Chapter 7. Concrete Mix Design and Fresh Concrete Properties

Transcription

1 Chapter 7 Concrete Mix Design and Fresh Concrete Properties 7.1 GENERAL This chapter describes the conceptual concrete mix design procedures adopted for different lightweight s produced in this study. Aggregate packing models were used to arrive the concrete ingredients for various types of fly ash concrete mixes. The workability of fresh concrete properties in terms of slump value was measured and a comparative assessment on the fresh concrete properties is discussed in this section. 7.2 CONCRETE MIX DESIGN Lightweight concrete containing porous s requires a careful estimation of mix water as it needs additional water for initial saturation of s. The additional water required for filling the inherent pores of s to obtain the desired level of saturated surface dry condition was estimated. In order to compensate the initial water required for obtaining saturated surface dry conditions the s was pre-wetted initially before preparing concrete. Further, studies were made to arrive a systematic procedure to arrive the various concrete constituents using packing models as discussed in further section. 7.3 CONCEPTUAL MIX DESIGN PROCEDURE FOR LWFAC AGGREGATE PACKING MODEL In order to arrive a theoretical volume of s required, packing models were estimated for a given volume of the concrete. Maximum packing density model adopted in LWFAC mix design procedure provides a reliable estimate on the volume of the coarse and the corresponding mortar required for a unit volume of concrete. It can be noted that the density of concrete was decreased when the maximum volume is occupied by the coarse s. The advantage of using maximum packing model leads to 110

2 economical consumption of mortar in the concrete mix design. An appropriate packing model was developed for optimizing concrete mixes by void minimization THEORETICAL VOID RATIO The voids ratio in a unit volume of a packed material is defined as the ratio of volume of voids (including internal pores) to that of total volume of solids PACKING MODELS FOR THREE PHASE SYSTEM The packing models developed for different fly ash-bentonite concrete mixes (LWFAC) were shown in Figures 7.1 (a), (b) and (c), which represent the theoretical packing model. An approximate maximum packing volume of 62% was obtained for all the type of s (particle size of 12.5 mm and down). This resulted in 38% of the total volume of voids occupied for the given 1 m 3 of concrete. Further assessment on the volume of s and the corresponding mortar was calculated for 50% and 40% of the total volume in order to study the effect of fly ash on the concrete properties. The detailed calculations on the mix proportions are provided in Appendix B. The basic theoretical calculations for arriving the proportions are given in Equations 7.1 and 7.2. The detailed concrete mix proportions for different types of fly ash concretes obtained from the theoretical calculations are provided in Tables 7.1 to 7.5. (a) (b) 111

3 (c) Figure 7.1 (a, b & c) Different volume proportions of three phase materials adopted V CA + V M = (7.1) Therefore, V M = V C + V FA + V W (7.2) For 1 m 3 of concrete, the theoretical maximum packing of coarse (V CA ) was found to be 62%, It can be noted that, V CA + V M = 1; and therefore, V M = 38%. Substituting the value of V M in equation (7.2), Where, C/FA = 1/ (7.3) W/C = (7.4) C + FA + W = 38% C + 3 C C = 38% C + 3 C C = 38% 112

4 Therefore, V C = 8.74% Then, V W and V FA is calculated from equation (7.3) and (7.4), Hence, V FA = 26.21% and V W = 3.06% WORKABILITY OF LWFAC The fresh concrete workability was assessed using slump cone method and the results for different types of fly ash concrete mixes are provided in Tables 7.1 to 7.5. For the maximum volume occupied in a given concrete mix exhibited a reduction in the slump value (as shown in Figure 7.2). However, the addition of super plasticizing admixtures in concrete had shown an improvement in workability (as shown in Figure 7.3). Also, the true slump of concrete was observed in concrete mixtures with higher fly ash containing various types of fly ash concrete mixes. It was also noted that, sintered fly ash based s exhibited higher workability than cold-bonded s due to lesser consumption of water required for pre wetting of s. This showed an increased workability in the case of sintered glass fibre reinforced fly ash metakaolin concrete (40MT-GFS) which exhibited a slump value of 80 mm as observed in Figure 7.4. Cohesiveness of concrete mixes primarily depends on the free water present around the s to reduce inter particle friction. 113

5 Concrete Mix ID Table 7.1 Different mix proportions of fly ash-bentonite concrete used in the study Type of Cement Sand Coarse Water W/C ratio V CA V M SP Slump (mm) Density of concrete 62BT-CB 20BT BT-CB 20BT BT-CB 20BT BT-S 20BT2-S BT-S 20BT2-S BT-S 20BT2-S BT-GFS 20BT2-GFS BT-GFS 20BT2-GFS BT-GFS 20BT2-GFS Note: W/C - water to cement ratio, V CA - volume of coarse, V M - volume of mortar, SP - super plasticizer 114

6 Concrete Mix ID Table 7.2 Different mix proportions of fly ash-metakaolin concrete used in the study Type of Cement Sand Coarse Water W/C ratio V CA V M SP Slump (mm) Density of concrete 62MT-CB 30MT MT-CB 30MT MT-CB 30MT MT-S 30MT3-S MT-S 30MT3-S MT-S 30MT3-S MT-GFS 30MT3-GFS MT-GFS 30MT3-GFS MT-GFS 30MT3-GFS Note: W/C - water to cement ratio, V CA - volume of coarse, V M - volume of mortar, SP - super plasticizer 115

7 Concrete Mix ID Table 7.3 Different mix proportions of fly ash (without binder) concrete used in the study Type of Cement Sand Coarse Water W/C ratio V CA V M SP Slump (mm) Density of concrete 62FL-CB FL FL-CB FL FL-CB FL FL-S FL3-S FL-S FL3-S FL-S FL3-S FL-GFS FL3-GFS FL-GFS FL3-GFS FL-GFS FL3-GFS Note: W/C - water to cement ratio, V CA - volume of coarse, V M - volume of mortar, SP - super plasticizer 116

8 Concrete Mix ID Table 7.4 Different mix proportions of fly ash GGBS concrete used in the study Type of Cement Sand Coarse Water W/C ratio V CA V M SP Slump (mm) Density of concrete 62S-CB 30S S-CB 30S S-CB 30S S-S 30S2-S S-S 30S2-S S-S 30S2-S S-GFS 30S2-GFS S-GFS 30S2-GFS S-GFS 30S2-GFS Note: W/C - water to cement ratio, V CA - volume of coarse, V M - volume of mortar, SP - super plasticizer 117

9 Table 7.5 Different mix proportions of fly ash-cement concrete used in the study Concrete Mix ID Type of Cement Sand Coarse water W/C ratio V CA V M SP Slump (mm) Density of concrete 62C-CB 20C3-HC C-CB 20C3-HC C-CB 20C3-HC C-S 20C3-S C-S 20C3-S C-S 20C3-S C-GFS 20C3-GFS C-GFS 20C3-GFS C-GFS 20C3-GFS Note: W/C - water to cement ratio, V CA - volume of coarse, V M - volume of mortar, SP - super plasticizer 118

10 It is also inferred that a general trend showing an increase in workability for lesser volume of s was noticed for all the types of s. The workability of all fly ash concrete mixes was consistently improved with the addition of superplastizer and was adequate for the compaction in the moulds without any defects. Figure 7.2 Slump test for cold-bonded fly ash-metakaolin concrete (62MT-CB) Figure 7.3 Slump test for sintered fly ash-metakaolin concrete (50MT-S) 119

11 Figure 7.4 Slump test for sintered glass fibre reinforced fly ash-metakaolin concrete PRODUCTION OF LWFAC Light weight concrete production requires careful estimation of mix water since, additional water is required to achieve the desired workability level due to consumption of water for saturation of pores in the s. Hence, prior to dry mixing of concrete ingredients fly ash s were soaked in water for 30 minutes and taken out for normal air drying for 10 minutes. This ensures an initial saturated surface dry condition before adding into the concrete mixer. Concrete was prepared in a laboratory pan mixer with the dry ingredients (cement, fine and saturated fly ash s) added into the mixer and the mixing duration was kept for 5 minutes. The fresh concrete was then cast in 100 mm size cubes for estimating the hardened compressive strength properties after required curing. The concrete specimens were cured using four different methods such as normal water curing, hot air oven curing (100⁰C) up to 7 days, Hot water curing at 75⁰C (12 hours) and steam curing at 75⁰C (12 hours). 7.4 SUMMARY The following observations are noted from the above experimental study. Concrete Mix design procedure using maximum packing model adopted in this study leads to economical consumption of mortar in the concrete mix design. Workability of fresh concrete mixtures was affected with the addition of fly ash s. The true slump of concrete was observed in concrete mixtures with 120

12 higher fly ash. However, with the addition of superplastizer upto 1.5% (kept constant for all the concrete mix) the workability was found to be improved with a slump value ranging between 50 to 86 mm for different types of fly ash based. It is also inferred that an increase in workability of concrete for lesser volume of s was obtained for all types of fly ash concrete mixes. The workability of concrete mixes was enough for the formation of good concreting in the moulds without any defects. 121