Root cause analysis is an important component of the regular maintenance plan for reinforced concrete structures. When it comes to addressing problems in these structures, cosmetic repair should be avoided; instead, a proper root cause analysis must be carried out to identify and understand the actual source of the problem.
Only after the root cause has been identified, should one begin material selection, which is an important step in asset maintenance, restoration and refurbishment projects.
Typically, defects in reinforced concrete structures can be due to multiple factors. For instance, stomach infections, which are common during monsoons in Asia, can be traced to water contamination caused by the mixing of sewage water running in 100-year-old corroded sewage pipes with the municipal water in parallel running pipes. Treating the gastro patients with medicine or changing water filters would only provide a temporary solution when the root cause of the problem – the corroded sewage pipes – is not addressed.
Corrosion and cracking
When steel corrodes, it generates iron oxides and hydroxides, resulting in the volume increasing by 5 to 8 times and causing expansive forces to accumulate within the concrete around the reinforcement, which leads to cracking and even concrete spalling. These cracks provide an entryway to oxygen, moisture, chlorides and other corrosive agents that create conditions suitable for accelerating the electrochemical corrosion process.
Pre-tensioned concrete bridge girders may exhibit unexpected end cracking upon pre-stress release. These cracks may propagate into the bottom flange of the girder where strands are located and become wider over time with increased traffic loads. Severe corrosion is triggered when leakage from the bridge expansion joint penetrates the bottom flange cracks. This situation can be resolved by arresting the expansion joint leakage prior to repairing the crack and concrete.
New concrete decks can develop transverse cracking, typically appearing over the length of span above the transverse reinforcement. These cracks can accelerate corrosion rates, reduce the service life of the bridge deck and increase maintenance costs. Cracking occurs when a mass of concrete that shrinks as it ages is restrained. In this case, for instance, restraint of the concrete deck by an integral support girder against its volume change initiates cracking. Concrete materials and mix, ambient temperature changes, humidity, bridge design characteristics and construction practices can all contribute to volume change and/or to degree of restraint of concrete mass. However, only a root cause analysis will reveal the real reason contributing to transverse cracking.
Chlorides are often blamed for corrosion in bridge structures, which can ultimately weaken the structure. While chlorides may have triggered the corrosion, the root cause of the bridge defect could be due to several other contributing factors such as cracks in bridge girder web and flange, poor bridge drainage, failed bridge deck waterproofing membrane, inappropriate bridge joints, and/or void in the pre-stressed or post-tensioned cable ducts due to excessive grout bleed. At the macro level, such defects can even be traced back to design, choice of materials, the environment, and construction practices.
Another problem observed on concrete surfaces is efflorescence, usually seen as a patch of white chalky powder formed due to dampness in institutional buildings, hotels and residential apartment buildings. Dampness and the consequent efflorescence can occur from several factors including the absence of or damage to the damp-proof course, which allows moisture to enter from the ground below. Moisture can also seep through external walls from unsealed landscape planters outside hotel rooms, resulting in white patches of dampness along the perimeter of the internal wall. In such instances, repairing the damp-proof course is a more permanent solution than merely treating the damp patch from inside.
Balcony and roof leaks
Temporary fixes are only cosmetic and will last for a few months before the problem recurs. For instance, repairing the spalled concrete of a balcony with a quick-fix patch method using the best quality repair mortar will not solve the problem unless the root cause has been identified and addressed. Such problems can be caused by leakage due to failure of the waterproofing membrane, a broken drainpipe, leaking concealed pipe joints or a combination of these factors. Corrosion of reinforcement is only a consequence and not the root cause of spalling on the balcony.
Roof leakages are a major problem in buildings, with the resulting seepage causing discomfort to the occupants. These leakages are mostly blamed on poor workmanship or the use of inferior materials during roof waterproofing. Repairing the waterproofing is only a temporary solution because the leaks will appear again after some time.
A study conducted by Lo, Leung and Cui (2005) on roof construction defects highlighted cracks in the roof parapet wall as the root cause of failure of roof waterproofing membrane systems. The study also concluded that the design and choice of material for a roof parapet wall was critical to avoid waterproofing membrane failures on the roof slab. Roof leaks could also occur from slipshod termination of the waterproofing membrane at up-stands and drains, improper selection of the waterproofing system, and poor roof joint detailing.
Another problem observed on concrete structures is that of tiles falling off building facades, which can seriously injure pedestrians and damage property. Research by Ho, Lo and Yiu (2005) has identified several factors that could lead to external tile failures such as thermal and moisture effects inducing movement of tiles, inferior quality adhesive, poor workmanship, improper joints, weathering, vibration and substrate properties. Only a root cause analysis will identify the real cause of failure in the de-bonding and falling of tiles.
Establishing root cause for a permanent fix
A successful repair and refurbishment project is one where the real cause of a concrete problem is identified and fixed instead of simply rectifying the symptoms. This article aims to create awareness among civil contractors and engineers about the importance of establishing the actual root cause of concrete defects through analysis and then fixing the problem for a complete, permanent and durable solution.
Based on the article by Hamid Khan, Brand Manager - Concrete Durability, Parchem Construction Supplies.