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Another typical phone call. An owner wants to know how long it will take for the lightweight concrete in the elevated slabs of his new building to dry before he can place the floor covering. The slabs were placed 4 months ago, but tests still show a moisture-vapor-emission rate of 8 pounds per 1000 square feet in 24 hours. The floor-covering manufacturer requires the concrete to be at 3 lbs/1000 sf/24 hrs. Delaying floorcovering installation will delay building occupancy. The owner has never had this problem in other buildings he has constructed. Why won’t this concrete dry? Concrete is concrete, right?

Well, unfortunately it’s not. Many owners and contractors have told us they’ve experienced project delays while waiting for lightweight concrete to dry. Though we couldn’t find any data regarding the drying time of lightweight concrete, field experience tells us that lightweight concrete takes longer to dry than normal-weight concrete. To help fill this information gap, CONCRETE CONSTRUCTION devised a testing program to find out how long it takes lightweight concrete to dry.

Three normal-weight concretes with water-cement ratios of 0.31, 0.37, and 0.40 were delivered to a testing lab in 1-cubic-yard loads. A lightweight concrete mix with a water cement ratio of 0.40 was also delivered to the testing lab. The proprietary mixes were supplied by the ready-mix division of CAMAS Colorado, Denver. The normal-weight concrete moisture-vapor-emission test results were reported in THE CONCRETE PRODUCER (Ref. 1). Here we compare the lightweight concrete moisture-vapor-emission test results with those for normal-weight concrete having the same water-cement ratio. The producer tightly controlled water content and water-cement ratio by closely monitoring aggregate moisture and water left in the drum. The fresh and hardened properties of the lightweight concrete were as follows:

■ 3-inch slump

■ 4.5% air content

■ 124-pound-per-cubic-foot unit weight

■ 48° F temperature

■ 6850-psi 28-day compressive strength

Workers placed and vibrated the concrete in 3-foot-square test slabs 2, 4, 6, and 8 inches thick. After striking off the surface with a 2x4 and floating it by hand, they covered the slabs with plastic sheeting for 3 days. Calciumchloride moisture-emission testing began after the sheeting was removed.

Moisture-emission tests were conducted in accordance with the test manufacturer’s instructions. Technicians ran one test on each slab at the 3-day age but thereafter ran two tests on each test slab and reported an average test value. 

The calcium-chloride test kits were left in place for 72 hours on slabs stored inside the test lab at a 70±3° F and a relative humidity of 28±5%. This is the normal indoor environment during the winter in the Rocky Mountain region, where the tests were conducted, and represents the environment found in many buildings without relative-humidity controls. 

The table shows moisture-vapor emission rates for the test slabs after drying for up to 183 days. Drying times are measured from the end of the 3-day curing period to the end of the 72-hour moisture-emission test. Thus the 3-day measurement was taken 6 days after concrete placement. The test data support the following conclusions.

Lightweight concrete dries slower. Regardless of the test slab thickness, the lightweight concrete took about 6 months to dry to a moisture-vapor emission rate of 3 lbs/1000 sf/24 hrs. Normal-weight concrete of the same water-cement ratio took only 6 weeks of drying in laboratory air to reach the same level (Ref. 1). From previous work (Ref. 2), we know that laboratory drying represents the fastest drying time. So field conditions that include wet-dry cycles will increase the actual time for the slab to reach the specified moisture-vapor-emission limits. 

As with normal-weight concrete, the moisture-vapor-emission rates were unaffected by slab thickness. 

Lightweight concrete offers many advantages. However, any owner using lightweight concrete that’s to be covered by a moisture-sensitive floor covering or any architect/engineer specifying this combination should consider the slower drying time as an important part of the construction schedule. Once lightweight concrete is specified, the contractor can’t change its drying characteristics. If owners want the benefits of lightweight concrete and a fast-track schedule, they may need to consider applying a polymer coating or sheet product to reduce moisture emissions.

Hence it is very essential during the concreting in cold weather conditions to ensure that the concrete will not undergo freezing in its plastic state.

There are two methods for carrying out cold weather concreting:

1. Provision of normal ambient temperatures for the concrete. This can be done through the heating of the concrete ingredients or bley providing heating enclosures.
2. The addition of chemical admixtures.

Conventional Chemical Admixtures in Cold Weather Concrete

 

Conventional concrete used calcium chloride as accelerating admixtures to offset the retarding effects of slow hydration of concrete in low temperatures. This admixture is not effective below the freezing temperatures.

This is found to be a drawback in the conventional form of admixtures. Hence, for arctic weather conditions, special admixtures are necessary. One such is antifreeze admixtures.
Antifreeze Admixtures for Concrete

The antifreeze admixtures affect the physical condition of the mix water used in the concreting. These can depress the freezing point of the water to a large extent and can be used in temperatures lesser than -30 degrees Celsius. This can enable the extension of the period of the construction activity.

Chemical Composition and Action of Antifreeze Admixtures

There are two groups of antifreeze admixtures that provide the characteristics of antifreeze and the accelerated setting and hardening properties.

They are:

1. First Group

This includes chemicals, weak electrolytes, sodium nitrite, sodium chloride and non-electrolytic organic compounds which lower the freezing point of the water used in the concrete. But these group acts as weak accelerators to promote the setting and hardening.

2. Second Group

These include binary as well as ternary admixtures which contains potash and additives based on calcium chloride, sodium nitrite, calcium chloride with sodium nitrite, calcium nitrite -nitrate-urea and other chemicals.

These have effective antifreeze properties and accelerating property to promote the setting and hardening. These are used in larger dosages compared to that of conventional admixtures.

One such example is the use of 8% of sodium nitrite to keep the liquid at a temperature of -15-degree Celsius.

These admixtures function by lowering the liquid phase freezing point and by accelerating the cement hydration at the freezing temperatures.

Based on the dosage in the mixture, the non-chloride admixture enables the mix (concrete or the mortar) to be placed at sub-freezing temperatures. This hence reduces the need of protective measures required during the cold weathering works.

The method improves the quality of the concrete and as it facilitates early setting, early stripping of formworks can also be carried out. This helps in the reuse of the form within a small duration and hence speed up the construction.

The table-1 shows the significant difference is strength gain at 3, 7 and 28 days for plain concrete and antifreeze admixture used concrete.

Table.1: Concrete Compressive strength with and without antifreeze admixture

 

(As per Ratinov and Rosenburg)

Property	Plain Concrete	Freeze-protection Admixture
Set time (-4 degree Celsius)	–	–
Compressive strength (MPa)
-4 degree Celsius (3 days)	3.4	9.24
-10 degree Celsius (7 days)	8.3	39.3
-10 degree Celsius (28 days)	18.1	49.9

It is possible for the incorporation of other admixtures that contains superplasticizers to be incorporated with the antifreeze admixtures. The main advantage of such combination is that in totality there will be a reduction of water.

The water reduction will reduce the freezable free water content in the mix. This freezable water content is the one that serves as the heat sink for the heat liberated by the initial hydration reactions. This will hence reduce the number of antifreeze admixtures.

Selection of Antifreeze admixtures

The factors based on which the selection of antifreeze admixtures is carried out are:

1. The type of structure
2. The operating Conditions
3. Protecting methods employed in winter concreting
4. Cement brand and aggregate types

A laboratory test must be carried out with the operating materials and the dosage of antifreeze admixtures that are intended to be used in the field.

The incorporation of other admixtures like retarders, superplasticizers with antifreeze admixtures is not restricted in cold weather concreting. The dosage of all the admixture that are used must be established experimentally.

Application and Advantages of Antifreeze admixtures

The antifreeze admixtures are technologically simple and beneficial for cold weather concreting. The admixture helps in improving the cohesiveness, cold joint minimizations, sand streaking, and plasticity. These are estimated to provide large cost saving than other methods of steam curing or concrete enclosures.

The combination of antifreeze agents with water reducing agents or air-entraining agents will help in increasing the resistance of concrete towards the frost action and corrosion.

Introduction

Placing concrete under water is a specialized subject and should be avoided whenever practical to do so. Where concrete works have to be constructed below water level as in the case of marine works, deep foundations of bridges etc; one of the two courses may be adopted. Either water may be excluded temporarily from the site by using cofferdams, caissons, pumps, dewatering equipment OR Concrete may be placed in water using special methods.

Whilst concrete will set and harden under water its placing presents several problems. The most difficult is the prevention of segregation and loss of cement. Formwork except for simpler forms of construction, is difficult to place accurately and in all cases must be anchored firmly. In view of these difficulties, underwater concreting in generally confined to mass un-reinforced work and consideration should always be given to use of pre-cast block for the whole work or as permanent formwork.

General Requirements 

• Concrete should not be placed underwater when the temperature of water is below 4°C.
• It requires a very workable concrete with slump as high as 7" and cement content upto 650 Ibs/cuyd.
• Placing is done in caissons, confer dam or forms.
• Foundation clean up is required using hydraulic jets or pumps.
• Concrete must not be placed in running water.
• Concrete must not be allowed to fall in water.
• Concrete should not flow horizontally by more than 10 ft.

Equipment and Techniques of Concreting Under Water

• Under Water Bucket

It consist of lowering a special bucket containing concrete to the bottom of foundation and opening it there slowly to allow concrete to flow out gently and without causing turbulence. Subsequent buckets are lowered(landed) on the previously placed concrete. Top of the bucket should be covered and should have means to open the gate from above water. Special buckets are made for under water placing with a sloping top having small opening to minimize water surge. see figure attached.

Concrete for the purpose will have 6-7 sacks of cement per cuyd, 6“ - 7" slump, 1.5“ _ 2" maximum size of aggregate and sand content higher than normal; 40% or more. Concrete for foundation of Franciso Bay Bridge was placed in water as deep as 240 ft with buckets. Bucket placement is a good way to start a placement by the tremie method. The later is probably faster and possibly superior once started. Tremie is operated when concrete layer 2 to 3 ft thick is placed.

• Tremie

It consist of placing concrete through a vertical pipe, allowing the concrete to flow from the bottom. The bottom of the tremie is kept submerged in the concrete all the times, and the concrete flows into the mass of previously placed concrete.A tremie consist of a pipe usually 10"-12" inch diameter with a hopper or funnel at the upper end. The equipment is adequately supported and arranged so that it can be raised and lengths of pipe can be removed if necessary as the level of concrete rises in the form. The spacing of the tremie should not be more than 12 ft to 16 ft apart or 8' from the sides or ends of enclosure. 

Concrete is delivered to the hopper by bucket, truck mixer, pump or conveyor. The delivery must be at a good rate and without interruption. Delay for any reason impairs the free flowing mobility of the mass into which concrete is being placed. Plugged pipe and cold joints can result from delays.There are two basic methods of starting a tremie, which may be called the " wet pipe" and "dry pipe" method.

• Pumps

Pumps can carry out under water concreting satisfactorily. Muck should be removed. The end of the concrete pump line should be plugged and lowered to the bottom. The line should be filled with concrete, forcing the plug out. Concrete pumping continues and pump line is raised periodically as resistance increases. Sections of pipe line are removed above water as work progresses.

Concrete Pump

• Pre placed aggregate concrete

Originally developed for structural concrete repairs, pre-placed or pre packed aggregate concrete, is composed of coarse aggregate placed in the form and a special mortar is then injected into the form starting at the bottom to fill the voids and create a unit mass. It effects saving in time and money.Some advantages include,

• Economical in cement content. 200 - 250 Ibs /cuyd of cement is used as against 600-800 Ibs/cuyd. Normal strength is 2900 psi.
• Uniform properties through out the mass. 
• High durability 
• Low permeability
• Suitable for placing in congested areas and where instruments are to be located precisely. 
• Suitable for under water concreting.
• Reduced shrinkage due to point contact of aggregate particles. Drying shrinkage 200 x 10-6 
• No cracking because of low shrinkage. 
• Aggregate, while in place can be cooled or heated. 
• Suitable for mass construction. 
• Suitable for repair of old structures. 

The concrete industry is the largest user of natural resources in the world and thus has a considerable environmental impact. Each ton of Portland cement requires about 1.5 tons of raw material for its production. This industry is not only energy intensive but is also a major contributor of greenhouse gases, in the form of CO₂. Each ton of Portland cement that is produced involves the release into the atmosphere of about one ton of CO2. Indeed, according to Mehta (1999), the cement industry is responsible for about 7% of global CO₂ emissions; thus, there is considerable interest now in developing cements that are more environmentally friendly. One such cement (CEMROC), based on blast-furnace slag, has recently been described by Gebauer et al. (2005). This cement, produced by Holcim in Europe, is reported to show close to zero CO₂ emission during its production (only about 100 pounds per ton of cement).

It is similar to the supersulfated cement described above and is particularly well suited for use in structures exposed to aggressive environments. Other cements of this general type will almost certainly be developed in the future. Another (and simpler) approach is to use much greater proportions of fly ash in concrete. A great deal of development is being conducted on what is referred to as high-performance, high-volume fly ash concrete (Malhotra, 2002; Malhotra and Mehta, 2002). Such concretes may be defined as:

a. General

Preplaced-aggregate (PA) concrete is produced by placing coarse aggregate in a form and later injecting a portland-cement-sand fly ash grout, usually with chemical admixtures, to fill the voids. The smaller-size coarse aggregate is not used in the mixture to facilitate grout injection. It is primarily applicable to the repair of existing concrete structures. PA concrete may be particularly suitable for underwater construction, placement in areas with closely spaced reinforcing steel and cavities where overhead contact is necessary, and in areas where low volume change is required. It differs from conventional concrete in that it contains a higher percentage of coarse aggregate since the coarse aggregate is placed directly into the forms with point-to- point contact rather than being contained in a flowable plastic mixture. Therefore, hardened PA concrete properties are more dependent on the coarse aggregate properties. Drying shrinkage of PA concrete may be less than one-half that of conventional concrete, which partially accounts for the excellent bond between PA concrete and existing roughened concrete. The compressive strength of PA concrete is dependent on the quality, proportioning, and handling of materials but is generally comparable to that achieved with conventional concrete. The frost resistance of PA concrete is also comparable to conventional air-entrained concrete assuming the grout mixture has an air content, as determined by ASTM C 231 (CRD-C 41) of approximately 9 percent. PA concrete may be particularly applicable to underwater repair of old structures and underwater new construction where dewatering may be difficult, expensive, or impractical. Bridge piers and abutments are typical of applications for underwater PA concrete construction or repair. A detailed discussion of PA concrete is provided in ACI 304.R.

b. Applications

PA concrete has been used on different types of civil works construction including:

(1) Resurfacing of lock chamber walls.

(2) Underwater repair to lock guide walls.

(3) Resurfacing of spillways.

(4) Construction of plugs to close temporary sluices through a dam.

(5) Filling of temporary fish ladders through a dam.

(6) Scroll case embedment.

c. Materials and proportioning

Intrusion grout mixtures should be proportioned in accordance with ASTM C 938 (CRD-C 615) to obtain the specified consistency, air content, and compressive strength. The grout mixture should also be proportioned such that the maximum w/c complies with appropriate standards. Compressive strength specimens should be made in accordance with ASTM C 943 (CRD-C 84). Compressive strength testing of the grout alone should not be done to estimate the PA concrete strength because it does not reveal the weakening effect of bleeding. However, such testing may provide useful information on the potential suitability of grout mixtures. The ratio of cementitious material to fine aggregate will usually range from about 1 for structural PA concrete to 0.67 for mass PA concrete. A grout fluidifier meeting the requirements of ASTM C 937 (CRD-C 619) is commonly used in the intrusion grout mixtures to offset bleeding, to reduce the w/c and still provide a given consistency, and to retard stiffening so that handling times can be extended. Grout fluidifiers typically contain a water reducing admixture, a suspending agent, aluminum powder, and a chemical buffer to assure timed reaction of the aluminum powder with the alkalies in the portland cement. Products proposed for use as fluidifiers which have no record of successful prior use in PA concrete may be accepted contingent on successful field use. ASTM C 937 requires that intrusion grout made as prescribed for acceptance testing of fluidifiers have an expansion within certain specified limits which may be dependent on the alkali content of the cement used in the test. Experience has shown, however, that because of the difference in mixing time and other factors, expansion of the field-mixed grout ordinarily will range from 3 to 5 percent. If, under field conditions, expansion of less than 2 percent or more than 6 percent occurs, adjustments to the fluidifier should be made to bring the expansion within these limits. The fluidifier should be tested under field conditions with job materials and equipment as soon as practicable so that sufficient time is available to make adjustments in the fluidifier if necessary. If the aggregates are potentially alkali reactive, the total alkali content of the portland cement plus fluidifier added to increase expansion should not exceed 0.60 percent, calculated as equivalent sodium oxide by mass of cement. The grout submitted for use may exhibit excess bleeding if its cementitious material to fine aggregate ratio is different than that of the grout mixture used to evaluate the fluidifier. Expansion of the grout mixture should exceed bleeding at the expected in-place temperatures. Grout should be placed in an environment where the temperature will rise above 40 °F, since expansion caused by the fluidifier ceases at temperatures below 40°F. This condition is normally readily obtainable when PA concrete is placed in massive sections or placements are enclosed by timber forms. If an airentraining admixture is used in the PA concrete, adjustments in the grout mixture proportions may be necessary to compensate for a significant strength reduction caused by the combined effects of entrained air and the hydrogen generated by the aluminum powder in the fluidifier. However, these adjustments must not reduce the air content of the mixture to a level that compromises its frost resistance. The largest practical NMSA should be used to increase the economy of the PA concrete. A 37.5-mm (1- 1/2-in.) NMSA will typically be used in much of the PA concrete; however, provisions are made for the use of 75-mm (3-in.) NMSA when it is considered appropriate. It is not expected that many situations will arise where the use of aggregate larger than 50 mm (2 in.) will be practical. Pozzolan is usually specified to increase flowability of the grout.

d. Preplacing aggregate

Care is necessary in preplacing the coarse aggregate if excessive breakage and objectionable segregation are to be avoided. The difficulties are magnified as the nominal maximum size of the aggregate increases, particularly when two or more sizes are blended. Therefore, the Contractor’s proposed methods of placing aggregate should be carefully reviewed to ensure that satisfactory results will be obtained. Coarse aggregate must be washed, screened, and saturated immediately prior to placement to remove dust and dirt, and to eliminate coatings and undersize particles. Washing in forms should never be permitted because fines may accumulate at the bottom.

e. Contaminated water

Contaminated water is a matter of concern when PA concrete is placed underwater. Contaminants present in the water may coat the aggregate and adversely affect the setting of the cement or the bonding of the mortar to the coarse aggregate. If contaminants in the water are suspected, the water should be tested before construction is permitted. If contaminants are present in such quantity or of such character that the harmful effects cannot be eliminated or controlled, or if the construction schedule imposes a long delay between aggregate placement and grout injection, PA concrete should not be used.

f. Preparation of underwater foundations

Difficulty has been experienced in the past with cleanup of foundations in underwater construction when the foundation material was glacial till or similar material. The difficulty develops when as a result of prior operations, an appreciable quantity of loose, fine material is left on the foundation or in heavy suspension just above the foundation. The fine material is displaced upward into the aggregate as it is being placed. The dispersed fine material coats the aggregate or settles and becomes concentrated in the void spaces in the aggregate just above the foundation thus precluding proper intrusion and bond. Care must, therefore, be exercised to ensure that all loose, fine material is removed insofar as possible before placement of aggregate is allowed to commence.

g. Pumping

Pumping of grout should be continuous insofar as practical; however, minor stoppages are permissible and ordinarily will not present any difficulties when proper precautions are taken to avoid plugging of grout lines. The rate of pumping should be regulated by use of sounding wells so that the preplaced aggregate is slowly intruded to allow complete and uniform filling of all voids. The rate of grout rise within the aggregate should be controlled to eliminate cascading of grout and to avoid form pressures greater than those for which the forms were designed. For a particular application, the grout injection rate will depend on form configuration, aggregate grading, and grout fluidity.

h. Joint construction

A cold joint is formed in PA concrete when pumping is stopped for longer than the time it takes for the grout to harden. When delays in grouting occur, the insert pipes should be pulled just above the grout surface before the grout stiffens, and then rodded clear. When pumping is ready to resume, the pipes should be worked back to near contact with the hardened grout surface and then pumping resumed slowly for a few minutes. Construction joints are formed in a similar manner by stopping grout rise approximately 12 in. below the aggregate surface. Care must be taken to prevent dirt and debris from collecting on the aggregate surface or filtering down to the grout surface. If construction joints are made by bringing the grout to the surface of the coarse aggregate, the joint surfaces should be cleaned and prepared properly.

i. Grouting procedure

The two patterns for grout injection are the horizontal layer and the advancing slope. Regardless of the system used, grouting should start from the lowest point in the form.

(1) Horizontal layer 

In this method grout is injected through an insert pipe to raise the grout until it flows from the next insert hole 3 to 4 ft above the point of injection. Grout is then injected into the next horizontally adjacent hole, 4 or 5 ft away, and the procedure is repeated sequentially around the member until a layer of coarse aggregate is grouted. Successive layers of aggregate are grouted until all aggregate in the form has been grouted.

(2) Advancing slope

The horizontal layer method is not practical for construction of such slabs when the horizontal dimensions are large. In situations such as this, it becomes necessary to use an advancing slope method of injecting grout. In this method, intrusion is started at one end of the form and pumping continued until the grout emerges on the top of the aggregate for the full width of the form and assumes a slope which is advanced and maintained by pumping through successive rows of intrusion pipes until the entire mass is grouted. In advancing the slope, the pumping pattern is started first in the row of holes nearest the toe of the slope and continued row by row up the slope (opposite to the direction of advance of slope) to the last row of pipes where grouting has not been completed. This process is repeated, moving ahead one row of pipes at a time as intrusion is completed.

(3) Grout insert pipes and sounding devices

The number required and the location and arrangement of grout insert pipes will depend on the size and shape of the work being constructed. For most work, grout insert pipes will consist of pipes arranged vertically and at various inclinations to suit the configurations of the work. The guide specification provides for the option of the diameter of the grout insert pipes being either 3/4, 1, or 1-1/2 in. Generally, either a diameter of 3/4 or 1 in. would be allowed for structural concrete having a maximum size aggregate of 37.5 mm (1-1/2 in.) or less. If the preplaced aggregate has a maximum size larger than 37.5 mm (1-1/2 in.), the grout insert pipes should be 1-1/2 in. in diameter. Intrusion points should be spaced about 6 ft apart; however, spacing wider than 6 ft may be permissible under some circumstances, and spacings closer than 6 ft will be necessary in some situations. Normally, one sounding device should be provided for each four intrusion points; however, fewer sounding devices may be permissible under some circumstances. In any event, there should be enough sounding devices, and they should be arranged so that the level of the grout at all locations can be accurately determined at all times during construction. Accurate knowledge of the grout level is essential to:

(a) Check the rate of intrusion
(b) Avoid getting the grout too close to the level of the top of the aggregate when placement of the aggregate and intrusion are progressing simultaneously.
(c) Avoid damage to the work which would occur if a plugged intrusion line were washed out while the end of the line was within the grout zone.

Sounding devices usually consist of wells (slotted pipes) through which the level of the grout may be readily and accurately determined. If sounding devices other than wells are proposed, approval should be based on conclusive demonstration that such devices will readily and accurately indicate the level of the grout at all times. In repairing vertical surfaces, such as lock chamber walls or sloping surfaces which are substantial distances and are relatively thin (up to about 2 ft thick), the grout is brought up uniformly from the bottom. Intrusion points for such work should be arranged in horizontal rows with the rows spaced not more than 4 ft apart horizontally. Holes in adjacent horizontal rows should be staggered so that a hole in any row is at the midpoint of the space between holes in the adjacent rows above and below. Intrusion is controlled by pumping through all holes in each horizontal row until grout flows from all holes in the row above. Grouting then proceeds through the next row above after the holes below, which have just been grouted are plugged. The process is repeated until a section is completed. The bottom row of holes should be placed at the bottom of the form.

j. Finishing unformed surfaces

If a screeded or troweled finish is required, the grout should be brought up to flood the aggregate surface and any diluted grout should be removed. A thin layer of pea gravel or 3/8- to 1/2-inch crushed stone should then be worked into the surface by raking and tamping. After the surface has stiffened sufficiently, it may be finished as required. A finished surface may also be obtained on PA concrete by adding a bonded layer of conventional concrete of the prescribed thickness to the PA concrete surface. The PA concrete surface should be cleaned and grouted prior to receiving the topping.

In addition to quality control procedures that are routinely necessary to achieve quality concrete placement, there are additional requirements imposed by cold weather. Cold ambient temperatures decrease the rate of setting and increase the likelihood of deficient concrete due to exposure to temperatures below freezing. The rate of setting refers to how quickly the new concrete changes from a fluid to a hard mass. A decreased rate of setting results in increased time the concrete is exposed to the harmful effects of freezing temperatures. A general rule of thumb is that a decrease of concrete temperature of 20 degrees will result in the doubling of the setting time. This slower rate of setting and the accompanying slower rate of compressive strength gain must be accounted for when scheduling work activities. However, the temperature of the concrete should not be higher than allowed by the specifications since high concrete temperatures can cause other undesirable problems such as surface cracking.

The temperature of the concrete as initially mixed should be selected based on weather conditions such as expected ambient temperatures, precipitation, and wind conditions. Other factors to consider are travel time from the plant to the project site and the size and thickness of the concrete placement. The concrete supplier may have recommendations based on the preceding factors. The Quality Control manager and the concrete supplier will have to exercise a degree of judgment in selecting the appropriate mix design temperature for the concrete. If the concrete temperature as placed in the forms is dropping below the acceptable minimum, adjustments will have to be made. The heating of the mixing water or other adjustments may be necessary. Another factor to consider is the fact that winter days are relatively short in terms of daylight and therefore darkness and rapidly dropping temperatures must be considered.

Preparation for Placement

 
The placement of concrete should be planned and discussed prior to commencement of work. Typically, the representative of the owner, the Quality Control Manager, a technical representative of the concrete supplier, and the foreman of the concrete crew will meet to discuss placement procedures. The size of the placement, the expected weather conditions, the equipment and materials needed, and the timing of the ready mix trucks should all be discussed and a plan of action adopted. Is the size of the placement reasonable and manageable considering the resources and manpower available? Are overnight temperatures expected to be below freezing? What is the expected high temperature during the next few days? Are the necessary equipment and materials, such as insulated blankets and heating equipment, on site? 

 

The placement plan should identify what work is going to be done, who is going to do it, and when the placement is going to be done to avoid errors due to a lack of communication. Planning should also include a plan of action for emergencies and problems that may arise during placement. For example, extra vibrators need to be available in the event that the vibrators being used to consolidate the concrete should fail or malfunction. Is a spare generator available for use in case the primary source of power fails? Where should construction joints be placed in the event that the delivery of concrete is interrupted? Temporary lighting should be available in the event that the concrete placement takes longer than anticipated. The technical representative of the concrete supplier should be consulted regarding the selection of a suitable mix design, anticipated travel time, haul routes and distances, and recommendations regarding cold weather practices. Concrete exposed to the elements should be air entrained. Air entrainment increases the durability of concrete. Entrained air will improve the resistance of concrete to the freeze thaw cycle. Entrained air also improves the workability of concrete. The workability of stiff, poorly graded concrete is improved by the use of entrained air. A minimum concrete temperature of 60 degrees Fahrenheit at the time of placement into the forms is generally considered desirable. 

 

In order to meet the minimum concrete temperature requirement, the temperature of the concrete when mixed has to be higher than 60 degrees because of the heat loss from the time of mixing to the time of actual placement into the forms. The concrete supplier may recommend an initial temperature of 70 degrees to compensate for heat loss. A maximum of 85 or 90 degrees is generally specified. The specifications generally stipulate a minimum and a maximum allowable concrete temperature. The temperature of the concrete can be increased by heating the water and the aggregate. However, because of the difficulty of heating aggregate, the heating of mixing water is a more practical solution. 

 

Because setting time is delayed by cold weather, the use of an accelerator may be considered. An accelerator is a chemical admixture that decreases the setting time of the concrete and by decreasing the setting time also indirectly decreases the risk of damage to the concrete due to freezing temperatures. Fresh concrete will freeze if its temperature drops under 25 degrees Fahrenheit and its compressive strength can be cut in half. Such a drastic loss of compressive strength is unacceptable. Admixtures, such as accelerators, are used to modify the characteristics of concrete in response to environmental circumstances such as cold temperatures at the time of the placement or other considerations. Non chloride accelerators can be used when possible corrosion of steel reinforcement is an issue. A misconception with respect to the use of accelerators is that accelerators prevent fresh concrete from freezing. Accelerators will not prevent concrete from freezing and so fresh concrete must be protected from cold weather by heating the ambient surroundings or by the use of insulation. Because of cold weather the substitution of type III cement in lieu of type I should be evaluated. Type III is high early strength cement. Another possible alternative is to increase the amount of type I cement in the mix design. However, the use of type III cement is generally more desirable because of cost. An increased amount of type I cement in the mix design is accompanied by a corresponding increased cost. Either solution is acceptable from the technical perspective. A higher cement content will increase the heat of hydration and decrease setting time. The concrete mix design selected should have the least amount of water possible to reduce setting time. Additionally, the compressive strength of concrete is inversely related to the amount of water in the mix. Higher water content results in lower compressive strength. A measure of this relationship is called the water cement ratio. The water cement ratio is calculated by dividing the weight of water by the weight of cement. Typical water cement ratios are .45 to .50. While the placement of concrete as early in the day as possible is desirable, circumstances may dictate the placing concrete in the afternoon in order to meet minimum ambient temperature requirements in the specifications. Typically, specifications will stipulate that ambient temperature be about 40 degrees Fahrenheit and rising before concrete placement is allowed. Once all of the preceding factors have been considered and discussed, a mix design and a placement plan can be selected.

Inspection of forms, subgrade, underground plumbing, and reinforcing steel for compliance with specifications should be done at least a day before concrete is ordered. It is advisable that the inspector use a concrete placement check list to determine if the contractor is ready. Forms should be checked for correct elevations. Immediately prior to the placement of concrete the surfaces of the forms should be covered with oil or sprayed with water. Oil will prevent the formation of a bond between the new concrete and the form surface. However, water should be used if the new concrete is to receive a coat of paint or any type of finish requiring a bond with the concrete. Floor slabs and footings should be checked to ensure correct location and dimensions. Floor slabs should be placed in one layer. Otherwise, concrete, such as walls, should be placed in layers not to exceed 12 inches. The limit on layer thickness is to ensure that the concrete is properly consolidated. Equipment to be used during the placement should be checked for proper operation and for adequate quantities to handle the placement. Vibrators should be checked for amplitude and frequency. Portable generators should be checked for operation and for fuel. Items that are to be embedded in the concrete, such as anchor bolts, should be checked for correct location, quantity, and elevation. The bottom of trenches where concrete footings are to be placed should be clean and free of debris. Ice and frozen earth are not acceptable. Reinforcing steel should be clean and free of rust. The ability of reinforcing steel to adhere to new concrete is adversely affected by rust, scale, or other foreign substances. Any reinforcing steel that is coated with concrete splattered during previous placements also needs to be thoroughly cleaned. It is also important that reinforcing steel be properly supported or braced to prevent movement during concrete placement. Spacers can be used to prevent unwanted displacement of reinforcing steel. It is important that the preparation for the placement of concrete be done in a deliberate and careful manner to avoid costly mistakes. The tying of reinforcing steel and other last minute adjustments while the ready mix truck is waiting is a practice that normally leads to mistakes and should be avoided.

 

Concrete temperatures should be checked and recorded for record and control purposes. An acceptable temperature for concrete placement is normally from 60 degrees to 85 degrees Fahrenheit. The temperature of the fresh concrete should be maintained at a minimum of 55 degrees until the compressive strength requirement is met. For transit mixed concrete, the specifications will typically require that the concrete meet the requirements of ASTM C94. One important requirement of C94 is that a delivery ticket be furnished with each delivery. It is important that the inspector keep a copy of the delivery ticket since the ticket will have important information regarding the time the concrete was batched, amount of concrete delivered, admixtures, air entrainment, any addition of water, and time of arrival at project site. ASTM C94 also establishes limits on the amount of concrete that can be mixed in transit based on the size of the mixing drum.

Once the compressive strength requirement is achieved, insulated blankets can be removed in a manner that will prevent a rapid decline in temperature. An acceptable rate of temperature loss is not more than 2 degrees Fahrenheit per hour. A simple pocket thermometer is handy. The recording of concrete temperatures, ambient temperature, and other weather conditions at the time of placement should be done by the contractor. These duties can become very important in the event that the concrete has to be removed due to exposure to freezing temperatures, unacceptable cracking, cold joints, segregation of aggregate, or failing compressive strength tests. Good documentation helps pinpoint the source of the problem and the liable party. Thorough documentation is an aid to determining if the problem was caused by exposure to cold temperatures, poor workmanship, poor or improper curing, defective curing compound, or faulty concrete because of mixing or batching problems. Samples of concrete should be taken in the frequency stipulated by contract to verify compliance with the slump, compressive strength, concrete temperature, air entrainment and ambient air temperature requirements. Slump is a measure of consistency and workability in concrete. Consistency is determined by comparing the latest batch of concrete with previous batches. Is the concrete consistent with previous batches in terms of air entrainment, temperature, and stiffness of the mix? It is desirable that concrete be consistent from batch to batch. Workability is a characteristic with respect to how easily the concrete can be placed into the forms. Concrete with a relatively higher slump is easier to place than stiff low slump concrete. Workability is effectively obtained by the use of air entrainment and by increasing the amount of fine aggregate. Workability of concrete is decreased by the use of poorly graded aggregates. The preparation and care of concrete cylinders has to be monitored for proper handling procedures. The concrete has to be deposited as closely as possible to the final location. Concrete for slabs on grade should be placed against the forms at one end of the slab with new deliveries placed against the previous batch. Concrete should not be dropped more than 5 feet. Concrete that is dropped from heights greater than 5 feet will segregate. An effective method of placing concrete is to pump the concrete into the forms. Pumping is effective because it places the concrete exactly where it is needed in a minimum amount of time. Concrete should not be dumped in separate piles and then moved to the final location by use of vibrators. This practice tends to cause segregation of the concrete Excessive vibration of concrete should be avoided. The proper consolidation can be obtained by vibration of concrete from 5 to 15 seconds. Vibration of over 15 seconds will result in the loss of air entrainment. During the placement of concrete into tall sender forms, such as columns, care must be taken to prevent segregation of particles caused by the dumping of the concrete into the form. The use of a tremie or chute may be required or advisable.

After concrete has been placed into the forms, concrete slabs on grade and concrete paving require additional work before curing. First, the concrete surface needs to be screeded to give a level surface at the correct elevation. Screeding is simply the striking off of excess concrete with a straightedge. A 2x4 wood stud is usually moved in a back and forth motion through the fresh concrete to remove humps. Afterwards, screeding the concrete surface is finished with a bullfloat. Large unformed concrete surfaces will require finishing of the surface. However, finishing should not start until surface water has disappeared. Premature finishing will result in a weak, watery, and eventually unacceptable surface and as time goes by the surface will flake and spall. A bullfloat has a flat blade with a long handle and is usually made of aluminum. The long handle allows the worker to reach areas in the middle of the slab. Bullfloating will give a flat even surface. In situations where a hard surface is desired, the floating of the surface is followed by steel troweling. Steel troweling will produce a smooth hard surface. Once the concrete slab is stiff enough to hold the weight of a person, the contraction joints can be sawed as indicated on the plans. Contraction joints provide a selected location for the concrete slab to crack in a predetermined manner instead of a random manner. Contraction joints are also referred to as control joints. Concrete which will remain hidden from view, such as footings, does not need a finished surface. The main concern with footings and grade beams is the correct location, line and grade. Concrete floors may have a straightness requirement. For ordinary floors the tolerance can be 1/8 inch per 10 feet of length. A gap of 1/8 inch or more would be out of tolerance and therefore unacceptable. Where a level surface is critical, the specifications may state an F number. The F number system consists of two standards. One standard, FF, refers to the bumpiness or waviness in a floor. The second standard, FL, refers to the tilt or inclination of the floor. F numbers are used when the levelness of a floor is important, such as a warehouse. When finishing of the concrete is completed and the surface water has disappeared, the next step is curing.

 

Once the concrete has been delivered, placed, and finished, the next operation left to accomplish is curing. Curing is defined as the process of retaining the moisture in the concrete to allow for the proper hydration of cement and the corresponding increase in compressive strength. The degree to which this chemical reaction is completed determines the quality of the new concrete. Specifications will generally require that new concrete be cured by an approved method for at least 3 to 7 days. Concrete that is not protected from freezing temperatures may not hydrate properly and deficient concrete that will not meet compressive strength requirements will result. The compressive strength of the properly cured concrete will increase rapidly in the first 7 days and at a slower rate thereafter. The specifications will require that the concrete cylinders taken at
the time the concrete was placed be tested for compressive strength at 7 and 28 days. The tests done at 7 days will give a good indication of whether the concrete will meet the 28 day compressive requirements in the contract. The tests done at 28 days after the placement must meet the compressive strength specified in the contract. For example, the specifications might require that the cylinders tested at 28 days break at 3,500 psi. During the curing period, concrete also needs to be protected from extreme changes in temperature and from damage caused by foot or vehicular traffic. Concrete members such as beams need to be supported by forms for a minimum of six days or maybe longer depending on test results. Concrete members must be able to support their own weight and the weight of the anticipated load before the forms are removed. Any exposed concrete surface on these members has to be properly cured. Fresh concrete must be protected from freezing temperatures. Insulated blankets are used to cover exposed concrete slabs and paving and other concrete placements. These blankets can also be wrapped around formed concrete placements. Care should be taken to ensure that corners and edges are adequately insulated to prevent heat loss. If heating devices are used to protect fresh concrete from freezing, care must be used to prevent drying the concrete surface excessively and causing surface cracking. Additionally, no sources of heat, such as torches, should be allowed near new concrete because of the rapid drying effect on the concrete surface induced by such devices.

In practice, concrete is seldom placed and cured under ideal conditions. Nevertheless, curing must prevent the loss of moisture and in cold weather an unacceptable drop in concrete temperature. Additionally, fresh concrete must be protected from damage caused by the weather or other sources. The most common method of curing concrete is by moist curing. The surface is kept moist by a continuous fog of moist air or by burlap that has been soaked in water. A fog of moist air is an effective method of curing concrete. Another environmental condition necessary for moist curing is that the ambient temperature be well above freezing. On formed concrete, the wood forms must be kept wet during the curing period. Once the forms are removed, the concrete can be cured with curing compound. Forms must be removed carefully to avoid damaging green concrete. Another moist curing method involves the use of burlap blankets. Burlap is placed on the new concrete as soon as the surface is hard enough to prevent damage from the weight of the mat and is kept wet. If placed too early, the burlap mat will damage the new surface. Burlap should be clean and free of contamination such as dirt and oil. The disadvantage of these methods is that cold temperatures prevent the use of moist curing methods.

In theory the best curing method is moist curing. In practice, moist curing will work well if environmental factors are favorable and all the necessary procedures are properly followed. Another, less complicated, method of curing concrete is by the use of curing compound. This method involves the use of a pigmented curing compound or a clear compound with a dye. The dye is used as a visual aid so that proper application of the compound is assured. The timing of the application of the curing compound on the concrete surface is important. The spraying of the compound should happen after the concrete surface has been properly finished. The concrete surface should be free of standing water. If applied too early curing compound will coagulate into clumps and fail to provide the desired impermeability on the concrete surface. However, the surface should not be allowed to dry. The correct time for application of the curing compound is as soon as the surface water has disappeared, but before the concrete surface is dry. The compound is applied with a sprayer. Generally two coats are applied to ensure proper coverage of the surface. The coats should be applied at 90 degrees to each other to ensure proper coverage.Curing compounds are also used after an initial period of moist curing or to cure concrete after the removal of forms. On paving projects, curing compound is applied with equipment specifically designed for this work. On small concrete placements, a hand held sprayer is generally used. There must be complete and uniform coverage of the concrete surface. In
situations where vinyl composition tile or paint is to be applied on the concrete surface, such as a slab on grade, a technical representative of the curing compound manufacturer should be consulted for proper selection of the compound. Curing compounds should be mixed well before use and should meet ASTM specifications for compounds used for curing concrete. A check should be done to ensure that the shelf life of the compound has not expired. The curing of concrete with old curing compound may result in concrete that will not meet the required compressive strength of the specifications. The most common deficiency when curing compound is used is the lack of adequate coverage. An unacceptable loss of moisture will result on the concrete surface from improper spraying of curing compound.

Adherence to proper placement procedures is critical during cold weather. The placement of quality concrete can be successfully accomplished even under adverse weather conditions such as cold weather. The key to success for placing concrete in cold weather is an understanding of the challenges, a viable plan of action, implementation of adopted procedures and compliance with the plan. For further information on placing concrete under adverse weather conditions or concrete issues in general, the reader is encouraged to visit the web sites of the American Concrete Institute at www.aci-int.org and the Portland Cement Association at
www.portcement.org.