Abstract— The adverse effects of elevated temperatures on the properties of the fresh concrete include increased water demand, shorter setting time and increased slump loss. Superplasticizers (SP) and retarders are important to enhance the workability and setting time of concrete under hot weather, hence, an experimental investigation was conducted to determine the optimum dosage for the admixtures and to study the effect of over dosage of the mentioned admixtures. Concrete mixes with SP and retarder dosages of 600, 1200, 1800, and 2500 ml/100 kg of cement were prepared, together with control mix (water/cement ratio were 0.50 ). After casting, normal curing was carried out on the concrete samples. Properties such as compressive strength, was determined, besides determining the workability of the fresh concrete. Over dosage of SP or retarder were found to deteriorate the properties of concrete with indication of lower compressive strength. However, if the dosage levels are lower than the optimum dosage, increase in admixture dosage might help to enhance the concrete characteristics. Keywords— Concrete; Admixtures; Superplasticizers; Retarders; Compressive Strength. I. INTRODUCTION N Libya, about 90-95 percent of the construction materials market for both structural and non-structural applications is made of concrete compared with other materials used for similar functions. Concrete, generally, is a product made from cement, water and aggregates and an additional material known as admixture, is sometimes added to modify certain properties of concrete. Cement is the chemically active constituent but its reactivity is only brought into effect upon mixing with water. The aggregate plays no important roles in chemical reaction but its usefulness arises because it is an economical filler material or hard composite material with good resistance to volume changes which take place within the concrete after mixing, besides improving durability of concrete. In hardened state, concrete is a rock like material with a high compressive strength. In its plastic state, concrete may be moulded into any form of shapes, it may be used to advantages architecturally or solely for decorative purposes. Salahaldein Alsadey is with department of civil engineering, faculty of engineering at Azzaytuna University (Libya), PO Box 5338, Tripoli-Libya. (e-mail: salahalsadey@yahoo.com). Concrete has low tensile strength, and hence, this is the reason why it is used with steel bar to resist any tensile forces in the reinforced concrete. Concrete is usually used in building for foundations, columns, beams and slabs, in shell structures, bridges, sewerage treatment plants, roads, cooling towers, railway sleepers and so on. In precast concrete industry, concrete is widely used as concrete blocks, cladding panels, pipes, piles and lamp posts 1. Nowadays, more than 70% of in-situ concrete in Libya is produced by the ready mixed concrete industry. The ready mixed concrete producers are using retarding and superplasticizing (SP) admixtures which are readily available from various manufacturers. Retarding admixture decreases the initial rate of reaction between cement and water, and hence, retards the setting time of concrete. Retarders are admixtures which will lengthen the setting time and workability retention, particularly important for concreting in hot weather. A retarding admixture holds back the hydration process, leaving more water for workability and allowing sufficient time for the concrete to be placed, compacted and finished. Superplasticiser (SP) is used to increase the workability without changing the water/ cement ratio. Or, it can be used to increase the ultimate strength of concrete by reducing water content while maintaining adequate workability. This experimental work is carried out to study the effect of dosage of retarder and SP on properties of concrete. Since in some cases at job site, it is found that the concrete is left unset for 2 to 3 days due to over dosage of retarding admixture and it becomes very common and serious in the concrete practice. Superplasticizer is a type of water reducers; however, the difference between superplasticizer and water reducer is that superplasticizer will significantly reduce the water required for concrete mixing 2. Generally, there are four main categories of superplasticizer: sulfonated melamine- formaldehyde condensates, sulfonated naphthalene- formaldehyde condensaes, modified lignosulfonates and others such as sulfonic- acid esters and carbohydrate esters. Effects of superplasticizer are obvious, i.e. to produce concrete with a very high workability or concrete with a very high strength. Mechanism of superplasticizer is through giving the cement particles highly negative charge so that they repel each other due to the same electrostatic charge. By deflocculating the cement particles, more water is provided for concrete mixing 2. For general usage, dosage of superplasticizer is between 1- Effects of Super Plasticizing and Retarding Admixtures on Properties of Concrete Salahaldein Alsadey I International Conference on Innovations in Engineering and Technology (ICIET'2013) Dec. 25-26, 2013 Bangkok (Thailand) http://dx.doi.org/10.15242/IIE.E1213607 271