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1 of weakness. Internal vibrators used shall be the largest size and the most powerful that 2 can be properly used in the work, as described in Table 5.1.4 of AC1309. They shall be 3 operated by competent workmen. Use of vibrators to transport concrete within forms 4 shall not be allowed. The vibrator shall be inserted vertically at uniform spacing over 5 the entire area of the placement. The distance between insertions shall generally be 6 about 1-1/2 times the radius of action, or such that the area visibly affected by the 7 vibrator overlaps the adjacent just-vibrated area by a few inches. In slabs, the vibrator a shall be sloped toward the horizontal as necessary to operate in a fully embedded 9 position. 10 11 3. The vibrator shall penesrate rapidly to the bottom of the layer, and at least 6 in. into the 12 preceding layer if there is such. At each insertion, the vibrator shall be held stationary 13 for a time sufficient to consolidate the concrete but not cause segregation, generally ' 14 from 5 to 15 seconds. The vibrator shall then be withdrawn slowly, at the rate of is approximately 3 inches per second. 16 17 4. A spare vibrator in good working condition shall be kept on the job site during all 1e concrete placing operations. Where the concrete is to have an as-cast finish, a full 19 surface of mortar shall be brought against the form by the vibration process, 20 supplemented if necessary by spading to work the coarse aggregate back from the 21 formed surface. 22 23 5. The use of external vibrators for consolidating concrete will be permitted when the ' 24 concrete is inaccessible for adequate consolidation provided the forms are constructed 25 sufficiently rigid to resist displacement or damage from external vibration. 26 27 D. Cold Weather Concrete Work: Concrete work may be carried on during cold weather, but 26 only with the express permission of the Engineer after approval of a plan of operation. 29 Precautions shall be taken to see that the concrete is properly protected after pouring and 30 during the cure period. In general, the requirements of the State Specifications in this regard 31 will apply. 32 33 E. Hot Weather Concrete Work: During hot weather, when the temperature of the concrete 34 would exceed 900F., the Contractor shall employ effective means, such as precooling of 35 aggregates and mixing the water with ice,or placing at night, as necessary to maintain the 36 temperature of the concrete, as it is placed, below 90°F. Proper attention shall also be given 37 to handling, placing, protection, and wiring to prevent excessive concrete temperatures or 39 water evaporation which will impair the required strength or serviceability of the member or 39 structure. 40 41 3.05 FORMWORK 42 43 A. Arrange formwork construction to allow for proper sequencing and removal without damage. 44 Use orderly and symmetrical panel arrangement with minimum number of joints. Before 1 45 proceeding, secure approval of formwork and procedures. 46 47 B. Forms: Lumber, prefabricated wood panels, metal, or plastic-lined panels shall be sound and 46 free from any defects that will mar or detract from the surface of the finished concrete. 49 Construct forms sufficiently tight to prevent loss of mortar. Design forms to withstand vibrator 50 action. Treat forms with a nonstaining material to eliminate absorption of water and to act 51 as a form release agent. 52 CONCRETE 042894 03100-9