Who cracked my concrete?

The other day I met Professor McCracken, an eminent concrete technologist and an avid concrete crack spotter! He was coming back after conducting a site inspection and had a very smug expression on his face. To my questioning looks, he said with immense pride that he had seen a variety of cracks in concrete that day. Not only did he find cracks in the freshly placed concrete on a bridge deck, he also saw a network of cracks on the pier of the old bridge adjacent to the one under construction. To add to his inventory, the beam in the lobby of the guest house had a long parallel crack on the underside and the concrete pavement that he walked on while boarding the flight had diagonal cracks on many panels! The next hour and a half on the flight were very educative for me and forms the basis of what follows in this article!

Serious crack

***image: serious cause for concern***

 

The title of this article is inspired by my interaction with Prof. McCracken and the popular book ‘Who moved my cheese’, much of the contents are however less metaphorical than the ‘cheese’ book. The idea is to put ‘cracking’ of concrete in perspective so that the concrete engineers respond appropriately to any crack they encounter. As Prof. McCracken put it succinctly the other day, anyone can repair a concrete crack but to understand the “how” and “why” of it and decide to repair or not is not everyone’s plate of crackjacks!

Concrete has become such a widely-used construction material that most lay-persons and even Engineers automatically assume it to be highly durable and something that is solid, strong and defect free! Cracks in concrete therefore are viewed as something very serious by many. On the other hand, one also comes across serious cracks which are ignored with the attitude ‘nothing is going to happen’ or ‘nothing can be done’! The reason behind these extreme responses perhaps is because concrete is under the care of Civil Engineers may be for a fraction of its life; a few months out of a few decades!!! For most of its life, concrete is either with end users who are mostly lay-persons as far as concrete is concerned or maintenance staff who are generally not concrete experts.

Let us look at cracks in concrete in greater details. Cracks in concrete can be wide or narrow, deep or shallow, dry or wet, patterned or crazy, dead or active and based on our knowledge ‘not to worry’ or ‘Oh my God’ types. Most literature on concrete technology would contain the following list:

  • Plastic shrinkage
  • Plastic settlement
  • Early thermal
  • Drying shrinkage
  • Structural
  • Settlement
  • Corrosion induced
  • Chemical attack induced (sulphate attack, Alkali silica reaction)

As one can see, cracks are classified primarily based on the causative factor. The trick is to be able to confidently diagnose the root cause and decide if the crack can be ignored or is serious enough to lose one’s sleep. The following process would help achieve these objectives more often than not.

First and foremost, one should aim to obtain reliable information about the age of concrete at the occurrence of the crack. Notice the emphasis on the word occurrence. Often, we get information about age at first sighting/observation. While this may be useful, age at first occurrence is a far more useful input.  The other relevant information is to find the orientation (with regard to rebar/members/joints/stress path), size (width/depth/length), pattern (single, straight, many in parallel, crazy etc.) and condition (dry/wet/stained/live etc).

Severe leakage due to poor compaction of slab

***image: severe leakage due to poor compaction of slab***

 

Armed with the above set of data, the best approach is to use the method of elimination. It is here that the age at first occurrence comes handy. Most of the cracks will fall under three age zones namely plastic concrete, early age concrete and late age concrete. The knowledge about age of concrete at crack occurrence can eliminate two out of these three groups and can narrow down the diagnosis to two or maximum three types of cracks!

The next step is to develop a hypothesis around the causes which are still in the contention. To do this, knowledge of concrete as a material as well as fundamentals of structural engineering are required. Remember, concrete will crack as and when the tensile stress/strain in concrete exceeds it tensile strain/stress capacity. One should then look for supporting signs and measurements to confirm or reject the hypothesis. It would help share the diagnosis with others and let them pick holes in your argument. Once the diagnosis is firmed up, the prognosis about the structure or the crack can be made with greater confidence.

Settlement crack 2

***image: settlement crack***

 

Extensive and expensive measures like use of rebound hammer, ultrasonic pulse velocity test, cover meter and chemical analysis of concrete sample etc. are often not necessary. Yet, if Prof. McCracken is to be believed, he had come across some experts undertaking such a time consuming, tedious and costly exercise to arrive at the same conclusion which he claimed to have done based on his skill of deductive logic! I find a parallel to this in the medical world. Good physicians make accurate diagnosis about the disease by careful questioning, smart observation and in-depth knowledge of the human body.

In an article of this nature, and the limited time that I got with Prof. McCracken, I am afraid I cannot elaborate further. The only advice that he gave me was to read and re-read manuals, codes and books on concrete and think & dream about concrete!

That night I had trouble sleeping and lay gazing at the ceiling. For the first time, I noticed a crack in the roof slab, starting from the hook of the fan in the ceiling all the way to the edge beam! I kept wondering as to when this crack had first occurred and if it would progressively lead to failure. I could not sleep till I realized that the crack was along the longer span of the slab, and hence could not have been a structural crack! Finally with this thought, I fell asleep and once again met Prof. McCracken in my dream...

 

Shashank Vaidya

Director

GEM Engserv Pvt. Ltd.