Difference between revisions of "701 - Portland Cement Concrete for Structures"
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Revision as of 15:55, 16 September 2014
Concrete used in bridge construction will typically be one of three grades. Grade T is used for concrete being placed underwater, such as tremie pours. Concrete Grade S2 is typically used for all substructure concrete pours and concrete Grade D is used for bridge deck, railing, sidewalk, sleeper slab, and approach slab pours.
Sack refers to a 94 pound sack of cement. Refer to Section 601 of this Construction Manual for equipment requirements for mixing and placing concrete.
The Contractor will provide communication service from the work site to the batching plant that will always be available to the Engineer during the concreting operations.
Sections 601 and 701 of the Standard Specifications for Construction, and 12SP604(B) QUALITY CONTROL AND ACCEPTANCE OF PORTLAND CEMENT CONCRETE provide the specifications related to strength requirements, quality control, quality assurance and acceptance related to structural concrete.
Concrete materials must be accordance with subsection 701.02 of the Standard Specifications for Construction.
|Coarse Aggregate 6A, 6AA, 17A||902|
|Fine Aggregate 2NS, 2SS||902|
All material must be inspected and approved before being incorporated in the work. Materials should be checked for damage in shipment or storage and to insure conformity to the required dimension, appearance and condition. Following are four main methods of material inspection acceptance:
- Material may be tested or sampled at the point of production, manufacture or supplier's storage by the Bureau of Field Services or an appointed representative. The project office should receive the resulting test report before using the material. The material should be checked for markings, such as lot number, to identify with the test report.
- Material may be certified by the supplier as meeting the specified requirements. The certification should be received in the project office before using.
- Material may be sampled at the job site. The sample is tested by Bureau of Field Services and a report is sent to the Engineer before using.
- The material may be visually inspected at the site by project personnel to insure conformance with specified requirements. Then, a visual inspection report is prepared and submitted. The required method of inspection is listed for all materials in Division 9 of the Construction Manual and is to be followed. If in doubt, the Region materials staff should be consulted.
Ensure the contractor adheres to the following actions when furnishing and handling aggregate:
- Fine and Coarse - must be furnished, stocked piled and handled in such manner as to prevent segregation.
- Aggregate Secured for Stockpiles - The storage area must be firm, reasonably level, and thoroughly cleaned of all foreign materials.
- Bottom 1 foot of the Stockpile – must not be used unless a paved area is used for stockpiling.
- Frozen Aggregate Lumps – must not be used.
- Equipment – Causing contamination or degradation must not be used.
- Aggregate Moisture Content - must be kept uniform for each day's run without evidence of any surplus water.
- Slag Stockpiles or Other Highly Absorptive Aggregates - must be kept continuously wet to maintain uniform moisture content.
- MDOT Fine & Coarse Aggregates - must be stocked in separate piles or bins apart from aggregates to be used in other work.
- Coarse Aggregate (Blend of Two or More Gradations) - must be stockpiled separately for each gradation.
Cement, slag cement and fly ash furnished to the project in bulk form must be stored in separate weatherproof bins. Wet or contaminated material may not be used.
If the Contractor changes the cement type or source, fly ash source or class or slag cement source or grade, the bin must be emptied to within the quantity used for two concrete batches before refilling with new material. Notice of the material change must be recorded on the first ticket.
The Contractor must furnish the Engineer with a shipment notice (copy) with the following information:
- The quantities of cement, slag cement or fly ash.
- A certification that the material meets MDOT requirements.
Ready mixed concrete is batched and delivered as central mixed or truck mixed concrete.
Central Mixed - Completely mixed in a central mixer and transported to the delivery point in a truck agitator, a truck mixer, or in approved, non-agitating equipment.
Truck Mixed - Completely mixed in a truck mixer with an approved revolution counter, at the plant site.
Functions of Parts:
Drum or blade - Must revolve at the speed or within the speed range recommended by the manufacturer and shown on a metal plate attached to the mixer.
Mixing Time - Equals the time all cement and aggregates are in the drum until beginning of discharging the concrete. For multi-compartment mixers, the mixing time must include the transfer time between drums.
|Batch Mixing Method||Minimum Mixing Time|
|Central Mix-Turbine||45 seconds|
|Central Mixed-Revolving drums & pugmills||60 seconds|
|Central Mixed-Revolving drums & pugmills 1 yd; (0.75 m3)||90 seconds|
|Truck Mixed Concrete||70 revolutions|
Ingredients - Must be charged into the mixer so that some water will enter before cement and aggregate; substantially all the water must be in the drum before one third the specified mixing time has elapsed.
Ensure the mixer drum and truck mixers are completely emptied after each batch before recharging.
The maximum batch size may not exceed the mixer capacity as shown on the attached metal plate. Batch sizes for agitating units and truck mixers used to transport central mixed concrete may not exceed the manufacturer's recommendation for maximum agitating capacity.
If Volumetric Batching and Continuous Mixing methods are used, the concrete must be proportioned, mixed and discharged according to ASTM C 685.
The time between charging the mixer and concrete discharge completion into the work must not exceed that specified in Table 601.01 of the Standard Specifications for Construction.
Charging time is the time the cement contacts the mixing water or damp aggregates.
When the anticipated time between charging and discharge completion exceeds 30 minutes, the concrete must be continuously agitated.
For concrete transported in truck mixers:
If additional mixing water is required to obtain the specified slump and the truck mixer is not loaded to more than its rated mixing capacity, water may be added. Water must only be added prior to any discharge.
After all water is added, a minimum of 30 revolutions at mixing speed will be required before discharging any concrete. The additional water will be added and the additional mixing completed at the work site within the maximum time specified in Table 601.01 in the Standard Specifications for Construction.
Except for fog curing, no water may be added to the concrete during discharge or placement. In particular, no water may be added to truck chutes or to pump or slip form hoppers.
The Contractor is responsible for determining the concrete mixture proportions (mix design) on projects containing Quality Control (QC) provisions.
A Contractor mix design, produced by a testing laboratory, conforming to ASTM C 1077, must be submitted for the Engineer’s approval. The submittal must include documentation of the slump, air content and a 28-day compressive strength. The compressive strength in Table 701.01 in the Standard Specifications for Construction must be exceeded by at least 500 psi.
Type I or IA Portland cement must be used.
The cement quantity for Type I or Type IA shown in Table 701.01 in the Standard Specifications for Construction may be reduced up to a maximum of 40 percent for fly ash or slag cement substitution.
The fly ash/slag cement weight must be equal to the weight of the cement removed.
If fly ash and slag cement are used in the same mixture, the cement quantity shown in Table 701.1 in the Standard Specifications for Construction may be reduced up to a maximum of 40 percent.
Structural concrete, including concrete placed in steel shell piles require an air content of 5.0 to 8.0 percent air.
When the Contractor adjusts the amount of air entraining admixtures used, the actual amount used per sack of cement or per concrete batch will be recorded on the first delivery ticket following the adjustment.
When concrete having low air content is received at the work site in truck mixers, the Contractor may add additional air entraining admixture at the site, mixed in a small amount of water [quart (liter)], with a minimum of 30 mix revolutions until adjustments can be made at the batching plant.
When a water-reducing admixture is used, it must be added according to the manufacturer’s recommendations. The dosage used must not be less than the amount shown on the Qualified Products List for Admixtures for Concrete found in the Materials Source Guide. Admixture dosage rates will be based on the total cementitious material (cement plus fly ash or granulated blast furnace slag) in the concrete.
The contractor is responsible for Quality Control (QC) of the concrete. The Department is responsible for administering Quality Assurance (QA) of the concrete. The contractor and the department will correlate testing by conducting side by side testing of the same concrete representing the first production lot to verify test results for temperature and air content of the concrete.
The QC sampling and testing must be independent from the Department’s QA sampling and testing. For QC testing, the contractor must sample and test a representative haul unit of concrete immediately after its discharge but before the pump hopper and again after the pump hopper. If the difference in air content between the two samples is greater than 2.0 percent by volume of concrete, the contractor must suspend operations and administer corrective action to determine the cause of the excessive air loss through the pump and to make the necessary adjustments to their operations in efforts to reduce the excessive air loss to less than 2.0 percent.
The engineer will conduct daily QA startup sampling and testing of temperature, slump and air content; QA sampling and testing; monitoring the contractor’s adherence to the QC plan; and inspection of field placed materials.
QA samples must be taken from the concrete as close to its final placement as practical (after the pump). In general, the sampling rate will be approximately one sample per 50 cubic yards, as determined by the engineer, with a minimum of one sampling for each day of production. The QA sampling rate and sample location will be based on cubic yard quantities. Samples will be taken from the concrete at the location as close to its final placement into the forms or on the grade as practical. If sampling from the discharge of the haul unit, the sample will be taken from approximately the middle one-third of the load.
QA sample locations will be determined as described in the ”Random Sampling for Quality Control/Quality Assurance Projects” section of the Materials Quality Assurance Procedures Manual
All concrete QC and QA must be in accordance with the requirements for non-PWL in the Special Provision for QUALITY CONTROL AND ACCEPTANCE OF PORTLAND CEMENT CONCRETE (12SP604B).
Sampling and testing must be done in accordance with the Special Provision for QUALITY CONTROL AND ACCEPTANCE OF PORTLAND CEMENT CONCRETE (12SP604B).
The following ASTM test methods apply to testing. The Department’s established procedures for sampling and testing are acceptable alternatives.
- C 31 Practice for Making and Curing Test Specimens in the Field
- C 39 Test Method for Compressive Strength of Cylindrical Concrete Specimens
- C 78 Test Method for Flexural Strength of Concrete
- C 138 Test Method for Density, Yield and Air Content of Concrete
- C 143 Test Method for Slump of Hydraulic-Cement Concrete
- C 172 Practice for Sampling Freshly Mixed Concrete
- C 173 Test Method for Air Content of Freshly Mixed Concrete (Volumetric)
- C 231 Test Method for Air Content of Freshly Mixed Concrete (Pressure)
- C 293 Test Method for Flexural Strength of Concrete