Category:360 Construction Inspection for Bases and Aggregate Surfaces
Base courses are structural foundations and provide drainage to maintain stability for the various types of roadway surfaces.
Pavement bases include all types of aggregate courses or stabilized permeable bases treated with portland cement or bituminous binders. The specific type is specified in the contract.
360.3 Construction Requirements
The construction procedures are common to most types of bases. Uniformity is an essential part of the construction of satisfactory base courses and shall receive the strict attention of the inspection forces as well as the contractor’s forces, during all the phases of production, placing, blending and mixing of the materials.
360.3.1 Inspection Checklist
1. Check and record finished subgrade for proper crown and grade base for conformance with allowable tolerance of 1/2 inch. prior to starting base operations. Spot-check areas both between stations where stakes are set as well as at staked locations. Document in bound field book.
2. If blading and manipulation of the base on the subgrade is necessary to achieve proper crown, sufficient moisture must be present in the material to prevent segregation.
3. Aggregate should not be deposited in excessive thicknesses in one lane with the top half of the material then bladed to the adjacent lane. Excessive manipulation promotes segregation and the material in place must meet gradation requirements in order to provide the necessary drainage and structure.
4. In the case where the material was deposited on the subgrade or a prior lift of base, and the desired density was not obtained, the aggregate can be aerated, wet down in place and rerolled to obtain the required required density. However, it must not become an accepted part of the normal operation. This practice typically produces segregation of the material.
5. A stable subgrade is essential and it can be secured only if adequate attention is given to surface drainage. Water pockets create soft or spongy spots in the subgrade that are unfit for pavement. Inspect the rolling operation. If the base is pumping, cracking or is showing other signs of failure, determine the magnitude of the failure and take corrective action to address the problem. Rutting will produce areas with deficient thickness and will result in a poorly drained subgrade.
6. Perform the required density tests for control purposes. Inspector should at all times be sure that minimum density requirements are being met and that compactive effort is uniform.
7. Inspect finished section and grade for conformance with acceptable tolerance.
8. All checks made and test results obtained should be properly documented in diaries or in appropriate inspection books.
360.3.2 Frequent Problems
Frequent problems occurring during base construction are:
1. Contractor produces or purchases aggregate base material within, but close to, specification limits, leaving no tolerance. In some cases handling and manipulating material causes segregation or breakdown so that it is no longer within specification limits when tested on the roadbed. This is the contractor's responsibility. If this occurs, the inspector should immediately contact the resident engineer for his handling of this matter.
2. Failure to add sufficient water to base material before hauling to grade for laying. This problem is created by attempting to avoid hauling costs for excess moisture which will be deducted from hauling weight. It is, however, imperative that proper moisture content be uniformly distributed in the material to obtain consistent and satisfactory density. If proper moisture content is not originally put in the delivered base or maintained through laying and compacting operations, the contractor must be required to take immediate corrective action.
3. Segregation of base material frequently occurs during the laying operation. Uniform density cannot then be obtained. Corrective action should be taken immediately. This problem normally occurs when insufficient moisture is in the base material, when water has not been thoroughly and uniformly mixed throughout the base material, or when equipment is used on-site to spread and manipulate the base material.
4. Insufficient or inconsistent compactive effort sometimes occurs. This is often due to high production in the laying operation without sufficient compactive equipment. Observe if the compactive equipment sits idle too often. In either case, the inspector should immediately inform the contractor that corrective measures are needed. If uniform density is to be obtained, compactive effort must be consistent at all times.
5. Controlling crown, width, and grade in base work is essential. This work is of a finishing nature and is often treated too lightly. Tolerance for finish grade for bases on which flexible pavement is to be constructed should generally be a maximum of + 1/2 inch.