concrete has been placed. The surface of the pavement shall be thoroughly wetted prior to the placing of the paper.

(c) Straw curing.—When this type of curing is used, the pavement shall be cured initially with burlap or cotton mats, as specified in (a) until after final set of the concrete or, in any case, for 12 hours after placing the concrete. As soon as the mats are removed, the surface and sides of the pavement shall be thoroughly wetted and covered with at least 8 inches of straw or hay, thickness of which is to be measured after wetting. If the straw or hay covering becomes displaced during the curing period, it shall be replaced to the original depth and saturated. It shall be kept thoroughly saturated with water for 3 days and thoroughly wetted down during the morning of the fourth day, and the cover shall remain in place until the concrete has attained the required strength. When permission is given to open the pavement to traffic, the covering shall be removed and the pavement swept clean. The covering shall be removed and disposed of in such manner as to leave the right-of-way in a neat and presentable condition. The straw or hay shall not be disposed of by burning on, or adjacent to, the pavement.

(d) Impervious membrane method. The entire surface of the pavement shall be sprayed uniformly with white pigmented curing compound immediately after the finishing of the surface and before the set of the concrete has taken place, or if the pavement is cured initially with jute or cotton mats, it may be applied upon removal of the mats. The curing compound shall not be applied during rainfall.

Curing compound shall be applied under pressure at the rate of one gallon to not more than 150 square feet by mechanical sprayers. The spraying equipment shall be of the fully atomizing type equipped with a tank agitator. At the time of use, the compound shall be in a thoroughly mixed condition with the pigment uniformly dispersed throughout the vehicle. During application the compound shall be stirred continuously by effective mechanical means. Hand spraying of odd widths or shapes and concrete surfaces exposed by the removal of forms will be permitted. Curing compound shall not be applied to the inside faces of joints to be sealed, but approved means shall be used to insure proper curing and to prevent the intrusion of foreign material into the joint before sealing has been completed.

The curing compound shall be of such character that the film will harden within 30 minutes after application. Should the film become damaged from any cause within the 72-hour curing period, the damaged portions shall be repaired immediately with additional compound.

Upon removal of side forms, the sides of the slabs exposed

shall be protected immediately to provide a curing treatment equal to that provided for the surface.

(e) White polyethylene sheeting.—The top surface and sides of the pavement shall be entirely covered with polyethylene sheeting. The units used shall be lapped at least 18 inches. The sheeting shall be so placed and weighted down as to cause it to remain in intimate contact with the surface covered. The sheeting as prepared for use shall have such dimension that each unit as laid will extend beyond the edges of the slab at least twice the thickness of the pavement. Unless otherwise specified, the covering shall be maintained in place for 72 hours after the concrete has been placed.

(f) Curing in cold weather.-When concrete is being placed and the air temperature may be expected to drop below 35° F., a sufficient supply of straw, hay, grass, or other suitable blanketing material shall be provided along the work, and any time the temperature may be expected to reach the freezing point during the day or night, the material so provided shall be spread over the pavement to a sufficient depth to prevent freezing of the concrete. The period of time such protection shall be maintained shall be not less than 10 days. The Contractor shall be responsible for the quality and strength of the concrete placed during cold weather, and any concrete injured by frost action shall be removed and replaced at the Contractor's expense.

501.18 Removing Forms. Unless otherwise provided, forms shall not be removed from freshly placed concrete until it has set for at least 12 hours, except auxiliary forms used temporarily in widened areas. Forms shall be removed carefully so as to avoid damage to the pavement. After the forms have been removed, the sides of the slab shall be cured as outlined in one of the methods indicated above.

Minor areas of deep honeycomb near reinforcing steel shall be cleaned, wetted, and neatly patched with stiff mortar in the proportions of 1 part cement to 2 parts fine aggregate.

Major honeycombed areas will be considered as defective work and shall be removed and replaced. Any area or section so removed shall not be less than 10 feet in length nor less than full width of the lane involved. When it is necessary to remove and replace a section of pavement, any remaining portion of the slab adjacent to the joints that is less than 10 feet in length, shall also be removed and replaced.

501.19 Sealing Joints. If the joints are to be sealed, they shall be filled with joint sealing material before the pavement is opened to traffic, and as soon after completion of the curing period as is feasible. Just prior to sealing, each joint shall be thoroughly cleaned of all foreign material, including membrane curing compound and the joint faces shall be

clean and surface dry when the seal is applied. Material for seal applied hot shall be stirred during heating so that localized overheating does not occur.

The sealing material shall be applied to each joint opening to conform to the details shown on the plans or as directed by the Engineer. The pouring shall be done in such a manner that the material will not be spilled on the exposed surfaces of the concrete. Any excess material on the surface of the concrete pavement shall be removed immediately and the pavement surface cleaned. The use of sand or similar material as a cover for the seal will not be permitted. Poured joint-sealing material shall not be placed when the air temperature in the shade is less than 50° F., unless approved by the Engineer.

501.20 Protection of Pavement. The Contractor shall protect the pavement and its appurtenances against both public traffic and traffic caused by his own employees and agents. This shall include watchmen to direct traffic and the erection and maintenance of warning signs, lights, pavement bridges, or crossovers, etc. The plans or special provisions will indicate the location and type of device or facility required to protect the work and provide adequately for traffic.

Any damage to the pavement, occurring prior to final acceptance, shall be repaired or the pavement replaced, as provided in subsection 107.16.

501.21 Opening to Traffic. The Engineer will decide when the pavement may be opened to traffic. The pavement will not be opened to traffic until specimen beams conforming to the requirements of subsection 501.12 have attained a flexural strength of 550 pounds per square inch when tested by the third-point method, or specimen cylinders have attained a compressive strength of 3500 pounds per square inch when tested in accordance with AASHO T97 or T22, respectively. If such tests are not conducted, the pavement shall not be opened to traffic until 14 days after the concrete was placed. Prior to opening to traffic, the pavement shall be cleaned and joint sealing completed.

501.22 Concrete Pavement-Slip Form Method. The pavement may be constructed without the use of fixed forms, unless otherwise provided in the contract. If slip form pavers are used, the following provisions shall apply:

(a) Grade. After the grade or base has been placed and compacted to the required density, the areas which will support the paving machine shall be cut to the proper elevation by means of a properly designed machine. The grade on which the pavement is to be constructed shall then be brought to the proper profile by means of a properly designed machine.

If the density of the base is disturbed by the grading operations, it shall be corrected by additional compaction before concrete is placed. The grade should be constructed sufficiently in advance of the placing of the concrete to permit grade checking and continuous advance of the paver. If any traffic is allowed to use the prepared grade, the grade shall be checked and corrected immediately ahead of the placing of the concrete.

(b) Placing concrete.-The concrete shall be placed with an approved slip-form paver designed to spread, consolidate, screed, and float-finish the freshly placed concrete in one complete pass of the machine in such manner that a minimum of hand finish will be necessary to provide a dense and homogeneous pavement in conformance with the plans and specifications. The machine shall vibrate the concrete for the full width and depth of the strip of pavement being placed. Such vibration shall be accomplished with vibrating tubes or arms working in the concrete or with a vibrating screed or pan operating on the surface of the concrete. The sliding forms shall be rigidly held together laterally to prevent spreading of the forms. [The forms shall trail behind the paver for such a distance that no appreciable slumping of the concrete will occur, and that necessary final finishing can be accomplished while the concrete is still within the forms.] Any edge slump of the pavement, exclusive of edge rounding, in excess of 0.02-foot shall be corrected before the concrete has hardened.

The concrete shall be held at a uniform consistency, having a slump of not more than 11⁄2 inches. The slip-form paver shall be operated with as nearly a continuous forward movement as possible and all operations of mixing, delivering and spreading concrete shall be so coordinated as to provide uniform progress with stopping and starting of the paver held to a minimum. If, for any reason, it is necessary to stop the forward movement of the paver, the vibratory and tamping elements shall also be stopped immediately. No tractive force shall be applied to the machine, except that which is controlled from the machine.

(c) Finishing.-The surface smoothness and texture shall meet the requirements of subsection 501.15(g) and section 501.16.

(d) Curing.-Unless otherwise specified, curing shall be done in accordance with one of the methods included in section 501.17. The curing media shall be applied at the appropriate time and shall be applied uniformly and completely to all surfaces and edges of the pavement.

(e) Joints. All joints shall be constructed in accordance with section 501.14.

(f) Protection against rain.-In order that the concrete may be properly protected against the effects of rain before the concrete is sufficiently hardened, the Contractor will be required to have available at all times materials for the protection of the edges and surface of the unhardened concrete. Such protective materials shall consist of standard metal forms or wood plank having a nominal thickness of not less than 2 inches and a nominal width of not less than the thickness of the pavement at its edge for the protection of the pavement edges, and covering material such as burlap or cotton mats, curing paper, or plastic sheeting material for the protection of the surface of the pavement. When rain appears imminent, all paving operations shall stop and all available personnel shall begin placing forms against the sides of the pavement and covering the surface of the unhardened concrete with the protective covering.

501.23 Tolerance in Pavement Thickness. The thickness of the pavement will be determined by average caliper measurement of cores tested in accordance with AASHO T 148.

For the purpose of establishing an adjusted unit price for pavement, units to be considered separately are defined as 1000 lineal feet of pavement in each traffic lane starting at the end of the pavement bearing the smaller station number. The last unit in each lane shall be 1000 feet plus the fractional part of 1000 feet remaining. One core will be taken at random by the Government in each unit. When the measurement of the core from a unit is not deficient more than 0.2 inch from the plan thickness, full payment will be made. When such measurement is deficient more than 0.2 inch and not more than 1.0 inch from the plan thickness, two additional cores at intervals not less than 300 feet will be taken and used in the average thickness for that unit. An adjusted unit price as provided in subsection 501.25(b) will be paid for the unit represented.

Other areas such as intersections, entrances, crossovers, ramps, etc., will be considered as one unit, and the thickness of each unit will be determined separately. Small irregular unit areas may be included as part of another unit. At such points as the Engineer may select in each unit, one core will be taken for each 1000 square yards of pavement, or fraction thereof, in the unit. If the core so taken is not deficient more than 0.2 inch from the plan thickness, full payment will be made. If the core is deficient in thickness by more than 0.2 inch but not more than 1.0 inch from the plan thickness, two additional cores will be taken from the area represented and the average of the three cores determined. If the average measurement of these three cores is not deficient