Prediction of crack resistance of massive concrete structures based on cements different types

Троян Вячеслав Васильевич — Mon, 16 Feb 2015 13:16:39 +0000

Introduction
Important factor determining durability of structures from cast-in-situ concrete is their cracking resistance [1]. Cracking of the massive cast-in-situ concrete is usually a result of high temperatures and shrinkage. Self-heating of the concrete structures during hardening results in their central part in almost adiabatic conditions, whereas the surface layers of the concrete due to cooling by air and evaporation are in partially adiabatic conditions and their temperature is lower. As a result of temperature and humidity gradients, deformations occur in the concrete, which in the conditions of restrains will cause in one part of the concrete element compressing stresses, and in other part- tensile stresses. When these stresses exceed a maximum tensile strength, then crack formation and propagation can take place on the surface of the concrete element. Purpose of the study was prediction parameters of cracking of the plasticized portland cements and slag cement concretes with different heat evolution.

Examination procedures and raw materials
Concrete mix compositions differed content of portland cements CEM I 42.5, CEM II/A-S 32.5 and slag cement CEM III/ A 32.5. Studies on heat evolution in the concrete of different compositions were conducted in an adiabatic calorimeter. The following expression was used to model a heat evolution:

Expression 1: formula applied for modeling heat evolution

where: - cumulative heat evolution of the concrete; ф –time;

k_Q and n_Q – coefficients, defined by a Monte-Carlo method.
A numerical modeling of the cracking was performed by application of a theory of fracture mechanics and a finite element method (the professional software product “Elcut” [2]).

Discussion of examination and test results
The results of temperature measurement of the concretes made using different cements (Fig.1) showed that a temperature of slag cements at early stages of hardening were lower compared to that of the concretes of similar compressive strength class made using clinker-containing cements.

Fig. 1: Temperature evolution in the concretes from different cements

Analyzing a singularity of stresses in the crack tips in the most stressed parts of a cross section of the concrete structure showed that values of the critical stress intensity factor (SIF) [3] had not been reached, thus, crack formation in the concrete structure has low probability.
Lower temperature of slag cement in the conditions of hot climate causes the less diffusion of water vapours into the ambient environment and, hence, the less acceleration of shrinkage processes in the concrete. So, after 40 hours hardening in case of slag cement concretes when a difference between the temperature of the concrete and of the ambient environment of 20 ^ос, the rate of water diffusion into the ambient environment, in accordance with the Fick’s Law [4], can show a 12- fold increase. At the same time, in case of portland cement concrete, when a difference between the concrete temperature and the temperature of ambient environment is 40 ^ос, the rate of water diffusion can give a more than 38- fold increase. Probable depth and width of the cracks defined through optimization of the models by a criterion of the crack tip critical stress-intensity factor due to shrinkage in the conditions of internal restrictions are shown in Figs. 2. As it follows from Fig. 2, when the reinforcement bars spacing is less than 0.125 m, the crack tip stress intensity factor of the crack proceeding to a depth exceeding 30 mm (Fig.2, a), does not exceed the critical stress intensity factor for the concrete, and the width of crack opening in these conditions will not exceed 0.02 mm (Fig. 2, b).


a)	b)
Fig. 2: Crack tip critical stress intensity factor (a) and width of crack opening (b) vs. reinforcement bar spacing and crack depth in the slag cement concrete

Concluding remarks
Compared with portland cement concrete, slag cement allows to reduce the temperature of self-heating concrete during 2 days from 40 to 25 оС. Low self-heating of the slag cement allows minimise shrinkage, and cracking of concrete massive structures in hot climate conditions.

Электронный научно-практический журнал «Современная техника и технологии» » Троян Вячеслав Васильевич

Prediction of crack resistance of massive concrete structures based on cements different types