**Construction Specification 32---Structure Concrete**

**1. Scope**

The work shall consist of furnishing, forming, placing, finishing, and
curing portland cement concrete as required to build the structures
described in section 24 of this specification.

**2. Material**

**Aggregates** shall conform to the requirements of Material
Specification 522, Aggregates for Portland Cement Concrete, unless
otherwise specified. The grading of coarse aggregates shall be as
specified in section 24.

**Portland cement** shall conform to the requirements of Material
Specification 531, Portland Cement, for the specified type.

**Fly ash** shall conform to the requirements of Material Specification
532, Mineral Admixtures for Concrete.

**Air-entraining admixtures** shall conform to the requirements of
Material Specification 533, Chemical Admixtures for Concrete. If
air-entraining cement is used, any additional air-entraining admixture
shall be of the same type as that in the cement.

**Water reducing and/or retarding admixtures** shall conform to the
requirements of Material Specification 533, Chemical Admixtures for
Concrete.

**Curing compound** shall conform to the requirements of Material
Specification 534, Concrete Curing Compound.

**Preformed expansion joint filler** shall conform to the requirements
of Material Specification 535, Preformed Expansion Joint Filler.

**Waterstops** shall conform to the requirements of Material
Specifications 537, Nonmetallic Waterstops, and 538, Metal Waterstops,
for the specified kinds.

**Water used in mixing and curing concrete** shall be clean and free
from injurious amounts of oil, salt, acid, alkali, organic matter, or
other deleterious substances.

**3. Class of concrete**

Concrete for structure concrete shall be classified as follows:

> Class of Maximum net    Minimum\
> concrete water content cement content\
>   (gal/bag)    (bags/yd3)
>
> 3000M 6 5.5
>
> 4000M 6 6

**4. Air content and consistency**

Unless otherwise specified, the slump shall be 3 to 5 inches. If air
entrainment is specified, the air content, by volume, shall be 4 to 7
percent of the volume of the concrete. When specified, directed, or
approved by the engineer, a water-reducing, set-retarding, or other
admixture shall be used. High range, water reducing agents
(superplasticizers) may be used to increase workability, reduce water
content, and control concrete temperature in hot weather. The maximum
slump after adding high range water reducing agents shall be 7.5 inches.

**5. Design of the concrete mix**

The proportions of the aggregates shall be such as to produce a concrete
mixture that works readily into the corners and angles of the forms and
around reinforcement when consolidated, but does not segregate or exude
free water during consolidation.

Fly ash may be used as a partial substitution for portland cement in an
amount of no more than 25 percent (by weight) of the cement in the
concrete mix, unless otherwise specified.

Before the concrete is placed, the contractor shall furnish the
contracting officer, for approval, a statement of the materials and mix
proportions (including admixtures, if any) intended for use. The
statement shall include evidence satisfactory to the contracting officer
that the materials and proportions will produce concrete conforming to
this specification. The materials and proportions so stated shall
constitute the \"job mix.\" After a job mix has been approved, neither
the source, character, or grading of the aggregates nor the type or
brand of cement or admixture shall be changed without prior notice to
the contracting officer. If such changes are necessary, no concrete
containing such new or altered material shall be placed until the
contracting officer has approved a revised job mix.

**6. Inspection and testing**

The engineer shall have free entry to the plant and equipment furnishing
concrete under the contract. Proper facilities shall be provided for the
engineer to inspect materials, equipment, and processes and to obtain
samples of the concrete. All tests and inspections will be conducted so
as not to interfere unnecessarily with manufacture and delivery of the
concrete.

**7. Handling and measurement of material**

Materials shall be stockpiled and batched by methods that prevent
segregation or contamination of aggregates and ensure accurate
proportioning of the ingredients of the mix. Except as otherwise
provided in section 8, cement and aggregates shall be measured as
follows:

**Cement** shall be measured by weight or in bags of 94 pounds each.
When cement is measured in bags, no fraction of a bag shall be used
unless weighed.

**Aggregates** shall be measured by weight. Mix proportions shall be
based on saturated, surface-dry weight. The batch weight of each
aggregate shall be the required saturated, surface-dry weight plus the
weight of surface moisture it contains.

**Water** shall be measured, by volume or by weight, to an accuracy
within 1 percent of the total quantity of water required for the batch.

**Admixtures** shall be measured within a limit of accuracy of 3
percent.

**8. Mixers and mixing**

Concrete shall be uniform and thoroughly mixed when delivered to the
work site. Variations in slump of more than 1 inch within a batch are
considered evidence of inadequate mixing and shall be corrected by
increasing mixing time or other acceptable alternative.

For stationary mixers, the mixing time after all cement and aggregates
are in the mixer drum shall be not less than 1.5 minutes. When concrete
is mixed in a truck mixer, the number of revolutions of the drum or
blades at mixing speed shall be not less than 70 nor more than l00.

Unless otherwise specified, volumetric batching and continuous mixing at
the construction site are permitted. To produce concrete meeting the
specified proportioning and uniformity requirements, the batching and
mixing equipment shall conform to the requirements of ASTM Specification
C 685 and shall be demonstrated by tests with the job mix before the
concrete is placed. Concrete made by this method shall be produced,
inspected, and certified in conformance with sections 6, 7, 8, 13, and
14 of ASTM Specification C 685.

No mixing water in excess of the amount called for by the job mix shall
be added to the concrete during mixing or hauling or after arrival at
the delivery point.

**9. Forms**

Forms shall be of wood, plywood, steel, or other approved material and
shall be mortar tight. The forms and associated falsework shall be
substantial and unyielding and shall be constructed so that the finished
concrete will conform to the specified dimensions and contours. Form
surfaces shall be smooth and free from holes, dents, sags, or other
irregularities. Forms shall be coated with a nonstaining form release
agent before being set into place.

Metal ties or anchorages within the forms shall be equipped with cones,
she-bolts or other devices that permit their removal to a depth of at
least 1 inch without injury to the concrete. Ties designed to break off
below the surface of the concrete shall not be used without cones.

All edges that will be exposed to view when the structure is completed
shall be chamfered, unless finished with molding tools as specified in
Section l8.

**10. Preparation of forms and subgrade**

Prior to placement of concrete, the forms and subgrade shall be free of
chips, sawdust, debris, water, ice, snow, extraneous oil, mortar, or
other harmful substances or coatings and the temperature of all surfaces
to be in contact with the new concrete shall be not be less than 40
degrees Fahrenheit. Any oil on the reinforcing steel or other surfaces
required to be bonded to the concrete shall be removed. Rock surfaces
shall be cleaned by air-water cutting, wet sandblasting, or wire brush
scrubbing, as necessary, and shall be wetted immediately before
placement of concrete. The earth surface shall be firm and damp.
Placement of concrete on mud, dried earth, or uncompacted fill or frozen
subgrade is not permitted.

Items to be embedded in the concrete shall be positioned accurately and
anchored firmly.

Weepholes in walls or slabs shall be formed with nonferrous material.

**11. Conveying**

Concrete shall be delivered to the site and discharged into the forms
within l-1/2 hours after the introduction of the cement to the
aggregates. In hot weather or under conditions contributing to quick
stiffening of the concrete, the time between the introduction of the
cement to the aggregates and discharge shall not exceed 45 minutes.

The engineer may allow a longer time, provided the setting time of the
concrete is increased a corresponding amount by the addition of an
approved set-retarding admixture. In any case, concrete shall be
conveyed from the mixer to the forms as rapidly as practicable by
methods that prevent segregation of the aggregates and assure no loss of
mortar occurs.

**12. Placing**

Concrete shall not be placed until the subgrade, forms, steel
reinforcement, and embedded items have been inspected and approved. No
concrete shall be placed except in the presence of the engineer. The
contractor shall give reasonable notice to the engineer each time
concrete is to be placed. Such notice shall provide sufficient time for
the engineer to inspect the subgrade, forms, steel reinforcement, and
other preparations for compliance with the specifications. Other
preparations include, but are not limited to, the concrete mixing plant;
delivery equipment system; placing, finishing, and curing equipment and
system; schedule of work; workforce; and heating or cooling facilities,
if applicable. Deficiencies are to be corrected before concrete is
delivered for placing.

The concrete shall be deposited as closely as possible to its final
position in the forms. It shall be worked into the corners and angles of
the forms and around all reinforcement and embedded items in a manner to
prevent segregation of aggregates or excessive laitance. Formed concrete
shall be placed in horizontal layers not more than 20 inches thick.
Concrete shall not be dropped more than 5 feet vertically unless
suitable equipment is used to prevent segregation. When high range water
reducing agents are used, the concrete shall not be allowed to drop more
than 10 feet. Hoppers and chutes, pipes, or \"elephant trunks\" shall be
used as necessary to prevent segregation and the splashing of mortar on
the forms and reinforcing steel above the layer being placed.

Immediately after the concrete is placed in the forms, it shall be
consolidated by spading, hand tamping, or vibration as necessary to
ensure a smooth surface and dense concrete. Each layer shall be
consolidated to ensure monolithic bond with the preceding layer. If the
surface of a layer of concrete in place sets to the degree that it will
not flow and merge with the succeeding layer when spaded or vibrated,
the contractor shall discontinue placing concrete and shall make a
construction joint according to the procedure specified in section 13.

If placing is discontinued when an incomplete horizontal layer is in
place, the unfinished end of the layer shall be formed by a vertical
bulkhead.

**13. Construction joints**

Construction joints shall be made at the locations shown on the
drawings. If construction joints are needed that are not shown on the
drawings, they shall be placed in locations approved by the engineer.

Where a feather edge would be produced at a construction joint, as in
the top surface of a sloping wall, an insert form shall be used so that
the resulting edge thickness on either side of the joint is not less
than 6 inches.

In walls and columns, as each lift is completed, the top surface shall
be immediately and carefully protected from any condition that might
adversely affect the hardening of the concrete.

Steel tying and form construction adjacent to concrete in place shall
not be started until the concrete has cured at least 12 hours. Before
new concrete is deposited on or against concrete that has hardened, the
forms shall be retightened. New concrete shall not be placed until the
hardened concrete has cured at least 12 hours.

The surface of construction joints shall be cleaned of all
unsatisfactory concrete, laitance, coatings, or debris by washing and
scrubbing with a wire brush or wire broom or by other means approved by
the engineer. The surface shall be kept moist for at least 1 hour before
the new concrete is placed.

**14. Expansion and contraction joints**

Expansion and contraction joints shall be made only at locations shown
on the drawings.

Exposed concrete edges at expansion and contraction joints shall be
carefully tooled or chamfered, and the joints shall be free of mortar
and concrete. Joint filler shall be left exposed for its full length
with clean and true edges.

Preformed expansion joint filler shall be held firmly in the correct
position as the concrete is placed.

When open joints are specified, they shall be constructed by the
insertion and subsequent removal of a wooden strip, metal plate, or
other suitable template in such a manner that the corners of the
concrete are not chipped or broken. The edges of open joints shall be
finished with an edging tool before the joint strips are removed.

**15. Waterstops**

Waterstops shall be held firmly in the correct position as the concrete
is placed. Joints in metal waterstops shall be soldered, brazed, or
welded. Joints in rubber or plastic waterstops shall be cemented,
welded, or vulcanized as recommended by the manufacturer.

**16. Removal of forms**

Forms shall not be removed without the approval of the engineer. Forms
shall be removed in such a way as to prevent damage to the concrete.
Supports shall be removed in a manner that permits the concrete to take
the stresses of its own weight uniformly and gradually.

**17. Finishing formed surfaces**

Immediately after the forms are removed:

a\. All fins and irregular projections shall be removed from exposed
surfaces.

b\. The holes produced on all surfaces by the removal of form ties,
cone-bolts, and she-bolts shall be cleaned, wetted, and filled with a
dry-pack mortar. The mortar will consist of one part portland cement,
three parts sand that will pass a No. 16 sieve, and just sufficient
water to produce a consistency such that the filling is at the point of
becoming rubbery when the material is solidly packed.

**18. Finishing unformed surfaces**

All exposed surfaces of the concrete shall be accurately screeded to
grade and then float finished, unless specified otherwise.

Excessive floating or troweling of surfaces while the concrete is soft
is not permitted.

Adding dry cement or water to the surface of the screeded concrete to
expedite finishing is not allowed.

Joints and edges on unformed surfaces that will be exposed to view shall
be chamfered or finished with molding tools.

**19. Curing**

Concrete shall be prevented from drying for a curing period of at least
7 days after it is placed. Exposed surfaces shall be kept continuously
moist for the entire period, or until curing compound is applied as
specified below. Moisture shall be maintained by sprinkling, flooding,
or fog spraying, or by covering with continuously moistened canvas,
cloth mats, straw, sand, or other approved material. Wood forms left in
place during the curing period shall be kept continuously wet. A formed
surface shall be thoroughly wetted immediately after forms are removed
and shall be kept wet until patching and repairs are completed. Water or
covering shall be applied in such a way that the concrete surface is not
eroded or otherwise damaged.

Concrete, except at construction joints, may be coated with the approved
curing compound instead of continued application of moisture, except as
otherwise specified in section 24. The compound shall be sprayed on the
moist concrete surface as soon as free water has disappeared, but shall
not be applied to any surface until patching, repairs, and finishing of
that surface are completed. The compound shall be applied at a uniform
rate of not less than 1 gallon per 175 square feet of surface and shall
form a continuous adherent membrane over the entire surface. Curing
compound shall be thoroughly mixed before applying and continuously
agitated during application. Curing compound shall not be applied to a
surface requiring bond to subsequently placed concrete, such as
construction joints, shear plates, reinforcing steel, and other embedded
items. If the membrane is damaged during the curing period, the damaged
area shall be resprayed at the rate of application specified above. Any
surface covered by the membrane shall not be trafficked unless protected
from wear.

**20. Removal and replacement or repair**

When concrete is honeycombed, damaged, or otherwise defective, the
contractor shall remove and replace the structure or structural member
containing the defective concrete or, where feasible, correct or repair
the defective parts. The contracting officer determines the required
extent of removal, replacement, or repair. Before starting repair work,
the contractor shall obtain the contracting officer\'s approval of the
plan for repairs. The contractor shall perform all repair work in the
presence of the engineer.

**21. Concreting in cold weather**

Concrete shall not be mixed nor placed when the daily minimum
atmospheric temperature is less than 40 degrees Fahrenheit unless
facilities are provided to prevent the concrete from freezing. The use
of accelerators or antifreeze compounds is not allowed.

**22. Concreting in hot weather**

The contractor shall apply effective means to maintain the temperature
of the concrete below 90 degrees Fahrenheit during mixing, conveying,
and placing.

**23. Measurement and payment**

For items of work for which specific unit prices are established in the
contract, concrete is measured to the neat lines shown on the drawings
and the volume of concrete is computed to the nearest 0.l cubic yard.
Measurement of concrete placed against the sides of an excavation
without using intervening forms is made only to the neatness or pay
limits shown on the drawings. No deduction in volume is made for
chamfers, rounded or beveled edges, or for any void or embedded item
that is less than 5 cubic feet in volume.

Payment for each item of structure concrete is made at the contract unit
price or the contract lump sum; whichever is applicable for that item.
Such payment constitutes full compensation for all labor, material,
equipment, transportation, tools, forms, falsework, bracing, and all
other items necessary and incidental to the completion of the work
except items listed for payment elsewhere in the contract. Compensation
for any item of work described in the contract, but not listed in the
bid schedule, is included in the payment for the item of work to which
it is made subsidiary. Such items and the items to which they are made
subsidiary are identified in section 24 of this specification.

**24. Items of work and construction details**

Work shall be done in accordance with NRCS Plans and Specifications.

This specification shall be used for all reinforced concrete within the
Construction Plans.

Reinforcing steel shall be US Grade 60, or higher, deformed bars.
Reinforcing steel shall be free from loose rust, concrete, form oil,
grease, paint, or other deleterious coating. No welding or heating of
steel shall be permitted.

Minimum splice length shall be as shown on the plans. If minimum splice
is not shown on the drawings, the following minimum(s) shall be applied:

#4 19 inches

#5 24 inches

*(ACI and CRSI Manuals of Standard Practice)*

NRCS shall be contacted 24 hours prior to concrete placement.

Concrete shall be 4000 psi Type II Portland cement with 4-7% air
entraining. Coarse aggregates meet AASHTO #67.

No mixing water in excess of the amount called for by the job mix shall
be added to the concrete during mixing or hauling or after arrival at
the delivery point.

Concrete shall be vibrated. Vibrator head shall be kept vertical when
inserted into concrete and shall penetrate at least six (6) inches into
previous layer.

Curing compound shall be applied to concrete surfaces. Curing compound
shall be applied after free water has disappeared, and within 24 hours
of pour. Curing compound shall not be applied at construction joints.

Heavy equipment may not be operated within 3 feet of new concrete wall.
Compaction within 3 feet will be by means of hand tamping or small
handheld tamping or vibrating equipment. Backfilling and compaction of
fill adjacent to new concrete walls shall not begin in less than ten
(10) days after concrete placement. Backfill material shall be free of
large stones and debris.