Influence of the microstructural changes on the durability of precast products due to heat curing

Author(s): M.C. Alonso, J.L. Garcia, M. Robles, L. Fernandez-Luco
Paper category: Conference
Book title: 2nd International conference on Microstructural-related Durability of Cementitious Composites
Editor(s): Guang Ye, K. Van Breugel, Wei Sun, Changwen Miao
Print ISBN: 978-2-35158-129-2
e-ISBN: 978-2-35158-123-0
Publisher: RILEM Publications SARL
Pages: 1220 - 1229
Total Pages: 9
Language: English

Heat curing processes are often used to ensure the production rate in industrial plants that fabricate precast concrete elements. However, heat curing may influence the microstructural properties and as a result the durability parameters of the reinforced concrete. This paper deals with the evaluation of the influence of the heat curing process in several microstructural parameters of the concrete such as the type of micro-cracking formed and the possible delayed ettringite formation (DEF).
Different types of samples, made of identical concrete composition, have been evaluated:
- Concrete directly cast at the plant and stored in standard curing conditions.
- Concrete made in the laboratory and stored in standard curing conditions.
- Concretes cores extracted from a precast structure made in plant, after the heat curing process.
- Concrete made in the laboratory, submitted to a heat curing process that reproduced the one carried out in the plant. In these concretes, two different maximum temperatures of curing have been used, 73ºC and 90ºC.

These concretes have been studied through the evaluation of the influence of the heat curing process on several macro and microstructural parameters of the concrete such as, mechanical strength, pore size distribution, the type of micro-cracking formed and contribute with additional evidence to the consequence derived from the delayed ettringite formation (DEF).

The compressive strength of the concretes has been determined at 28 days. The microstructure of the fabricated concrete samples has been evaluated by means of Back Scattered Electron Microscopy, BSEM, and X- ray energy dispersive spectroscopy microanalysis, EDAX, and X-ray difraction. The total porosity and the pore size distribution of the concrete samples have been also evaluated using a Mercury Intrusion Porosimeter.

To evaluate if the maximum temperature reached during the heat curing process has some after-effects in the formation of delayed or massive ettringite in the concrete matrix, the ettringite presence has been studied submitting the concrete to two different maximum temperatures of curing: 73ºC and 90ºC.

The obtained results show that the heat curing process carried out on a precast concrete influences the physico-chemical characteristics of the concrete. Compensating measures must be considered in the concrete mix design to compensate the loss in mechanical properties associated with heat curing and in microstructure. The formation of microcraks is located in specific places: around coarse aggregates and perpendicular to paste; they are consequences of heat curing but not of the maximum temperature. The pore size is altered by the heat curing but the maximum temperature of heating has no influence. The risk of delayed ettringite formation increases with the heat temperature being critical the maximum temperature of 73ºC in present work. At 90ºC the ettringite formation increases with age by formation of nodules that increased microcracking in paste and filled cracks at interface with aggregates.

Online publication: 2013
Publication Type: full_text
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