Chapter 1800. Portland Cement Concrete.

 

1801. Scope. This section of the specifications defines materials to be used in all Portland cement concrete work and requirements for mixing, placing, finishing and curing.

 

1802. Description. Portland Cement Concrete shall consist of a mixture of Portland Cement, fine and coarse aggregates, water and authorized admixtures mixed as hereinafter specified.

 

1803. Portland Cement. Portland cement shall conform to the requirements of AASHTO Designation M‑85, for low alkali cement. Unless otherwise specified, Type I & II cement shall be used. When Type V cement is specified, it shall conform to the requirement of AASHTO Designation M‑85, Table I, except that the tri-calcium aluminate shall not exceed 5.0 percent. When Type III cement is specified, it shall conform to the requirements of AASHTO Designation M‑85, Table IA. concrete shall be clean and

 

Coarse aggregate shall consist of gravel, crushed slag, crushed stone or other approved inert materials, composed of hard, strong and durable particles, free of injurious coatings, and conforming to the requirements of AASHTO Designation M‑80, except as modified herein.

 

(1) The amount of deleterious substances included in the aggregate shall not exceed the following:

 

Percent (by weight)

 

Soft fragments 2.0

Coal and lignite 0.3

Clay lumps 0.3

Other deleterious substances 2.0

 

The material passing the No. 200 sieve both fine and coarse aggregates shall not exceed l.75 percent by weight.

 

(2) The coarse aggregate shall meet one of the following gradations when tested with laboratory screens having square openings conforming to the requirements of AASHTO Designation M‑92.

 

Adequate supplies of coarse aggregate shall be produced, and separately stockpiled, sufficiently in advance of construction operations to permit sampling and testing before use. The stockpiles shall be of such size as to adequately supply the mixing plant when it is operating at full capacity, and to provide continuous placing of concrete. Gradation samples may be taken from the belt.

(3) The requirements of soundness and percentage of wear specified in AASHTO Designation M‑80 shall be used for determining the suitability of coarse aggregate sources. They shall not be used for routine control testing.

 

TABLE 18‑l

 

Percentage Passing (by weight)

 

Coarse

Aggregate

Size

2 1/2"

2"

1 1/2"

1"

3/4"

1/2"

3/8"

No. 4

2" - #4

100

95‑100

 

35‑37

 

10‑30

 

0‑5

1 1/2"- #4

 

100

95‑100

 

35‑70

 

10‑30

0-5

1" - #4

 

 

100

95‑100

 

25‑60

 

0-10

3/4" - #4

 

 

 

100

90‑100

 

20‑55

0-10

 

Fine aggregate shall consist of natural sand, or other approved inert materials, composed of hard, strong and durable particles conforming to the requirements of AASHTO Designation M‑6, except as modified herein.

 

The amount of deleterious substances included in the aggregate shall not exceed the following:

 

Percent (by weight)

Clay lumps 0.5

Coal and lignite 0.3

Other deleterious substances 2.0

 

Fine aggregate shall be uniformly graded from coarse to fine within the following gradation, when tested with laboratory sieves having square openings conforming to the requirements of AASHTO Designation M‑92.

 

Sieve Size (by weight)

Percent Passing

3/8"

100

No. 4

95‑100

No. 16

45‑ 80

No. 50

10‑ 30

No. 100

2‑ 10

 

Adequate supplies of fine aggregate shall be produced and stockpiled sufficiently in advance of construction operations to permit sampling and testing before use. The stockpiles shall be of such size as to adequately supply the mixing plant when it is operating at full capacity and to provide continuous placing of concrete.

 

The requirements for soundness specified in AASHTO Designation M‑6 shall be used only for determining the suitability of fine aggregate sources and shall not be used for routine control testing.

 

Water for washing aggregates and for mixing concrete shall not contain impurities in an amount sufficient to cause a change in the setting time of Portland Cement of more than 25 percent nor a reduction in the compressive strength of mortar of more than 10 percent when compared to results obtained with distilled water, tested in accordance with AASHTO Designation T‑26. Water known to be of potable quality may be used without testing. Where the source of water is relatively shallow, the intake shall be so enclosed as to exclude silt, mud, grass, or other foreign substances.

 

Air‑entraining agents shall conform to the requirements of AASHTO Designation M‑154. The agent shall be thoroughly mixed prior to use. Any agent which will not remain in suspension will be constantly agitated during batching.

 

The agent shall be added at the mixer. The use of air‑entraining cement will not be permitted.

 

Water Reducing or Water Reducing and Retarding Admixtures may be used to increase workability of the concrete when approved by the Engineer. Such admixtures shall be a product of a company with not less than five years experience in the manufacture and successful field use of this type admixture.

Water Reducing Admixtures shall conform to AASHTO M‑914, Type A, except that the relative durability factor shall not be less than 90 and the chlorides content (as Cl ‑) shall not exceed 1 percent by weight of the admixture.

 

Water Reducing and Retarding Admixtures shall conform to AASHTO M‑194, Type D, except that the relative durability factor shall not be less than 90 and the chlorides content (as Cl‑) shall not exceed 1 percent by weight of the admixture.

 

Admixtures, other than air‑entraining agents, may be used when the type and amount to be used are approved. Calcium chloride will not be allowed as an admixture. Admixtures shall be used at the Contractor's expense.

 


 

1804. Classes of Concrete. The various classes of concrete shall be used as follows:

 

Class "AA" (AE) concrete shall be used for all bridges, reinforced concrete box culverts, curb and gutter, sidewalks, and concrete pavement unless shown otherwise on the plans.

 

Classes "A" or "A" (AE) concrete shall be used for all reinforced structures except for those areas where Class "AA" (AE) concrete is to be used. The items of use include small miscellaneous structures such as catch basins and cleanout boxes as well as retaining walls, piles, and cribbing.

 

Class "B", "B"(AE), "C" or "C"(AE) shall be used only where specifically designated. The symbol (AE) designates air‑entrainment.

 

1805. Composition of Concrete: The coarse aggregate shall be uniformly graded within the limits specified in Section _______ NLC Code. The gradation to be used shall be at the option of the Contractor unless otherwise specifically specified in the plans or special Provisions.

 

The Contractor's choice of the maximum size of coarse aggregate to be used shall be subject to the following:

 

The nominal maximum size of the aggregate shall not be larger than one‑fifth of the narrowest dimension between sides of forms, one‑third the depth of slabs, nor three‑fourths of the minimum clear distance between reinforcing bars or between bars and forms, whichever is least.


 

The Contractor shall be responsible for the mix design. In establishing the proportions of cement, aggregate and water to be used, the Contractor shall comply with the following design criteria:

 

TABLE NO. 18‑2

 


Class

Coarse Aggregate Size

 

 

Water/Cement

(Gals./Sack )

Minimum

Cement

Content

(Sacks/C.Y.)


AA (AE)

2" to No. 4

5.0 max. *

6.0

 

1 1/2 to No. 4

5.0 max. *

6.0

 

1" to No. 4

5.0 max. *

6.5

 

3/4" to No. 4

5.0 max. *

6.5

A or A(AE)

1 1/2 to No. 4

6.0 max.

5

 

1" to No. 4

6.0 max.

5

B or B(AE)

 

7.0 max.

4

C or C(AE) shape.

 

8.0 max.

4

 

* For Portland Cement Concrete Pavement, the water/cement ratio should not exceed 5.5 gallons/sack.

 

For all classes of concrete: Concrete deposited in water shall contain one bag of cement per cubic yard more than the design required for concrete placed above water.

 

Prior to placing concrete, the Contractor shall furnish the Engineer a mix design and information based on trial batch test results to verify the concrete mix design strength. Changes in the mix design may be made only with written approval from the Engineer. The proportioning and mixing of concrete shall be subject to inspection by the Engineer. The Engineer may provide such laboratory data and other information that may be available to aid in preparing the mix design.


 

Concrete used on the project shall conform to the following requirements:

 

TABLE NO. 18‑3

 

 

Class

 

Slump

 

Air

Content

Required

Mix Design

Strength

Moving average

Strength (Average of 3)

Minimum

Strength

 

Inches

%

PSI

PSI

PSI

AA(AE)

1‑3.5

6+/-1 *

5210

4550

3650

A

2‑4

 

3910

3410

3000

A(AE)

2‑4

5+/‑1

3910

3410

3000

B

2‑5

 

3260

2840

2500

B(AE)

2‑5

4.5+/‑1.5

3260

2840

2500

C

2‑5

 

2610

2270

2000

C(AE)

2‑5

4.5+/‑1.5

2610

2270

2000

 

* For Portland Cement Concrete Pavement, the air content shall be 7+/‑ 1%.

 

When Type V Cement, is specified, the required twenty‑eight day strength shall be 90 percent of the strength given in Table No. 18‑3 for the class of concrete specified.

 

Table 18‑3 is based on coefficient of variation of 10 percent and allowable number of tests below the minimum strength of 1 in 100. (In order to decrease the probability of a strength test falling below the minimum strength to 1 in 100, the moving average strengths must meet the specifications shown above.)

 

1806. Sampling and Testing. All concrete, except that placed with a pump, shall be sampled in accordance with AASHTO Designation T‑141. Concrete sampled from a concrete pump shall be sampled from the hose after all of the priming grout has been wasted. The end of the hose shall be placed in a horizontal position before the concrete is discharged into the sampling pan. The concrete shall not be allowed to fall into the sampling pan. The Contractor, at his expense, shall furnish the concrete required for testing.

 


 

Strength, slump and air tests for Class AA and A concrete, except concrete pavement and pre-stressed concrete members, shall be taken in accordance with the placement rate per day as shown below:

 

TABLE NO. 18‑4

 

Minimum Number of Tests

Rate/Day (C.Y.)

Air

Slump

Strength

0‑8

1

1

As directed by Proj. Eng.

8‑50

1

1

1

For each 50 cu. yds. or fraction thereof

1

1

1

 

For Class B and C concrete one air, one slump, and one strength test shall be made at the time concrete is first placed. Additional tests shall be made as determined by the Engineer.

 

Yield test for Class AA and A Concrete: One test per mix design. For Class B and C no test required.

 

Additional tests may be made at the discretion of the Engineer.

 

If a slump test does not meet the specification, a second slump test shall be made immediately on the same load. The concrete shall be accepted if the second slump test meets the specification or rejected and removed from the project if the second slump test does not meet the specification.

 

If an air test does not meet the specification, a second air test shall be made immediately on the same load. The concrete shall be accepted if the second air test meets the specification or rejected and removed from the project if the second air test does not meet the specification.

 

Slump and air tests shall be made in accordance with AASHTO Designation T‑119 and T‑152, respectively. Alternate air tests may be made in accordance with AASHTO Designation T‑121.

 

Making, curing, and determining the compressive strength of concrete cylinders shall be in accordance with AASHTO Designations T‑22 and T‑23. Compressive strength of concrete shall be determined from "strength tests", in which the average strength of three standard cylinders shall constitute one strength test. Cylinders for strength tests shall be taken by the Engineer, who shall also inspect the batching procedure to ascertain compliance with the mix design.

 


 

1807. Acceptance. The average of any three consecutive strength tests should equal or exceed the moving average strength specified in Table No. 18‑3. If the average of any three consecutive strength tests is below specification, the Contractor should increase the strength of the concrete at his expense. When Type V cement is specified, the required 28 day strength shall be 90 percent the strength in Table 18‑3 for the class of concrete specified.

 

If any individual strength test is below the specified strength shown in Table No. 18‑3, the concrete may be accepted at a reduced price. The price reduction shall apply to the amount of concrete represented by the strength test in accordance with the following schedule:

 

PSI Below Specified Strength Specification

Pay Factor

1‑100

98

101‑200

94

201‑300

88

301‑400

80

 

Concrete with a compressive strength of more than 400 psi below the required specified strength shall be evaluated by the Engineer for capabilities necessary to the integrity of the structure. The Engineer may accept this concrete at a pay factor of 0.80, or require that it be replaced with acceptable material. The Engineer shall make the final decision.

 

If a strength test is below the required specified strength, the Engineer shall immediately notify the Contractor or his authorized representative. If the Contractor desires re-testing, his request shall be in writing and shall be made within 35 days after the concrete was placed. No re-testing shall be done after 40 days have elapsed since the concrete was placed.

 

Strength tests for re-testing shall be made by coring or an acceptable nondestructive method, as determined by the Engineer. Cores shall be obtained and tested in accordance with AASHTO Designation T‑24 Only one set of three cores shall be obtained per test lot. The re-tested strength shall be either the average strength of three cores or the average of three nondestructive method determinations.

 

All costs incurred in re-sampling and re-testing shall be paid by the contractor if the re-tested strength is below the specified strength, and shall be assumed by the City if the re-tested strength is above the specified strength.

 

The final strength of the test log shall be either the original strength of the re-tested strength, whichever is higher. If the final strength is below the specified strength shown in Table No. 18‑3, the concrete shall be accepted at a reduced price or replaced as specified above.

 

1808. Reserved

 

1809. Batching Materials. The weighing and batching of materials shall conform to the applicable provisions of AASHTO Designation M‑157. The beams and scales on the batching plant shall be inspected and sealed as required by the Engineer. If the beams and scale fail to meet the requirements, the Contractor shall pay the cost of inspection. If the beams and scales do meet the requirements, the City shall pay the cost of inspection.

 

Where cement is batched in bulk, it shall be handled and loaded in such a manner that none will be lost in transporting to the mixer or in the dumping process.

 

1810. Mixing. Mixing shall be accomplished in a batch mixer of approved type and capacity. The mixing period shall not be less than one minute after all materials are in the drum for mixers of one cubic yard or less capacity. For dual drum and single drum mixers of more than one cubic yard capacity the minimum mixing time shall be 45 seconds. The mixer shall be operated at the drum speed recommended by the manufacturer. Pick‑up and throw‑over blades shall be replaced when they are worn down 3/4 inch or more. Drums and blades shall be kept free of excessive build‑up of cement and mortar.

 

Water measuring devices shall be accurate within 1 percent by weight and shall not be affected by variations in line pressure or construction conditions.

 

The batch materials shall be so charged into the mixer that a portion of the water will enter the drum in advance of the cement and aggregates and the water shall continue to flow for at least five seconds after all other materials are in the drum. Air‑entraining agents and other approved admixtures shall be added to the water before it enters the mixer by means of an automatic dispenser which shall be capable of accurately metering the material.

 

The Engineer shall have the right to order mixing efficiency tests at the beginning of concrete operations, or any time there is reason to believe that the mixer is not operating properly. Mixing efficiency tests shall be made in accordance with ASTM Designation C‑94. The Contractor shall make the necessary corrections, if the tests show poor mixing efficiency.

 

When two admixtures are used in the concrete, they shall be added to the water in such a manner that the two admixtures will not be in contact prior to entering the mixing drum.

 

After the transit mixer leaves the batch plant, additional water may be added, only with the approval of the Engineer. However, the amount of water added shall not exceed the maximum allowed on the batch ticket. When water is added, the drum shall be rotated at least twenty revolutions at the mixing speed recommended by the manufacturer. Water shall not be added after more than 1/2 cubic yard of concrete has been discharged from the drum.

 

Concrete shall be mixed only in such quantities as are required for immediate use. The maximum allowable time between charging of the material in the mixing drum and final placing shall be ninety minutes for air temperatures below 80 degrees F and sixty minutes for temperatures above 80 degrees F. Concrete not placed within these time limits, or if an initial set has developed shall not be used. Tempering concrete by adding water or by other means will not be permitted.

 

181l. Ready‑Mixed Concrete. Ready‑mixed concrete will be permitted subject to the provisions in AASHTO Designation M‑157 regarding batching, mixers and agitators, mixing and delivery, inspection consistency and air content and certification of batches.

 

Transit mixers shall be equipped with a water meter and revolution counter. The meter shall be installed to measure all water discharged from the tank on the truck.

 

1812. Hand Mixing. Hand‑mixed batches shall not exceed 0.5 cubic yard in volume.

 

Hand mixing shall be done only on a water tight platform. The fine aggregate shall be spread evenly on the platform and the cement shall be thoroughly mixed in while dry, until the mixture is of a uniform color.. The water shall then be added and the entire mass turned until all the stone are thoroughly covered with mortar and the mixture is of uniform color and consistency.

 

1813. Temperature Control. Concrete to be placed in bridge structure decks and approach slabs shall not be placed when the temperate exceeds 80 degrees F. in the shade. The temperature of the concrete as placed in forms shall not be less than 50 degrees F. nor more than 90 degrees F.

 

1814. Heating Aggregate. If heated aggregates are required because of cold weather operations, the Contractor shall provide and operate suitable heating devices with no additional compensation. The aggregates may be heated either by steam or dry heat. The products of combustion of any fuel shall not contact the aggregate. The aggregates and mixing water shall be heated to a temperature of not less than 70 degrees F. nor more than 150 degrees F. when introduced into the mixer. Regardless of the heating method used, the aggregates shall be heated uniformly and overheating or the development of "hot spots" shall be avoided. The temperature of the mixed concrete at the time of placing shall not be less than 60 degrees F. nor more than 90 degrees F.

 

1815. Forms. Forms shall be substantially built and adequately braced so as to withstand the liquid weight of concrete. All linings, studding, walling and bracing shall be such as to prevent bulging, spreading, or loss of true alignment while pouring and displacement of concrete while setting.

 

Metal forms shall be used for curb and gutter work except at curves. All edge forms for sidewalk pavement, curbs and gutters shall be of sufficient rigidity and adequately braced to accurately maintain line and grade.

 

Forms for curved sections shall be so constructed and placed that the finish surface of walls and edge of sidewalks, curbs and gutters will not deviate appreciably from the arc of the curve.

 

1816. Joints. Joints shall be provided for sidewalk and curb and gutter as follows:

 

(1) Sidewalks. Sidewalks shall be ruled at 5 foot intervals. In addition, one half inch expansion joints shall be provided at twenty (20) foot intervals and at locations where sidewalks adjoin curbs or existing sidewalks.

 

(2) Curb and Gutters. Curb and Gutters shall be ruled at 10' intervals. In addition, one half inch expansion joints shall be provided at 100 foot intervals and at locations where curb and gutter adjoin drop inlet boxes, cross drains, and diversion structures.

 

Material for one‑half inch expansion joints shall be as defined in AASHTO M‑33 and shall be installed at 100 foot intervals with the top approximately one‑fourth inch below the concrete surface.

 

1817. Reinforcement and Embedded Items. Reinforcing steel shall be clean and free from rust, scale, paint, grease, or other foreign matter which might impair the bond. It shall be accurately bent and shall be tied to prevent displacement when concrete is poured. Reinforcing steel shall be held in place by only metal or concrete ties, braces and supports. No steel shall extend from or be visible on any finished surface.

 

The Contractor shall use concrete chairs for holding the steel away from the subgrade, and spreader or other type bars for securing the steel in place. The spreader bar shall be not less than three‑eighths inch in diameter.

 

1818. Preparations. Before batching and placing concrete, all equipment for mixing and transporting the concrete shall be cleaned, all debris and ice shall be removed from the places to be occupied by the concrete, forms shall be thoroughly wetted (except in freezing weather), or oiled, and masonry filler units that will be in contact with concrete shall be well drenched (except in freezing weather), and the reinforcement shall be thoroughly cleaned of ice or other coatings. Water shall be removed from spaces to receive concrete.

 

When placing concrete on earth, or granular surfaces, the surfaces shall be free from frost, ice, mud and water. When the support surface is dry material, it shall be wetted prior to placing the concrete. No concrete shall be placed until the surfaces have been inspected and approved by the Engineer or Inspector.

 

1819. Concrete Mixing. The concrete shall be mixed until there is a uniform distribution of the materials. Ready‑mixed concrete shall be mixed and delivered in accordance with the requirements set forth in the Specifications for ready‑mixed concrete AASHTO Designation M‑157. Concrete shall be delivered and deposited in its final position within sixty (60) minutes after adding the cement and water to the mixture.

 

1820. Deposition. Concrete shall be deposited as nearly as practical in its final position to avoid segregation due to re-handling or flowing. The concrete placing shall be carried on at such a rate that the concrete is at all times plastic and flows readily into the corners of forms and reinforcing bars. No concrete that has partially hardened or been contaminated by foreign material shall be deposited in the work, nor shall re-tempered concrete be used.

 

All concrete in structures shall be vibrator compacted during the operation of placing, and shall be thoroughly worked around reinforcement and embedded fixtures and into the corners of the forms.

 

1821. Placing Concrete in Cold Weather. No concrete shall be [poured where the air temperature is lower than 40 degrees F. at location where the concrete cannot be covered or protected from the surrounding air. When concrete is poured below a temperature of 35 degrees Fahrenheit, the ingredients of the concrete shall be heated so that the temperature of the mixture shall not be less than 50 degrees or more than 100 degrees Fahrenheit. Before mixing, the heated aggregates shall not exceed 125 degrees Fahrenheit and the temperature of the heated water shall not exceed 175 degrees Fahrenheit. Cement shall not be added while the temperature of the mixed aggregates and water is greater than 100 degrees Fahrenheit. When there is likelihood of freezing during the curing period, the concrete shall be protected by means of an insulating covering to prevent freezing of the concrete for a period of not less than seven days after placing.

 

Equipment for protecting concrete from freezing shall be available at the job site prior to placing concrete. Particular care shall be exercised to protect edges and exposed corners from freezing. In the event heating is employed, care shall be taken to insure that no part of the concrete becomes dried out or is heated to temperatures above 90 degrees Fahrenheit. The housing, covering, or other protection used shall remain in place and intact at least 24 hours after the artificial heating is discontinued.

 

1822. Finishing. After the concrete for slabs has been brought to the established grade and screened, it shall be worked float and then given a light broom finish. In no case shall cement or a mixture of dry cement and sand be sprinkled on the surface to absorb moisture or hasten hardening. Water shall not be added to the surface to facilitate finishing. Surface edges of all slabs shall be rounded to a radius of one‑half inch.

 

After concrete has been poured in curb and gutter forms, it shall be hand tamped to eliminate honeycomb surfaces. The front and top sides shall be finished with a float or steel trowel to make a uniform finished surface and shall be given a broom or brush finish. Wherever corners are to be rounded, special steel trowels shall be used while the concrete is workable and the corners constructed to the dimensions specified. Forms shall be removed as soon as the concrete has set sufficiently to hold its shape.

 

The top and face of the curb and also the top of the apron on combined curb and gutter must be finished true to line and grade and without any irregularities of surface noticeable to the eye. The gutter shall not hold water to a depth of more than one‑fourth of an inch from a straight edge ten feet in length, placed on the curb parallel to the center line of the street, nor shall any part of the exposed surface present a wavy appearance.

 

1823. Curing and Protection. All newly‑placed concrete shall be cured by the water method or the membrane curing compound method as herein described and specified unless written approval to use any other method herein described and specified has been obtained from the Engineer.

 

(A) Membrane Curing Compound Method. Surfaces of newly placed concrete, to be cured by the membrane curing compound method, shall be kept moist or wet until the curing compound is applied and the curing compound shall not be applied until all patching or surface finishing has been completed.

 

The entire surface of the concrete shall be sprayed uniformly with a membrane curing compound. Should the film of compound be damaged from any cause before the expiration of seven days after the concrete is placed, the damaged portions shall be repaired immediately with additional compound.

 

The curing compound shall be delivered to the work in ready‑ mixed form. At the time of use, the compound shall be in a thoroughly mixed condition with the pigment uniformly dispersed throughout the vehicle. The compound shall not be diluted or altered in any manner.

 

Curing compound that has become chilled to such an extent that it is too viscous for satisfactory application shall be warmed to a temperature not exceeding 100 degrees Fahrenheit.

 

The curing compound shall be applied to the exposed surface at a uniform rate of 1 gallon per 150 square feet of area.

 

In the event that the application of curing compound is delayed, the application of water as provided in this section shall be started immediately and shall be continued until application of the compound is resumed or started.

 

(B) Water Method. The concrete shall be kept continuously wet by the application of water for a minimum period of seven days after the concrete has been placed.

 

Cotton mats, burlap or earth or sand blankets may be used as a curing medium to retain the moisture during the curing period. The cotton mats or burlap shall be of such character that they will retain water but not prevent evaporation. When cotton mats, burlap or earth or sand blankets are to be used to retain the moisture, the entire surface of the concrete shall be kept damp by applying water with a nozzle that so atomizes the flow that mist, and not a spray, is formed until the surface of the concrete is covered with the curing medium. The moisture from the nozzle shall not be applied under pressure directly upon the concrete and shall not be allowed to accumulate on the concrete in quantity sufficient to cause a flow or wash the surface. At the expiration of the curing period, the concrete surfaces shall be cleared of all curing mediums.

 

1824. Reserved.