Hitech Concrete Solutions Chennai Pvt Ltd is an NABL Accredited Laboratory for Mechanical and NDT testing and a consulting company specialized in providing solutions to the various concreting issues through in-situ and laboratory testing. Managing Director, Dr. K. Balasubramanian has been a former Scientist & Deputy Director at Structural Engineering Research Centre (CSIR-SERC), Chennai. At present, he also heads an IIT Madras incubated company ‘Concrete Quality Concepts Pvt. Ltd.’ He speaks to Buildotech on the current concrete technology, testing methods available and repair & rehabilitation challenges of RC structures.
Any inherent defect introduced at the time of construction will manifest itself in the form of cracking/distress after a few years of service. Hence, greater care has to be shown right at the time of construction to achieve the desired service life of the structure. There are instances that despite the best efforts put in by the engineers, due to the aggressive environment, the structures get deteriorated over a period of time. Some of the major environmental factors which cause corrosion are the penetration of chlorides and carbon dioxide into the concrete. When these reach the rebar inside concrete, depassivation of the steel occurs, and this initiates the corrosion process. This is the reason the country is divided into five environmental zones and for each zone, a certain cover has been specified in IS 456:2000. If these covers are not strictly adhered to during the design as well as during the construction, due to the diffusion of the above agents, the structures are bound to face distress over a period of time. This will result in the need for their rehabilitation, causing discomfort to the occupants and sometimes resulting in productivity loss in the case of process buildings. To make it simple, if proper cover is maintained, correct water is added, concrete is well compacted and placed within the specified time and cured, we can avoid many quality issues and improve the service life of the structures.
How to address premature repair failures in concrete?
Premature failure of repairs takes place due to improper material specification as well as poor workmanship during the actual execution. Premature repair failures also take place due to improper repair methodology suggested and adopted, viz., jacketing of structural elements without shear connectors. The poor maintenance of the repaired structures by the user agency is another reason for the premature failure of the repairs.
By adopting the appropriate NDT testing procedure, a proper understanding of the in-situ quality of the concrete can be arrived at for critical projects and based on the results of the investigation, a suitable rehabilitation methodology can be implemented. This should be followed up with post repair evaluation on a limited basis using NDT to check the efficiency of the repair methodology as well as the execution carried out. This is more important for all structures, more so for high rise structures like chimneys.
However, the focus of the industry should be towards avoiding repairs to the extent possible. The only recourse towards avoiding repair/rehabilitation is adopting and ensuring strict quality control at the time of construction. Depending on the environment in which the structure is situated, stronger specifications to ensure the durability of the concrete should be included at the time of drafting the tender specifications. For example, most of the Metro projects in the country specify higher retention of workability of concrete as well as concrete meeting certain RCPT and Permeability values.
High performance self compacting concrete will make it possible to cast intricate structures, where the form is difficult to attain by conventional vibration. Further, because of self-compactability, the uniformity and integrity of high performance concrete, which is absolutely critical for good long term performance, will be maintained. As in any other case involving SCC, considerable savings in construction time can also be realized. The use of high performance SCC would also necessitate the establishment of very strict quality procedures on the jobsite, as special concretes tend to be less forgiving than conventional concrete. Special formwork and continuous monitoring of the process would be absolutely necessary.
Advantages & Disadvantages of Fly ash in concrete
Fly ash improves the long term durability of concrete structures, besides reducing the heat of hydration, particularly in the case of mass concrete, viz., rafts etc. But there is an apprehension in the midst of consultants and architects that fly ash may result in reduced strengths. However, concrete mix incorporating fly ash can be suitably designed to achieve the same 28 day compressive strength as that of the same grade of concrete with only OPC. Another aspect which has to be looked into, is the quality of fly ash being used in the concrete and we should ensure that it satisfies the codal recommendations laid down in IS 3812 (Part I). Even though fly ash per se does not possess any disadvantage as a mineral admixture, procuring and using fly ash not conforming to the Indian standards and not taking into account their influence in the strength development during the design of concrete mixes can prove to be counterproductive.
What are the tests available for assessing the strength of in-situ concrete?
The only test which can assess the in-situ strength of concrete is the core drilling test. All other tests, such as rebound hammer and ultrasonic pulse velocity, either describe the surface quality, or the overall uniformity of the concrete. Even the more sophisticated tests such as pull out or pull off test are only restricted to the cover zone. None of the non-destructive methods can provide an accurate assessment of the in-situ compressive strength. Core extraction is the best alternative.
Civil engineers have to clearly understand that non destructive testing like Rebound hammer are used to measure the near surface quality of the concrete while Ultrasonic pulse velocity test is used to check the integrity of concrete. Partial destructive test like core drilling test can be used to evaluate the equivalent cube compressive strength of the in-situ concrete. However, sometimes priorities get shifted to close the various non conformities arising out of quality issues in the site and engineers resort to improper methods/techniques to evaluate the in-situ strength.
How to achieve sustainability in concrete construction
Sustainability in concrete construction can be achieved in numerous ways. Use of alternative cementing materials such as fly ash, slag and silica fume is probably the best option. The use of such materials helps in achieving economic, environmental and social benefits, which are all necessary in sustainability. Since these are mostly by-products, the cost of concrete is reduced because cement consumption decreases. Further, the benefit to cost ratio over the life cycle of the concrete structure improves. The reduced cement consumption also leads to reduction in the level of carbon dioxide emissions associated with the production of cement clinker. Further, since waste or by-products are used, there is a distinct reduction in environmental degradation that is brought about by dumping materials such as fly ash. On the social front, the crack free infrastructure, which serves over a long time period, can be brought about by the use of such materials.
We offer solutions to the construction industry in the areas of Concrete Technology and Non Destructive Testing for quality related issues. The company provides the condition assessment of concrete structures in the areas of Bridges, Power Plant structures, Marine structures, Buildings and Factories, Chennai Metro Rail Project, etc. We are also involved in the testing of the constituent materials of concrete, viz., cement, fine and coarse aggregates and steel. We also carry out design of concrete mixes, taking into consideration the requirement of the site from the pumping point of view and check the retention of the workability so that the site is aware about the time at which the concrete should be placed from the time of batching. This is a major issue besting the construction industry as sometimes 3-4 hours is taken from the time of batching to place the concrete despite using SNF based admixtures, which may hold the retention of workability only up to 2 hours unless specifically designed for higher retention.
We also provide opportunity to students on a selective basis to use our lab facility for their Post Graduate Thesis to focus on Research. Construction Chemical Companies use the Lab Facility for the development of their products and trials.
Are our structural engineers geared up for the challenges of the profession?
The fresh engineering graduates require a lot of training in the various aspects of concrete production, placement and post concreting issues. The skill the engineers possess is not matched with their knowledge on the various aspects of concrete. For example, some of the rudimentary aspects like how the concrete gains strength, why cover blocks are provided, why mineral admixtures like fly ash are used, what is the purpose of using superplasticiers/water reducing admixtures are not well understood by the site engineers.
Despite the best efforts put in by the various construction companies in training their engineers through in-house as well as external experts, there is a gap in knowledge. Thus, major construction companies have collaborated with Hitech Concrete Solutions for the onsite training of engineers in the area of concrete technology. We conduct training programme for a day in the site on the various aspects of fundamentals of concrete technology as well as the issues involved in the production and post production.
Specific (but fundamentally simple) issues that keep cropping up on a regular basis
Plastic shrinkage cracks are one of the most common types of cracking that can be observed, particularly in slabs. This is due to temperature, humidity and wind velocity and occurs when the rate of evaporation of water is greater than the rate at which the bleed water comes to the surface. This can be offset by floating and sprinkling with water within 30 minutes from laying the concrete. But invariably this is not adhered to and we find many construction sites with plastic shrinkage cracks.
The other common issue is not laying the concrete within one and a half hours to two hours from the time of batching using RMC. This leads to addition of water at the site resulting in lower concrete strengths. Sometimes cubes are taken the moment the transit mixers reach the site, but concrete is placed after nearly three hours and leading to the actual in-situ strength issues being swept under the table.