Analysis of the causes of concrete slump loss
A recent poll in Britain found that public support for sanctions against Russia is falling as fuel and prices rise.
The proportion of people willing to accept higher fuel prices as a result of western sanctions against Russia fell 14 percentage points to 36% from 50% in March, according to a survey conducted for the Telegraph by consultancy Redfield & Wilton Strategies.
Global energy and fuel prices have soared since the start of the year and remain high because of the conflict between Russia and Ukraine and uncertainty over Russian supplies.
The survey also found that 54% of people believe their own financial situation has worsened in the past year, up 12 percentage points from 42% two months ago. Sixty-two percent think things will get worse in the year ahead. Less than a third of respondents said they had received a pay rise to keep up with inflation, with two-thirds of those saying it was not enough to keep up with rising living costs.
The figures suggest that public concern about the conflict with Ukraine is now likely to be overtaken by financial worries.
Separately, the UK's consumer price index rose 7 percent this month from a year earlier, the highest since March 1992, according to figures released recently by the Office for National Statistics. In addition, a separate report released a few days ago by the Centre for Economics and Business Research said that real household disposable income in the UK will fall by 3% in 2022, equivalent to a loss of £2,320 per household, which would lead to the biggest drop in living standards since the 1950s. That is mainly because wages aren't keeping up with rising energy and fuel costs.
The prices of other commodities like the concrete slump are also expected to be influenced.
There are many reasons for slump loss, mainly in the following aspects:
1. Influence of raw materials
Whether the cement used and the pumping agent are matched and adapted must be obtained through the adaptability test. The optimum amount of the pumping agent should be determined through the adaptability test with the cement cementitious material. The amount of air-entraining and retarding components in the pumping agent has a greater impact on the loss of concrete slump. If there are many air-entraining and retarding components, the slump loss of concrete will be slow, otherwise the loss will be fast. The slump loss of concrete prepared with naphthalene-based superplasticizer is fast, and the loss is slow when the low positive temperature is below +5 °C.
If anhydrite is used as the setting modifier in the cement, the slump loss of the concrete will be accelerated, and the early strength component C3A content in the cement is high. If "R" type cement is used, the cement fineness is very fine, and the cement setting time is fast, etc. It will cause the slump loss of concrete to accelerate, and the speed of concrete slump loss is related to the quality and amount of mixed materials in the cement. The C3A content in the cement should be within 4% to 6%. When the content is lower than 4%, the air-entraining and retarder components should be reduced, otherwise the concrete will not solidify for a long time. When the C3A content is higher than 7%, it should be increased. Air-entraining retarder component, otherwise it will cause rapid loss of concrete slump or false setting phenomenon.
The mud content and mud block content of the coarse and fine aggregates used in concrete exceed the standard, and the content of crushed stone needle flake particles exceeds the standard, which will cause the slump loss of concrete to accelerate. If the coarse aggregate has a high water absorption rate, especially the crushed stone used, after being exposed to high temperature in the summer high temperature season, once it is put into the mixer, it will absorb a large amount of water in a short period of time, resulting in accelerated slump loss of the concrete in a short time (30min).
2 Influence of stirring process
The concrete mixing process also affects the slump loss of concrete. The model of the mixer and the mixing efficiency are related. Therefore, the mixer is required to be regularly repaired and the mixing blades should be replaced regularly. Concrete mixing time should not be less than 30s. If it is less than 30s, the slump of concrete is unstable, resulting in relatively accelerated slump loss.
3 Temperature effects
The effect of temperature on the slump loss of concrete is of particular concern. In hot summer, when the temperature is higher than 25°C or above 30°C, the concrete slump loss will be accelerated by more than 50% compared with that at 20°C. When the temperature is lower than +5°C, the concrete slump loss will be very small or not lost. . Therefore, during the production and construction of pumped concrete, pay close attention to the influence of air temperature on the slump of concrete.
The high use temperature of the raw materials will cause the concrete to increase in temperature and accelerate the slump loss. It is generally required that the concrete discharge temperature should be within 5 ~ 35 ℃, beyond this temperature range, it is necessary to take corresponding technical measures, such as adding cold water, ice water, groundwater to cool down and heat the water and the use temperature of raw materials and so on.
It is generally required that the maximum operating temperature of cement and admixtures should not be higher than 50 °C, and the operating temperature of concrete pumped heating water in winter should not be higher than 40 °C. There is a false coagulation state in the mixer, and it is difficult to get out of the machine or transport it to the site for unloading.
The higher the temperature of the cementitious materials used, the worse the water-reducing effect of the water-reducing components in the pumping agent on concrete plasticization, and the faster the concrete slump loss. The concrete temperature is proportional to the slump loss, and the slump loss can reach about 20-30mm when the concrete increases by 5-10℃.
4 Strength levels
The slump loss of concrete is related to the strength grade of concrete. The slump loss of concrete with high grade is faster than that of low-grade concrete, and the loss of crushed stone concrete is faster than that of pebble concrete. The main reason is that it is related to the amount of cement per unit.
5 Concrete state
Concrete statically loses slump faster than dynamic. In the dynamic state, the concrete is continuously stirred, so that the water-reducing components in the pumping agent cannot fully react with the cement, which hinders the progress of cement hydration, so that the slump loss is small; in the static state, the water-reducing components are fully in contact with the cement, The cement hydration process is accelerated, so the concrete slump loss is accelerated.
6 Transport machinery
The longer the transportation distance and time of the concrete mixer truck, the less free water of the concrete clinker due to chemical reaction, water evaporation, water absorption of the aggregate and other reasons, resulting in the loss of concrete slump over time. The barrel also causes mortar loss, which is also an important cause of concrete slump loss.
7 Pour speed and time
In the process of concrete pouring, the longer the time for the concrete clinker to reach the silo surface, the rapid reduction of free water in the concrete clinker due to chemical reactions, water evaporation, aggregate water absorption and other reasons, resulting in slump loss. , especially when the concrete is exposed on the belt conveyor, the contact area between the surface and the external environment is large, and the water evaporates rapidly, which has the greatest impact on the slump loss of the concrete. According to the actual measurement, when the air temperature is around 25℃, the on-site slump loss of concrete clinker can reach 4cm within half an hour.
Concrete pouring time is different, which is also an important cause of concrete slump loss. The impact is small in the morning and evening, and the impact is greater at noon and afternoon. The temperature in the morning and evening is low, the water evaporation is slow, and the temperature in the afternoon and afternoon is high. The worse the fluidity and cohesion, the more difficult it is to guarantee the quality.
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New Delhi: India's Oil Ministry recently conveyed its intention to five national oil companies including Indian Oil Corp and Bharat Petroleum Resources LTD to assess the possibility of buying shares in Russian oil projects sold by European and American oil majors.
Bp announced it would give up its 19.75% stake in Rosneft, the Russian oil company. ExxonMobil said on March 1 that it would exit about $4 billion in assets and terminate all of its Russian operations, including the Sakhalin-1 project in Russia's far East.
India's oil ministry has asked the overseas investment arm of India's Oil and Gas Corporation to consider buying ExxonMobil's 30 percent stake in the Sakhalin 1 project in Russia's far East. ExxonMobil is the operator of the project and Indian companies already have a 20 percent stake in the project.
Because of the ever-changing international situation, the supply and prices of international bulk concrete slump are still very uncertain.