Category:304 Aggregate Base Course

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304.1 Construction Inspection for Sec 304

304.1.1 Description (Sec 304.1)

Aggregate bases are used to provide additional structure and drainage.

304.1.2 Material (Sec 304.2)

Production of aggregate base material is normally inspected by Materials inspectors. The resident engineer will receive through SiteManager results copies of test reports showing quantity, gradation, and plastic index (PI) of the material produced.

Construction personnel are responsible for QA inspection of material when it arrives on the project and laying operations begin. Refer to Quality Control/Quality Assurance (QC/QA) in regard to QA testing requirements. There will be times during base laying operations when Materials inspectors will be on the project to take record samples of base. If difficulties are encountered with material degrading, or varying appreciably in plastic index or gradation, the project inspector should not hesitate to ask their assistance or advice.

304.1.3 Construction Requirements (Sec 304.3)

Compaction and rolling requirements will be in accordance with Sec 304.3.5. Review the general requirements in Construction Inspection for Bases and Aggregate Surfaces.

304.1.3.1 Field Laboratory (Sec 304.3.1)

No additional guidance - refer to specifications.

304.1.3.2 Subgrade (Sec 304.3.2)

All subgrade on which the base is to be constructed shall be completed in accordance with Sec 209 prior to placing base material. Aggregate base is not to be placed on frozen subgrade.

(Sec 304.3.2.1)

Examine the subgrade to ensure that it has not deteriorated since it was tested. Ensure that the material underlying the grading plane is firm, stable and unfrozen. A rutted surface does not provide an acceptable surface.

304.1.3.3 Placing (Sec 304.3.3)

Ensure Type 5 Aggregate Base is being spread without significant segregation. Normally, you would observe segregation, but if problems persist, support your contractor corrective active notice with a sieve analysis.

(Sec 304.3.3.1) A subgrade without cohesion and stability may need special considerations. Frequent watering of a sand subgrade to obtain a stable surface may be necessary. If the subgrade doesn’t stabilize, a material belt spreader may be necessary or the aggregate base may be dumped in piles and spread ahead. It is important the base has sufficient moisture to minimize segregation. Authorization to construct stabilized subgrade may be the only viable option.

(Sec 304.3.3.2) Aggregate base used for shoulders adjacent to rigid or flexible type pavement, including pavement resurfacing, shall be simultaneously deposited and spread on the subgrade. Because of segregation concerns and potential damage to the pavement, aggregate is not to be deposited on the pavement and bladed or dozed into place.

If earth subgrade is too low and the contractor elects to use base material to bring it to grade, the cost of bringing it to plan subgrade elevation must be borne by the contractor. Such work must be done before any base operations proceed in that area. When base material is used, normal base manipulation is necessary to get required compaction. If the subgrade tolerance encountered is excessive, the quantity of material required, the area effected and the contractor’s time and equipment should be noted in the inspector's diary so there will be a record in case of a later claim for additional costs. The production in linear feet / day in the area in question can be compared to the contractor’s normal daily production to justify any additional costs.

304.1.3.4 Shaping and Compacting (Sec 304.3.4)

Compaction operations usually follow immediately behind placing operations. It is imperative that satisfactory moisture content be maintained during compaction operations if proper density is to be obtained. Each layer shall be compacted to the specified density before another layer is placed.

(Sec 304.3.4.1) Segregated surface areas constructed of Type 1 aggregate base may be corrected by adding and compacting limestone screenings of such gradation and quantity as required to fill the surface voids, and firmly bind the loose material in place. Type 5 aggregate base is intended to provide drainage and must not be segregated. Trimmed Type 5 aggregate base may not be reused until the material is verified as meeting the required specifications. Base material contaminated to the extent that the material no longer complies with the specifications must be removed and replaced with satisfactory material at the contractor’s expense.

(Sec 304.3.4.2) No visible consolidation shall be evident after the final rolling.

(Sec 304.3.4.3) If at any time the compacted aggregate base or subgrade becomes unstable, the contractor, at the contractor's expense, shall restore the earth subgrade and the aggregate base to the required grade, cross section and density.

Density test is to be made at a minimum frequency specified. Additional tests may be necessary to insure that minimum density requirements are being met.

Density tests using the nuclear moisture / density gauges are documented with Form C-709-ND.

304.1.3.5 Substitutions for Aggregate Base (Sec 304.3.5)

If available, the contractor may substitute bituminous pavement cold millings or crushed recycled concrete in lieu of aggregate base for any temporary surface, regardless of the type or thickness of aggregate shown on the plans. If this option is exercised, the contractor shall notify the engineer at least two weeks prior to using the millings or recycled concrete, and shall identify the location from where the millings or concrete will be removed. The millings or recycled concrete shall be installed to the same dimensions shown on the plans for the aggregate base. Millings or recycled concrete shall be placed in maximum 4-inch (100 mm) lifts, and each lift shall be compacted by a minimum of three passes with a 10-ton (9 Mg) roller.

304.1.3.6 Maintenance (Sec 304.3.6)

The contractor is responsible to maintain the required density and surface condition of any portion of the completed aggregate base until either the prime coat or a succeeding course or pavement is placed. If a prime coat is specified in the contract, the contractor will be required to apply the prime coat on any completed portion of the aggregate base as soon as practical, or as otherwise specified. The contractor will not be permitted to apply prime if the moisture in the top 2 in. (50 mm) of the aggregate base exceeds the higher of either (1) the average of the optimum moisture as determined by the Standard Compaction Test and the absorption of the plus No. 4 (4.75 mm) fraction, or (2) two-thirds of the optimum moisture as determined by the Standard Compaction Test.

304.1.4 Quality Control/Quality Assurance (QC/QA) (Sec 304.4)

The contractor is responsible for density, thickness, gradation, deleterious and plasticity requirements of the aggregate base. The frequency, documentation requirements and submittals are specified in Sec 304.4.

304.1.4.1 Quality Control (Sec 304.4.1)

The contractor shall control operations to ensure the aggregate base, in place, meets the specified requirements for density, thickness, gradation, deleterious, and plasticity index. Tests shall be taken at random locations designated by the engineer at the frequency required by the specifications.

(Sec 304.4.1.1) The contractor shall provide copies of the test results, including all raw data, to the engineer the next business day following testing or sampling.

(Sec 304.4.1.2) The contractor or the contractor’s representative shall also determine the standard maximum dry density and the optimum moisture content and supply all test data to the engineer.

(Sec 304.4.1.3) When density tests are less than specified or when thickness measurements indicate the thickness is deficient by more than 1/2 in. (13 mm) from the plan thickness, additional measurements will be taken at the frequency specified. Each measurement will be assumed as representative of the base thickness for a distance extending one-half the distance to the next measurement, measured along centerline, or in the case of a beginning or ending measurement, the distance will extend to the end of the base section. Any deficient areas shall be corrected by reworking or adding material within the limits of the deficiency.

(Sec 304.4.1.4) When two consecutive tests for gradation, deleterious material, or plasticity index do not meet the specification limits, the material shall be removed beginning at the point where the first test was conducted.

(Sec 304.4.1.5) The contractor shall retain the untested portion of the plasticity index, gradation and deleterious sample for the engineer’s use.

304.1.4.2 Quality Assurance (Sec 304.4.2)

A vital element in acceptance of aggregate base is the quality assurance. Material properties are to be tested at the frequency stated in Sec 304.4. Gradation, presence of deleterious material and the PI need to be checked and documented. Test the compaction of aggregate base. Observe the compacting operation to ensure the material forms a firm, stable base. Measure and record the random thickness and template of the finished aggregate base for conformance with specifications, in addition to any inspections performed on items that are not otherwise part of a permanent record.

The contractor’s QC test results and the engineer’s QA test results shall meet the specifications and the following. The contractor’s compaction standard tests shall compare within 3.0 pounds of the maximum density of the MoDOT determined compaction standard. For retained samples, the contractor’s test results and the engineer’s test results shall compare within the limits specified.

304.1.5 Method of Measurement (Sec 304.5)

Final measurement of the completed aggregate base course will not be made except for authorized changes during construction, or where appreciable errors are found in the contract quantity. Where required, measurement of aggregate base course, complete in place, will be made to the nearest square yard (m2). Where the aggregate base course extends to the inslope of the shoulder, the pay limit of the aggregate base course will be measured from the mid-point of the sloped portion. The revision or correction will be computed and added to or deducted from the contract quantity.

304.1.6 Basis of Payment (sec 304.6)

The accepted quantities of aggregate base course of the thickness and type specified will be paid for at the contract unit price for each of the pay items included in the contract. When bituminous pavement cold millings or recycled crushed concrete are substituted for aggregate base, payment will be made for the aggregate base quantity provided in the plans, regardless of whether millings, recycled crushed concrete or the aggregate base is used.

304.2 Material Inspection for Sec 304

304.2.1 Scope

This guidance establishes procedures for inspection and acceptance of material used in aggregate base. Aggregate for use in base courses is to be inspected in accordance with Aggregate for Base.

304.2.2 Apparatus

The apparatus and materials required are listed in General Requirements for Material.

304.2.3 Procedure

304.2.3.1 Quality Control/Quality Assurance (QC/QA)

The contractor (QC) shall control operations to ensure the aggregate base, in place, meets the specified requirements for density, thickness, gradation, deleterious and plasticity index. Tests are to be taken at random locations designated by the engineer. The inspector (QA) shall take test, at random locations and at a reduced frequency, to accept the contractor’s results. The inspector (QA) shall also test, at least once per week, one of the contractor’s retained samples to assure that they are running the tests correctly.

304.2.3.2 Compaction Standard

Aggregate base course construction, except as noted in the Standard Specifications, requires that a certain density be achieved. In order that this density can be checked in the field, a sample must be submitted to the Laboratory for standard maximum density determination. The contractor is also required to determine the compaction standard. The contractor’s compaction standard shall be within 3.0 pounds of the compaction standard determined by the Central Laboratory.

The inspector shall obtain representative samples and submit two full sample bags of material in accordance with the procedures outlined in General Requirements for Material. In addition, the sample record in SiteManager shall request that a compaction standard test be performed.

304.2.3.3 Random Sampling

The inspector shall generate random numbers for the testing of each "lot" of material. A "lot" is defined in Sec 304.4.1. For example, a "lot" for the contractor’s determination of gradation and deleterious is defined as 2,000 tons or a days production, whichever is greater.

The inspector shall generate the numbers either using a random number table or with a random number generator on a calculator or computer. Using a random number generator is the preferred method.

The inspector shall generate two (2) random numbers for each lot. One to determine the longitudinal offset and one for the transverse offset. Only one set of random numbers needs to be determined for the Density and Thickness "lot". Determine the density and thickness of the base at the same location.

304.2.3.4 Sampling

Samples for gradation, deleterious and Plasticity Index (PI) shall be taken at the roadway, behind the placing operation, prior to compaction. Care should be taken to not contaminate sample with sub-grade material when extracting a sample from the roadway. The recommended sample size is outlined in General Requirements for Material. The contractor’s sample shall be large enough that one half is retained for QA testing.

304.2.3.5 Testing

Tests are to be run in accordance with the applicable test methods at the frequency listed in Sec 304.4.1. Please note that the frequencies listed are minimums. If material is approaching specification limits or if problems are encountered the inspector should increase the testing frequency.

Inspectors shall test one of the contractor’s retained QC samples at the following frequencies:

Test Frequency
Gradation and Deleterious 1 per 16,000 tons, minimum 1 per project
PI 1 per 80,000 tons, minimum 1 per project

For determination of thickness a rule with suitable graduations should be used to accurately measure the material to be inspected.

For the determination of density the inspector should use AASHTO T 310, Direct Transmission, for wet density. In order to determine the wet density the inspector must have first submitted a sample for the determination of the Compaction Standard. At the start of the job, a moisture-offset factor shall be determined in accordance with MoDOT Test Method T35.

304.2.3.6 Failing Tests

Procedures for failing test results are outlined in Sec 304.4.

304.2.4 Trimmed Base

Trimmed base may be reused as base material but must be checked for specification compliance prior to use. The material should be stockpiled and held pending testing. Material not meeting gradation can be reconditioned. Material that has been contaminated to such an extent that it no longer complies with the specification cannot be used.

304.3 Laboratory Testing for Sec 304

For aggregate base course, refer to Laboratory Testing for Sec 1007 for testing and reporting of standard compaction samples.