How preventative maintenance can prolong the concrete structure life, preserve integrity, and reduce repair cost.
Concrete construction offers inherent benefits, such as resilience, durability, and strength. However, those benefits are dependent on the quality control of placing concrete during construction and the maintenance during its service life. During construction, concrete’s quality depends on the care taken during mixing the ingredients, handling, and curing the fresh concrete.
What is Concrete?
Concrete consists of a mixture of cement, sand, aggregate, and water; additional chemical substances, called admixtures, can be added to accommodate a specific need for a design application. The primary functionality of admixture is to change the properties of concrete to make it more workable, inhibit corrosion, and achieve higher strength or other improvements.
After all the ingredients are mixed, the concrete will be poured (placed) into forms with properly positioned conventional or prestressed steel reinforcement. Conventional reinforced concrete is constructed with non-prestressed steel reinforcement (i.e., reinforcing bars or welded wire fabric). Prestressed concrete contains high-strength reinforcement (i.e., bars, wires, or strands), which can be either pre-tensioned or post-tensioned. The pre-tensioning process consists of pre-tensioning the reinforcement to a desired stress level before pouring the fresh concrete. The post-tensioning process consists of placing the concrete before the reinforcement is tensioned. The reinforcement is placed inside a conduit or coated with a lubricant to prevent it from bonding with concrete. After the concrete is poured and reaches optimum strength, the reinforcement is post-tensioned to the desired stresses.
During the process of concrete mixing, handling, and curing, the following precautions should be taken to achieve the desired concrete quality,
A- Mixing and Handling
- Accurate mix ingredients proportions.
- Adequate mixing.
- Appropriate delivery time to avoid segregation.
- Vibrating to reduce excess entrapped air to avoid poor consolidation.
- Sampling and testing
B- Reinforcement details.
- Maintain a proper clear cover of reinforcement.
- Maintain adequate reinforcement spacing.
- Adhere to design documents and specifications.
What Is Concrete Curing?
The process begins after the fresh concrete is placed into the forms and hydration begins at the appropriate temperature. The hydration process should be controlled to retain moisture within the concrete and avoid rapid temperature changes. As the hydration process continues, the concrete properties will change, and the concrete strength will increase over time (see Figure 1).
Other improvements during the hydration process are the property changes (i.e., increase resistance to abrasion resistance, improved water tightness, etc.). A defective curing process may impair the concrete quality significantly. For example, placing the concrete in an environment with high temperature and low humidity may cause rapid water loss or fast evaporation. The rapid water loss will result in quick concrete volume changes at an early age and may lead to cracks. Cracks are unfavorable results that can immediately be a significant issue for structures that store liquids and chemicals. Other problems that increase over the structure’s lifetime are discussed in the following concrete defects section.
The concrete quality depends on the concrete ingredients’ mix proportions and the means and methods of mixing and handling. Better quality can be achieved by implementing an effective plan for quality assurance and quality control (QA/QC), including workmanship, inspection, sampling, and laboratory testing.
Concrete Defects
During or after the concrete placement.
- Cracking: This may result during the curing process or at other times during the structure’s lifetime. During the structure’s lifetime, the gradient changes between low and high temperatures may result in cracking. For example, an expansion typically occurs during the summer, and a contraction typically occurs during the winter. These phenomena are the main causes of cracks in concrete. Other cracks may result from loading that exceeds the structural capacity, resulting in overstresses such as shear, flexure, or torsion.
- Honeycombs: This results from poor consolidation, such as a lack of vibration while placing the concrete. Proper vibration is needed to reduce excessive entrapped air. Excessive air leads to voids that increases permeability, exposes the reinforcement, and reduces the strength of the concrete.
- Spalling: This may result from excessive loading and poor bonding, resulting in a member separating a shell or a section. Other causes during the service life may include deterioration (weathering) and corrosion of reinforcing steel.
During the service life – deterioration.
- Efflorescence: Crystalline, white material that appears on the surface of the concrete (see Figure 2). It results from water penetrating the cracks over time, and a chemical reaction occurs with the cement paste leaching out to the surface.
- Freeze-thaw failure occurs when the water penetrates the voids, pores, or cracks and freezes at low temperatures. As water freezes within the concrete voids it increases in volume and induces forces inside the concrete, propagating the crack size, and may lead to spalling.
- Scaling occurs when a concrete surface breaks down, or erodes, exposing the aggregates.
- Delamination occurs where cracks and splits are generated parallel to the surface, caused by overloading and/or corrosion of steel reinforcement.
These defects are aesthetically unpleasant and in many cases jeopardize the structural integrity of the reinforced concrete member. For example, corroded reinforcement can reduce the cross-sectional area of the member, expose the steel reinforcement to corrosive element and break off the concrete cover (see Figure 3). The cross-sectional area reduction of reinforcement can be visually seen when it expands or becomes smaller. Not all defects can be visually seen; in many cases, specific testing with a thorough inspection program is required to detect those defects.
Other defects not discussed in this article that result from utilization include tears and chemical attacks such as chloride and sulfate. How do we avoid those defects and reduce their impact? Those can be avoided by implementing the program below.
Preventative Maintenance
All those defects are structural and safety concerns, and it should be the priority of the structure’s owner to implement an effective process to ensure those are addressed and avoided. The process should consist of routine inspection and maintenance to prolong the structure’s life. The routine inspection is an evaluation process of the entire structure to identify potential and existing defects. Defect findings should be presented with a condition rating system such as (good, fair, and poor) and maintenance priority (immediate or routine).
Preventive maintenance offers several critical benefits to the property owner, not limited to structures in poor condition; it also applies to structures in good condition (Figure 4).
The most crucial benefit is maintaining safety and structural integrity; this is achieved by preserving functionality, strength, and stability. An alternate advantage is that less effort is required to address an issue or defects early in the process and prevent it from worsening its condition. For example, cracks are an issue but can be prevented by sealing them prior to the water penetrating and corroding the reinforcement that propagates to a spall. Other minor efforts that will help avoid a significant structural issue are cleaning and ensuring the drainage system is functional. This will also prevent water ponding (water accumulation). Ponding clearly leads to water intrusion; it may also induce loads that exceed the structural capacity and lead to structural instability and failure.
Major repairs are not only a structural concern and higher cost but also interrupt functionality and inconvenience the occupants. Interruption occurs when a portion of the structure needs to be vacant during the repair process, such as temporary shoring. Temporary shoring is a structural support or frame designed to support the structure (partial/entire) and prevent it from collapsing until the repair is finished.
Therefore, preventative maintenance substantially benefits the property owner by maintaining safety and the structure’s expected life cycle. It will also be cost-effective as significantly high-cost repairs will be avoided, as well as offers better budget planning, maintains the structure’s aesthetic, and provides a reliable environment for the occupants.■
References
Michael S. Mamlouk and John P. Zaniewski, Materials for Civil and Construction Engineers, Second Edition, 2006.
FHWA – NHI-13055, Safety Inspection of In-Service Bridges, Publication No. FHWA-NHI-15-011, July 2015.