1 ) Air- Entraining admixturesAdmixtures in concrete could be used for many situations including enhancing strength, workability, curing time or sustainability and longevity these factors are just a start of a long list of other factors admixtures can alter. Engineers use admixtures when the current situation is considered an unnatural environment or situation where the concrete needs certain alterations. Air entraining admixtures in concrete are related to adding certain chemicals in the concrete which intern create small air bubbles.
“Air entraining was first started in the 1930s for road making, and the civil engineers of that day found Vinsol resin that had been neutralized or other salts of wood resins effective for the purpose”(Row.J (E.d). (2010).
Today many different chemicals are used as an air entrainment agents such as many other synthetic materials including salts acids and other chemical which are manufactured and prove as an effective means of creating air entainment. These products seem to be effective in many ways when used to air entrained concrete mixtures. In air entrained concrete The air bubbles are created when concrete is mixed usually for a set amount of time depending on the requirements of the mixture being compounded and the air remains in the concrete even after the curing process which creates small holes in the concrete, if too much air is entrained the strength of the concrete is reduced by a factor of 5 percent to every 1 percent of entainment but can be adjusted by reducing the water content in the mixture or adjusting the size of aggregates (Row.J (E.
d). (2010). The loss of strength is based on the air pockets reducing the amount of concrete mixture in the batch. Some air is lost during the delivery process, pumping, placing, vibrating and finishing process so a sufficient amount of air is added considering these factors before hand. The benefit of these air bubbles have proven to- help resist damage caused by de-icer chemicals – Reduce effects made by freezing and thawing from colder temperatures- increase workability in fresh concrete material – also reduce bleeding and segregation between aggregate materials and binders within the concrete caused by over saturaturation in the concrete mix.
The air bubbles generally range in size but are usually under a few millimeters and can be measured in billions per cubic foot. Usually on exterior applications such as foundation walls, patio slabs, driveways and applications which are similar and the concrete is subject to deicing materials or freezing and thawing temperatures air entrainment is required. air entainment levels of 4-7% is considered best for resistance to freezing and thawing but variations are always an option for different measures (Row.
J (E.d). (2010). Air entrainment is not required in areas that are not subject to deicing materials or freezing and thawing temperatures such as interior structural walls, interior floor slabs, interior posts and other applications which are not subject to exterior conditions which include freezing and thawing and harsh deicing materials. In conclusion when using concrete admixtures each have positive benefits and each have their negatives depending on the situation. When making and applying air entrained concrete one should know the exact specifications which are required for the concrete mixture. Using concrete admixtures is a very quantity sensitive procedure similar to baking because if the right mixture is not made many problems could stem in the future including premature cracking and under strength concrete . It is common practice to leave concrete ad-mixing to engineers as they have the knowledge to creating the proper mix for the batch of concrete.
2) Water-reducing admixtureThe Concrete formula is often altered by adjusting the chemical properties by either increasing the amount of a certain chemical in the basic concrete mix or adding certain chemicals in the mixture. To much water in concrete usually causes imperfections premature cracking, reduced strength, bleeding and other problems within the bonding of the mixture. When using water-reducing admixture and using less water the cement material will be – more dense, which is a result of higher quality and also – stronger concrete paste material – also less water in concrete will also reduce the curing time and harden faster intern justifying the slump test at a much faster rate. – increase flexural and compressive values – lower permeability- increase resistance to weathering – improve bonding between concrete and reinforcement – reduce volume change- reduce cracking and shrinkage Water reducing admixtures are considered a Type A admixture according to ASTM standards. Water reducing admixtures have been used for more than 40 years and have been a success in the industry.
Many different manufactured chemicals are used today but are based on chemicals such as lignosulfonates, naphthalene, and hydroxylated carboxylic acids. Based on proven studies an increase in paste quality tends to produce a higher compressive and flexural strength value, and also produces a less permeable batch of concrete which increases resistance to effects caused by the weather. When reducing the amount of water in the concrete it should be done in a way which does no affect the workability or placement of the concrete.
The use of these water reducing admixtures accelerate the curing process of concrete due to the low volume of water. In conclusion when adding or reducing the amount of water in the chemical mixture the concrete formula is being altered. Depending on the situation either adding or reducing the amount of liquid in the concrete can be beneficial.