How many days/hours should I let my cement driveway dry before I can drive on it. Basically 60 degrees during the day and 35 degrees at nite. Thanks an everyone have a Happy Turkey Day
Cement ??--Ray??
- Thread starter hawkeye
- Start date
Hawkeye,
This coming from a GC, my subs tell me a minimum of 48 hours, if it didnt matter, I would give it a week....
Hope that helps.......
Franky
This coming from a GC, my subs tell me a minimum of 48 hours, if it didnt matter, I would give it a week....
Hope that helps.......
Franky
Concrete does not "dry," it undergoes a reaction called hydration,
which requires water. The longer you can keep the concrete moist,
the longer it hydrates, and the stronger it gets. If it dries, then the
reaction stops and it stops gaining compressive strength. The
concrete manual I have shows the graph of compressive strength
versus time for wet-cured concrete, data points as follows:
Days Strength
-------------------------
3 25%
7 60%
28 100%
90 120%
180 125%
73 Mike N6MZ
--
--------------------------------------------------------------------------------
which requires water. The longer you can keep the concrete moist,
the longer it hydrates, and the stronger it gets. If it dries, then the
reaction stops and it stops gaining compressive strength. The
concrete manual I have shows the graph of compressive strength
versus time for wet-cured concrete, data points as follows:
Days Strength
-------------------------
3 25%
7 60%
28 100%
90 120%
180 125%
73 Mike N6MZ
--
--------------------------------------------------------------------------------
:mj07: :mj07: :mj07:
Thats it?:shrug: :SIB . Im not sure what kind of dildo got stuck up your Rectum, or what I said that got you on my bad side, but you need to get a life and stop stalking my posts:142smilie ...................
FYI, he asked when he could drive on it, not when he could park an Army Tank on it:SIB .....
Im done playing marbles with your sorry replies, so why dont you go TITFD on another sorry-ass Georgia team, have some Taco-Bell, and fill up your Neon-Green Geo Metro with some ethanol and have a good time:scared
Oh, and here is the rest of the info I left out;
LENGTH OF CURING
Fixed Time Intervals
The traditional prescriptive way of specifying length of curing is with fixed time periods. The requirement is usually accompanied by a minimum temperature during the specified time interval, typically 10 ?C.
AASHTO
The AASHTO Guide Specifications for Highway Construction requires 3 days of curing, without comment on temperature.(14)
State DOTs
More than half of State guidance reviewed requires 3 days of curing, with no requirements on temperature during the curing period, although some DOTs had coldweather provisions requiring concrete temperatures be 10 ?C. One DOT requires the temperature during those 3 days to be at ?C. Several States require 4 days, without qualifications on temperature. About 25 percent of States require 7-14 days of curing, but most of these DOTs allow for a shorter period if strength reaches a prescribed level, as determined by field-cured cylinders or maturity methods.
ACI
ACI guidance is quite variable, and some standards provide for several options.
? ACI 318 (Building Code).7 days at T 10 ?C, or 3 days at T 10 ?C for high early-strength concrete (not specifically defined).(31)
? ACI 301 (Standard Specification for Structural Concrete).7 days or 3 days for high early strength concrete.(32)
o ACI 308 (Standard Practice for Curing Concrete)(4)
o 3 days with type III cement.
o 7 days with type I cement.
o 14 days with type II cement.
o Types IV and V, or w/ pozzolan.no recommendation.
o Pavements.7 days at 5 ?C.
? ACI 325.9 R (Guide for Construction of Concrete Pavements and Concrete Bases).7 days at 4 ?C.(33)
? ACI 330 R (Guide for Design and Construction of Concrete Parking Lots)-3 days for auto traffic, 7 days for all other traffic.(34)
Time to a Specified Strength
The option to cure until a certain fraction of design strength is attained is common in ACI guidance, summarized as follows.
? ACI 301 (Standard Specification for Concrete).(32)
o Time to 70 percent f'c using field cured specimens.
o Time to 85 percent f'c using lab specimens, with field temperatures 10 ?C.
o Time to 100 percent f'c using NDT methods (methods unspecified).
? ACI 308 (Standard Practice for Curing Concrete).time to 70 percent f'c? option for pavements.(4)
? ACI 325.9 R (Guide for Construction of Concrete Pavements and Concrete Bases).time to 70 percent f'c.(33)
? ACI 330 R (Guide for Design and Construction of Concrete Parking Lots).until compressive strength 21 MPa.(34)
Maturity
The maturity method is a calculation based on the concept that time-temperature history, rather than simple time, determines the strength development of concrete. By monitoring time-temperature histories of in-place concrete, real-time strength development can be indirectly monitored. The method is calibrated using strength development of laboratoryor field-cured specimens with a known time-temperature history. ASTM C 1074 describes the method.(35) Hardware and software are manufactured that automates much of the work, and consulting firms specializing in this procedure exist.
Equations in ASTM C 1074 can be written into a spreadsheet to simplify exploratory calculations.(35) Exploratory calculations are useful for approximate planning purposes and investigating likely effects of different temperature histories. For exploratory work, inputs of daily high and low concrete temperatures and of standard laboratory strength determinations can be used to estimate strength development for the first 7 days after placing. Predictions become more prone to error at later ages and should not be used.
In actual field application, the maturity method normally takes temperature input from inplace thermocouples located at critical points in the pavement. Determining critical locations is an important part of the application. Pavement corners, sections of elevated pavement, and most recently placed pavements are particularly sensitive to low temperature events.
VERIFICATION OF CURING
Although strength is the primary variable around which curing specifications are based, verifying adequacy of a curing program on pavements may not be best measured by strength. Several approaches are described below.
Strength of Cores (ASTM C 42)(36)
The strength of concrete is strongly affected by inadequate curing, and, in theory, could be detected by measuring strength of cores taken from a concrete pavement. However, the effects of poor curing are only strongly apparent in the properties of the top 50 mm of concrete, and sometimes even less. Therefore, only thin pavements are likely to be well represented by strength testing. Compressive strength is not likely to be an effective procedure for typical highway pavements.
Rebound Hammer (ASTM C 805)(37)
The rebound hammer method basically measures the modulus of elasticity of the nearsurface concrete. It is often criticized as being unduly affected by near-surface properties and insensitive to the strength of the entire section of concrete under the test point. This may actually recommend the method for use in evaluating the curing of concrete pavements, where near-surface effects are considered most important. The test method is suitable for in-place measurements and has been found in laboratory tests to be well suited for detecting curing deficiencies in near-surface pavement. There is a considerable amount of scatter in rebound numbers because of the heterogeneous nature of nearsurface properties (principally due to near-surface aggregate particles). The method directs that an average over 10 readings be taken to smooth out this effect. The method requires at least modest maturity of the concrete for the instrument to register readings, typically 1-2 days depending on the concrete mixture and temperature.
A reasonable approach to using this technique for field verification of pavements would be to select one or a few small sections of pavement over which strict curing control could be maintained. Then, using the rebound numbers in these well-cured sections as a reference, the near-surface development of the remainder of the pavement could be evaluated through a random sampling scheme.
Laboratory work has shown that rebound numbers of uncured concrete exposed to modestly severe drying are reduced by about 50 percent at 7 days relative to well-cured concrete.
Surface Water Absorption (ASTM C 1151, withdrawn)(38)
It has been well established in laboratory work that the amount of water a dry concrete specimen absorbs in the first minute or so after contact with liquid water is related to the quality of the curing of the near-surface zone of the concrete. In theory, then, this method should have direct applicability to verifying curing. A number of field methods have been developed, but most suffer because of lack of control over the moisture content of the in-place concrete. The method is reasonably applied to cores, which can be dried to a constant low moisture content before testing.
The procedure is relatively simple. The top 50 mm of concrete pavement is removed by coring or sawing. The water applied during the short interval of taking the core is not significant if the core is dried in an oven (>60 ?C) within no more than a few hours after extraction. The core is so dried for 24 hours, cooled, and weighed, and then the surface of the core representing the surface of the pavement is placed on a towel saturated with water. Sixty seconds is a reasonable exposure time. The core is then reweighed and the mass of water absorbed and the surface area of the concrete are calculated. The result is expressed in units of kg/m2. If curing compound is on the surface of the core, it must be removed prior to testing. A powered wire brush is suitable for this. Sometimes a surface cut more than 50 mm from the finished surface is used as a well-cured standard. Although such a surface is probably well cured, it has probably experienced a different type of mechanical action during placing and finishing that make it not strictly comparable with the finished surface.
Well-cured concrete can serve as a control. As with the rebound hammer method, described above, select a small section of concrete over which control of curing can be assured, then take cores and use them as a reference.
Ultrasonic Pulse Velocity
The ultrasonic pulse velocity (UPV) method is an indirect measure of the modulus of elasticity of concrete. The modulus of elasticity of concrete tends to increase with increasing hydration (or quality of curing) of the cement paste fraction of the concrete. UPV testing can be set up in a number of configurations, each of which tends to focus on slightly different features of the concrete. A simple pulse velocity taken through a piece of concrete, which is the traditional way of using UPV to investigate concrete properties, gives information on the average quality of the concrete. This method would be difficult to apply to concrete pavements. UPV testing can be configured to measure the speed of wave propagation in the near-surface zone of the concrete. This configuration should be quite useful for monitoring curing.
Equipment for executing the latter type of analysis is not widely available at the commercial level, but has been mostly used in research applications. The hardware and analysis software could be developed into a practical technology if there were sufficient interest to warrant the commercial development.
Abrasion Resistance
The degree of curing has been shown in numerous research publications to be strongly reflected in the abrasion resistance of the cement-paste fraction of concrete. This truth is easily verified qualitatively using an electrically powered wire brush and observing the ease with which the near-surface mortar can be removed from a small spot of concrete. Poorly cured concrete is easily abraded away, while well-cured concrete is quite difficult to abrade away with such equipment. One major difficulty with this technology is in quantifying the forces involved and the results on the concrete. The test is also sensitive to the moisture condition of the concrete.
These shortcomings could be overcome if cores were taken and standard procedures were developed for laboratory testing, but it is doubtful that the results would be a better indicator than those derived from the other tests described above.
EFFECTIVE ONSITE ADJUSTMENTS TO CORRECT FOR EXCESSIVE DRYING
Two onsite adjustments can be useful in reducing evaporation rates of bleed water: reducing concrete placing temperatures and use of evaporation reducers.
Concrete Placing Temperatures
Of the variables affecting evaporation rate of bleed water from freshly placed concrete, concrete temperature is one of the most important and probably the only one that can be practically applied at the jobsite for large paving operations. Cooling aggregate stockpiles, cooling mixing water, or using ice for mixing water are very effective ways of reducing concrete temperatures.
The amount of cooling that can be expected from these measures, and its probable effect on evaporation rates, can be estimated from the calculations in ACI 305 R(5) and the evaporation-rate nomograph in ACI 308,(4) both of which can be programmed into a spreadsheet to simplify the calculation. The equation shown in figure 6, above, reproduces the information in the ACI 308 nomograph.(4) The ACI 305 R calculation of concrete placing temperature from ingredient temperatures is reproduced below in figure 12.(5)
Figure 12. Equation. Temperature of fresh concrete from ingredients.
where:
T = concrete placing temperature
Tca = temperature of coarse aggregate
Tfa = temperature of fine aggregate
Tc = temperature of cement
Tp = temperature of pozzolan
Tw = temperature of mixing water, excluding ice
Ti = temperature of ice
Wca = dry mass of coarse aggregate
Wfa = dry mass of fine aggregate
Wc = mass of cement
Wp = mass of pozzolan
Wi = mass of ice
Ww = mass of mixing water
Wcam = mass of free and absorbed moisture in coarse aggregate
Wfam = mass of free and absorbed moisture in fine aggregate
Evaporation Reducers
Evaporation reducers are a relatively new product developed to specifically address the condition of excessive evaporation rates. The approach is to apply evaporation reducers in sufficient quantity and frequency that the concrete does not ever lose critical amounts of water to evaporation. Application is made using the same (or similar) equipment as that used to apply curing compounds.
Evaporation reducers are water emulsions of film-forming compounds. The film-forming compound is the active ingredient that slows down evaporation of water. There is also a benefit from the water fraction of the evaporation reducers, in that it compensates to a small degree for losses of mixing water to evaporation.
Evaporation reducers may need to be applied several times, depending on the conditions. The equation in figure 13, below, yields a conservative estimate of the frequency of application for a given condition.
Figure 13. Equation. Frequency of application of evaporation reducer.
where:
F = frequency of application, h
AR = application rate, kg/m2
ER = evaporation rate of bleed water, kg/m2/h
BR = bleed rate of concrete, kg/m2/h
The constant, 0.4, is taken to be the reduction in evaporation rate affected by an
evaporation reducer. The exact value is difficult to know in the absence of test methods
and specifications, but most manufacturers claim at least a 50 percent reduction in
evaporation. Therefore, this equation is probably conservative. An application of 0.2
kilograms per square meter (kg/m2) also expressed as 5 square meters per liter (m2/L), is
a commonly recommended rate. This is also often near the maximum that can be applied
practically without runoff.
Alternative Curing Compound Practice
The relatively common practice of applying some or all of the curing compound very soon after placing will serve as an effective evaporation reducer. However, there may be problems associated with this practice, as described in chapter 4. If used, this practice should be verified as part of the laboratory verification of curing compound properties, as also described in chapter 4.
CONCRETE MATERIALS AND MIXTURE PROPORTIONS?EFFECT ON CURING
General Comment
Properties of cementitious materials (cement and pozzolan) and chemical admixtures are important to consider in anticipating problems with curing. Variation in aggregate properties is probably less important (except possibly for lightweight aggregate, which is not commonly used in paving), although there may be subtle effects. None of the properties described in this section necessarily requires specific action when values deviate from the acceptable limits, but being aware of effects may help anticipate a problem.
Cement Types
The cement properties that are most important in determining curing requirements are strength gain, time of setting, and fineness. Most paving is made with types I, II, or I/II portland cement; guidance is found in publications from the American Association of State Highway and Transportation Officials (AASHTO M 85) (6) and the American Society for Testing and Materials (ASTM C 150).(7) Type V is used where soils are high in sulfate. The strength gain rates among types I, II, and I/II tend to all converge within a given geographic area, so the user really has very little choice in this property. Blended cements specified by AASHTO 240(8) and ASTM C 595(9) have strength-gain behaviors that are essentially equivalent to the M 85/C 150 types. ASTM C 1157, which has no AASHTO equivalent, is a general specification for hydraulic cement (portland and blended cements).(10) Requirements are based on performance properties, with little or no prescriptive specifications. Strength development of the various grades is essentially equivalent to C 150 types (e.g., type O is approximately equivalent to C 150 type I in performance).
Strength Gain of Cement
The length of required curing of a concrete structure is sometimes directly tied to the strength-gain rate of the cementitious materials. In most guidance, the length of curing is either a prescribed amount of time or the time required to achieve a given strength of the concrete. The strength-gain rate of cementitious materials can affect the strength gain of concrete, but other variables are also involved, most notably the water-cement ratio. The strength-gain rate of cement also affects the amount of cement necessary in a concrete mixture to obtain a given strength in a required time interval. High cement content can result in large amounts of long-term drying shrinkage, particularly if the cement is well hydrated. Hydrated cement paste contributes strongly to drying shrinkage.
Mortar strengths of about 24 MPa at 3 days and 31 MPa at 7 days are most common for types I, II, and I/II cements. Strengths for type V cements are typically about 21 MPa at 3 days and 28 MPa at 7 days. Strengths of available cements can range from about 3.5 MPa less than these values to about 7 MPa higher, but these are less common. Within a geographical area, cement strengths among producers tend to converge on similar values. Some specifications that are based on fixed-time curing requirements cite the need for extra curing time of concrete made with type II cement. Before 1980, type II cement was usually made with a composition that gained strength at a significantly slower rate than type I cement. Typical 3-day mortar strengths were about 14 MPa. This is now rarely true except in custom-made cements, usually produced for mass concrete applications. Except when the optional heat of hydration requirement is cited, the only practical distinction between type I and type II cement has to do with sulfate resistance.
Fineness
The principal direct effect of fineness on curing has to do with its effect on bleeding and, in concretes with a very low water-cement (w/c) ratio, on development of internal desiccation due to early consumption of mixing water. Modest bleeding tends to buffer the effects of early-age drying and help prevent PSC. Since finer cements tend to hydrate faster, they also generate more heat and potentially cause temperature gradients in the concrete depending on ambient conditions and curing procedures employed (see discussion of HIPERPAVTM in chapter 4.
Blaine fineness values for portland cements tend to range between 325 and 375 square meters per kilogram (m2/kg). Values higher than 400 m2/kg may indicate a problem with development of too little bleed water when drying conditions are high, and/or internal desiccation if water-cement ratios are less than about 0.40. Pozzolans are sometimes very fine and can contribute significantly to this problem. Silica fume is particularly noted for this property, but is rarely used in slip-form paving because of workability and cost issues. Slag can also be fine enough, particularly in grade 120, to have a detectable effect on water demand. Fly ashes are typically not so fine as to be problematic, although ultrafine products that may have some noticeable effect are being introduced into the market.
Pozzolans
Class F pozzolan (AASHTO M 295,(11) ASTM C 618(12)) was the major type of pozzolan used in paving until recent years. The major effect of this class of pozzolan is that setting times are usually delayed by 1 to several hours, and strength gain can be retarded relative to concrete made without pozzolan. The major effect of delayed setting time is that the optimal time for applying final curing is also delayed, hence more time for occurrence of PSC. Slow strength gain can result in prolonged curing-time requirements, unless concrete temperatures are warm. These properties have typically limited the amount used in paving to about 20, by mass of total cementitious materials.
In the last 10 years, class C fly ash has become an abundant product in concrete construction. This class of fly ash is often popular in paving concrete because strength gain is higher than with class F pozzolan; however, setting times may be delayed by times similar to class F. Some of these materials contain chemical phases that hydrate very rapidly upon contact with water, and may tend to tie up the water in the concrete within a few minutes of mixing. This property normally causes some early stiffening.
Class N pozzolan is not commonly available, but some of the products available in the past have been very finely divided, giving good early strengths but seriously affecting water demand.
Chemical Admixtures
Water-reducing admixtures (WRAs) can have two effects on curing. One effect is that they facilitate reducing the w/c rating which impacts curing requirements as discussed below. The other effect involves the occasional case of cement-admixture interaction. Occasionally certain cements and certain WRAs interact badly, resulting in very rapid early hydration of the cement. This may result in rapid consumption of a significant amount of the free mixing water and substantially reduce or eliminate bleeding. Under certain drying conditions (described below), this will make the concrete more susceptible to plastic shrinkage cracking.
WRAs are sometimes advertised as being helpful in reducing drying shrinkage cracking. This effects stems from the fact that if the water-cementitious materials
Cont'...
(w/cm) ratios are low enough, most of the mixing water is either chemically bound or tightly bound as surface water in gel pores, and not available to evaporate and cause shrinkage. Unfortunately, when taken to extremes, this overconsumption of mixing water creates internal desiccation, which is similar in effects to atmospheric drying.
Mixture Proportions
The w/cm ratio, total cementitious materials content, and percentage of cement replaced by pozzolan are the three most important mixture-design variables affecting curing requirements, as discussed in the following paragraphs.
Water-Cementitious Materials Ratio
The amount of bleeding is highly dependent on the w/cm ratio. Small to moderate bleeding is effective in buffering excessive drying when concrete is in the plastic state and susceptible to PSC. Excessive bleeding can be detrimental because it tends to result in deposition of a layer of weak material on the surface of the concrete. The w/cm ratio of paving concrete mixtures is rarely high enough to result in this problem.
The relationship between bleed rate and w/c ratio is approximately linear. The empirically developed equation shown in figure 2 approximately relates the average rate of bleeding, in kilograms per square meter per hour (kg/m2/h), to the w/cm ratio.(13) T is pavement thickness in centimeters.
Figure 2. Equation. Bleeding rate from water-cement ratio.
Paving concretes tend to have w/cm ratio between 0.38 and 0.48. For 30-cm thick pavement, bleeding would then range from about 0.13 to 0.28 kg/m2/h. These are lower average bleeding rates than found in more general-use concretes, which range from about 0.5 to 1.5 kg/m2/h. The result is that paving concretes are more susceptible to losing excessive or unsafe amounts of bleed water to evaporation. ACI 308(4) states that drying conditions of less than 0.5 kg/m2/h represent a mild threat to most concrete. A safer upper limit for paving would be about 0.3 kg/m2/h. Recommendations on how to evaluate danger of excessive drying are described below.
Cementitious Materials Content
The cementitious materials content of paving concrete typically ranges from 325 to 385 kg/m3. It is relatively common practice to compensate for slow strength gain, particularly when using flexural strength as the design property, by adding more cement. High cementitious materials contents, particularly if cementitious materials are very finely divided, tend to contribute to a reduced amount of bleeding.
A major effect of high cementitious materials contents is long-term drying shrinkage, even if the concrete is well cured. Since hydration ties up free water and results in a volume decrease, hydrated cement paste is the major component of concrete causing drying shrinkage. Long-term drying shrinkage is almost totally dependent on the fraction of hydrated cement in the concrete.
High cement contents can also contribute significant heat of hydration, particularly if the concrete is placed several hours before the hottest part of the day. Portland cement typically reaches its most intense hydration period (and hence heating) 2-4 hours after time of initial setting. Given that time of setting is typically 2-4 hours after mixing, the period of peak heat evolution is approximately 4-8 hours after placing. Temperaturerelated problems begin after the concrete reaches peak temperature. As the concrete begins to cool, temperature stresses go from compressive to tensile (a situation where concrete is relatively weak).
In general, cement contents as low as compatible with adequate strength gain and durability are beneficial in reducing effects of drying shrinkage and thermal heating effects.
Pozzolan Contents
Retardation of early strength gain is strongly related to the amount of pozzolan replacement of portland cement, particularly if Class F pozzolan is used. Class C pozzolan tends to make a strength contribution at an earlier age than Class F pozzolan. The AASHTO Guide Specifications for Highway Construction recommends 3 days' extra curing if substantial (greater than 10) replacement amounts of pozzolan are used.(14) However, preconstruction mixture verification studies should be used to verify this effect. Calculations based on maturity concepts can help anticipate required curing time. The strength gain rate of pozzolan concretes reported to be more temperature sensitive than pure PPCs.(15) If the temperature is expected to be in the 5-15 ?C range, then some preliminary exploration of strength gain using maturity calculations may help quantify potential delays in strength gain. See chapter 5 for a discussion of the maturity method.
DETERMINATION OF BLEEDING FOR JOB CONCRETE
It is important to determine the bleeding behavior of concrete intended for use in paving because this indicates the amount of water that can be safely lost to evaporation. Plotting bleeding over time allows one to identify potentially critical intervals during the bleeding period, which occurs between placing and initial time of setting. Concrete ceases to bleed after the time of initial setting.
Bleeding of job concrete is easily measured during mixture verification testing. The basic method is described in AASHTO T 158(2) and ASTM C 232,(16) but several modifications make the data more useful for the present purposes. The standard test method stipulates using a unit-weight bucket as a test apparatus.
The procedure calls for fabricating a test specimen from job concrete using the same procedures as used in making strength cylinders (AASHTO T 23,(17) ASTM C 31(18)). Make the specimen about the same height as the thickness of the pavement. If the pavement is to be placed on a porous base, then a layer of sand in the bottom of the mold can be used to simulate this drainage potential. Control evaporative losses by keeping the container covered except when taking measurements. About every 30 minutes between fabrication and time of initial setting, tilt the cylinder slightly to one side and let the bleed water collect for about 5 minutes. Draw off the bleed water with a syringe or medicine dropper and measure, either by volume using a small graduated cylinder (5-10 milliliters (mL)) or by weighing. Making a slight depression on the downhill side of the specimen surface will facilitate collecting and drawing off the bleed water.
Calculate the average bleed rate over each time interval using the equation shown in figure 3.
Figure 3. Equation. Time-averaged bleed rate.
where:
V = the amount of bleed water (in kg)
A = the surface area of the specimen (m2)
t = time (h)
Units of bleeding are kg/m2/h for that specific thickness of pavement
Plotting the amount of bleed during each time interval gives a time profile of bleeding. Periods when bleeding is less than 0.3 kg/m2/h may be potentially critical periods. However, the level of criticality depends on drying conditions. Figure 4, using data found in volume II, (13) shows such a plot for a paving mixture.
Figure 4. Graph. Plot of bleed water formation v. time for a typical paving mixture.
For this concrete, bleeding rates are low during the first hour and again immediately before time of setting, which occurred at 5 hours. Even at the peak, the bleeding rate was less than the 0.5 kg/m2/h cited in ACI 308(4) as a limit below which caution should be exercised. Additional information on interpreting such data and accounting for drying conditions is found later in this chapter.
IMPORTANCE OF TIME OF INITIAL SETTING
Time of initial setting is an important property in paving practice because it indicates bleeding is complete and final curing procedures can be initiated. This detail is not usually part of standard guidance on the start of final curing. Application of final curing is usually directed to start when final finishing is complete and the surface sheen is gone. In conventional concreting, final finishing is typically not executed until about the time of initial setting. In slip-form paving, final finishing is usually completed within a few minutes of placing the concrete, well before the time of initial setting and the end of the bleeding period. If bleeding rates are low relative to evaporation rates, then loss of surface sheen will appear rather soon after placing, suggesting that final curing should be initiated even though bleeding is continuing.
Starting final curing before the time of initial setting can lead to several problems. With water and sheet curing, the surface can be damaged due to lack of strength. Water tends to wash out fines, and sheet materials can mar the surface. With curing compounds, continued bleeding under an applied membrane can lead either to poor membrane formation (and loss of critical mixing water) or to spalling of surface mortar. See chapter 4 for a discussion of this phenomenon.
Time of setting is measured as described in AASHTO T 197(3) and ASTM C 403,(19) and is conveniently done during mixture verification work prior to the start of construction. The time of setting is strongly affected by the concrete temperature, and therefore the field time of setting will differ from the laboratory-determined time if the two temperatures differ. This is important in field application, since in-place concrete temperatures can differ significantly from laboratory concrete temperatures, and the effect can be substantial. Laboratory values can be adjusted for actual concrete temperature using the following equation.(13)
Figure 5. Equation. Time of setting-adjustment for concrete temperature.
where:
TOS = time of setting at temperature of in-place concrete, same units as in standard test
TOSStdTemp = time of setting under standard conditions, any units
CT = temperature of in-place concrete, K
StdTemp = temperature of concrete during laboratory test, K
R = constant
The constant, R, can be determined empirically, but a value of 5,000 Kelvins (K) works well. This equation can be programmed into a spreadsheet to simplify the calculation for use in exploratory work.
ANTICIPATE PROBABLE DRYING AND THERMAL STRESS CONDITIONS ON THE JOB
It is important to be able to anticipate likely drying conditions immediately after placing to determine whether water in excess of bleed water is likely to be lost, making the concrete vulnerable to PSC.
A nomograph ACI has been found to be reasonably accurate in estimating drying conditions for inputs of wind velocity (0.5 m above the concrete surface), concrete temperature, air temperature, and relative humidity of the air above the concrete.
The range of probable drying conditions can be forecast for a given locale based on typical range of weather conditions and projected concrete temperatures. Drying rates of greater than 0.3 kg/m2/h may present a problem for paving concrete, depending on bleeding rates during the same time (see below).
The information in the nomograph can be represented by the equation shown in figure 6. This equation can be programmed into a spreadsheet to simplify the calculation. The nomograph from ACI 308 is shown in figure 7.(4)
Figure 6. Equation. Evaporation rate of bleed water-effect of environmental conditions.
where:
ER = evaporation rate (kg/m2/h)
WS = the wind speed (m/s)
CT = concrete temperature (?C)
AT = air temperature (?C)
RH = relative humidity (%)
Figure 7. Chart. Evaporation rate nomograph from ACI 308.(4)
It is very instructive to explore the effects of ranges of environmental conditions expected in a given construction location on the evaporation of the bleed water. This information, along with bleeding data and time of setting, allow the engineer to anticipate critical conditions. Wind and concrete temperature are usually found to be the most critical variables. The temperature of freshly placed concrete is a property over which a producer has some control by adjusting the temperature of concrete-making materials. ACI 305 R contains equations that relate the temperature of materials to temperature of concrete.(5)
Anticipating thermal stress conditions on the job can be complicated because of the many variables involved. The Federal Highway Administration (FHWA) has developed a software program called HIPERPAVTM that allows the user to enter plausible data on concrete and site conditions and get thermal-stress output back, in the form of warnings on critical times after placing when cracks may develop (see chapter 4). This program also includes an evaporation-rate calculator similar to the results obtained from the equation shown in figure 6.
ANALYSIS OF MULTIPLE FACTORS AFFECTING EARLY MOISTURE-LOSS MANAGEMENT
Comparing bleeding behavior with probable drying conditions will identify potential critical points during construction. The time of initial setting indicates the end of this critical period. Figure 8 shows the cumulative bleed rate calculated from the data shown in figure 4 plotted along with the cumulative evaporation rate, assuming a constant evaporation rate of 0.30 kg/m2/h.
In this example, evaporation rates exceed bleeding rates for the first hour after placing and again after about 3.5 hours. The time of setting is about 5.2 hours. These two periods represent critical periods from a PSC perspective. Sometimes concrete will endure the first critical period because the mixture is plastic enough to adjust to evaporative losses by simply shrinking into a thinner placement. However, the period after about 3.5 hours may result in cracking because the concrete may have developed some stiffness at this point, and cannot adjust to the loss of water by simply reducing volume.
Figure 8. Graph. Plot of cumulative bleed and cumulative evaporation v. time.
PLANNING FOR POTENTIAL CORRECTIVE ACTION
Standard guidance recommends that when evaporation exceeds bleeding, something must be done to reduce evaporation rates. Standard remedies include use of fogging and wind breaks. Neither of these methods is particularly useful for large paving projects. Three practices are potentially useful in paving large areas. One is to shift paving operations to a time of day when the drying conditions are less severe. Nighttime placement is often attractive because relative humidity is usually higher than during the day.
Another effective option is to reduce the temperature of the concrete at placing. This is a very strong variable affecting evaporation rates. ACI 305 R gives guidance on calculating placing temperature (also discussed in chapter 3). This calculation can be used to explore the amount of benefit expected from cooling concrete ingredients. For example, figure 9 shows the effect of reducing the concrete temperature by 5 ?C (using data from the nomograph in figure 3) on evaporation rates. Evaporation rates become less critical as a result of this adjustment.
Figure 9. Graph. Effect of reducing concrete placing temperature from 30 ?C to 25 ?C.
Still another approach is to use evaporation reducers. Evaporation reducers can reduce evaporation rates by as much as 65 percent. (13) Figure 10 shows the effect of a 50 percent reduction in evaporation, using the data shown in figure 4. The cumulative effect is similar to the reducing concrete placing temperature by 5 ?C. Currently, there are no test methods or specifications for evaporation reducers, and the user must rely on the manufacturer's guidance.
Figure 10. Graph. Effect of reducing evaporation by 50 percent by using an evaporation reducer.
A limited laboratory investigation demonstrated that evaporation reducers can reduce evaporation rates by an amount ranging from 0 to 65 percent. Evaporation reducers must be reapplied if the time of setting is extended and drying rates are high. The approximate application interval is discussed in chapter 3.
In conclusion, reducing concrete placing temperature and use of evaporation reducers can have a relatively strong influence on potential for plastic shrinkage cracking.
I hope your happy now dipshit
Dont bother cause your on ignore:nono:
(w/cm) ratios are low enough, most of the mixing water is either chemically bound or tightly bound as surface water in gel pores, and not available to evaporate and cause shrinkage. Unfortunately, when taken to extremes, this overconsumption of mixing water creates internal desiccation, which is similar in effects to atmospheric drying.
Mixture Proportions
The w/cm ratio, total cementitious materials content, and percentage of cement replaced by pozzolan are the three most important mixture-design variables affecting curing requirements, as discussed in the following paragraphs.
Water-Cementitious Materials Ratio
The amount of bleeding is highly dependent on the w/cm ratio. Small to moderate bleeding is effective in buffering excessive drying when concrete is in the plastic state and susceptible to PSC. Excessive bleeding can be detrimental because it tends to result in deposition of a layer of weak material on the surface of the concrete. The w/cm ratio of paving concrete mixtures is rarely high enough to result in this problem.
The relationship between bleed rate and w/c ratio is approximately linear. The empirically developed equation shown in figure 2 approximately relates the average rate of bleeding, in kilograms per square meter per hour (kg/m2/h), to the w/cm ratio.(13) T is pavement thickness in centimeters.
Figure 2. Equation. Bleeding rate from water-cement ratio.
Paving concretes tend to have w/cm ratio between 0.38 and 0.48. For 30-cm thick pavement, bleeding would then range from about 0.13 to 0.28 kg/m2/h. These are lower average bleeding rates than found in more general-use concretes, which range from about 0.5 to 1.5 kg/m2/h. The result is that paving concretes are more susceptible to losing excessive or unsafe amounts of bleed water to evaporation. ACI 308(4) states that drying conditions of less than 0.5 kg/m2/h represent a mild threat to most concrete. A safer upper limit for paving would be about 0.3 kg/m2/h. Recommendations on how to evaluate danger of excessive drying are described below.
Cementitious Materials Content
The cementitious materials content of paving concrete typically ranges from 325 to 385 kg/m3. It is relatively common practice to compensate for slow strength gain, particularly when using flexural strength as the design property, by adding more cement. High cementitious materials contents, particularly if cementitious materials are very finely divided, tend to contribute to a reduced amount of bleeding.
A major effect of high cementitious materials contents is long-term drying shrinkage, even if the concrete is well cured. Since hydration ties up free water and results in a volume decrease, hydrated cement paste is the major component of concrete causing drying shrinkage. Long-term drying shrinkage is almost totally dependent on the fraction of hydrated cement in the concrete.
High cement contents can also contribute significant heat of hydration, particularly if the concrete is placed several hours before the hottest part of the day. Portland cement typically reaches its most intense hydration period (and hence heating) 2-4 hours after time of initial setting. Given that time of setting is typically 2-4 hours after mixing, the period of peak heat evolution is approximately 4-8 hours after placing. Temperaturerelated problems begin after the concrete reaches peak temperature. As the concrete begins to cool, temperature stresses go from compressive to tensile (a situation where concrete is relatively weak).
In general, cement contents as low as compatible with adequate strength gain and durability are beneficial in reducing effects of drying shrinkage and thermal heating effects.
Pozzolan Contents
Retardation of early strength gain is strongly related to the amount of pozzolan replacement of portland cement, particularly if Class F pozzolan is used. Class C pozzolan tends to make a strength contribution at an earlier age than Class F pozzolan. The AASHTO Guide Specifications for Highway Construction recommends 3 days' extra curing if substantial (greater than 10) replacement amounts of pozzolan are used.(14) However, preconstruction mixture verification studies should be used to verify this effect. Calculations based on maturity concepts can help anticipate required curing time. The strength gain rate of pozzolan concretes reported to be more temperature sensitive than pure PPCs.(15) If the temperature is expected to be in the 5-15 ?C range, then some preliminary exploration of strength gain using maturity calculations may help quantify potential delays in strength gain. See chapter 5 for a discussion of the maturity method.
DETERMINATION OF BLEEDING FOR JOB CONCRETE
It is important to determine the bleeding behavior of concrete intended for use in paving because this indicates the amount of water that can be safely lost to evaporation. Plotting bleeding over time allows one to identify potentially critical intervals during the bleeding period, which occurs between placing and initial time of setting. Concrete ceases to bleed after the time of initial setting.
Bleeding of job concrete is easily measured during mixture verification testing. The basic method is described in AASHTO T 158(2) and ASTM C 232,(16) but several modifications make the data more useful for the present purposes. The standard test method stipulates using a unit-weight bucket as a test apparatus.
The procedure calls for fabricating a test specimen from job concrete using the same procedures as used in making strength cylinders (AASHTO T 23,(17) ASTM C 31(18)). Make the specimen about the same height as the thickness of the pavement. If the pavement is to be placed on a porous base, then a layer of sand in the bottom of the mold can be used to simulate this drainage potential. Control evaporative losses by keeping the container covered except when taking measurements. About every 30 minutes between fabrication and time of initial setting, tilt the cylinder slightly to one side and let the bleed water collect for about 5 minutes. Draw off the bleed water with a syringe or medicine dropper and measure, either by volume using a small graduated cylinder (5-10 milliliters (mL)) or by weighing. Making a slight depression on the downhill side of the specimen surface will facilitate collecting and drawing off the bleed water.
Calculate the average bleed rate over each time interval using the equation shown in figure 3.
Figure 3. Equation. Time-averaged bleed rate.
where:
V = the amount of bleed water (in kg)
A = the surface area of the specimen (m2)
t = time (h)
Units of bleeding are kg/m2/h for that specific thickness of pavement
Plotting the amount of bleed during each time interval gives a time profile of bleeding. Periods when bleeding is less than 0.3 kg/m2/h may be potentially critical periods. However, the level of criticality depends on drying conditions. Figure 4, using data found in volume II, (13) shows such a plot for a paving mixture.
Figure 4. Graph. Plot of bleed water formation v. time for a typical paving mixture.
For this concrete, bleeding rates are low during the first hour and again immediately before time of setting, which occurred at 5 hours. Even at the peak, the bleeding rate was less than the 0.5 kg/m2/h cited in ACI 308(4) as a limit below which caution should be exercised. Additional information on interpreting such data and accounting for drying conditions is found later in this chapter.
IMPORTANCE OF TIME OF INITIAL SETTING
Time of initial setting is an important property in paving practice because it indicates bleeding is complete and final curing procedures can be initiated. This detail is not usually part of standard guidance on the start of final curing. Application of final curing is usually directed to start when final finishing is complete and the surface sheen is gone. In conventional concreting, final finishing is typically not executed until about the time of initial setting. In slip-form paving, final finishing is usually completed within a few minutes of placing the concrete, well before the time of initial setting and the end of the bleeding period. If bleeding rates are low relative to evaporation rates, then loss of surface sheen will appear rather soon after placing, suggesting that final curing should be initiated even though bleeding is continuing.
Starting final curing before the time of initial setting can lead to several problems. With water and sheet curing, the surface can be damaged due to lack of strength. Water tends to wash out fines, and sheet materials can mar the surface. With curing compounds, continued bleeding under an applied membrane can lead either to poor membrane formation (and loss of critical mixing water) or to spalling of surface mortar. See chapter 4 for a discussion of this phenomenon.
Time of setting is measured as described in AASHTO T 197(3) and ASTM C 403,(19) and is conveniently done during mixture verification work prior to the start of construction. The time of setting is strongly affected by the concrete temperature, and therefore the field time of setting will differ from the laboratory-determined time if the two temperatures differ. This is important in field application, since in-place concrete temperatures can differ significantly from laboratory concrete temperatures, and the effect can be substantial. Laboratory values can be adjusted for actual concrete temperature using the following equation.(13)
Figure 5. Equation. Time of setting-adjustment for concrete temperature.
where:
TOS = time of setting at temperature of in-place concrete, same units as in standard test
TOSStdTemp = time of setting under standard conditions, any units
CT = temperature of in-place concrete, K
StdTemp = temperature of concrete during laboratory test, K
R = constant
The constant, R, can be determined empirically, but a value of 5,000 Kelvins (K) works well. This equation can be programmed into a spreadsheet to simplify the calculation for use in exploratory work.
ANTICIPATE PROBABLE DRYING AND THERMAL STRESS CONDITIONS ON THE JOB
It is important to be able to anticipate likely drying conditions immediately after placing to determine whether water in excess of bleed water is likely to be lost, making the concrete vulnerable to PSC.
A nomograph ACI has been found to be reasonably accurate in estimating drying conditions for inputs of wind velocity (0.5 m above the concrete surface), concrete temperature, air temperature, and relative humidity of the air above the concrete.
The range of probable drying conditions can be forecast for a given locale based on typical range of weather conditions and projected concrete temperatures. Drying rates of greater than 0.3 kg/m2/h may present a problem for paving concrete, depending on bleeding rates during the same time (see below).
The information in the nomograph can be represented by the equation shown in figure 6. This equation can be programmed into a spreadsheet to simplify the calculation. The nomograph from ACI 308 is shown in figure 7.(4)
Figure 6. Equation. Evaporation rate of bleed water-effect of environmental conditions.
where:
ER = evaporation rate (kg/m2/h)
WS = the wind speed (m/s)
CT = concrete temperature (?C)
AT = air temperature (?C)
RH = relative humidity (%)
Figure 7. Chart. Evaporation rate nomograph from ACI 308.(4)
It is very instructive to explore the effects of ranges of environmental conditions expected in a given construction location on the evaporation of the bleed water. This information, along with bleeding data and time of setting, allow the engineer to anticipate critical conditions. Wind and concrete temperature are usually found to be the most critical variables. The temperature of freshly placed concrete is a property over which a producer has some control by adjusting the temperature of concrete-making materials. ACI 305 R contains equations that relate the temperature of materials to temperature of concrete.(5)
Anticipating thermal stress conditions on the job can be complicated because of the many variables involved. The Federal Highway Administration (FHWA) has developed a software program called HIPERPAVTM that allows the user to enter plausible data on concrete and site conditions and get thermal-stress output back, in the form of warnings on critical times after placing when cracks may develop (see chapter 4). This program also includes an evaporation-rate calculator similar to the results obtained from the equation shown in figure 6.
ANALYSIS OF MULTIPLE FACTORS AFFECTING EARLY MOISTURE-LOSS MANAGEMENT
Comparing bleeding behavior with probable drying conditions will identify potential critical points during construction. The time of initial setting indicates the end of this critical period. Figure 8 shows the cumulative bleed rate calculated from the data shown in figure 4 plotted along with the cumulative evaporation rate, assuming a constant evaporation rate of 0.30 kg/m2/h.
In this example, evaporation rates exceed bleeding rates for the first hour after placing and again after about 3.5 hours. The time of setting is about 5.2 hours. These two periods represent critical periods from a PSC perspective. Sometimes concrete will endure the first critical period because the mixture is plastic enough to adjust to evaporative losses by simply shrinking into a thinner placement. However, the period after about 3.5 hours may result in cracking because the concrete may have developed some stiffness at this point, and cannot adjust to the loss of water by simply reducing volume.
Figure 8. Graph. Plot of cumulative bleed and cumulative evaporation v. time.
PLANNING FOR POTENTIAL CORRECTIVE ACTION
Standard guidance recommends that when evaporation exceeds bleeding, something must be done to reduce evaporation rates. Standard remedies include use of fogging and wind breaks. Neither of these methods is particularly useful for large paving projects. Three practices are potentially useful in paving large areas. One is to shift paving operations to a time of day when the drying conditions are less severe. Nighttime placement is often attractive because relative humidity is usually higher than during the day.
Another effective option is to reduce the temperature of the concrete at placing. This is a very strong variable affecting evaporation rates. ACI 305 R gives guidance on calculating placing temperature (also discussed in chapter 3). This calculation can be used to explore the amount of benefit expected from cooling concrete ingredients. For example, figure 9 shows the effect of reducing the concrete temperature by 5 ?C (using data from the nomograph in figure 3) on evaporation rates. Evaporation rates become less critical as a result of this adjustment.
Figure 9. Graph. Effect of reducing concrete placing temperature from 30 ?C to 25 ?C.
Still another approach is to use evaporation reducers. Evaporation reducers can reduce evaporation rates by as much as 65 percent. (13) Figure 10 shows the effect of a 50 percent reduction in evaporation, using the data shown in figure 4. The cumulative effect is similar to the reducing concrete placing temperature by 5 ?C. Currently, there are no test methods or specifications for evaporation reducers, and the user must rely on the manufacturer's guidance.
Figure 10. Graph. Effect of reducing evaporation by 50 percent by using an evaporation reducer.
A limited laboratory investigation demonstrated that evaporation reducers can reduce evaporation rates by an amount ranging from 0 to 65 percent. Evaporation reducers must be reapplied if the time of setting is extended and drying rates are high. The approximate application interval is discussed in chapter 3.
In conclusion, reducing concrete placing temperature and use of evaporation reducers can have a relatively strong influence on potential for plastic shrinkage cracking.
I hope your happy now dipshit
Dont bother cause your on ignore:nono:
Thanks guys--I will water it a couple of times a day and now have decided to wait 10 days to drive on it. Gobble Gobble!!
