NANO-SILICA (SiO2) CONTRIBUTION TO MECHANICAL AND MICROSTRUCTURAL CHARACTERISTICS OF HIGH PERFORMANCE CONCRETE PAVEMENT

High performance concrete pavement appears to be the best choice for layer with high strength and low cost. In this paper, the mechanical properties were investigated which considered essential for design and requirements of rigid highway pavement. These properties were examined by compressive strength, flexural strength and modulus of elasticity. Previous test investigations were imposed for concrete mixes with different Nano SiO2 (size of 30 nm) percentage (0, 0.5, 1,1.5, & 2) % by wright of cement as mineral admixture . Also, XRay Diffraction (XRD) and X-Ray Fluorescence (XRF) tests were conducted to indicate the chemical composition of the used admixture. Energy Dispersive Spectroscopy (EDS) was conducted to visualize the micro structural behaviour of concrete during hydration process. Wet and dry mixing methods were utilised to prepare the modified concrete mixes and the results of the more effective method, were adopted. Scanning electron microscopy (SEM) was utilized to investigate the microstructure and morphology of nano concrete mixes. The results of SEM indicated that the microstructural distribution of C-S-H gel were comparable for all the nano mixes. The decrease in thickness of nano concrete layer was associated with the increment in the percentage of nano silica add to the mixes when the other factors remain the same. The best percentage of nano silica was 1.5% by weight of cementitious materials (by using dry mixing method). This percentage satisfied all requirements of fresh and hardened concrete mixes for highway pavement and achieved well microstructure with significant decrement in the thickness of concrete layer which positively reflected on construction cost .


Introduction
Rigid Highway pavement plays a significant role in the nation's infrastructure. The construction and maintenance of them consume large amount of energy resulting in environmental impact and reduction of covetable source. Therefore, any method reducing the above harmful impact may reflect positivity on sustainable development [1] . One of main recent technology is the use of nano concrete mixes to construct rigid highway pavement. The use of nano materials in cement-based mixes have sustainable merits due to the advantages related to the final product. The incorporation of nano materials into matrix improves the mechanical properties of nano composite [2]. Usually, the high specific surface of nano silica particles cause high adsorption of Ca+ ions as well as accelerate the dissolution rate of C3S which in turn increase the hydration rate and the formation of the cement hydration products. In addition, nano silica acts as reactive filler which increase the packing density and reduce the voids within the concrete mass [3]. Pozzolanics are known as siliceous and alumina particles reacting with Ca(OH)2-CH to form cementitious products. The mineral characteristics such as particle size and the amorphousness are the main parameter related to Pozzolanic activities. The ultra-size of nano silica causes packing that work as filler inside the matrix of hardened cement paste increasing the strength, durability and sulphate resistance of mixes [6].  [7].  Cement: Al Jeser sulfate resisting cement produced in Iraq was used. The cement conforms to the Iraqi requirements (No.5/1984-type V) [8].  Nano material: Nano silica (SiO2) has high purity reaches to 99% and particle size of about (30 nm) with different contents (0%, 0.5%, 1%, 1.5%, 2%) by weight of cement.  High range water reduces admixture (superplasticizer). Hyperplastic (PC 175) is high performance super plasticizer used as dispersing addition in order to overcome the nano particles agglomeration and ensure reactive surface for the filling effect and C-S-H formation. [9].  Colloidal polymer resins admixture Poly Vinyl Alcohol (PVA) is colloidal material (solid white granules) in water help dispersion of micro material [10].

Concrete Mix
The concrete mix was prepared to achieve the requirements of concrete mixture construction (C30) according to ACI 211.1) [11].

Types of Nano Mixing Method
For better performance, the following main mixing methods [6], which were used to solve the problem of nano agglomeration during addition to cement binder within nano modified concrete mixture: 1. Wet Mixing Method Nano silica admixture was mixed with first part of mixing water in the presence of Polyvinyl alcohol (PVA) (1% by weight of cement binder) by using stirring device as shown in plate (1-a) for (10 min period) . Then, the solution was treated by sonication device with 50W power for (5 min period) and temperature (25 ℃ ) as shown in plate (1-b).  X-Ray Detraction Test of Nano Silica Admixture The crystallinity of Nano-silica particle was determined by X-ray diffraction (XRD) which is considered the most useful technique for identification of microstructure was shown in Figure (1). The analysis was compatible with the powder is amorphous silica with little impurities. It is a simple non-distractive method for testing the quantitative and qualitative analysis of chemical composition of Nano silica material to achieve the purity and chemical composition of the admixture as shown in

Fresh Properties of Control and Nano Cement Mix
Due to the large surface area of Nano silica (smaller particle size of Nano silica admixture), the initial setting time is reduced and the slump value is decreased that have a significant effect on workability behaviour of Nano concrete mix due to high degree of agglomeration but still within the adopted limitation (25-75 mm slump) except for2% Nano silica content. Thus, there was a greater demand for the superplastizer to achieve the required workability as shown in Figure (2).

Hardened Properties of Control and Nano Concrete Mixes
Nano silica gives both pozzolanic activities due to high surface area and packing ability with significant role in filling voids, thus it increases the mechanical properties. Moreover, it can participate in the hydration process to create C-S-H product through reaction with Ca(OH)2 [17] . The increase of strength is the most important benefit of using nano materials like Nano silica as shown in Table (2) and Figure

Microstructure Analysis of Control and Nano Concrete Mixes
In SEM analysis of concrete mixes, it can be noticed the absence of well-developed Ca(OH)2 plates and the presence of relatively high porosity. When nano particles are uniformly distributed in concrete mix, they fill cement panes and act as concrete core. Due to its intense activity, cement hydration becomes rapid and concrete strength measures especially modulus of elasticity increase that agree with [18]. The microstructure of hydrated cement paste of M3 (1.5% of nano silica addition) was shown in Figure (6-a). The considerable strength obtained in this mix was due to the pozzolanic activity of the Nano silica. The chemical reaction of nano silica with the portlandite Ca(OH)2 leads to production of additional C-S-H gel and less voids. The nano structure is very dense and many crystalic lumps can be observed. From EDS test shown in Figure (6-b), the purity observed matches the specification which presents the predominant phases Oxygen (O), silicon (Si) and Calcium (Ca). That shows a good reaction between silica and Ca(OH)2.
b. EDS Images of M3 From SEM micrograph shown in Figure (7-a), it was observed that C-S-H gel was rapidly spread on the M0 (control mix) which was the main cause for the effective strength but with Ettringite appearance extend in relatively high air voids. The microstructure looks to contain many amorphous substances. EDS test given in Figure

Thickness Design of Nano Concrete Layer
The thickness of the concrete layer within rigid highway pavement is fabricated in accordance to AASHTO Guide [19]. All mixes are designed to withstand the same conditions. Table (3) shows the comparison which is based on layer thickness between control mix (0% Nano silica addition) and mixes contain different percentages of Nano silica admixture (0.5, 1, 1.5, and 2% %). As given in previous table, the mixes contain Nano silica admixture gives lower thickness than control mixes due to high value of modulus of elasticity and Flexural strength.