Q: How does chemistry affect the life of concrete (bridges)?
A: Portland cement concrete gains and losses its properties through chemical processes.
Concrete is composed of Portland cement (500lbs), fine aggregate (1000lbs) and large aggregate (2500lbs) for 4000lbs per Yd3. It is strong in compression (3500 to 6500 psi), but weak in tensile (400 to 600psi) and flexural (800 to 1000psi) strengths. For this reason most concrete structures are reinforced with steel bars or rebar. Fresh hard concrete has an alkaline pH of 13 and will react with acids.
Portland cement under goes a transition from the wet paste state to a solid by an exothermic reaction with water. The main component, tricalcium silicate (1lb) reacts with water (0.3 lbs) to form monocalcium silicate tetrahydrate and calcium hydroxide, which accounts for the cement high pH. More than 0.3 lbs of water per pound of cement are required to form a workable concrete mix. Typically 0.5 lbs of water are used. When this 0.2 lbs of excess water evaporates it leaves behind a microscopic pore structure in the concrete cement phase and is referred to as gel pores. It is through and in the gel pores where air water and salts can migrate into the concrete surface and where future chemical changes in the cement will take place.
At a water cement ratio of 0.5 the cement phase of concrete is about 20% porous and allows air containing oxygen and carbon dioxide to penetrate and along with water cause corrosion on the concrete rebar and a reaction with the cement alkaline component calcium hydroxide called carbonation, which is the conversion calcium hydroxide into calcium carbonate. This results in a reduction of the cement pH from 13 down to 8. Other destructive materials such a deicing salt or chlorides, and sulfates from ground water can also penetrate into the concrete through the cement pores.
Freeze thaw cycles in the winter cause damage to the surface of concrete bridge decks due to the expansive pressure of water freezing in the cement gel pore structure. Deicing salts increase the impact of the freeze thaw cycles. Resistance to freeze thaw cycle damage increases with the reduction in cement porosity. This can be accomplished in an original pour by reducing the water cement ratio down to 0.4 and by the addition of silica fume which reacts with the cement calcium hydroxide to fill up the cement pores. Porosity in hard concrete can be reduce by the application of reactive penetrates to the surface which will penetrate the gel pores and react inside with the calcium hydroxide to reduce the pore size. Water repellents can also be applied to inhibit surface wetting, but they will not prevent water penetration under hydrostatic pressure such as vehicle tire pressure on a wet bridge deck.
Alkali Silica reaction or ASR is seen in bridge concrete in the form of a random crack pattern. ASR takes place when concrete contains large aggregate with an acidic surface chemistry such as quartz chert. The acid aggregate reacts with the cement alkaline components such as sodium and potassium to form a byproduct that causes expansive force on the concrete. Reinforced concrete relieves this pressure by crack formation. If the concrete is not reinforced the concrete just grows. The best way to avoid ASR is to only use aggregate that has been tested to verify that is does not have ASR potential. ASR can be minimized if the aggregate is suspicious by the addition of fly ash to the concrete mix which will reduce the pH down to a level where the ASR reaction will not take place. The addition of lithium compounds to the concrete mix or to the surface of hard concrete has proven to also reduce the impact of ASR.
Most damage to reinforced concrete bridges occurs due to rebar corrosion. When the rebar is surrounded by fresh concrete with a pH of 13 it forms a stable corrosion resistant surface due to abundance of hydroxyl ions. Over time air containing carbon dioxide penetrates the concrete pores and caused carbonation to reach the rebar level and reduce the pH to a level (11) where the natural corrosion resistance is lost and rebar will corrode in the presence of air and water. Deicing salt can penetrate the bridge decks and the associated chloride ions will react on the rebar surfaced to accelerate rebar, which corrosion appears in the form of concrete delaminating and spalling from a bridge deck or supporting structures. The onset to rebar corrosion can be delayed by deep and uniform concrete cover over the first rebar level usually at least 2 inches, using a concrete mix having a low water cement ratio or by the addition of silica fume or fly ash to achieve a low level of cement porosity. Corrosion inhibitors can be added to the concrete mix where they will be available when the conditions for corrosion are reached. It is also important to inspect bridge decks for cracks that can act as access ways to the rebar level. Most bridges built in the past 10 years have used rebar with an epoxy coating which protects the rebar from deicing salts. It also prevents contact with the alkaline cement and surface damage to the coating can introduce cathodic sites and result in corrosion at anodic sites under the coating surface.
Steel corrosion is an electrochemical oxidation process that is inhibited by an alkaline environment and accelerated in environments containing acidic substances and chlorides. There are 2 main types of rebar corrosion macro and micro cell. On macrocell corrosion the anodic and cathodic sites are on different t section of rebar and on microcell corrosion the anodic and cathodic sited are on the same rebar section. Rebar corrosion on bridges is primarily microcell and pitting in type. Steel (iron) is oxidizing at the anodic sites to the ferrous state with the release of 2 electrons per iron atom oxidized. The corresponding cathode site has oxygen and water reacting to form 2 hydroxyl ions which combine with the ferrous iron to form ferrous hydroxide. This is referred to as the passive oxide film and must undergo further oxidation to the ferric state or be decomposed by reaction with chlorides in order for further oxidation to occur at the site. Ferric oxide occupies about 6 times the volume of steel and it is the volume expansion that causes the concrete to delaminate from the rebar level and the spall from the bridge surface and cause pot holes in the deck.
The potential for rebar corrosion in bridges can be predicted in advance of spalling by measuring concrete pH and chloride content at the first rebar level. When pH reaches 11 or the water soluble chloride content in the cement reaches 300ppm the conditions for corrosion will exist. The electrical potential for corrosion can be determined using copper or silver half-cell measurements. The rate of corrosion can be measured in a microcell condition by measuring the polarization resistance of a section of rebar, and then using the DC current equation convert it into corrosion current in uA/cm2 and then in to the rate of steel loss in terms of micrometers per year.
The life of a bridge can be extended by paying attention during construction to the chemical factors controlling cement curing, avoiding ASR active aggregate, protecting the rebar surface with substantial concrete cover and the use of admixtures to inhibit rebar corrosion and enhance cement properties. The life of an existing bridge can be extended by the application of migratory corrosion inhibitors and surface applied strength builders and porosity sealers.
Robert A. Walde, Chief Technical Officer, Surtreat Holding, LLC contact
Surtreat Holding, LLC 437 Grant Street, Pittsburgh, PA 15219; 412.281.1202, official website www.surtreat.info
Max Merzlikin, Surtreat Holding, LLC 2015