Even with the best of materials and construction methods, architectural concrete is likely to have imperfections and defects. Some imperfections are acceptable as part of the character and nature of the concrete. Objectionable defects, however, will require some type of corrective action which must be planned and carried out as carefully as any other phase of construction.
PATCH MIX DESIGN AND COLOR SELECTION
Patching mixes for shallow surface voids are generally mortars batched by weight using a small, accurate set of scales. As a starting point, use the cement-sand ratios from the actual project design mix. Vary the patching mix color by blending white cement with cement originally used for the job concrete. Lime, plaster of paris, or accelerators such as calcium chloride should not be used in patching mixes. They can have an adverse effect on color and texture.
For mixing use rubberized mixing pans and buckets that are easy to clean. Dry mix each batch to thoroughly blend the materials. Next add clean water slowly while hand mixing. Acrylic or latex bonding agents are often added to the stiff-ball consistency batch to improve bonding properties and workability.
Smooth, as-cast concrete surfaces are the most difficult to patch. Use a hand-held hammer or a pneumatic chipping hammer to dress the hole and square the edges. Apply the patch mix and allow it to take an initial set. Finally, press a small section of form panel into the patch to duplicate the as-cast surface. For exposed aggregate surfaces, prewetted aggregate particles are hand placed to match the surrounding concrete, then pushed into the mortar mix with a trowel. After the patch has taken an initial set, the first texturing step is done with a stiff-bristle nylon brush. A wet paint brush is used to further blend the patch and surrounding concrete.