Treatment of cracking in rigid highway pavements using knowledge-based System

Highway engineers encounter numerous problems in the quality of rigid pavements; cracking types are the most critical ones. Thus, deciding on appropriate controlling measures represents an essential mission. Normally, cracking problems affect the quality of pavements as well as increase initial cost. Although experts can control and solve these problems by using their tacit knowledge, novice engineers cannot. Transfer of expertise from experts to novices is difficult pavements domain. Therefore, a system which experts could use to share their experience with other engineers both during and after a project is necessary. Without such transfer of expertise and knowledge, novices may repeat mistakes that experts have already learned to avoid. Documentation, classification, and computerization of these problems, their causes, treatments, and preventive actions can be very helpful in controlling and preventing them. This study aims to describe the development of a knowledge-based system that can be used by novice engineers to overcome cracking of rigid pavements. The system can also be used as an instructional tool for prospective highway engineers. In addition, the system can archive and organize raw knowledge from experts to be utilized by engineers who work in this field. Domain experts can use the system to share their experiences. The knowledge includes problems encountered in the domain, their causes, preventive actions, treatments, and their effects, which were presented in a classified format. The knowledge was presented in the form of rules and coded in software by using Visual Basic. The system was tested by different users involved in highway engineering, including experts and novice engineers. The mean values of the overall system evaluation by the four types of users based on the 5-point Likert scale were 4 and 4.5 respectively. These values reflect high level of satisfaction by end-users.


Introduction
The expansion of highways, which are main infrastructure elements (Mohammed, Ambak, Mosa, & Syamsunur, 2018a, 2018bLubna Abdulrahman Salem, Taher, Mosa, & Banyhussan, 2020), is essential in improving a country's economy (Banyhussan, Tayh, & Mosa, 2020; Ahmed Mancy Mosa, 2017b; A. M. Mosa, Salem, & Waryosh, 2020;A. M. Mosa, Taher, & Al-Jaberi, 2017). Fast urbanization becomes an increasingly problematic issue of economic development (Mohammed, Ambak, Mosa, & Syamsunur, 2019a;. Urbanization and increase in population need efficient transportation facilities (Hatem, Mosa, & Al-Dahlaki, 2018;Mohammed, Ambak, Mosa, & Syamsunur, 2019b). Highway networks are the main facility in transportation systems (Ahmed Mancy Mosa, 2017c;A. M. Mosa, Atiq, Raihantaha, & Ismail, 2011a). Therefore, constructing highways with high level of serviceability is necessary Lubna Abdulrahman Salem, et al., 2020) to connect cities with each other and with the new constructed districts. Pavement is known as the most decisive part of highway construction (Choi, 2011). Pavements construction needs team work (Ahmed Mancy Mosa, 2017a). Successful construction can be attained by applying the efforts wisely at a practical price within a specified time schedule (Hatem, et al., 2018;. However, pavement engineers face numerous problems with rigid pavements (Ahmed Mancy Mosa, 2017;A. M. Mosa et al., 2015;A. M. Mosa, Rahmat, Ismail, & Taha, 2013). Among those problems, cracking types can be the most critical category as their occurrence reduces the pavements serviceability. Therefore, the engineers must decide appropriate controlling measures, suggest effective treatments, identify causes, specify preventive actions to prevent recurrent problems, and anticipate the effects of uncontrolled problems. In addition, adoption of preventive action for these problems prior to their occurrence is very helpful. Experts are able to overcome those problems using their expertise (Miller, Huerne, & Dorée, 2007;A. M. Mosa, Taha, Ismail, & Rahmat, 2013a, 2013b. However, junior engineers cannot yet (A. M. Mosa, et al., 2011a;A. M. Mosa, Atiq, Raihantaha, & Ismail, 2011b). Therefore, creating a knowledge-based system in this domain is very helpful to assist the pavements engineers to overcome domain problems. Numerous knowledge-based systems were created in the domain of transportation engineering such as transportation management, traffic control, highway project management, highway geometric design, and management of rigid and flexible pavements. Pavement management involves structural design, material selection, and maintenance and rehabilitation (Al-Dahlaki & Mosa, 2016;A. Mosa, Jawadb, & Salema, 2018;Ahmed Mancy Mosa, 2017b;Lubna A Salem, Taher, & Mosa, 2018;Taher, Al-Jaberi, & Mosa, 2018). However, gaps still exist in the domain of rigid pavement. Among these gaps, cracking of rigid pavements is on the top of the list. Therefore, this study aims to develop a knowledge-based system to overcome cracking problems of rigid pavements. The proposed system contains a knowledge base of study domain problems, their description, treatments, causes, preventive actions to avoid their recurrence, and their effects if not controlled.

Knowledge Elicitation
The knowledge used to create the knowledge base which is the core of the system was attained from multiple sources to ensure an adequate agreement on ideas and procedures. The advantages of applying multiple sources include the elimination of bias towards a single view, less reliance on a single domain expert for knowledge elicitation, and representation of the real situation in the field in case of contradictory views in which further explore is regularly needed. Knowledge acquisition mostly consists of two steps. First step is written knowledge gaining; it was implemented by detailed analysis of written sources such as textbooks, manuals, guides, research publications, and so on. Second step is human expertise elicitation; it was executed via 4 manners: unstructured interviews, structured interviews, questionnaire forms, and focus interviews. Six experts participated in this step as abstracted in Table 1. The acquired knowledge was analyzed and classified as shown in subsection 2.2.

Classification of Cracking Types
Through deep review and frequent analysis of the elicited knowledge, the cracking types were defined and classified based on their forms, locations, effects, and other common characteristics. Based on these characteristics, engineers can visually diagnose the crack type or by measurements. Furthermore, the description of the cracks, their likely causes, preventive actions, instantaneous treatments, and possible effects if they remained uncontrolled are stated. Repeated classification stages were argued with the experts (1, 2, and 3 in Table 1) for adjustment. Those experts reviewed the final classification during focus interviews and accepted it. Fig. 1 presents the classification diagram of the cracking types. The cracks were identified according to their characteristics, which could be visually diagnosed according to their appearance or through measurements. The name of each problem reflects its description. Experts can easily recognize cracking types and rapidly find the treatments whereas novices cannot.

Description of Domain Problems
Crack is the complete or incomplete separation of concrete into two or more parts produced by breaking or fracturing. Cracks may appear in plastic concrete or hardened concrete surface, and can be visually diagnosed with aid of measurements.
Plastic shrinkage cracks occur at the surface of fresh concrete during the interval after concrete placing and before concrete setting, when the concrete is plastic and can be remolded. This interval may range from 1 hour to 12 hours, depending on air temperature, water content in the mixture, and the use of accelerators or retarders. These cracks are usually diagonal in accordance with the panel's border and do not reach slab edges. They are relatively shallow (25 mm to 75 mm deep) and short (a few centimeters to 1 meter long). They are discontinuous and usually 30 cm to 100 cm apart. Generally, they are perpendicular to the wind direction and parallel to each other. They may occur throughout the panel surface and rarely intersect its perimeter. Treatment of this type can be implemented based on conditions; generally, two cases are considered. First case, if the problem was diagnosed before concrete setting (when the concrete is still plastic), three sub-cases is to be considered. First sub-case, if work was performed by fixed-form paving, the concrete can be re-vibrated to close the cracks. Afterward, the concrete surface can be finished. Second subcase, if work was performed by slip-form paving and the problem was diagnosed in a small area, the cracks can be closed manually by trowel. Third sub-case, if work is performed by slip-form paving and the problem was diagnosed in a large area, side forms should be installed firmly to the edges of the pavement. Correct elevations and alignment of the forms should be ensured. The concrete should then be re-vibrated to close the cracks and then finished. Second case, if problem was diagnosed after concrete setting, three sub-cases is to be considered. First sub-case, if cracks are shallow (less than 20 mm deep) and narrow (a width of 1 mm or less), or are hairline cracks, the problem can be neglected. Second sub-case, if cracks are intermediate (no more than 75 mm deep, no more than 3 mm wide, not continuous, and do not reach the slab edge), the cracks can be sealed with expansive waterproof cement slurry or by injecting low-viscosity epoxy according to the manufacturer's instructions. Third sub-case, if cracks are severe, deep, or extensive (more than 75 mm deep, more than 3 mm wide, continuous, or reach the slab edge), the defective panels should be removed and reconstructed.
Map Cracking (Craze Cracks) is a network of fine shallow (usually approximately 3 mm deep) fissures on concrete which encloses small (12 mm to 20 mm), asymmetrical, and hexagonal areas. This type of cracking may occur after concrete setting throughout the panel surface and are generally apparent the day after placement or by the end of the first week. Craze cracking can be removed by diamond grinding.
Transverse and oblique cracks within the panel appear at the transverse, oblique, or random direction of hardened concrete highway panels. Usually, these cracks are evenly spaced and continuous from the centerline to the pavement edge (perpendicular to centerline). The crack may fork into a "Y" shape. This type of cracking occurs after the concrete has set but before the pavement is opened to traffic. It normally appears within the first 72 hours and extends into the full depth of slab. However, it may be a hairline (nearly invisible or open crack. The slab should be removed and reconstructed in three cases: if a vertical movement or displacement along the crack is detected, if more than one transverse, longitudinal, or corner crack is present in the panel, or if the panel is divided into more than two pieces. However, if only one not working crack is present in the panel and no vertical movement along the crack sides is observed, the crack should be sawed and sealed. Random longitudinal cracks within the panel appear at the longitudinal direction of the hardened concrete highway panels and parallel to the centerline. This type of cracking occurs after the concrete has set but before the pavement is opened to traffic and may appear within the first 24 hours or after several months). It extends into the full depth of the slab. However, the cracks may be hairline (and nearly invisible) or open. The treatments for random longitudinal cracks are similar to those for random transverse cracks.
Corner crack (break at panel corner) intersects the adjacent transverse and longitudinal joints at approximately 45°. It ranges from 0.3 m long to half the slab width on each side and extends into the full depth. Corner breaks can occur after setting before the pavement is opened to the public. However, corner breaks can continue to form for years, anytime the pavement loses support due to soil settling, erosion, frost heaving, expansive soils, or excessive curling and warping. The panel should be removed and reconstructed.
Random transverse cracks at or near transverse joint is either at the transverse joint or located 30 cm to 45 cm from the joint and along its length. The crack may be parallel to or intersect with the transverse joint. It may start from the pavement edge and end at the joint, or continue to the edge or centerline of the pavement. It may extend into the full depth of the slab or continue towards the joint in a vertical direction and intersect with the joint, thus forming a triangle shape. It may cause the separation of a concrete triangle block between the crack and the joint. It occurs after the concrete has set but before the pavement is opened to traffic and normally appears within the first 72 hours. The panel should be removed and reconstructed.
Random longitudinal crack at or near longitudinal joint is either at the longitudinal joint or located 30 cm to 45 cm from the joint and along its length. The crack may be parallel to or intersect with the transverse joint. It may extend into the full depth of the slab or continue towards the joint in the vertical direction and intersect with the joint, thus forming a triangle shape. It may be hairline (and nearly invisible) or open. It occurs after concrete has set but before the pavement is opened to traffic, and it may appear within the first 24 hours or after several months). The panel should be removed and reconstructed.
Cracks at the intersection of joints occur after the concrete has set, but before the pavement is opened to traffic. They appear at the intersection point between longitudinal and transverse joints where intersected joints (longitudinal and transverse joints) are discontinuous or misaligned. Usually, this type of crack occurs in small projects when joints are not sawed. In such works, the contraction joints are formed by wet-forming, or the insertion of a strip in the plastic concrete to form the joint groove.
Treatment of this type can be implemented based on conditions; generally, three cases are considered. First case, if no sapling is observed at the cracked area, a saw cut should be applied to complete the joint grooves. Second case, if there is sapling at the defective area and dimensions of any defective part along the panel joint is not more than 300 mm and the total length of the defective parts along the joint is not more than 10% of the joint length, partial depth repair can be performed. Third case, if the conditions mentioned in case 2 is exceeded, the panel should be removed and reconstructed.
Cracks in front of saw during joint cutting are fine cracks appear in front of the saw direction during early sawing of the joint before the concrete has gained sufficient strength. The treatment of this type consists of two steps. First, joint sawing should be stopped and resumed only after the concrete has gained sufficient strength. Joint sawing should then be performed within the sawing window by using light saws such as earlyentry or handle saws. Second, if the crack occurred below the sawed area, it can be repaired by increasing the width of the sawn groove in the second saw cut.

Knowledge Representation
To use the acquired knowledge, the knowledge is represented symbolically to be manipulated in an automated manner by knowledge-based system reasoning. Therefore, the implementation environment of the system must be selected prior to the selection of the knowledge representation method. In this study, Visual Basic was selected to develop the proposed system as it is an effective and flexible tool to develop such systems. The domain knowledge was represented in the form of rules. When problems arise, experts need to collect information regarding the issue and then make a decision. Therefore, a forward chaining inference engine (which is data driven) is suitable for knowledge representation. This procedure works in accordance with the facts in the knowledge base towards the goal or conclusion. Reasoning originates from the given information and then continues forward with it.
This approach depends on IF-THEN relationships. That is, if the IF condition is matched in a rule, the action in its THEN part shall be applied. This process can be modeled as IF (condition) THEN (conclusion). Two or more rules can be logically connected by connection terms AND, OR, and ELSE to form the combined rule. The connection of combined rules can produce composite rules. The classified knowledge, which represents the core of the proposed system, was prepared in the form of rules coded in a computer environment.

The Structure of the System
The system's structure composed of a graphical user-interface, working memory, inference engine, and knowledge base. The components of the system work in cooperation to supply the end-user with rapid, obvious, and effective recommendations. The graphical user-interface is the part that the end-user sees and interacts with. Via the graphical interface, the user can input data and achieve outputs. End-user inputs are manipulated in working memory via the interface. The inference engine uses inputs via the working memory as well as searches the rules in the knowledge base to match the problem description and provide recommendations.
In general, the system attains the information needed to solve a particular problem by asking the user to answer questions or prompting the user to provide relevant information in a prearranged way via the interface. The user-interface was constructed in a manner that it effectively collects information and structurally presents the whole system without restricting the overall performance. It was designed with an attractive graphic to eliminate the time needed for learning and minimize mistakes made by first-time users. Regardless of how well the knowledge base is organized or how effective the system's performance is, the quality of user interface design alone can specify the destiny of the system.
The system was built by creating a new project under the Visual Basic environment. Visual Basic creates at least one form with each project. Each form includes the interface module and code module. Fig. 2 and Fig.  3 present examples of the interface module and the code module respectively. The interface modules were designed to be clear, user-friendly, and interactive. The interface module includes a number of components or controls (each represents a tool) such as frames, labels, text boxes, images, option buttons (radio buttons), checkboxes, command buttons, windows media player, adobe acrobat reader, combo boxes, and message boxes. Each control was designed to perform a specific function in the form. Forms and controls are called objects. Visual Basic was utilized to design clear interface modules by using a variety of colours for background and texts. Changes in colors, fonts, and other visual effects were used to bring the user's attention to some points.
The inference engine is the control mechanism that organizes the problem data and searches the knowledge base for applicable rules. The inference engine reaches a conclusion by matching appropriate rules under a set of specific facts. Basically, the inference engine operates as follow: IF the premise of a rule is true, THEN the conclusion is true.

System Evaluation
A questionnaire survey was designed to test the satisfaction of 4 experts in rigid pavements with the system. The rigid pavement experts evaluated the system by using the questionnaire described in Table 2. The evaluation reflects their satisfaction through their high ratings (more than 3). The satisfaction of experts indicates that they positively evaluated the user interface; this result also verifies the system. Table 2 summarizes the testing and evaluation of the system by the domain experts.
Another questionnaire was designed to test the satisfaction of 10 novice highway engineers with the system. The highway engineers evaluated the system by using the questionnaire described in Table 3. The evaluation reflects the satisfaction of the users through their high ratings (more than 3). The satisfaction of the participants indicates that they positively evaluated the user interface; this result also verifies the system.

Conclusions
The system was constructed by computerization of the elicited knowledge using Visual Basic programming language. It involves a sophisticated knowledge to work as: 1. A system to help highway engineers overcome domain problems, identify their causes, preventive actions, and their effects on pavement quality, and its future conditions. A simple graphical user interface was designed for the system to simplify the user's task.