Performance of Fresh And Hardened Concrete - Essay Example

Design and Technology – CE2140 Performance of fresh and hardened concrete with recycled aggregates included {KU Number} IntroductionThere is an increasing trend for using recycled materials to create new concrete in order to increase sustainability and decrease costs among other factors. Recycled materials are being used as aggregate in concrete. The effects of these recycled aggregates on the created concrete can be minimal to significant depending on a host of factors. The properties of concrete are affected both in the fresh state and in the hardened state and so there is great need to understand how the use of recycled aggregates affects concrete. The various kinds of recycled aggregates in use include recycled concrete aggregate (RCA), recycled wash glass sand, recycled rubber tyres and others. This text will attempt to analyse the effects of recycled aggregates on the fresh properties of concrete (such as workability, compatibility, segregation, bleeding etc.) and on the properties of hardened concrete (such as strength, durability etc.). Effects of using recycled aggregate (RA) Recycled aggregates are composed of a host of materials but in general recycled aggregates are composed of common building materials in particular. These effects have been studied in detail in a number of studies. (Mindess & Young, 1981) (Back, 1977) (ACPA, 1993) Recycled aggregates composed of building materials are often composed of wasted concrete, tiles and bricks. In addition materials such as steel, wood, plastics, paper and other similar materials can also become part of the recycled aggregate. All of these materials affect the fresh and hardened properties of concrete significantly. Therefore these materials are generally excluded from recycled aggregates through fine selection by screening through magnetised means, wind separation, manual picking up etc. (Chen et al., 2003). Chen et al studied these effects by creating five mixtures of concrete with recycled aggregates composed of 0%, 17%, 33%, 50% and 67% by weight. Furthermore two groups of aggregate were created having different compositions of aggregate components as Group A (greater amount of tiles included) and Group B (greater amount of bricks included). Recycled aggregates such as sand, mortar, gypsum etc. were kept constant throughout both groups. The properties of both groups of recycled aggregates are shown in the table below and the properties of natural aggregate are shown below it. Figure 1 - Recycled Aggregate Properties Figure 2 - Natural Aggregate Properties Based on the tables above it is apparent that all properties remain nearly the same except for absorption which represents a major effect on fresh concrete. Recycled aggregates are far more absorbent because of their larger particle sizes. This indicates that nearly all of the properties of fresh concrete will be affected negatively. The results of the study also indicated that the properties of hardened concrete would also be affected as the proportion of aggregate included increased. More specifically, the compressive strength of hardened concrete decreases as the recycled aggregate content in it increases. The total change in compressive strength of the hardened concrete was within 10% of the compressive strength of concrete made out of natural aggregates. This is depicted in the graph below. Figure 3 - Change in Compressive Strength of Concrete with Increasing Recycled Aggregate Components Another study by Kovler and Roussel (2011) on the properties of fresh and hardened concrete ended up with similar findings. (Kovler & Roussel, 2011) The study also related that increasing non building based recycled aggregates reduced the properties of hardened concrete by as much as 30% especially in cases of adding rubber as the recycled aggregate. The study suggested that concrete composed of recycled aggregates especially with plastic and rubber components should be used for as low structural loading as warranted. However, in cases that building material based substances are added under 50% by weight as recycled aggregates such as marble powder, then the resulting change in properties of fresh and hardened concrete is not highly different. (Topcu et al., 2009) Effects of using recycled concrete aggregate (RCA) The use of waste concrete as a means of creating aggregate for creating new concrete is becoming increasingly popular. Such practices allow the reduction of landfill disposal, help to conserve primary resources and also aid in reducing transport costs significantly. Investigations into the use of RCA concluded that the cement attached to aggregate particles is the primary determinant of concrete performance. (Rasheeduzzaffar & Khan, 1984) Moreover compressive strength was seen to decrease by some 10% while the drying shrinkage was increased by some 70% commonly through the use of RCA. A physical investigation by Sagoe-Crentsil et al (2001) into this issue revealed was conducted by considering for primary mixture types. These included ordinary Portland cement (OPC) with basalt, OPC with RCA, slag cement with RCA and OPC with 5% RCA. Based on the results of this study, the investigators concluded that the properties of fresh concrete lay within close limits of each other. The amount of trapped air was similar (within 5%) for all samples. However the absorption of RCA based concrete was found to be greater than that of the base sample. However, there were no problems in achieving the desired amount of consistency and compaction for all of the concrete samples. These results are reflected in the table shown below which was sourced from the original investigation. (Sagoe-Crentsil et al., 2001) Figure 4 - Mix Designations and Mixture Details of Concrete Specimens The study also investigated the strength and durability of concrete with varying proportions of RCA over a period of 365 days where all concrete samples were stored under similar conditions. The results from this are graphed below for the compressive strength and the tensile strength respectively. Figure 5 - Development of concrete compressive strength with age Figure 6 - Relationship between splitting tensile strength of recycled concrete and reference basalt concrete with time The plot of mean compressive strength of all the samples clearly indicates that the compressive strength does not change significantly with the age of the concrete. The total drop in strength is well within the 10% limit of the original sample. The slag cement based sample achieves more strength than the other samples owing to hydraulic properties. Moreover, the strength achieved by OPC with RCAs remains nearly the same at the 5% significance level after the 28 days. Similar results are achieved for the tensile strength where the strength of three samples (excluding slag cement with RCA) remains within close margins of each other. However the slag cement with RCA mixture is seen to gain far more strength which can be attributed to the binder rather than the kind of aggregate in use. In much the same way as above, the drying shrinkage strain of these samples decreased consistently but the spread was around 15%. The expansion of the samples was also consistently within close limits of each other. In terms of the abrasion of the samples in question, the investigation found out that accelerated carbonation had similar effects on all samples. Moreover the total abrasion resistance of all samples remained similar as given below in the histogram depicting the total material removed. Figure 7 - Abrasion resistance of recycled concrete and reference concrete A similar investigation by APCA concluded the same results as those mentioned above. (ACPA, 2010) These results are shown below in tabular format from the original report for fresh concrete and hardened concrete. Figure 8 - Properties of RCA based fresh concrete Figure 9 - Properties of RCA based hardened concrete Effects of using recycled wash glass sand (WGS) The problems associated with disposing of waste glass are huge when compared to other materials. However, if these materials are used in varying proportions in concrete creation an amount of sustainability can be attained. Glass is composed of sand which is also used to create concrete although the sand in glass has already undergone major transformation. However there are possibilities for using glass sand in concrete. The particular glass sand under consideration in this case is washed glass sand (WSG) which indicates that the subject glass sand has been treated through screening and washing to create a more consistent aggregate mixture. A study conducted by Limbachiya (2009) has investigated the link between the usage of WSG as a concrete aggregate and the resulting properties of fresh and hardened concrete. The subject investigation utilized multiple aggregate mixes with the baseline at 0% and the maximum utilized aggregate level of 50% by weight. (Limbachiya, 2009) The resulting properties of fresh concrete are delineated in the table below that was sourced from the original investigation. Figure 10 - Properties of fresh concrete with NA and WGS mixes The results above clearly indicate that the workability of the aggregate mixes remained steady till 30% by weight aggregate but deteriorated following that mark. However, the specified tolerance levels of 25 mm were not crossed. However the consistency and the wet densities of the concrete mixes with WGS can be seen to deteriorate beyond the 20% aggregate by weight mark. The shape of WSG was identified as the greatest culprit behind the achieved trends. The use of a filler material is prescribed as the best solution to these deviations. The bulk properties of the WSG based mixtures can be seen in the graph below. It is apparent from the graph above that the strength remains consistent up to an aggregate mix of 20% by weight after which the strength suffers significantly. Moreover the strength of the WSG based concrete mixes was observed to be stable well over a year after the casting. The results of this study are also confirmed consistently by other studies on the matter. (Shayan & Xu, 2006) (Taha & Nounu, 2008) Effects of using recycled rubber tyres Disposing of scrap rubber products such as rubber tyres is a mounting challenge for sustainability today. Colossal volumes of rubber tyres are produced as scrap each year throughout the world. This is exacerbated by the fact that vulcanised rubber is non biodegradable. Land filling has not emerged as a sustainable solution for dealing with rubber tyres. There has been some interest in using rubber tyres in concrete as an aggregate material. The resulting properties after using rubber as an aggregate material have been investigated by Siddique and Naik (2004) through literature review only. The results are seen as encouraging to the use of rubber as an aggregate in concrete especially for applications such as pavements. The investigation reveals that fresh concrete shows improved workability with the addition of rubber as an aggregate material. However slump was seen to decrease as the rubber content increased as part of volume of the mixture. Notably slump was almost zero at rubber content of 40% or more and so concrete was not manually workable anymore. Air content also increases as more and more rubber is added. This is thought to occur as rubber does not possess a string affinity for water. Another major finding was that the weight of the cast blocks with rubber content was lower than those without rubber. However the effects of weight are notable only after rubber content goes up more than 20% as total aggregate volume. In contrast the investigation shows that the hardened properties of concrete are affected negatively by the addition of rubber. The compressive and tensile strengths of rubber are seen to decrease significantly when rubber is added. A comparison is presented in the table below. Reduction Rubber Content Compressive Strength Tensile Strength Coarse 85% 50% Fine 65% 50% Figure 11 - Change in properties of concrete with the addition of rubber The addition of rubber also aids in reducing the plastic shrinkage of concrete. Moreover rubber addition increases the toughness of concrete though not highly significantly. Concrete becomes less stiff and less brittle when rubber is added. (Siddique & Naik, 2004) Conclusion Based on the investigations and results presented above it is obvious that the addition of recycled aggregates affects the properties of concrete significantly. The inclusion of recycled aggregates to concrete helps to increase the sustainability and reduce the cost of the build but compromises on the strength achieved. Therefore concrete created with these methods can be utilised for less demanding tasks that require lower loading. In some cases however, the addition of aggregates has helped improve some of concrete’s abilities especially fresh concrete’s workability and other properties. More investigation into creating concrete with the inclusion of aggregates can help to boost sustainability and decrease costs further. Bibliography ACPA, 1993. Concrete Paving Technology: Recycling Concrete Pavement. Skokie, Illinois: American Concrete Pavement Association American Concrete Pavement Association. ACPA, 2010. Properties of Concrete containing RCA. Investigation. Skokia, Illinois: ACPA American Concrete Pavement Association. Back, D., 1977. Recycle concrete as a source of aggregate. ACI Mater Journal, 74(22), pp.212-19. Chen, H.-J., Yen, T. & Chen, K.-H., 2003. Use of building rubbles as recycled aggregates. Cement and Concrete Research, 33, pp.125-32. Kovler, K. & Roussel, N., 2011. Properties of fresh and hardened concrete. Concrete and Cement Research, 41, pp.775-82. Limbachiya, M.C., 2009. Bulk engineering and durability properties of washed glass sand and concrete. Construction and Building Materials, 23, pp.1078-83. Mindess, S. & Young, J.F., 1981. Concrete. New Jersey: Prentice Hall. Rasheeduzzaffar, I.B. & Khan, A., 1984. Recycled concrete - a source of new concrete. ASTM Cement, Concrete, Aggregrates, 6(1), pp.17-27. Sagoe-Crentsil, K.K., Brown, T. & Tayor, A.H., 2001. Performance of concrete made with commercially produced recycled concrete aggregate. Cement and Concrete Research, 31, pp.707-12. Shayan, A. & Xu, A., 2006. Performance of glass powder as a pozzolanic material in concrete: A field trial on concrete slabs. Cement Concrete, 36, pp.457-68. Siddique, R. & Naik, T.R., 2004. Properties of concrete containing scrap tire rubber - an overview. Waste Management, 24, pp.563-69. Taha, B. & Nounu, G., 2008. Properties of concrete containing mixed colour waste recycled glass as sand and cement replacement. Construct Build Mater, 22, pp.713-20. Topcu, I.B., Bilir, T. & Uygunoglu, T., 2009. Effect of waste marble dust content as filler on properties of self compacting concrete. Construction and Building Materials, 23(2), pp.981-88. Read More