In order to effectively assess, plan, prepare and execute a successful repair, protection and strengthening (RPS) strategy for concrete and/or masonry, it is important to understand the root cause of the deterioration. Understanding the service conditions of the structure and matching the appropriate product(s) to the performance requirements of the project will ensure a lasting repair tailored to the specific needs of the owner.
The two main factors that influence concrete performance are:
Concrete failures can be the result of a combination of placement and/or service condition issues. Cracking, spalling, leaking, premature or excessive wear, scaling, settlement, deflection and disintegration are examples of conditions that lead to concrete failure. Understanding these failure modes and their root cause is necessary to develop a successful repair strategy.
Proper concrete placement is essential not only to satisfy the design and service requirements of the structure, but also to decrease the potential for premature deterioration and reduced lifespan. Identifying defects in the assessment stage of an RPS project is important in developing the proper repair plan to address the specific cause of the damaged concrete.
One of the most common defects in concrete is cracking. Cracking can be caused by inadequate substrate or subbase preparation, high water-to-cement ratio, improper curing methods, poor concrete consolidation, timing of control-joint installation and many other placement factors. Cracking can also be caused by design-related issues such as inadequate reinforcement or insufficient control-joint spacing.
Excess water increases the water-to-cement ratio of concrete and increases workability; however, this benefit is not without cost. The additional water eventually evaporates in the hydration (curing) process, leaving a porous network of capillary voids and lower overall strength.
Excess water can also lead to segregation, or the settling of large aggregates to the bottom of the matrix, resulting in reduced structural capacity and increased likelihood of cracking.
Proper curing involves maintaining an adequate moisture content and temperature in freshly-placed concrete to allow for full hydration (curing) of the cement while the concrete develops its intended design strength. The absence of moisture stops the hydration process which can lead to lower strengths. Rapid moisture loss of freshly-placed concrete can also lead to plastic shrinkage cracking, which are random shallow cracks on the surface of the concrete.
Poor consolidation can result from improper vibration and/or poor mix design. If the concrete does not fully consolidate around the reinforcing steel, design capacity may not be reached, and cracking can result.
Concrete expands and contracts with temperature changes. When these contraction forces exceed the tensile strength of the concrete, cracking can occur. To influence where this cracking will occur, control joints are installed, but the design and timing of control-joint installation are vital to their effectiveness.
Reinforcing steel (rebar) is used to increase the tensile strength of concrete. When tensile forces acting upon a concrete element exceed its tensile strength, a crack may occur. Reinforcing steel controls the width of the crack and can prevent complete failure of the element.
Cause: Entrapment of air bubbles on the formed concrete surface.
Effect: Uneven concrete surface, increased porosity due to increased surface area and poor appearance.
Cause: Improper consolidation between large aggregates and cement mortar during the placement process creates large voids in concrete.
Effect: Reduction in strength and increased potential for water intrusion, corrosion of reinforcing steel and reduced abrasion resistance.
Cause: Improper consolidation of concrete due to lack of vibration or improper mix design.
Effect: Reduction in strength andincreased potential for water intrusion, corrosion of reinforcing steel and reduced abrasion resistance.
Cause: Delays in concrete placement where the initial lift of concrete hardens before subsequent lifts are placed, resulting in a visible lineation and substandard bond between layers.
Effect: Water intrusion through the cold joint and corrosion of reinforcing steel.
Common service condition issues with concrete
Causes: Chloride intrusion, oxygen-water exposure, corrosive chemical environment surrounding reinforcing steel, drop in alkalinity, improper placement or concrete coverage of steel reinforcement (rebar).
Effects: Corrosion and subsequent expansion of reinforcing steel leads to cracking and spalling, reducing the structural capacity of the concrete.
Using concrete with a low water-to-cement ratio and proper rebar placement with adequate concrete cover are common measures used to prevent metal corrosion.
Causes: Water-saturated concrete, combined with temperature cycling above and below 32°F (0°C).
Effects: Concrete absorbs water into its pores. When the water in the pores freezes, it expands, causing scaling and delaminations.
High-strength concrete mix designs that incorporate air-entraining admixtures yield the best defense against freeze-thaw damage. Specialty coatings and water repellents are also commonly used to minimize water absorption and prevent freeze-thaw damage.
Causes: Exposure to acid rain, de-icing salts, in-service chemicals and naturally occurring sulfates in soils and ground water dissolve the cement matrix which results in aggregate loss.
Effects: Loss of concrete cover and subsequent corrosion of reinforcing steel. Assessing potential exposure before construction or repair can prevent premature deterioration.
Specific cement types, water-repellent sealers or chemically-resistant barrier coatings are all common preventative measures to protect concrete against chemical attack.
Cause: A chemical reaction between the alkali in cement and high-silica content aggregate forms a gel around the aggregate, preventing proper bond between the aggregate and cement.
Effects: The gel expands in the presence of water, creating tension cracking around surface aggregate and delaminations. Surface cracking promotes water intrusion which can lead to metal corrosion and spalling.
Careful selection of aggregates and consideration of silica content can help prevent alkali-aggregate reactions.
Causes: Airborne or waterborne debris moving across the concrete surface.
Effects: Progressive section loss can lead to inadequate concrete cover and corrosion of the reinforcing steel, reducing structural capacity.
High-strength, dense concrete and specialty abrasion-resistant coatings are the most effective means to resist erosion and abrasion.