CVL 2407 Faculty of Applied Engineering and Urban Planning Civil Engineering Department Dr. Eng. Mustafa Maher Al-tayeb 2nd Semester 2013/2014 CVL 2407

Chapter 6: Admixtures for Concrete Definition A material other than water, aggregates and hydraulic cement that is used as an ingredient in concrete or mortar and is added to the batch immediately before or during its mixing (ASTM C125). What are admixtures: Admixture are natural or manufactured chemicals which are added to concrete before or during mixing. the most often used admixtures are air-entraining agents, water reducers, water-reducing retarders and accelerators.

General Groupings of admixtures 1. Air-entraining agents group: There are used primarily to improve freeze-thaw durability. They are liquid chemicals added during batching concrete to produce microscopic air bubbles called entrained air, mixing state these air bubbles improve the concrete's resistance to damage caused by freezing and thawing plastic concrete state (improve workability, reduce bleeding and segregation)

General Groupings of admixtures (Cont.) 2. Chemical Admixtures group: There are water soluble compounds added primarily to control setting and early hardening of fresh concrete or to reduce the water requirements. 1- Water reducer: are used to different purposes to lower the water content in plastic concrete and increase its strength. to obtain higher slump without adding water. 2- Retarders: are chemicals that delay initial setting of concrete by an hour or more ( are often used in hot weather to counter the rapid setting caused by high temperatures) 3- Accelerators: reduce the initial setting time of concrete and give higher strength. 4- High range water-reducers(HRWR): often called superlasticizers, HRWRs reduce the water content of a given concrete mixture between 12 and 25%. It are used to increase strength and reduce permeability of concrete by reducing the water content in the mixture.

General Groupings of admixtures (Cont.) 3. Mineral admixtures group: There are finely divided solids to improve workability, durability, or provide additional cementing properties. (i.e. slags, silica fume, fly ash, and pozzolans). 4. Miscellaneous admixtures group: Those admixtures that don't fall under the above categories.

Air Entraining Admixtures Air entrainment refers to the introduction of large quantities of tiny air bubbles in the concrete matrix. The main reason for air entrainment is to improve the durability of the concrete to freeze-thaw degradation.

Air Entraining Admixtures

Air Entraining Admixtures

As un-reacted water freezes it expands 9 % by volume on phase change. The Air-Void System: As un-reacted water freezes it expands 9 % by volume on phase change. This internal volume expansion causes internal stresses in the matrix. It can generate cracks in the concrete, which may allow water to infiltrate and the process can get progressively worse. It can lead to significant degradation of the concrete. The formation of ice in the pore spaces generates pressure on any remaining unfrozen water. Introducing a large quantity of air bubbles provides a place for this water to move in to relieving the internal pressure.

The Air-Void System: (Cont.) It's been determined that the optimum air content for frost protection is about 9% by volume of the mortar fraction. With respect to the concrete volume, the air content should be in the range of 4-8% by volume. The concrete normally has entrained air, the admixture increases the total volume of the air voids by 3-4% of the concrete volume.

Chemical Admixtures Type A: Water-reducing (WR) Type B: Set retarding (SR) Type C: Set accelerating (SA) Type D: WR + SR Type E: WR + SA Type F: High-range water-reducing (HRWR) Type G: HRWR + SR

Mineral Admixtures Raw or calcined pozzolans Fly ash produced from burning bituminous coal Fly ash normally produced from burning lignite (subbituminous) coal. (both pozzolanic and cementatious). Natural pozzolan Industrial pozzolan Clay & Shale الطين و الطفلة Fly ash رماد متتطاير Opaltine materials مواد أوبالية Burn oil زيت محروق Volcanic tuffs رواسب بركانية