ITEM 405 – STRUCTURAL CONCRETE
405.1 Description
405.1.1 Scope
This Item shall consist of furnishing, bending, placing and finishing
concrete in all structures except pavements in accordance with this
Specification and conforming to the lines, grades, and dimensions shown on
the Plans. Concrete shall consist of a mixture of Portland Cement, fine
aggregate, coarse aggregate, admixture when specified, and water mixed in
the proportions specified or approved by the Engineer.
405.1.2 Classes and Uses of Concrete
Five classes of concrete are provided for in this Item, namely: A, B, C, P
and Seal. Each class shall be used in that part of the structure as called
for on the Plans.
The classes of concrete will generally be used as follows:
Class A – All superstructures and heavily reinforced substructures. The
important parts of the structure included are slabs, beams, girders,
columns, arch ribs, box culverts, reinforced abutments, retaining walls,
and reinforced footings.
Class B – Footings, pedestals, massive pier shafts, pipe bedding, and
gravity walls, unreinforced or with only a small amount of reinforcement.
Class C – Thin reinforced sections, railings, precast R.C. piles and
cribbing and for filler in steel grid floors.
Class P – Prestressed concrete structures and members.
Seal – Concrete deposited in water.
405.2 Material Requirements
405.2.1 Portland Cement
It shall conform to all the requirements of Subsection 311.2.1.
405.2.2 Fine Aggregate
It shall conform to all the requirements of Subsection 311.2.2.
405.2.3 Coarse Aggregate
It shall conform all the requirements of Subsection 311.2.3 except that
gradation shall conform to Table 405.1.
Table 405.1 – Grading Requirements for Coarse Aggregate
| Sieve Designation | Mass Percent Passing | |||||
| Standard mm | Alternate US Standard | Class A | Class B | Class C | Class P | Class Seal |
| 63
50 37.5 25 19.0 12.5 9.5 4.75 |
2-1/2”
2” 1-1/2” 1” ¾” ½” 3/8” No.4 |
100
95 – 100 - 35 – 70 - 10 – 30 0 - 5 |
100
- 35 – 70 - 10 – 30 - 0 - 5 |
100
90 – 100 40 – 70 0 – 15* |
100
- - 20 – 55 0 – 10* |
100
95 – 100 - 25 – 60 - 0 – 10* |
* The measured cement content shall be within plus (+) or minus (-) 2 mass
percent of the design cement content.
405.2.4 Water
It shall conform to the requirements of Subsection 311.2.4
405.2.5 Reinforcing Steel
It shall conform to the requirements of Item 710, Reinforcing Steel and
Wire Rope.
405.2.6 Admixtures
Admixtures shall conform to the requirements of Subsection 311.2.7
405.2.7 Curing Materials
Curing materials shall conform to the requirements of Subsection 311.2.8.
405.2.8 Expansion Joint Materials
Expansion joint materials shall be:
- Preformed Sponge Rubber and Cork, conforming to AASHTO M 153.
- Hot-Poured Elastic Type, conforming to AASHTO M 173.
- Preformed Fillers, conforming to AASHTO M 213.
405.2.9 Elastomeric Compression Joint Seals
These shall conform to AASHTO M 220.
405.2.10 Elastomeric Bearing Pads
These shall conform to AASHTO M 251 or Item 412 – Elastomeric Bearing Pads.
405.2.11 Storage of Cement and Aggregates
Storage of cement and aggregates shall conform to all the requirements of
Subsection 311.2.10.
405.3 Sampling and Testing of Structural Concrete
As work progresses, at least one (1) sample consisting of three (3)
concrete cylinder test specimens, 150 x 300 mm, shall be taken from each
seventy five (75) cubic meters of each class of concrete or fraction
thereof placed each day.
Compliance with the requirements of this Section shall be determined in
accordance with the following standard methods of AASHTO:
Sampling of fresh concrete T 141
Weight per cubic meter and air content T 121
(gravi-Metric) of concrete
Sieve analysis of fine and coarse aggregates T 27
Slump of Portland Cement Concrete T 119
Specific gravity and absorption of fine aggregate T 84
Tests for strength shall be made in accordance with the following:
Making and curing concrete compressive T 23
and flexural tests specimens in the field
Compressive strength of molded concrete Cylinders T 22
405.4 Production Requirements
405.4.1 Proportioning and Strength of Structural Concrete
The concrete materials shall be proportioned in accordance with the
requirements for each class of concrete as specified in Table 405.2, using
the absolute volume method as outlined in the American Concrete Institute
(ACI) Standard 211.1. “Recommended Practice for Selecting Proportions for
Normal and Heavyweight Concrete”. Other methods of proportioning may be
employed in the mix design with prior approval of the Engineer. The mix
shall either be designed or approved by the Engineer. A change in the
source of materials during the progress of work may necessitate a new mix
design.
The strength requirements for each class of concrete shall be as specified
in Table 405.2.
Table 405.2 - Composition and Strength of Concrete for Use in Structures
Class Of Concrete
|
Minimum Cement Content Per m3 kg(bag**)
|
Maximum Water/
Cement Ratio kg/kg
|
Consistency Range in Slump mm
|
Designated Size of Coarse Aggregate Square Opening Std. mm
|
Minimum Compressive Strength of 150x300 mm Concrete
Cylinder Specimen at 28 days, MN/m2
|
| A
B C P Seal |
360
(9 bags) 320 (8 bags) 380 (9.5 bags) 440 (11 bags) 380 (9.5 bags) |
0.53
0.58 0.55 0.49 0.58 |
50 – 100
50 – 100 50 – 100 100 max 100 – 200 |
37.5 – 4.75
(1-1/2” – No. 4) 50 – 4.75 (2” – No. 4) 12.5 – 4.75 (1/2” – No. 4) 19.0 – 4.75 (3/4” – No. 4) 25 – 4.75 (1” – No. 4) |
20.7
16.5 20.7 37.7 20.7 |
* The measured cement content shall be within plus or minus 2 mass percent
of the design cement content.
** Based on 40 kg/bag
405.4.2 Consistency
Concrete shall have a consistency such that it will be workable in the
required position. It shall be of such a consistency that it will flow
around reinforcing steel but individual particles of the coarse aggregate
when isolated shall show a coating of mortar containing its proportionate
amount of sand. The consistency of concrete shall be gauged by the ability
of the equipment to properly place it and not by the difficulty in mixing
and transporting. The quantity of mixing water shall be determined by the
Engineer and shall not be varied without his consent. Concrete as dry as it
is practical to place with the equipment specified shall be used.
405.4.3 Batching
Measuring and batching of materials shall be done at a batching plant.
1. Portland Cement
Either sacked or bulk cement may be used. No fraction of a sack of cement
shall be used in a batch of concrete unless the cement is weighed. All bulk
cement shall be weighed on an approved weighing device. The bulk cement
weighing hopper shall be properly sealed and vented to preclude dusting
operation. The discharge chute shall not be suspended from the weighing
hopper and shall be so arranged that cement will neither be lodged in it
nor leak from it.
Accuracy of batching shall be within plus (+) or minus (-) 1 mass percent.
2. Water
Water may be measured either by volume or by weight. The accuracy of
measuring the water shall be within a range of error of not more than 1
percent.
3. Aggregates
Stockpiling of aggregates shall be in accordance with Subsection311.2.10.
All aggregates whether produced or handled by hydraulic methods or washed,
shall be stockpiled or binned for draining for at least 12 hours prior to
batching. Rail shipment requiring more than12hours will be accepted as
adequate binning only if the car bodies permit free drainage. If the
aggregates contain high or non-uniform moisture content, storage or
stockpile period in excess of 12 hours may be required by the Engineer.
Batching shall be conducted as to result in a 2 mass percent maximum
tolerance for the required materials.
4. Bins and Scales
The batching plant shall include separate bins for bulk cement, fine
aggregate and for each size of coarse aggregate, a weighing hopper, and
scales capable of determining accurately the mass of each component of the
batch.
Scales shall be accurate to one-half (0.5) percent throughout the range
used.
5. Batching
When batches are hauled to the mixer, bulk cement shall be transported
either in waterproof compartments or between the fine and coarse aggregate.
When cement is placed in contact with moist aggregates, batches will be
rejected unless mixed within 1-1/2 hours of such contact. Sacked cement may
be transported on top of the aggregates.
Batches shall be delivered to the mixer separate and intact. Each batch
shall be dumped cleanly into the mixer without loss, and, when more than
one batch is carried on the truck, without spilling of material from one
batch compartment into another.
6. Admixtures
The Contractor shall follow an approved procedure for adding the specified
amount of admixture to each batch and will be responsible for its uniform
operation during the progress of the work. He shall provide separate scales
for the admixtures which are to be proportioned by weight, and accurate
measures for those to be proportioned by volume. Admixtures shall be
measured into the mixer with an accuracy of plus or minus three (3)
percent.
The use of Calcium Chloride as an admixture will not be permitted.
405.4.4 Mixing and Delivery
Concrete may be mixed at the site of construction, at a central point or by
a combination of central point and truck mixing or by a combination of
central point mixing and truck agitating. Mixing and delivery of concrete
shall be in accordance with the appropriate requirements of AASHTO M 157
except as modified in the following paragraphs of this section, for truck
mixing or a combination of central point and truck mixing or truck
agitating. Delivery of concrete shall be regulated so that placing is at a
continuous rate unless delayed by the placing operations. The intervals
between delivery of batches shall not be so great as to allow the concrete
in place to harden partially, and in no case shall such an interval exceed
30 minutes.
In exceptional cases and when volumetric measurements are authorized, for
small project requiring less than 75 cu.m. per day of pouring, the weight
proportions shall be converted to equivalent volumetric proportions. In
such cases, suitable allowance shall be made for variations in the moisture
condition of the aggregates, including the bulking effect in the fine
aggregate. Batching admixing shall be in accordance with ASTM C 685,
Section 6 through 9.
Concrete mixing, by chute is allowed provided that a weighing scales for
determining the batch weight will be used.
For batch mixing at the site of construction or at a central point, a batch
mixer of an approved type shall be used. Mixer having a rated capacity of
less than a one-bag batch shall not be used. The volume of concrete mixed
per batch shall not exceed the mixer’s nominal capacity as shown on the
manufacturer’s standard rating plate on the mixer except that an overload
up to 10 percent above the mixer’s nominal capacity may be permitted,
provided concrete test data for strength, segregation, and uniform
consistency are satisfactory and provided no spillage of concrete takes
place. The batch shall be so charge into the drum that a portion of the
water shall enter in advance of the cement and aggregates. The flow of
water shall be uniform and all water shall be in the drum by the end of the
first 15 seconds of the mixing period. Mixing time shall be measured from
the time all materials, except water, are in the drum. Mixing time shall
not be less than 60 seconds for mixers having a capacity of 1.5m3 or less. For mixers having a capacity greater than 1.5m 3, the mixing time shall not be less than 90 seconds. If timing
starts, the instant the skip reaches its maximum raised position, 4 seconds
shall be added to the specified mixing time. Mixing time ends when the
discharge chute opens.
The mixer shall be operated at the drum speed as shown on the
manufacturer’s name plate on the mixer. Any concrete mixed less than the
specified time shall be discarded and disposed off by the Contractor at his
own expenses.
The timing device on stationary mixers shall be equipped with a bell or
other suitable warning device adjusted to give a clearly audible signal
each time the lock is released. In case of failure of the timing device,
the Contractor will be permitted to continue operations while it is being
repaired, provided he furnishes an approved timepiece equipped with minute
and second hands. If the timing device is not placed in good working order
within 24 hours, further use of the mixer will be prohibited until repairs
are made.
Retempering concrete will not be permitted. Admixtures for increasing the
workability, for retarding the set, or for accelerating the set or
improving the pumping characteristics of the concrete will be permitted
only when specifically provided for in the Contract, or authorized in
writing by the Engineer.
1. Mixing Concrete: General
Concrete shall be thoroughly mixed in a mixer of an approved size and type
that will insure a uniform distribution of the materials throughout the
mass.
All concrete shall be mixed in mechanically operated mixers. Mixing plant
and equipment for transporting and placing concrete shall be arranged with
an ample auxiliary installation to provide a minimum supply of concrete in
case of breakdown of machinery or in case the normal supply of concrete is
disrupted. The auxiliary supply of concrete shall be sufficient to complete
the casting of a section up to a construction joint that will meet the
approval of the Engineer.
Equipment having components made of aluminum or magnesium alloys, which
would have contact with plastic concrete during mixing, transporting or
pumping of Portland Cement concrete, shall not be used.
Concrete mixers shall be equipped with adequate water storage and a device
of accurately measuring and automatically controlling the amount of water
used.
Materials shall be measured by weighing. The apparatus provided for
weighing the aggregates and cement shall be suitably designed and
constructed for this purpose. The accuracy of all weighing devices except
that for water shall be such that successive quantities can be measured to
within one percent of the desired amounts. The water measuring device shall
be accurate to plus or minus 0.5 mass percent. All measuring devices shall
be subject to the approval of the Engineer. Scales and measuring devices
shall be tested at the expense of the Contractor as frequently as the
Engineer may deem necessary to insure their accuracy.
Weighing equipment shall be insulated against vibration or movement of
other operating equipment in the plant. When the entire plant is running,
the scale reading at cut-off shall not vary from the weight designated by
the Engineer more than one mass percent for cement, 1-1/2 mass percent for
any size of aggregate, or one (1) mass percent for the total aggregate in
any batch.
2. Mixing Concrete at Site Concrete mixers may be of the revolving drum or
the revolving blade type and the mixing drum or blades shall be operated
uniformly at the mixing speed recommended by the manufacturer. The pick-up
and throw-over blades of mixers shall be restored or replaced when any part
or section is worn 20mm or more below the original height of the
manufacturer’s design. Mixers and agitators which have an accumulation of
hard concrete or mortar shall not be used.
When bulk cement is used and volume of the batch is 0.5m3 or
more, the scale and weigh hopper for Portland Cement shall be separate and
distinct from the aggregate hopper or hoppers. The discharge mechanism of
the bulk cement weigh hopper shall be interlocked against opening before
the full amount of cement is in the hopper. The discharging mechanism shall
also be interlocked against opening when the amount of cement in the hopper
is underweight by more than one (1) mass percent or overweight by more than
3 mass percent of the amount specified.
When the aggregate contains more water than the quantity necessary to
produce a saturated surface dry condition, representative samples shall be
taken and the moisture content determined for each kind of aggregate.
The batch shall be so charged into the mixer that some water will enter in
advance of cement and aggregate. All water shall be in the drum by the end
of the first quarter of the specified mixing time.
Cement shall be batched and charged into the mixer so that it will not
result in loss of cement due to the effect of wind, or in accumulation of
cement on surface of conveyors or hoppers, or in other conditions which
reduce or vary the required quantity of cement in the concretemixture.
The entire content of a batch mixer shall be removed from the drum before
materials for a succeeding batch are placed therein. The materials
composing a batch except water shall be deposited simultaneously into the
mixer.
All concrete shall be mixed for a period of not less than 1-1/2 minutes
after all materials, including water, are in the mixer. During the period
of mixing, the mixer shall operate at the speed for which it has been
designed.
Mixers shall be operated with an automatic timing device that can be locked
by the Engineer. The time device and discharge mechanics shall be so
interlocked that during normal operation no part of the batch will be
charged until the specified mixing time has elapsed.
The first batch of concrete materials placed in the mixer shall contain a
sufficient excess of cement, sand, and water to coat inside of the drum
without reducing the required mortar content of the mix. When mixing is to
cease for a period of one hour or more, the mixer shall be thoroughly
cleaned.
3. Mixing Concrete at Central Plant Mixing at central plant shall conform
to the requirements for mixing at the site.
4. Mixing Concrete in Truck
Truck mixers, unless otherwise authorized by the Engineer, shall be of the
revolving drum type, water-tight, and so constructed that the concrete can
be mixed to insure a uniform distribution of materials throughout the mass.
All solid materials for the concrete shall be accurately measured and
charged into the drum at the proportioning plant. Except as subsequently
provided, the truck mixer shall be equipped with a device by which the
quantity of water added can be readily verified. The mixing water may be
added directly to the batch, in which case a tank is not required. Truck
mixers may be required to be provided with a means of which the mixing time
can be readily verified by the Engineer.
The maximum size of batch in truck mixers shall not exceed the minimum
rated capacity of the mixer as stated by the manufacturer and stamped in
metal on the mixer. Truck mixing, shall, unless other-wise directed be
continued for not less than 100 revolutions after all ingredients,
including water, are in the drum. The mixing speed shall not be less than 4
rpm, nor more than 6 rpm.
Mixing shall begin within 30 minutes after the cement has been added either
to the water or aggregate, but when cement is charged into a mixer drum
containing water or surface wet aggregate and when the temperature is above
320C, this limit shall be reduced to 15 minutes. The limitation
in time between the introduction of the cement to the aggregate and the
beginning of the mixing may be waived when, in the judgment of the
Engineer, the aggregate is sufficiently free from moisture, so that there
will be no harmful effects on the cement.
When a truck mixer is used for transportation, the mixing time specified in
Subsection 405.4.4 (3) at a stationary mixer may be reduced to 30 seconds
and the mixing completed in a truck mixer. The mixing time in the truck
mixer shall be as specified for truck mixing.
5. Transporting Mixed Concrete
Mixed concrete may only be transported to the delivery point in truck
agitators or truck mixers operating at the speed designated by the
manufacturers of the equipment as agitating speed, or in non-agitating
hauling equipment, provided the consistency and workability of the mixed
concrete upon discharge at the delivery point is suitable point for
adequate placement and consolidation in place.
Truck agitators shall be loaded not to exceed the manufacturer’s guaranteed
capacity. They shall maintain the mixed concrete in a thoroughly mixed and
uniform mass during hauling.
No additional mixing water shall be incorporated into the concrete during
hauling or after arrival at the delivery point.
The rate of discharge of mixed concrete from truck mixers or agitators
shall be controlled by the speed of rotation of the drum in the discharge
direction with the discharge gate fully open.
When a truck mixer or agitator is used for transporting concrete to the
delivery point, discharge shall be completed within one hour, or before 250
revolutions of the drum or blades, whichever comes first, after the
introduction of the cement to the aggregates. Under conditions contributing
to quick stiffening of the concrete or when the temperature of the concrete
is 300C, or above, a time less than one hour will berequired.
6. Delivery of Mixed Concrete
The Contractor shall have sufficient plant capacity and transportation
apparatus to insure continuous delivery at the rate required. The rate of
delivery of concrete during concreting operations shall be such as to
provide for the proper handling, placing and finishing of the concrete. The
rate shall be such that the interval between batches shall not exceed 20
minutes. The methods of delivering and handling the concrete shall be such
as will facilitate placing of the minimum handling.
405.5 Method of Measurement
The quantity of structural concrete to be paid for will be the final
quantity placed and accepted in the completed structure. No deduction will
be made for the volume occupied by pipe less than 100mm (4 inches) in
diameter or by reinforcing steel, anchors, conduits, weep holes or
expansion joint materials.
405.6 Basis of Payment
The accepted quantities, measured as prescribed in Section 405.5, shall be
paid for at the contract unit price for each of the Pay Item listed below
that is included in the Bill of Quantities.
Payment shall constitute full compensation for furnishing, placing and
finishing concrete including all labor, equipment, tools and incidentals
necessary to complete the work prescribed in the item.
Payment will be made under:
Pay Item Number
|
Description
|
Unit of Measurement
|
405 (1)
405 (2)
405 (3)
405 (4)
405 (5)
|
Structural Concrete, Class A
Structural Concrete, Class B Structural Concrete, Class C Structural Concrete, Class P Seal Concrete |
Cubic Meter
Cubic Meter
Cubic Meter
Cubic Meter
Cubic Meter
|
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