Concrete slab crack FAQs:
Questions & answers on diagnosing & repairing cracked, settling, spalling, heaving concrete roofs, floors, slabs.
This article series describes methods for repair of cracks that occur in poured concrete slabs or floors and explains the need for accurate crack cause diagnosis and impact on structure before repairs are attempted.
Green links show where you are. © Copyright 2017 InspectApedia.com, All Rights Reserved.
Our photo (left) shows sealant being applied to a sound but leaking concrete roof on a home in central Mexico.
(May 3, 2016) Mahta said:
Hair Crack on roof slab
(May 10, 2016) aditi said:
Sir, i have constructed my house roof one year back n in starting of rainy season i found cracks on my roof due to which there is leakage in rainy days. what is the cause n remedy for the cracks in my house roof.
I can't diagnose nor suggest repair with just the information in your question.
Risks involved include at least these
Start at SHRINKAGE CRACKS in SLABS
You're asking a roof question on a concrete slab article. IF your roof is a flat concrete slab and is cracking, there are a variety of causes: sagging from improper construction or inadequate reinforcement would require structural repairs; cracks from concrete shrinkage can typically be patched and sealed.
If a concrete roof is leaking typical repair is to use a combination of synethtic fabric membrane and a concrete roof sealant mastic or paint.
Details are at FLAT ROOF LEAK REPAIR
Watch out you are describing what sounds like risk of structural collapse. A structural engineer is needed to make an on-site assessment and then describe what is a safe repair.
Typically we see emergency support provided from below (posts and beams below the roof).
A final repair could require either roof removal and then reconstruction or construction of a properly-reinforced concrete roof above or below the existing one.
(May 28, 2016) Jesse Vasquez said:
I just had a concrete slab poured, 14 by 30, bromed! It looks like crap, how do I fix it ? There's dips n it's not good
If you're not satisfied with the concrete work I hope you haven't paid the contractor.
What repair is needed depends on the nature of the problems. Brooming may be proper on some pours - such as a sidewalk, as an anti-slip measure - but would be an odd finish for a building interior floor.
Dips - depending on the amout of out of level a new cap slab could be needed, or simply a top coat with a concrete leveling compound.
(Aug 17, 2016) shallu razdan said:
Sir plz provide any solution for crack on concrete roof which has occured since construction due to delection of beam support at that time
At FLAT ROOF LEAK REPAIR you'll find detailed procedures for successful sealing of leaks in a flat or low slope concrete roof, PROVIDED that structural repairs are not needed.
Watch out: A deflecting beam below a concrete roof needs evaluation by a civil or structural engineer. The risk is catastrophic structural collapse and injury or death to occupants.
First, you need an expert diagnosis of the crack that you described to answer at least these questions:
1. what caused the crack?
2. has that cause been reparied so that the cracking won't continue?
3. has the amount of movement caused other damage that needs repair?
4. How big is the existing crack? As that may affect the repair approach?
Once those questions are satisified, for hairline or small cracks, say up to a few mm in width, you might try any of the sealants described in the article above. On a roof surface I like to apply a synthetic fabric membrane over the crack area, sealing it to the roof surface, and then coating the membrane upper surface with a good quality roof sealant formulated for use on concrete. That membrane offers additional protection against re-opening of the repaired area and thus against leaks.
(Sept 4, 2016) Pranav said:
Sir, my house is constructed six years before , now these days of rainy season slap of the drawing room is breaking and large pieces of concrete (broken reinforcement started falling and portion where slab is bent and water leakage is there through crack.
It came to know now because there is POP in between. I have some photos of slab captured. Should i have to reconstruct slab or theirs any other way? How can diagnose broken rebars? Please give if theirs any solution.
Watch out: you are describing crumbing, collapsing concrete roof construction. I cannot know and am not a concrete engineer, but from your description it sounds as if the concrete mix was improperly done, perhaps without enough concrete or reinforcement or both.
The risk is that someone could be killed by a falling roof. You need an onsite civil or structural engineer to assess the damage and then to help you decide what repairs are needed such as
- removal and reconstruction of the roof
- added reinforcement and support
- a new top layer of sealant or fabic + sealant
Continue reading at CONCRETE SLAB CRACK REPAIR or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see FOUNDATION CRACK DICTIONARY since those pages also assist in distinguishing among types of cracking in concrete foundations (vertical supporting walls and footings).
Or see these
Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
Please see the main text version of this article at Shrinkage Cracks in Slabs
We discuss the recognition and significance of concrete shrinkage cracks in detail at Shrinkage Cracks in Slabs. Just below you can read a summary of this topic.
Shrinkage cracks in poured concrete are easily recognizable and can be distinguished from other types of cracks that occur later in the life of a foundation wall or floor slab.
The photograph of cracks above were taken of shrinkage cracks in a concrete slab floor in a home built in 2006. The cracks in this case ranged in width (measured across the crack) from "hairline" (less than 1/16") to about 3/32" in the basement floor slab of this particular home. They may appear larger.
What is unique about shrinkage cracks in concrete is that they usually appear to be discontinuous, as shown in this photo. The crack will meander along in the concrete, taper to a stop, and then continue beginning in a parallel line to the first crack, meandering again through the concrete. This is characteristic of concrete (or mud) shrinking while giving up its moisture.
You can see the shrinkage of even a perfect concrete floor slab with no visible cracks in the field of its surface if the floor was poured inside of an existing foundation. Look for the gap between the edges of the slab and the foundation wall? Look also for the stains or concrete debris on the wall at the slab level? These confirm that at the time the slab was poured it was touching the wall.
Depending on the mix and pour conditions as well as the site work preparation significant concrete shrinkage cracks can appear in a basement slab. On occasion we may also see vertical dislocation in a floor slab crack, that is, one side has settled or tipped away from the other side of the cracked concrete.
Our photographs above show shrinkage cracking and ensuing minor settlement cracks occurring in the same poured concrete basement floor. We suspected that site drainage defects or possibly nearby site blasting for additional construction contributed to the 1/16" to 1/8" vertical dislocation across some of the cracks in this floor where the cracks were about 3/16" wide.
Happily the building owner at the site where we made the photos above did not report water entry through these openings, though it certainly might be expected as the home ages and its footing drains stop working, particularly if surface runoff and roof runoff are not kept away from the building.
The photo at left shows some concrete shrinkage cracks that are larger than usual. Note that these cracks begin at building foundation wall inside projection corners - a condition that probably creates stress points as the slab cures.
If you click on and enlarge the photo you can see clear evidence that the cracks are discontinuous, multiple, and very roughly parallel in this area - good evidence that they were caused during the concrete curing process and not by a later event which "broke" the slab.
When we see combined slab shrinkage and slab settlement condition we suspect that the concrete pour not only allowed for excessive shrinkage, or perhaps shrinkage without control joints, but also the pour was made on top of poorly prepared soils. For example if a slab is poured on poorly-compacted soil, after shrinkage cracking occurs, we may see uneven settlement among sections of the cracked slab.
Settlement cracking following shrinkage cracking may also occur. In both of these cases we anticipate that significant horizontal dislocation in a poured concrete slab should be small unless steel reinforcement was omitted or was improperly installed. Since some contractors use a fiber-reinforced cement and may omit steel reinforcement in floor slabs, this condition may occur.
We discuss the recognition and significance of control joints in poured concrete slabs in detail at CONTROL JOINT CRACKS in CONCRETE. Just below you can read a summary of this topic.
The first photograph shows an expansion joint in a basement floor slab. Notice that we do not see other cracks in this slab. Shrinkage cracks that occur at control joints such as shown in the second, close up photo here, are occurring where they are supposed-to, although the width of this particular crack was surprisingly large. These cracks are not normally a defect in the slab but may be a source of water or radon entry into the building and may need to be sealed. Use a flexible sealant.
We discuss the recognition and significance of settlement cracks in poured concrete slabs in detail at Settlement Cracks in Slabs. Just below you can read a summary of this topic.
Settlement cracks in a conventional concrete floor slab which has been poured inside a separate foundation wall (and often resting at its edges on the building's foundation wall footings) are usually not connected to the foundation wall and are not supporting any structure [except possibly Lally columns, discussed next].
These cracks may not be a structural concern, but there are cases where a serious hazard can be present, such as garage floor cracking when the floor was poured over soft, loose, inadequately-compacted fill and where the floor slab was not pinned to the garage foundation walls. Soil settlement under a garage floor, perhaps aggravated by groundwater which can increase soil settlement, can lead to first hollowing-out of space below the floor and second, sudden collapse of the floor structure.
We have used a heavy chain, dragging it across the garage floor and listening to changes in the sound it produces, to find areas of significant soil voids below the floor. The pitch of the chain noise drops significantly when passing over a void below the concrete.]
Settlement cracks in a concrete floor around a supporting Lally column might be indicative of a serious problem such as building settlement if the columns are settling. Independent footings may have been provided supporting Lally columns in the building interior and those may be settling independently of the floor slab which may have been poured around and even over them (See photo above). But beware, where slab thickness and local building codes allow, supporting columns may bear directly on a poured floor slab without their own (deeper) pier or footing. In that case floor slab cracking and settling can cause column movement and may be a structural concern.
Settlement cracks in a monolithic slab or floating slab floor may be more serious, depending on their extent since in this case the edges and other portions of the slab are, unlike the cases above) expected to support the upper portions of the building structure.
A monolithic concrete slab is one which includes the building footing as part of the slab, created in a single continuous pour of concrete.
A floating concrete slab is one which is poured at a (generally) uniform thickness on the ground without a separate footing. [Beware, in areas of wet soils, expansive clays, freezing climates, or unstable soils, floating slabs may be exposed to extra stresses and may tip or crack. Proper site work and drainage are important as is proper engineering design of such structures.
We discuss the recognition and significance of frost heaving damage and cracks in poured concrete slabs in detail at Frost Heave/Expansive Soil Cracks in Slabs. Just below you can read a summary of this topic.
Frost heaves or expansive soils damage to building floor slabs can range from minor to extensive in buildings depending on soil and weather conditions, site preparation, and slab construction details, as we elaborate here.
The photograph above shows a rather straight crack across a garage slab near the garage entry door. What is happening here and why is this particular crack straight if it's a frost or soil heave crack?
In freezing climates building foundations include a footing which extends below the frost line. This is true for both the occupied space as well as garages. When a concrete slab is poured either abutting the top of such a foundation, or poured extending over the edges of such a foundation, there is risk of cracking across the concrete at the interior edge of the buried footing.
The combination of water under a garage floor (watch out for driveways and sites that slope towards the garage or home) and freezing can cause the portion of the slab which rests directly on the soil to move up and down during freeze/thaw cycles. Since a garage is often colder near the garage entry door than in other areas, there is extra risk of these cracks occurring there, but they can occur anywhere. When there is freezing and heaving of a slab, particularly one which omitted reinforcing steel, or did not extend the steel over the footing, these garage floor slab cracks may appear during freeze/thaw cycles.
Similar floor slab damage might occur in areas of expansive clay soils if the proper moisture level is not maintained.
Basement floor heave patterns - frozen floor drains: Basement floors can be frost heaved in other patterns in buildings which are unheated or which lose heat. We have found basement floor slabs broken and heaved above buried drain lines which ran below the basement floor of a home which remained unheated during freezing weather.
A clogged drain sitting full of liquid combined with prolonged freezing weather was the culprit in most of these cases. The heaved concrete was raised following exactly the path of the frozen (and burst) buried, clogged drain line. This problem can be epidemic in older homes which were constructed using a downspout drain line extending below the basement slab.
Basement and garage floor random heave and crack patterns: Cracked and heaved concrete or settled concrete can occur in more random patterns in any concrete floor where there has been frost heaving, soil contraction/expansion, or simple soil settlement, as shown in this photograph.
Garage or basement floor sloped or semi-uniform settlement may also produce a tipped floor even if the concrete is not cracked, or the floor may settle uniformly. This condition occurs if the concrete was reinforced by steel or fiber cement, but was poured inside of a separate concrete or masonry block foundation. We see this condition more often in garages in which the slab was reinforced but poured on poorly-compacted soil. The problem may be worst if in addition to poor compaction, water runs under the slab, causing additional or more rapid soil settlement.
My first construction job (for pay) was to rake level the backfill soil that the contractor had dumped inside of the newly-completed garage foundation in a series of homes. No compaction of any kind was performed. When a lot of fill, several feet or more in depth, was required to bring the slab to the desired height, there was a good chance that the slab would settle or tip in the future.
Garage slabs which were poured inside of the foundation walls but which were pinned to the foundation sides (typically using re-bar set into holes punched into the masonry block foundation), the slab was resistant to settlement or movement even if there was modest soil settlement below.
In a garage where the slab has settled you can often spot the original level of the slab and thus can measure the amount of settlement. Look for a concrete line above the level of the top of the slab and found along the masonry block or poured concrete foundation wall. we have seen this line ranging from a fraction of an inch to six to eight inches above the current level of the slab!
Settlement cracks may form a tripping hazard even if they are not traced to a structural
concern. While there are few construction acceptance standards for floor slab cracking, one, "Quality Standards for the Professional
Remodeling Industry" NAHB, , recommends that cracks in basement floors which exceed 3/16" in width or 1/8" in vertical displacement should be repaired.
The same standard provides that cracks in slab-on-grade floors shall not exceed 1/16" in width or in vertical displacement. The same standard
provides that cracks occurring in control joints in concrete slabs are normal and acceptable. [We suggest that this last
criteria should apply to crack width but not to vertical displacement.]