# Performance Evaluation of Silane in Concrete Bridge Decks Using Transmission X-ray Microscopy
## 1 Introduction  
Concrete structures have many durability problems associated with their service life due to cracking and external fluid penetration [1; 2; 3; 4; 5; 6]. The vulnerability of concrete structures further increases due to material deterioration phenomena occurring over the structural lifetime, primarily fluid transportation into concrete. These fluids may contain harmful ionic species, exposing the concrete to corrosion of steel elements (reinforcement bars of reinforced concrete or prestressing strands of prestressed concrete girders), alkali-silica reactions, sulfate attacks, bulk freezing and thawing damage, and salt scaling [7; 8; 9; 10; 11]. By resisting the penetration of these harmful ionic species, the durability of concrete structures can be increased [3; 12; 13]. Increasing the concrete durability is a practical approach toward sustainability [14]. Using protective coatings to decrease the fluid penetration could be a cost-effective strategy to increase the durability and prolong the service life of concrete, for which filling or lining the pores with a hydrophobic surface is a common approach [3; 7; 15; 16]. Silanes and siloxanes are the main constituents in most penetrating sealants available on the market [17]. Silane with monomeric silicon is commonly used as a coating material to reduce fluid penetration into the concrete matrix. It consists of chemicals that contain at least one hydrolytically stable carbon-silicon bond. The organic groups of the silane are bound to the silicon atoms by carbon-silicon bonds, and they coat the surface of the pores with a nonpolar, low-surface-energy, and hydrophobic lining [7; 17; 18]. This coating acts as abarrier that mitigates water penetration, but allows vapor transport from the concrete [7; 19]. To ensure uniform and deeper penetration of silane into the concrete, silanes are dissolved in a solvent, such as water or alcohol. The amounts of active ingredients and the penetration depth of the carrier into the concrete mainly determine the performance of silanes [7; 17; 18; 19; 20; 21]. Using silane to repair concrete is an effective method of reducing the ingress of external fluids into concrete [22; 23; 24; 25]. Although numerous studies have been completed on silane coating materials or sealers, the results of laboratory and field tests have generally been varied or nonconclusive [22; 26; 27; 28; 29]. This variability may be due to using different test procedures because a standard test method has not been developed to evaluate the long-term performance of such sealers when applied on concrete. It has also been shown that sealer performance is sensitive to the environment, substrate, sealer chemical properties, and application variables [28]. In 2019, the Oklahoma Department of Transportation (ODOT) reported that silane coatings are not resisting water penetration and the reasons are unknown [30]. This suggests that silane coatings are not effective, and this puts the long-term performance of the bridge in question. Moreover, only a small collection of information exists for the field performance of silane treatments [31; 32; 33]. In addition, applying silane does not alter the porosity of the concrete. Therefore, porosity measurements cannot evaluate the efficiency of silane. Another study showed that porosity is not a useful parameter for evaluating the properties of a cementitious system against fluid transport [34]. This suggests that the efficiency of silane should be studied with a technique that considers different mass transport mechanisms, porosity, pore interconnectivity, and chemical binding. Therefore, the aim of this study was to utilize a direct observation technique to rapidly identify the protective effectiveness of silane coatings to reduce ion penetration and figure out the reasons for the poor performance of silane in some projects as reported by the ODOT.