Corrosion in Concrete - how BFRP can help

The Romans used a form of concrete incorporating pozzolans to build some impressive structures such as the domed roof of the Pantheon, the art of using concrete in construction was not widely used until 1756 when John Smeaton rediscovered hydraulic cement.

Whilst the Romans utilised the high compressive of concrete they did not reinforce it to compensate for the relatively low tensile strength. (They did add horse with limited results). Portland cement which we associate with modern concrete was invented by Joseph Aspdin in 1824, using the name of the quarry for his invention. In the present day we use steel rebar, welded wire mesh and fibres to reinforce concrete which allow building designers a flexibility to construct with concrete.

Rebar

The first recorded use of steel to reinforce concrete was in 1854 when William Wilkinson used iron bars to reinforce a concrete floor and roof of a servants cottage in Ellison Place Newcastle, subsequently patenting his idea thus inventing reinforced concrete.

Introducing steel bars into concrete overcame the problem of low tensile strength opening up new opportunities for the use of concrete. Steel bars work well as a reinforcement due to the high tensile strength of the steel but also because of the similar coefficients of linear expansion of both concrete and steel. This means that as the temperature changes the thermal expansion of both materials remain the same size in relation to each other preventing the steel inducing stress and cracking.

For reinforcement to be effective there must be a good bond between the concrete and steel. In addition, the steel must be protected from corrosion.

Present day rebar is ribbed to produce a good bond but corrosion still remains a problem in reinforced concrete structures. When steel rusts it results in ferric oxide Fe₂O₃  which can be up to six times the volume of steel. When this occurs in steel reinforcement within concrete the results are highly destructive, the expanding rebar creates such a force that it fractures the concrete which allows water in and accelerates the process.

The corrosion process produces two problems, firstly the transformation of steel to rust reduces the tensile strength rebar and secondly the face of the concrete spalls to expose the steel both reduce the composite effect of the structure ultimately resulting in catastrophic failure.

Concrete does provide some protection against corrosion of steel on its own but atmospheric CO₂ and the presence of chlorides can dramatically reduce the service life of a concrete structure.

Atmospheric CO₂reacts with calcium hydroxide within the cement leading to carbonation. This reaction occurs in a solution (carbonic acid) therefore the process is slow in dry concrete but also in saturated concrete as the additional moisture acts as a barrier to the CO₂slowing the process.  The carbonation process reduces the alkalinity of the concrete, research suggests that concrete with a PH of 12-13 requires chlorides at 7000-8000 ppm to begin corrosion, whereas in concrete with a PH of 10-11 only chlorides at 100ppm are required to begin corrosion.

Carbonation in itself is not a bad thing as it increases the tensile and compressive strength of the concrete but has a devasting effect on the steel reinforcement.

Chloride attack of concrete again corrodes the steel reinforcement but is normally due to the presence of chlorides from factors such as chemical manufacturing, marine environments or from de-icing salts. The chlorides migrate through the pores within the concrete by capillary action and once absorbed they will again reduce the PH of the concrete, leading to corrosion of the any steel reinforcement.

Steel within the concrete develops a passivity layer which protects the steel, chloride ions attack and destroy this layer. As with carbonation moisture is required for the corrosion process to take place therefore, dry concrete with a relative humidity of 60% or where concrete is fully immersed in water will slow the corrosion rate.

Various techniques are employed to attempt to slow down or prevent corrosion, such as increased cover of concrete, less permeable concrete or anti-carbonation coatings. In some cases where reinforced concrete structures are subject to aggressive environments such as marine, car parks, chemical bunds etc stainless steel or epoxy coated steel is used.

Recently the use of fibre-reinforced polymer rebar for precast structures has been implemented especially in aggressive environments like those mentioned above. FRP rebar is manufactured from composite materials and as such is corrosion-resistant, resulting in increased service life and enhanced durability.

GFRP (Glass Fibre Reinforced Polymer) is lightweight and has a higher tensile strength than steel. This makes it useful in a variety of applications, including bridge decks, caissons and seawalls. However, the elastic modulus of GFRP is relatively low, for this reason many designers are turning to BFRP (Basalt Fibre Reinforced Polymer) which has a higher elastic modulus and a greater resistance to alkalies than GFRP. One important property of BFRP is the coefficient of linear thermal expansion, which was noted earlier as an important element of reinforcement within concrete, is similar to that of concrete.

Steel rebar is a heavy material compared to BFRP, which makes construction and fabrication more labour intensive offsetting the additional cost of the BFRP reinforcement.  Reinforced concrete structures built with BFRP do not suffer from corrosion and research shows that BFRP does not suffer any loss of performance over a 100 year period and does not require any maintenance over that period.

To this effect BFRP rebar from Galen Composites was used to construct a bridge in County Fermanagh in 2010, the first of its kind in the UK.

In addition BFRP is dielectric, non magnetic, has a very low thermal conductivity and uses forty times less CO₂in production compared with steel.

Welded Wire Reinforcement (WWR)

Welded wire reinforcement arose as a direct response to the perceived shortcomings of rebar. The material is produced from a series of longitudinal and transverse high-strength steel wires which is welded at all intersections. The resulting steel lattice is stronger, by weight, than straightforward rebar. design steel areas (and hence weight) when permitted by specification.

The grid nature of WWR provides welded wire with excellent bonding to concrete, added to this the welded wire nodes, there are multiple surfaces – in multiple orientations – for concrete to bond to.

As WWR is manufactured in sheets (normally 4.8M x 2.4M) it is easier to locate in moulds during the precast process as opposed to rebar reducing the labour required.

WWR is used as a crack control and reinforcement in screeds and concrete slabs and is recognised as a good alternative to lacing steel rebar to achieve the same result.

However, WWR is still manufactured from steel in many cases and suffers from exactly the same corrosion problems described above.

FRP products have not been available as a mesh until recently and one manufacturer, Galen Composites, has developed a method of joining BFRP strands to create a mesh much the same as steel WWR. Development of this procedure has result in 2.2mm and 3.0mm BFRP being available in mesh form but with the added benefit of being supplied on a roll.

BFRP has a tensile strength of between 1250 and 1350 MPa some 2 ½ time stronger than steel mesh whilst being some six times lighter than steel (360g/m²opposed to 2220g/m²). This means that thinner diameters of mesh can be used in place of steel mesh for crack control of ground bearing concrete slabs.

BFRP has a lower modulus of elasticity compared to steel and should not be used as a direct replacement for steel in suspended concrete floors or bridge decks but when used in a composite design with for example steel decking it provides an ideal solution reducing weight of the floor or bridge deck.

Concrete in its current form is still a relatively new building material with most of the innovation occurring in the 20^thCentury. For those who design and work with concrete there are many changes and advances in technology but while we still use steel as a reinforcement there will still be the problems of protecting that steel from the devastating effects of corrosion.

Christopher Hirst 2018