## 1 Introduction  
Given the present development of construction chemistry, the thesis that nearly every reinforced concrete (RC) structure can be repaired even if its technical condition is very bad can be proposed. By technical condition, the authors understand the actual condition of the RC element, taking into account its original manufacturing defects, mechanical damage, and chemical pollution--in relation to a new, properly made element. A repair is defined herein as restoration of original features of the structure, especially its load-bearing capacity and rigidity. The proposed thesis is true to a certain extent, but it requires answering the following questions.  
* Is the repair cost-effective from an economic point of view?
* Does the scope of the necessary repair works actually cover the replacement of substantial parts of the structure (which would be reconstruction or modernization rather than a repair)?
* What will the real structure durability be after the repair and is it satisfactory concerning the anticipated further service life of the facility?  
Certainly, the above questions do not cover all the doubts. This paper discusses the real durability of repaired RC structures from which a relatively long period of further service life is required.  
Examples presented in the paper include RC support structures of power boilers and trestle bridges under industrial systems. In those cases, industrial devices are fairly often replaced entirely--this raises a question concerning the possibility of performing an overhaul of the RC support structure and using it further. This is caused both by the willingness to save money and the time pressure related to the performed works. However, the economic aspect is usually less important because repairing a severely damaged structure is quite costly as well--which results from the cost analysis of repairs of real objects that the authors performed during their many years of practice in evaluating and planning various overhauls of RC structures. The willingness to save time is more significant--demolition and reconstruction of an RC structure last much longer than its repair. Therefore, an investor relatively often forces a repair of RC support structures even if their technical condition is very bad.  
Both object owners, as well as designers dealing with such repairs, often think that appropriately applied repair materials allow restoring the original features of a damaged structure. In principle, such a stance is justified if one takes into account a period lasting a few years. However, the service life of new power boilers, process pipelines, and other industrial devices is defined as 20-30 years and sometimes even more. Thus, the use of support structures should be planned for the same period. This creates a problem of assessing the suitability of a damaged structure for repairs guaranteeing reliable operation throughout an anticipated long period. From a practical point of view, it is important that the assessment method should be relatively quick and cheap and does not require specialized equipment or examination techniques.  
This study investigated the influence of basic concrete chemical tests on RC structure suitability for an overhaul through literature review and case-studies verification. The basic threats to RC structure durability, with particular reference to chemical factors, are presented and discussed. This stems from the fact that any mechanical damage can be repaired because its causes are usually temporary--as an example, damage to the hall's supporting column as a result of being hit by a forklift [1]. On the other hand, chemical agents usually cause permanent changes that hinder durable repairs or reinforcements and sometimes even make those impossible--as shown in this paper. The main thesis of this paper that the factor determining the possibility of effective and durable repairs of a corroded RC structure is its chemical condition is discussed and confirmed by actual cases of damaged industrial structures.