A rubble stone, or rubble trench foundation is an ancient construction practice, popularized in the early 20th century by American architect, Frank Lloyd Wright (June 8, 1867 – April 9, 1959.) Loose stone or rubble reduce the volume of concrete and promote drainage, however this practice is not suitable for some soil conditions, or for excavated basement areas which will not support the rubble from both side of the foundation wall. Other probelms include poor, or unsuitable mixtures of cement including lime mortar, the use of un-washed, native sand and other aggregates, and the use of salts to promote curing in cold weather.
Masonry mortar and concrete contain the same basic ingredients. On the other hand, lime mortar has low compressive strength and high water retention characteristics.
I believe the "oatmeal cookie" phenomenon that we see in some of the 60+ year-old rubble-concrete foundations in east-central Ontario may be a result of "fines" or clay in the aggregate, or use of lime mortar, or possibly an overdose of fly ash or other fillers. Cold winter conditions could also be a contributing factor, with the lime mortars not generating enough heat to complete the curing. The presence of iron pyrites or alkali, or the use of slag and other deleterious substances seem less likely in this locale, however can be identified in other parts of the country. In many cases cement, or even lime mortar was poured with a rubble mix, into wooden forms to emulate at poured concrete foundation.
Constant wetting due to poor back-fill, poor grading and drainage and the absence of eves trough accelerate the deterioration by keeping the cement wet. Insulation and interior finishes which impede the drying of these walls may also accelerate the deterioration. I am not familiar with any commonly available bonding agents or polymers which will stick to this kind of "dirt."
It can be very costly and disruptive to remediate this type of foundation, and purchase of a home with a visibly soft rubble foundation should be approached with caution.
About the strength of concrete
The strength is more difficult to assess when supplementary cementitious materials are used. For example, with fly ash, strength gain will usually be slower over a much longer period because of the slower reaction of the fly ash--so 60-day strength gains are used to gauge what was determined in 28 days. When ground granulated blast-furnace slag is used, temperature plays an influencing role in strength gain because of its effect on the speed of the chemical reactions that provide the strength. During warm summer temperatures, 3-day strengths may be equivalent to 7-day strengths, and 14-day strengths equal to or exceed what would be expected for 28-day strengths. At winter temperatures, 7-day strengths may be equivalent to 3-day strengths because of the sluggish chemical reactions.
Although Type F fly ashes slow strength development, they also do many good things, including lowering concrete costs, increasing resistance to water penetration (decreased permeability), increasing resistance to aggressive chemicals, and slowing and reducing heat development in mass concrete. Fly ash is a "patriotic" material because it extends the use of portland cement, which consumes a lot of energy in the making, and making use of this major waste product improves the environment.
Ground granulated blast-furnace slag is also a patriotic material for the same reasons as fly ash. Nowadays, concrete mixtures are often designed with two (ternary) or three (tertiary) supplementary cementitious materials. How patriotic can you get?
We recall a concrete foundation placed decades ago during low winter temperatures using a 25 percent fly ash replacement of the portland cement. This same mixture had worked well at summer temperatures, but when placed at winter temperatures, the concrete crumbled and fell away when forms were removed after a week. Why? A straight portland cement mixture would have developed enough heat during hydration to warm the concrete sufficiently to allow hydration to continue. But the portland cement diluted by the fly ash greatly curtailed heat development. A lesson learned the hard way shouldn't let you forget--but it still happens.
Although 28-day strength requirements are usually the controlling specification, unexpected things can sometimes happen. For example, unacceptable 28-day strengths can result because of cold days, excessive air contents, badly handled test specimens, poor testing procedures, and "accidental" over-dosages of admixtures. In many cases, petrographic studies and chemical analyses permit reasonable prognostications of future strength gain in cases of low strength. We recall a second-floor cast-in-place, thickened edge beam that someone called about because the concrete failed to harden. We found an overdose of a water reducer, which acts as a retarder when used in excessive amounts--more isn't always better. The soft concrete was well put together except for its nonstrength. We told the contractor that if he kept the concrete curing, it would not only reach the intended strength but probably would be even stronger than it might have been under more normal circumstances. He did and it was. That edge beam now serves at the entrance to a major public building in Washington, D.C.
Strength "requirements," of course, is a subject to itself. House foundation concrete, for example, may have a code requirement of 2000 psi (at 28 days). But the foundation will never carry a load greater than a few hundred psi. That looks like a significant strength safety factor; however, there are other things to consider so that 2000 psi may be adequate in strength but inadequate when environmental requirements enter into the picture. In fact, requirements other than strength usually do dictate.
Gil Strachan is a professional home inspector, representing Electrospec Home Inspection Services in east-central Ontario since 1994.