A large section of a major highway in my area went through reconstruction for nearly two years, and seeing millions of tons of existing concrete being hauled by trucks through local roads made me wonder, where do they dump these and what is its future? Driving over the newly paved highway, one thing to notice was the absence of potholes (which we were so used to navigating around) and the nature of the road surface. Not only do the tires of my vehicle seem to grip well on the road surface, but also the ride is quieter. We all have experienced driving on roads which are very noisy. While structural requirements are the primary focus for building roads, other functionalities like smooth riding, road safety, noise level and appropriate skid resistance (read friction!) are also important. There are considerable research activities balancing these conflicting functionalities.

So, what is the secret of a road which provides sufficient grip (skid resistance) while providing a quieter ride? The secret lies in texturing. Texturing, in general, is used mostly for reducing friction in the tribology world; however, in this road construction application it is used for increasing friction (skid resistance). Before we get into the details, let us understand what a concrete mix is. It is a mixture of cement, water, aggregates and chemical admixtures. The aggregate consists of crushed stone (granite, limestone, etc.) of various sizes, recycled concrete and may contain sand and clay particles. The admixtures are a blend of different chemical compounds which modify concrete properties such as setting time, strength development, permeability, durability and workability. 

There are two types of texturing: micro-texturing and macro-texturing.¹ The fine aggregate of concrete mortar exposed after initial wearing-in period refers to micro-texturing, and it is good for vehicle speed less than 50 mph on a wet pavement. For higher vehicle speed, macro-texturing (with grooves of sufficient depth and spacing) is required to prevent hydroplaning. So, how is texturing done? Texturing is usually done after the concrete is poured and allowed to sit for curing. But timing is critical, so that the concrete is not fully set and it is still workable with appropriate moisture level. The texturing can be done by different methods;² one method is to drag a piece of burlap, a broom or artificial turf using a construction bridge that spans over the width of the road to create a uniform gritty texture. This results in macro-texture and used in heavy-duty roads with high traffic loads. This type of texture has very high initial skid resistances, but it wears off gradually and is associated with significantly high noise emissions. Texturing can be followed by tining, a process of creating parallel striation marks (see Figure 1). Tining is done by pulling a metal rake attached to a mechanical device that moves across the road width. The rake has a series of fixed wires which creates grooves when dragged along the concrete surface. Tining can be longitudinal, transverse or at an angle. Longitudinal tining has been shown to produce excellent long-term skid resistance and much lower tire/road noise qualities on high speed road and highway pavements. Transverse tining exhibits good frictional characteristics in wet pavements and reduces the potential for hydroplaning because of reduced water film thickness, but the drawback is high noise level. The depth and spacing of grooves are critical for attaining appropriate road noise level; narrow and deeper grooves produce less noise than wider and shallower grooves. 


Figure 1. Transverse tining operation creating a texture.³

In addition to high friction and low noise level, improved wear resistance is also important. The wear resistance of concrete with quartzite is better than that of concrete with granite, which is better than that of concrete with limestone. 

Localized surface treatments such as high friction surface treatments (HFST) are also applied in specific sections of concrete roads (such as curves or frequently brake application areas) prone to accidents and crashes. The objective is to provide additional skid resistance, and it consists of replacing the existing concrete with an aggregate of calcined bauxite mixed with a polymeric resin binder.⁴ 

So, there are a lot of tribological considerations in building and maintenance of roads and highways. 
REFERENCES
1. Leng, Z., Fan, Z., Liu, P., Kollmann, J., Oeser, M., Wang, D. and Jiang, X. (2023), “Texturing and evaluation of concrete pavement surface: A state-of-the-art review,” Journal of Road Engineering, 3 (3), pp. 252-265.
2. https://pavementinteractive.org/the-feel-of-the-road-texturing-concrete-pavements/#:~:text=Without%20additional%20work%2C%20the%20surface,pavement%20surface%20at%20an%20angle
3. Concrete Pavement Texturing, May 2019, https://www.fhwa.dot.gov/pavement/pubs/hif17011.pdf
4. https://hfsta.org/66D82-088%20addA%20IL%20DOT%2066D82.pdf