Effect of additional agitation on the development of strength of cement mortar studied using ultrasonic technique

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1 Effect of additional agitation on the development of strength of cement mortar studied using ultrasonic technique 1 Mohit Gupta; Dept. of Civil Engineering BRCM College Bahal, Haryana(INDIA) 2 Dr. Arabinda Sharma; Dept. of Civil Engineering Associate Professor,BRCM College Bahal,Haryana(INDIA) Abstract: In this paper, the effect of additional agitation on the development of strength of cement mortar studied using ultrasonic techniques. Mortars having water to cement ratio (w/c) of 0.40 were produced in the laboratory and were agitated for defined periods before being cast into cubes. The strength of these cubes at 7 days and 28 days were measured. The development of the mechanical properties of these mortars was also investigated using the ultrasonic pulse velocity method. It was found that the effect of additional agitation on the compressional wave velocity is also investigated through the ultrasonic pulse velocity method. The technique developed in this study is a first proof of concept for assessing the effect of delay in placing of concrete on its strength development at the site. The technique if further researched has the potential to generate indicators towards the acceptability of the concrete at the site after the delay. Research work presents an ultrasonic evolution of the fresh mortar. The mortar having various sand to cement ratios (0.25, 0.50, 3) and constant water to cement ratio of Key words: ultrasonic technique, additional agitation,strength development, compressive strength Introduction: The ultrasonic method can be used to follow setting. The change of ultrasonic velocity over time is sensitive to the differences in setting behavior of the tested mixtures. Initial setting seems to correspond with the point of inflection of the velocity-vs.-time graph and final setting time with point at which the velocity becomes constant. It is difficult to determine the usability of concrete on site after transit stage between batching plant and site. A delay in the placing of concrete on site could have a significant effect on the strength and durability of concrete. In this study, it is proposed to monitor the process of setting and strength development in concretes through an initial study on mortars. Literature Review: Reinhardt (2004) developed test setup for the exact determination of transmitted ultrasonic pulse. It was shown that the good estimate of initial setting time can be determined from the velocity vs. age of 2016, IRJET Impact Factor value: 4.45 ISO 9001:2008 Certified Journal Page 3188

2 cement paste by mathematical procedure and final setting time can be determines using empirical experience relationships. Many researchers successfully worked on setting and hardening of cementitious material. Reinhardt et al. (2004) have done continuous monitoring of hardening process of mortar and concrete. They developed a container that helps to measure transmitted ultrasonic pulse accurately. Subramanian et al. (2010) investigated microstructure evolution in cement paste by using ultrasonic technique and rheological measurements. He concluded that ultrasonic method and rheological measurements allows for relating changes in the microstructure with the mechanical properties of hydrating cement paste through setting and hardening.. Figure 1 Development of Strength and Stiffness Source: (Nenad B., 2012) Methodology: Effect of additional agitation on setting in mortar was observed with water to cement ratio (w/c=0.40) and sand to cement ratio (s/c=3) also using the same mixtures for measurement of compressive strength. Compressive strength was measured after 7 days and 28 days curing at 27 degrees centigrade temperature. Experimental Program In a Ready Mix plant, the concrete is agitated continuously before delivery at the site. Sometimes concrete may reach the site late, due to various reasons. At present no method exists that can ascertain whether the concrete is usable or not after the prolonged agitation. The method that is currently existing involves the slump test for checking the workability of the concrete. This technique may not be very accurate and may lead to either wastage of usable concrete or acceptance of concrete that is no longer usable. The method proposed in this work is a first step towards a more accurate technique of ascertaining the extent of setting of an agitated sample of concrete in a Ready Mix plant. All the experimental investigations are performed on cement mortar. 2016, IRJET Impact Factor value: 4.45 ISO 9001:2008 Certified Journal Page 3189

3 Experimental Investigations on Agitated Samples The effect of agitation on the setting characteristics of the cement mortar is investigated by:- 1. Conduction of compressive strength tests to complement the UPV investigations. The mortar is in a mixer, the inside surface of which does not absorb moisture and nor does the mortar stick to the internal surface. The mixer is fitted in external blades that lift and tumble the ingredient on to itself. The ingredients are first put in the mixer and the mixer is rotated and subsequently some water is added while the mixer rotates. After some time the remaining water is added to the mixer and it is rotated for some more time. The total mixing time of the mortar was about 3 minutes. Three cubes are cast from the mortar mix for testing the 7 days compressive strength. The ultrasonic pulse velocity is also monitored every 3 minutes continuously for a period of 6 hours. The data generated from these tests correspond to the mortar sample which is Non- Agitated. The remaining mixture is agitated in the mixer for 3 minutes every 10 minutes interval. Two sets of three cubes, each set corresponding to 7 and 28 days strength tests, are cast from the mix 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330 and 360 minutes after the commencement of the mixing. The samples subjected to varying durations of agitation are once again prepared on different days and the UPV is monitored according to the following schedule- day 2: the sample agitated for 1 hour, day 3: on 2 hr. agitated sample, on day 4 on 3 hr. agitated sample, on day 5 on 4 hr. agitated sample, and on day 6 on 5 hr. agitated sample. Compressive Strength Test The prepared samples were cast in metallic cubes moulds with non-absorbent surfaces of dimensions 50mm x 50mm x 50mm. Compaction was done with a mechanical shaker. Prior to filling the moulds with mortar, oil was applied to the internal surfaces to ensure that the mortar cubes could be easily demoulded. The mortar cubes were cured in a curing tank and the samples were demoulded 24 hours after casting. Some samples were kept in the curing tank for 7 days and the others for a period of 28 days. The casting date and specimen name were written above the specimen once demoulded before putting in the curing tank. The cubes were taken out of the curing tank and the moisture was wiped off the surface using a clean dry cloth. The cubes were tested for their compressive strengths in the compression testing machine. 2016, IRJET Impact Factor value: 4.45 ISO 9001:2008 Certified Journal Page 3190

4 Figure 2: Automatic Compression Testing Machine Using rate of loading.007 Kg/cm2/min for 50mm mortar sample. Three cubes were tested for 7 days and 28 days compressive strength, the average of three sample was considered to be the compressive strength of sample. The load was applied uniformly at the said rate 28 days. The casting date and specimen name were written above the specimen once demoulded before putting in the curing tank. The cubes were taken out of the curing tank and the moisture was wiped off the surface using a clean dry cloth. The cubes were tested for their compressive strengths in the compression testing machine. Combine graph UPV vs Duration of Agitation Duration of agiitation (min) velocity(non agitated) V (60 min) V(120 min) V (180 min) V (240 min) Figure 3 Plot of UPV vs. Duration of Agitation Figure 3 shows the values of evolution of the UPV of the samples agitated for various durations. In the non-agitated sample, ultrasonic pulse velocity reaches to a velocity of 200 m/s at approximately , IRJET Impact Factor value: 4.45 ISO 9001:2008 Certified Journal Page 3191

5 minutes after the sample is poured inside the container. While the sample that is agitated for 1 hour, the same velocity is attained 126 minutes after pouring the sample in the container. The corresponding values for samples agitated for 2, 3 and 4 hours are 120, 93 and 90 minutes. This in turn means that some setting has happened earlier that causes the compressional wave arrival faster at early age for these agitated samples. Table 1: Ultimate velocity attained for various duration of agitation S. No. Duration of agitation (min) Ultimate velocity attained (m/s) 1 Non agitated sample Table 1 shows that longer duration of agitation is showing a lower ultimate compressional wave velocity; this is an indirect indicator of lower strength SLOPE vs TIME Time (min.) slope non agitated 60 min 120 min 180min 240 min Figure 4: Evolution of the slope of the UPV curve plotted as a function of time Figure 4: shows that rate of development of the UPV as a function of the age of the mortar. This may be considered as an indirect indicator of the development of strength of the sample which is related to the UPV. In the Non-Agitated sample, the rate of increase of the UPV is slower, indicating that the strength development is slow compared to the samples subjected to prolonged agitation. However, the latter part of 2016, IRJET Impact Factor value: 4.45 ISO 9001:2008 Certified Journal Page 3192

6 Compressive Strenghth m 2 ) International Research Journal of Engineering and Technology (IRJET) e-issn: the graph is almost straight indicating a linear increase in the velocity with the progression of time. The curve corresponding to rate of increase of the UPV for the sample agitated for 60 minutes, is comparable to the Non-Agitated sample. The rate of increase of the UPV for the sample subjected to 120 minutes of agitation is higher in comparison to both the Non-Agitated and 60 minutes agitated samples. However, since the duration of the experiment for this sample is shorter due to some technical difficulties developed during the experimentation, not much can be inferred about the ultimate velocity developed. However, the nature of the curve indicates better development of strength possibly due to faster hydration caused by better mixing of the ingredients. For the samples agitated for 180 and 240 minutes, the rate of increase is high in the initial part of the curve and then the rate decreases with time. Longer duration of agitation allows air voids to be mixed in the mixture which results in reduced rate of strength gain. Results and Discussion- Result obtained from the experimental investigation is: Compressive Strength Test to Complement Ultrasonic Test The compressive strength tests are carried out on the cast samples to complement the results obtained from the ultrasonic tests days Compressive Strength (N/mm 2 ) Duration of agitation (min.) Figure 5: 28 Days Compressive Strength vs. Duration of Agitation for Mortar Figure 5 shows the 28 day compressive strength obtained from mortar samples with different durations of agitation. It is observed that approximately beyond 210 minutes of agitation, the compressive strength 2016, IRJET Impact Factor value: 4.45 ISO 9001:2008 Certified Journal Page 3193

7 Compressive Strength (N/mm 2 ) International Research Journal of Engineering and Technology (IRJET) e-issn: developed in the samples starts to decrease with increasing duration of agitation. This means that after this duration of agitation (210 minutes), the mortar may not be useable.it can be observed in Fig. 3, that the ultimate compressional wave velocity developed in the sample agitated for 240 minutes is lower than the ultimate velocity developed in the samples agitated for lower durations. 7 Days Compressive Strength (N/mm 2 ) Duration of agitation (min) Figure 6; 7 Days Compressive Strength vs. Duration of Agitation for Mortar Figure 6 shows the 7 day compressive strengths of the mortar samples subjected to different durations of agitation. In this figure also, approximately beyond 210 minutes, the developed compressive strength decreases with increasing durations of agitation. This confirms that the mortar may not be useable if agitated for more than 210 minutes. There is a clear relationship between the compressive strength and duration of agitation. As the duration of agitation increases, compressive strength decreases which is also indicated by reduced ultimate wave velocity. 2016, IRJET Impact Factor value: 4.45 ISO 9001:2008 Certified Journal Page 3194

8 Conclusion The problem of a large amount of Ready mix concrete going waste due to rejection based on the testing of a small sample may be better addressed using NDT tests. At the same time it can ensure greater durability of built structures by providing a better control on the mix to use. Monitoring of early age strength development using ultrasonic pulses can be highly effective to predict the long term strength and durability of the structures. Prediction of long-term strength can also be advantageously made in a much shorter duration by ultrasonic pulse velocity measurement at 7 days and 28 days can be avoided. This actually can save a lot of material and labour wastage and corresponding costs in constructing structure and demolishing it later based on cube test results. The effect of the prolonged agitation on the development of strength was studied using ultrasonic techniques. It was found that the effect of delay in placing the mortars could be captured using the ultrasonic pulse velocity method. This technique would be useful in assessing the effect of delay in placing concretes on the strength development of concretes at site and the acceptability of the concrete after the delay. Sand to cement content decreases, the velocity of the compressional wave s increases. Hence, it may be concluded that sand attenuation at early age is very high through transmission measurements. Hence the ultrasonic waves are influenced by the attenuation of sand. As the sand content is decreased, the attenuation decreases and ultrasonic pulse velocity increases. It is found that longer duration of agitation not only is reducing the strength but also showing a lower ultimate compressional wave velocity; this is an indirect indicator of 36 lower strength. Also the method is an early pointer towards the condition of the mortar compared to the 7days and 28 days tests which is a big advantage. References 1. Al-Mufti, R.L. and Fried, A.N. (2012), The early age Non-destructive testing of concrete made with recycled concrete aggregate, Construction and Building Materials, 37, Boumiz, A., Vernet, C. and Tenoudjif, F.C. (1996) Mechanical properties of cement paste and mortar, Advanced Cement Based Material, 3, Bishnoi, S., (2013), Mechanical Properties of Concrete, personal slides, CivilEngineering Department, IIT, Delhi, slide no., Chowdhury, S., Jayasankar, K., Keshkar S.V. and Kadam, S.D. (2010), Influence of fine aggregates 2016, IRJET Impact Factor value: 4.45 ISO 9001:2008 Certified Journal Page 3195

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