Answer: This is not true. In-fact the advantages of addition of fly ash are many, a few of which are listed below:

Typically, concrete designers use fly ash a partial replacement for portland cement at values up to 30 percent of the total cementitious composition. The use of high percentages (high volumes) of fly ash in concrete have been studied extensively over the last 15 years and the benefits have been well documented.

By reacting with free lime to form additional binder material, the permeability of concrete made with fly ash is lower than that made without it. Since virtually all durability aspects of concrete are improved with a reduction in permeability, the use of fly ash improves concrete durability. In addition, concrete water demand is typically reduced when using fly ash, leading to a lower water/cementitious ratio thus increasing strength.

Answer: Fly ash is a by-product from coal fired electric power generating plants The inorganic or inderal constituents of the coal, such as clay, quartz and shale, fuse and chemically recombine during burning to produce various crystalline and glassyphases of fly ash. The fly ash is entrained in the flue gas and cools into spherical, usually hollow shaped particles.

For mix design purposes, fly ash itself should be considered like portland cement, except that the specific gravity for fly ash is different. The specific gravity of portland cement is typically 3.15, while the specific gravity of fly ash may range from 2.2 to 2.8, depending on fly ash composition. Therefore, if a certain percentage of cement is replaced with fly ash on a mass basis, simply multiply the initial portland cement quantity by the percent replacement.

The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely effected by adding more water.

Fly Ash is a pozzolan. A pozzolan is a siliceous and/ or siliceous and aluminous material which in itself is not cementitious but will, in the presence of moisture and temperature > 40°F (> 5C), combine with calcium hydroxide to form cementitious compounds. The major use of fly ash is as a partial replacement for cement in concrete. The amount of cement replaced depends on the fly ash type, the fly ash quality, the application and the atmospheric conditions.

Answer: Concrete's most rapid period of strength gain occurs in the first 7 days. The accepted time standard for measuring concrete strength is 28 days. However, concrete will continue to gain strength for long periods of time (months, even years!) As long as moisture is present to continue the chemical process of hydration.

Answer: The water cement ratio is very important with regards to the strength of concrete. The strength of concrete is inversely proportional to the water cement ratio i.e. with teh increase in water content the compressive strength of concrete decreases and vice versa.

Fly ash is produced through the combustion of coal used to generate electricity. After coal is pulverized, it enters a boiler where flame temperatures reach up to 1500 degrees Celsius. Upon cooling, the inorganic matter transforms from a vapor state to a liquid and solid state. During this process individual, spherical particles are formed. This is fly ash. It is then collected by either using electrostatic precipitators, baghouses or a combination of both.

Fly ash is classified by the EPA as a non-hazardous product. As indicated by the MSDS, it is a relatively inert material. Fly ash is a product of coal combustion thus possesses no significant risk of fire or explosion. Fly ash is similar to sand in composition and consistency. When transporting and handling fly ash, the recommended precautions for safe handling as outlined on the product MSDS should be followed.

Concrete strength is determined by it's water/cement ratio. Water/cement ratio is defined as pounds of water per pound of cement, or gallons of water per bag of cement in the mix. The higher the water content, the lower the strength of the mix.

The primary components that control the strength of concrete are the water and the cement contents. These two components are commonly referred to as the water/cement ratio. While properly graded sand and stone, and their relative amounts can affect the overall strength of the mix, the water/cement ratio has the greatest effect on the strength of the mix.

The impact of fly ash on air entraining admixture (AEA) is primarily due to the carbon, represented as loss-on-ignition (LOI), remaining in the fly ash after combustion of the coal fuel. Due to the high rate of absorption of surfactants such as air entrainment into carbon, the loss-on-ignition generally has a positive correlation with AEA dosage demand.

Fly ash, particularly Class C, can effectively be used to stabilize soil for various types of construction. High calcium fly ash (Class C) acts as a good source of calcium hydroxide which "self activates", reacting with silica and alumina in the fly ash and soil to form a cementitious hydration product. In addition, C3A in fly ash (Class C) can react with sulfates to gain strength relatively quickly.

Generally walls will require 3000 psi concrete or as specified by your engineer or code. Maximum aggregate size is 3/4" and placed at a 5-6" slump.

Concrete can be placed using any one of several methods: concrete pump, conveyer belt, crane and bucket or by a chute directly from the truck. A boom pump is far and away the easiest method using a 3" or 2.5" reducer in the hose to reduce the concrete velocity. Question: What kind of vibrator should be used? An electric vibrator with a 1-inch to 1.25 inch head and a 12-14 foot shaft.

There is an excellent guide from Concrete Construction magazine that answers that. Click here to view.

What does “Global” in the lower right-hand drop-down list refer to? It refers to the coordinate system being used with the current display. Users can have multiple coordinate systems associated with the same model, cartesian and/or cylindrical. When more than one coordinate system has been defined, users can switch between them using the drop-down list in the bottom right corner of the SAP2000 screen which usually defaults to “Global”.