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ISRN Civil Engineering
Volume 2013 (2013), Article ID 369387, 7 pages
Pulse Velocity Measurements in Fly Ash Blended Cementitious Systems Containing 43 Grade Cement
Structural Engineering Division, School of Mechanical and Building Sciences, VIT University, Vellore, Tamil Nadu 632014, India
Received 9 March 2013; Accepted 18 April 2013
Academic Editors: I. G. Raftoyiannis and I. Smith
Copyright © 2013 V. M. Sounthararajan and A. Sivakumar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Investigations on the different supplementary cementitious materials based on the hardening properties and the optimized dosage in cementitious systems find the right choice of pozzolanic material. It is essential to combine various additive/admixtures in concrete in proper proportions to maximize the benefits resulting in cost savings in construction. In the recent years, production technology and composition of hydraulic cements affect the setting and early age behavior of cementitious material. The addition of fly ash in cement is one viable technology to derive maximum benefits in terms of the economy and improved pozzolanic reaction. Ultrasonic pulse velocity testing is a feasible method for evaluating the hardening properties of cementitious materials. In this study, an attempt was made to derive the engineering basis for understanding the development of hardness during hydration of fly ash (FA) based cementitious systems. The tests conducted using pulse velocity technique proved to be an effective method for characterizing the early strength gain properties of different cementitious systems.
During recent years many variations have been made in the production technology, and its performance exceeded expectations. In addition, mineral admixtures like fly ash have been combined with Portland cement content improve the demonstrated excellent performance, both in mechanical as well as in durability aspects [1, 2]. The addition of pozzolanic admixtures converts the leachable calcium hydroxide into insoluble nonleachable calcium hydroxide in the cementitious products . One such effect relates to the setting and early strength gain of concrete. Such delays are not desirable on site as tasks cannot be completed as per schedule. It is well known that chemical reactions between cement and water react with cement pastes, mortars, and concrete from fluids to rigid bodies . The term setting has been used to describe the onset of rigidity in fresh cement pastes, mortar, and concrete. The setting phenomena are altered some factors, such as water/binder (w/b) ratio, curing regime temperature, and admixtures [5–8]. Thermodynamically, the initial set is marked by a rapid temperature rise, which corresponds roughly to the beginning of the main factor of chemical reactions that temperature rise will reach a maximum rate around the final set . Chemical admixture becomes inevitably an important component to make a high early strength of hardened concrete . The objective of this study is to establish the scientific and engineering bases for understanding to improve the hardness during the hydration of cement-based materials by using the Vicat apparatus for determining the normal consistency and setting time for cement and measuring the pulse velocity through concrete using ultrasonic equipment. The detailed cement compositions of different percentage with different mixture proportions were studied.
2. Preparation of Specimens
Ordinary Portland cement 43 grade available in the local market was used in investigations. The cement used has been tested for various properties as per IS 4031-1988. The consistency of cement was 30% both ordinary Portland cement and Portland pozzolana cement and normal river sand passing through 2.36 mm IS sieve and conforming (IS 383-1978) to zone III and fineness modulus of 2.89 was used as fine aggregate and normal potable water was used. Mineral admixture like fly ash class F were used for pozzolanic material was greatly increased the workability at different level of replacement was 10%, 20%, 30%, 40%, and 50% by weight of cement and consistency of fly ash was 48%. Cement mortar mixture proportions were 1 : 3 (one part cement which is equally divided into number of percentage by replacement of cement and used as fly ash) and dosage of accelerator 1%, 2%, and 3% by weight of binder material and size of specimens as per standard mm were prepared by using a special steel mould designed and cement mortar was made by mixing of water, cement with or without fly ash and also accelerator, an each mix three cubes were cast and tested. The glass plate mould fabricated the size of the mould mm. The fluidity of cement paste after finishing the top surface was measured using the ultrasonic pulse velocity (m/sec) by the direct transmission method and the arragement was securred to ensure uniform contact with the transducer and the surface of the plexiglass mould.
3. Results and Discussions
3.1. Measurement of Ultrasonic Pulse Velocity Test in Cement Mortar
The cement mortar specimens were tested for 1, 3, 7, 14, and 28 days after sufficient curing in water tank, tested on the smooth surface of the both sides, fixed the length of specimen that was 0.076 m in UPV machine, recoded the values by using the ultrasonic pulse velocity techniques, and after that tested for the same specimen by using a universal testing machine for determining the compressive strength for each specimen with respect to day wise as shown in Table 1. It can be also observed that various mixture proportions of cement mortar cubes were recorded that the ultrasonic pulse velocity values with different curing days for all values are presented in Table 2.
3.2. Fresh Cement Pastes Monitoring by UPV Test
The experimental results obtained from the fresh cement paste studies using ultrasonic pulse velocity are shown in Figure 1. To observe the stiffens of early age properties of different cementitious materials by using Plexiglass, the size of the Plexiglass mould was mm. The fresh cement paste was gently poured into the glass mould with three layer, for each layer compacted gently after finishing the top surface and measured the Ultrasonic pulse velocity in terms of m/sec and for each different mixtures were carried out immediately after finishing the top surface of the cement past by attaching the transmitter and the reading was taken at every 5 minute intervals for all mixture proportion investigated. A different mixture proportion of fresh cement paste was monitored, and all the values are tabulated in Tables 3, 4, 5, 6, 7, and 8 and plotted graphically in Figures 2, 3, 4, 5, 6, 7, 8, and 9. In general the fly ash-based mortar mixtures with different percentage of fly ash contents showed a decrease in UPV corresponding to setting times. This indicates that the degree of hydration at the same duration is reduced in the case of fly ash-based mixtures due to the prolonged dormant period and delay during the acceleration period of hydration. Some of the mixes showed different behavior. The fly ash-based mixes also showed normal changes in the ultrasonic pulse velocity with decrease in water to binder ratio. The mixtures incorporating fly ash as a binder at 10%, 20%, 30%, 40%, and 50% with a suitable selection of the water to binder ratio showed better improvement in the rate of hardening with the addition of accelerator. Also the grade of cement used plays a major role in the early setting reaction and faster reactivity due to the fineness of cement particles. However, an optimal addition of fly ash in cement shows a gradual increase in the early reaction with the cement hydration products and helps in improved microstructural formation.
The important conclusions arrived based on these investigations are as follows.
The UPV values show a varied pattern, which is very much affected by setting time, which in turn is affected by water-binder ratio and chemical admixture dosage. There is a need for development of a technique based on more reliable methods for prediction of setting times. The ultrasonic pulse velocity techniques the test result values observed a different pattern, which is very much affected by setting time, which in turn is affected by water-binder ratio and chemical admixture dosage. The pulse velocity measurements showed an increased rate of hardening in cementitious systems and thereby proves to be a worthy method of assessment of cement hydration. It was observed from the test results of UPV that the influence of accelerator provided a constant increase in strength gain at the early ages. It may be observed from the graphs that the hydration process occurs in 3 stages. In the 1st stage, the hydration occurs very rapidly in the first few minutes. In the 2nd stage, the hydration process ceases or remains constant for another few minutes. This period is known as “dormant period.” After this stage, the rate of hydration starts to increase again. At the end of the dormant period, the C2S and C3S in the cement start to react, with the formation of calcium silicate hydrate (C–S–H) and calcium hydroxide Ca(OH)2. It observed that the test result value for 0PC with 20 percentage of fly ash showed an increase in UPV values up to 60 minutes, after that 60 to 90 minutes had a dormant period; that is, the heat of hydration was decreased, after further increasing the setting time will develop the stiffness of the cement paste.
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