Why sidewalks have two kinds of cracks

Source: can a video on sidewalks change your life?, Rabbit Hole, 25:35, uploaded 2026-11-02.

A person who walks a mile of sidewalk a week passes close to a million cracks in eighteen years, yet most never notice that there are two different kinds. Rabbit Hole starts from that observation. Most lines repeat at regular intervals, while every so often a different one appears, filled with a rubbery strip. Roads have no such lines, although they are also large slabs of stone laid on the ground. The video follows a New York City Department of Transportation crew building a new sidewalk in eastern Queens to explain why concrete needs both kinds of crack and why the cracks are made on purpose.

Concrete and its weakness

Grady Hillhouse of Practical Engineering explains that concrete combines four ingredients: cement, sand as the small aggregate, gravel as the large aggregate, and water. The ingredients are easy to get and need little processing, so concrete is cheap, durable and strong. Asphalt is cheaper still because it combines only a gravel mix with bitumen, a sticky black liquid made from oil. It is messier and weaker, but it can be driven on quickly, whereas concrete usually takes about a month to cure. Roads carry heavy loads and need to return to service fast, so they use asphalt. Sidewalks carry lighter loads, which allows the slower and stronger material.

Concrete is strong in compression and weak in tension. It is not a smooth homogeneous material like steel but a mixture of chunks that leaves tiny gaps. Compression keeps those gaps in place or tightens them, while pulling spreads them into cracks. Kai Yin Tam, deputy director of the NYC DOT citywide concrete unit, lists the common causes in the city: delivery trucks parked on the sidewalk, tree roots that heave the slabs, water that expands by about ten per cent when it freezes and pushes the concrete apart, and road salt. Her unit has close to 400 people working across the five boroughs on sidewalk repair and pedestrian islands, all of it within about three feet of the surface. The sidewalks are designed for a twenty-five-year service life rather than built to resist every crack. The department instead accepts cracking as inevitable and tries to decide where it happens first.

Expansion joints

The crew starts by compacting a base of recycled concrete aggregate, made from old sidewalks that have been dug up, crushed and refined. Before compaction it feels like sand on a beach; afterwards it feels like pavement. Wooden boards then divide the empty ground and hold strips of asphalt-impregnated fibreboard, made of wood and paper fibres bound with the same bitumen used in asphalt. These strips are the expansion joints. Concrete expands with heat, and a long continuous stretch could buckle, which would put it into tension and crack it. A full break every so often gives each section room to move.

Tam shows how the spacing can be estimated from the local temperature range alone. The change in length relative to the original length equals the coefficient of thermal expansion multiplied by the change in temperature. She assumes a worst-case coefficient of 7.3 × 10⁻⁶ per degree Fahrenheit, a pour at no lower than 40°F (about 4°C) and a maximum of 110°F (about 43°C) in New York, a range of 70°F. A half-inch joint gives a quarter inch (about 6mm) of room on each side, and she allows for the board compressing by about half. The result is a maximum span of roughly 40 feet (about 12m), so the 20-foot (about 6m) spacing used in practice is very safe. Expansion joints also run along every solid border, such as the curb, light poles and buildings, so that different materials move independently. A light pole swaying in the wind, for example, should not transfer stress to the slab around it. The boards and their pins only hold the fibreboard in place against the weight of the wet concrete and are pulled out during the pour.

Control joints

Once the concrete leaves the truck, the cement and water are already reacting. The water dissolves the surfaces of the cement grains and reacts with them, slowly at first so the concrete can be shaped and then quickly and permanently. The crew has about ninety minutes as a rule of thumb before the concrete can no longer stay in the truck. As some water reacts and the excess evaporates, the concrete shrinks, which again puts it in tension. Control joints answer that problem. They are grooves rather than full breaks, cut about an inch deep, roughly a quarter of the sidewalk’s depth. They do not separate the slab and ideally it never cracks at all, but if it must crack, the groove tells it where. Because they are less invasive, control joints can be placed more often: every five feet in New York, compared with twenty for expansion joints. Tam explains that early shrinkage is only their first job. In a city, tree roots and freeze-thaw cycles keep inducing stress, and the control joints accommodate that over time. The resulting five-by-five-foot squares, called flags, also became the unit for counting repairs and violations.

Control joints can be cut with a saw once the concrete has cured for hours, which is loud and messy, or tooled by hand into the wet surface. The cement mason on the crew works freehand by eye, with different tools for handle length and scoring depth, and redraws the lines to match the narrower joints of the neighbouring sidewalk. The work needs a light touch, since a tool angled even slightly wrong digs into the concrete instead of pressing a groove. The final steps are brooming the surface for traction and sealing the expansion joints with a waterproof sealant. That sealant is the easiest way to tell the two joints apart: a whitish rubber line marks an expansion joint, although weeds can take over when the seal is not maintained.

The city’s share

Homeowners in New York are responsible for the sidewalk in front of their property, yet the department’s job remains large. The city has roughly 12,000 miles of sidewalk, and the department maintains public staircases, bus stops, the frontage of public buildings and every street corner quadrant. It rebuilds about 3,000 corners a year, which is why corners often look different from the sidewalk beside them. Tam asks people not to drive onto sidewalks and not to scratch names or symbols into wet concrete, since offensive marks have to be ground down or the section rebuilt.

Tradition as evidence

The expansion joints rest on a calculation, but the five-foot spacing of control joints does not. Tam says it usually matches the width of the sidewalk, so the flags come out square. The video found no formula in the literature either. The consistent finding is only that more distance between joints means more stress and more cracking. If a sidewalk is already five feet wide, closer joints add little because the slab will still crack across its longest dimension, so joints usually need to be no closer than square. Climate also matters. Places with milder temperature swings can use fewer joints, and a sidewalk in Orlando alternates control and expansion joints. Spacing therefore varies with climate, sidewalk dimensions and other factors that remain partly unexplained.

The same uncertainty applies to the basic choice of concrete over asphalt. Tam does not know the reason and says such decisions were above her pay grade. She thinks the answer lies largely in tradition. Reasons offered online, which the video treats as uncertain, include that asphalt is darker, hotter and rougher on feet and on pets’ paws. The video does not dismiss tradition as inertia. A standard has passed the test of time in real use and captures side benefits that an engineer might miss, such as the comfort of animals walking on it. Tam adds that when nobody notices a piece of infrastructure, it means the job has been done well. The video still argues that questioning and innovation matter, but it ends with the reminder that no calculation predicts every outcome and that sometimes the best course is simply to try something and see whether it holds.