1. Applicability
Examples of such specifications and guides are contained in Standard
Specifications for Highway Bridges, AASHTO and the ACI Manual of Concrete
Practice.
2. Material Specifications
3. Items to be Included in Contract Specifications and Drawings
a. Complete engineering and structural detail drawings of the structure.
(See Section 6, National Engineering Handbook.)
b. Type, size and quality of joint filler, waterstops, and metal plates.
c. Deviations, if any, from the specified concrete temperature ranges in
Section 21, CONCRETING IN COLD WEATHER.
d. Specify dowel size, spacing, length, and treatment of moveable dowel
ends (plastic sleeve, grease, etc.). Also specify deviations, if any, from
dowels specified.
e. Deviations, if any, from specifications requiring:
(1) Placement of slab concrete in a single layer.
(2) Consolidation of concrete with vibrators.
f. Section 3, Concrete Mix Design, Method 1. Specify and/or consider the
following items:
(1) Class of concrete.
(2) Type of cement.
(3) If water-cement ratios greater than 0.50 will be allowed, they must
be specified in Section 25.
(4) Any deviation from the air content and slump specified in Section 3,
Method 1, must be specified in Section 25. (The air content specified in
Section 3 may not be reduced for structures where the average annual minimum
air temperature is below 20o F.)
(5)Aggregate Data. Specify only the nominal maximum size of coarse
aggregate and not the ASTM size designation for coarse
aggregates.
If Material Specification 522 does not apply, specify the applicable
specification and/or the salient properties for aggregate.
(6) Types of admixtures, if any.
(7) Fly Ash and Slag Considerations. Specify in Section 25 if fly ash
or ground blast-furnace slag is required or is not allowed in the design
mix as a partial substitute for cement. Specify in Section 25 if the fly
ash class is restricted to either Class F or C (ASTM C 618). If there is
no mention of fly ash or slag in Section 25, the Contractor has the
option to use either fly ash (Class F or C) or slag (any grade) in the
design mix under Method 1, Section 3.
g. Section 3, Concrete Mix Design, Method 2. Specify the material
proportions and batch weights for the initial job mix which includes:
water-cement ratio; cement type and content/cy; coarse aggregate nominal
maximum size and content/cy; fine aggregate content/cy; percent air
content required; slump requirements; fly ash class and content/cy, if
any; slag grade, and content/cy, if any; and any other admixtures that
will be used.
h. Section 9, Conveying. The hot weather limitations for non-plasticized
concrete may be waived by the Engineer if the concrete continues to remain
within the allowable slump range and the temperature of the concrete does
not exceed 90o F.
This is consistent with industry practices but the waiver must be based on
the performance of the concrete onsite and the Engineer should exercise
caution and judgment when waiving the limitations.
i. Section 10, Placing. Specify a placement plan when required.
Placement plans should be considered only when complex placement and/or
control is required or expected or when a high volume of concrete is
involved.
Section 16, Removal of Forms, Supports, and Protective Coverings.
Strength Test option. Specify a minimum concrete compressive
strength for the form removal of structure member in Section 25. The
designer and the government assume the risk of form removal at that
designated strength.
Cumulative Time option. Form removal time for this option is based
upon the structural live load (the final in service load the member must
support) being substantially greater than the structural dead load (load
of the member only) and no significant horizontal loads on the member when
the forms are removed. If the structural dead load is near to or larger
than the structural live load, identify in Section 25 the members that
will require longer form removal times (See Footnote 2/ for the
Accumulative Form Removal Time table.). For walls and columns where forms
may be removed quickly and may be exposed to significant horizontal loads;
such as, wind loads, different removal times should be specified or the
Strength Test option required (See Footnote 3/ for the Accumulative
Form Removal Time table.). ACI 347R, Guide to Formwork for Concrete,
Paragraph 3.7.2.3 may be used as a reference for removal times.
k. Specify the finish in Section 25 if a finish other than that
required in Section 17 and 18 is needed; such as, an architectural
surface, a special finish, or other necessary restrictions. USBR Concrete
Manual, Chapter VI, Sections 119 and 121 may be used as a reference for
different finishes.
l. Section 22, Concreting in Hot Weather. Extreme conditions for formed
concrete is a manner of professional judgment on the part of the Engineer.
Extreme conditions for flatwork and slab construction is defined as when
the evaporation rate exceeding 0.2 lb/ft2/hr. Another method
that may be specified in Section 25 or approved is Figure 11-8, page 135,
"Design and Control of Concrete", Thirteenth Edition, Portland
Cement Association, 5420 Old Orchard Road, Skokie, IL 60077-1083.
m. Section 23, Acceptance of Concrete Work.
Concrete Strength. The basic premise for acceptance is the approved
job mix is delivered to the structure. Strength test failing to meet the
required criteria will occasionally occur even though concrete strength
and uniformity are satisfactory. The probabilities of a low strength test
result from a job mix meeting the criteria in Section 3 are: A 9 percent
chance (1 in 11) that an individual strength test will fall below f'c,
a 1 percent chance (1 in 100) that an individual strength test will fall
more than 500 psi below f'c, and a 1 percent chance that
the average of three consecutive strength tests will fall below f'c.
Allowances should be made for such statistically expected variations in
deciding whether the strength level being produced is adequate. The
criterion of an individual strength test falling more than 500 psi below f'c
adapts well to small numbers of tests (small
concrete volumes) and is the strongest indicator that something is wrong
with the concrete mix. Although there is a 1% chance that concrete
strength will not meet this criteria, verification of the in place strength
should be required if it does not meet the criteria and the contractor
required to take actions to improve the strength test averages. The
criterion of the average of three consecutive strength tests being equal
to or greater than the f'c is a good trend indicator and not as
critical as the other criterion. An occasional average below f'c can be tolerated; however, if it occurs two
or more consecutive times, consideration should be given to verifying the in place
concrete strength and requiring the contractor to take actions to
improve the strength test averages.
Structure Dimensions. Specify acceptable tolerances for the
structure if the tolerances shown under Structure Dimensions affect the
function, strength, or appearance of the structure.
4. Discussion of Methods
a. Section 3, Concrete Mix Design
(1) Method 1 is intended for use when strength is to be used as
one of the criteria for acceptance of the concrete and the Contractor is to
be responsible for the mix.
(2) Method 2 is intended for use when the Engineer is to be
responsible for and prescribe the concrete mix and strength will not govern
acceptance from the Contractor.
b. Section 13, Construction Joints
(1) Method 1 is intended for use in circumstances where maximum
bond between old and new concrete is desired. When such joint treatment is
specified, it is permissible to design horizontal construction joints as
flat plane surfaces without keyways or metal plates. Method 1 is preferred
for all structures that are continuously or intermittently exposed to water.
(2) Method 2 is intended for use in circumstances where bond
between old and new concrete is not a critical element.
c. Section 24, Measurement and Payment
(1) Method 1 is intended for use with Method 1 in Section 3, when
the design and control of the concrete mix is the responsibility of the
Contractor (that is, when the compressive strength of the concrete is one of
the criteria determining acceptability).
(2) Method 2 is intended for use with Method 2 in Section 3, when
the job mix is designed and controlled by the Engineer. A bid item for
cement should be included in the specification and the bid schedule.
When specifications are prepared using electronic procedures and all but
one method are deleted for use in a contract specification, delete "All
Methods The following provisions apply to all methods of measurement and
payment." and left justify the remaining text.
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