable of measuring, displaying, and discharging the required amount of admixture into the mix. Dispensing systems shall be kept clean and in good operating condition. The dispensing system shall be either:
1. Manually operated, self contained; or
2. Semi automatic or automatic, self contained; or
3. Interfaced to operate automatically with the concrete batching control panel.
The admixture dispenser shall dispense the required quantity of admixture for each concrete batch within an accuracy of plus or minus 3 percent. It shall be the Contractor's responsibility to check the accuracy of the dispenser as provided below. The accuracy shall be checked at the point of discharge, or through a bypass valve suitable for obtaining a calibrated sample of admixture. The accuracy shall be checked at the volumes normally used for one half mixer capacity and for full mixer capacity. Accuracy shall be determined at the time of installation, and shall be checked daily during the early part of each day's operation.
Each system shall include a graduated measuring unit into which the admixture is batched to permit a quick visual check of accuracy prior to its discharge. The measuring unit shall be clearly graduated and shall be of sufficient size to hold the maximum anticipated dose for 1 batch. The measuring unit shall be clearly marked for the type of admixture to be used.
The discharge sequence shall be controlled so that an admixture will not be brought into contact with raw cement or another admixture before being diluted through contact with the mixing water in the mixer. Where 2 types of admixtures are being used, they shall not be discharged into the mix simultaneously. The air entraining agent shall be added with the first addition of water. Any other chemical admixture shall be added with the final addition of water, unless otherwise permitted by the Engineer.
The discharge lines shall be constructed to completely empty after each cycle. The admixture dispensing systems shall be so located that the batching plant operator will have a visual verification of the actual quantity of admixture batched.
Air entraining admixtures shall be used in accordance with the manufacturer's recommendations and in such quantity to provide the specified air content in freshly mixed concrete. The quantity of set retarding admixture and of water reducing admixture per 100 lb. of cement shall be within the range recommended on the current list of approved admixtures issued by the Materials and Tests Unit.
(A) Bins and Hoppers:
Bins with separate compartments for fine aggregates and for each required size of coarse aggregate shall be provided in the batching plant. Each compartment shall be designed to discharge efficiently and freely into the weighing hopper. Control shall be provided so that, as the quantity desired is being approached, the material may be added slowly and shut off with precision. Weighing hoppers shall be constructed to eliminate accumulation of tare materials and to discharge fully unless otherwise permitted by the Engineer. A port or other opening for removing an overload of any one of the several materials from the hopper shall be provided.
(B) Scales:
The scales for weighing aggregates and cement shall be of either the beam type or the springless dial type. The minimum graduation on beam or dial shall be not more than 0.1 percent of the total capacity of the scale. Methods of weighing, other than beam or springless dial scales, may be approved by the Engineer provided they meet the required weighing tolerances. Scales shall be accurate within 0.5 percent under operating conditions. Ten 50 pound test weights shall be available at the plant for checking accuracy. Test weights shall meet the US Bureau of Standards requirements for calibrating and testing equipment. All exposed fulcrums, clevises, and similar working parts of scales shall be kept clean. When beam type scales are used, provision shall be made for indicating to the operator that the required load in the weighing hopper is being approached. The device shall indicate at least the last 50 pounds of load, and shall also be designed to give a positive indication of overload of the scales. During charging of the hopper all indicating devices shall be in full view of the operator and he shall have convenient access to all controls. The indicating devices shall be in the immediate vicinity of the operator so that they are easily readable by the operator.
(C) Water Measuring Devices:
The device for measurement of the water shall be readily adjustable and shall be capable of being set to deliver the required amount and cut off the flow automatically when this amount has been discharged. Under all operating conditions the device shall have an accuracy within 1 percent of the quantity of water required for the batch. The device shall be so arranged that the measurements will not be affected by variable pressures in the water supply line. Measuring tanks shall be of adequate capacity to furnish the maximum mixing water required and shall be equipped with outside taps and valves to provide for checking their calibration unless other means are provided for readily and accurately determining the amounts in the tank.
(A) General:
Mixers are defined as equipment to mix concrete and may be stationary or truck mounted. Agitators are defined as equipment used to haul central mixed concrete and may be truck mixers or truck agitators. Each mixer and agitator shall have attached thereto in a prominent place a metal plate or plates on which the manufacturer has plainly marked the mixing speed of the drum or paddles and the maximum capacity of the drum or container in terms of volume of mixed concrete. On truck mixers and agitators, the manufacturer's recommended agitating and mixing speed of rotation of the mixing drum or blades shall also be shown. Stationary mixers shall be equipped with an acceptable timing device that will not permit the batch to be discharged until the specified mixing time has elapsed. Truck mixers shall be equipped with counters to verify the number of revolutions of the drum or blades. The counters shall be actuated at the initial time the drums have reached mixing speed.
Mixers and agitators shall be examined periodically for changes in condition due to accumulation of hard concrete or mortar, wear of blades, or any other condition which decreases mixing efficiency. Mixers shall be unacceptable when the radial height or other dimension of the blade has worn below 90 percent of the original dimension. This radial height does not include any lips on the blade, and shall be the height of the blade running perpendicular to the shell of the drum. Where such conditions are found, the units shall not be used until they are corrected.
Mixers and agitators shall also be examined periodically for general mechanical condition, including water measuring and discharge apparatus, condition of the blades, speed of rotation of the drum, and condition of the drum.
(B) Mixer Capacity:
Truck mixers when loaded with concrete shall not contain more than 63 percent of the gross volume of the drum. The mixer shall be capable of combining the ingredients of the concrete into a thoroughly mixed and uniform mass and of discharging the concrete with a satisfactory degree of uniformity. The stationary mixer, when loaded at the manufacturer's guaranteed mixing capacity and the concrete mixed for the prescribed mixing time, shall be capable of combining the ingredients of the concrete into a thoroughly mixed and uniform mass and discharging the concrete with satisfactory uniformity.
The minimum quantity of concrete permitted to be mixed or agitated in any mixer shall be not less than 20 percent of the rated mixing capacity.
(C) Agitator Capacity:
The agitator shall be loaded to not exceed 80 percent of the gross drum volume and shall be capable of maintaining the concrete in a thoroughly mixed and uniform mass and of discharging the concrete with a satisfactory degree of uniformity.
(D) Consistency Tests:
The Engineer may, from time to time, make slump tests to measure consistency of the concrete. Individual samples shall be taken at approximately the 1/5th point, the midpoint, and the 4/5th point of the load, using AASHTO T119. Such tests will be made within 20 minutes of discharge of that portion of the load. If the results vary by more than 1 inch in slump, the mixer or agitator shall not be used unless the condition is corrected.
(A) General:
Concrete shall be mixed and delivered to the site of the work by one of the following methods, except where other methods are approved by the Engineer. The Contractor shall be responsible for controlling the materials and operations as to produce uniform concrete meeting specification requirements.
1. Central Mixed Concrete:
Mixed completely in a stationary mixer and the mixed concrete transported to the point of delivery in a truck agitator or in a truck mixer operating at agitating speed or in non agitating equipment approved by the Engineer. Mixing shall be done within the capacity and at the mixing speeds recommended by the manufacturer.
2. Transit Mixed Concrete:
Mixed completely in a truck mixer while at the batching plant, in transit, or at the work site.
3. Shrink Mixed Concrete:
Mixed partially in a stationary mixer at a central mixing plant and completed as transit mixed concrete. All ingredients for a batch shall be in the stationary mixer and partially mixed before any concrete is discharged to the truck mixer, and the size of the batch shall not exceed the rated capacity of the equipment. The mixing time at the stationary mixer may be reduced to the minimum necessary to intermingle the ingredients, and the mixing completed in the truck mixer. The number of mixing revolutions in the truck mixer shall be as specified for transit mixed concrete or may be reduced as indicated by mixer performance tests.
When concrete is being produced for structures and incidental construction in accordance with the provisions of Article 10004, the Contractor shall have present during all batching operations a Certified Concrete Technician employed by the Contractor or concrete supplier. During batching and delivery, the sole duty of this employee shall be to supervise the production and control of the concrete. He shall perform moisture tests, adjust mix proportions of aggregates for free moisture, complete and sign Batch Tickets (M & T Form 903) or approved delivery tickets, and assure quality control of the batching. Delivery tickets will be permitted in lieu of batch tickets (M & T Form 903) provided they have been reviewed and approved by the Materials and Tests Unit. The Division of Highways certifies technicians who satisfactorily complete examinations prepared and administered by the Division of Highways.
(B) Mixing Time for Central Mixed Concrete:
Mixing time begins when all solid materials are in the mixing compartment and ends when any part of the concrete begins to discharge. In charging the mixer water will enter in advance of cement and aggregate, and substantially all the water shall be in the drum before one-third of the specified mixing time has elapsed. Transfer time in multiple drum mixers shall be counted as part of the mixing time.
Where mixer performance tests are not made, minimum mixing time shall be 90 seconds, providing that blending of materials during charging is achieved to the satisfaction of the Engineer. The minimum mixing time for an individual mixer is that which, as shown by mixer performance tests, will produce concrete meeting the requirements of Table 10003, except that the mixing time shall not be less than 50 seconds under any circumstances. Maximum mixing time excluding discharge time shall be 150 seconds.
Sampling and testing for mixer performance tests will be done as provided below. The mixer shall be charged to its rated capacity with the materials and proportions to be used in the work and mixed at the recommended mixing speed to the target time. Mixing shall then be stopped and discharging shall begin. Two samples of sufficient size to make the required tests will be taken after discharge of approximately 15 and 85 percent of the load.
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| Air content, percent by volume of
concrete (AASHTO T152) | |
| Slump, inches (AASHTO T119) | |
| Coarse aggregate content, portion
by weight of each sample retained on the No. 4 sieve, percent | |
| Weight per cubic foot (AASHTO
T121) | |
| Average compressive strength
at 7 days, percent of average (AASHTO T22 and T23) |
* Tentative approval may be granted pending 7 day compressive strength tests.
Each of the 2 samples of concrete will be separately tested for the properties listed in Table 10003. Tests will be conducted in accordance with the test procedures specified in Table 10003 or procedures established by the Materials and Tests Unit.
The mixer performance test described above will be performed on a minimum of 2 batches of concrete. For the performance test to be acceptable, all tests in each batch tested shall meet the requirements listed above.
The Engineer may recheck mixer performance at any time when in his opinion satisfactory mixing is not being accomplished.
Where satisfactory mixing cannot be accomplished in 90 seconds, the Engineer may increase the mixing time or require that the mixer be repaired or replaced before any further mixing can be done.
(C) Mixing; Truck Mixers and Truck Agitators:
When a truck mixer is used for complete mixing, each batch of concrete shall be mixed for not less than 70 revolutions of the drum or blades at the rate of rotation designated by the manufacturer of the equipment as mixing speed, unless otherwise directed by the Engineer. Unless the mixer is equipped with a counter which will distinguish between mixing and agitating speeds, the minimum required number of revolutions of the drum at mixing speed shall be done as directed by the Engineer, either at the batching plant before the mixer leaves for the work site and/or at the work site before the concrete is discharged. Any additional mixing shall be at the speed designated by the manufacturer of the equipment as agitating speed. All materials including mixing water shall be in the drum before actuating the revolution counter for determining the number of revolutions of the drum.
When a truck mixer or truck agitator is used to transport concrete that has been completely mixed in a stationary mixer, mixing during transport shall be at agitating speed.
The concrete, when discharged from truck mixers or truck agitators, shall be of the consistency and workability required for the work. The rate of discharge of the plastic concrete from the mixer drum shall be controlled by the speed or rotation of the drum in the discharge direction with the discharge gate fully open. If additional mixing water is necessary to produce the slump necessary for proper placement it shall be added only with the permission of the Engineer, and the truck mixer drum shall rotate a minimum of 25 revolutions at mixing speed before discharge of any concrete. Additional mixing water will be allowed only if the maximum specified water content per cubic yard is not exceeded.
(D) Delivery:
The Contractor shall use a ticket system for recording the transportation of batches from the proportioning plant to the site of the work. The tickets used shall be those furnished by the Engineer and shall be filled out in accordance with instructions issued by the Engineer. The tickets shall be issued to the truck operator at the proportioning plant for each load and will be signed by the plant inspector, which will signify that the concrete in the truck has been inspected prior to departure. Each ticket shall show the time batching was completed and if transit mixed, the number of revolutions at mixing speed, if any, at the plant. The tickets shall be delivered to the inspector at the site of the work. Loads which do not carry such tickets and loads which do not arrive in satisfactory condition within the time limits specified shall not be used in the work.
Ready mixed concrete plants shall have been inspected and approved
by the Department before they are used to produce concrete, either
paving, structural or incidental, for the project. Plants shall
meet all the applicable requirements of the specifications, and
in addition each ready mix plant shall have at least 3 acceptable
truck mixers or truck agitators available for use. Plants approved
by the Department will be placed on a list of approved plants
which will be made available to the Contractor. All plants will
be subject to reinspection at intervals selected by the Engineer.
Reapproval after each inspection will be contingent on continuing
compliance with the specifications.