Concrete
Concrete Pavement and Concrete Structures
Quality Control Plan
The quality control plan must be produced and submitted according to the
provision.
Contractor Concrete Mix Design
The contractor is responsible for providing the design of the concrete
mixture for use on the project and
for any necessary adjustments during production. A mix design may be a new
design, or one used on
a previous project. New mixtures are those that use different material
sources or quantities than a
previously used mix.
A PCC Technician II, hired or employed by the contractor, is required to
develop and submit the mix
design report to the engineer before the production of concrete for the
project. The mix design must
meet the conditions specified in the QMP provision. The report can include a
number of different mix
designs, but each mix design is required to have supporting laboratory or
field test results. Multiple mix
designs will enable using the most appropriate mix on a project, for given
conditions.
Concrete Mix Design, and submit to the engineer. The
engineer's signature verifies that the engineer had an opportunity to review
the mix design.
A mix design may be transferred from one project to another if the quality
control and verification test
results verify consistent, acceptable performance. To be used on a new
project, a transferred mix
should contain the same materials and proportions as that used on the
previous project. The
contractor should submit a written and signed request for transfer of a mix
design. The written request
must certify that the source and characteristics of the materials have not
changed since the original
mix design was issued. All supporting documentation should be included with
the request. This
includes a summary of the quality control and verification test results from
the previous project(s).
With the initial use of a mixture in a production capacity, it is the
contractor's responsibility to test the
properties of the mixture in a trial batch before mass production. Trial
mixtures must use the same
materials proposed for the work. When necessary, minor adjustments may be
made to a mix formula.
The adjustments should be determined from the quality control test results.
The adjusted mix formula
must meet the conditions specified of the mix design in the QMP
provision.
A copy of the mix design must be made available to all the interested
project parties (i.e. engineer,
contractor, QC Technician, QA Technician, and Independent Assurance
Technician). For concrete
structures, fly ash or slag is required to be used as a partial replacement
for portland cement. Use the
appropriate materials and proportions as specified in the provision. For
concrete pavement, fly ash or
slag may be used as a partial replacement for portland cement concrete. Use
the appropriate
materials and proportions as specified in the provision.
Concrete Plants
Plant start up includes calibration of the plant and testing equipment.
Before production, the contractor
should inspect the plant and test equipment. The engineer may choose to
waive his inspection based
on the results of the contractor's report.
In addition, the concrete producer is required to record the quantity of the
materials used in each batch.
The contractor is required to measure, monitor, and record the addition of
materials to the mix after
discharge from the plant.
Aggregate Sampling & Testing
Aggregate gradation sampling and testing must be performed according to the
QMP provision.
Combined Gradation
A combined aggregate gradation analysis should only be conducted on samples
collected during the
production of concrete. This analysis is performed using the as-batched
aggregate proportions for a
production load of concrete. The batch proportions used for the analysis
should be recorded from the
plant at the time the aggregate samples are collected from the working
of the stockpiles. After
performing gradation testing for each aggregate sample, the combined
aggregate gradation is
calculated according to the form instructions. Record project data on
department worksheet WS3012,
Combined Concrete Aggregate Gradation.
Specification Limits
Lower and upper (specification) limits for the combined gradation should be
calculated as follows:
1. Determine the as-batched fractional portion of each aggregate
gradation, by dividing the weight of the
aggregate gradation by the weight of the total aggregate used in the batch.
2. For each gradation control sieve, multiply the upper and lower
specification limits for each gradation by the
fractional portion of that aggregate being used.
3. For each control sieve, add the resultant products, from step 2, for each
aggregate's fractional upper
specification limit and lower specification limit.
The specification part of the calculation sheet (lower part) will remain fixed
unless a change is made in
the aggregate proportioning. In which case, it will be necessary to
re-calculate the specification limits.
Analysis of Combined Gradation Data
The data resulting from the combined gradation analysis is used by the QC
personnel to evaluate the
mixture quality and for control chart plotting. Analysis should be conducted
as follows.
First, complete and determine the specification limits, by summarizing the
principle gradations
and performing the indicated calculations for the percent total retained and
percent between sieves.
The Aggregate Gradation Chart is used as a visual of where the combined
gradation lies within the
specification limits. If any blend changes are made the control chart running
average values will start
over.
The provision requires the contractor to notify the engineer of adjustments
made in the batching process.
While movement within the specification envelope will be allowed to benefit
the contractor's use of
aggregate, any blend change resulting in a combined gradation outside the
established envelope will
constitute a significant adjustment to the mixture design. These adjustments
will require approval of
the engineer and re-establishment of the specification limits, following the
previously outlined
procedures.
The gradation summary table and the aggregate gradation chart are intended to
help the contractor make
quality control decisions.
Aggregate Moisture and P200 Testing
During concrete production for pavement and structures, P200 tests are
required. In addition, moisture
content tests and Water cementitious ratio (W/Cm) calculations are required
for class I structures
concrete. Use department worksheet WS3010 "Worksheet for Calculating:
Aggregate Moisture
Content, Combined % Passing #200 Sieve, and Water/Cementitious Ratio" to
calculate moisture
content and combined P200. The quantities used must reflect a specific batch
of concrete (not mix
design quantities); therefore, as aggregate samples are collected the
technician must also obtain
current batch quantities.
Concrete Testing
Materials Reporting System
The contractor submits mix information and test results for concrete pavement
and concrete structures
using the department's Materials Reporting System (MRS)
Water Cementitious Ratio
Water cementitious ratio (W/Cm) is an indicator of concrete quality. High
water contents result in lower
strength. W/Cm below 0.42 is desirable.
The W/Cm is calculated according to the formula below. Quantities used must
reflect target batch weights
for production concrete; therefore, when an individual aggregate moisture
content changes
significantly, the technician must also obtain current target batch quantities
and adjust the target batch
weights to maintain the design W/Cm. If using mobile transit mixer trucks, be
sure that the technician
includes the water added on-site to the mix drum.
In order to make this information useful to the batch operator, timely
results are necessary. Work should
begin immediately after the samples are collected and results should be
shared as soon as they are
available.
Concrete Pavement Lots & Sublots
The contractor must define lot and sublot locations before placing any QMP
concrete. Lots and sublots
may contain concrete placed on more than one day of paving. A mainline
sublot is 1000 lane feet in
size. Therefore, depending on the paving operation, a sublot will be 500
linear feet for 2 lanes being
paved simultaneously, or 1000 linear feet if the paving operation is one
lane wide. For non-mainline
surfaces a sublot is a maximum of 250 cubic yards.
Lots will consist of a maximum of 8 sublots and contain material from a
single mix design. If a lot contains
less than 4 sublots, there is not enough information to establish a
meaningful percent within limits
(PWL) statistic, and therefore there is no opportunity for the contractor to
earn a strength incentive for
that lot.
Concrete Structures Lots & Sublot
The contractor must define all lots on the project before placing any QMP
concrete. The contractor may
need to adjust the planned lot sizes and locations to match the actual
construction operations. These
adjustments are allowable if they do not introduce bias.
The contractor must define lots that do not exceed 500 CY of material from a
single mix design. Each lot
must be divided into sublots that do not exceed 50 CY. If a lot contains
less than 4 sublots, there is not
enough information to establish a meaningful percent within limits (PWL)
statistic, and therefore there
is no opportunity for the contractor to earn a strength incentive for that
lot.
The contractor should try to create lots that logically correspond to their
construction operations.
Encourage the contractor to define smaller lots if the work is spread out
over time or if a number of
smaller individual components are being constructed. Within each lot, the
contractor should try to
designate sublots that are all about the same size. Each sublot, however, is
weighted by its volume for
pay determination.
Contractors will usually prefer to do the minimum required testing. Remind
the contractor that under a
statistical specification, it may be in their interest to define their lots
and sublots rationally to reduce
potential variability.
Examples:
1. A series of bridge footings is poured in a day. These footings contain 40
CY of material. For this pour, the
contractor may want to create one sublot to represent the concrete that is
placed on that day.
2. A 700 CY deck is planned. This pour requires at least two lots. The
contractor may want to divide the pour
into two lots of 350 CY each and subdivide each lot into 7 sublots of 50 CY
each.
3. A small project contains two 35 CY abutments and an 80 CY deck. Since the
total quantity of concrete
under the Concrete Masonry Bridges bid item for the project is 150 CY,
"small quantity" provisions apply.
Here the contractor is instructed in the special provision to divide the
project into at least 3 approximately
uniformly sized sublots.
If the contractor wants the benefit of a full statistical analysis and a
possible strength incentive, they must
create 4 or more sublots. In this example, it may make sense to define two
35 CY sublots for the
abutment work and two 40 CY sublots for the deck.
Slump
Slump test results must be documented with appropriate sample identification
information on a copy of
the Air Content Control Chart.
Temperature
High concrete temperatures result in fast hydration of the concrete and can
result in shrinkage cracking
and low strengths. Temperature data must be recorded on a copy of the Air
Content Control Chart.
Air Content
The contractor plots air content data using the department's MRS
software.
Compressive Strength
The contractor QC staff is responsible for fabrication, curing, and strength
testing for standard-cured
cylinders required under the QMP. These cylinders are independent of the
field-cured opening
strength cylinders that the contractor casts and breaks to determine when to
remove forms, falsework,
or open to service.
Fabricating & Curing Cylinders
The contractor QC staff fabricates, cures, and tests cylinders to determine
the 28-day compressive
strength for each sublot. A set of three 6X12 inch QC cylinders is required.
The contractor selects 2 of
the 3 QC cylinders at random and breaks them. If the breaking strengths are
close to the same, the
average strength of those 2 cylinders defines the sublot strength. If the 2
breaking strengths are
significantly different, the contractor breaks the third QC cylinder and
determines the sublot strength
as the average of the 2 highest strength cylinders.
The contractor records cylinder fabrication data using the department's MRS
software.
Care should be taken during casting, curing, transporting, and breaking
cylinders to avoid anything that
might bias the strength results. If vibrating cylinders, the technician
should take particular care to avoid
over-vibration that can cause segregation and lower strength. Although poor
technique generally gives
inconsistent and lower compressive strengths, which will hurt the
contractor, some irregularities may
benefit the contractor.
All HTCP certified technicians are trained to follow the same standard
procedures. The department's
independent assurance staff is charged with monitoring all project testing,
whether by the contractor,
the department, or a consultant, to make sure that those standard procedures
are followed.
Strength Test Results
The 28-day strength is the benchmark strength the department uses for
design, to measure the concrete
quality, and to determine incentive/disincentive pay adjustment. The average
strength of the 2 QMP
cylinders from each sublot defines the 28-day compressive strength for that
sublot.
Pay Adjustment for Strength
The department determines a pay adjustment for 28-day compressive strength.
For lots with less than 4
sublots, each sublot is evaluated individually. For lots with 4 or more
sublots a statistical analysis is
done to determine a lot-by-lot pay adjustment. After verifying the
contractor's data, the department
calculates pay adjustments using the department's MRS software. The
contractor must submit the
required strength test information electronically using the MRS .
The department administers incentives and disincentives under different
items. The unit for both items is
dollars. The engineer should always use these items for pay adjustment. On
smaller jobs, there may
be a single pay adjustment done for the entire project. On larger projects
pay adjustments may be
issued with progress payments.
The incentive items are included in the contract schedule of items as
predetermined prices fixed at
bidding. The fixed costs for the items are estimated at 60% of the maximum
available incentives for
the project. These items allow the engineer to pay incentives without a
construction change order.
Because a contractor can earn 0% to 100% of the maximum strength incentives
attainable for the
project and the contract bid items were at 60% of the maximum attainable, a
project can result in more
or less pay for the compressive strength incentives.
The disincentive items are administrative items included in the Field
Manager reference files to allow the
engineer to assess disincentives but require the addition of the
administrative items by change order.
Pay Adjustment for Small Lots (less than 4 sublots)
The contractor is free to establish lots with less than 4 sublots. With 4
sublots a statistical analysis is still
meaningful, but with less than 4 sublots it is of questionable value. The
department calculates the payadjustment for a lot with less than 4 sublots
by treating each sublot individually. Sublots with average
sublot strength greater than or equal to the specification limit receive no
adjustment. Sublots with an
average sublot strength less than the specification limit receive a
disincentive.
Statistical Pay Adjustment (4 or more sublots)
The department calculates the pay adjustment for a lot with 4 or more
sublots using a percent within limits
analysis (PWL) based on lot statistics, the lot mean strength, and the lot
sample standard deviation.
Only those lots with a standard deviation below a specified threshold are
eligible for incentive
payment. The lower quality index, how many standard deviations the lot mean
is above the
specification limit, is calculated and used to determine the PWL for a given
sample size. The resultant
PWL is applied to a pay equation to determine the appropriate pay adjustment
for the lot.
The basis for the analysis is the sublot average strength, the average of 2
QC cylinders for each sublot.
Weighted lot statistics are developed from the set of sublot average
strengths as follows:
Additional Payment Considerations
Special circumstances may require the engineer to modify the pay adjustment
using the MRS software.
Material that is represented by out-of-spec test results is not eligible for
incentive payment. The
engineer must deduct the appropriate amount from the lot pay adjustment that
the MRS calculates.
Testing frequencies for those other properties (aggregate gradation, P200,
air content, slump, and
concrete temperature) may not correspond to the strength sublots. The
engineer should note what
additional adjustments were made and how the quantity was determined using
the MRS software
"redlining tools."
Department Testing
Verification and independent assurance sampling and testing will be
performed by the department or a
department representative as described in the provision. Sampling and
testing will be performed by an
HTCP certified technician.
Verification Testing
Verification testing is performed by a department representative on samples
collected independently of
the contractor's samples according to the provision. Testing of the material
is conducted in a separate laboratory and with separate equipment from the
contractor's tests. The fabrication, curing, and
strength testing of QV cylinders will follow the same procedures specified
for QC cylinders.
Independent Assurance Review
Independent assurance reviews are conducted by a department representative
according to the provision
and the department's Independent Assurance Program. These reviews are made
of the contractor's
quality control and the department's verification sampling and testing
equipment and personnel.
Ancillary Concrete
Acceptance by Certification
According to the provision, certain ancillary concrete items can be accepted
based on a contractor's
certificate of compliance. Not all ancillary concrete items can be accepted
with this method.
Concrete Mix Design
The contractor may elect to use a concrete mix from standard spec 501 or,
where one of the grade A
mixes is allowed under standard spec 501.3.1, an approved QMP mix design for
concrete pavement
or structural concrete from the current contract may be used.
QC Documentation
Document all observations, inspection records, mix adjustments, cylinder
identification, and test results
daily for the engineer . Submit original testing records to the
engineer in a neat and orderly manner within 10 days after completing
concrete production.
Sampling Frequency
Randomly choose sample locations using the procedures. Tests must be
performed at
the following frequencies:
1. A minimum of one slump and air test per 100 cubic yards per mix
grade and placement method.
2. A minimum of one set of cylinders per 200 cubic yards per mix grade
and placement method.
3. For deck overlays, one set of tests and one set of cylinders per 50
cubic yards.
4. For concrete base and base patching, one set of tests and one set
of cylinders per 250 cubic yards.
Compressive Strength
The contractor QC staff is responsible for casting, field curing, and
breaking cylinders for each sample
location.
Air Content
The air content data must be plotted on a copy .
Temperature
The recording of concrete temperature during the fabrication of strength
cylinders will provide the
contractor information that may be useful on future projects. High concrete
temperatures result in fast
hydration of the concrete, thus reducing the time before sawing must occur.
If the hydration process is
too fast, it can result in shrinkage cracking and low strength. The
temperature data should be plotted
on a copy .
Slump
A certified PCC technician I or IA should measure slump according to AASHTO
T119. Do not use cut-off
components of a metal roll-up tape. The measuring device must start at
"zero" inches. The contractor
need not test slump for slip-formed concrete unless the engineer requests.
Provide material
conforming to standard spec 501.3.7.1. Slump should be recorded on a copy of
the WS5013.
Aggregate Gradation Sampling and Testing
Aggregate gradations must be performed according to the provision and using
AASHTO T11 and T27.
Department Testing
Verification and independent assurance sampling and testing are performed by
the department or a
department representative as described in the provision. Sampling and
testing must be performed by
an HTCP certified technician.
Verification Testing
Verification testing is performed by a department representative on samples
collected independently of
the contractor's samples according to the provision. Testing of the material
is conducted in a separate
laboratory and with separate equipment from the contractor's tests. The
fabrication, curing, and
strength testing of QV cylinders will follow the same procedures specified
for QC cylinders.
Independent Assurance Review
Independent assurance reviews are conducted by a department representative
according to the provision
and the department's Independent Assurance Program. These reviews are made
of the contractor's
quality control and the department's verification sampling and testing
equipment and personnel.
Dispute Resolution
Dispute resolution is conducted according to the provision.