On the thermo-mechanical characterization of cement mortars exosed to high temperature

Some recent technical documents on fire-resistant walls made of either cementitious blocks or clay bricks are bringing onto the stage the thermo-mechanical behavior of the mortars. Information on mortars decay at high temperature, however, is either contradictory or can hardly be found in the technical-scientific literature. This study aims to provide information on the thermo-mechanical behaviour of mortars in residual conditions. Three mortars are investigated (a reference mortar, cube strength fcc » 5 MPa, and two higher-grade mortars, fcc » 10 MPa and fcc » 15 MPa, respectively). All mortars are tested past a high-temperature cycle at 200, 400 and 600°C. On the whole, the mechanical decay turns out to be very close to that of typical ordinary concretes, while the thermal diffusivity is markedly lower. A worked example about a concrete-block fire-resistant wall ends the paper

Corner spalling and tension stiffening in heat-damaged R/C members: a preliminary investigation

Corner spalling in fire and tension stiffening past a fire are addressed in this paper by investigating reinforced and unreinforced prismatic specimens in the former case, and by testing reinforced tension members in the latter case. An experimental procedure to asses concrete sensitivity to spalling due to pore pressure is proposed as well. The focus is on the material as such, in order to allow different cementitious mixes to be compared in terms of spalling sensitivity under realistic conditions regarding the thermal gradients, the pore pressure and the moisture transfer (thermal and load-induced stresses are not a primary concern). Concrete spalling and tension stiffening are investigated within the same project, because (a) a bar—whose cover has been subjected to the risk of spalling during a fire—exhibits reduced tension stiffening, and (b) the same prismatic specimens were used (firstly thermal shock at 750 °C to provoke spalling and later rest at high temperature to investigate tension stiffening past cooling). Since only 10–20 % of the twenty-one specimens suffered medium/light spalling, nearly all the reinforced specimens were later tested to investigate tension stiffening in residual conditions. The prismatic specimens were made of SCC (target strength fc = 50, 80 and 90 MPa). The results show that (a) during a thermal shock the spalling tendency is rather weak if no load-induced stresses and/or thermal self-stresses occur, (b) tension stiffening appears to be still effective in 50 % of the reinforced specimens and (c) bond stiffness is a roughly linear function of the actual compressive strength of the concrete, be it heat damaged or not

Innovation in Concrete Structures and Cementitious Materials

ACI Italy Chapter - Federbeton Awar

Design Issues about the Bar Lay-Out at the Dapped Ends of Simply-Supported One-Way R/C Slabs

Directly- or indirectly-supported R/C slabs are frequently used in the covers of small-tomedium underground facilities, where the service loads are often rather limited, but very severe loads cannot be ruled out in accidental situations. To have fresh information on the structural performance and cracking of R/C slabs simply supported along their short sides via corbels (dapped ends), four rectangular slabs subjected to transversely distributed or concentrated loads have been recently tested in Milan (b x L = 1.30 x 2.20 m; simply-supported span = 2.10 m; thickness = 15 cm). The load was applied either at midspan (in the bending tests, not presented in this paper), or at 1/10 of the span (in the shear tests). Two bar arrangements were investigated with straight or bent-up bars in the main body, and straight bars or hooks in the corbels. In the D zones of the slabs tested in shear, the concentrated load induces a rather complex 3-D crack pattern, with a limited reduction in the bearing capacity, compared with the cross-wise distributed load. Rather simple strut-and-tie models applied to the slabs with bent-up bars and hooks, and the equations provided by European and North-American codes for the beams devoid of transverse reinforcement are shown to yield realistic predictions for the bearing capacity at the ultimate limit state. In the former case, however, modelling the bond along the tension reinforcement of the corbel requires some care, because of bond local and global effects. Last but not least, refining bar layout confirms - once more - that slab performance can be markedly improved with little extra effort

Thermo-mechanical behavior of baritic concrete exposed to high temperature

A number of structures close to the core of the reactor in nuclear power plants or designed to confine X rays in medical facilities are required to have radiation-shielding capabilities. To this aim, dense concretes are used, and - among them - baritic concretes containing barite aggregates, with a mass per unit volume from 25% to 50% higher than that of ordinary concrete. Information on their mechanical and thermal properties at high temperature - however - is scarce and rather outdated, something that should be looked at, as required by the ongoing revamping of several nuclear power plants and by the increasing use of heavy concrete in waste repositories and in medical facilities. A research project on the high-temperature behavior of a rather typical baritic concrete (target strength on cylinders = 30 MPa, mass per unit volume = 3100-3200 kg/m3) has been recently completed in Milan. After the usual curing period, the specimens were kept for three years either in air or in a moist environment. The strength in tension/compression and the elastic modulus after heating and cooling down to room temperature (= residual mechanical properties), as well as the mass loss, the thermal diffusivity and the porosity were investigated up to 750 °C, and damage indices were introduced to quantify the mechanical damage. Compared to ordinary concrete (as indicated in ACI and European documents, and by previous tests performed by the authors), baritic concrete is shown to have better insulation properties, a slightly better residual compressive strength above 500°C, a similar tensile strength by splitting and a slightly lower stiffness (elastic modulus). In general, however, the high-temperature performance of baritic concrete is similar to that of any good ordinary concrete, thanks to the closeness of the thermal coefficients of baritic aggregate and baritic mortar. © 2014 Elsevier Ltd. All rights reserved

High-temperature behavior of structural and non-structural shotcretes

Sprayed concrete (shotcrete) is well known as a reliable and effective material for rock stabilization, fire proofing of metallic structures and jacketing of R/C members. Shotcrete structural applications, however, have been so far very limited mainly because of some concerns about material's durability and high-temperature behavior. The latter issue is the starting point of this research project aimed to investigate the thermo-mechanical properties of three shotcretes containing different accelerating agents (based on sodium silicates in one mix – C1, and on sulfo-aluminates in two mixes – C2/C2F, no steel fibers/with steel fibers). The objective is to check (a) whether the heat-triggered mechanical decay of shotcrete is similar to that of ordinary concrete, and (b) how shotcrete low thermal diffusivity and relatively large porosity evolve at high temperature. The mechanical properties in compression are investigated both at high temperature (hot tests, C2 and C2F) and past cooling (residual tests, all mixes). In terms of normalized mechanical decay, the two shotcretes containing an alkali-free accelerating agent behave similarly to ordinary concrete, while the shotcrete with an alkaline accelerating agent is more heat and age sensitive. Up to 850°C, all mixes exhibit a markedly lower thermal diffusivity compared with ordinary concrete, and a higher porosity. Furthermore, the rather low mechanical properties of the shotcrete with an alkaline accelerator with respect to the base material make it hardly fit for structural purposes, while the two shotcretes with an alkali-free accelerator are as good as any ordinary concrete even at high temperature, as demonstrated by the basic structural application presented at the end of the paper, concerning the lining of a circular deep R/C tunnel exposed to the standard fire