Post-Tensioning Concrete

Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands or bars, typically referred to as tendons. Post-tensioning applications are commonly used in office and apartment buildings, parking structures, slabs-on-ground, bridges, sports stadiums, rock and soil anchors, and water-tanks. In many cases, post-tensioning allows construction that would otherwise be impossible due to either site constraints or architectural requirements.

In building construction, post-tensioning allows longer clear spans, thinner slabs, fewer beams, and more slender, dramatic elements. Thinner slabs mean less concrete is required. In addition, it means a lower overall building height for the same floor-to-floor height.

Post-tensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors. This reduces the foundation load and can be a major advantage in seismic areas.

A lower building height can also translate to considerable savings in mechanical systems and façade costs.

Another advantage of post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other. Structurally, this is much more efficient than having a beam that just goes from one column to the next.

Jim Graham, Las Cruces Builder

Using Fly Ash to Improve Concrete Quality and Reduce Environmental Impact

This article is about using fly ash to improve the quality of concrete while reducing its environmental impact. We are all concerned about the quality of Portland cement concrete used in our projects, and I hope also about reducing the environmental impacts of our built environment. There is a way to achieve both of these at the same time by the addition of fly ash to concrete.

Fly ash is a byproduct of the emission controls of coal fired power plants. Most fly ash is landfilled but can be recycled as an ingredient of concrete. It consists basically of tiny spheres of silica and is considered a “pozzolan” additive. Pozzolans are essentially inert fillers that take the place of much more reactive Portland cement and have been used since the Romans learned to make extremely durable concrete 2,000 years ago, even in aggressive marine environments. Portland cement production uses lots of energy and produces massive amounts of carbon dioxide.

Typically Fly Ash is used to replace 10-60% of the Portland cement in concrete. The effect on the concrete is to reduce shrinkage and reduce heat from the hydration of the more active Portland cement. It can lead to a much more durable concrete with less cracking and increased resistance to adverse reactions including Alkali Silica, chloride, and sulfates. It is widely used in highway construction, pre-casting plants, and dam construction. It is less widely used by residential concrete contractors, primarily as a minor hot weather additive to retard setting and reduce cracking, however, it can be used in higher amounts and in cooler weather. The key to its successful use is by controlling the water/cement ratio, as are so many other aspects of good concrete.

There is often considerable resistance to fly ash use by trade contractors and even concrete producers. Let’s face it, construction is often very conservative about adopting “new” practices and there is a learning curve to fly ash adoption.

The most common concerns about using Fly Ash is that it will slow concrete setting time and will reduce high early strengths. These can both be addressed with water-reducing additives. We can classify water reducers as standard or superplasticizers. Essentially their purpose is to make concrete more workable while using less water. Concrete use is often a conflict between more workability, which make the subs life easier, and a lower water to cement ratio, which does good things for the concrete and makes the engineer happy. Plasticizers can make everyone happier. So how do we use them, how much do they cost, and what is this about a learning curve?

Standard plasticizer is often included with regular concrete mixes, is added during batching at the plant, and shouldn’t have much effect on the cost. Superplasticizer can be added either at the batch plant or at the job site, is usually used at 40-90 0unces per yard, and might cost $.10/ounce. This incremental cost increase should be offset by the substituting of inexpensive fly ash for more expensive Portland cement.

Superplasticizer can be thought of as dehydrated water and is useful for other places in residential construction such as making concrete countertops and Portland based tile grouts on floors.

I would recommend learning to use superplasticizer in smaller projects at the job site such as sidewalks or driveway sections, before doing a large slab or foundation. The most challenging aspect of its use is probably timing. When used at the batch plant it may lose its effect by the time the concrete is placed. When added at the site it may delay setting time unacceptably. Avoid the temptation to add more water if it sets too soon and instead add more superplasticizer, which is a reason to keep some on the job, even when most of it is added at the plant. If the setting is delayed it may be that the plant is adding too much water to improve mixability and should add “super” early on. Properly used fly ash with super makes for a more workable concrete with higher early and ultimate strengths and improved pump-ability. Since fly ash is a recycled product it can be used to achieve points in LEED rating.

Hopefully, this article on fly ash use in concrete will help you make better concrete while reducing its environmental impact.

Sustainable Construction For A New Millennia

Jim Graham Sun & Earth Construction LLC.