placing concrete cold temperature poland

7/29/2019 Placing Concrete Cold Temperature Poland

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Placing concrete in low temperatures

1. Specification of concrete works in low temperatures

Ambient temperatures of +5oC are considered low temperatures, as far as the construction of

concrete elements is concerned. Temperatures below +10C affect the properties of the concrete mix

and of the set of concrete. In general, reduced temperatures, in particular, involve the following:

overextended setting time, especially for cement types with normal strength gain (N) and a

high content of mineral additives

significantly slower process of gaining strength

This entails major risks and significant inconveniences in the performance of construction

works. Reduced strength of the composite in the initial period requires a longer formwork time, a

prolonged construction cycle and slows down the rotation of formworks or inhibits the use of more

formworks.

The most serious problems, however, may occur at sub-zero temperatures. If no appropriate

measures are undertaken, a single instance of the freezing of the concrete within the initial twenty-four-hour cycles following the placing often causes disruption of the structure, as the concrete sets too

slowly. As maintained by T. C. Powers [4], a recognised expert in the field of concrete, at

temperatures of approx. -10C, 90% of water undergoes a change of phase. National regulations

specify the so called critical temperature, at which 50% of water freezes, at -1C. It is widely known

that ice crystallisation causes the initial water volume to increase, directly leading to stress in the

material, which significantly exceeds the strength (mainly tensile strength) of young concrete. On the

other hand, frozen water does not contribute to the hydrolysis and hydration of the binding agent.

Unprotected concrete is thus unable to gain the required strength fast enough.

Performing concrete works at low temperatures is duly considered one of the greatest

challenges in concrete technology. Over the years, certain methods, some more effective than others,

have been developed:

o modifying the composition and the procedure of making the concrete mix

o heat retention method

o re-heating method

o method of heat insulation elements

Each methods goal is to reduce the time required for the concrete composite to reach the so

called relative strength (critical strength) and full strength. Relative strength allows the material to

resist the destructive effect of concrete freezing, provided that the water from the environment does

not penetrate the material. When water penetrates the concrete (as a result of rain or snow fall, snow

melting, etc.) it must reach its full strength before freezing. Approximate strength values, identified as

the compression strength of materials, are provided in table 1, after Jamroy [1].

Strength type Compression strength [N/mm2

] for cementCEM I CEM II, III, IV and V

Relative 2.0 8.0

Full 5.0 10.0

Table 1. Recommended concrete strength values ensuring resistance to the negative effects of freezing

In practice, the most frequently applied approaches are the modification of the composition

and the procedure of the concrete mix, and the heat retention method. The procedure and the

conditions to be met when applying the two methods are described below.


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2. Practical approach to placing concrete in sub-zero temperatures

One of the simplest methods of improving the concretes strength gain dynamics at low

temperatures is to use chemical admixtures. The widely known accelerators and anti-frost

admixtures operate based on 2 main mechanisms: they reduce the temperature at which water changes its phase (ice crystallisation), and

accelerate the solution of the cements active phases, which initiates the hydration of

the binding agent and speeds up the materials strength gain

Chryso Polska Sp. z o.o. offers highly effective admixtures from this group, the so called Xel series:

CHRYSOXel AD

CHRYSOXel 650

CHRYSOXel GTX

CHRYSOXel 384

and

CHRYSOSpolos X.

These admixtures significantly boost the strength gain of young concrete, which is decisive for theelements resistance to frost and/or deformation. Technical advisers at Chryso Polska Sp. z o.o. can

provide the results of many comparative tests, which define the strength gain, ranging from a few

dozen to over one hundred per cent, for the various admixture compositions. Other similar purpose

admixtures available on the market were also found to be effective.

Anti-frost admixtures allow the concrete to quickly reach its appropriate resistance to freezing

at ambient temperatures as low as -10C. However, certain limits for which specific composition and

admixture requirements apply should be established. These are well illustrated by standards [6] in

table 2 for, among others, the minimum temperature of the concrete at the time of placing.

Ambient conditions or composition

propertiesRequirements

Air temperature ranging from -3C to+5C

Concrete mix temperature: +5oC

As above; in addition, cement content of< 240 kg/m3 or cements with low

hydration temperature

Concrete mix temperature: +10oC

Air temperature: < -3C

Concrete mix temperature: +10C; in addition, temperature of

+10C for three, 24-hour cycles; otherwise, provide sufficientprotection until the required strength is reached.

Air temperature: < +5C Concrete mix temperature: + 30oC

Cement content: 270 kg/m3, w/c 0.6;

external moisture penetration prior toconcrete freezing

Curing temperature of the setting mixture: +10C for at least three

days (fast-set concretes with high hydration temperature) or until thecompression strength of 5.0 N/mm2 is reached

Table 2. Recommended temperatures of the concrete mix delivered to a construction site and

recommended compression strength necessary to ensure the required resistance

The recommendations form an essential part for correctly constructing a given object/element at low

temperatures. Even the most effective procedures may fail to produce the desired effect, if other

recommendations are not met. Therefore, applying chemical admixtures or heat insulation elements is

necessary. These measures, however, are merely stages in a broader process. A successful end product

is the sum of appropriate actions taken at individual stages.

Steps necessary to achieve the required mixture parameters and produce set concrete:

I. Designing the mix composition:

Selecting cement type and quantity: fast-set cements (R) are recommended; if possible, avoidcements with high content of blast furnace slag (and mineral additives)


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Selecting admixture type and quantity: liquidising or anti-frost admixtures it is

recommended to reduce the quantity of water and use a higher than required concrete class;

when using the concrete class corresponding to the class required for normal conditions, it is

necessary to use an anti-frost admixture;

II. Mix production:

The temperature of the mix leaving a concrete-mixing plant: using heated-up components;

frequently, the make-up water is heated up or the places for storing the aggregate is covered

with roofing; particular attention must be paid in the case of frozen aggregates (melting ice

consumes a substantial portion of heat from its environment)

III. Transporting the concrete mix:

Reducing heat loss of the mix during transport to the construction site: special installations on

mobile concrete mixers (among others, heating up the mixer with exhaust fumes); eliminating

vehicle stoppages; minimising the exposure of the mix to cold air (pump transport, cover,

short mix discharge time);

IV. Installing formwork and placing the concrete mix:

Selecting the type of formwork and heat insulation: tar board, covering the formwork, can

delay the concretes temperature decrease; widely available and used heat and wind insulation

materials: mineral wool and polystyrene foam, straw or reed matting, canvas tarpaulin, thick

foil, etc. For higher requirements, the type and thickness of the insulation material can be

specified by using the physical properties of the structure itself (heat conductivity and

penetration factors).

After placing the concrete, ensure that the curing conditions are met; monitor the actual

temperatures of the elements and the tightness of the insulation elements on the formwork. Re-vibration of the concrete mix is often used as an effective method to reduce the effects of

concrete freezing in the initial phase of setting. The procedure should be conducted once the

mix has thawed and a temperature of approx. +10C has been reached. For even better results,

use mixes with lower cement content [2].

The elements should be shielded from ambient moisture, such as rainfall and water splashes.

External heating methods include heat generators, radiators or air blowers.

V. Removing the formwork

The time of removing the formwork or the time when the concrete reaches an appropriate

resistance to sub-zero temperatures can be determined by way of an experiment. It is a

common practice to leave sampled material to cure in similar conditions; this enables crediblemonitoring of the condition of the concrete in the structure and eliminates risky methods

involving approximate estimation of parameters of the cement composites exposed to low

temperatures.

placing concrete cold temperature poland

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