Hot Weather Concreting Practice

Hot weather problems

During hot weather conditions a number of on-site factors can work against deriving optimum performance from concrete. When combined with low relative humidity and strong winds placing and finishing requires special care.

However, provided your premixed concrete producer supplies concrete made with sound, well-graded aggregates, with an adequate cement content, and with the precise water content needed to give sufficient workability for efficient placing and consolidation, there should be few problems in placing and finishing if reasonable care is taken.

There are a few simple precautions which will protect "summer" concrete and will make it easier to obtain the best concrete job.

The main problems arising during summer concreting are (a) to prevent the early loss of water from concrete, and (b) to prevent early setting through too-rapid drying. If these problems are not anticipated, there may be -

• strength reduction
• crazing or cracking
• shrinkage cracks
• finishing difficulties.

Precautions for Hot Weather Concreting

Planning ahead and preparation for each job will minimise the problems mentioned above, and will avoid irritating on-site delays.

Probably any experienced concrete contractor will know why it is sound sense to observe each one of the precautions set out below. Put together, they provide a time-saving and work-saving check list for supervisors and, perhaps, a guide for people not yet familiar with hot weather conditions in this country -

A first and very necessary step in retaining the water in mixed concrete (as delivered by the producer) is to thoroughly moisten the sub-grade, reinforcing steel and wooden forms before placing the concrete. Avoid delay in placing the concrete. Have sufficient labour and equipment on hand to perform the placing quickly. Don't order or try to place more ready-mixed concrete than you can reasonably expect to finish and cover. Discharge concrete as soon as possible from the Ahlcon Readymix truck. Excessive temperature build-up may result from prolonged agitation on the job-site. Care should be exercised with vibrators, to avoid over- vibration. Five to fifteen seconds of vibration, depending on the depth of the concrete, should give the desired compaction During a pour in very hot weather, try to shade the concrete from direct sunlight. Use wet coverings until final finishing can be completed, or spray with an alaphatic curing compound. If a flat finish is required, uncover only a small section immediately ahead of the finishers. Cover again at once after final finish. Keep covers wet. Have sufficient labour and equipment on hand to finish the concrete. In cases of extreme hot weather it may be wise to start jobs in the afternoon to take advantage of lower temperatures in the evening. Keep a "weather eye" open. A gentle breeze on a hot, dry day cannot be ignored. The evaporation rate of moisture from freshly placed concrete will increase to four times when wind velocity rises from zero to only 15km per hour on a hot day. Start curing as soon as possible, using a method that will ensure moisture losses are minimised and protects the concrete from temperature extremes.

Sub-grade should be damp, but not muddy. Saturate beforehand then sprinkle again just before concrete is placed. Discharge concrete from waiting trucks as soon as possible. Heat evolution from cement hydration and continuous agitation results in temperature rises in the concrete which can cause a rapid loss in workability. In very hot weather, shade concrete from sunlight or use wet coverings until finishing can be completed.

Curing techniques

Curing is the protection of fresh concrete from evaporation and temperature extremes which might adversely affect cement hydration. If concrete is to gain potential strength and durability it must have -

1. Sufficient water for the hydration of the cement, and
2. A temperature conducive to maintaining this chemical reaction at a rapid, continuous rate.

To ensure the existence of these conditions, the concrete must be protected from the harmful influences of wind, sun and variable weather. As 23ºC is considered the ideal temperature for hydration, it is desirable to maintain concrete temperature at or about this figure as curing proceeds.

Concrete curing techniques fall into two groups - those designed to prevent loss of water, such as the application of impermeable membranes; and those that supply moisture throughout the early stages of the hydration process, such as ponding or the application of wet sand or hessian. Selecting the method of curing is generally a matter of economics, but another consideration is that the method used should cause the least interference to other operations on the site.		Water can be retained longer by using an absorptive cover.

Absorptive covers

An absorptive medium such as sand, hessian or canvas will hold water on the concrete surface while curing progresses.

Any such medium must be kept damp constantly during the curing period, for if drying is permitted the cover itself will absorb moisture from the concrete. Alternate drying out and wetting of the cover may cause cracking.

The use of sawdust as a cover is not advisable, for it has on occasion retarded the hardening of concrete through the action of sugar in the sap still present in the sawdust.

Water addition curing

Theoretically, flooding, ponding or mist spraying are better than the retention methods mentioned above. But they are not always practical because of job conditions.

Care should be exercised to prevent large temperature differentials between the concrete mass and curing environment so as to avoid potential cracking due to temperature gradients within the concrete. This is generally known as thermal shock cracking.

Water retaining materials

Chemical or liquid membranes are gaining in popularity because they are convenient to use. They can be applied by hand or power sprays.

These membranes come in four general categories: wax based; chlorinated-rubber based; resin based and PVA based.

When it dries, a membrane compound forms a vapour seal on the surface of the concrete, the water in the concrete is sealed in and good curing conditions are established. Care should be exercised in the selection of an appropriate membrane coating in that compatibility with the intended applied finish to the concrete must be taken into account.

Chemical or liquid membranes reduce evaporation by seating the concrete.

Mechanical barriers

The use of waterproof building papers or plastic film (polyethylene sheeting) will also prevent the evaporation of moisture from concrete.

Any material used as a mechanical barrier to evaporation should be placed over the concrete as soon as the placing of it will not cause surface damage. The edges of the material should overlap several inches, and should be tightly sealed with sand, tape, mastic or wooden planks. It is good practice, though one not always followed, to moisten the surface of the concrete with an atomising spray of water immediately prior to placing of the sheeting on the concrete.

Mechanical barriers should be placed over concrete as soon as the surface is set.

Avoid adding water to mixes

When handling low-slump concrete in hot weather, the placers will often ask for "more water". Excessive water added to the components of a mix can destroy the quality of poured concrete. Wet concrete has a tendency to segregate and exhibit excessive bleeding properties. As the water bleeds to the surface and under certain conditions evaporates quickly cracking can and does result.

Better workability and longer setting times are best achieved using special chemical admixtures.

Inform your Ahlcon Readymix supplier of your special requirements, as most of these admixtures must be added at the concrete plant.

Effects of too much mixing water

ADVANTAGES OF ADDED WATER

• Easier Placing

DISADVANTAGES OF ADDED WATER

• Lower compressive strengths
• Segregation of the concrete mix under certain conditions resulting in variable quality throughout the concrete mass.
• Cracking - with too much water, there will be lower tensile strength, and a tendency towards high shrinkage and subsequent cracking.
• Dusting and scaling - bleeding of excess water brings too many fines to the surface of floors
• Sand streaks. Excess water bleeding up the sides of forms washes out cement paste and leaves an unsightly streaked surface.
• Contamination. Too much water in concrete placed on grades causes contamination from the subgrade with the concrete leading to an array of quality problems
• Permeability. Voids left as excess water evaporates invite water to seep through walls and floors
• Dead losses - costly repairs, or in extreme cases, demolition and re-building at contractor's expense.

Reasons for curing

To sum up the advantages of careful control of moisture and temperature in curing

1. The strength of concrete increases with age if curing conditions are favourable. Compressive strength of properly cured concrete is 80 to 100 per cent greater than the strength of concrete which has not been cured at all.
2. Properly cured concrete surfaces wear well.
3. Drying shrinkage cracking is reduced.
4. Greater watertightness of constructions is assured.

Points to keep in mind when curing

Start curing operations as soon as possible after concrete has been placed For proper curing concrete needs moisture Continuity in curing is a must, alternations of wetting and drying promote the development of cracking If during curing the concrete is allowed to dry out, as may happen in hot weather, the chemical change stops right at the point where the concrete loses its moisture. The ideal curing temperature is 23°C. Cure concrete for at least 7 days.

Note: 35oC is the maximum concrete temperature for field placing alowed under AS1379.

The vicious cycle in inadequate curing must be obvious. If enough water evaporates from the concrete before it has attained its maximum strength, there will not be sufficient water remaining in the concrete to fully hydrate the cement and so achieve that maximum strength.

Techniques of Cold Weather Concreting

Cold weather concreting problems

Few areas in Australia experience temperatures low enough to warrant elaborate and expensive protection of freshly-placed concrete which are common practice in Europe and North America. However occasional frosts, abrupt drops in ambient temperature, and/or prolonged periods of cold weather, do occur in our winter seasons. Harmful effects of these conditions on fresh concrete can be avoided by relatively simple measures in ordering, placing and curing.

Hardening of concrete is a chemical process and as in many chemical reactions the rate is temperature dependent. The lower the temperature, the slower is the process of hardening or setting of concrete.

At an ambient temperature just above 0°C the development of strength in unprotected freshly-placed concrete is very slow. If the ambient temperature drops below 0°C some of the water in the concrete may freeze; setting will virtually stop until it thaws, and this interruption of hydration increases porosity and reduces strength and durability.

Because some heat is generated during the hydration process, ordinary concrete has a minor inherent resistance to the freezing of its water after placing. But when the temperature of the concrete surface itself falls below freezing point, the water near the surface will solidify, increasing in volume and causing high pressures in concrete, which is no longer plastic. Scaling or spalling will follow, and will be severe if several freezing and thawing cycles occur.

Reduced Permeability

Of all the factors affecting freeze resistance of concrete, permeability plays by far the most important role. Impermeable concrete has only small amounts of free moisture in its pores and thus the destructive action of freezing and expanding water is largely eliminated. There are three basic methods of reducing permeability and hence increasing freeze resistance of concrete, viz:

Use of air-entraining admixtures. These prevent formation of continuous capillary passages by replacing them with minute, discrete (not interconnected) air voids. Reduction in water to cement ratio, which in turn reduces the bleeding rate (and bleeding capacity) of concrete. The presence of relatively large and continuous capillaries is usually closely related to bleeding of concrete. Use of pozzolans, such as fly ash, in order to replace part of the cement (generally fifteen to twenty per cent) resulting in a slight increase in the amount of
hydraulically active material. Pozolans react with soluble products of cement-water reaction and form water-insoluble and hence water-impermeable substances. With proper use of pozzolans, permeability of concrete can be reduced by a factor larger than ten. However, as pozzolanic reaction is very temperature- sensitive, use of fly ash can reduce the rate of strength gain (depress early strength) in cold water concreting.

Water is at its maximum density at approximately 4°C, i.e. it has minimum volume for a given mass at that temperature. Therefore disruption to hardened concrete structure due to the increase in volume of freezing water (or ice) is possible at very low temperatures only. Hence, at temperatures above 5°C, long-term durability and strength of concrete are not going to suffer (ultimate strength of concrete moist cured in cool storage is generally superior to conventionally cured concrete).

Early Age Strengths

However, the rate of strength gain of concrete at low temperatures is relatively slow (refer to the graph above) and this can adversely affect construction pace (delay in removal of formwork, disruption to "critical path" etc...). To overcome this problem, several methods of producing higher early strength can be employed.

The methods of achieving faster setting times and high early strengths of concrete vary with particular applications, viz: local climatic conditions in different regions, availability of certain raw materials (e.g. cement and admixture types etc.), as well as layout of plant and machinery. Therefore it is important to discuss all the special requirements of cold weather concreting with Ahlcon RMC production or technical staff.

These methods include:

• The use of accelerating admixtures
• The introduction of hot water at the concrete batch plant
• Covering or heating of form areas prior to pouring.

This particularly applies to the inclusion of set accelerating admixtures such as calcium chloride, improper use of which can produce an adverse effect both in plastic and in hardened concrete.

Preparing for pouring

Small oversights at the pouring stage can result in disputes and dissatisfaction over cold-weather concreting.

After rain, free water lying on the surface, or lying in porous sub-grades, will be slow to evaporate, and its total volume may be substantial. If the concrete pour causes much of the free (and cold) water to accumulate in one end or corner of formwork and combine with low-slump concrete there, a critical weakness may develop.

Accumulations of ice at the bottom of holes prepared for concrete piers may be overlooked, and structural movement may follow.

Concrete should not be poured on frozen ground, or on reinforcing steel or formwork which has a temperature near freezing point. Covering or heating of form areas prior to concreting, a not uncommon winter practice in Hobart, parts of Victoria, the Snowy Mountains area, and Canberra, is less usual in coastal areas further north. But successive frosts in Sydney's western suburbs can cause ground temperatures there to drop to low levels, particularly where ground is shaded throughout the day by adjacent buildings or trees. In Sydney generally, if form areas are covered overnight, frosts will not delay pouring the following morning. Wherever possible, monolithic floor finishes should be placed after walls and roof enclose the area.

Maintaining temperature

After placing concrete in cold weather its temperature must be maintained at a consistent high level if strength gain is to be normal.

Where ambient temperatures can be expected to be near or below freezing point for several days, insulation by batts or commercial blankets is indicated. Such insulation should be in close contact with surfaces and forms, and should itself be covered with strong, moisture-proof material. Steel projecting from forms should also be covered where possible.

Where minimum daily temperatures are unlikely to fall much below 5°C, less elaborate means of maintaining concrete temperatures can be used. It may be sufficient to lay waterproof paper on the form area, cover the paper with straw or sawdust to a depth of three or four inches and cover this with more waterproof paper, or sufficient merely to create dead air space between the form area and tarpaulins suspended above it. Heating and curing by exhaust system requires the building of an enclosure to keep cold air out. Any breakdown in the process may permit surface icing or rapid temperature changes in the concrete, with subsequent cracking.

Heating by fires placed at intervals provides uneven temperatures and is not favoured. The absorptive ability of cold air is low but increases rapidly as the air is heated. If heated air causes excessive evaporation from the concrete surface, shrinkage cracks will occur. Also, carbon dioxide produced by fires may carbonate the concrete surface, causing it to become chalky.

Five or six days after pouring, insulation should be removed at a time of day and in a manner which will allow the drop in temperature of any area of the concrete to be gradual. Membrane-curing compounds can be applied at this stage if necessary.

Stripping formwork

Strength gains of concrete will vary with the type of cement and type of mix, the use of accelerators, the ratio of mass to surface area, and other factors apart from temperature.

Generally it will be advantageous to leave formwork in position longer than the minimum period specified. Formwork will foster rather than retard curing in cold weather, and while it remains in position it is a reminder that any one section of a new slab should not be loaded too early or too abruptly.

Concrete mixes with some air entrainment, with the minimum practical water content and adequate cement, minimise problems in cold-weather work.

The ability of the concrete supplier to design and supply consistent batches of such mixes is important. The need to ensure that the mixes are at or close to ideal curing temperature is no less important.

Techniques of Curing Concretes

Curing Techniques

Curing is the protection of fresh concrete from evaporation and temperature extremes which might adversely affect cement hydration. If concrete is to gain potential strength and durability it must have -

Sufficient water for the hydration of the cement, and a temperature conducive to maintain this chemical reaction at a rapid, continuous rate.

To ensure the existence of these conditions, the concrete must be protected from the harmful influences of wind, sun and variable weather. As 23°C is considered the ideal temperature for hydration, it is desirable to maintain concrete temperature at or about this figure as curing proceeds.

Concrete curing techniques fall into two groups - those designed to prevent loss of water, such as the application of impermeable membranes; and those that supply moisture throughout the early stages of the hydration process, such as ponding or the application of wet sand or hessian.

Selecting the method of curing is generally a matter of economics, but another consideration is that the method used should cause the least interference to other operations on the site

Evaporation from concrete freshly placed on site

The graph shows the effects of air temperature, humidity, concrete temperature, and wind velocity together on the rate of evaporation of water from freshly placed and unprotected concrete.

Example:

• With air temperature at 25°C
• With relative humidity at 40%
• With concrete temperature at 25°C
• With a wind velocity of 20 km/hr the rate of evaporation would be 1.1 kg/m² hr. The highlighted line plots the example described above. The nomogram can be used for calculation of evaporation rates if other variables are known.

To determine the evaporation rate from the graph enter the graph at the air temperature (in this case 25°C and move vertically to intersect the curve for relative humidity encountered - here 40%. From this point move horizontally to the respective line for concrete temperature - here 25°C. Move vertically down to the respective wind velocity curve and then horizontally to the left to intersect the scale for rate of evaporation.

Trouble with plastic cracking is potentially in the making when the rate of evaporation exceeds 0.5 kg/m² hr.

When the evaporation rate exceeds 1.0 kg/ m² hr precautionary measures to prevent plastic shrinkage are almost mandatory.

Water retaining materials

Chemical or liquid membranes are gaining in popularity because they are convenient to use. They can be applied by hand or power sprays.

These membranes come in four general categories: water based; chlorinated-rubber based; resin based and PVA based.

In addition to the above classification, curing compounds can be purchased in three different types: Type 1 is clear and translucent without dye; Type1-D clear and translucent with fugitive dye; Type 2, white pigmented.

The clear types usually preferred for surfaces that must be architecturally attractive have one major disadvantage; it is difficult to determine whether they have covered an area of concrete completely.

White compounds reflect the rays of the sun, greatly reducing the surface temperature of the concrete. The white colour will make the covered areas easily distinguishable. After a few days of exposure to direct sunlight the colour will fade. When it dries, a membrane compound forms a vapour seal on the surface, and the hardness of the sealer film when dry, are important factors.

Chemical or liquid membranes reduce evaporation by seating the concrete.

Care should be exercised in the selection of an appropriate membrane coating in that compatability with the intended applied finish to the concrete must be taken into account.

Mechanical barriers

The use of waterproof building papers or plastic film (polyethylene sheeting) will also prevent the evaporation of moisture from concrete.

Pigmented polyethylene sheeting provides a good curing medium as it is impervious to moisture, light in weight, and can be re-used under good conditions.

Plastic sheeting also has the advantage of flexibility. It is easy to drape over complex shapes, and the progress of curing and condition of the concrete can be checked easily at any time.

Any material used as a mechanical barrier to evaporation should be placed over the concrete as soon as the placing of it will not cause surface damage. The edges of the material should overlap several inches, and should be tightly sealed with sand, tape, mastic or wooden planks. It is a good practice - though one not always followed - to moisten the surface of the concrete with an atomising spray of water immediately prior to placing of the sheeting on the concrete.

Mechanical barriers should be placed over concrete as soon as the surface is set.

Water addition curing

Theoretically, flooding, ponding or mist spraying are better than the retention methods mentioned above. But they are not always practical because of job conditions.

On roads, pavements or floors, the method of flooding or ponding is simple. A small dam of earth or other water retaining material is placed around the perimeter of the surface and the enclosed area is kept flooded with water.

Continuous spraying with water, while somewhat easier to control than ponding, is subject to such difficulties as nozzle clogging and loss of water pressure. In the spraying method the water is forced through horizontal pipes to various locations and out through nozzles which disperse it over the concrete surface.

Fogging is essentially the same, but the nozzles deliver a finer, mist like spray. Both methods are affected by changes in wind velocity, which can considerably reduce their effectiveness. Care should be exercised to prevent large temperature differentials between the concrete mass and curing environment so as to avoid potential cracking due to temperature gradients within the concrete. This is generally known as thermal shock cracking.

Flooding with water is effective but can be impractical on some sites

Absorptive covers

An absorptive medium such as sand, hessian or canvas will hold water on the concrete surface while curing progresses.

Any such medium must be kept damp constantly during the curing period, for if drying is permitted, the cover itself will absorb moisture from the concrete. Alternate drying out and wetting of the cover may cause cracking. The use of sawdust as a cover is not advisable, for it has on occasion retarded the hardening of the concrete through the action of sugar in the sap still present in the sawdust.

Water can be retained longer by using an absorptive cover.

Structural concrete

The curing of structural concrete does not present as many problems as may be experienced in curing thick floor slabs or pavements, because the exposed surfaces are usually smaller.

Many concrete structures receive no curing other than that effected by leaving forms in place for several days. The insulating value of formwork is self-evident, but as a curing medium formwork is effective only for the first day or so. As drying proceeds, water vapour can escape along the gaps between the forms and concrete, as well as through gaps between boards.

If formwork is to have a complete curing function it must be kept wet continuously.

Exposed surfaces of the structure should be covered with tarpaulins, plastic sheeting or waterproof paper, and kept wet.

When a concrete structure is built without protection of any kind, the concrete being allowed to harden while exposed by turns of sunlight, evening temperatures, wind and rain, inferior strength and durability of concrete frequently results.

Reasons for Curing

To sum up the advantages of careful control of moisture and temperature in curing:

• The strength of concrete increases with age if curing conditions are favourable. Compressive strength of properly cured concrete is 80 to 100 per cent greater than the strength of concrete which has not been cured at all.
• Properly cured concrete surfaces wear well.
• Drying, shrinkage, cracking is reduced.
• Greater watertightness of constructions is assured.

Points to keep in mind when curing:

• Start curing operations as soon as possible after concrete has been placed.
• For proper curing concrete needs moisture.
• Continuity in curing is a must; alterations of wetting and drying promote the development of cracking.
• If during curing the concrete is allowed to dry out - as may happen in hot weather - the chemical stops right at the point where the concrete loses its moisture.
• The ideal curing temperature is 23°C.
• Cure concrete for at least 7 days.

The vicious cycle in inadequate curing must be obvious. If enough water evaporates from the concrete before it has attained its maximum strength, there will not be sufficient water remaining in the concrete to fully hydrate the cement and so achieve that maximum strength.

The Effect of Excess Water in Concrete

Nothing is easier than adding excess mixing water to premixed concrete at building sites. And nothing is more likely to reduce the strength of the concrete, or make repairs to a concrete construction necessary, or more likely to damage a contractor's reputation for efficiency and reliability.

Concrete supplied by Ahlcon RMC is carefully proportioned and mixed to produce strength according to specifications. Less than half the water it contains is needed for the hydration of cement. The rest of the water is there to make transporting and workability easier (by providing lubrication between sand/aggregate particles) and to ensure that there will be sufficient inherent water for the curing process.

The plant supplying the mix will adjust the water content fractionally to meet summer or winter conditions of transporting and placing.

"Wetness" of concrete as measured by the slump test is directly related to its compressive strength - the 28 day compressive strength of concrete is reduced by about 1.5 MPa for each additional 20mm of slump produced by adding water.

To put this another way, each additional 10 litres of water per cubic metre will reduce the strength of concrete by about 2.5 MPa.

Unless extreme conditions make it necessary, site supervisors should not permit water to be added to premixed concrete without their approval.

The ever present site problem is that all members placing teams quickly learn about the labour saving effect that "a little" added water has on workability of concrete.

The site supervisor can't tie knots in the hose(s) needed for cleaning equipment so 'a little' water may be added to concrete during a pour.

With this unofficial system operating it doesn't take long to add 100 litres of water to a truck load say (5m3) of mixed concrete. The effects will be (a) an average increase of about 80mm of slump over the slump specified, (b) an average reduction in compressive strength of about 5 MPa and (c) uneven strength throughout the concrete mass comprising a number of truck loads with varying slumps.

The only answer is to make at least one responsible member of each placing team fully aware of the harm excess water can cause.

Perhaps the most dangerous of all practices is the use of extra water to help concrete "flow" along elevated forms to lower points. In this case, if shores/toms are removed after a normal curing period, severe structural cracking and/or collapse are more than possibilities.

Effects of too much mixing water

Whilst adding water will in some cases facilitate easier placing, the disadvantages of this include the following:-

• Lower compressive strengths
• Segregation of the concrete mix under certain conditions resulting in variable quality throughout the concrete mass.
• Cracking - with too much water, there will be lower tensile strength, and a tendency towards high shrinkage and subsequent cracking.
• Dusting and scaling - Bleeding of excess water brings too many fines to the surface of floors
• Sand streaks. Excess water bleeding up the sides of forms washes out cement paste and leaves an unsightly streaked surface.
• Contamination. Too much water in concrete placed on grades causes contamination from the subgrade with the concrete leading to an array of quality problems
• Permeability. Voids left as excess water evaporates invite water to seep through walls and floors
• Dead losses - costly repairs, or in extreme cases, demolition and re-building at contractor's expense.

Approximate compressive strengths for given water-cementitious ratios are shown below. Cementitious binder needs less than half its own weight of water to turn concrete into durable construction material.

The 'wetter' this cementitious paste is, the weaker it is. The chart below shows how strength decreases as water content of a mix increases.

Four ways to minimise temptation to add excess water

Be ready to accept premixed concrete as it arrives. Prolonged mixing in the truck agitator makes concrete stiffer, and harder to work. Where high slump concrete or special placement conditions exist consult Ahlcon RMC Technical staff for advice about mix designs.		All concrete requires proper compaction using correct equipment and techniques. Make sure that enough men are available on site to transport , place, compact and finish fresh concrete.
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