Preconstruction Testing (520.09)
Documentation Requirements – 520 Pneumatically Placed Mortar
This item of work consists of repairing concrete structures by spraying the area to be repaired with dry premixed sand and cement that is blended with water in a mixing nozzle. The pneumatically placed mortar is then finished and cured. This type of procedure is often referred to generically as “shotcrete” by the industry.
Prior to placement of pneumatically placed concrete, the area to be repaired is to be properly prepared. All soft, loose, and disintegrated concrete plus an additional depth of 1/4 inch (6 mm) of sound concrete must be removed. Failure to remove soft, loose, and disintegrated concrete will adversely affect the bond of the mortar and shorten the life of the repair.
The edges or shoulders of the repair areas must be square or slightly undercut. If this is not accomplished, the mortar placed at the edges of the repaired area will be feathered. These feathered areas will not have adequate strength and will scale off.
After all concrete has been removed from the repair area, all dowels and expansion hooks placed, all steel areas restored, and not more than 24 hours prior to placement of mortar, the area to be repaired must be abrasive blast cleaned. The abrasive blast cleaning must be done to remove spalls, latence, and any other foreign material that might be detrimental to achieving a bond with the pneumatically placed mortar. The Contractor should select an abrasive blast method that will control or minimize the amount of fugitive escaping into the atmosphere. Suitable blast methods may include high-pressure water blasting with abrasives in the water, abrasive blasting with containment, or vacuum abrasive blasting.
Unless otherwise specified, the Contractor shall wet the area to be repaired with water for at least 2 hours prior to placing the mortar. The area must be kept wet until the mortar is placed. At the time of placement of the mortar, all free water must be removed.
All existing reinforcing steel bars must have a minimum cover of 1 inch (25 mm). If the existing location of the reinforcing bars would result in less than 1 inch (25 mm) of cover, they are to be driven back into recesses cut into the existing concrete to achieve that coverage. If this is not practical due to the large number of reinforcing bars, the coverage must be obtained by modifying the finished surface. Not that if the reinforcing steel is epoxy coated care should be taken to minimize the damage to the existing coating.
Where the depth of the patch exceeds 1 ½ inch (38 mm) in addition to any existing reinforcing steel, wire fabric is also required. Where the depth of the patch exceeds 4 inches (100 mm) a layer of fabric is to be placed for each 4 inches (100 mm) thickness of patch or fraction thereof.
Due to past experiences with pneumatically placed mortar that was improperly placed and prematurely failed, each operator must demonstrate their ability to construct a sound, durable repair prior to being allowing to place mortar on the structure. This is accomplished by gunning the mortar onto a test panel. The mortar on this test panel is then tested for strength and examined for hollow areas, sand pockets, and bond to the reinforcement. The cores taken for compressive strength samples can not contain any reinforcing steel. If the reinforcing steel spacing is too tight to retrieve a non-reinforced core, it may be necessary to construct a portion of the test panel without reinforcing. The easiest means to examine the test panel for mixing and consolidation issues is to pull a core at the intersection of the reinforcing steel or to simply saw the test panel in half. It is important to look at the cross section of the reinforcing steel, as the backside of the reinforcing steel is usually the most suspect area. The test panel should be water cured for 7 days and handled in the same manner as a cylinder. The sample should not be cored for at least 7 days and the cores should also be handled in the same manner as a cylinder.
After the mortar is placed, it must be cured. This curing shall consist of covering the patch with burlap or cotton mats and keeping them wet for 7 days. If it is not practicable to use mats, the surface of the patch must be kept wet by sprinkling the surface with water for 7 days. If it is determined that the above methods are impracticable due to isolated areas being inaccessible, they must be cured according to the requirements of 511.17, Method (b).
After the curing of the patched areas has been completed and before they are accepted, they must be sounded and every 200 sq. ft. (20 m²) cored. All unsound areas, or areas that exhibit cracking, must be removed and replaced. The cores must be inspected for hollow areas, sand pockets, voids around reinforcing steel, and lack of bond to the underlying concrete. The cores are also to be tested for compressive strength. Any defective patches as determined by the cores must also be removed and replaced at the Contractor’s expense.
1. Unsound concrete removed plus 1/4 inch (1 cm)
2. 1 inch (2.5 cm) minimum clearance to reinforcing
3. 1 layer wire fabric for each 4 inches (10 cm) of patch depth
a. Reinforcing fabric lapped 6 inches (15 cm) minimum
4. Surface cleaned by water or sand blast
5. Mortar composed of 3 parts sand to 1 part cement
6. Mortar placed as dry as possible
a. No one coat greater that 1 inch (2.5 cm) in thickness
7. Wet burlap cure- 7-day minimum, or membrane cured with Engineer approval
8. Inspect and test per 520.11
a. Sounding
b. Core taken for every 200 square feet of repair
9. Measure patch area and pay by the square foot (square meter)