The Design of Reinforced Cement-Based Protective Coatings - Paul Bennison
THE DESIGN OF REINFORCED CEMENT-BASED PROTECTIVE COATINGS
PAUL BENNISON, B.Sc., C.Eng., M.I.C.E., M.I.Struct.E., F.F.B.
JOINT MANAGING DIRECTOR
P.O. BOX 7
This 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.
They 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.
The 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.
MATERIALS USED IN CEMENTITIOUS COATINGS
- Cement (a) Ordinary Portland
a. Rapid Hardening Portland
b. Ultra finely ground Portland
c. Calcium Sulphoaluminate
d. Sulphate Resisting Portland
- Resins (a) Polyester
- Polymers (a) Styrene Butadiene Rubber
b. Modified Acrylic
- Pozzolans (a) Microsilica
a. Pulverised Fuel Ash
b. Ground Granulated Blast Furnace Slag
- Migrating Corrosion Inhibitors (a) Amino Carboxylates
DURABILITY OF CEMENT BASED PRODUCTS
The following factors will significantly affect the durability of cement based products:-
1. The mix design.
2. The cement content and type.
3. The water/cement ratio.
4. The degree and type of compaction.
5. The quality and type of aggregates.
7. The use of chemical admixtures.
8. The use of polymers.
9. The use of pozzolanic materials.
POROSITY AND PERMEABILITY
For 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 gel pores.
Small 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.
Capillary and gel pores are as a result of the hydration process of cement and water.
10 - 2.5nm
2.5 - 0.5nm
Micro pores (interlayer)
10 - 0.5m m
50 - 10nm
0.01 - 0.2mm
In 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).
It is, however, important to note that, while strength and diffusion are affected by porosity, they are not uniquely inter-related.
Thus, the nature of the hardened cement can be summarised as follows:-
Immediately 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 "gel water".
Despite 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).
The 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).
Fig. 1 Illustration of Permeability and Porosity
Fig. 2 Typical Relationship between Cube Strength between Cube Strength
Fig. 3 Typical Relationship and Water/Cement Ratio and Capillary Porosity
CEMENT CONSTITUENTS PROPERTIES AFFECTED
C3S Tricalcium Silicate 55% Strength up to 28 days/setting time.
C2S Dicalcium Silicate 20% Long term strength, i.e. after 28 days.
C3A Tricalcium Aluminate 10% Setting time/24 hours strength.
C4AF Tetracalcium Aluminoferrite 8% Very little effect.
MAIN HYDRATION PRODUCTS
C-S-H Calcium Silicate Hydrates : Form the "gel" structure.
C3AH6 Calcium Aluminate Hydrate.
Ca(OH)2 Calcium Hydroxide : Gives the cement paste its high alkalinity
Permeability and porosity can, therefore, be influenced in several ways, i.e.:-
1. The use of chemical admixtures
a) To 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.
A 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 shr