Effect of elevated temperatures on the flexural strength of crushed rock dust concrete N. Venkata Sairam Kumar ⇑ , S.V. Satyanarayana Department of Civil Engineering, R.V.R & J.C College of Engineering, Guntur, Andhra Pradesh, India article info Article history: Received 25 September 2020 Received in revised form 8 December 2020 Accepted 13 December 2020 Available online 24 January 2021 Keywords: Flexural strength Crushed rock dust Elevated temperature Green concrete abstract This experimental study presents the effect of partial replacement of Ordinary Portland Cement (OPC) by crushed rock dust (CRD) as filler material on the flexural strength of concrete when subjected to elevated temperatures of 200 °C, 400 °C, 600 °C and 800 °C for duration of 2 h using an electrically controlled fur- nace. The OPC replacement percentages are: 0% (T20), 10% (T21), 20% (T22), 30% (T23) and 40% (T24) by weight. Ultrasonic pulse velocity (UPV), Mass loss, flexural strength, and scanning electron microscope (SEM) are evaluated at the targeted elevated temperatures. At ambient temperature, up to 40% CRD, a dense microstructure with less pores is observed using SEM micrographs. Both T20 and CRD concrete beams begin to crack when temperature reached to 600 °C and pronounced surface cracks are observed at 800 °C. UPV values obtained with T20 and CRD concrete beams at elevated temperatures are in good agreement with flexural strength and mass loss values. SEM micrographs signify the use of CRD in con- crete at elevated temperatures. The results of T20 and CRD concrete beams at elevated temperatures are found to be acceptable when exposed up to 400 °C. Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the Second International Conference on Recent Advances in Materials and Manufacturing 2020. 1. Introduction Concrete when exposed to fire or elevated temperature creates a severe potential problem to the safety of the structure. Concrete is an incombustible construction material than wood and steel, but when exposed to elevated temperatures, its cement matrix and constituent properties are affected, resulting in the deterioration of its physical, mechanical and durability properties [1]. The main factors influencing the behaviour of concrete at elevated tempera- tures are moisture content, type of aggregates, relative proportions of cement and aggregates, thermal compatibility between cement paste and aggregate, peak temperature and exposed duration as well as size and shape of member [2]. The deterioration of concrete at higher temperatures includes crack formations, causing spalls, large pores and reduction of bonding between cement matrix and aggregates. This results in the development of high internal tensile stresses causing damage, cracking and a significant reduc- tion in compressive strength. Hence, the residual compressive strength of concrete exposed to fire plays an important role to sig- nify the suitability [3]. The thermal parameters such as specific heat, the coefficient of thermal expansion, thermal conductivity and diffusivity are similar to normal strength concrete and high strength concrete, but some studies showed poor performance and high risk of spalling of high strength concrete at elevated temperatures due to its high brittle- ness and low permeability. Concrete spalling at elevated tempera- tures is majorly influenced by moisture gradients and free water. However, concrete with 3 – 4% of moisture by weight has a high risk of spalling than concrete having moisture content less than 3% by weight. On the other hand, high performance concrete or dense microstructure concrete having zero moisture content may spall at elevated temperatures [4,5]. It is well known that concrete has great vulnerability towards spalling when prepared with low water to cement ratio than high water to cement ratio. The changes occur in cement paste when exposed to fire or elevated tempera- ture is as follows: (1) The evication of evaporable water at 100 °C temperature. (2) At a temperature of 180 °C, the hydrates of cal- cium silicate start dehydration. (3) The disintegration of calcium hydroxide takes place at 500 °C. (4) At around 700 °C, the decom- position of calcium-silicate-hydrate begins. (5) At the elevated temperature of 400 °C, calcium hydroxide in hardened cement paste of concrete starts dissociation, and the change persists up https://doi.org/10.1016/j.matpr.2020.12.535 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the Second International Conference on Recent Advances in Materials and Manufacturing 2020. ⇑ Corresponding author. Materials Today: Proceedings 42 (2021) 1176–1183 Contents lists available at ScienceDirect Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr