Technology Coordination Technology (TCT)
We conducted a technical discussion in Part III on cracks developing on putty finished surface. The root cause of the same was noticed, and the remedial measure will be discussed serially in the forthcoming issues. Another problem we often face is the crack development on AAC Blocks; sometimes this is noticed after installation and before installation.
Mostly the blocks, which are cracked before installation are rejected or cut in suitable size and shape. However, in a few case studies, it was observed that the AAC blocks, which are cracked after installation, cause cracks on finished surface (plaster or putty). The following figures highlight the same.
The new building materials such as AAC Blocks have replaced red brick, M Sand has replaced natural sand which is necessary from environment point of view, and Fly ash brick helps to prevent environmental pollution. But, one effort was necessary to find something which is compatible with such materials and different putty. Also, we must know properties of each material component to get rid of any disadvantage issues in the course of period.
It is an autoclaved Aerated Concrete Block which is manufactured under a controlled condition composed of quartz sand, calcined gypsum, lime, cement, water and aluminium powder or protein enzyme.
The reasons behind the crack development are contraction and shrinkage. So, few questions appear:
- What are the sources of contraction?
- How to handle Shrinkage?
- Why do we see cracks through the bricks?
- Why aeration increases shrinkage?
- What are the sources of expansion?
AAC blocks are designed to be with low density for economic design of building. It reduces density, by-products being low strength, high water absorption and very high shrinkage. Further, these blocks are produced using aerated cementitious composite, which is autoclaved. Immediately after production, AAC Blocks are usually with high moisture content.
Effect Of Material Content And Process
Materials used in production of AAC blocks, autoclaving process affect shrinkage. For example, poorly graded aggregates will have high shrinkage. Thus, it is apt to dry out the AAC blocks sufficiently before putting them into use. Additionally, no water shall be supplied to AAC blocks during construction.
Now, it is common sense that effective water curing affects high cracking in AAC. In that case, block fixing agent is used, strength of which is higher than AAC block. As a result, cracks appear at other places. Normal cement sand weak mortar with self-curing nano composite based compound should be used for fixing blocks as air increases porosity. When pores in the block receive water, it expands and shrinks on drying.
Due to this marginal expansion, contraction in length and tensile stress, crack appears. Further, while storing at factory/site, blocks are stacked in piles and the blocks in the outer periphery dry out well but the blocks deep inside the pile do not dry enough.
The sources of expansion include curing and rain water is another source. Further, sun is an important source of drying. Wind is also a source, depending on relative humidity.
In addition, diagonal cracks/cracks through bricks are due to handling of the bricks by labour. AAC blocks are poor in compressive strength, and do not resist shocks when dumped from a height. They develop minor fissures internally. Crack propagation is always through the weakest zone. Brick binding mortar will be stronger than the cracked block, so crack goes through the brick. Therefore, we see diagonal cracks; otherwise, cracks in AAC blocks are always in straight lines. Steps may be observed, depending on weaker zones for crack propagation.
If the wall is exposed to wind movement/heavy sun, susceptibility to cracking is high. As the height increases due to heavy wind, cracking is promoted.
Cutting the bricks for electrical conduits if not done carefully, lead to appearance of cracks.
Shocks due to lack of sufficient self weight, make the walls move heavily even with smallest shocks. If nails are used to finished surface, there is a chance of appearing cracks.
Packing/filling materials are used to plug the holes of electrical fittings, etc., which should be treated as weak zones through which cracks appear.
AAC blocks should be stored under a shade away from rain. These blocks should be dried well before use.
All the cracks appeared on the AAC Block finished surface on internal and external wall only. I visited the project during the construction of the building. Only six to seven months after completion of that project, I was requested to visit the same building and I observed the above cracks developed on finished AAC Blocks.
There are many other case studies of the same problem. TCT can answer and solve these types of different problems, which will be serially answered in next issues.
A valuable technical comment from eminent engineers in this article is very important. We must read their technical comments.
Mr. Subhas Chandra Basu, Vice President – Engineering (Tata Housing Development Company Ltd.) - “AAC Blocks is a unique and excellent type of building material due to its super heat, fire and sound resistance. AAC blocks are lightweight and offer ultimate workability, flexibility and durability. Its main ingredients include sand, water, quicklime, cement and gypsum. The chemical reaction due to the aluminium paste provide AAC its distinct porous structure, lightness, and insulation properties, which is completely different when compared to other lightweight concrete materials. Further, AAC offers incredible opportunities to increase building quality and at the same time, reduce costs at the design stage. Excellent thermal insulation, light weight, faster construction, environment friendly are the key takeaways.
Although the advantages of AAC blocks far outweigh its disadvantages, it does have following notable disadvantages:
Cracking in walls: we need to take precautions to eliminate drying shrinkage cracks and stress concentration cracks.
- Not perfectly suitable for load bearing walls: the lightweight-ness becomes a disadvantage for load bearing structures and use of AAC blocks has to be done judiciously for load bearing structures.
- Choosing right mortar for laying of blocks, along with the possibility of using nano-polymer in the mortar.
- Although AAC wall do not need curing, conventional idea of water curing has to change.
- In case of cracks on the wall, it needs a special treatment for crack repairs.
We have engaged in special treatment wherever we came across such cracks and received desirable results.”
Mr. Avijit Dutta, Deputy General Manager (Construction), Shapoorji Pallonji And Co. Pvt. Ltd. - “Thank you for considering the article on construction with AAC blocks. My views are as follows:
Your analysis on this subject is very relevant and helpful for all construction engineers. Use of AAC blocks in building construction is a recent development due to various problems (viz. non-availability of good quality clay bricks, economic designing of buildings & environmental issues). But as the masons are not accustomed with such type of blockwork, hence, workmanship and their skill development is also very important. So, for sustainable construction, I propose to train these masons and supervisors with the ‘DOs & DON’Ts’ before starting construction with AAC blocks. This can be successfully carried out by sharing a ‘Standard Methodology’ by the consultants and engineers. I am thankful and must appreciate your efforts in ushering some lights on this subject.”
To be continued in Part V...
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