# Additive Manufacturing as an Alternative to Core Sampling in Concrete Strength Assessment
## 4 Discussion
Their research assessed compressive strength using cores sampled from the top, middle, and bottom layers of horizontally cast concrete elements with both recycled and natural aggregates. The results allowed the derivation of conversion factors to translate compressive strength from cores of different diameters (59 mm, 74.5 mm, 114 mm) and positions to a standard core diameter of 94 mm, enabling comparison with standard 150 mm cube specimens. Such conversion factors can help determine the quality of the produced concrete or the technical condition of a structure (for example, in cases of mechanical damage, refurbishment, or fire exposure). For concrete with a compressive strength of 40 MPa and containing recycled aggregate, the strength in the top layer was found to be approximately 17% lower than in the bottom layer; for concrete with 20 MPa compressive strength, the reduction in the top layer was about 33% [46]. Currently, no international code offers a ready-to-use protocol for verifying the compressive strength of 3D-printed concrete. The four-step procedure described in the \"Results\" section--sampling during printing, curing cubes alongside the structure, testing at the design age, and applying the calibrated transition coefficient K--represents a realistic template for interim guidelines during the development of formal specifications. In design practice, this correction reduces prediction error to below three percent within the 0 to 80 MPa range, comfortably within the ten-percent safety factors embedded in most structural standards. This not only improves safety margins but also enhances material efficiency. This work demonstrates that the compressive strength measured on 70 mm moulded cubes consistently exceeds the strength obtained from \(\varnothing\) 28 mm cores drilled from printed elements, confirming a systematic gap that cannot be ignored in design or quality control. The cube-to-core ratio, expressed here as the transition coefficient K, remained remarkably stable across five binders and three curing ages, with average values ranging from 0.82 to 0.85 (Table 11). These magnitudes align with earlier reports on reduced interlayer compaction and uneven moisture exchange in printable concretes [18; 22; 41], supporting the view that the layered fabrication process alters structural integrity in a predictable yet significant way. The observed variations in mix density may result from differences in composition, the inclusion of mineral additives (such as TMF), and print stability. These factors affect compaction density and porosity, both of which are critical for mechanical performance and long-term durability. Across the full range of explored parameters--porosity (30-80%), mix composition (from CP100 to hybrid C+I systems), and print settings--the predicted in situ strength deviated from measured core strength by less than 10% for each data point. The aggregate coefficient of determination R\({}^{2}\) = 1.000 and a root-mean-square error of just 5.5 \(\times\) 10\({}^{-14}\) MPa are several orders of magnitude below standard concrete testing repeatability. The absence of trend related to colour or marker size confirms that neither binder type, water-cement ratio, layer height, nor induced porosity content disrupt the linear cube-to-core relationship captured in Equation (2). Two practical conclusions follow:  
First, a single-factor correction is sufficient. Since the slope term alone (0.97) reduces the systematic gap between cube and core to the level of experimental noise, strength control for additively manufactured elements can be reduced to: (i) one standard cube test per batch and (ii) the application of Equation (2). This eliminates the need for coring, cutting, and end-grinding--saving time, material, labour, and preventing damage to the finished part. Second, structural safety margins are preserved. Even in the worst case (highest porosity and lowest strength), the residual error remains below 2 MPa (<3%), while design standards typically assume safety factors \(\geq\) 10%.