When was AMCOS Geotechnical (AG) founded?  ANS: Founded in 1978 by Nils Jansma, AG offered a specialized service that was largely restricted to the Geotechnical community.  However, in 1988, AG opened up its services to the general public and has been successfully serving in that regard ever since.

What does AMCOS do?  ANS:  AG provides a top-notch quick, dependable solution to the problem of cracked slabs or foundations, great customer support, and prices that can't be beat!  Generally, even the most serious problems can be fully repaired within one week.

 What is the most commonly asked question about a cracked or broken slab?  ANS:  "IS MY HOUSE SAFE TO LIVE IN?"  The answer generally, under most circumstances, is “YES,"

Do these questions and answers apply to all concrete foundations and slabs?  ANS:  No!  You have to distinguish between “above-grade” and “slab-on-grade” foundations.  Most of the problems or damage associated with cracked or broken foundations involve slab-on-grade structures.  Back

What is the difference between an “above-grade” foundation and a “slab-on-grade” foundation?  ANS:  An “above-grade” foundation is used to support a wood floor and wall system while a typical “slab-on-grade” is made entirely of concrete that includes a “perimeter” foundation.  The wood floor system generally covers an 18-inch crawl space that requires the interior part of the house to be supported on regularly spaced “piers.”   The concrete slab-on-grade generally rests directly on the ground or “grade” and does not include a crawl space.  Back

Is a broken “above-grade” foundation as serious as a broken “slab-on-grade” foundation?  ANS:  No!  Often, the wood “floor and wall system” has sufficient strength to bridge over a broken perimeter foundation.  Differential movement of the broken perimeter foundation is more likely to be evenly distributed through the wood floor and wall system.  This arrangement often prevents damage from being transmitted from the broken foundation to the walls above.  However, this is not the case with a slab-on-grade foundation.  When differential movements occur, the wall system above is immediately impacted and often damaged.  Back

When is damage to an “above-grade” foundation considered serious?  ANS:  When there is obvious damage to the upper structure that is clearly associated with the foundation damage.  This is generally due to differential settlements of the structure that has caused the foundation to break.  Back

What causes concrete to break?  ANS:  There are a variety of potential causes many of which will be discussed in more detail in connection with other more specific questions.  However, in general, there are three basic problems that probably cause most of the serious concrete damage associated with residential structures.  These are concrete shrinkage, expansive soils (clays), and soil consolidation or settlements.  Often, these work in combination.  Back

What is generally not a cause of cracked or broken concrete in the San Diego area?  ANS:  Cracks and breaks due to “earthquake” or seismic activity.  Earthquakes intense enough to break slabs and foundations rarely occur in San Diego county.  If a local structure is damaged by seismic activity, it is probably because of an already existing weakened condition.  Back

I have been told that "all concrete cracks sooner or later," and so not to worry about it.  Is that true?  ANS:  Yes and no or "YO."  More often than not, concrete cracks develop during the "curing" stage due to "shrinkage."   However, there are “cracks” and then there are “cracks.”  Just because they happen due to “shrinkage” does not mean that they are not “serious.”  So whether you should “worry” about them or not depends upon where they are located and how “bad” they really are.  Back

Can cracks due to “concrete shrinkage” be avoided?  ANS:  Yes but it does require a lot of care before, during, and after the concrete is poured.  Aside from unanticipated site conditions probably the second greatest reason why “concrete curing” causes cracks is because the slab was not properly reinforced with steel.  Back

Does reinforcing slabs with steel guarantee that the concrete will not crack?  ANS:  NO.  However, if a slab is adequately reinforced, the cracks that occur will usually not be serious and may not need to be repaired “structurally.”  Usually, properly reinforced concrete only allows numerous, relatively small harmless cracks to occur rather than one big one.  Back

Don’t local codes usually require slabs and foundations to be reinforced?  ANS:  YES for the slab and NO for the foundation.  The big “problem” is that most slab reinforcement has not been properly placed and so is ineffective.  Unless the steel reinforcement is physically “lifted” to the middle of the slab during the concrete pour, it will not work.  Steel that is walked on by the installers and allowed to remain at the “bottom” of the slab is next to “useless.”  That is probably why I have never repaired a slab with properly placed steel reinforcement.  Back

 Why are broken slabs and foundations a cause for concern?  ANS:  The answer is four fold: 

       A.  The damage reduces the value of the house compared to an undamaged house.   

       B.  Finance companies will generally require that the damage be fixed by a reputable party before they will offer full financing. 

       C.  All Sellers are required by law to disclose the damage to a future buyer.  A Prospective Buyer knows that the damage will have to be fixed sometime in the future to avoid a loss of resale value.  What is saved now will be lost then.

        D.  The damage to the slab and foundation may actually be a “symptom” that there are site-problems that could significantly reduce the over-all value of the property at some time in the future.  Back

    Is there any good reason why the Damage should be fixed now rather than later?  ANS:  YES.   Imagine if you were considering the purchase of a house and that you were told it had significant repairs made to the slab or foundation .  What would you prefer to hear as a buyer, a repair with a 5-year history of no movement or one made recently?  Obviously, the longer a repair proves to be a success the less concern it becomes.  Most finance agencies disregard the adverse impact of a repair if it has a successful history of at least three or more years.  Back

 How can I tell if I have a serious problem?  ANS:  If you observe interior or exterior wall and/or ceiling cracks that are 1/8-inch or more, then you can anticipate that your problems may be serious.  If you observe that such damage is accompanied by a visually evident tilting floor slab, the probability of serious damage increases significantly.  Back

  Is there any way I can look at a house and tell if it has a marginally damaged slab or foundation?     ANS:  Yes.  If you see cracks longer than 24-inches around exterior windows and /or corresponding interior damage along with ceiling cracks, then that may be an indication of foundation damage.  Additionally, if the exterior foundation is visible, you generally see vertical cracks of any size in it also.  Back

 What are some of the symptoms of a house with a significantly damaged slab or foundation.  ANS:  Often, you will see corresponding exterior and interior wall cracks.  The house may or may not have interior ceiling cracks.  If the house was built since 1960, you will likely be able to visually see breaks in the exterior foundation.  If built before 1960, you may see exterior wall cracks that extend down to the ground elevation.  Often you will see exterior wall cracks around windows that extend both up and down to the wall edges.  Back

 Can a slab be cracked and still not have to be repaired?  ANS:  Slab cracks that do not go through the exterior foundation often do not need to be repaired.  Generally, they do not completely penetrate the slab.  Back

 Can a foundation be cracked and still not have to be repaired?  ANS:  Foundation breaks are different from slab breaks.  Whether to repair them or not depends upon subjective (personal) criteria.  If non seismic safety is your concern, then many foundation cracks do not have to be repaired.  However, such unrepaired foundations may cause the house to experience up to 100 times more damage during a significant earthquake.  Additionally, normal ground vibrations cause by air and/or road traffic along with seasonal site moisture fluctuations may also result in wall and ceiling cracks that cannot be repaired permanently.  Back

What damage often happens to a house because of a broken slab or foundation?  ANS:  This depends upon the cause.  The breaks in the slab and foundation act like “hinges.”  This means that the slab and foundation become extremely “flexible.”  This flexibility forces the house frame to help keep the foundation and slab together.  This results in predictable damage in the form of cracks and breaks to occur to the house frame.  Back

What is the primary cause for a slab or foundation to break?  ANS:  The primary cause is “normal” concrete shrinkage due to misplacement of concrete reinforcement.  The shrinkage problem may be made worse by pouring “wet” concrete on a “hot” day.  Back 

 After “concrete shrinkage” what other factors cause concrete to break?  ANS:  Generally, there are two other reasons for concrete to break if not due to normal shrinkage.  These would be, expansive soils and/or settlement of artificial site fills.  Back

 Why do cracks appear in the walls and ceilings of a house supported by a broken “slab-on-grade” foundation?  ANS:  When a slab-on-grade foundation and slab breaks, the house slab is divided into multiple sections separated by a measurable space.  This allows the individual pieces of the slab to move independently (differentially) of one another.  The differential movement of all the slab pieces requires that the upper “house structure” hold them together during movement episodes.  Movement episodes can be caused by earthquakes, wind buffeting, vehicular road traffic, sonic booms and other various “transient” energy sources.  Because the house structure was not designed to hold the slab together, its rigid surfaces crack in various locations.  If the foundation is not repaired, these upper structure cracks cannot be permanently repaired.  Back

 Why would a house with broken foundation experience significantly more damage during an earthquake than a house whose foundation has been properly repaired?  ANS:  During an earthquake, a house foundation is subjected to both “push” and “shake” forces.  As long as the foundation is not broken, the push and shake forces do not cause major damage to the upper structure.  However, when the foundation is finally broken by the earthquake, the house foundation is segmented into separate pieces which begin to move independently of one another.  This independent or more properly called “differential movement” results in considerable damage to the upper structure in a very short period of time.  Consequently, a house with an "already-broken" foundation is subjected to “differential movement” during the entire earthquake period and will likely experience up to 100 or more times the damage in comparison to a house whose foundation and slab does not break.  Back

 What is the best way to repair a broken slab and/or foundation?  ANS:  The best way to repair a broken slab and/or foundation is by having the separate slab pieces bound together with “structural steel” that equals or exceeds the strength of the original slab before it was broken.  Back

What is meant by a “structural” repair?  ANS:  A structural repair must bind the broken slab parts together in such a way to make it as strong or stronger than before it was broken.  Back

What is an “epoxy” repair?  ANS:  Epoxy is a type of very strong glue.  An “epoxy repair” is suppose to glue the two slab portions together.  Back

 Is an epoxy repair a “structural” repair?  ANS:  An epoxy repair is generally not considered a structural repair for normal residential construction.  This is because the two broken edges of the slab pieces are usually not completely covered by the bonding epoxy.  Therefore, without exceptional application techniques, the epoxy repair can never restore 100% of the slabs original strength. 

        All of the epoxy repairs examined by AMCOS have only penetrated the crack approximately ˝ inch at most.  This amounts to a restoration of approximately a maximum of 12% of the original strength of the slab.  However, due to poor preparation of the slab edges, it is estimated that such a repair restores no more than 5% of the original slab’s strength.  Nevertheless, even if credible proof can be offered of 100% penetration of the applied epoxy, it is estimated that only 30% of the original strength of an unbroken concrete slab can be restored under residential placement conditions. 

       This deficiency is due to inadequate cleaning of the full depth of the broken pieces of the concrete slab.  This conclusion can be confirmed by testing.  When epoxy repairs are tested by breaking them apart, the broken surface is observed to have only a small number of tiny pieces of broken concrete attached to it.  This means that the actual break occurs mostly in the weaker epoxy-bond zone and not in the concrete.      Back

 Can I trust a contractor or Engineer who says that an observed foundation and/or slab break will not move in the future?  ANS:  YES, if he or she will give you a written guarantee.  Without a written guarantee, the answer is an emphatic NO!  A person who will not put their opinion in writing is at best guessing and at worst, conspiring.   Back

 How can a contractor or Engineer say with assurance that a broken slab or foundation will not move in the future?  ANS:  By repairing it.  Generally, no one can predict the future.  However, if a slab or foundation is properly repaired, one can guarantee that the repair will not move again within a specified period of time.  If the repaired break does “reactivate” then it should be repaired again without charge.  That is what a guarantee should provide.  Back

 Are all guarantees the same?  ANS:  No.  A guarantee is only good for the items it covers.  A good guarantee covers the repaired crack itself which is generally what is expected.  However, other guarantees are for “workmanship” or “products.”  These are of no use if the repair fails.  One should carefully examine a guarantee and make sure that it is not limited to the “workmanship” employed or the “products” used.  Back

 If an Engineering report accompanies a repair, does that mean it has been properly done?  ANS:  YES, most of the time.  However, some engineering reports are carefully worded to transfer responsibility for selecting the repair to the Real Estate Agent or the Seller.  If a choice of solutions are given by the Engineer that have a significant price range, probably only the most expensive repair carries a guarantee.  Should the Agent or Seller select a less expensive repair solution, they might find that they have voided the warranty.  These type reports may not give the expected protection and should be used with caution.  Back 

Is it necessary to remove carpet/tile in order to perform an inspection of a slab-on-grade foundation?  ANS: Not necessarily.  If the foundation is visible outside, we generally can discover any serious breaks by just walking around the house.  Or, sometimes, when there is obvious interior wall or ceiling cracks that might indicate a foundation break, we will use a sub-carpet probe to locate the edges of a cracked or broken slab.  However, if you are serious about buying a house, the very best way to find all the foundation damage is to loosen the edges of the carpeting adjacent to the exterior walls and expose about 12 inches of the concrete slab.  Any such work is the responsibility of others and should be done in advance of inspections performed by AMCOS.   Back

 

Copyright © 2004 Nils M. Jansma, all Rights Reserved

 

 

 

 

 

 

 

 

 

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Slab On & Above Grade
 


 

Causes For Cracks