# Prediction Models for Mechanical Properties of Cement-Bound Aggregate with Waste Rubber
## 1 Introduction  
In semi-flexible pavements, cement-bound aggregate (CBA) is used as a bearing layer. This layer provides improved bearing capacity and freeze--thaw resistance while presenting an even surface for installing asphalt layers. Despite all the benefits, these materials are prone to cracking due to cement hydration and the expansion of these cracks under the influence of repeated traffic loads [1]. Recently, waste rubber has been used in this material to release internal stresses and reduce the occurrence of cracks. The quality of this material is primarily described by its compressive strength. The compressive strength of CBA is usually tested after 7 and 28 days [2], but it is often measured after 90, 180 and 360 curing days for CBA modified with materials possessing postponed pozzolanic activity. The strength is significantly lower than that of conventional concrete, and satisfactory 7-day compressive strength ranges from 2.1 to 2.8 MPa [3; 4]. Determining the compressive strength implies measuring the breaking force of a sample exposed to a uniaxial compressive load. The destruction of a sample is an acceptable way of testing laboratory-prepared samples when a sufficient number of samples can be produced. However, when evaluating the material incorporated in a pavement, there is a limited number of cored specimens. Preserving thesample for many test procedures is very useful in this case. Additionally, conducting a field evaluation of the inbuilt bearing layer using non-destructive testing is preferable. The other important characteristic of cement-bound aggregate is its elasticity modulus (E). The dynamic (E\({}_{\text{dyn}}\)) and static (E\({}_{\text{st}}\)) moduli of elasticity can be measured within cement-based materials [5]. The E\({}_{\text{st}}\) is determined from the linear relationship of the stresses and strains during the compression strength test. An obstacle in determining the static modulus of elasticity is the rough surface of this material [6], because the procedure entails the precise measurement of microscopic vertical displacements of points on the sample derivatives during the change in compression force. In addition to the difficulty of ensuring precise measurements, this is also a destructive method. At the same time, the static modulus of elasticity is significantly lower than the dynamic modulus [6]. On the other hand, the dynamic modulus of elasticity is usually measured using the ultrasonic pulse velocity. This is a non-destructive method, usually used in concrete testing, after which the sample is ready for further testing, and it can be applied to various materials. Ultrasonic pulses are pulses with frequencies over 20 Hz. There are several conventional ultrasonic testing methods, such as the pulse-echo ultrasonic, pitch-catch ultrasonic, immersion-based ultrasonic, air-coupled ultrasonic, oblique incidence, phase array ultrasonic and laser-ultrasonics and non-contact laser-ultrasonic techniques [7]. The choice of method depends on the tested material, the size of the specimen and external conditions. As mentioned above, cement hydration is a time-dependent process, so the passage of time significantly affects the development of the material stiffness. [PERSON] et al. [8] explain three typical stages of UPV development during the first 55 h of cement-stabilized aggregate microstructure formation. In the first stage, the UPV is stable at low values, followed by the second stage, where, due to cement hydration, the UPV rapidly increases. In the third stage, the UPV gradually becomes stable due to a rigid and stable matrix. These methods apply to all materials used in road construction, starting from the stabilized soil through bearing layers to asphalt materials. [PERSON] and [PERSON] [9] developed prediction models for compressive and indirect tensile strength estimation of unstabilized and stabilized rammed earth based on UPV measurement. The authors also encourage using UPV measurement as an effective strength-estimation method.