1. Introduction
Concrete is one of the most extensively utilised building materials in the world. Aggregates, cement, water, and any other additives required to achieve the specified specification of concrete. C
2S, C
3A, C
3S, C
4AF, and CSH
2 are the five major components of cement which determines the strength of concrete. In concrete, porosity is one of the important characteristics that directly affects the strength and durability of cement-based materials [
1,
2]. Gel pores, capillary pores, and air Voids make up the pore structure of cement-based materials, and the pores are randomly sized, organised, and connected [
3]. Two- and three-dimensional (2D and 3D) imaging methods were used to compare the pore volume dispersion of concrete with varied air entraining admixture doses to anticipate the material characteristics. The relationship between the pore features of the specimens and their mechanical properties was investigated. As concrete buildings are subjected to a variety of hazardous conditions, research into the long-term durability of cementitious composites has attracted much interest. Concrete is prone to degradation during freeze/thaw cycles [
4], and as a result, it can degrade visibly in the winter. The hazard of concrete degradation due to freeze-thaw cycles is significant in critical applications, including foundations, dams, pavements, and bridges [
5,
6,
7,
8,
9,
10]. Xie et al. [
11] investigated the high-temperature resistance of carbonise concrete and proposed a constitutive equation for predicting the compressive properties of concrete during high-temperature exposure. The air-void system of concrete determines how well it performs under freezing and thawing situations. Furthermore, in terms of longevity and the mechanical/ physical characteristics of concrete buildings, it is essential to determine the distribution of air holes and pore volume [
12].
The interquartile range between a point in concrete mixture and the nearest void’s edge is determined by spacing factor,
L. It is the essential defining characteristic required for safeguarding concrete against thawing and freezing activities. Furthermore, air pore concentration, size, and distribution are important for longevity and concrete buildings’ physical/mechanical characteristics [
13]. For appropriate design and evaluation of concrete durability and performance in an open environment, accurate measurements of void characteristics and microscope-based standard are required as per ASTM C-457 [
14]. ASTM C-457 [
14] and its European counterpart EN 480-11 [
15] are the most frequently utilised methods for assessing the air void arrangement in concrete at present.
The use of destructive testing procedures to determine compressive strength reduces the cross-section of structural components due to spalling, lowering load bearing capability. These methods are also time-consuming and costly. Instead, quicker and more cost-effective methods for estimating concrete’s compressive strength are required. Non- destructive testing like pin penetration, ultrasonic velocity, and pull-off provide estimated findings on concrete compressive strength using numerical formulae involving a series of experiments conducted in a laboratory setting [
16,
17,
18,
19,
20].
Image processing is the computer-assisted replication of human vision-based activities. Many techniques, like image generation, image digitalisation, categorisation, image augmentation, classification, recording, and imagining, are included in image processing methods [
21]. Several studies have been published in the literature that use image processing techniques to analyse concrete’s physical and mechanical characteristics. Jennings et al. [
22] employed a range of image processing approaches to determine the type of pore structure in back-scattered electron pictures containing bright sections. Two-point correlation and fracture analysis are two image-processing techniques they utilised for dimensioning. Gaydecki et al. [
23] employed a portable scanning device with an inductive sensor to identify steel bars embedded in concrete. With this technique, it is able to generate 16-bit gray-leveled images and it is able to extract numerous characteristics about the steel bars buried in concrete using image processing techniques on these pictures. To investigate the mechanical behaviour of concrete, mortar and high-performance concrete specimens under semi-static stress, Huon et al. employed infrared thermography and digital image correlation [
22]. Surface polishing, cutting, surface contrasting, and drying take time in the preparation of samples for testing [
24,
25,
26]. The incorporation of 2D based image techniques using a linear traverse method may not be as representative as the predicted air-void system, since in this method, measurements are limited to one dimension (1D), while air-voids are actually distributed in a three-dimensional (3D) space in hardened concrete. To overcome the limitations mentioned above and to more effectively investigate target materials, a different non-destructive approach which can describe both 2D and 3D objects is needed. X-ray microcomputed tomography (micro-CT) can be used for further investigation of the pore structure of concrete without damaging the material [
27,
28,
29,
30].
Microcomputed tomography produces a sequence of cross-sectional pictures of the desired material, from which a 3D image is created by stacking 2D images. The three-dimensional dispersion of concrete substances, such as aggregates and pores, may be characterised and analysed in 3D using the micro-CT image produced. The data acquired can be used to explore more complicated studies, like those involving component size and morphology distributions, qualitatively and quantitatively. Micro-CT has been utilised to examine the pore volume of cement composites because of these benefits [
31,
32,
33,
34].
Most research has utilised volume-based measurement to investigate pore properties using microcomputed tomography, notably pore volume distribution, where the diameter/radius of pores is estimated using the volume of the particle and the formula for estimating sphere volume [
10,
35,
36,
37,
38]. The pore volume distribution of a focused material has been described using the volume-based method by utilising the maximum ball algorithm. Traditional volume-based measurements, presume that pores are completely spherical, limiting their ability to describe anisotropy or variability in pore properties. As a result, a complementary method for characterising pore structures in 3D is required [
39]. Several characterisation approaches have been utilised to measure the porous properties of cement-based materials [
40,
41,
42,
43,
44,
45]. Chord length dispersion is a way to express the geometric features of composite material that may be used to analyse material pore size distribution. Chords are indeed the distances of line crossings with the interfaces, and chord information may be used to calculate the diameter of a pore in a particle. This method allows for a more precise description of particle size properties and pore since it is based on the measured length. Despite its advantages for characterising material properties, chord length distribution has seldom been used to examine the pore architectures of concrete due to its computational difficulty when compared to traditional approaches [
46,
47,
48].
The pore volume dispersion of concrete specimens of varied air-entraining doses was investigated in this work by utilising varying water/cement ratios. Concrete compressive strength values rise in tandem with the amount of cement in the mix, as is well known. The 2D pore volume dispersion was calculated according to EN 480-11 [
49]. The specimens were imaged using micro-CT to produce a series of cross-sectional pictures without causing damage [
50]. The specimens’ pore volume dispersion was calculated by volume-based technique and the chord-length distribution in 3D using microcomputed tomography images. As a result, grey colour values in concrete pictures are projected to rise. However, this research aims to better understand pore size distribution in cement-based materials. The acquired samples were subjected to mechanical, and durability tests and image processing techniques were employed on samples with identical mixes. Doing so attempted to discover if image analysis may be utilised as an auxiliary approach to disruptive and non-disruptive procedures, as well as the connections between image analysis findings and concrete mechanical strength values. The porosity in concrete samples is the one of the important factor influencing concrete strength character, however pores with diameters smaller than 10 mm has modest effect on the ultimate strength [
10]. Due to the specific surface area, there is no discernible influence on strength. The distribution of air-voids in concrete specimens predicted using 3D image technique has excellent precision. Thus, by utilizing the technique of image processing the pore structure of concrete can be investigated without damage to the specimen.
Furthermore, we attempted to determine the cement matrix and ratio of aggregate. The results of the micro-CT scan were compared to the outcomes of the experiment. The efficacy of the investigations employed in this study was proven by demonstrating a link between the pore features acquired and the mechanical strength of the samples. The research findings of Arularasi et al. [
51] show that paste and mortar rheological behaviour is identical in the presence of a mineral combination of fly-ash and GGBS
Both cement–fly ash and ternary combinations have relatively better shear and impact resistance as compared to OPC fly-ash based composites [
51,
52,
53,
54,
55,
56,
57]. Siva et al. [
50] conducted experiments to overcome the limitations mentioned above and to more effectively investigate target materials a different non-destructive approach which can describe both 2D and 3D objects is needed. X-ray microcomputed tomography (micro-CT) can be used for further investigation of the pore structure of concrete without damaging the material [
24,
25,
26,
27]. The work of Jarosław Michałek et al. focuses on methods for evaluating the internal structure of spun concrete that can be used in practise to ensure that the spinning parameters chosen are correct. Following that, imaging techniques and appropriate image processing were used to assess the spatial distribution of pores, aggregate, and cement matrix based on early results reported in [
58,
59]. Images were used to examine the microstructure of concrete and forecast its interior properties. For the cost-effective rehabilitation of ageing and disintegrating concrete structures, new approaches for precise and timely condition evaluation, performance prediction, and maintenance management are continually being developed. To this purpose, imaging techniques are becoming more popular due to their high potential for providing accurate information on a structure’s viability. To obtain concrete imagery, non-destructive testing techniques such as optical images, which present surface image data, infrared thermography and acoustics, which are used for subsurface conditions, and ground penetrating radar, which is used to obtain below-surface information of a structure, are used. This study provides a technical overview of imaging principles and potential methodologies for the automated identification, measurement, and characterization of pores in concrete.