Theory Associated with Concrete Mix Design and Using Pulverised Fly Ash as Cement Replacement - Math Problem Example

1.0 Introduction The process of producing concrete may be seen as a very important component with respect to quality control in many construction works. Cement hydration is the main reaction that leads to formation of concrete (ASTM C 702-98, 2003). The three components of concrete are water , aggregate and cement with the later serving as a bonding agent. Water is very important in ensuring that hydration process takes place. When calcium silicates and water reacts with fine fibres where crystals are formed thus resulting to a stiff building material (Babor, D., Plian, D. and Judele, L. ,2009). Concrete mixtures can be designed with the aim of providing a variety of mechanical as well as durability properties so as the design requirements may be achieved (Concrete in Practice Series). In the design of building and other structures engineers most often use compressive strength as the performance measure. The measurement of compressive strength involves breaking concrete specimens in a compression testing machine with the cylindrical shaped samples being preferred (Portland Cement Association,2015).. The calculation of compressive strength involves dividing the failure load by the cross-sectional area of the specimen. The required concrete compressive strength may vary from 17Mpa (mostly in residential concrete) to 28MPa and above for commercial and industrial structures (NRMCA Publication 133). Higher values that goes beyond 70MPa may be required for some specific applications. Experiment calculations NB: 1 N mm-2 = 1Mpa Aims and objectives This experiment aimed at investigating and validating the theory that is associated with concrete mix design and using Pulverised Fly Ash (PFA) as partial cement replacement. The objectives were 1. The design and batch a normal concrete trial mix using BRE method and use PFA as a partial replacement and the mix would be then be modified to come up with a self compacting concrete (SCC). 2. Conducting standard workability tests with outcome being reported 3. Cast concrete samples and perform standard tests these samples so as to establish mechanical properties of hardened concrete 4. Re-design the trial mix by use of the observed workability and strength results 5. Perform analysis of the concrete results for the mixes used in the experiment by the module cohort and make conclusions on the need of using PFA as a partial cement replacement and self-compacting concrete to improve the sustainable use of concrete in practice. 1.2 Experiment procedure There was dry mixing of coarse aggregate, fine aggregate and cement, the process being carefully undertaken so as to avoid any dust being produced. Addition of water was done gradually as the mixing of the concrete was being done up to the point a uniform colour and consistency was realized. The 100mm standard cube mould was filled in three equal layers and compacting each of the layers with at least 25 strokes using a tamping bar. Cylindrical shaped samples were also made by using cylindrical moulds. Surplus concrete was removed and the top surface was leveled using a trowel and the concrete was then left for 7 days. There was recording of corresponding mould numbers in a table against the appropriate mix After elapsing of seven days samples were retrieved from the curing tank where dimensions of the samples were measured, their masses were recorded and their volumes and densities were calculated with the data being entered into a table. The specimens were placed in a vacuum chamber which was evacuated for 10 minutes. The chamber was then flooded with water that saturated the specimens and upon 10 minutes lapse the specimens were removed. The surplus water was wiped off, the specimens were then weighed and their saturated masses recorded in a table. 1.3 Calculations Characteristic strength : 20Mpa Target age : 28 days Cement type = CEM1 class 52.5 Coarse aggregate: maximum size of 10mm Fine Aggregate = 50% passing 600µm sieve Specific gravity Cement = 3.15 Fine Aggregate = 2.61 Coarse Aggregate (10mm) = 2.66 Minimum Cement =400 kg / m3 Maximum water cement ratio = 0.45 Mix Calculation: – Target Mean Strength = 20+ ( 5 X 1.65 ) = 28.25 Mpa Water cement ratio selection Assumed water cement ratio = 0.4 Water content calculation Approximate water content for the case of use of 10mm maximum size aggregate = 208 kg /m3 Cement content calculation Water cement ratio = 0.4 Water content per m3 of concrete = 208 kg Cement content = 208/0.4 = 520 kg / m3 Calculation for C.A. & F.A. Concrete volume = 1 m3 Cement volume = 520 / (3.15 X 1000) = 0.1650 m3 Water volume = 208 / (1 x 1000) = 0.208 m3 Total volume of other materials with aggregate excluded= 0.208 + 0.1650 = 0.373 m3 Coarse and fine aggregate volume = 1 – 0.373 = 0.627 m3 Volume of fine aggregate = 0.7016 X 0.33 = 0.2315 m3 (with the assumption that 33% by volume of total aggregate) Volume of coarse aggregate = 0.7216 – 0.2315 = 0.4901 m3 Thus weight of fine aggregate = 0.2315 X 2.61 x 1000 = 604.215 kg/ m3 Say weight of fine aggregate = 604 kg/ m3 Therefore weight of coarse = 0.4901 x 2.655 X 1000 = 1301.2155 kg/ m3 Say weight of coarse aggregate = 1301 kg/ m3 Density = (208+520+604+1301)/1=2633 km/m3 Changes with admixture As plasticizer is proposed we can reduce water content by 20%. Now water content level= 208 X 0.8 = 166.4 kg /m3 Cement content = 166.4/0.4 = 416 kg / m3 Cement volume = 416 / (3.15 x 1000) = 0.1320 m3 Water volume = 166.4 / (1 x 1000) = 0.1664 m3 Admixture volume = 4.994 / (1.145 x 1000) = 0.0043 m3 Total volume of other materials excluding aggregate admixture included = 0.1320 + 0.1664 +0.0043 = 0.3027 m3 Total weight of aggregate =1- 0.3027 m3=0.6973 m3 Volume of fine aggregate (with admixture included) = 0.6973 x 0.33 = 0.2301 m3 (with the assumption that the fine aggregate is 33% by volume of total aggregate) Volume of coarse (with admixture) = 0.6973 – 0.2301 = 0.4672 m3 Therefore weight of fine aggregate (with admixture) = 0.2301 x 2.61 X 1000 = 600.561 kg/ m3 Say weight of fine aggregate = 600 kg/ m3 Therefore weight of coarse aggregate with admixture) = 0.4672 x 2.655 x 1000 = 1240.416 kg/ m3 Say weight of coarse aggregate = 1240 kg/ m3 Density= 166.4+416+600+1240.4+4.994=2427.794kg/m3 1.5 Results Table 2 Sample Mould type Slump(mm) Cubecross-sectional area (mm2) Cube volume Cube mass(kg) Cube density Average compressive strength (Nmm-2) Average cube density (mm3) (m3) 1 1 wet 40 10000 1000000 0.001 2.281 2281 33.73 1771.3 2 dry 10000 1000000 0.001 1.2616 1261.6 2 1 wet 45mm 10000 1000000 0.001 2.2923 2292.3 32.006 1775 2 dry 10000 1000000 0.001 1.2577 1257.7 3 1 wet 40 10000 1000000 0.001 2.2837 2283.7 34.671 1770 2 dry 10000 1000000 0.001 1.2563 1256.3 Table 3 Sample Mould number Slump(mm) Cubecross-sectional area (mm2) cylinder volume Cube mass(kg) Cube density Average compressive strength (Nmm-2) Average cube density (mm3) (m3) 1 1 wet 45 7854 1570800 0.00157 3.6225 2307.325 23.73 1797.2 2 dry 7854 1570800 0.00157 2.0206 1287.006 2 1 wet 40 7854 1570800 0.00157 3.5902 2286.752 17.46 1781.27 2 dry 7854 1570800 0.00157 2.0030 1275.796 3 1 wet 55 7854 1570800 0.00157 3.8442 2448.535 19.39 1872.99 2 dry 7854 1570800 0.00157 2.037 1297.452 1.6 Ultrasonic test results The modulus of elasticity is given by the relationship Where  is the Poisson’s ratio,  represents the density of specimen and  gives the longitudinal wave velocity for the sample materials From the experiment  is determined from the calculated densities while  is obtained by choosing appropriate Poisson’s ratio. Upon obtaining E we use the relationship  Through this procedure the following were the concrete strength for the cubical and cylindrical samples Table 4: Cubical samples strength Sample Strength 1 22.1 2 23.4 3 22.8 Table 4: cylindrical samples strength Sample Strength 1 23.739 2 17.463 3 19.393 1.7 Discussion From the calculation involved in coming up with a it was observed that that with admixture there can be reduction of the water content by up to 20%. The wet density of a mix with admixture is seen to have a lower density compared to that without admixture. Usually when there is use of admixture the amount of cement material is to be increased by about 10%this lower density is supposed to be taken care of by increasing the amount of cementious materials by about 10%. From the results it was seen that the cubical samples had higher strength compared to the samples with cylindrical sections. This could be an a pointer that for the case of a cylindrical section there is a smaller cross sectional area is and thus leading to failure occuring at a lower load. The results also indicates that samples with higher strength had higher slump value. This could be as a result of samples with higher strength having higher cement and high cement increased workability of the mix which resulted to a higher slump value. Conclusion From the experiment it has come out that it is possible to come up with a concrete mix of the desired strength. To achieve this it is required to come up with several mixes and to make adjustment in the desired direction. References ASTM C 702-98(2003), “Standard Practice for Reducing Samples of Aggregate to Testing Size.” ASTM C 637-98a (2003), “Standard Specification for Aggregates for Radiation-Shielding Concrete.” ASTM C31, C39,C617,C1077,C1231, Annual Book of ASTM Standards, Volume 04.02, ASTM, West Conshohocken, PA Babor, D., Plian, D. and Judele, L. (2009) “Environmental Impact of Concrete”, Publicat de Universitatea Tehnică „Gheorghe Asachi” din Iaşi Concrete in Practice Series, NRMCA, Silver Spring, MD, www.nrmca.org Hansen, T. C. and Narud, H. (1983) “Strength of Recycled Concrete Made from Crushed Concrete Coarse Aggregate”, Concrete International, Vol. 5, Iss. 1, pp 79–83. In-place Strength Evaluation- A recommended Practice, NRMCA Publication 133, NRMCA RES Committee, NRMCA, Silver Spring , MD Tips on Control Tests for quality Concrete, PA015, Portland Cement Association, Skokie, IL, www.cement.org Read More

There was recording of corresponding mould numbers in a table against the appropriate mix After elapsing of seven days samples were retrieved from the curing tank where dimensions of the samples were measured, their masses were recorded and their volumes and densities were calculated with the data being entered into a table. The specimens were placed in a vacuum chamber which was evacuated for 10 minutes. The chamber was then flooded with water that saturated the specimens and upon 10 minutes lapse the specimens were removed.

The surplus water was wiped off, the specimens were then weighed and their saturated masses recorded in a table. 1.3 Calculations Characteristic strength : 20Mpa Target age : 28 days Cement type = CEM1 class 52.5 Coarse aggregate: maximum size of 10mm Fine Aggregate = 50% passing 600µm sieve Specific gravity Cement = 3.15 Fine Aggregate = 2.61 Coarse Aggregate (10mm) = 2.66 Minimum Cement =400 kg / m3 Maximum water cement ratio = 0.45 Mix Calculation: – Target Mean Strength = 20+ ( 5 X 1.65 ) = 28.25 Mpa Water cement ratio selection Assumed water cement ratio = 0.

4 Water content calculation Approximate water content for the case of use of 10mm maximum size aggregate = 208 kg /m3 Cement content calculation Water cement ratio = 0.4 Water content per m3 of concrete = 208 kg Cement content = 208/0.4 = 520 kg / m3 Calculation for C.A. & F.A. Concrete volume = 1 m3 Cement volume = 520 / (3.15 X 1000) = 0.1650 m3 Water volume = 208 / (1 x 1000) = 0.208 m3 Total volume of other materials with aggregate excluded= 0.208 + 0.1650 = 0.373 m3 Coarse and fine aggregate volume = 1 – 0.373 = 0.627 m3 Volume of fine aggregate = 0.7016 X 0.33 = 0.

2315 m3 (with the assumption that 33% by volume of total aggregate) Volume of coarse aggregate = 0.7216 – 0.2315 = 0.4901 m3 Thus weight of fine aggregate = 0.2315 X 2.61 x 1000 = 604.215 kg/ m3 Say weight of fine aggregate = 604 kg/ m3 Therefore weight of coarse = 0.4901 x 2.655 X 1000 = 1301.2155 kg/ m3 Say weight of coarse aggregate = 1301 kg/ m3 Density = (208+520+604+1301)/1=2633 km/m3 Changes with admixture As plasticizer is proposed we can reduce water content by 20%. Now water content level= 208 X 0.8 = 166.4 kg /m3 Cement content = 166.4/0.4 = 416 kg / m3 Cement volume = 416 / (3.15 x 1000) = 0.

1320 m3 Water volume = 166.4 / (1 x 1000) = 0.1664 m3 Admixture volume = 4.994 / (1.145 x 1000) = 0.0043 m3 Total volume of other materials excluding aggregate admixture included = 0.1320 + 0.1664 +0.0043 = 0.3027 m3 Total weight of aggregate =1- 0.3027 m3=0.6973 m3 Volume of fine aggregate (with admixture included) = 0.6973 x 0.33 = 0.2301 m3 (with the assumption that the fine aggregate is 33% by volume of total aggregate) Volume of coarse (with admixture) = 0.6973 – 0.2301 = 0.4672 m3 Therefore weight of fine aggregate (with admixture) = 0.2301 x 2.61 X 1000 = 600.

561 kg/ m3 Say weight of fine aggregate = 600 kg/ m3 Therefore weight of coarse aggregate with admixture) = 0.4672 x 2.655 x 1000 = 1240.416 kg/ m3 Say weight of coarse aggregate = 1240 kg/ m3 Density= 166.4+416+600+1240.4+4.994=2427.794kg/m3 1.5 Results Table 2 Sample Mould type Slump(mm) Cubecross-sectional area (mm2) Cube volume Cube mass(kg) Cube density Average compressive strength (Nmm-2) Average cube density (mm3) (m3) 1 1 wet 40 10000 1000000 0.001 2.281 2281 33.73 1771.3 2 dry 10000 1000000 0.001 1.2616 1261.

6 2 1 wet 45mm 10000 1000000 0.001 2.2923 2292.3 32.006 1775 2 dry 10000 1000000 0.001 1.2577 1257.7 3 1 wet 40 10000 1000000 0.001 2.2837 2283.7 34.671 1770 2 dry 10000 1000000 0.001 1.2563 1256.3 Table 3 Sample Mould number Slump(mm) Cubecross-sectional area (mm2) cylinder volume Cube mass(kg) Cube density Average compressive strength (Nmm-2) Average cube density (mm3) (m3) 1 1 wet 45 7854 1570800 0.00157 3.6225 2307.325 23.73 1797.2 2 dry 7854 1570800 0.00157 2.0206 1287.006 2 1 wet 40 7854 1570800 0.00157 3.5902 2286.752 17.46 1781.

27 2 dry 7854 1570800 0.00157 2.0030 1275.

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