Protective coating for reinforced concrete

A protective coating for application to steel reinforced concrete structures, comprising a liquid composition of elastomeric polymeric material having electrically conductive particulate matter evenly dispersed therein. A cathodic protection system embodying the coating comprises a moisture impervious membrane of elastomeric polymeric material having electrically conductive particulate material evenly dispersed therein, an electrically conductive grid embedded in the membrane or contiguous thereto, and direct current means connecting the grid electrically with the reinforcing bars.

FIELD OF THE INVENTION 
This invention relates to a protective coating for steel reinforced 
concrete. 
BACKGROUND OF THE INVENTION 
All suspended concrete decks in parking facilities contain reinforcing 
steel. When steel encounters both oxygen and water, rusting can occur. The 
process may be referred to as galvanic action, electrochemical corrosion, 
or ionic activity. If sodium chloride (de-icing salt) is used on roadways, 
it is carried into the garage on the underside of vehicles, drops to the 
deck surface, and eventually permeates through the concrete to the 
reinforcing steel (rebars), acting as an electrolyte in the presence of 
moisture and oxygen. This process greatly accelerates the rusting, which 
in turn reduces the steel to scale. This transformation of the steel 
increases its volume and produces extreme stress within the concrete slab, 
initially creating cracks and eventually spalled areas or potholes. The 
pressure created by corroding rebars can reach several thousand pounds per 
square inch. 
Since no present system can eliminate oxygen from the concrete slab, it is 
customary to employ a waterproofing system using an impervious membrane 
and/or coating with a sealer, which stops the penetration of both moisture 
and chlorides into the slab. Another method of inhibiting corrosion is to 
apply a constant negative electric voltage to the reinforcing steel to 
reverse its anodic property. 
An example of such a coating and voltage application presently in use is 
disclosed in U.S. Pat. No. 4,506,485 issued Mar. 26, 1985 to J. Apostolos 
which shows a coating of molten metal and a direct current circuit joining 
the coating and the embedded rebar. The Apostolos system suffers from the 
disadvantage that the coating provides a sacrificial anode and ablates 
over a period of time. Also the coating of Apostolos is relatively rigid 
and would be ineffective to bridge larger cracks often occurring in 
concrete. 
The present invention provides a moisture impervious membrane or coating of 
improved conductivity which when given an electropositive charge reverses 
the anodic property of the reinforcing steel and thus all the reinforcing 
steel network becomes cathodic whereby no corrosion occurs. 
Known waterproofing coating systems also suffer from the disadvantage that 
they are relatively inflexible and inelastic. Since hairline cracks in 
concrete can develop which are one-eighth of an inch or more in width, 
such coatings will fracture when stressed at the site of the crack. 
Shrinkage cracking of the coating may also occur. 
The present invention provides a moisture impervious membrane or coating 
with inherent flexibility to bridge cracks without itself being fractured 
under normal extension under stress. 
SUMMARY OF THE INVENTION 
Essentially the invention consists of a protective coating for steel 
reinforced concrete structures, comprising a liquid composition of 
elastomeric polymeric material having electrically conductive particulate 
matter evenly dispersed therein and curable at ambient temperature to form 
a moisture impervious conductive membrane. 
In another aspect the invention consists of a concrete structure containing 
steel reinforcing bars, a cathodic protection system comprising: a 
moisture impervious membrane of elastomeric polymeric material applied to 
the structure, the membrane having electrically conductive particulate 
matter evenly dispersed therein; an electrically conductive grid embedded 
in the membrane or contiguous thereto; and direct current means connecting 
the grid electrically with the reinforcing bars.

DESCRIPTION OF PREFERRED EMBODIMENT 
As seen in FIG. 1 of the drawings an impermeable membrane or coating 10 of 
the invention is applied to a concrete structure 12 containing reinforcing 
bars 14. Coating 10 has embedded in it, or contiguous to it, a primary 
anode of conductive wire material which is formed in a grid 16 connected 
electrically by a conductor 18 to a source 20 of direct current the source 
also being connected by a conductor 22 to reinforcing bars 14. A low 
electrical current flows between grid 16 and rebars 14 substantially equal 
and opposite to the current which would result from an electrical 
connection between the rebars and the grid, which inhibits electrolytic 
action on the rebars. A wear course 24 may be laid over coating 10. 
The composition of coating 10 allows the current to be uniformly 
distributed throughout the coating. Examples of suitable formulations for 
forming the coatings of the invention are set forth below. 
______________________________________ 
I CHLOROPRENE MEMBRANES 
______________________________________ 
EXAMPLE I 
INGREDIENT PERCENT BY WEIGHT 
chloroprene latex solids 
28.8 
conductive carbon black 
8.7 
clay 1.35 
thickening agent (polyacrylate) 
0.45 
water 60.3 
anti-oxidant 0.4 
PROPERTIES 
electrical resistivity 
20-100 ohm-cm 
tensile strength 870 psi 
elongation at 72.degree. F. 
500% 
elongation at 0.degree. F. 
150% 
EXAMPLE II 
INGREDIENT PERCENT BY WEIGHT 
chloroprene latex solids 
31.0 
nickel powder (particulate) 
33.5 
water 33.39 
thickening agent 0.6 
anti-oxidant 0.51 
wetting agent 0.2 
PROPERTIES 
resistivity 1-15 ohm-cm 
tensile strength 800 psi 
elongation at 72.degree. F. 
500% 
elongation at 0.degree. F. 
300% 
EXAMPLE III 
INGREDIENT PERCENT BY WEIGHT 
chloroprene latex solids 
38.03 
conductive carbon black 
3.7 
conductive carbon fibers 
8.1 
water 49.55 
anti-oxidant 0.42 
wetting agent 0.2 
PROPERTIES 
resistivity 10-20 ohm-cm 
tensile strength 600 psi 
elongation at 72.degree. F. 
450% 
EXAMPLE IV 
INGREDIENT PERCENT BY WEIGHT 
chloroprene latex solids 
39.0 
nickel coated graphite fibers 
0.36 
water 45.74 
clay 14.9 
PROPERTIES 
resistivity 20-80 ohm-cm 
tensile strength 840 psi 
elongation at 72.degree. F. 
550% 
EXAMPLE V 
INGREDIENT PERCENT BY WEIGHT 
chloroprene latex solids 
30 
clay 8.7 
conductive carbon black 
5.45 
nickel coated graphite 
0.3 
water 55 
wetting agent 0.05 
PROPERTIES 
resistivity 40-50 ohm-cm 
tensile strength 800 psi 
elongation at 72.degree. F. 
450% 
EXAMPLE VI 
INGREDIENT PERCENT BY WEIGHT 
chloroprene latex solids 
25.44 
clay 0.17 
nickel powder 4.23 
other inorganic filler 
1.36 
conductive carbon black 
10.6 
water 58.0 
wetting agent 0.2 
PROPERTIES 
resistivity 16-30 ohm-cm 
tensile strength 500 psi 
elongation at 72.degree. F. 
400% 
______________________________________ 
II URETHANE MEMBRANES 
______________________________________ 
EXAMPLE I 
INGREDIENT PERCENT BY WEIGHT 
blocked urethane prepolymer 
46.91 
conductive nickel powder 
24.27 
nickel coated graphite fibers 
0.21 
xylene 21.86 
curing agent 3.88 
anti-oxidant 1.00 
other 1.87 
PROPERTIES 
resistivity 0.5 ohm-cm 
tensile strength 420 psi 
elongation at 72.degree. F. 
240% 
EXAMPLE II 
INGREDIENT PERCENT BY WEIGHT 
blocked urethane prepolymer 
58.91 
conductive nickel powder 
6.09 
nickel coated graphite fibers 
0.15 
conductive carbon fibers 
5.69 
conductivity enhancer 
0.10 
xylene 19.95 
curing agent 4.87 
anti-oxidant 1.00 
other 2.36 
PROPERTIES 
resistivity 0.5-4 ohm-cm 
tensile strength 505 psi 
elongation at 72.degree. F. 
835% 
EXAMPLE III 
INGREDIENT PERCENT BY WEIGHT 
blocked urethane prepolymer 
60.21 
conductive nickel powder 
7.79 
nickel coated graphite fibers 
0.11 
conductive carbon fibers 
3.11 
xylene 20.39 
curing agent 4.98 
anti-oxidant 1.00 
other 2.41 
PROPERTIES 
resistivity 2-10 ohm-cm 
tensile strength 355 psi 
elongation at 72.degree. F. 
1000+% 
EXAMPLE IV 
INGREDIENT PERCENT BY WEIGHT 
blocked urethane prepolymer 
56.78 
conductive carbon black 
7.84 
nickel coated graphite fibers 
0.44 
wetting agent 0.29 
xylene 26.67 
curing agent 4.7 
anti-oxidant 1.00 
other 2.28 
PROPERTIES 
resistivity 2-5 ohm-cm 
tensile strength 245 psi 
elongation at 72.degree. F. 
195% 
______________________________________ 
A suitable chloroprene polymer is sold by Dupont de Nemours Inc. under the 
trade mark NEOPRENE. A suitable blocked urethane prepolymer is sold by 
Bayer AG under the trade mark DESMOCAP. 
The composition for the coatings of the invention are liquid when 
formulated and are curable (that is, dryable) at ambient temperatures 
(that is, at temperatures in the range about 40.degree.-120.degree. F.) in 
order to form a moisture impervious conductive membrane. 
The thickness of membrane 10 depends upon the size of cracks in concrete 12 
to be bridged. Generally membrane thickness of 0.020 inch to 0.030 inch 
would be required. In case of excessive roughness a parge coating on the 
concrete may be required to prepare the surface to receive membrane 10. 
The wire of grid 16 may be encapsulated in (i.e. embedded within) membrane 
10 or the membrane may be laid as a secondary anode on top of the grid 
which would then be contiguous with the membrane. A suitable material for 
grid 16 is platinum (over niobium copper wire) which may be laid in 
strands about twenty feet apart, the spacing and gauge of the wire being 
dependent on the specific requirements of the system. The composition of 
coating 10 allows the current to be distributed evenly throughout the 
coating and reduces its resistivity which may be as low as 10 ohm-cm. 
By providing uniform distribution of electric current localized 
disintegration, and the consequent destruction of the adjacent concrete, 
is avoided where chloride ions already exist in the concrete from salt 
penetration before the membrane is applied. Consequently the membrane 
performs the dual function of keeping water and salt from penetrating the 
concrete and inhibits the corrosive effects of chloride ions already 
present in the concrete. 
A separate wear course such as a coal tar epoxy (sand) may be applied over 
membrane 10 in known manner. 
If desired, membrane 10 may be prefabricated in sheets or sections and 
applied to the concrete substrate using an auxiliary binding agent such as 
a conductive primer adhesive. 
The invention provides a moisture impervious membrane which is resistant to 
chloride ion penetration and neutralizes chloride ions already present in 
the concrete, has a high flexibility and elongation characteristics, and 
has low electrical resistivity. Additionally, the membrane has the ability 
to bond to concrete substrates and to have wear courses bond to it. 
The term "impervious" means highly resistant to moisture transmission.