Rehabilitation of reinforced concrete structures #5 Why do concrete cracks?

They are the professionals!

In reinforced concrete structures, cracking is a common phenomenon. As mentioned earlier, there are structural and superficial cracks. The one we should worry about is the structural crack. Hence, if encounter such crack, it requires our concern which translate into the requirement of an inspection and investigation.

Why do concrete cracks? There are four main reasons:

1. Excess water in the mix
-  The associated problem related to this is wet shrinkage.As concrete hardens and dries, it shrinks.

2. Rapid Drying of the concrete
- This occur during the curing process. Curing is an important process; serving two pusposes: 1. It retains moisture in the slab so that the concrete continues to gain strength and 2. It delays drying shrinkage until the concrete is strong enough to resist shrinkage cracking. Improper curing will result is crack to appear.

3. Improper strength concrete poured on the job
- Concrete can be engineered to a specific strength depending on the purpose it was intended for. Make sure that the concrete you require is the same as what is ordered (from the factory) or designed (in-situ casting). Excessive load exerted on the structures will eventually leads to cracking.

4. Lack of control joints.
-  Control joints are needed when you are expecting expansion/shrinkage of concrete due to temperature changes and drying shrinkage. Spaced the joint correctly.

Dr. Ibrisam Akbar

Rehabilitation of reinforced concrete structures #4 Visual Inspection

What type of problem that you can see in your building?

Rehabilitation of reinforced concrete structures #3 Evaluation

 To the untrained eyes, an architectural drawing will look the same with the structural drawing. But there are huge differences between them. Do you know how to differentiate?

The next question when you suspect a deterioration of a reinforced concrete structures is how and what to evaluate? First, we must evaluate whether the damage is structural or superficial. The latter is not serious and a simple rectification can overcome the problem. It only affect the "cosmetic" of the structures. A crack on the wall may be due to the crack of the paint and does not affect the structural behaviour of the wall.

However, if the crack occur at the wall itself, then you might have a problem. But sometimes, it is quite hard for the untrained eyes to differentiate between the structural and superficial cracks. If you don't know what you are looking at, the cheapest way is to invite a friend (who is also a structural engineer) to you house for a lunch (or dinner) and ask for his opinions.

If you see something fishy with the crack, observe and record it behaviour such as the length, the direction it starts and propagates, the depth of the crack etc if you cannot find an engineer to do the inspection. This may help him to evaluate further.

Basically, there are three ways to evaluate the requirements for repairs. The first one,  is the visual inspections to see any abnormalities which then requires further evaluation and testing. The second is when a confirmed visual defects such as cracks, concrete spalling or exposed beam appearing; then a full investigation and evaluation are required. Finally a periodic testing which incorporates non-destructive testing (NDT) using specialised electronic instruments which is the safest way to check the strength of the structures.

Another important aspect of evaluation is to keep the records of construction particularly the structural drawing. It is a part of the construction documents that are typically filed before permits for construction are issued. This is important so that a thorough investigation can be performed properly. Some of these documents include:

• Design documents
• Structural drawings
• Site inspections records
• Laboratory test reports
• Maintenance reports

Next, schedule a site visit (with an engineer). Make sure you have the relevant documents ready. The engineer will visually inspect the damage. He will probably have a check list to record the inspection which includes cracks, discolouration or joint deterioration. A sketch may also be produced.

Dr. Ibrisam Akbar

Rehabilitation of reinforced concrete structures #2 Reinforced concrete

Mexico City Earthquake, September 19, 1985. Nuevo Leon fifteen-story reinforced concrete structure. Part of the building was only slightly damaged, while another part of it collapsed. Can this building be rehabilitated?

Before we can start, first we need to know what is a reinforced concrete and how it behave under common loads. This will formed the understanding on why it deteriorates and finally lead us how it can be rehabilitated.

Reinforced concrete is a composite material. Composite materials mean that it is derived from a few materials to form a new engineered material. The few materials have significantly different physical and chemical properties which remains separate at the microscopic or the microscopic level. All these materials in concrete formed a concrete. These materials must be mixed together at a certain composition in order to produce a concrete that is fit for the intended purpose. A variations of mixture may result in difference of strengths. More importantly, an error in the mixtures may lead to structural failures.

The common materials for a reinforced concrete consists of cement, reinforcement bar (or steel - the same thing), aggregates and water. Separately, all these materials work and behave differently. They may not work as intended if one of it is removed even if the proportions were not mixed properly.

Reinforced concrete was derived from a concrete. The difference between these two was the introduction of the steel element (reinforcement bar). When steel is introduced, we called it reinforced. Concrete is strong when being compressed but weak if tensioned while steel behave the exact opposite way. This is the beauty. Hence, reinforced concrete not only resist compression but also tension, which are the two inherent stresses acting on a structures.

Funnily, reinforced concrete was not invented by an engineer but rather a French gardener named Joseph Monier in 1849. Of course, further work after that was carried out by engineers. As I say before, let the professional do the work. You may invent something, but that's it (no bad intention to the original inventor).

Reinforced concrete structures are everywhere. Look around you.

Dr. Ibrisam Akbar

Rehabilitation of reinforced concrete structures #1 Introduction

 MRR2 - rehabilitation work

In this sharing of knowledge series, I will try to explain how reinforced concrete structures can be rehabilitated. I will try to explain in layman terms without too much incorporating engineering jargon.

The reason is simply for my own benefits first which is that this series will be my personal notes for my reference. Hopefully, the readers of this blog will enhance their knowledge as well.

Why concrete structures need to be rehabilitated? Logic says that everything will ends. Same as concrete. However, concrete structures were designed for a period of time. Maybe 100 years. While reaching to that age, deterioration may occur, just like human getting old. That deterioration, depending on what it is, can be slowed down, stopped or in the worse case, removed and changed with a new one.

Of course, removing and building a new one will cost more, just like erecting a new building. So the wise way is to rehabilitate. Maintenance and retrofit are the simile to rehabilitation (of structures) - you may find those words be used as well.

Most of the structural defects are visible. With this, eyes are your greatest tools. Can you see a crack? Or a group of cracks? Do you see discoloration on the concrete surface? Can you see the reinforcement bar exposed from a concrete beam? These are some of the things that you can observe.

Since the real rehabilitation of concrete structures involve in engineering inspection and analysis by professionals, the actual work to determine the level of deterioration and what types of rehabilitation should be conducted. However, some general knowledge on it should also be understood by the clients (you, readers) so that you know what you are facing and what are the expectations should you encounter such problems. Hence, do not try to do the rehabilitation works yourself because what you thought was the problem may not be the problem at all. Further, it may be dangerous as well. Just leave it to the professionals.

Honestly, I am not the expert. I humbly call for comments and discussions. If there are discrepancies and errors in my writing, please correct me.

Finally, I hope that this series will be of some beneficial knowledge to all of us especially the readers (non-engineers) of this blog. My English is not that good, so my grammar may be off sometimes but I hope this will not be a hindrance for you to extract the knowledge. I pray that may Allah s.w.t. see this as part of my good interaction with the society. For He is The All Seeing.

Dr. Ibrisam Akbar