Is slab lifting or replacing concrete the better choice?

When lifting is the better choice

The Federal Highway Administration (FHWA) says the ideal candidates for slab jacking are slabs with localized settlement that are generally free of cracking.^[1] At home, that looks like a sidewalk square that has dropped at one end, a patio that now tilts toward the house, or a driveway section sitting an inch below its neighbour, with the concrete itself still sound.

Lifting has real advantages in those cases:

• It keeps the original surface. No demolition, hauling or new pour, and the slab still matches its neighbours.
• It is quick. One foam product's data sheet says it is traffic ready in 15 minutes.^[4] New concrete has to cure before it carries a car; see how long new concrete should cure.
• It adds little weight. FHWA lists polyurethane as a lightweight foam that does not add to the settlement.^[1]

How mudjacking and foam lifting work explains the two methods.

When replacement is the better choice

• The slab is broken into loose pieces that rock or shift underfoot. Lifting raises a slab as a unit; pieces move separately and leave gaps and edges.
• It has heaved rather than sunk. Lifting only raises. In Fairfax County, the county says swelling clay under a floor slab can push it up hard enough that the slab sometimes has to be replaced.^[2]
• The base itself is the problem. Fairfax County says slab damage on those clays can be reduced by removing the high shrink-swell clays under slabs, usually to a depth of one to two feet, and replacing them with compacted non-swelling soils.^[2] That cannot be done through injection holes.
• The slope is wrong and cannot be corrected. Fairfax County also says it is important to maintain positive drainage away from houses, and that rainfall runoff may stand for long periods in areas that do not have at least a two percent slope.^[2] A slab that has always sent water toward the house may be a candidate for a new one, set to a proper fall.
• The surface is failing anyway. Deep flaking, spreading cracks or exposed steel are surface or reinforcement problems a lift will not fix; whether to repair or replace cracked concrete covers them.

A new slab only lasts if the base is right. For the sidewalks it builds, the Virginia Department of Transportation requires: "The Contractor shall excavate, shape and compact the earthen foundation to a firm, even surface."^[3] VDOT also requires that unsuitable material is to be removed and replaced.^[3] Ask any contractor quoting a replacement how they will prepare and compact the base, and what they will do with soft or unsuitable soil.

Neither works if the cause is still there

FHWA warns that if the underlying causes of the settlement are not dealt with, the same problems will come back.^[1] Downspouts emptying beside the slab, a yard that drains toward it, clay soils that shrink in dry summers, and poorly compacted fill are the usual suspects; why concrete slabs sink goes through each, and how to keep a slab from sinking again covers the follow-up.

Permits and the utility locate request

Approval rules for flatwork are local, and a replacement is more likely than a lift to raise the question. Prince William County, for example, lists concrete patio slabs on grade that are not designed to support a future building addition among work that needs no building permit, while noting that zoning or watershed approval will be required.^[5] Rules vary, so ask your own county or town, especially for a driveway apron or a public sidewalk.

Replacement also means digging. Virginia's utility damage prevention law says no one may begin any excavation or demolition without first submitting a locate request to the notification center,^[7] and defines excavation to include grading, digging and drilling, and moving or removing any structure.^[6] Your contractor normally submits that locate request before breaking out the old slab.

When to call a licensed pro

Both jobs are for a licensed contractor. Lifting needs pressurised injection equipment, and replacement means demolition, base work and a new pour. Virginia's concrete contracting specialty (CEM) covers placing, finishing and repairing concrete, including grouting,^[9] and the Board for Contractors says that if the work is valued at $1,000 or more, a valid Virginia contractor's license is required for the license category the contractor works in.^[11] A big replacement can also move a job up a class: the Code of Virginia defines Class B contractors as those working on single contracts of $30,000 or more but less than $150,000.^[8]

To choose well:

• Ask for both prices. The Board for Contractors advises getting at least three written bids or estimates using identical plans and specifications,^[11] so ask each contractor to quote a lift and a replacement for the same slabs.
• Ask what they will do about the cause, and whether that is in the price.
• Ask about permits. Virginia's residential contract rules require a statement of assurance that the contractor will comply with all local requirements for building permits, inspections and zoning.^[10]
• Check the licence with DPOR before you sign; how to check a contractor's licence shows how.

For prices, see concrete leveling costs and new concrete slab costs.

Loclis lists DPOR-licensed concrete businesses with their licence number and class. Browse slab lifting and leveling, look up concrete contractors in Blacksburg, or see all concrete and hardscaping answers.

Describe the slab once and licensed local businesses quote a lift, a replacement, or both.