Model for predicting strength development of concrete incorporating fly ash of variable chemical composition and fineness | |
| Author | Tahir, Muhammad Akram |
| Call Number | AIT Diss. no. ST-98-03 |
| Subject(s) | Fly ash Aggregates (Building materials) Strength of materials |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering, School of Civil Engineering |
| Publisher | Asian Institute of Technology |
| Abstract | The investigation is conducted to formulate a model for predicting the compressive strength of fly ash-concretes by taking into account the chemical composition and fineness of fly ash. The proposed model recognizes the role of the chemical composition and fineness of cement in the strength development of concrete. The influence of coarse and fine aggregates on the strength development of fly ash-concrete is also considered. A rigorous experimental program was conducted in which 135 concrete mixtures were prepared by incorporating three fly ashes possessing different chemical compositions and fineness. The strength development of these concretes was investigated using, as the main variables, the binder content, water-binder ratio, fly ash-binder ratio, and fineness and chemical composition of fly ash. The data obtained from the testing program were used to develop the basic relationships between the strength and other relevant variables namely water-binder ratio, fly ash-cement ratio and age. Besides the concrete mixtures, the strength developments of 21 mortars incorporating two fly ashes with three grades of fineness were also investigated. The relationships between the pozzolanic strength of concrete, and the fineness and chemical composition of fly ash were developed from these test results and subsequently incorporated in the proposed model. The same data along with those adopted from other researchers were used for calibrating and validating the model. I The strength of fly ash-concrete is expressed as a function of the solid volume of the hardened fly ash-cement paste (referred to as the solid volume hereinafter). It is shown that strength of fly ash concrete is proportional to the square of the concentration of the solid volume, which is defined as the ratio of the solid volume to the volume of fresh fly ash-cement paste. The formulation of the model is based on the chemistry of hydration and pozzolanic reactions. The four mechanisms that contribute to the solid volume are classified as hydration, pozzolanic reaction, physical action of water-binder and fly ash-binder ratios, and influence of the minor chemical compounds on the solid volume. The contributions of the hydration to the solid volume are formulated as functions of the four cement compounds namely C2S, C3S, C3A and C4AF. The hydration parameters are defined by the interaction of the two silicates with each other and with the two aluminates from the cement. The contribution of the pozzolanic _reaction to the solid volume is formulated as a function of the silica from fly ash or the calcium hydrate liberated from the hydration of both cement and fly ash. The interaction of the silica· with botn the 'alumina and ferric oxide is also considered. The fineness of cement controls the rate of its hydration, and consequently the solid volume contributed by hydration of cement is proportional to the square root of its Blaine surface area. It is established from the results of fly ash-mortar tests that the solid volume contributed by the pozzolanic reaction is proportional to the third root of the Blaine surface area of the fly ash. When the silica content of a mixture is less than its silica demand, the solid volume given by the pozzolanic reaction is governed by the amount of the silica. The silica demand is the amount of silica just sufficient to take up the total calcium hydrate liberated from the hydration of cement and fly ash. On the other hand when silica present in a mixture is more than or equal to its silica demand, the pozzolanic solid volume is controlled by the amount of calcium hydrate and the silica richness coefficient. The latter is defined as a function of the reactivity of the fly ash and the ratio between the silica present in fly ash and the silica demand of the mixture. The effect of the fly ash-cement ratio is also included in the formulation. From iv " .. the results obtained from the tests on fly ash-concrete mixtures, it is found that the water-binder ratio is directly proportional to the porosity of hardened fly ash-cement pastes. The formulation of the model is proposed in two steps. First, the model is developed to predict the 28-day strength and subsequently it is generalized to predict the strength at any age. The strengths are predicted from the solid volume of hardened paste. The 28-day solid volume can be calculated using fourteen variables, which consist of hydration, pozzolanic, minor chemical and physical parameters. The six hydration parameters are taken from the hydration of the pure C2S and C3S, and their interaction with each other in the presence of C3A and C4AF. The four pozzolanic parameters are calculated from the pozzolanic reactions between the silica and the calcium hydrate separately released from the hydration of fly ash and cement, and the interaction of both the Al20 3 and Fe20 3 with the silica during the pozzolanic reaction. The only minor chemical parameter is obtained from the alkali content of the mixture expressed as NaiO equivalent. The three physical parameters considered in the formulation are the water-binder ratio, the fly ash-cement ratio and its square root. The generalized strength model calculates the change in the solid volume with reference to the 28-day solid volume. It is found from the test results that the change in the solid volume is directly proportional to the logarithm of the age of concrete in days divided by 28. The change in solid volume obtained from the same variables as of the 28-day strength model along with the logarithm of the age of concrete expressed in days divided by 28. Finally, the proposed model is calibrated using part of the data obtained both from the present experimental program and from other researchers. The model is validated with the remaining data and the quality of validation like that of the calibration is found to be satisfactory. The proposed model conforms to the present state of knowledge about the chemistry of the hydration and pozzolanic reactions. The excellent fit and high correlation obtained with the experimental results indicates that there is a fair similarity between the actual strength development process and the one presented in this study. It is concluded that the proposed model is capable of predicting the strength development of concrete, with a reasonable degree of accuracy, incorporating fly ash of variable chemical composition and fineness from different sources. |
| Year | 1998 |
| Type | Dissertation |
| School | School of Civil Engineering |
| Department | Department of Civil and Infrastucture Engineering (DCIE) |
| Academic Program/FoS | Structural Engineering (STE) /Former Name = Structural Engineering and Construction (ST) |
| Chairperson(s) | Pichai Nimityongskul; |
| Examination Committee(s) | Balasubramaniam, A. S. ;Wijeyewickrema, Anil C. ;Loo, Yew Chaye ; |
| Scholarship Donor(s) | The Government of Pakistan; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1998 |