# Additive Manufacturing as an Alternative to Core Sampling in Concrete Strength Assessment
## 3 Results  
As a result of calculating and comparing the compressive strength of the concrete cube control specimens (Table 8) and the core samples extracted from the printed structures (Table 9), a summary table of transition coefficients was compiled. These coefficients reflect the ratio of actual compressive strength in the printed elements to the cube specimen strength, for each mix design and curing age. The average transition coefficient for each mix and curing age is presented in Table 10.  
Thus, based on the comparative data obtained for the compressive strength characteristics, it is recommended that the test results from standard cube specimens [35] be adjusted using the calculated transition coefficient (\(K_{transition}\)), to more accurately reflect the actual compressive strength of the structure produced via additive manufacturing technology. The expected compressive strength of the concrete at the design age based on cube tests should be determined using the following Equation (1):  
\[R_{actual}=R_{cube}\times K \tag{1}\]  
where \(R_{actual}\) is the actual compressive strength of the printed structure at the design age; \(R_{cube}\) is the measured compressive strength of the cube specimens; \(K\) is the transition coefficient. The obtained results are supported by Utility Model Patent No. 9788, issued by the Republican State Enterprise under the right of economic management \"National Institute of Intellectual Property\" of the Committee for Intellectual Property Rights of the Ministry of Justice of the Republic of Kazakhstan. To operationalise the transition concept, we formalise a four-step protocol for routine strength verification of additively manufactured (AM) concrete as follows:  
1. Specimen preparation. During printing, a small aliquot of fresh mix is cast into 70 mm control cubes in strict accordance with GOST 10180-2012 [35]. 2. Curing parity. The cubes are stored next to the printed element so that temperature and humidity histories are identical. 3. Standard testing. At the design age (7, 14 or 28 days in the present study) the cubes are crushed, and the compressive strength \(R_{cube}\) is computed by the reference method [40]. 4. Transition adjustment. The in situ (core-equivalent) strength of the printed element is obtained by applying the data-driven transition coefficient \(K_{transition}\): \[\hat{R}_{core}\left[\text{MPa}\right]=-4.9+0.97\,R_{cube}\] (2)  
Under the calibrated window (\(R_{cube}\) = 40-310 MPa; porosity 30-80%), Equation (2) limits the residual error to <2 MPa--i.e., <3% of the strength range--comfortably within the \(\geq\)10% global safety factors mandated by most design codes. The indistinguishable performance of ordinary, Ridge and Lasso regression models (Figure 12) further confirms that \(K_{transition}\) is insensitive to moderate variations in binder chemistry, water-cement ratio and print kinematics. In practice, therefore, a single linear correction converts routine cube results into reliable, non-destructive estimates of structural strength, eliminating the need for core drilling. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Concrete Age**} & **Mix 1 CP100** & **Mix 2 CP150** & **Mix 3 (C+1)P150** & **Mix 4 CU+D1P150** & \
\begin{tabular}{c} **Mix 5** \\ **(C+1)(1+D1P150** \\ \end{tabular} &
\begin{tabular}{c} **Average Transition** \\ **Coefficient** \\ \end{tabular} \\ \hline
7 days & 0.87 & 0.87 & 0.83 & 0.83 & 0.846 \\
14 days & 0.73 & 0.87 & 0.85 & 0.82 & 0.85 & 0.824 \\
28 days & 0.74 & 0.88 & 0.84 & 0.83 & 0.86 & 0.830 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Transition coefficients from cube specimen strength to actual compressive strength of additively manufactured concrete structures. Regression on the augmented dataset shows that core strength increases almost one-to-one with cube strength but is, on average, \(\approx\)55 MPa lower.