**1. Introduction**

The assessment and verification of constancy of performance (AVCP) of products in EU countries is a complex and multidimensional process. The rules for placing construction products on the market define the Construction Products Regulation (CPR) [1]. This document describes a harmonized system of assessing, performance expression, and conditions for CE marking, while controlling the constancy of the assessment results, which should remain constant.

The laying down of a product on the market is always associated with a risk, which is considered in different categories. Risk is an ambiguous concept, challenging to define. However, risk assessment is a fundamental technical framework for systematically analyzing the risk associated with an industrial activity [2]. Effective risk management requires essential knowledge about people's perceptions of risk in their industry [3]. The construction industry, in general, is risky, and the risks involved in building construction objects are highly complex [4]. The risk related to the quality of construction products is only one out of several dozen identified in the construction process [5]. In the scientific literature, the risk related to the construction product's non-compliant evaluation criteria is often not presented [6–10], very rarely from the manufacturer's perspective [11,12].

It is essential for all market participants that the products are safe. Knowledge from internal and external sources should be considered to create any product. In most cases,

**Citation:** Stancu, C.; Michalak, J. Interlaboratory Comparison as a Source of Information for the Product Evaluation Process. Case Study of Ceramic Tiles Adhesives. *Materials* **2022**, *15*, 253. https://doi.org/ 10.3390/ma15010253

Academic Editor: Krzysztof Schabowicz

Received: 3 December 2021 Accepted: 27 December 2021 Published: 29 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in-house knowledge is dominant, and, of course, producers know the level of safety/risk related to their product development. From the manufacturer's perspective, it is a problem when market supervision bodies will negatively assess their product. A particular situation is when this happens due to negative test results ordered by market surveillance authorities, and, of course, it may occur in many cases. One of them is when the actual values of the product's performance are close to the threshold value of the evaluation criterion (declared value), and when the evaluation methods are incorrectly selected. A proper, holistic understanding of risk allows manufacturers to make appropriate decisions to protect against adverse effects, including uncertainty analysis [6,7,13]. Such understanding is crucial to avoid contentious situations. Conducting a risk analysis by the product's manufacturer, including the measurement uncertainty (MU) related to the measured and declared performance of the product, is fundamental for the correct determination of the stability of the product's performance and safe use. It is also necessary for the continuous maintenance of the product as compliant with the assessment criteria. All measurements are erroneous; it is essential to know what size of measurement error accompanies the measurement [14]. However, since it is not possible to identify the sources and values of all systematic errors (and their directions) for any given measurement result, as well as the value of the random error occurring at a given time, the MU criterion is commonly used, inter alia, in clinical chemistry [15–17]. This parameter characterizes the dispersion of the quantity values attributed to a measurand (quantity intended to be measured) based on the information used [18]. It is essential to add that MU is a "non-negative" parameter [14]. Failure by the manufacturer to consider the consequences of uncertainty, including MU, during product development and the production process may result in a situation in which the product assessed as conforming may be non-compliant. It is also possible that a product rejected as non-compliant may be a compliant product [19,20]. Note that uncertainty in measurements is an operational concept that only relates to quantitative values assigned to the measurand based on the available information, the model of the measurement procedure, and the probabilistic assumptions used [21]. However, it is also essential to be aware that the measurement information provides only partial information about the actual product characteristic. When taken into account during the product assessment, it minimizes the adverse effects of incorrect evaluation. Still, it is not always sufficient to permanently deliver a product that complies with the assessment criteria [22]. Knowing the MU value for a given measurement method minimizes the risk of obtaining results that do not meet the evaluation criterion; for example, modifying the product recipe resulting in a change (usually increase) of the product parameter value. Such an operation, however, is generally associated with an increase in production costs. Of course, the MU related to the testing method of product properties is one of the many factors that the manufacturer needs to consider.

All laboratories aim to provide reliable information. Most of the measurements aim to assess compliance with a specification or regulation [23]. In this case, the measurement itself is not the goal but the basis for making objective decisions [24]. Conformity decisions are made for different products in many application areas without transparent and harmonized risk sharing, due to MU between the consumer and the producer/supplier of the product [24]. Performing measurements to assess compliance with specifications requires further development. Mainly, where shortcomings are observed, i.e., in the description of the measurement process, recognition that confidence in the measured result depends not only on its uncertainty but also on its integrity and further development of validated methods for the performance assessment [25,26]. Interlaboratory research plays an essential role in ensuring the quality of laboratory testing. The growing interest in this topic reflects the scientific literature [9,27–36]. In interlaboratory comparisons (ILCs) dominate clinical chemistry, biochemistry, medicine, and pharmacy researches. Construction products are much less the subject of the ILCs. In addition, the testing and assessment of construction products is still not precisely defined in terms of uncertainty, methods of estimation, and taking into account [8]. For construction products, a small number of tests and a limited

number of participants are ILC limitations [9,36]. An additional difficulty is that most of the measurement methods used for assessment with evaluation criteria are destructive in the case of construction products. Thus, it is not possible to repeat the test with the same sample. In the future, the situation will change due to the standardization regulations creating new requirements for selected construction products related to the possibility of ILC/PT performance [37].

ILC is considered primarily in the aspect of proficiency testing (PT), understood as part of a quality system that provides an external assessment that a laboratory's performance meets the requirements. PT is also understood to mean that an individual laboratory evaluates its performance for the intended purposes. A significant tenet of ILC is to prove the laboratory's ability to reproduce the results generated by the other laboratories. The ILC is also considered a learning exercise and is associated with terms such as quality control and certification. Those, as mentioned above, are the main goals of ILCs. In the scientific literature, ILCs are not perceived as a tool providing information to the manufacturer that can be used to verify the product recipe, so that the product meets the evaluation criteria during external evaluation. ILCs are also not considered a tool to verify the test methods specified in the standards for product assessment.

This study compares the results of the ILCs of ceramic tile adhesives (CTAs) organized by Ceprocim, carried out in two editions, i.e., in 2019–2020 and 2020–2021. The research organized by Ceprocim aimed to demonstrate that the systematic participation of laboratories in ILC improves the quality of their work. Based on the ILC results, the importance of the laboratories' participation in the PT will be analyzed. The authors will analyze the obtained results from the perspective of laboratories participating in the ILC. The ILC results will also be considered in the manufacturer's risk analysis, accompanying the product evaluation process. Based on the results, the authors will discuss the potential need to modify the requirements and methodology specified in EN 12004 [38]. There arises the need for discussion due to the application by market surveillance authorities of the simple acceptance rule, which does not consider the variability resulting from measurement uncertainty.

As mentioned before, few scientific papers discuss ILC for construction materials. Additionally, the articles published so far consider the ILC/PT in terms of assessing the competence of the work of laboratories. Conclusions resulting from PT are discussed between laboratories and institutions that granted laboratory accreditation. There are no studies in the scientific literature discussing how the results obtained in the ILC/PT can be the subject of a risk analysis conducted by the producer. Results of the ILC/PT also indicate possible modifications to the standards to improve them and make them more useful for both producers and users of products.
