DESIGN OF REINFORCED CEMENT-BASED PROTECTIVE COATINGS
BENNISON, B.Sc., C.Eng., M.I.C.E., M.I.Struct.E., F.F.B.
paper describes the design of cement-based coatings which have a
high diffusion resistance to gases such as Carbon Dioxide and Oxygen,
as well as Chloride Ions.
are also impermeable to water molecules even under a 10 bar pressure,
although they will allow the diffusion of vapour. By reinforcing
with a variety of fibres and meshes, tough protective membranes
can be formed with all the similarities to concrete.
incorporation of Migratory Corrosion Inhibitors also enhances their
abilities to retard the corrosion mechanisms of the steel reinforcement
by providing a protective film having diffused through the pore
system in the vapour phase.
USED IN CEMENTITIOUS COATINGS
- Cement (a) Ordinary
a. Rapid Hardening Portland
b. Ultra finely ground Portland
c. Calcium Sulphoaluminate
d. Sulphate Resisting Portland
- Resins (a) Polyester
- Polymers (a) Styrene
b. Modified Acrylic
- Pozzolans (a) Microsilica
a. Pulverised Fuel Ash
b. Ground Granulated Blast Furnace Slag
Corrosion Inhibitors (a) Amino Carboxylates
OF CEMENT BASED PRODUCTS
following factors will significantly affect the durability of cement
cement content and type.
degree and type of compaction.
quality and type of aggregates.
use of chemical admixtures.
use of polymers.
use of pozzolanic materials.
a cement matrix having a coherent pore system (porosity) to become
permeable, there must be some form of interconnecting system of
tubes or canals. Obviously, discrete sealed pores will not lead
to either air or water permeability (1) (see Fig. 1). Also
the size and width of pores have an influence on permeability, because
the narrower the pores, the higher must be the pressure to force
a liquid (generally water) through the pore system. It is important,
therefore, to distinguish between air pores, capillary pores and
air pores are usually artificially entrained into the matrix by
chemical admixtures in order to increase workability and enhance
resistance to the effects of frost. Large, irregular air pores are
caused by poor placing and compaction of concrete.
and gel pores are as a result of the hydration process of cement
- 0.5m m
fact, the porosity and permeability of the hardened cement paste
depends upon the water/cement ratio. There is, therefore, a relationship
between water/cement ratio and compressive strength (2), capillary
porosity and compressive strength (3), and permeability and water/cement
ratio (4) (see Figs. 2, 3 and 4).
is, however, important to note that, while strength and diffusion
are affected by porosity, they are not uniquely inter-related.
the nature of the hardened cement can be summarised as follows:-
after mixing the cement and water together, an agglomeration of
particles and water is formed. The water in the interconnecting
cavities is termed "capillary water". Until it has been completely
hydrated, Portland cement chemically binds water equivalent to approximately
a quarter of its weight. Therefore, the water loses approximately
a quarter of its volume. In addition to this chemically bound water,
Portland cement loosely binds approximately 15% of its weight as
its loose chemical bond, gel water is not able to react with unhydrated
cement and, in fact, evaporates in dry air or in an oven at 105°C.
It is this gel water and the additional water required to "lubricate"
the mix to form a workable material, that create more capillaries
as they become evaporable water. Therefore, the greater the water/cement
ratio, the greater the permeability (see Fig. 4).
hydration by-products of the cement/water reaction form a coherent,
homogeneous mass, the "cement gel". The cement gel is, however,
made up of approximately 25% by volume of finely distributed pores,
namely "gel pores". The total porosity (large capillary and, to
a lesser degree, gel pores) of the hardened cement paste, therefore
determines its strength (5) (see Fig. 5).
1 Illustration of Permeability and Porosity
2 Typical Relationship between Cube Strength between
3 Typical Relationship and Water/Cement Ratio and
CONSTITUENTS PROPERTIES AFFECTED
Tricalcium Silicate 55% Strength up to 28 days/setting
Dicalcium Silicate 20% Long term strength, i.e. after
Tricalcium Aluminate 10% Setting time/24 hours strength.
Tetracalcium Aluminoferrite 8% Very little effect.
Silicate Hydrates : Form the "gel" structure.
Hydroxide : Gives the cement paste its high alkalinity
and porosity can, therefore, be influenced in several ways, i.e.:-
The use of chemical admixtures
lower numerically the water/cement ratio and reduce the
amount of water in the mix;
- To entrain
small, discrete air pores to enhance resistance to freeze/thaw
cycles in vulnerable areas;
- To modify
the calcium silicate hydrate crystal formation during the hydration
induction phase and, therefore, the gel pore formation.
grain of cement will only hydrate when it is totally surrounded
by a membrane of water. If this membrane is ruptured by evaporation,
then the hydration process is adversely affected. The effects of
poor curing manifest themselves in durability of the concrete in
terms of possibly surface drying, causing plastic shrinkage cracking
and, also, unhydrated cement, which may cause further disruption
4 Relationship between Permeability and Water/Cement Ratio
for Mature Cement Pastes
5 Hydration of Cement
WHICH INFLUENCE POROSITY AND PERMEABILITY
is defined as the total volume fraction of hollow spaces (i.e. pores)
in a material. A fluid cannot pass through the material if the pores
is defined as the rate of flow of a fluid under a pressure differential
through a material with inter-connected pores.
* Microsilica * Fibres
Silicates * Long chain Polymers
Fuel Ash * Low water/cement ratio
* Polymers * Polymers
6 Influence of Microsilica on Porosity of Cement Pastes -
Total Pore Volume
7 Influence of Microsilica on Porosity of Cement Pastes -
Pore Size Distribution
8 Hydration of Portland Cement and Portland Cement
with Pozzolanic Materials
stated earlier, pozzolanic materials have been used for construction
since ancient times. Natural pozzolans, or in some cases calcined
earths, were blended with lime (calcium hydroxide) and water to
make strong cementitious materials (8).
pozzolan may be defined as a siliceous or siliceous and aluminous
material which, in itself, possesses little or no cementitious value
but which will, in finely divided form in the presence of moisture,
react chemically with calcium hydroxide at ordinary temperature
to form compounds possessing cementitious properties.
is a particularly interesting material, which can pore block (it
has a mean particle size of 0.1m m) in its own right but, more
importantly, by reacting with the dissolved calcium hydroxide liberated
by the cement hydration, it forms stable calcium silicate hydrates
(gel) which further reduce and refine the pore capillary system
(see Figs. 6 and 7). It is the by-product of the manufacture
of silicon and ferro silicon alloys. These processes require large
quantities of electrical energy for the furnaces and, therefore,
tend to be situated in countries with cheaper, hydro-generated power,
i.e. Norway, Iceland and Canada. High purity quartz is introduced
into the furnace and, at these very high temperatures (2000¡
C), vaporises to form silicon monoxide. This then oxidises above
the furnace and condenses to form microspheres of amorphous silica.
This almost pure (85-92%) silicon dioxide, in the form of a fume,
is then collected in bag filters.
fuel ash is collected in a different way to that of microsilica
and is a waste ash in the flue gases of coal burning power stations.
The flue gases enter very large concrete precipitators horizontally,
at a temperature of approximately 160¡ C. The hot gases then
pass across large, vertical steel plates, which have a high voltage
alternately switched across them. The charge attracts ash from the
gas and the gas exits the precipitator at the other side, at a temperature
of approximately 60¡ C. Once the charge has been switched off,
the ash falls into a holding hopper below the precipitator. This
is then usually air graded to give the acceptable particle sizes.
Pulverised Fuel Ash has a similar action in cement pastes to that
of microsilica, in that it produces stable calcium silicate and
aluminium hydrates (see Fig. 8).
materials produced from calcined china clays are more reactive than
the above (see Fig. 9).
materials are manufactured from selected kaolins which are refined
and calcined under conditions designed to give optimum pozzolanic
reactivity, and are subject to strict quality control procedures.
By contrast, as stated above, most other pozzolanic materials used
in concrete are by-products from other industrial processes, and
are well known to be highly variable in chemical composition.
of Pozzolanic Materials (Chappelle Test)
Ca(OH2) per g
china clays react rapidly with calcium hydroxide, converting portlandite
to stable cementitious compounds. At the same time they significantly
reduce the porosity and permeability of the cement hydrates and
the concentration of alkali metal ions (K+ and Na+)
in the pore water.
can be seen from the pore size distribution from mercury intrusion
porosimetry in Fig. 10, the total pore volume decreases significantly
when 10% of cement is replaced by a calcined china clay based on
10 Effect of Metakaolin on Pore Size Distribution of a Mortar
11 Physical Data for Cementitious Materials
Area (m² /kg)
Density (kg/m³ )
of the by-products of cement hydration is heat. This can micro-crack
the paste, forming continuities in the pore structure, thus increasing
permeability. By acting as randomly orientated tensile reinforcement,
the propagation of cracking is halted by stress re-distribution
along the fibres. They also influence durability, impact resistance
and tensile strengths of cement pastes. Fibres come in a variety
of different sizes and materials.
Common Materials for Fibres
water, over and above the chemically bound water in a cement mix,
migrates to become evaporable water. This moisture movement again
forms continuities in the pore system and increases permeability,
hence the relationship between water/cement and permeability (see
chain polymers can also control the movement of moisture by chemically
binding water molecules so that fewer are free to migrate.
when used in cement-bound materials, come in two forms:
dried polymer powders
dried polymers are dispersion which are dried to form a water-re-dispersible
powder which then acts in a similar way to a dispersion.
dispersions as used in a cementitious environment, are a series
of discrete "plastic" spheres in a solution of water.
The spheres can come in a variety of different degrees of "hardness"
and "stickiness", which impart different properties to
the modified cement paste. As these spheres can be very small, of
the order of 0.1m m in diameter, a stabilisation system is
required in the water suspension in order to stop the polymer particles
sticking together and forming large agglomerates. These usually
take the form of ionic surfactants or colloids. As stated earlier
in this paper, a grain of cement will only hydrate when it is totally
surrounded by a membrane of water. However, the water membrane now
contains polymer particles which gradually come together to form
a film on the surface of the hydrates as the water becomes chemically
bound, (it is generally accepted that it requires 10% polymer solids
by weight of cement to reach this state). Once above minimum film-forming
temperature, chemical bonding takes place between the particles
(coalescing) to form coherent thermoplastic films coating the hydrates.
Based upon this understanding, the mechanisms by which polymers
enhance the performance of cement pastes can be explained.
introduction of "sticky" plastic spheres makes the cement paste
more adhesive and cohesive.
soft but strong plastic films give a greater degree of elasticity,
but also act as reinforcement, giving increased flexural strength.
It should, however, be noted that they are low modulus films
and the implication that this has on the stress/strain relationship.
polymer rich surface has, essentially, a plastic coating which
is better at resisting abrasion than an unmodified cement paste.
polymer spheres are small and can therefore block pores and
capillaries and subsequently prevent water loss, reducing shrinkage.
plastic coating on the surface has a much greater resistance
to a variety of chemical attack than an unmodified cement paste.
similar mechanism to (d), capillary pores are blocked, reducing
OF POLYMERS USED IN CEMENT-BOUND MATERIALS
are many different generic types of polymer dispersions and spray
dried polymer powders now commercially available worldwide. However,
the historic generations were as follows:
Generation: Polyvinyl Acetate (P.V.A.)
Generation: Styrene Butadiene Rubber (S.B.R.)
Generation: Styrene Acrylic Copolymers
the polymer molecule is made up from more than one "building
block" (monomer), then in the case of two monomers it becomes
a copolymer or, in the case of three, a terpolymer.
stated earlier, the polymer particles vary considerably in their
physical properties, for example a S.B.R. being a synthetic rubber
latex, is relatively soft and sticky, whereas a pure acrylic is
hard and not very sticky. Therefore, the films which are formed
also vary in their characteristics, necessitating the performance
requirements to be matched for individual applications.
cement blended with one of the following:-
Granulated Blast Furnace Slag
dramatically reduce the permeability and, hence, chloride ion diffusion
group, when combined with the benefits of polymer modification,
can offer good engineering solutions.
RESIN MODIFIED CEMENT-BASED MATERIALS
this discovery in 1911, epoxides have enjoyed considerable success
in a wide variety of applications, including surface coatings and
main attributes are toughness, low shrinkage on cure, high adhesion
and good chemical resistance.
term "epoxy" or "epoxide" is applied to any
resin containing a ring consisting of an oxygen atom attached to
two carbon atoms on an adjacent position.
first and still the most important commercial epoxy resins are reaction
products of Bisphenol A and Epichlorohydrin. In order to thermo-set
the resin, a reaction with a curing agent is required which transforms
them from low molecular weight materials to a highly cross-linked
network. This network is composed of segments involving both epoxide
and cross-linking agents.
work has shown that this epoxy/curing agent adduct polymer when
residing in a cementitious matrix, takes the form of short rod-like
materials randomly interspersed within the hydrates. It is thought
that these strands of cured epoxy polymer, act as reinforcement
and help explain the increased strength properties of epoxy/cement
the technology described earlier, it is now possible to produce
cementitious coatings which have outstanding protective performance
characteristics, whilst maintaining all the desirable features of
being similar physically and chemically to base concrete and also,
of even greater relevance, being totally "green" products
as they are water-based.
can also be reinforced with a variety of meshes made to enhance
the tensile strain capacity.
the incorporation of MCIs existing in the vapour phase, sufficient
active constituents can migrate and penetrate into the substrate
and form protective films on the steel reinforcement.
MECHANICAL CHARACTERISTICS OF CEMENT-BOUND COATINGS
Strength at 20¡ C:
1 day 5-10N/mm²
7 days 30-40N/mm²
28 days 50-60N/mm²
Strength: 2N/mm² (concrete)
Permeability Coefficient: 1.43 x 10-16 m/sec
i.e. 2mm of coating º 6,000mm of typical concrete
Dioxide Gas Diffusion
Resistance Coefficient: m CO2 = 2,600,000
equivalent air thickness value R at 2mm
thickness (s) = m CO2 x s = 5,200m
on Engelfried Technique an effective
barrier to carbon dioxide is R ³ 50m.
Diffusion Coefficient: 4.42 x 10-5cm² s-1
Ion Diffusion: No steady state flux of chloride ions after
approximately 12_ years.
R.F.M., Illustration of Permeability and Porosity. Diffusion
within and into Concrete. Paper presented at 13th Annual Convention
of the Institute of Concrete Technology, University of Loughborough,
25-27th March 1985. Slough Institute of Concrete Technology, 1985.
F.D., Concrete Mix Design, Applied Science Publishers Limited,
G.J., and Vol. 3, Proceedings of the 5th Helmuth, R.H., International
Symposium on the Chemistry of Cement, Tokyo 1968.
T.C., Copeland, Permeability of Portland Cement Paste ACI L.E.,
Hayes J.C., and Journal Proceedings, V.51. Mann, H.M.,
T.C., Chemistry of Cement Proceedings, 4th International Symposium,
H., Evaluation of Coatings as Carbonation Barriers, Construction
Repair, February 1987, Vol. 1, No. 1.
R., Preventive Protection by Low Permeability Coatings, The Concrete
Society One Day Conference, Permeability of Concrete and its Control,
London, 12th December 1985.
J.A., Jones, T.R. New pozzolanic materials for the concrete industry.
Corporation Concrete Manual.