CoatingsPro Magazine Supplements

Concrete Dec 2017

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Page 31 of 40

COATINGSPRO CONCRETE COVERED 2017 31 Concrete and Water complexity of the mix design. Concrete that is too stiff is difficult to place uniformly, resulting in voids and weak layers after placement. Concrete that is too fluid may strat- ify or "segregate" into layers of coarse aggregate alternating with layers of high water and fines content. e wrong mix design (i.e., too much or too little water) is the second latent construction defect. To facilitate placement and consolida- tion, vibration is commonly used, but this may incompletely compact the layers, resulting in " honeycomb" voids in the concrete, or it may increase segregation and produce non-uni- form concrete. Water is also necessary for the curing of concrete. e hydration of hydraulic cement occurs over a time span of weeks, with typical design strengths required at 28 or even 90 days after placement. During this hydration period, moisture is consumed by the cement filling in the spaces between aggregate particles. If water is not present during this time, improper hydration of the cement will result in low strength and poor durability. Improper curing is the third latent construction defect contributing to concrete issues. Concrete is strong in compression and comparatively weak in tension. Reinforcement, usually in the form of steel bars (aka rebar), is added to improve the concrete's tensile properties. Steel that is used for concrete contains mainly iron, which will eventually rust if sufficient water and oxygen are present. ere is a synergy between the two materials in steel-reinforced concrete: e alkalinity of the concrete (high pH) causes a protective (passivating) layer to remain on the steel/concrete interface that prevents rusting of the steel. is passivating layer will remain to protect the steel until the pH around it is reduced, chloride (or other damag- ing chemicals) enter the concrete and penetrate to the steel, or stray electrical currents cause the layer to dissipate. Once the passivating layer is no longer present, rust will form. Rust occupies a larger volume than the iron, oxygen, and water that form the rust, creating an expansive force in the concrete that causes cracking in the covering concrete. Once cracking occurs, further deterioration is acceler- ated since now these deleterious materials can easily reach the steel. erefore, the steel reinforcement must be covered by enough concrete of suitable quality to provide adequate protection for the anticipated life cycle of the structure. Frequently during concrete placement, the reinforcement may shift or move closer to the surface of the concrete, result- ing in shallow cover. is is called improper cover, and it is the fourth latent construction defect in this discussion. A ll of these latent construction defects contribute to a greater or lesser extent to the overall durability of the concrete structure, but in a synergistic fashion. Concrete Improper cover is the final defect. This occurs when not enough concrete protects the steel reinforcement. These hidden defects can cause concrete to under-perform. The wrong design mix is another defect. If it's too fluid, layers may form separating the aggregate and fine materials. Concrete that's too stiff is difficult to place uniformly. If the wrong design mix is a problem in the concrete, vibration may compact the layers incompletely, resulting in "honeycomb" voids.

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