ABSTRACT
Hot weather, which is characterized by high ambient temperature and low relative humidity causes rapid setting of cement paste and difficulties during concreting. Admixtures that retard rapid setting of cement paste and modify workability of concrete are necessary to prevent fresh concrete from the adverse effects of hot weather. However, in Nigeria the retarding admixtures are imported and also expensive. Sugar a locally sourced admixture is another substitute that has been established to be a retarder, but its effects varies with the chemical composition of cement. This research therefore assessed the effects of sugar as retarder on properties of concretes made with selected brands of Portland cement produced in Nigeria. The concrete specimens were prepared by using 0%, 0.06% and 0.1% additions of sugar by weight of cement. A nominal mix of 1:2:4 with water cement ratio of 0.5 was used for the research. Setting time tests were performed at two different temperatures; at normal laboratory temperature of 27oC and simulated hot ambient temperature of 43oC (similar to that of Sokoto, Birnin Kebbi, Maiduguri, etc.). A total of 540 concrete cubes of size 100mm× 100mm × 100mm were produced, out of this number 360 cubes were used for compressive and split tensile strengths tests at 3, 7, 28 and 56 days curing periods. The remaining 180 cubes were also used for abrasion and water absorption test at 28 and 56 days curing periods. The results showed that the initial setting times of the control pastes made with cements A, B, C, D and E at 43oC reduced by 45.94%, 40%, 38%, 24.73% and 51.85% respectively when compared with the control pastes at normal laboratory temperature of 27oC. The reduction effects were completely eliminated by the addition of 0.1% and 0.06% of sugar in all the brands of cement at 43oC. Thus, significant increase in initial setting times were recorded. Similar trends were observed for the final setting time in all the brands of cement. The results further revealed that the 0.06% addition of sugar had the best results in strengths tests of the specimens made with cements A, B and D therefore 0.06% proved to be optimum dosage. On the other hand, 0.1% addition of sugar had the best results in strengths test of the specimens made with cement C which implies 0.1% as optimum dosage. However, cement E showed a decrease in strengths at every percentage additions of sugar at all the curing periods. The research concluded that the use of sugar as a retarding admixture in concretes made with the brands of cement enhanced the properties of concretes when an optimum dosage was used. It is recommended that, sugar can be used as retarder when concreting in hot dry weather regions in Nigeria, but should not be used as a retarding admixture in cement E brand.
CHAPTER 1
INTRODUCTION
1.1 Background of the Study
Concrete is a heterogeneous material which consists of a chosen mixture of binding materials such as lime or cement, well graded fine and coarse aggregates, water and admixtures to produce concrete with special properties (Duggal, 2008). Khan (2006) opines that concrete is the most widely used construction material in the world and has gained a unique place in the construction industry. The most important characteristics of fresh and hardened concrete are workability, setting time, rate of strength development, ultimate strength, durability and impermeability of the concrete. The desired properties of fresh and hardened concrete can often be economically achieved through proper mixing of the ingredients, placing, and compacting. However, there are instances where the aforementioned properties may be adversely affected during the production of the concrete due to hot weather effect (Khan, 2006).
Furthermore, Okereke (2003) noted that extreme North East and North Western parts of Nigeria such as Maiduguri, Yola, Sokoto, Birnin kebbi etc. are categorized as hot dry regions with temperature which ranges between 35 to 43oC with the low relative humidity of 40%. According to ACI 305R (1991) hot weather is any combination of high air temperature, low relative humidity, wind velocity, and solar radiation tending to impair the quality of fresh or hardened concrete. Also, Neville and Brooks (2010) defined hot weather concreting as concreting at an atmospheric temperature above 35oC. BS 8110-1 (1997) specifies that the maximum temperature for placing concrete should not exceed 30oC. Therefore, to achieve good properties of concrete it depends upon the climatic conditions under which concrete is produced. High temperature is one of the important factors that adversely affects setting times, ultimate strength and durability of cement concrete (Khan, 2006). In addition, Suryawanshi et al. (2014) noted that another effect of hot weather concreting is the problem of cold joint which is a plane of weakness in concrete caused by an interruption, delay and rapid loss of workability in the fresh concrete. It occurs when the first batch of concrete has begun to set as a result of rapid evaporation of mix water before the next batch is added, so that the two batches do not intermix. A more serious problem associated with a cold joint is the possibility of moisture intrusion into the concrete section (Rathi and Kolase, 2013). But, Rana (2014) noted that it is more practical and economical to achieve such desired properties in concrete by adding extra materials such as admixture to the basic concrete making materials during the process of concrete production. Thus, retarder admixtures are used to slow down the speed of the reaction between cement and water by affecting the growth of the hydration products and reducing the rate of water penetration to the cement particles (Myrdal, 2007).
Alsadey (2013) defined retarder admixture as a type of chemical admixture that delays the initial rate of reaction between cement and water and hence, retards the setting time of concrete and also lengthens the setting time and workability retention. It is 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. Also, it stop the rapid set shown by tricalcium aluminate which causes false set of the cement paste (Abalaka, 2011b). Sugar (sucrose) is a very efficient retarder admixture which is capable of delaying or prolonging the setting of cement paste in concrete (Gupta and Gupta, 2004). The use of sugar as a retarder admixture also, slow down the chemical process of hydration, so that concrete may remain in plastic state and workable for longer period (Suryawanshi et al., 2014). The mechanism of function of sugar as a retarding admixture is that its retardation arises from the adsorption of sugar molecules on to the surfaces of growing particles of hydrating products (calcium silicate hydrate) and hence the hydration process slows down (Khan and Ullah, 2004). Due to the slow in hydration process the cement paste remains in plastic state for a longer time. Moreover, Greesan et al. (2014) states another mechanism of precipitation theory that the sugar increases the concentrations of calcium, aluminum and iron in concrete. The sugar molecules combine with these metals to form insoluble chemical complexes that coat the cement grains. Several key chemical processes that harden the concrete are then impeded. Hydration process is slow down and therefore, the concrete takes longer to set. For this reason, refined sugar is known as one of the best retarder (Greesan et al., 2014).
Abalaka (2011a) noted that sugar content of 0.06% by weight of cement can improve compressive strength of concrete by 3.62% as compared with control at 28 days and delay initial setting time by 1.556 hours (94 minutes). No adverse effect on concrete and cement paste have been observed at this level of sugar concentration for the age of 28days curing period. In addition, Rana (2014) found that 0.1% of sugar by weight of cement delays both the setting times and also shows around 4% increase in compressive strength, but above 0.13% by weight of cement accelerated the setting time of cement paste. Shetty (2009) opines that when sugar is added to concrete mix in a careful controlled manner (about 0.05% to 0.1% by weight of cement), the setting time of the mix will be delayed by about 4hours. However, Shetty (2009) asserted that the exact effects of sugar as retarding agent on concrete mix depends on the chemical composition of cement. Furthermore, research conducted by Khan and Baradan (2002) in Pakistan on three different types of cement under the same curing condition (namely; PKC/A42.5 cement, PKC/B32.5 cement and PC 42.5 cement), the findings revealed that 0.8% sugar content acted as an optimum sugar content for retarding the setting of PKC/A42.5 cement, 0.1% for PKC/B32.5 cement and 0.2% for PC42.5 cement. This means the retardation effects of sugar in concrete depends on the brand of OPC used. Hence, this research work assessed the effects of sugar as retarder on the properties of concretes made with selected brands of Portland cement in Nigeria.
1.2 Statement of Research Problem
As highlighted by Okereke (2003) the extreme North West and North Eastern parts of Nigeria are considered as hot dry zones in Nigeria with approximate maximum mean ambient temperature of 43oC and low relative humidity of 45%. Alsadey (2013) opines that hot weather causes rapid setting of cement paste and difficulties during concreting. Admixtures that retard rapid setting of cement paste and modify workability of concrete are necessary. However, in Nigeria the retarder admixtures are imported and also expensive. Sugar is another substitute that has been established to be a retarding admixture (Rathi and Kolase, 2013), but its effects varies with the chemical composition of cement (Neville and Brooks, 2010). Because, cement with low tricalcium aluminate (C3A) and alkali contents are easier to retard compared to cement with large amounts of these constituents (Myrdal, 2007). One explanation might be that at lower C3A contents, smaller amounts of retarder are adsorbed, leaving larger amounts of the admixture to affect and retard the hydration of the C3S component. In line with this, the research question arise, which brand of cement in Nigeria can give an optimum results when sugar is used as admixture in it? Therefore, research needs to be carried out on selected brands of cement in Nigeria to ascertain the best cement brand to be used so as to achieve effectiveness in terms of setting as well as desirable strength results.
1.3 Justification of the study
The use of sugar to retard the rapid setting of cement paste and to modify the workability of fresh concrete at construction site seems reasonable as it is cheap and readily available as compared to other retarding admixtures (Rana, 2014). Thus, the need for proper guide for its use as admixture in concrete becomes necessary due to the fact that its retarding effect depends on the type of cement brand used in the concrete production (Dransfield, 2012). The results of this research work could provide some basic information to professionals in construction industry on the best and appropriate cement brand as well as the optimum dosage of sugar to be used as a retarding admixture when concreting in hot dry regions in Nigeria. Another importance of this study is to improve the quality of concrete produce in hot weather regions in Nigeria. This could be attained by using sugar as retarding admixture in concrete during the production stage of concrete so that to prevent it from adverse effects of high temperature.
1.4 Aim and Objectives
1.4.1 Aim
The aim of this research is to assess the effects of sugar as retarder on the properties of concretes made with selected brands of Portland cements with a view to establishing its effectiveness in retardating setting time of cement.
1.4.2 Objectives
The specific objectives are:
- To determine the chemical composition of the different brands of Portland cement.
- To assess the effects of sugar on the setting times of the different brands of the cement at two different temperatures of 27oC and 43
- To determine both the fresh and hardened properties of concretes made with the different brands of cements and sugar as retarding admixture.
- To ascertain an optimum dosage of sugar as an admixture in concretes made with the different brands of cement.
- To compare the properties of concretes made with the different brands of cement and sugar as retarding admixture.
1.5 Scope and Limitations
1.5.1 Scope
The research focused on the effects of sugar as retarder on the properties of concretes made with selected brands of Portland cement in Nigeria. The five different brands of cement selected for the research were; Ashaka, Bua, Dangote, Elephant and Sokoto cements. The cement brands were chosen because they are the ones readily available in the Northern parts of Nigeria. The tests carried out in order to achieve the objectives of this research includes; workability test, setting time test, density test, compressive strength test, split tensile test, abrasion test and water adsorption test and the percentage addition of sugar were 0%, 0.06% and 0.1% by weight of cement. The setting time tests were performed at two different temperatures; at normal laboratory temperature of 27oC and at an oven temperature of 43oC ̶ approximate maximum ambient temperature for Sokoto, Birnin Kebbi, Maiduguri, Yola etc.
1.5.2 Limitations
The hardened concretes were not subjected to the temperature of 43oC, they were prepared at treated normal room temperature of 27oC. Other properties such as shrinkage, durability index, water content and elastic properties were not checked due to equipment constraints.
