Formation of structure of heavy concrete for manufacture of wall panels

Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration. It may be either after it has been placed in position (or during the manufacture of concrete products), thereby providing time for the hydration of the cement to occur. Since the hydration of cement does take time – days, and even weeks rather than hours – curing must be undertaken for a reasonable period of time if the concrete is to achieve its potential strength and durability. Curing may also encompass the control of temperature since this affects the rate at which cement hydrates.

In recent times, the increased usage of the high strength concrete elements coupled with demand for high productivity of the projects has resulted in adequate curing time not being followed, which has affected the strength and durability of the concrete structures all over Russia.

This problem can be tackled to some extent through the application of steam curing to the concrete mix. Steam curing is the process by which concrete is cured at high temperatures at atmospheric pressure in steam. The hydration rate of cement increases with temperature and hence the gain of strength can be speeded up. The process can be regarded as a special case of moist curing in vapour saturated atmosphere which ensures a supply of water [1, 2]. Application of steam curing to concrete specimens makes them develop sufficient compressive strength with the advantage that they can mature properly without application of any further form of curing. At the same time, with regards to the precast industry, moulds can be removed earlier and less curing storage is required which gives an economic advantage. Also, the concrete can start to perform its function soon after casting which significantly saves time.

The typical parameters of this steam curing process are curing time, maximum steam temperature and duration at the maximum temperature [2]. The basic process consists of a delay period of 2 to 5 hours, followed by heating at a rate of 22 to 44ºC per hour up to a maximum of 50 to 85 ºC, maintaining it at that temperature for a period of up to 10 hours followed by a cooling period with the total cycle lasting not more than 18 hours [3].

We studied the effect of heat and humidity treatment process on the concrete structure formation. Samples were tested by heat treatment at different temperature rise rates. By analyzing the experimental data, we realized that the choice of the optimal rate of rise in temperature of the coolant in the first period of heat and humidity treatment of the products determines such properties as strength, water absorption and frost resistance due to insignificant influence on the process of concrete structure formation. Thus, choosing heat and humidity of concrete processing regime, not only the kinetics of the hardening process in terms of providing the required strength must be taken into account, but it also should be considered how the temperature conditions affect the structure formation processes, which ultimately determines the basic performance characteristics of the concrete. According to the results of the study, the optimal treatment regimen of heat and humidity include the rate of temperature rise within 10-15°С/h.

 

The rate of rise of coolant temperature, °C/h	Tensile strength, MPa	Water absorption, %	Frost resistance, cycles
bending	compression
	5,7	32,7	4,7
	5,6	31,3	4,7
	5,4	29,8	4,8
	4,7	27,5	5,2
	5,0	27,2	5,3
Hardening in normal conditions	5,8	33,0	4,8

Научный руководитель – И.Л. Чулкова, канд. тех. наук, СибАДИ, Т.Г. Галкина, канд. филологич. наук, доцент кафедры «Иностранные языки» СибАДИ, Омск

 

Библиографический список

 

1. Neville A. M. (1997), Properties of Concrete, Pitman publishing, London.

2. Hanson J. A. (1963), “Optimum steam curing procedure in pre-casting plants”, ACI J. Proc. 60, 75-100.

3. ACI 517.2 R-87 (1992), “Accelerated Curing of Concrete at Atmospheric Pressure- State of the Art”, ACI Manual of Concrete, Revised.

4. Торопова М.В. «Влияние тепло­влажностной обработки на структурообразование и эксплуатационные свойства бетона». Дис. … канд. техн. наук: 05.23.05 / Ивановская государственная архитектурно-строительная академия. – Иваново, 2002. – 130 с.

 

УДК 691.328

студент Тришин С.А.

Сибирская государственная автомобильно-дорожная академия (СибАДИ), Омск

Abstract. Open stirrups are provided principally to resist shear forces in concrete beams and they are applied in locations in which the effect of torsion is insignificant. U-shaped stirrups are placed in the tension side of concrete beams in which shear cracks would occur. However, when concrete beams are designed to resist a substantial amount of torsion, closed stirrups should be used instead.

 

Аннотация: В статье рассматриваются различные виды арматурных хомутов: открытый арматурный хомут, замкнутый арматурный хомут, U-образный арматурный хомут. Обсуждаются особенности их применения.

 

Keywords: open stirrups, closed stirrups, concrete beams, flexural failure, shear failure.

Ключевые слова: открытый арматурный хомут; замкнутый арматурный хомут; бетонная балка; разрушение при изгибе; разрушение при сдвиге