10-1 Division 10
MATERIALS PORTLAND CEMENT CONCRETE PRODUCTION AND DELIVERY 1000 -1 DESCRIPTION This section addresses Portland cement concrete to be used for pavement, structures and precast and incidental construction. Produce Portland cement concrete composed of Portland cement, fine and coarse aggregates, and water. Include supplementary cementitious material (SCM) and chemical admixtures as required o r needed. SCMs consist of g round granulated blast furnace slag, fly ash or silica fume and may be substituted for a portion of the Portland cement. Type (Portland -Limestone Cement ) IL, (Portland -Pozzolan Cement ) IP, (Portland -Slag Cement ) IS or (Ternary Blended Cement ) IT blended cement may be used instead of Portland cement (see Section 1024 for details) . Cement, fine and coarse aggregate, and SCMs shall be approved by the Department prior to use. Only use admixtures that are currently on the N CDOT APL. All Portland cement concrete mixtures shall be designed by a NCDOT Certified Concrete Mix Design Technician or an engineer licensed by the State of North Carolina. For approved mixture designs requiring a major change , submit a new mix design accompanied by the applicable test results indicating the mix conforms to the design requirements for the indicated class of concrete. Define a major change as:
A.A source change in co arse aggregate or fine aggregate.
B.A change in cement type (e.g. from Type I /II to Type IL).
C.A change in SCM class or type change (e.g. Class F fly ash to Class C fly ash)
D.A quantitative change in coa rse aggregate, fine aggregate, or pozzolan greater than 5% (by weight) .
E.A quantitative change in water (applies to increase only) or cement (applies to decrease only) . For approved mix designs requiring a minor change, submit a Materials and Tests Form 312M. Define a m inor change as:
A.A change in source of the same type of cement or SCM .
B.A change in source or brand of admixtur es. When concrete for any one pour is furnished by multiple concrete plants, use the same mix design for all concrete, including sources and quantities of ingredients. Use materials which do not produce a mottled appearance through rusting or other staining of the finished concrete surface.
10-2 1000 -2 Materials
Refer to Division 10. Item Section Air Entraining Agent 1024 -3 Calcium Nitrite Corrosion Inhibitor 1024 -3 Chemical Admixtures 1024 -3 Coarse Aggregate 1014 -2 Fine Aggregate 1014 -1 Fly Ash 1024 -5 Ground Granulated Blast Furnace Slag 1024 -6 Portland Cement 1024 -1 Silica Fume 1024 -7 Type IL Blended Cement 1024 -1 Type IP Blended Cement 1024 -1 Type IS Blended Cement 1024 -1 Type IT Blended Cement 1024 -1 Water 1024 -4 1000 -3 PORTLAND CEMENT CONCRETE FOR STRUCTURES AND INCIDENTAL CONSTRUCTION
A.Composition and Design Provide the class of concrete required by the contract. Higher strength classes of concrete may be permissible as approved by the Engineer. Submit proposed concrete mix designs for each class of concrete to be used in the work. Mix proportions shall meet the design requirements provi ded in Table 1000- 1 and the applicable portions of this section . Laboratory trial batches shall be created to confirm the proposed mix design meets the requirements of the plastic and hardened concrete . Submit mix designs in terms of saturated surface dry weights on Materials and Tests Form 312U at least 35 days before proposed use. Adjust batch proportions 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 approved. Accompany Materials and Tests Form 312U with a listing of laboratory test results of aggregate gradation, air content, slump and compressive strength from a certified laboratory. List the compressive strength of at least three 6 inch x 12 inch or 4 inch x 8 inch cylinders at the age of 7 and 28 days. Perform laboratory tests in accordance with the following test procedures: Property Test Method Aggregate Gradation AASHTO T Air Content AASHTO T 152 Slump AASHTO T 119 Compressive Strength AASHTO T 2 2 and R100 The Engineer will review the mix design for compliance with the s pecifications and notify the Contractor as to its acceptability. Do not use a mix until written notice has been received. Acceptance of the mix design does not relieve the Contractor of his responsibility to furnish a product that meets the contract.
10-3 (B) Air Entrainment
Entrain air in the concrete unless otherwise indicated in the plan s or in the Specifications. Add an air entraining agent at the time of mixing to produce an air content in the freshly mixed concrete of 6.0% ± 1.5% when tested at the job site. Determine the air content in accordance with AASHTO T 1 21, T1 52 or T196. Measurement of air content may also be performed by the Chace Indicator, in which case sufficient tests will be made in accordance with AASHTO T 1 21, T1 52 or T196 to establish correlation with the Chace Indicator. Concrete for structures will not be rejected based on tests made with the Chace Indicator . Concrete for incidental construction may be rejected based on an average of 3 or more tests made with the Chace Indicator . Air entraining agent may be added at the job site when permitted by the Engineer.
C.Strength of Concrete The compressive strength of the concrete will be considered the average compressive strength test results of two 6 inch x 12 inch cylinders, or two 4 inch x 8 inch cylinders if the aggregate size is not larger than size 57 or 57M. Make cylinders in accordance with AASHTO R 100 from the concrete delivered to the work. Make cylinders at such frequencies as the Engineer may determine and cure them in accordance with AASHTO R 100 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 for the class of concrete 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 below the specification, the in -place concrete will be investigated . Based on these investigation results, the concrete will be accepted with no reduction in payment , accepted at a reduced unit price or rejected as set forth in Article 105 -3.
D.Temperature Requirements The concrete temperature at the time of placement shall be not less than 50°F and no more than 95°F except where other temperatures are required by Articles 420- 4, 420- 7, 420- 14 and 420- 15. Do not place concrete without permission when the air temperature m easured at the location of the concrete operation in the shade away from artificial heat is below 35°F. When such permission is granted, uniformly heat the aggregates and/or water to a temperature not higher than 150°F. Heated concrete shall be between 55°F and 80°F at the time of placement.
10-4 Table 1000 -1
DESIGN REQUIREMENTS FOR CONCRETE Class of Concrete Min. Compressive Strength at 28 days Maximum Water - Cementitious Ratio Maximum Slump Min. Cementitious Content Coarse Aggregate Sizes Vibrated Non- Vibrated Units psi inch inch lb/cy AA 4500 0.426 1.5 slip form 3.5 A --- 639 57, 67, or 78M Drilled Pier B 4500 0.450 --- 5 – dry 7 - wet 640 78M Sand Light - weight 4500 0.420 4.0 A --- 715 Lightweight (see Table 1014 -1) A B,C 3000 0.532 3.5 A 4.0 564 57, 67, or 78M BB 2500 0.567 1.5 machine placed 2.5 A hand placed 4.0 508 57, 67, or 78M
A.A slump of 6 inches is allowed only by use of an approved admixture meeting
Article 1024 — 3. In no case shall the water -cement ratio on the approved design be
exceeded.
B.Drilled Pier mixes are non -air entrained. Class A and Class B mixes may be non -air entrained if specified by the contract or special provision.
C.Per Article 450-2, Class A concrete used for drilled -in piles shall have a slump of 6- 8 inches and is achieved using an approved water reducer. Do not exceed the water - cement ratio on the approved design.
E.Elapsed Time for Placing Concrete Regulate the delivery so the maximum interval between the placing of batches at the work site does not exceed 20 minutes. Place concrete before exceeding the times in Table 1000-
2.Measure the elapsed time as the time between adding the mixing water to the mix and placing the concrete.
10-5 Table 1000 -2
ELAPSED TIME FOR PLACING CONCRETE Air or Concrete Temperature Whichever is Higher Maximum Elapsed Time No Retarding Admixture Used Retarding Admixture Used 90°F or above 30 minutes 1 hr. 15 minutes 80°F through 89°F 45 minutes 1 hr. 30 minutes 79°F or belowA 60 minutes 1 hr. 45 minutes 70°F through 79°FB 60 minutes 1 hr. 45 minutes 69°F or belowB 1 hr. 30 minutes 2 hr. 15 minutes
A.Applicable to Class AA, A and Drilled Pier concrete.
B.Applicable to Class B concrete.
F.Use of Set Retarding Admixtures Use an approved set retarding admixture in all concrete placed in the superstructure of bridges such that the concrete will remain workable until the entire operation of placing and finishing, including corrective measures, if necessary, has been completed. The Engineer may waive the use of set retarding admixture when conditions clearly indicate that it is not needed. Other structural concrete may contain an approved set retarding admixture when permitted by the Engineer. Use a set retarding admixture on the NCDOT APL following the manufacturer’s recommended dosage rate.
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. Use a water reducing admixture on the NCDOT APL following the manufacturer’s recommended dosage rate. Concrete containing water reducing admixtures that exhibits segregation and/or excessive bleeding will be rejected. Utilizing an admixture to modify slump does not relinquish the contractor’s responsibility to ensure the final product quality and overall configuration meets design specifications. Caution should be taken when placing these mixes on steep grades to prevent unintended changes to the set slope.
H.Use of Calcium Chloride Calcium chloride may be used as a set accelerating agent where permitted by the Engineer. Use one lb. of calcium chloride per 100 lbs. of cement except where lesser amounts are directed by the Engineer . Do not use calcium chloride where steel reinforcement, metal conduit or other metals will be in contact with the concrete. Do not use calcium chloride in concrete that has a temperature higher than 70°F, or when the air temperature is greater than 70°F. Provide cold weather protection for concrete containing calcium chloride in the same manner as is provided for concrete without calcium chloride. Use cal cium chloride in liquid form. Dissolve solid calcium chloride using one lb. or less of calcium chloride per one quart of water and mix well to form a liquid solution . To avoid incompatibility with other additives, add the calcium chloride to the batch af ter all other ingredients have been put into the mixer.
I.Use of Supplementary Cementitious Materials (SCMs) SCMs may be substituted for cement in the mix design at a rate of 1.0 lb. of SCM to each pound of cement replaced, up to the maximum amount shown in Table 1024 -1. Concrete mixes using SCMs shall not exceed the maximum allowable water/cementitious material
10-6 ratio for the designated concrete class. Use Table 1000 -1 to determine the maximum
allowable water -cementitious material ratio for the classes of c oncrete listed.
J.Use of Calcium Nitrite Corrosion Inhibitor Units with calcium nitrite in a quantity less than specified are subject to rejection. Furnish concrete powder drilled from representative cylinders to the Engineer, in a quantity to be specified , to verify the concentrations of calcium nitrite in hardened concrete. Concrete that fails to contain calcium nitrite at the required concentrations as tested is subject to rejection. Use air -entraining, water -reducing and/or set -controlling admixtures compatible with calcium nitrite solutions. Strictly adhere to the manufacturer’s written recommendations regarding the use of admixtures, including storage, transportation and method of mixing. If preferred, use calcium nitrite, which acts as an accelerat or, in conjunction with a retarder to control the set of concrete, as per the manufacturer’s recommendation. Add an approved calcium nitrite corrosion inhibitor (30% solids) to the concrete mix at the batch plant for the bridge elements identified by the plan notes. Use the inhibitor at a minimum rate of 3.0 gal/cy . Ensure that the hardened concrete contains at least 5. 8 lbs/cy nitrite (NO 2) when tested in accordance with Materials and Tests Method Chem. C-20.0. The preceding paragraph does not apply to c oncrete used in prestressed concrete members. Concrete used in prestressed concrete members shall be tested in accordance with Subarticle 1078 -4(G). 1000 -4 PORTLAND CEMENT CONCRETE FOR PAVEMENT
A.Composition and Design Submit a concrete paving mix design in terms of saturated surface dry weights on Materials and Tests Form 312U to the Engineer for approval at least 3 5 days before proposed use. Mix proportions shall meet the design requirements provided in Table 1000- 3 and the applicable portions of this section. TABLE 1000 -3 DESIGN REQUIREMENTS FOR CONCRETE PAVEMENT Class of Concrete Min. Compressive Strength at 28 days Min. Flexural Strength at days Maximum Water/ Cementitious Ratio Max . Slump Min. Cementitious Content Air Content Coarse Aggregate Sizes Units psi psi lb/lb inch lb/cy % Pavement 4500 650 0.559 1.5 slip form 3.0 hand placed 526 5.0% ± 1.5% 57, 67, or 78M Very High Early Strength for Pavement Repair 4500 650 400 at hoursA 0.500 1.5 slip form 3.0 hand placed 600 5.0% ± 1.5% 57, 67, or 78M
A.Use of a high alkali cement or reactive aggregate is prohibited unless the supplier can achieve the required flexural strength in 4 hours while substituting the minimum supplementary cementitious material (SCM) amount specified in Section 1024 -1 for mitiga ting ASR. Include in the mix design the source of aggregates, cement, SCM , water and admixtures; the gradation and specific gravity of the aggregates; the fineness modulus of the fine aggregate; and the dry rodded unit weight and size of the coarse aggrega te. Submit test
10-7 results showing that the mix design conforms to the criteria, including the 1, 3, 7, 14 and
28-day strengths of the average of two 6 inch x 6 inch x 20 inch beams and the average of two 6 inch x 12 inch cylinders for each age made and tested in accordance with AASHTO R 39, T22 and T97 from a certified laboratory . Design the mix to produce an average strength sufficient to indicate that a minimum strength of 650 psi in flexure and 4,500 psi in compression will be achieved in the field within 28 days. Where concrete with a higher slump for hand methods of placing and finishing is necessary, submit an adjusted mix design for approval to provide a maximum slump of 3 inches and to maintain the water /cementitiou s material ratio established by the original mix design. The water/cementitious ratio refers to the ratio of the weight of water (lb) in concrete to the combined weight of cement and SCMs in the concrete. For mixes that use only cement (e.g. no SCMs), th e water/cementitious ratio refers to the ratio of the weight of water (lb) to the weight of cement (lb) in the concrete.
B.Air Entrainment Entrain air in the concrete by the use of an approved air entraining agent dispensed with the mixing water, unless prohibited. Provide an air content of 5.0% ± 1.5% in the freshly mixed concrete. The air content will be determined in accordance with AASHTO T 121, T1 52 or T196. At the option of the Engineer, the air content may be measured by the Chace I ndicator, in which case sufficient tests will be made to establish correlation with the test methods of AASHTO T 121, T1 52 or T196 . Concrete will not be rejected based on tests from the Chace Indicator .
C.Slump Provide concrete with a maximum slump of 1. 5 inches where placed by a fully mechanized paving train and no more than 3 inches where placed by hand methods. The sample taken for determination of slump will be obtained immediately after the concrete has been discharged onto the road.
D.Set Retarding Admixture and Water Reducing Admixture With permission, the Contractor may use an approved set retarding admixture, an approved water reducing admixture or both to facilitate placing and finishing. Use a quantity of set retarding admixture or water reducing admixture within the range shown on the current list of approved admixtures maintained by the Materials and Tests Unit.
E.Use of Supplementary Cementitious Materials (SCMs) SCMs may be substituted for cement in the mix design at a rate of 1.0 lb. of SCM to each pound of cement replaced, up to the maximum amount shown in Table 1024 -1. Concrete mixes using SCMs shall not exceed the maximum allowable water -cementitious material ratio for the designated concrete class. Use Table 1000 -3 to determine the maximum allowable water -cementitious material ratio for the classes of concrete listed.
F.Contractor ’s Responsibility for Process Control Before or at the preconstruction conference, s ubmit a plan detailing the process control and the type and frequency of testing and inspection necessary to produce concrete that meets the specifications . During all batching and delivery operations assign a Certified Concrete Batch Technician on site whose sole duty is 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 adm ixture dispensing systems, weighing systems and water gauge s.
3.Tests and adjustments of mix proportions for moisture content of aggregates.
10-8 (4) Mixer performance tests before reducing mixing time of central mix plant to less than
90 seconds and at other times w hen 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. Determine air content at least twice each day.
8.Tests for depth of the pavement in the plastic state.
9.Furnishing data to verify that the approved theoretical cement content has been met at intervals not to exc eed 50,000 sy of pavement.
10.Signing all plant reports, batch tickets and delivery tickets. The Department certifies technicians who satisfactorily complete examinations prepared and administered by the Department . Perform all test procedures in compliance w ith the appropriate articles of Section 1000. Tests may be witnessed by the Engineer. Document the results of all tests and inspections and make a copy available to the Engineer upon request. Take prompt action to correct conditions that have resulted in or could result in the submission of materials, products, or completed construction that do not conform to the s pecifications .
G.Contractor Not Relieved of Responsibility for End Result The Contractor will not be relieved of his obligation to produce a unifo rm 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 requir ements by the Contractor; or
3.The failure of the Engineer to perform any tests in the process control, nor the performance of any tests in the process control that indicate compliance with the specifications . 1000 -5 VERY HIGH EARLY STRENGTH CONCRETE FOR CONCRETE PAVEMENT REPAIR Mix proportions shall meet the design requirements provided in Table 1000- 3 and the applicable portions of this section. Use cement, fine aggregate, coarse aggregate, admixtures and SCMs that have been approved by the Department. SCMs shall be used according to Section 1024. Accompany Materials and Tests Form 312U with 4 hour flexural strength results of at least 6 beams made and tested in accordance with AASHTO R 39 and T97. In addition, submit 4 hour compressive strength results of at least six 4 inch by 8 inch or 6 inch by 12 inch cylinders and maturity test re sults of the mix. With permission of the Engineer, compressive strength testing and maturity testing may be used in lieu of or concurrent with flexural strength testing to determine the acceptability of the concrete in the field. 1000 -6 HIGH EARLY STRENGTH PORTLAND CEMENT CONCRETE Use high early strength Portland cement concrete when required by contract. When not required, it may be used at the Contractor ’s option with approval of the Engineer.
10-9 For all classes of concrete, high early strength concrete may be produced by using
Type III Portland cement. To produce high early strength concrete with regular cement, use a higher class of concrete as follows: For Class A and Class B, use Class AA with a cement content of at least 677 lbs /cy. Other classes that lend themselves to high early strength with regular cement will be reviewed by the Engineer on a case- by-case basis. 1000 -7 FLOWABLE FILL Flowable fill consists of Portland cement, water, supplementary cementitious materials and/or fine aggregate and, optionally, concrete admixtures. Submit the proposed mix design on Materials and Tests Form 312U at least 35 days before use. State on Materials and Tests Form 312U the intended use of the material (excavatable or non- excavatable) . Excavatable flowable fill shall have a maximum strength of 150 psi at 56 days of age. Non- excavatable flowable fill shall have a minimum strength of 125 psi at 28 days of age. Accompany Materials and Tests Form 312U with a listing of compressive strength of at least three 4 inch x 8 inch cylinders at the age of 28 or 56 days, depending on whether the mix is to be excavated or not. Air cure the cylinders during the entire period before testing. The Engineer will advise the Contractor in writing of the acceptability of the mix design. 1000 -8 LATEX MODIFIED CONCRETE (LMC)
A.Materials Refer to Division 10. Item Section Coarse Aggregate, standard size No. 78M 1014 -2 Fine Aggregate 1014 -1 Portland Cement 1024 -1 Type IL Blended Cement 1024 -1 Type IP Blended Cement 1024 -1 Type IS Blended Cement 1024 -1 Type IT Blended Cement 1024 -1 Water 1024 -4 Use a formulated latex admixture that is a non -hazardous, film forming and polymeric emulsion in water and is homogeneous and uniform in composition. Add all stabilizers at the point of manufacture. The use of Type III high early strength cement in LMC is only permitted as allowed by the contract . Use a latex modifier conforming to Table 1000- 4.
10-10 Table 1000 -4
PROPERTIES OF LATEX MODIFIER FOR CONCRETE Property Requirement Polymer Type Styrene Butadiene: 68 ± 4% Styrene 32 ± 4% Butadiene Average Polymer Particle Size 1500 to 2500 Angstroms Emulsion Stabilizers Anionic and non-ionic surfactants Percent Solids 46.5% to 49.0 % Weight per gallon at 75°F 8.40 to 8.60 lb pH 9.5 to 11.0 Shelf Life 2 Years Color White Provide a Type 5 material certification for each load of latex emulsion admixture in accordance with Article 106 -3. Test admixture samples to verify compliance with the requirements before use. Allow 7 days for sampling and testing after delivery to the project. Do not allow the temperature of latex emulsion admixture to fall below 35°F at any time or exceed 85°F after delivery to the project. For latex emulsion that has been in storage, use a transfer pump and lines to recirculate it before using and sampling. For LMC , use a workable mixture that meets Table 1000 -5. Measure the slump 4 to 5 minutes after discharge from the mixer. Submit the LMC mix design, including laboratory compressive strength data for a minimum of six (6) 4- inch by 8- inch cylinders at seven (7) days for normal setting concrete to the Engineer for review. Include test results for the slump and air content of the laboratory mix. Perform tests in accordance with AASHTO T 22, T119 and T152. TABLE 1000 -5 PROPERTIES OF LATEX MODIFIED CONCRETE Property Requirement Cement Content, lb/cy 658 min. Latex Emulsion Admixture, gal/ cy 24.5 min. Air Content of Plastic Mix, % 3.5 - 6.5 Slump, inches 3 - % Fine Aggregate as percent of total aggregate by weight 50 - 7 day Compressive S trength, psi 3,000 min. Water -Cement Ratio by weight 0.40 max.
B.Equipment Before beginning any work, obtain approval for all equipment to be used for deck preparation, mixing, placing, finishing and curing the latex modified concrete. Use sandblasting equipment capable of removing all clay, salt deposits, oil and grease deposits and all other foreign matter. Provide traps or separators to remove oil and water from the compressed air. Use traps or separators of adequate size and drain t hem periodically during operations. For proportioning and mixing, use self -contained, mobile and continuously mixing equipment that meets the following requirements:
1.Use a self -propelled mixer that is capable of carrying sufficient unmixed dry, bulk cement, sand, coarse aggregate, latex modifier and water to produce at least 6 cy of concrete on site.
10-11 (2) Use a mixer that is capable of positive measurement of cement introduced into the
mix. Use a recording meter that is visible at all times and equipped w ith a ticket printout to indicate the quantity of cement.
3.Calibrate the mixers to accurately proportion the specified mix. Before placing latex modified concrete, perform calibration and yield tests under the Engineer ’s supervision in accordance with the Department ’s written instructions. Copies of these written instructions are available from the Materials and Tests Unit. Perform the calibration and yield tests using the material to be used on the project. Recalibrate the mixer after any major maintenance operation on the mixer, anytime the source of materials changes or as directed by the Engineer . Furnish all materials and equipment necessary to perform the calibrations and yield tests.
4.Use a mixer that controls the flow of water and latex emulsion i nto the mix. Measure the flow rate of water and the latex emulsion with a calibrated flowmeter coordinated with both the cement and aggregate feeding mechanisms and the mixer. Adjust the flow rate, as necessary, to control the slump and ensure that the w ater-cement ratios are met. In addition to flowmeters, use mixers with accumulative water and latex meters capable of indicating the number of gallons, to the nearest 0.1 gallon, introduced into the mixer. Filter water and latex with a suitable mesh filter before it flows through the accumulative water and latex meters.
5.Calibrate the mixer to automatically proportion and blend all components of the indicated composition on a continuous or intermittent basis as the finishing operation requires. Provide a mixer that discharges mixed material through a conventional chute and is capable of spraying water over the placement width as it moves ahead to ensure that the surface to be overlaid is wet before receiving the modified material.
6.Mount a tachometer on th e unit to indicate the drive shaft speed.
7.Use adequate hand tools for placing and leveling concrete down to approximately the correct level for striking off with the screed.
8.Use a finishing machine that meets the approval of the Engineer and the requirements of the contract. Use a self -propelled finishing machine capable of forward and reverse movement under positive control. Use a machine with at least 2 finishing devices, one that is a vibrating screed and the other either a vibrating screed, oscillatin g screed, or one or more rotating cylindrical drums 48 inches long or less and operating between 1,500 and 2,500 vpm. Make certain the finishing machine can finish the surface to within 1 foot of the edges of the area being placed. Raise all screeds when the finishing machine is moving backwards over the screeded surface.
9.Use screeds with a vibration frequency that is variable between 3,000 and 6,000 vpm with positive controls. Use screeds with a metal covered bottom face not less than 4 inches wide. Provide screeds with positive control of the vertical position.
10.Use supporting rails for travelling of the finishing machine rigid enough to eliminate deflection from the weight of the machine.
C.Proportioning and Mixing of Modified Compositions Use mobile continuous mixers that accurately proportion all materials for the specified mixture. Operate the proportioning equipment at the manufacturer ’s recommended speed verified with the tachometer during calibration and normal operations. Yield checks an d other checks are permitted.
D.Contractor’s Responsibility for Process Control Before or at the preconstruction conference, submit a plan detailing the process control and the type and frequency of testing and inspection necessary to produce concrete that m eets the specifications . During all batching and delivery operations assign a Certified Concrete
10-12 Batch Technician on site whose sole duty is to supervise the production and control of the
concrete. This duty includes the following:
1.Tests and inspecti ons necessary to maintain the stockpiles of aggregates in an unsegregated and uncontaminated condition.
2.Calibration of admixture dispensing systems, weighing systems and water gauges.
3.Tests and adjustments of mix proportions for moisture content of aggregates.
4.Mixer performance tests before 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 introdu ced 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. Determine air content at least twice each day.
8.Tests for depth of the pavement in the plastic state.
9.Furnishing data to verify that the approved theoretical cement content has been met at intervals not to exceed 50,000 sy of pavement.
10.Signing all plant reports, batch tickets and delivery tickets. The Department certifies technicians who satisfactorily complete examinations prepared and administered by the Division of Highways. Perform all test procedures in compliance with the appropriate articles of Section 1000. Tests may be witnessed by the Engine er. Document the results of all tests and inspections and make a copy available to the Engineer upon request. Take prompt action to correct conditions that have resulted in or could result in the submission of materials, products, or completed constructi on that do not conform to the se specifications . 1000 -9 MEASURING MATERIALS
A.Weighing Cement Measure cement by weight on scales separate from those used for other materials and in a hopper that is entirely free and independent of the hoppers used for weighing the aggregates. When the quantity of cement in a batch exceeds 30% of the full capacity of the scale, ensure the quantity of cement as indicated by the scale is within ± 1% of the required weight. For smaller batches, ensure the quantity of cement as indicated by the scale be not less than the required amount or more than 4% in excess. Equip all beam type scales with a tare beam.
B.Weighing Aggregates Measure aggregates by weight. Base batch weights on saturated surface dry materials which is the required weight plus the total weight of surface moisture contained in the aggregate. Ensure the individual aggregates, as weighed, are within ± 2% of the required weights.
C.Water Measure water by volume or by weight. Ensure the quantity of water measured is within ± 1% of the required amount.
D.Admixture Dispensing Systems Provide a separate dispensing system with separate fill and discharge lines for each type of admixture to be us ed, except that admixtures may be measured and introduced into the mix
10-13 manually if approval has been obtained. Ensure each system is capable of measuring,
displaying and discharging the required amount of admixture into the mix. Keep dispensing systems c lean and in good operating condition. Use a dispensing system that is 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. Have the admixture dispenser dispense the required quantity of admixture for each concrete batch within an accuracy of ± 3%. Check the accuracy of the dispenser as provided below. Check the accuracy at the point of discharge, or through a bypass valve suitable for obtaining a calibrated sample of admixture and at the volumes normally used for one half mixer capacity and for full mixer capacity. Determine the accuracy at the time of installation and check daily during the early part of each day ’s operatio n. Include in each system a graduated measuring unit into which the admixture is batched to permit a quick visual check of accuracy before its discharge. Ensure the measuring unit is clearly graduated and be of sufficient size to hold the maximum anticipa ted dose for one batch. Clearly mark the measuring unit for the type of admixture to be used. Control the discharge sequence so 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, do not discharge them into the mix simultaneously. Add the air entraining agent with the first addition of water and add any other chemical admixture with the final addition of wate r, unless otherwise permitted. Construct the discharge lines to completely empty after each cycle. Locate the admixture dispensing systems so the batching plant operator will have a visual verification of the actual quantity of admixture batched. Use air entraining admixtures in accordance with the manufacturer ’s recommendations and in such quantity to provide the specified air content in freshly mixed concrete. Use a quantity of set retarding admixture and of water reducing admixture per 100 lbs. of cement that is within the range recommended on the current list of approved admixtures issued by the Materials and Tests Unit. 1000 -10 BATCHING PLANT
A.General Plants located on the Department rights of way shall conform to Article 107 -3. Have ready mixed concrete plants inspected and approved by the Department before they are used to produce concrete, either paving, structural or incidental, for the project. P lants shall meet all the applicable requirements of the se Standard Specifications , and in addition, ensure each ready mix plant provide s at least three acceptable truck mixers or truck agitators available for use. Use trucks that have an identifying number. Plants approved by the Department will be pl aced on a list of approved plants 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 Standard Specifications .
B.Bins and Hoppers Provide bins with separate compartments for fine aggregates and for each required size of coarse aggregate in the batching plant. Design each compartment to discharge efficiently and freely into the weighing hopper. Provide control so , as the quantity desired is being approached, the material may be added slowly and shut off with precision. Construct weighing hoppers to eliminate accumulation of tare materials and to discharge fully unless
10-14 otherwise permitted. Provide a port or other opening for removing an overload of any one
of the several materials from the hopper.
C.Scales Use either the beam type, load cell type or the springless dial type scales for weighing aggregates and cement. Ensure the minimum graduation on beam or dial is not more than 0.1% 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. Ensure the scales are accurate within 0.5% under operating conditions. Make available ten 50 lb. test weights at the plant for checking accuracy. Use test weights which meet the U .S. Bureau of Standards requirements for calibrating and testing equipment. Keep all exposed fulcrums, clevises and similar w orking parts of scales clean. When beam type scales are used, make provisions for indicating to the operator that the required load in the weighing hopper is being approached. Ensure the device indicate s at least the last 50 lbs. of load and design it to give a positive indication of overload of the scales. During charging of the hopper, have all indicating devices in full view of the operator and provide convenient access to all controls. Ensure the indicating devices are in the immediate vicinity of the operator and easily readable by the operator.
D.Water Measuring Devices Use devices for measurement of the water which are readily adjustable and are capable of being set to deliver the required amount and cut off the flow automati cally when this amount has been discharged. Under all operating conditions the device shall have accuracy within 1% of the quantity of water required for the batch. Arrange the device so variable pressures in the water supply line will not affect the mea surements. Use measuring tanks of adequate capacity to furnish the maximum mixing water required and equip them with outside taps and valves to provide for checking their calibration unless other means are provided for readily and accurately determining t he amounts in the tank. 1000 -11 MIXERS AND AGITATORS
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 centra l mixed concrete and may be truck mixers or truck agitators. Provide a metal plate or plates attached to each mixer and agitator in a prominent place on which the manufacturer has plainly marked the mixing speed of the drum or paddles and the maximum capa city of the drum or container in terms of volume of mixed concrete. On truck mixers and agitators, show the manufacturer ’s recommended agitating and mixing speed of rotation of the mixing drum or blades. Equip stationary mixers with an acceptable timing device that will not permit the batch to be discharged until the specified mixing time has elapsed. Equip truck mixers with counters to verify the number of revolutions of the drum or blades. Actuate the counters at the initial time the drums have reache d mixing speed. Examine mixers and agitators 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 are unacceptable when the radial height or o ther dimension of the blade has worn below 90% of the original dimension. This radial height excludes any lips on the blade and is the height of the blade running perpendicular to the shell of the drum. Where such conditions are found, do not use the uni ts until they are corrected. Also examine mixers and agitators periodically for general mechanical condition, including water measuring and discharge apparatus, identifying number on trucks, condition of the blades, speed of rotation of the drum and condit ion of the drum.
10-15 (B) Mixer Capacity
Do not load truck mixers with concrete with more than 63% of the gross volume of the drum. Use mixers capable of combining the ingredients of the concrete into a thoroughly mixed and uniform mass and of discharging the conc rete with a satisfactory degree of uniformity. Use stationary mixers, when loaded at the manufacturers guaranteed mixing capacity and the concrete mixed for the prescribed mixing time, capable of combining the ingredients of the concrete into a thoroughly mixed and uniform mass and discharging the concrete with satisfactory uniformity. Use at least 20% of the rated mixing capacity as the minimum quantity of concrete permitted to be mixed or agitated in any mixer.
C.Agitator Capacity Load the agitator to not exceed 80% of the gross drum volume and have it 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 tim e, make slump tests to measure consistency of the concrete. Take individual samples at approximately the 1/5th point, the midpoint and the 4/5th point of the load, using AASHTO T 119. 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, do not use the mixer or agitator unless the condition is corrected. 1000 -12 MIXING AND DELIVERY
A.General Mix and deliver concrete to the sit e of the work by one of the following methods, except where other methods are approved by the Engineer . Maintain responsibility for controlling the materials and operations as to produce uniform concrete meeting s pecifications requirements. When concrete is being produced for structures and incidental construction in accordance with Article 1000 -3, have present during all batching operations a Certified Concrete Batch Technician employed by the Contractor or concrete supplier. During batching and delivery, the sole duty of this employee is to supervise the production and control of the concrete. Perform moisture tests, adjust mix proportions of aggregates for free moisture, complete and sign Batch Tickets ( Materials and Tests Form 903) or approve d delivery tickets and assure quality control of the batching. Delivery tickets will be permitted instead of batch tickets ( Materials and Tests Form 903) provided they have been reviewed and approved by the Materials and Tests Unit. The D epartment certif ies technicians who satisfactorily complete examinations prepared and administered by the D epartment .
1.Central Mixed Concrete Concrete that is mixed completely in a stationary mixer and the mixed concrete transported to the point of delivery in a truck agit ator or in a truck mixer operating at agitating speed or in non -agitating equipment approved by the Engineer. Perform mixing within the capacity and at the mixing speeds recommended by the manufacturer.
2.Transit Mixed Concrete Concrete that is mixed comple tely in a truck mixer while at the batching plant, in transit, or at the work site.
3.Shrink Mixed Concrete
10-16 Concrete that is mixed partially in a stationary mixer at a central mixing plant and
completed as transit mixed concrete. Place all ingredients for a batch in the stationary mixer , partially mix before any concrete is discharged to the truck mixer and do not exceed the rated capacity of the equipment for the batch size. The mixing time at the stationary mixer may be reduced to the minimum necessary to intermingle the ingredients, and the mixing may be completed in the truck mixer. Use the number of mixing revolutions in the truck mixer as specified for transit mixed concrete or reduce as indicated by mixer performance tests.
B.Mixing Time for Central Mix ed 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. Ensure all the water is substant ially in the drum before 1/3 of the specified mixing time has elapsed. Count transfer time in multiple drum mixers as part of the mixing time. Where mixer performance tests are not made, use a minimum mixing time of 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 in accordance with Table 1000- 6, except that the mixing time shall not be less than 50 seconds under any circumstances. Maximum mixing time excluding discharge time is 150 seconds. Sampling and testing for mixer performance tests will be done as provided below. Charge the mixer 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. Stop mixing and begin discharging. Two samples of sufficient size to make the required tests will be taken after discharge of approximately 15% and 85% of the load. TABLE 1000 -6 REQUIREMENTS FOR UNIFORMITY OF CONCRETE Property Requirement Test Method Difference in Test Samples Air Content, percent by volume of concrete 1.0% AASHTO T 152 Slump 1.0" AASHTO T 119 Coarse aggregate content, portion by weight of each sample retained on the No. 4 sieve 6.0% AASHTO M 157 Weight 1.0 lb AASHTO T 121 Average Compressive Strength at 7 days, percent of average 10.0%A AASHTO T AASHTO R 100
A.Tentative approval may be granted pending 7 day compressive strength tests. Each of the two samples of concrete will be separately tested for the properties listed in Table 1000- 6. Tests will be conducted in accordance with the test procedures specified in Table 1000- 6 or procedures established by the Materials and Tests Unit. The mixer perform ance test described above will be performed on at least two batches of concrete. For the performance test to be acceptable, have all tests in each batch tested 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.
10-17 (C) Truck Mixers and Truck Agitators
When a truck mixer is used for complete mixing, mix each batch of concrete for at least 70 revolutions of the drum or blades at the rate of rotation designated by the manufacturer of the equipment as mixing speed, unles s otherwise directed by the Engineer. Unless the mixer is equipped with a counter which will distinguish between mixing and agitating speeds, perform the minimum required number of revolutions of the drum at mixing speed as directed by the Engineer , eithe r at the batching plant before the mixer leaves for the work site and/or at the work site before the concrete is discharged. Perform any additional mixing at the speed designated by the manufacturer of the equipment as agitating speed. Put all materials including mixing water 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, perform mixing during transport at agitating speed. Provide concrete, when discharged from truck mixers or truck agitators, of the consistency and workability required for the work. Control the rate of discharge of the plastic concrete from the mixer drum by t he 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, add it only with permission and rotate the truck mixer drum at least 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 Use a ticket system for recording the transportation of bat ches from the proportioning plant to the site of the work. Use tickets furnished by the Engineer and fill it out in accordance with instructions issued by the Engineer. Issue the tickets to the truck operator at the proportioning plant for each load and have them signed by the plant inspector, which will signify that the concrete in the truck has been inspected before departure. Ensure each ticket show s the time batching was completed and if transit mixed, the number of revolutions at mixing speed, if any, at the plant. Deliver the tickets to the inspector at the site of the work. Do not use loads which do not carry such tickets and loads which do not arrive in satisfactory condition within the time limits specified in the work. 1000 -13 VOLUMETRIC MIXED CONCRETE Upon written request by the contractor, the Department may approve the use of concrete proportioned by volume. The volumetric producer must submit and have approved a process control plan and product quality control plan by the Materials and Tests Unit. If concrete is proportioned by volume, the other requirements of these specifications with the following modifications will apply. Unless otherwise approved by the Department, use of concrete proportioned by volume shall be limited to Class B concrete and no more than 30 cy per unit per day.
A.Materials Use materials that meet the requirements for the respective items except that they will be measured by a calibrated volume- weight relationship. Storage facilities for all material shall be designed to permit the Department to make necessary inspections before the batching operations. The facilities shall permit identification of approved material at all times and shall be designed to avoid mixing wi th, or contaminating by , unapproved material. Coarse and fine aggregate shall be furnished and handled so variations in the moisture content affecting the uniform consistency of the concrete is avoided. Moisture content of the coarse and fine aggregate wi ll be made available onsite for the Engineer ’s review for each load. The frequency of moisture testing will be dependent on
10-18 certain variables such as weather, season and source; however, moisture tests should be
performed at least once at the beginning of the work day for each source material. Additional daily moisture tests for the coarse and fine aggregate shall be performed if requested by the Engineer. Unused materials should be emptied from hopper daily. Concrete should not be mixed with materials left in the hopper overnight.
B.Equipment Provide volumetric mixers with rating plates indicating that the performance of the mixer is in accordance with the Volumetric Mixer Manufacturer Bureau or equivalent. Mixers must comply with ASTM C685. Unless otherw ise specified, all mixing operations must be in strict accordance with the manufacturer ’s recommended procedures. Such procedures shall be provided to the Department for review upon request. The volumetric mixer shall be capable of carrying sufficient unmi xed dry bulk cement, supplementary cementitious material (if required), fine aggregate, coarse aggregate, admixtures and water, in separate compartments and accurately proportioning the specified mix. Each batching or mixing unit (or both) shall carry in a prominent place a metal plate or plates on which are plainly marked the gross volume of the unit in terms of mixed concrete, discharge speed and the weight -calibrated constant of the machine in terms of a revolution counter or other output indicator. The concrete mixing device shall be an auger -type continuous mixer used in conjunction with volumetric proportioning. The mixer shall produce concrete, uniform in color and appearance, with homogeneous distribution of the material throughout the mixture. Mi xing time necessary to produce uniform concrete shall be established by the contractor and shall comply with other requirements of these specifications. Only equipment found acceptable in every respect and capable of producing uniform results will be perm itted. Each volumetric mixer shall be equipped with an onboard ticketing system that will electronically produce a record of all material used and their respective weights and the total volume of concrete placed. Alternate methods of recordation may be us ed if approved by the Engineer. Tickets shall identify at least the following information:
1.Contractor Name
2.Contractor Phone Number
3.NCDOT Project No. and TIP No.
4.Date
5.Truck No.
6.Ticket No.
7.Time Start/End of Pour
8.Mix ID and Description (Strength)
9.Aggregate Moisture Before Mixing
C.Proportioning Devices Volume proportioning devices, such as counters, calibrated gate openings or flow meters, shall be easily accessible for controlling and determining the quantities of the ingredients discharged. All in dicating devices that affect the accuracy of proportioning and mixing of concrete shall be in full view of and near enough to be read by the operator and Engineer while concrete is being produced. In operation, the entire measuring and dispensing mechanis m shall produce the specified proportions of each ingredient. Provide positive control of the flow of water and admixtures into the mixing chamber with a volumetric mixer. Indicate water flow by a flow meter and be readily adjustable to provide for slump control and/or minor variations in aggregate moisture. Provide a mixer capable of continuously circulating or mechanically agitating the admixtures.
10-19 Dispense liquid admixtures through a controlled, calibrated flow meter. A positive means
to observe the continuous flow of material shall be provided. If an admixture requires diluting, the admixture shall be diluted and thoroughly mixed before introducing the admixture into the dispenser. When admixtures are diluted, the ratio of dilution and the mixing sha ll be approved by and performed in the presence of the Department. The volumetric mixer shall be capable of measurement of cement, supplementary cementitious material (if required), liquids and aggregate being introduced into the mix.
D.Calibration Volume -weight relationships will be based on calibration. The proportioning devices shall be calibrated by the contractor before the start of each NCDOT job and subsequently at intervals recommended by the equipment manufacturer. Calibrations will be performed in the presence of the Department and subject to approval from the Department. Calibration of the cement and aggregate proportioning devices shall be accomplished by weighing (determining the mass of) each component. Calibration of the admixture and water proportioning devices shall be accomplished by weight (mass) or volume. Tolerances in proportioning the individual components will be as follows: TABLE 1000 -7 VOLUMETRIC MIXED CONCRE TE CALIBRATION TOLERANCES Item Tolerance Cement, Weight (Mass) percent 0 to +4 Fine Aggregate, Weight (Mass) percent ± Coarse Aggregate, Weight (Mass) percent ± Admixtures, Weight (Mass) or Volume percent ± Water, Weight (Mass) or Volume percent ± Each volumetric mixer must be accompanied at all times by completed calibration worksheets and they shall be made available to the Department upon request.
E.Verification of Yield Verification of the proportioning devices may be required at any time by the Department. Verification shall be accomplished by proportioning the rock and sand based on the cement meter count for each concrete mobile mixer. Once the count (revolutions) for 94 lbs. of cement has been determined then delivery of the corr ect amount of rock and sand can be verified.
F.Uniformity When concrete is produced, have present during all batching operations a Certified Concrete Batch Technician. During batching and placement, the sole duty of this employee is to supervise the production and control of the concrete, perform moisture tests, adjust mix proportions of aggregates for free moisture, complete and sign approved delivery tickets and assure quality control of the batching. Two samples of sufficient size to make the requi red tests will be taken after discharge of approximately 15% and 85% of the load. Each of the two samples of concrete will be separately tested for the properties listed in Table 1000- 7. Tests will be conducted in accordance with the test procedures specified in Table 1000- 7 or procedures established by the Materials and Tests Unit. The Engineer may recheck mixer performance at any time when , in his opinion , satisfactory mixing is not being accomplished.
Source: North Carolina Standard Specifications for Roads and Structures, 2024 Edition. Pages 417–435 of 857.