**Acoustic Emission for Determining Early Age Concrete Damage as an Important Indicator of Concrete Quality**/**Condition before Loading**

#### **Wiesław Tr ˛ampczy ´nski \*, Barbara Goszczy ´nska and Magdalena Bacharz**

Faculty of Civil Engineering and Architecture, Kielce University of Technology, Al. Tysi ˛aclecia Pa ´nstwa Polskiego 7, 25-314 Kielce, Poland; bgoszczynska@tu.kielce.pl (B.G.); mbacharz@tu.kielce.pl (M.B.) **\***Correspondence:wtramp@tu.kielce.pl;Tel.:+48-41-34-24-545

Received: 24 June 2020; Accepted: 5 August 2020; Published: 10 August 2020

**Abstract:** Phenomena occurring during the curing of concrete can decrease its mechanical properties, specifically strength, and serviceability, even before it is placed. This is due to excessive stresses caused by temperature gradients, moisture changes, and chemical processes arising during the concreting and in hardened concrete. At stress concentration sites, microcracks form in the interfacial transition zones (ITZ) in the early phase and propagate deeper into the cement paste or to the surface of the element. Microcracks can contribute to the development of larger cracks, reduce the durability of structures, limit their serviceability, and, in rare cases, lead to their failure. It is thus important to search for a tool that allows objective assessment of damage initiation and development in concrete. Objectivity of the assessment lies in it being independent of the constituents and additives used in the concrete or of external influences. The acoustic emission-based method presented in this paper allows damage detection and identification in the early age concrete (before loading) for di fferent concrete compositions, curing conditions, temperature variations, and in reinforced concrete. As such, this method is an objective and e ffective tool for damage processes detection.

**Keywords:** early age concrete; acoustic emission method; damage processes detection before loading; strength of structures
