Forensic Engineering Investigation into Decay of Cinder Aggregate Masonry Units

By: John J. Hare, R.A., P.E., P.P.
Copywrite 03-2018, John J. Hare, RA, PE, PP


Cinder aggregate concrete masonry units had been in use from the late 1890’s, but it was not until after World War II with the housing boom that came after peace, that cinder aggregate masonry units (cinder blocks) became the material of choice for inexpensive foundation walls as well as for party and fire walls. In the mid 1970’s the inherent problems with decay of cinder aggregate units became apparent, and the product was generally removed from the market; however, hundreds of thousands of foundations built using this material are still in use and are continuing to fail. This paper discusses the cause of the failure, an acceptable method of repair, and presentation of the forensic evidence of failure due to decay in the courts.

History of Cinder Aggregate Concrete Masonry Units

Concrete masonry units (C.M.U.) are formed units made of concrete typically sized as 8” wide by 8” high by 16” long, or 12” wide by 8” high by 16” long, although many other sizes and shapes are available. They are usually used to construct concrete masonry unit curtain walls and bearing walls both above and below grade (foundation walls). C.M.U.’s are made of four constituent ingredients; Portland Cement, a coarse aggregate, a fine aggregate (usually sand), and a carefully controlled amount of water (water/cement ratio). Coarse aggregate is usually one of four materials: originally, pea gravel stone up to 1⁄4 inch in diameter was used. Blast furnace slag was used from the late 1890’s – this material is left over from the making of steel. It became very popular for use in concrete after World War II in the late 1940’s because blast furnace slag was cheap. It was free. It recycled a waste product; something we try to do as much as possible today. Cinder aggregate was essentially slowly phased out. The reason for this, as originally discovered by the British Department of Scientific and Industrial Research, Royal Academy of Engineers in 1928, was that the blast furnace slag (breeze and clinker aggregate as they refer to cinders) deteriorates over time. Their report states:

In the building industry the term ‘clinker’ is employed to cover well-burnt furnace residues which have been fused into lumps of varying size; the term ‘breeze’ is widely employed to cover any type of furnace residue varying from disintegrated clinker to fine, poorly sintered ashes containing a large proportion of combustible matter. The content of combustible matter in clinker may fall as low as 1-2 percent, and is never high, but in breezes it varies over a wide range. Contents of combustible matter up to 40-50 percent may occur, and values from 15-20 percent are common. The mean combustible content of over forty samples of breeze and clinker, and concretes made from them, examined in the present work is about 21 percent. Seven of these samples, consisting of large well fused material, would be termed clinker. Their average combustible content was 4.0 percent. Sixteen other samples were composed of smaller, more friable material such as is commonly called breeze, and their average combustible content was 21 percent. The remainder were samples of concrete, seven of which were actual cases of failure and several others were unsound. The average combustible content of the aggregates in these was 28 percent.”

Blast furnace slag contains not only unfired coal, but also little iron balls from the smelting

process that gets into the cinder aggregate. Some of these can be as large as 1⁄2 inch in diameter. In the mid 1970’s, the National Concrete Masonry Association recommended that concrete masonry unit manufacturers phase out the use of blast furnace slag for C.M.U.’s because of the potential for deterioration due to internal fracturing and pop-outs (little cone shaped craters on the surface of the concrete units). This was because the unfired coal and iron balls in the cinder aggregate will expand when subjected to moisture absorption from high humidity from the atmosphere or through contact with damp soils. The expansion of the iron balls and particles of unfired coal causes pressure to develop inside the concrete units over time causing pop-outs (surface spalling) or internal fracturing of the face shells and webs. Not only did the unfired coal and iron balls cause surface damage, but the internal structural integrity of the masonry units was negatively affected. Many cinder aggregate block masonry walls have actually collapsed due to this problem. There were extensive differences in the quality of the concrete units made by various manufacturers of cinder aggregate units. If one finds masonry units with evidence of pop-out damage, it is likely that not only will extensive further damage due to pop-outs continue to occur, but also extensive internal damage to the units is probable. As stated, in lieu of cinder aggregate, the concrete unit masonry industry started using naturally occurring minerals expanded by a heat process to form a lightweight aggregate as a substitution for cinders in C.M.U.’s.

This was not the case back when cinder aggregate first became very popular in the late 1940’s and early 1950’s during the housing boom. This early use of cinder aggregates to make C.M.U.’s is the cause of extensive foundation wall failures today. The majority of basement foundation walls built from the 1940’s into the early 1970’s in the Northeast, were built using cinder aggregate C.M.U.’s with blast furnace slag, as Pittsburgh was the steel manufacturing capital of the world.

Cause of Failure

As an architect, professional engineer and general contractor, the author has seen extensive damage to cinder aggregate bearing walls, fire walls, and party and foundation walls, and has been involved in the repair or repair design of over 3,000 cinder aggregate C.M.U. walls, mostly in the states of New Jersey and Pennsylvania. Unfortunately, many of these types of walls have collapsed without notice, and the author has investigated the cause of numerous such collapses. Unlike structural steel or wood which has strong flexural qualities, as steel or wood begins to fail, the material deflects; obvious bending or sagging alerts building owners to the potential for failure and prompts investigation. On the other hand, masonry walls and foundations built of cinder aggregate masonry give little notice other than possibly slight to moderate horizontal joint cracking or inward movement (bowing) before they fail. It is well known that when concrete masonry fails, it fails catastrophically. In many cases, a careful inspection by an architect or engineer may lead to discovery of potential or existing problems before an actual collapse occurs, making a proper long-term repair possible.

In cinder aggregate masonry walls and foundations, most of the damage due to the deterioration of the masonry units is hidden. Hidden decay is most likely to develop towards the exterior of walls above and below grade. With C.M.U.’s above grade, the masonry is usually painted or stuccoed, but still subject to moisture infiltration and absorption. Below grade foundation walls are parged with cement pargetting and most times, but not all, are coated with dampproofing such as bituminous foundation coating. Dampproofing still allows some moisture migration through the wall. Very few basement foundation walls are ever actually waterproofed.
Indications of Aggregate Deterioration

From the above, it can readily be understood that the decay of the inclusions within the cinder

aggregate concrete masonry units causes an unseen gradual deterioration in the structural integrity of the units leading to a potential catastrophic collapse. In addition, as part of this deterioration, the bond between the cinder aggregate units and the mortar used to construct the wall slowly loses it adhesive qualities as the surface to the mortared- face shells deteriorates, causing the adhesive bond to decrease over time.

The author has seen this in numerous cases where an inward bowing of the masonry foundation wall occurs and the mortar can be plucked out with just two fingers from between the units – no tools necessary. The mortar has completely lost all tensile bond (See Photograph 1). The mortar tensile bond to cinder aggregate masonry units is, at best, 25 p.s.i., with a maximum of approximately 16 +/- square inches of area on the interior face shell of the unit, stressed in tension for a total of 400 pounds tensile strength to resist bending. Adding a 3/8” diameter reinforcing rod whose area is equal to 0.113 square inches, of either 60 or 80 k.s.i. steel yield strength, provides an increase in the capability of the wall to resist bending well above the original designed unreinforced strength. Adding these rods increases the tensile bond resistance to 0.6 times the yield strength of the steel used, i.e. 6.6 kips for 60 k.s.i. steel per 16 inches per wall. The majority of foundation wall failures repaired are those where long horizontal cracks in the masonry joints develop, usually one, two or three units below the exterior grade. (See Photograph 2). Typically, it has been a single longitudinal crack that becomes diagonal step crack up and down as it nears intersecting or end walls, although it is not unusual to have two or even three horizontal cracks in some cases. Usually, it is possible to repair walls with this problem if the cracks are open 3/8 inch or less. It is also possible to move the wall back to vertical by excavating the exterior backfill, then pushing or jacking the wall back into position. Once this is done, installing reinforcing in the cores and grouting restores the wall to well above its original strength.

Essentially, it could be considered a series of spaced concrete studs with infill panels to hold back the earth. The pea gravel grout bonds with the existing cinder aggregate concrete of the masonry units helping to re-solidify the fractured units.

As to the procedures for determining the conditions of cinder aggregate masonry walls, the first is visual inspection of the masonry units, looking for pop-outs and rust stains on the surface, an indication of iron inclusions under the surface (See Photograph 3). Looking closely at the surface, it is also possible to detect hairline cracks on the interior face shell of the cinder aggregate masonry wall indicating the possibility of internal fracturing of the webs and exterior face shells. Sounding, by lightly tapping on the face shell with a hammer, also provides an indication of internal fracturing. Typically, if the unit is not severely fractured internally, the sound produced will be a ping or ringing sound. If the sound is more like a dull thud, it is a sure indication of internal fracturing and deterioration. The amount of wall area that is determined to be damaged by sounding is an indication of the extent of the problem. Deciding whether to either repair or replace the wall is a matter of engineering judgment based on knowledge and experience.

Presentation of Evidence of Decay to Insurance Companies and the Courts

Although many homeowners’ insurance policies do state that the residence is covered for hidden decay, unfortunately, many insurance companies will deny any such decay claim for cinder aggregate masonry failure by attempting to claim that the term “decay” applies only to wood. Another

obstacle which must be argued and overcome with the insurance company involves “exclusions for bowing and earth movement.” Fortunately, the judges in some cases in which this author has been involved ruled in the homeowners’ favor stating that decay is decay, no matter what caused the decay or what decayed, and that cinder aggregate masonry deterioration due to swelling of unfired coal and rusting iron balls is decay. The definitions of “decay” used by two judges in two different cases were from the Merriam-Webster Dictionary which is as follows:

“Decay –

  1. 1:  to decline from a sound or prosperous condition
  2. 2:  to decrease usually gradually in size, quantity, activity, or force
  3. 3:  to fall into ruin
  4. 4:  to decline in health, strength, or vigor
  5. 5:  to undergo decomposition”

Judges have ruled that if the bowing and earth movement was a result of the hidden decay, the decay was the primary cause of failure, and that the bowing and earth movement exclusion was not applicable because it was the result of the hidden decay and not the primary cause of the foundation failure. As a forensic expert reviewing the condition of a failed or failing cinder aggregate masonry foundation or exterior bearing wall, one should consider, look for and sound for, signs of hidden decay and deterioration within the structure of the masonry units, and, with the client or their attorney, review the language of the policy to ascertain if it allows for hidden decay, which may be construed by the courts to be a legitimate claim and provide funds to cover the costs of the wall repair.


Although catastrophic failures may occur in foundations using cinder aggregate masonry units, it is possible for the forensic engineer or architect to investigate, detect, and propose a repair of the condition before such failures occur. Engineering judgment, based on experience, will determine if the wall should be repaired or replaced. It is also necessary to properly present such claims to insurance companies and/or the courts in a way which will allow a building owner to realize the full benefit of the “decay” provision of their insurance policy.