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Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > Filling
cracks in patios
Q: The ground underneath my patio has settled
on one side and one of the joints has opened very wide, about 2 inches.
What can I do to fill this gap?
 A:
The opening of the joint in the patio is due to large deflections
on one side of the slab. Pumping grout under the slab, often called
undersealing, can restore slab support and reduce the gap width. However,
this may only postpone the joint widening if the ground continues
to settle down the slope. It may be helpful to consider methods that
will stop the downward flow of the slab upon the soils that are moving.
A simple method to accomplish this goal may the excavation of 8 in.
to 12 in. diameter postholes (post piers) that extend below the frost
line (consult local codes for frost depth requirements). The holes
are located so that the edge of the slab will rest on the center of
the pier when completed. After excavation the holes are filled with
concrete to slightly higher than the bottom of the slab and the concrete
is then consolidated by vibration or trouncing with a suitable rod
(a simple 2 by 4 stud will do).The piers will provide long term support
for the slab to prevent further movement.
After the slab has been stabilized the open joint is cleaned thoroughly
and filled with a mortar consisting of one part portland cement, 2
¼ to 3 parts sand, (a latex polymer or bonding agent is optional)
and sufficient water to make a stiff mortar (consistency of a thick
oatmeal). The mortar is placed in the joint opening, compacted with
the thin edge of a wood block and then topped off and struck off with
a wood or magnesium float. Allow the mortar to set until it begins
to stiffen, and then use an edging tool to create a clean radius at
interface between existing and repair. After edging, float the surface
again. Allow a time lapse for additional stiffening of the mortar
and then finish with a broom.
Caution: due to the nature of this type of repair (long thin strip),
it is expected that the repair will crack across its width at multiple
locations along its length.
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