Is there Spalling at the Oroville Dam Spillway.
by Paul Preston
Over the last several months I have been asked the same question over and over again about the Oroville Dam spillway. “Is the spalling occurring on the spillway?”
Spalling is a form pf concrete “cancer”. Where because a wide variety of issues concrete which surrounds rebar starts to in a concrete structure pulls away from the rebar. This ultimately renders the concrete rebar structure useless.
Since there’s a lot of concrete surrounding rebar in the Oroville spillway the idea that spalling could happen to such a large structure almost seems impossible. Yet this question continues to surface week in and week out.
To start this discussion with our listeners and views we are going to bring you a study in spalling. Below the lesson begins because the question of spalling at the Oroville spillway is NOT doing to go away.
Spalling of Concrete – Causes, Prevention & Repair
ingress of moisture of air into concrete causes corrosion of reinforcement and results in the volume expansion of steel bars, consequently causing cracks & spalling of concrete cover
The word Spall in engineering, describes the chips or fragments of a material that is broken off a bigger object. The process of spalling also known as spallation is the surface failure that occurs when a material such as concrete, brick, or limestone is subjected to excess moisture, corrosion, weathering, and much more. Concrete spalling commonly known as (concrete cancer) is an eye-sore and has the potential to be an extreme hazard.
Left untreated and exposed to the elements will cause extensive structural damage and may cause it to break away from the façade, potentially injuring the public and damaging property. Spalling, at a low level, is mainly a cosmetic problem but it can lead to structural damage if not dealt with immediately. If left untreated, damage can occur to the reinforcing bars within the concrete. Also, large enough fragments could fall off which could lead to serious consequences.
Prevention of Spalling Concrete
- Corrosion protection is provided by the amount of concrete cover protecting the steel
- Preventing spallation is usually done at the outset of mixing the concrete, by using air-entrained concrete, curing well,
- making sure to apply a water-repellant sealer after the slab is cured. The water-repellant sealer is probably the most essential ingredient needed to prevent spallation.
- After the concrete is done though, one can use a Concrete Treat as a sealer on outdoor and indoor concrete in order to prevent moisture from entering.Spalling of a Brick Wall
The most common source of spalling in brick, for example, is from excess moisture in which water enters into the brick material causing pieces of it to
crumble. Causes of excess moisture may include consistent, heavy rainfall coming into contact with outdoor brick, or dark, damp areas such as basements in which the humidity and also salt will affect the walls.
Spalling of Concrete in Detail
A common but not well understood problem with older concrete structures is spalling. Concrete spalling is usually caused by corrosion of the steel reinforcement bars embedded in the concrete matrix, but can be caused by other ferrous elements either fully or partially embedded in the structure. Steel frame window systems, handrails, structural I-beams, metal pipes and conduits are among the most common of the damage causing building components.
Corrosion of the reinforcement steel however, is by far the most common cause of spalling and splitting in older concrete structures. Steel reinforcement bar (rebar) is used in concrete to provide strength to a material that is extremely high in compressive strength but has very limited flexural or tensile strength. One of the principles of reinforced concrete is that the high alkaline content of the concrete passivates and protects the
embedded steel from corrosion. One of the main components of this corrosion protection is provided by the amount of concrete cover protecting the steel. As a general rule of thumb one inch of cover is required to protect the steel rebar, in other words, no rebar should be nearer than one inch of the concrete surface. Another important factor for the protection rebar is to make sure that the concrete is properly consolidated during installation.
Besides environmental factors, poor installation also induces spall to form in concrete or limestone such as through structural overloading of the stone or not taking care to have the proper mixture of ingredients while pouring the concrete.
Good consolidation is achieved by installing concrete that is the proper consistency, and by vibration of the wet concrete before it begins to set. Concretethat is too stiff (has too little water) is often not consolidated properly which allows air bubbles and voids to be left in the finished matrix. If these voids (commonly known as rock pockets) are large enough or extend all the way through the wall structure, water can easily penetrate the finished wall and/or contact the rebar and cause corrosion.
