by Uday R. Bhate, P.E.
Senior Principal, BHATE Geosciences
Concrete is the construction material most widely used worldwide. All building owners want high-quality, high-performance durable concrete with minimal cracking, curling, and no voids. Good knowledge of building code requirements for plain and reinforced concrete is critical to successful concrete placement and performance on new construction.
Poor construction usually leads to reduced durability which will manifest itself in later years. The working life of the structure may be reduced or extensive maintenance may be required as a result of the deterioration of materials, usually steel subject to corrosion attack or concrete subject to aggressive chemicals. Evidence of this type of damage may appear after 15 or 20 years and is strongly environment dependent. Corrosion may be detectable at an early stage and before serious damage occurring to the extent that the functionality of the structure is affected.
An adequate design does not always guarantee a satisfactory function. Poor design details can cause a localized concentration of high stresses in structural members even if the design is adequate to meet the requirements. Detailing is an important component of the structural design. Poor detailing may or may not directly lead to a structural failure but it may contribute to the deterioration of the concrete.
Some of the general design and detailing problems include:
- Abrupt changes in section
- Insufficient reinforcement at reentrant corners and openings
- Inadequate provision for deflection
- Inadequate provisions for drainage
- Inadequate expansion joints
- Material incompatibility
Importance of Field Quality Assurance: An issue that comes up on many projects is the quality testing of concrete by properly credentialed and experienced technicians. In addition to basic certifications on knowing how to conduct field tests, properly cast, protect, transport and cure test cylinders, the technician should have a broader knowledge of fundamental concrete technology and standards. A concrete quality technician should also be familiar with and able to perform in-place non-destructive tests in the field. An experienced and credentialed technician is an essential part of successful high-quality concrete construction.
It is shocking and concerning to know how many testing laboratories send out non-certified and inexperienced technicians to perform concrete testing. This can be remedied at the outset by requiring the testing agency to submit ACI and ICC certifications of the testing personnel assigned to the project. The concrete contractor should see the technician as a valuable resource for information and for timely field communication of concrete problems.
Concrete Construction Problems: Problems are often created by human error. Lack of supervision on any given day can lead to something small being overlooked or something major not being noticed until it’s too late.
- Low concrete strength: If too much water is added to the concrete.
- Concrete cover: Sometimes reinforcement steel may not be secured properly. It may move during placement of the concrete, or the forms may move, causing the steel to lean against the form with only a thin layer of concrete covering it.
- Poor construction techniques: Education in industry best practice is vital to successful projects. Often, the people being trained are not the ones completing the task. Sometimes something as simple as where to place the welded wire mesh (WWM) in a slab on ground can be overlooked.
- Honeycombs and voids in concrete are created when the mortar does not fill all of the spaces between the coarse aggregate particles. This is occurring more frequently because of the changes in structural design and the need to change the concrete mixture design to suit the requirements of the specifications.
- Specifications: Unclear specifications can lead to mistakes in the field.
- Environmental causes:
- Cold weather: freezing and thawing and the limiting effects of cold conditions;
- Hot weather: early drying shrinkage and the limiting effects of hot conditions;
- Chemical attack: material degradation as a result of chemical agents such as bases, salts, and moisture;
- Carbonation: the conversion of calcium ions in hardened cementitious materials to calcium carbonate by reaction with atmospheric carbon causing rusting of reinforcing bars.
Concrete Degradation: Depending on the materials used and construction quality, with time, stress and environmental exposure, degradation of concrete will occur. Poor quality materials or construction will accelerate the degradation process significantly.
Inspection and Repair Strategy: Developing a proper repair strategy to address the concrete problem requires an understanding of the cause of the problem. Understanding the cause allows for a repair that addresses both cause and effect. One should keep in mind that the repair systems are not easily classified into a cookbook type design or repair procedures.
The concrete repair process involves establishing cause/effect relationships, concrete evaluation, and analysis and repair strategy. To develop a proper concrete repair strategy:
- Identify the degraded or damaged condition and determining the causes
- Assess whether the problem needs to be repaired and consequences.
- Perform a condition survey to quantify the problems
- Repair analysis and methodology
- Properly accomplishing the repair may include:
- Surface repair or concrete removal and replacement
- Future monitoring
A thorough and logical evaluation of the current conditions of a concrete structure is the first step in any repair project. Guidelines set out in procedures and manuals, such as the “Guide for Making a Condition Survey of Concrete in Service” (ACI 201.1 R92) are an excellent source of information. The American Concrete Institute (ACI) guide provides the specialist with a checklist to facilitate a thorough survey as well as photographic illustrations of various types of distress manifestations.
The system is designed to record the history of a project from inception through construction and subsequent life of the structure. However, experience is essential and just following the guide does not eliminate the need for intelligent observations and the use of sound judgment. The individuals performing the survey should be experienced and competent in this field.
Regular inspection and monitoring are essential to detect problems with concrete structures. It is important to keep written records of the dimensions and extent of deterioration. Structural cracks should be monitored more frequently and repaired if they are a threat to the stability of the structure. Photographs provide invaluable records of changing conditions. All maintenance and inspection records should be kept updated.