This Old House Magazine: Where the Sidewalk Mends
Fixing cracks on a concrete path and steps
By Nancy Lackman
From This Old House Magazine
Sharp bits of concrete fly off the front stoop of a Greenwich Village brownstone in New York City as mason Lenny Moreira chips away at a crack that zigzags across the bottom step like a lightning bolt. He is here to repair these stairs and a nearby fractured sidewalk, a day's work for this fourth-generation mason and owner of It Only Takes One Construction, in Manhattan. "Concrete's worst enemy is water," he says, as a steady ping, ping, ping of hammer and chisel echoes through the air. "This probably started as a tiny hairline break, but over the years water trapped inside turned to ice and expanded it."After clearing out the crack to create a smooth surface, Moreira mixes a batch of slurry, which will bond the new mortar to the old concrete, and brushes it into the crevice. The crack needs filling because it is deeper than 1/4 inch, he explains. Had it been caught when it started, it could have been checked with urethane sealant or silicone caulking. But now, Moreira insists, it must be refilled before it becomes a tripping hazard or a threat to the structural integrity. "Not only does water push the surface out, it also rusts the iron rebar in the concrete," he says. With the slurry coat still wet, Moreira fills the crack with a freshly mixed batch of mortar, smoothing it flush with the step to complete the patch.
Turning his attention to the adjacent sidewalk, he examines a two-inch-wide split that leads to a large severed chunk. As a temporary fix until the homeowner can replace the entire sidewalk, he will recast the 18-inch-long section. The numerous cracks and surface flaking—known as scaling—point to a problem with the original job: either too much water in the concrete mix or not enough cure time at consistent temperatures. "There's no way to correct that," he says.
Moreira chips out the broken pieces, fills the gap with concrete, and smooths it with a float. As he redraws the edge of the slab, the imperfections have nearly disappeared. Only a slight color variation remains to betray that the entire sidewalk is anything but freshly poured.
Kneeling in front of the damaged front step of a row house, Lenny Moreira uses a mason's cold chisel and hammer to carefully remove the crumbling pieces from the water-damaged concrete step. "It's just like filling a tooth," says Moreira. "You've got to apply the new mix to a solid, sound surface for it to bond properly." To free the last of the smallest broken pieces he uses a fine-pointed masonry nail in lieu of the chisel.
So that the slurry coat—a low-aggregate mix that acts as a bonding agent—adheres well, Moreira sweeps away dust from the crack and wets it with a brush. He makes the slurry by adding water to a mix of one-part Portland cement, two-parts mason's lime (which helps adhesion), and three-parts sand (see "Caution When Wet," below). He brushes a thin coating of the slurry into the crack, then removes excess with a wet sponge.
Moreira's finish mortar—a thicker, grittier filler—is water mixed with one-part Portland cement, one-part lime, and three-parts sand. He trowels it deep into the crack—only 5/8 inch at a time—then waits at least an hour before applying more layers (inset). When the crack is filled, he repeatedly swirls his float, a foot-long wood tool, along the surface of the wet mortar, as if wiping a counter. This marries the new mix with the old step while eliminating air pockets. Finally, Moreira sponges down the rest of the stair and rinses the surrounding sidewalk clean with a hose._______________________________
Caution When Wet
Mason Lenny Moreira's mantra when mixing mortar or concrete is "Wetness leads to weakness." Sidewalks and steps often break or chip because there was too little Portland cement and too much water in the mixture. For Moreira, knowing when there's too much water is not so much an exact science as a skill he's acquired with 30 years of experience.
He begins his mixes by blending the dry ingredients: Portland cement, sand, and mason's lime for mortar or aggregate for concrete. (The proportions change with the project.) Then he adds water slowly, sparingly, and with an eye to consistency, aiming for a spreadable mixture similar to cake frosting.To maintain control of the wet/dry fusion, Moreira makes a volcano shape in the dry mix and pours only one cup of water into its crater at a time. Chopping and sliding his pointing trowel, he pushes small sections of dry mix toward the center. With each turn of the trowel, the blend darkens and gets harder to work. Moreira says there's no magic formula for knowing when to stop. "Wet concrete should be stiff yet pliable, with no air bubbles—never soupy," he explains. "If the aggregate is swimming in the water, then it's too wet. To fix it, just add more dry ingredients mixed in the right proportions."
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Fixing A Hole In The Sidewalk
Moreira begins the sidewalk repair by using a 10-pound electric chipping hammer to break away the cracked and loose bits of concrete. He cuts at an angle to leave a beveled edge on the hole, which will give the new concrete more surface area to adhere to. After sweeping away the sand and debris, he wets the area with a paint brush and coats it with a new, stronger slurry coat made with 50 percent more Portland cement than the one used on the stairs.
Because this sidewalk is in front of a garage where cars will cross, Moreira mixes up concrete, a stronger finish coat, by leaving out the lime and adding one-part 3/4-inch gravel (nickel-size stones) to one-part cement and 2-1/4-parts sand. He works the mix into the hole, packing each trowelful by tapping it with the short edge of a wood float. After filling the void halfway, he lays metal lath (wire concrete mesh would work as well) on top of the wet concrete to lend strength and rigidity when the patch sets.
With the mesh in place, he fills the second half of the hole with his pointing trowel, flattening and packing often with the float as he works. "Not only does tapping release all the air and work the mix into the crevices," says Moreira, "it settles the solids and brings excess water to the surface, making a stronger slab." In order to prevent tripping hazards at the repair, Moreira works his mason's trowel in a back-and-forth sawing motion, gently compressing the new concrete until it's flush with the old.
After an hour's drying time, Moreira redraws the edge of the patched slab (inset). He slides an edger—a piece of metal with a soft-cornered right angle in the middle—along the joint, guided by a wood straightedge. Then he sponges down the repair's perimeter one last time. He recommends covering the slab with a wet cloth for 72 hours and setting a dribbling hose on it for another few weeks. "You can walk on it after three days," he says, "but it should stay damp for 28 days to cure properly."
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