(6) the patch should he resistant to weathering and be durable 

 in the environment in which it is exposed. 



b. Types of Repairs . 



(1) Concrete Replacement . The concrete replacement method 

 consists of replacing defective concrete with machine-mixed concrete of 

 suitable proportions and consistency, so that it will hecome integral with 

 the base concrete. Concrete replacement is the desired method if there is 

 honeycomb in new construction or deterioration of old concrete which goes 

 entirely through the wall or beyond the reinforcement, or if the quantity 

 is large. For new work, the repairs should be made immediately after 

 stripping the forms (Tuthill, 1960). Considerable concrete removal is always 

 required for this type of repair. Excavation of affected areas should con- 

 tinue until there is no question that sound concrete has been reached. Addi- 

 tional chipping may be necessary to accommodate the repair method and shape 

 the cavity properly. Although opinions differ on the value of wetting the 

 cavity before placing plastic mortar, most authorities believe it is advisable 

 to keep the faces of the cavity wet for several hours before placing opera- 

 tions are begun. No standing water should be present, however, at the time of 

 placement. Concrete for the repair should generally be similar to the old 

 concrete in maximum size of aggregate and water-cement ratio. 



(2) Dry Pack . The dry-pack method consists of ramming a very 

 stiff mix into place in thin layers. It is suitable for filling form tie- 

 rod holes and narrow slots, and for repairing any cavity which has a 

 relatively high ratio of depth to area. Practically no shrinkage will 

 occur with this mix, and it develops a strength equaling or exceeding that 

 of the parent concrete. 



(3) Preplaced Aggregate Concrete . Preplaced aggregate concrete 

 may be used advantageously for certain types of repairs. It bonds well to 

 concrete and has low drying shrinkage. It is also well adapted to under- 

 water repairs . 



(4) Shotcrete . Shotcrete or gunite has excellent bond with new or 

 old concrete and is frequently the most satisfactory and economical method 

 of making shallow repairs. It is particularly adapted to vertical or 

 overhead surfaces where it is capable of supporting itself (without a form) 

 without sagging or sloughing. Shotcrete repairs generally perform satis- 

 factorily where recommended procedures are followed. 



(5) Repair of Scaled Areas and Spalls in Slabs . Scaling of 

 concrete pavement surfaces is not unusual where they are subject to deicing 

 salts, particularly if the concrete is inadequately air-entrained. Such 

 areas may be satisfactorily repaired by a thin concrete overlay provided 

 the surface of the old concrete is sound, durable, and clean. A minimum 

 overlay thickness of about 38 millimeters is needed for good performance 

 The temperature of the underlying slab should be as close as possible to 

 that of the new concrete. 



Spalls may occur adjacent to pavement joints or cracks. Spalls usually 

 are several inches in depth, and even deeper excavation may be required to 



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