Repair of Concrete

Repair of Concrete Introduction

This guide first discusses Reclamation’s methodology for concrete repair. It then addresses the more common causes of damage to Reclamation concrete, including suggestions of the types of repair methods and materials most likely to be successful in repair of concrete damage resulting from those causes. Finally, the guide contains a detailed description of the uses, limitations, materials, and procedures of each of the standard repair methods/materials included in the “Standard Specifications for Repair of Concrete.”

Maintenance of Concrete.Modern concrete is a very durable construction material and, if properly proportioned and placed, will give very long service under normal conditions. Many Reclamation concrete structures, however, were constructed using early concrete technology, and they have already provided well over 50 years of service under harsh conditions. Such concrete must be inspected regularly to ensure that it is receiving the maintenance necessary to retain serviceability. Managers
and foremen of operation and maintenance crews must understand that, with respect to concrete, there is no such thing as economical deferred maintenance.

Experience has shown that there are certain portions of exposed concrete structures more vulnerable than others to deterioration from weathering in freezing climates. These are exposed surfaces of the top 2 feet of walls, piers, posts, handrails, and parapets; all of curbs, sills, ledges, copings, cornices, and corners; and surfaces in contact with spray or water at frequently changing levels during freezing weather. The durability of these surfaces can be considerably improved and serviceability greatly prolonged by preventive maintenance such as weatherproofing treatment with concrete sealing compounds.

Selecting the most satisfactory protective treatment depends to a considerable  extent upon correctly assessing the exposure environment. Concrete sealing compounds and coatings that provide good protection from weathering in an essentially dry environment may perform poorly in the presence of an abundance of water such as on some bridge curbs and railings, stilling basin walls, and piers. Freezing and thawing tests of concrete specimens protected by a variety of concrete sealing  compounds and coatings, including linseed oil, fluosilicates, epoxy and latex paints, chlorinated rubber, and water-proofing and penetrating sealers, have been performed in Reclamation laboratories. These tests indicate that proprietary epoxy formulations, siloxane and silane fomulations, and the high molecular weight methacrylate formulations.

General Requirements for Quality Repair of Concrete.—The term “concrete repair” refers to any replacing, restoring, or renewing of concrete or concrete surfaces after initial placement. The need for repairs can vary from such minor imperfections as she-bolt holes, snap-tie holes, or normal weathering to major damages resulting from water energy or structural failure. Although the procedures described may initially appear to be unnecessarily detailed, experience has repeatedly demonstrated that no step in a repair operation can be omitted or carelessly performed without detriment to the serviceability of the work. Inadequate workmanship, procedures, or materials will result in inferior repairs which will ultimately fail at significant cost.

• Workmanship.—It is the obligation of the construction contractor or operation and maintenance crew to repair imperfections or damage in concrete so that repairs will be serviceable and of a quality and durability comparable to the adjacent portions of the structure. Repair personnel are responsible for making repairs that are inconspicuous, durable, and well bonded to existing surfaces. Since most repair procedures involve predominantly manual operations, it is particularly important
  that both foremen and workmen be fully instructed concerning procedural details of repairing concrete and the reasons for the procedures. Workmen should also be apprized of the more critical aspects of repairing concrete.
• Procedures.—Serviceable concrete repairs can result only if correct methods are chosen and techniques are carefully performed. Wrong or ineffective repair or construction procedures, coupled with poor workmanship, lead to inferior repairs. Many proven procedures for making high quality repairs are detailed in this guide;
  however, not all procedures used in repair or maintenance are discussed. Therefore, it is incumbent upon the craftsmen doing the work to use procedures that have been successful or that have a proven high reliability factor.
• Materials.—Materials to be used in repair of concrete must be high quality, relatively fresh, and capable of meeting specifications requirements for the particular  application or intended use. Mill reports or testing laboratory reports should be required of the supplier or manufacturer as an indication of quality and suitability. Short of this requirement, certifications stating that the materials meet certain specifications should be required of the supplier.

This concrete damage was found to be a serious threat to the structural integrity of this spillway.

Reclamation’s Repair of Concrete System

1. Determine the cause(s) of damage
2. Evaluate the extent of damage
3. Evaluate the need to repair
4. Select the repair method
5. Prepare the old concrete for repair
6. Apply the repair method
7. Cure the repair properly

Standard Methods of Repair of concrete

Surface Grinding.—Surface grinding can be used to repair some bulges, offsets, and other irregularities that exceed the desired surface tolerances. Excessive surface grinding, however, may result in weakening of the
concrete surface, exposure of easily removed aggregate particles, or unsightly appearance.

Portland Cement Mortar.—Portland cement mortar may be used for repairing defects on surfaces not prominently exposed, where the defects are too wide for dry pack filling or where the defects are too shallow for concrete filling and no deeper than the far side of the reinforcement that is nearest the surface.
Repairs may be made either by use of shotcrete or by hand application methods. Replacement mortar can be used to make shallow, small size repairs to new or green concrete, provided that the repairs are performed within 24 hours of removing the concrete forms.

Dry Pack and Epoxy-Bonded Dry Pack.—Dry pack is a combination of portland cement and sand passing a No. 16 sieve mixed with just enough water to hydrate the cement. Dry pack should be used for filling holes having
a depth equal to, or greater than, the least surface dimension of the repair area; for cone bolt, she bolt, core holes, and grout-insert holes; for holes left by the removal of form ties; and for narrow slots cut for repair of cracks. Dry pack should not be used for relatively shallow depressions where lateral restraint cannot be obtained, for filling behind reinforcement, or for filling holes that extend completely through a concrete section.

Preplaced Aggregate Concrete.— Preplaced aggregate concrete is an excellent repair material that has not been used much in recent years. Preplaced aggregate concrete is made by injecting portland cement grout, with or without sand, into the voids of a formed, compacted mass of clean, graded, coarse aggregate. The preplaced aggregated is washed and screened to remove fines before placing into the forms. As the grout is injected or pumped into the forms, it displaces any included air or water and fills the voids around the aggregate, thus creating a dense concrete having a high aggregate content.

Shotcrete.—Shotcrete is defined as “mortar or concrete pneumatically projected at high speed onto a surface” (American Concrete Institute, 1990). There are two basic types of shotcrete—dry mix and wet mix. In dry mix shotcrete, the dry cement, sand, and coarse aggregate, if used, are premixed with only sufficient water to reduce dusting. This mixture is then forced through the delivery line to the nozzle by compressed air (figure 43). At the nozzle, sufficient water is added to the moving stream to meet the requirements of cement hydration. Figure 44 shows the nozzle and water ring of a dry mix shotcrete nozzle. For wet mix shotcrete, the cement, sand, and coarse aggregate are first conventionally mixed with water (figure 45), and the resulting concrete is then pumped to the nozzle where compressed air propels the wet mixture onto the desired surface (figure 46). The two types of shotcrete produce mixes with different water contents and different application characteristics as a result of the distinctly different mixing processes. Dry mix shotcrete suffers high dust generation and rebound losses varying from about 15 percent to up to 50 percent. Wet mix shotcrete must contain enough water to permit pumping through the delivery line. Wet mix shotcrete, as a result, may experience significantly more cracking problems due to the excess water and drying shrinkage. Advances in the development of the high range water reducing admixtures, pumping aids, and concrete pumping equipment since about 1960 have greatly reduced these problems, and wet mix shotcrete is now being used more frequently in repair construction.

Replacement Concrete.—Concrete repairs made by bonding new concrete to repair areas without use of an epoxy bonding agent or mortar grout applied on the  prepared surface should be made when the area exceeds 1 square foot and
has a depth greater than 6 inches and when the repair will be of appreciable continuous area. Replacement concrete repairs should also be used for: C Holes extending entirely through concrete sections C Holes in which no reinforcement is
encountered, or in which the depth extends 1 inch below or behind the backside of the reinforcing steel and which are greater in area than 1 square foot and deeper than 4 inches, except where epoxy- bonded concrete replacement is required or
permitted as an alternative to concrete replacement.Epoxy-Bonded Epoxy Mortar.— Epoxybonded epoxy mortar should be used where the depth of repair is less than 1-1/2 inches and the exposure conditions are such that relatively constant temperatures can be expected. Epoxy mortars have thermal coefficients of expansion that may be significantly different from  conventional concrete. If such mortars are used under conditions of wide and frequent temperature fluctuations, they will cause failure just below the bond surface in the base concrete. For this reason, current Reclamation practice precludes the use of  epoxy mortars under conditions of frequent or large temperature fluctuations.

Epoxy-Bonded Replacement Concrete. Epoxy-bonded concrete is used for repairs to concrete that are between 1.5 and 6 inches thick. Shallow replacement concrete repairs, less than 6 inches thick, are subject to poor curing conditions as a result of moisture loss to evaporation and to capillary absorption by the old base concrete. Such repairs seldom develop acceptable bond strength to the old concrete.

Resin Injection.—Resin injection is used to repair concrete that is cracked or delaminated and to seal cracks in concrete to water leakage. Two basic types of resin and injection techniques are used to repair Reclamation concrete.

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