Studying the Effect of Gradation and Macro Texture Generating by the Method of Gravel Distribution, on the Skid Resistance of the Concrete Pavements

1. There are two types of road pavement, asphalt and concrete, which have some advantages and disadvantages. Because of the problems of asphalt pavement under heavy axles loading, the concrete slab as alternative solution can been used. As a result, the tendency for using concrete pavements is increasing among road engineers worldwide. The skid resistance is one of the most important subjects influencing the safety of the roads having a special attention by road designers. In the lack of enough amounts of friction in the rainy weathers, the chance of accidents caused by vehicle on the road surface increases dramatically. The statistically obtained data shows that fatality is the worse outcome of the accidents. Unfortunately, the statistics of fatality in Iran has been very high and on a daily average basis 63 people had been killed in 2009 by the road accidents.The aim of this research is to find a relation between macro texture generating and skid resistance in order to upgrade the safety of concrete pavements by evaluating the methods of distribution of aggregate on fresh concrete. Also with respect to the sizes of the aggregates used in concrete mix and their effects on the skid resistance of the road different types of grading has been studied to identify which one has the highest friction. 2. The skid resistance of concrete pavement is affected by various factors such as, types of aggregates, water to cement ratio, voids percentage, processing methods and the most important of all, the method of macro texture generating on the topping of the pavement. Depth, distance and orientation of macro texture can have a huge impact on the friction characteristic, noise pollution and riding quality. In conclusion, macro texture generating on concrete pavement can be done in different methods, and aggregate distribution on fresh concrete is one of the most common methods in Europe. In this study, the best configuration of texture generating by the method of gravel distribution will be found by using two different types and sizes of aggregate. Therefore, one of the main objectives is to investigate the impact of aggregate’s diameter on the skid resistance. In this study, friction properties on concrete surfaces have been investigated by evaluating BPN (pendulum number index) and MTD (macro texture depth).To evaluate concrete sample’s macro texture of the concrete; the modified sand has been used according to ASTM E965 standard. In normal sand distribution method for specification of average texture depth, the diameter of the circle generated on the pavement surface is measured. But in the modified sand distribution, the sample diameter stays fixed and the volume of the sand will be measured.3. The results of British pendulum and the analyses3.1. The effect of aggregate type on pendulum number To investigate the effect of aggregate type on pavement skid resistance the relationship between average value of BPN and aggregate type was evaluated. As we can see in Fig. 1 skid resistance and gravel type have a significant relation, and this means in conclusion, if the aggregate type changes the skid resistance changes. 3.2. The effect of aggregate nominal diameter on pendulum numberTo check the aggregates maximum nominal size effect on pavement surface frictional resistance, the relationship between the average value of BPN and aggregate nominal diameter was evaluated. The procedure of BPN changing versus aggregate nominal diameter change is shown in Fig. 2 for 3 different mixes. According to Fig. 2, it can be understood that the skid resistance and aggregate nominal size have a direct relation.4. 1- In aggregates distribution method, using regular aggregate increases the pendulum number by 4.2% compared to RAP aggregates. Skid resistance and aggregate nominal diameter have a direct relationship, and if aggregate diameter increases the skid resistance increases. 2- By comparing changes of the aggregates diameters, types and mixed aggregate sizes, it can be concluded that the change of aggregate diameters has the most impact on BPN value changing and after this aggregate type changes, finally the size of mixed aggregate size changes. 3- It should be mentioned that the linear variation of type and diameters of gravels and aggregate sizes do not cause linear change of pendulum number. It means if these factors increase or decreases in fixed interval, the pendulum number does not change in fixed interval.4- Therefore, including the standards of noise pollution and riding quality in the optimization and design of the rough texture of the roads in necessary.