# Additive Manufacturing as an Alternative to Core Sampling in Concrete Strength Assessment
## 4 Discussion  
Concrete remains the most widely used construction material globally. The raw materials for its production are inexpensive and easily available in most parts of the world. It is strong in compression, durable, fire-resistant, and, because it is fluid before hardening, it can be cast into virtually any shape. The term \"concrete\" in fact covers a wide range of composite mixes whose common characteristic is that they consist of a filler--sand, gravel, or other granular materials--bonded by a matrix formed through the exothermic hydration reaction between cementitious materials (cement or supplementary cementitious materials such as fly ash) and water. Additional or alternative admixtures, additives, aggregates, and binders achieve the desired properties such as self-compaction, high strength, low CO\({}_{2}\) footprint, or plasticity. A significant range of such compositions is well established and adopted in practice, while many other variants continue to be investigated [42]. The work of [PERSON] et al. [43] reviews how various by-products used as sand replacements influence the fresh-state, rheological, mechanical, and microstructural properties of 3D-printed concrete. Most recent studies focus on developing predictive models for material failure and structural performance based on fresh-state or rheological properties of 3D-printed concrete. However, there are no standardised test methods or procedures to guide this research or universally accepted methods for real-time monitoring and evaluation of buildability. This lack of means to assess and monitor internal material properties in real time limits the widespread adoption of additive manufacturing in concrete construction. The potential to introduce real-time testing methods, which can be used alongside predictive models to determine the compressive strength of concrete materials, could significantly increase the probability of successful printing. [PERSON] et al. [44] have  
Figure 13: Parity plot comparing the predicted core compressive strength \(\hat{R}_{core}\) (y-axis) with the measured core strength \(R_{core,\;mns}\) (x-axis) for all mixes. The solid grey line denotes perfect agreement (1:1), dashed lines indicate \(\pm 10\%\) deviation, and the shaded band shows the 95% prediction interval. Point size encodes porosity; colours distinguish individual mix designs. also described a continuous monitoring method for the hydration reaction of 3D-printed concrete using ultrasonic pulse velocity (UPV) as an alternative test for setting time. UPV testing has previously been applied to conventional mortars. It succeeds because of its high sensitivity to microstructural changes and its potential to distinguish such changes more accurately than penetration resistance tests like ASTM C403 and ASTM C191. [PERSON] et al. [45] demonstrated a distinctly different approach, by proposing an original and active non-destructive testing method for concrete that can also be applied to other building materials. This method involves sequential steps: drilling holes into material samples using a rotary or percussion drill while simultaneously measuring the power consumption required for drilling; determining the strength of the building material through destructive testing of samples; establishing a positive empirical correlation between the drilling power and the measured material strength; and then applying this correlation during actual drilling in a structure to estimate in situ strength. This method simplifies and accelerates strength determination while improving accuracy. [PERSON] et al. [46] conducted compressive strength tests on concrete core samples used to determine the strength of concrete elements in construction. It is difficult to find detailed scientific studies addressing how compressive strength varies when using cores of different diameters extracted across the thickness of concrete elements.