Lightweight insulating structural concrete

A lightweight insulative, structural concrete is produced from a cement mix containing lightweight aggregates having precisely defined physical and chemical properties. These aggregates are very low in density and very high in amorphous SiO.sub.2 content. The concrete product produced using the mix of this invention has a unique combination of high structural strength (up to 3400 psi, 28-day compressive), low density (50 to 110 pounds per cubic foot) and high thermal resistance (R) values (up to 3.16/inch). Such a product is just as strong, one half the weight, and 36 times more insulative than ordinary hard rock concrete.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to concrete products, and more particularly to a 
structural insulative lightweight concrete. 
2. Description of the Prior Art 
Lightweight concrete compositions per se are known in the art. Frequently 
this type of product is produced by formulating a concrete mix with 
lightweight aggregates. Among the lightweight aggregate materials which 
have been employed in the prior art to produce lightweight concretes are 
expanded shale, pumice, volcanic tuffs, sintered diatomite, blast furnace 
slag, sintered flyash, perlite, and vermiculite. 
The lightweight concretes of the prior art employing these aggregate 
materials, however, have not achieved both high strength and high thermal 
resistance. In general, those materials possessing suitable structural 
strength (i.e., over 1000 psi code or 2500 psi design) have low thermal 
resistance (R) values, i.e., in the range of less than 1. (Hard rock 
concrete has an R value of 0.08.) Those lightweight concretes possessing 
higher R values such as perlite or vermiculite-containing materials (R=1 
to 2) do not possess sufficient strength for structural use. 
Applicant is aware of no lightweight concrete which has sufficient strength 
for structural applications and yet has a high enough thermal resistance 
to be a significantly insulating material. Thus, at a time when energy 
resources are becoming increasingly scarce, a structural lightweight 
concrete that is also significant in its insulative properties would be a 
welcome advance. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a low cost 
building material which will result in significant energy savings through 
higher thermal insulative capacity. 
It is also an object of the present invention to provide a novel building 
material which will significantly lower the cost of construction in terms 
of both raw materials and labor costs. 
It is another object of the present invention to provide a versatile 
building material which can be used in place of composite structures 
containing as many as 3 or 4 separate building materials. 
It is also an object of the present invention to provide a concrete product 
which has approximately one-half the density of ordinary structural 
concrete, which has a thermal insulative capacity 36 times that of 
ordinary structural concrete and which has nearly the compressive strength 
of ordinary structural concrete. 
According to this and other objects, the present invention provides a 
concrete mix for making structural insulative lightweight concrete which 
mix comprises Portland cement and a lightweight fine aggregate having an 
amorphous SiO.sub.2 content of at least about 50% by weight and a density 
of less than about 60 pounds per cubic foot. 
The present invention also provides a hardened lightweight insulative 
cementitious product having sufficient strength for structural 
application, a thermal resistance (R) value of at least about 2.5/inch and 
a density of less than about 110 pounds per cubic foot, this cementitious 
product comprising lightweight aggregates of low density and high 
amorphous SiO.sub.2 content bonded together with a hydrated Portland 
cement. 
The present invention also relates to a method for making a precast 
insulative lightweight cementitious product having sufficient strength for 
structural applications, a thermal resistance (R) value of at least about 
2.5/inch and a density of less than about 110 pounds per cubic foot, this 
method comprising the steps of (a) providing a concrete mix comprising 
Portland cement and lightweight aggregates having a low density and high 
amorphous SiO.sub.2 content; (b) adding sufficient water to the mix to 
form a castable concrete mixture; (c) pouring the concrete mixture into a 
mold of predetermined size and configuration; (d) drawing a vacuum on the 
concrete mixture in the mold to remove excess free water; (e) curing the 
concrete mixture in the mold for a period of time sufficient to impart 
handling strength to the product; and (f) drying the cured concrete 
mixture at a temperature and for a time sufficient to remove substantially 
all of the free water from the product. 
DESCRIPTION OF THE INVENTION 
The concrete building product of the present invention possesses a unique 
combination of structural and insulative properties: 
(1) Density: approximately half of that of conventional hard rock concrete; 
(2) Compressive strength: approximately equal to that of conventional hard 
rock concrete; 
(3) Modulus of elasticity: approximately one third that of concrete or 
approximately the same as wood; 
(4) Resistance to rapid freezing and thawing: approximately four times 
better than regular concrete; 
(5) Thermal transmittance: 
Thermal Conductivity--k Factor=0.316 BTU-inch/Hr-Ft.sup.2 -.degree.F. which 
corresponds to a thermal resistance (R) value of 3.16 per inch. This is 
approximately 36 times the resistance of conventional concrete. 
As an example of the potential savings in both material and labor costs, 
consider an exterior wall which must meet an R-30 insulation requirement. 
Using conventional building materials an R-30 rated wall would have an 
overall thickness of 18 inches and consist of an internal course of cinder 
blocks, a layer of rock wool or fiberglass insulation and an exterior 
course of brick. The cost for this type of construction is about $9.50 per 
square foot. Using the concrete product of the present invention, however, 
an R-30 rated wall structure can be achieved by simply employing a 10 inch 
thick unitary structure. Employing the concrete as precast panels results 
in a cost of $4.50 per square foot. 
The unusual combination of high strength, low density and high thermal 
resistance is achieved in the concrete product of the present invention by 
employing lightweight aggregates having particular physical and chemical 
properties. Broadly these aggregates are materials comprising a high 
percentage of amorphous SiO.sub.2 and containing an extremely large 
percentage of air-containing porosity, i.e., very low density. 
While not wishing to be bound by any particular theory applicant believes 
that the surprisingly high values of thermal resistance are achieved by a 
minute "thermopane" effect as a result of the aggregate structure which 
approximates numerous layers of glass separated by air spaces. 
One lightweight aggregate employed according to the present invention is a 
finely divided material having an amorphous silica content of at least 50% 
by weight and a density of less than 60 pounds per cubic foot. In this 
fine aggregate the preferred range of amorphous SiO.sub.2 content is at 
least 75% by weight. The closer to pure glass (100% amorphous SiO.sub.2) 
the better the material will be. 
The density of this fine siliceous aggregate should be less than about 60 
pounds per cubic foot. Densities much over this value result in reduced 
insulation properties. At the other end of the preferred density range is 
a practical limit of about 20 pounds per cubic foot. If the density of the 
fine lightweight aggregate is much below 20 pounds per cubic foot the 
strength of the resulting lightweight concrete is adversely affected. More 
preferred are fine aggregate densities in the range of about 30 to 50 
pounds per cubic foot. The most preferred density is about 40 pounds per 
cubic foot. 
The low density of the fine aggregates employed according to the present 
invention results from a highly porous structure which contains from about 
60 to about 85% void space. This type of material can be found in 
naturally occurring materials of volcanic origin, such as expanded 
volcanic tuffs or in other natural or man made materials. 
The fine aggregate of the present invention preferably should be employed 
in a particle size of from about 0.001 to 1.0 mm. Preferred is a fine 
aggregate particle size range of from about 0.001 to 0.2 mm. Most 
preferred are fine aggregate particle sizes in the range of about 0.005 to 
0.100 mm. It is believed that the smaller particle size as contemplated by 
the present invention positively contributes to the structural strength of 
the resulting concrete products. 
One fine aggregate material suitable for the practice of the present 
invention will now be described. This material is an expanded volcanic 
tuff which is used in its naturally occurring finely divided form as found 
in deposits in central Utah. This material has a density of about 39 
pounds per cubic foot and a relatively uniform particle size of 
approximately 0.005 to 0.100 mm. This fine aggregate material has a 
porosity of about 65 to 75% void space and the following composition (by 
weight): 
______________________________________ 
SiO.sub.2 (amorphous) 
79.9% 
Fe.sub.2 O.sub.3 4.8% 
(CaAl.sub.2 Si.sub.7).sub.18 . 6H.sub.2 O 
3.7% 
SiO.sub.2 (crystalline) 
3.1% 
KAlSi.sub.3 O.sub.8 
2.1% 
TiO.sub.2 2.0% 
BaSO.sub.4 1.4% 
______________________________________ 
The present invention also contemplates the use of a coarse aggregate 
having a high amorphous SiO.sub.2 content, generally of at least 50% and 
preferably at least 90% by weight of the aggregate. In general, the coarse 
aggregate should be in the particle size range of from about 1/8 inch up 
to about 2 inches. The preferred particle size range is from about 3/16 
inch to about 3/4 inch with most preferred aggregates being approximately 
3/8 inch in size. 
The coarse aggregates of the present invention should have a density of 
less than 70 pounds per cubic foot. Preferred are densities in the range 
of about 35 to 70 pounds per cubic foot with most preferred density range 
falling from about 40 to 50 pounds per cubic foot. 
As in the case of the fine aggregate described above, materials of a 
volcanic origin are the most prevalent supply of materials having the 
requisite characteristics. A particularly well suited material is a 
crushed volcanic rock mined in central Utah. This rock has a density of 
about 46 pounds per cubic foot when crushed to the 3/16 to 3/4 inch size 
range and has the following composition (by weight): 
______________________________________ 
SiO.sub.2 (amorphous) 
94.8% 
CaCO.sub.3 3.1% 
MgCa (SiO.sub.3).sub.2 
1.5% 
Fe.sub.2 O.sub.3 0.5% 
______________________________________ 
Another component of the concrete mix of the present invention is Portland 
cement. Any of the conventional Portland cement types, i.e., I-V or 
entrained air varieties thereof, can be employed according to the present 
invention. In addition, conventional concrete additives or modifiers such 
as hardeners, accelerators or retarders can be added to the cement or 
concrete mixture of the present invention with little adverse effect on 
the unique combination of properties achieved. 
The proportions of the major ingredients in the concrete mix of the present 
invention can be varied according to the intended use of the concrete 
material. In general, the lower the proportion of cement the lower the 
overall strength of the concrete. For most applications the cement will 
comprise at least about 20% by weight of the mix. However, lower 
proportions may be used if strength requirements permit. The upper end of 
the cement content range of the mix is dictated by economic factors. In 
general, amounts much greater than 55% by weight become economically 
unfeasible; however, for a particular application higher cement 
proportions may, of course, be employed. A typical cement mix formulation 
for use in precast panel manufacturing comprises about 35% by weight of 
Portland cement. 
The remainder of the cement mix formulation consists of the lightweight 
aggregates. The percentage of these materials may vary broadly according 
to the intended applications of the final product. Thus, for some 
applications, a product consisting entirely of fine aggregate and Portland 
cement is contemplated by the present invention. A minimum amount of fine 
aggregate in the range of about 10% by weight, however, should be employed 
in order to achieve suitable strength and insulating characteristics. In 
general, the fine aggregate comprises from about 10% to about 85% by 
weight of the mix. Preferred are fine aggregate additions in the range of 
from about 20% to about 35% by weight. The coarse aggregate can comprise 
from about 0 to about 70% by weight of the mix with the preferred amounts 
being in the range of from about 30% to about 50% by weight. A typical 
cement mix formulation according to the present invention which is 
suitable for precast panel applications comprises 35% by weight Portland 
cement, 26% by weight fine aggregate and 39% by weight coarse aggregate. 
Having described the ingredients of the dry cement mix, the process for 
producing a hardened cementitious product will now be described. 
In the first step of the process of the present invention the 
above-identified dry ingredients are mixed with sufficient water to form a 
castable cementitious mixture. Generally, because of the high porosity of 
the low density aggregates, more water than normally employed will be 
required. Typically, water requirements of up to about 80 gallons per 
square yard of product may be employed. This is approximately twice as 
much water as required in hard rock concrete mixes. 
In the next step of the process of the present invention the 
water-containing concrete mix is agitated gently for a brief period of 
time for example with a paddle mixer or corkscrew mixer to render the 
mixture homogeneous. Care should be taken to avoid violent physical 
agitation to prevent attrition of the coarse aggregate into more dense 
fine products. 
The agitated mixture is then poured into a suitable casting form. The mold 
can be of any conventional size and configuration such as those used to 
produce precast panels for tilt-up construction. A rubber form liner 
contained in the bottom of the casting form is conventionally employed to 
impart particular surface ornamentation to the precast panel. After the 
mixture has been poured into the casting form a small amount of vibration 
or shaking can be employed to remove air pockets. Care should be taken, 
however, to avoid excessive vibration as the aggregates employed according 
to the present invention are lighter than the water and tend to rise to 
the surface. Accordingly, vibration for periods of up to about 10 seconds 
have been found suitable. 
Next, the mold containing the concrete mixture is subjected to a vacuum 
dewatering step to remove excess free water. It has been found that the 
concrete product according to the present invention does not exhibit the 
marked improvement in thermal resistance values unless substantially all 
the free water is removed from the concrete product. The vaccum dewatering 
step assists in this ultimate water removal and is further advantageous in 
that it promotes the rapid attainment of strength in the cast product. 
Accordingly, a panel which has been vacuum dewatered can be removed from 
the casting form without damage within 24 hours whereas a panel which has 
not been vaccum dewatered does not have sufficient strength for removal 
until at least 2 or 3 days after casting. The vaccum dewatering step is 
preferably accomplished by placing a vacuum jig over the top of the 
casting form and drawing a vacuum, e.g., -0.5 atmosphere on the form. 
The next step in the process of the present invention comprises the air 
cure or hydration of the cast concrete mixture for a period of time 
sufficient to impart handling strength to the product. This air cure may 
take place at temperatures which can range from ambient temperature up to 
about 150.degree. F. Preferred are curing temperatures of about 
100.degree. F. The cast product should be left to air cure for at least 
about 24 hours before it is dried according to the step described below. 
It is preferred, however, to let the product cure at temperatures up to 
about 100.degree. F. for several days before subjecting the panel to the 
higher temperatures of the drying step. It appears that the longer the 
product is allowed to air cure the less effect the heat of drying has on 
the ultimate strength characteristics of the product. 
After a suitable air cure period the precast product is dried at a 
temperature and for a time sufficient to remove substantially all of the 
free water. Suitable temperatures are from at least about 212.degree. F. 
Preferably, the maximum drying temperature is about 300.degree. F., but 
higher temperatures may be employed as long as no thermal degradation of 
the product occurs. The preferred drying temperature is about 225.degree. 
F. This drying step is essential in order to achieve a product which has 
ultimate thermal resistance of the high values discussed above. 
Preferably, the heating process can be continued for a time period 
sufficient to drive off enough water to achieve a product in the desired 
density range, i.e., about 50 to 110 pounds per cubic foot. Typically, 
heating times of about 8 to 10 hours at 225.degree. F. result in a 
suitable product. As will be readily apparent, the higher temperatures 
will require less time and vice versa. 
The product of the above-described process is a structural lightweight 
insulative concrete which has a density in the range of 50 to 110 pounds 
per cubic foot, a thermal resistance value of at least 2.5/inch, and a 
28-day compressive strength of at least 1000 psi. The preferred product of 
this invention has an R value of at least 3.0/inch, a density of about 60 
to about 90 pounds per cubic foot, and a 28-day cure compressive strength 
of at least 2500 psi. 
In order to insure the maintenance of this high thermal resistance value 
after on-site installation of the precast panel, it is advisable to coat 
at least any weather-facing surfaces of this concrete panel with a 
conventional sealant to prevent moisture regain. The sealant application 
may be by spray, dipping or other known coating and impregnating means. 
Any of the conventionally employed concrete sealants such as Thompson's 
Water Seal sold by E. A. Thompson Co., Inc. may be employed. 
The coated product produced by the above-described invention may be used in 
any number of structural applications such as in the formation of 
structural members, i.e., beams and roof decks, or as tilt-up panels for 
exterior wall construction. When used in this manner, the product of the 
present invention provides a low-cost alternative for composite building 
structures comprising a plurality of components which result in marked 
savings of both material and labor.

The following specific example is intended to illustrate more fully the 
nature of the present invention without acting as a limitation on its 
scope. 
EXAMPLE 1 
A dry concrete mix was prepared by combining 211.5 pounds of type I 
Portland cement with 230 pounds of a coarse aggregate and 156 pounds of a 
fine aggregate. The coarse aggregate is a 3/8 inch uniform product which 
was crushed and sieved from a volcanic deposit located in central Utah. 
This volcanic rock has an approximate analysis as follows (by weight): 
______________________________________ 
SiO.sub.2 (amorphous) 
94.8% 
CaCO.sub.3 3.1% 
MgCa (SiO.sub.3).sub.2 
1.5% 
Fe.sub.2 O.sub.3 0.5% 
______________________________________ 
This particular volcanic material has a density of 46 pounds per cubic 
foot. 
The fine aggregate is a powder-like naturally occurring expanded volcanic 
tuff having a particle size in the range of about 0.005 to 0.100 mm. This 
aggregate material, which is also found in central Utah, has a bulk 
density of about 39 pounds per cubic foot and has the following 
approximate analysis (by weight): 
______________________________________ 
SiO.sub.2 (amorphous) 
79.9% 
Fe.sub.2 O.sub.3 4.8% 
(CaAl.sub.2 Si.sub.7).sub.18 . 6H.sub.2 O 
3.7% 
SiO.sub.2 (crystalline) 
3.1% 
KAlSi.sub.3 O.sub.8 
2.1% 
TiO.sub.2 2.0% 
BaSO.sub.4 1.4% 
______________________________________ 
Water was added to the mixture obtained above was added water at the rate 
of about 80 gallons per yard of finished product and the resulting slurry 
was briefly agitated in a paddle mixer. The mixed slurry was then poured 
into an 8'.times.16'.times.10" casting form having a rubber form liner 
with a fractured fin design on the surface. After about 10 seconds of 
vibration to remove air pockets, the concrete was vacuum dewatered under 
about -0.5 atmosphere and the dewatered product was set aside to cure for 
24 hours at about 100.degree. F. The cured product was then stripped from 
the mold and dried in a kiln for about 8 hours at 225.degree. F. The 
resulting concrete product has a bulk density of about 72 pounds per cubic 
foot, a thermal resistance (R) value of 3.16 per inch, and a 28-day air 
cure compressive strength of about 3400 psi. 
While certain specific embodiments of the invention have been described 
with particularity herein, it should be recognized that various 
modifications thereof will occur to those skilled in the art. Therefore, 
the scope of the invention is to be limited solely by the scope of the 
claims appended hereto.