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Planning and Execution of a Mass Concrete Placementutilizing Insulation Regimen

Ufuk Dilek Ph.D. , P.E.Concrete Subject Matter Expert

The Shaw Group128 S. Tryon Street Suite 600Charlotte, NC 28202

Tel: 1 (704) 343 46 26E-Mail: ufuk.dilek@shawgrp.com

Abstract

This paper summarizes the planning and execution stages of a critical mass concrete placement performed during summer months. The subject structure was a criticalcomponent of a large heavy industrial facility, consisting of large load bearing elevatedflexural members. The planning and execution of this critical mass placement consistedof multiple tasks.A laboratory study was performed for the purpose of making improvements to themixture proportions existing and currently in use, admixture dosages and investigating

placement temperature options. Adiabatic and semi adiabatic temperature rise was alsomeasured during the laboratory study along with set times. Final proportions andadmixture dosages were selected as a result of the laboratory phase. Primary outcomewas increase in fly ash percentage from the existing mix design to control heat ofhydration.Based on the findings of the measured adiabatic temperature rise, a thermal control planwas developed adapting the new approach to structural mass concrete placements. Athermal protection/insulation regimen was developed using the mix parameters,expected ambient temperatures following placement, member dimensions andformwork/blanket insulation properties. The pre-placement modifications to themixture proportions and the delivery temperature requirements protected the concreteagainst high internal temperatures and potential of Delayed Ettringite Formation (DEF),while the insulation regimen protected the concrete against rapid cooling andoccurrence of thermal gradients between core and perimeter.As part of the thermal control plan analysis, target placement temperatures wererecommended to control maximum temperatures to prevent occurrence of DEF, in lightof the heat rise of the modified mix. The placement temperature was accomplished bystarting the placement at night and the use of ice to draw the temperature down. Uponcompletion of finishing, a curing compound was applied in lieu of water curing and the

placement was insulated.

The thermal control plan simulation predicted a gradual reduction in the temperature ofthe placement, within limits of maximum internal temperatures and temperaturegradients. The actual placement was monitored for core and perimeter temperaturesusing maturity probes. Monitoring enabled the team to react to abrupt changes intemperature if any was to occur. The placement was completed successfully withinternal temperatures and gradients controlled within the desired ranges.

Keywords : mass concrete, adiabatic heat rise, thermal control plan, fly ash, thermalcracking, heat or hydration

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Introduction

This paper summarizes the planning and execution stages of a critical mass concrete placement performed during summer months. The subject structure was a criticalcomponent of a large heavy industrial facility, consisting of large load bearing elevatedflexural members, with minimum dimensions exceeding 7 ft [Figure 1]. The planningand execution of this critical mass placement consisted of multiple tasks.

Figure 1 Overview of beams showing reinforcement congestion.

Planning and execution consist of the following stages:

1. Laboratory Study2. Development of the Thermal Control Plan3. Post-Placement Thermal Monitoring4. Curing, Insulation and Protection against Temperature Changes

Laboratory Study

Subject mass concrete placement was scheduled to occur during summer months. Theexisting concrete mixtures used on site were not utilized to date in a mass concrete

placement of this significance and under the summer ambient conditions. Therefore theconcrete mixture planned for use was reviewed in light of the placement specifics. Alaboratory study was performed to revise the concrete mixture to achieve desirable

performance objectives in the mass concrete placement under summer ambientconditions.

Existing concrete mixture utilized class F ASTM C 618 (ASTM, 2008) fly ash at a 25%replacement rate. Existing concrete mixtures typically used type A (lignin based) waterreducers meeting ASTM C 494 (ASTM, 2010). Lignin based water reducer also offered

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set control benefits in addition to reduction of water. In addition, as warranted byreinforcement congestion and consolidation challenges at the bottom of sizeablereinforced concrete members a type F superplasticizer meeting ASTM C 494 (ASTM,2010) was utilized to achieve a maximum slump of 8 inches. Compressive strength ofthe concrete mixture ( fc) was 4000 psi (27 Mpa) and specification required water-to-cementitious ratio maximum was 0.45.

Variables Used during Laboratory Study During the laboratory study, concrete mixtures with 25% and 35% fly ash replacementrates were tested. In addition, the dosages of type A and type F water reducers weremodified in coordination to yield the same slump value, while increasing the relativedosage of type A in the mixture and achieving an increase in the time of set.

Laboratory study also included concrete mixtures batched at varying temperatures.Temperature of water and aggregates was modified to achieve resulting batchtemperatures ranging between 21 and 32 C (70 and 90 F) enabling evaluation of thechanges with temperature, in time of set and retention or loss of slump. A total of 10concrete trial batches were performed.

The laboratory study aimed to identify the combination of factors such as concrete placement temperature, admixture dosages, fly ash replacement rate prior to the placement. In addition to time of set and slump loss, adiabatic and semi-adiabatic heatrise of concrete mixtures was measured and the information gathered was used in the in-depth planning of the mass concrete placement.

Figure 2 shows semi adiabatic heat rise measurements performed using Adiacal TMapparatus by WR Grace. This device mainly contains multiple insulated cylinderhousing compartments for 4 x 8 in (10 cm x 20 cm) cylindirical specimens. Thehousing compartments are equipped with themocouples feeding temperature data to thedata acquisition hardware and associated software. The measurements are not fullyadiabatic as is evident from the eventual decline in temperature. However, a time of setdetermination made based on the shape of the curve provides an estimate of the time ofset, which can be compared or indexed to times of set determined per ASTM C 403(ASTM 2008). For this study, the subject device was not used for an absolutedetermination of time of set but for comparisons between various trial batches withdifferent attributes. In general, the comparisons between mixtures were made on the

basis of maximum magnitude of temperature achieved, the occurence time of this peak temperature, and occurence of the flat portions of the curve with respect to admixture

dosage and retardation effects.

These attributes can be observed in Figure 2.

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Figure 2 Thermal measurements using Adiacal.

Selected Mixture Proportions

Based on the findings of the study, the mixture proportions nominated for placementwere determined. These proportions are provided in Table 1 below. It was decided toutilize an increase fly ash replacement rate of %35. As a consequence of the increasedfly ash usage, an ( fc) of 4000 psi (27 Mpa) determined at 56 days was utilized in lieuof the typical 28 day mixture design cycle. This increase in length of the design cycleaimed to reduce the earlier heat of hydration which would be more pronounced if thedesign cycle was kept at 28 days and a more agressive water-to-cementitious ratio wasused for the 28 day design.

Based on the behavior of mixtures and Adiacal data, a likely upper limit for placementtemperature was initially considered to be 80 F (27 C). Under standard operatingmeasures, the supplying concrete plant was producing concrete just under 90 F (32 C)during the heat of the summer season. If this temperature limit of 80 F was to beconfirmed by the thermal control plan development, it would likely be necessary to usemanually loaded ice as a replacement to batch water and that the placement be

performed at nighttime.

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Table 1 Selected Mixture Proportions.

Code 1435 1435Strength 4000 psi 27.6 Mpa

lb/CY kg/m3Cement Type I/II 412 lb 245

Fly Ash 222 lb 132Fine Aggregate 1112 lb 660Coarse Agg. (67) 1852 lb 1100Water 285 lb 169W/(C+F) 0.45 0.45Air (%) 3.5 - 6.5 3.5 - 6.5Slump w/HRWR 5 to 8 in 12.5-20 cm

Subsequently, full adiabatic thermal measurements were performed on the final selected proportion. Figure 3 shows this measurement on the mixture containing 35% ash and amix containing 25% ash for comparison, demonstrating the difference made withincreasing the fly ash. An increase of 10% in fly ash results in a reduction of 3 C (5F).

Figure 3 Adiabatic measurements, 35 % fly ash ( left), 25% fly ash (right).

DEVELOPMENT OF THE THERMAL CONTROL PLAN

ACI 207 Mass Concrete Committee ReportIn general, ACI 207 (ACI, 2005) committee report (Guide to Mass Concrete)concentrates on unreinforced, large mass structures, such as dams with typically lowerstrength requirements than reinforced structural concrete. The guidelines provided formixture attributes (low heat, use of pozzolan, reduction of cement content etc) areappropriate. The recommended curing method is curing by water therefore aimingrapid removal of heat from the mass concrete structure.

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ACI 207 committee is currently working on a committee report entitled Guide toReinforced Concrete Structures that is geared towards reinforced mass concretestructures that are not sizeable as dams and possibly of a higher strength class. Thisreports approach is generally inline with the mass concrete practice in ACI 301-10(ACI, 2010). The fundamental difference in the new approach is in the curing regime.

The new approach states water curing cools the concrete surface rapidly andconcentrates on prevention of thermal cracking that can occur due to thermal differences

between a cooler perimeter and a hot core.

In general the new approach promotes use of pozzolans, reduction of cement content forcontrol of heat of hydration, as well as controlling delivered concrete temperature byway of shading or evaporative cooling of aggregates and use of ice or liquid nitrogen, ifwarranted. The internal temperature of concrete is not recommended to exceed 71 C(160 F) to eliminate the potential for the occurence of a phenomenon called delayedettringite formation (DEF).

In addition to the internal temperature limit, the second criteria is a temperaturedifferential between core and perimeter. This value is typically 19 C (35 F) althoughcan be revised based on aggregate mineralogy and coefficient of thermal expansion(Bamforth) The recommended method of curing to prevent rapid drop of temperaturesinvolves insulation of the concrete surface and use of moisture retentive barriers orcuring compounds as a method of curing. The recommended method of curing withinsulation is counter intuitive for mass concrete placements that generate heat, however,it is intended to control temperature drop of the surface, unlike the water curing method.

Development of the Thermal Control Plan

A commercially available system, Quadrel TM was used in development of the thermalcontrol plan recommendations (CES, 2010). This system incorporates the adiabaticthermal measurement for the concrete mixture, concrete delivery temperature, memberdimensions, expected ambient temperature ranges during and following placement, andinsulating values of forms and blankets as input. The associated software aims todevelop a simulation of thermal profiles for a given set of parameters in an effort todevelop thermal control plan specifics such as duration of protection, extent ofinsulation etc. The simulation allows sensitivity analysis by modifying input parametersand observing the effects on the outcome, in an effort to identify a combination of

parameters that are achievable and provide the desired outcome.

The thermal control plan aimed to protect the concrete against thermal cracking by aninsulation regimen, over the prescribed protection period, under the given ambient

conditions, while satisfying the two objectives of core temperature and temperaturedifferential.

Upon completion of the thermal control plan the upper limit for placement temperaturewas determined to be 80 F (27 C). Under standard operating measures, the supplyingconcrete plant was producing concrete just under 90 F (32 C) during the heat of thesummer season. It was therefore decided to use manually loaded ice as a replacement to

batch water and that the placement be performed at nighttime.

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In addition to the 27 C (80 F) placement temperature limit, an ambient dailytemperature range of 16 38 C (60 -100 F), typical wind speeds, and insulating valuesof forms and blankets (R value 5 F-ft2-h/Btu) were input into the simulation. Uponcompletion of the simulation, as shown in Table 2, under daily temperature range of 16

38 C (60 -100 F), concrete placed at 27 C (80 F) was predicted to reach amaximum internal temperature of 67 C (152 F). (CES, 2010)

The insulation was required to be kept on the structure for a minimum of 7 days tomaintain temperature gradients within the established differential limit. This period wasconcident with the specification required curing period which could occur concurrentlywith insulated protection.

Table 2. Maximum internal temperatures and required insulation period for 16 38C (60 -100 F) daily temperature range.

Figure 4 shows results of one of the simulation runs. Based on the results of theanalysis, insulation can be removed and the protection can be terminated after 7 days,when the temperature of the surface and core have reduced relatively and somewhatconverged. The analysis suggests upon removal of the insulation, the temperature of thesurface would start to demonstrate more pronounced fluctuations in response to theambient temperature fluctuations, however this would not constitute a violation of thetemperature differential limit since the core values have dropped as well. If the concretewas not insulated during the early age period, the exterior temperatures would rapidlydrop and approach 16 38 C (60 -100 F) range. The rapid reduction of the externaltemperatures would constitute a large temperature gradient relative to the hot core at thetime and impose thermal strains on the concrete that is of relatively young age and oflower strength.

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Figure 4 Simulation run for 16 38 C (60 -100 F) ambient temperature range and

7 day protection period and 80 F placement temperature

Temperature Monitoring Locations

In order to monitor temperatures following placement, the two largest beams wereequipped with temperature sensors, typically used for maturity metering per ASTM1074 (ASTM, 2010). Sensors were placed at perimeter and core locations (Figure 5),(CES, 2010)..

Figure 5 Monitoring Locations

.

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EXECUTION OF CONCRETE PLACEMENT

Concrete Placement

The thermal control plan and the associated analysis required a placement temperatureno higher than 27 C (80 F) and that the placement be performed at night with use ofice to achieve this placement temperature. Figure 6 shows the placement during nighttime.

Figure 6 Nighttime Placement.

Due to the hot summer ambient conditions, the aggregate stockpiles were sprayedduring the day to benefit from evaporative cooling. Up to 75 % of the batch water wasreplaced with ice during batching. Concrete temperatures were monitored both duringthe record testing as well as in-situ by the field crew using manual infraredthermometers. Figure 7 shows concrete in-situ temperature after running through the

pump, measured at 25 C (77 F).

Figure 7 Concrete temperature in the form during nighttime placement.

After the concrete placement was complete, a curing compound was applied and theconcrete finished surfaces as well as the vertical surfaces of the side forms wereinsulated for a period of 7 days per the requirements of the thermal control plan.

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Monitoring

Sensors were placed at the pre-determined locations, fixed to the reinforcing steel cage,or supported on dedicated supports and activated prior to the placement. Sensor cableswere extended to a central location for monitoring and a communication box fed thedata to the dedicated computer at the job-site office over the cell phone network.

Figure 8 Maturity sensors and centralized monitoring junction box.

The measured data was monitored and reviewed periodically. This real-time monitoring provided site crew the ability to react and modify the extent of insulation if temperatureswere observed to exceed the prescribed limits.

The temperature data indicated that maximum temperatures did not exceed 71 C (160F) which is the limit for prevention of occurence of DEF. (Figure 9). The measureddata was in general agreement with the simulated temperature values. It was noted thatthe measured perimeter temperatures exhibited fluctuations following ambienttemperatures, as forecasted by the simulation. The measured data also indicated that thetemperature differential between core and perimeter did not exceed (25 F) and wasgenerally controlled within the prescribed limit.

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Figure 9 Measured temperatures.

Results

The measures developed as a result of the thermal control plan and the associatedanalysis enabled a controlled reduction of the concrete temperature. If insulation regimewas not adapted, temperature differentials between core and exterior could haveexceeded the prescribed limit. In general maximum internal temperature and thetemperature differential was kept within the prescribed limits. Temperature levels weremonitored during the protection period and following the protection period usingmaturity sensors. Real-time monitoring provided the capability to react and modify theextent of insulation if temperatures were observed to exceed the prescribed limits. The

placement and protection period was completed succesfully with no apparent thermalcracking and without exceeding the limits on temperature magnitude and differential.

The planning leading up to the placement contained several stages. During thelaboratory phase, the heat of hydration was reduced based on the modifications made tothe concrete mixture. Subsequently, measures such as evaporative cooling of theaggregates, use of ice, nightime placement enabled reduction of the placementtemperature. These measures were effective in preventing the concrete from reachingdetrimental temperature levels which could lead to formation of DEF. The thermalsimulation utilizing, adiabatic heat rise, member sizes, insulation properties, ambientconditions as input provided estimates of the expected core temperatures andtemperature gradients. While the core temperatures were kept under control withmixture modifications and delivery temperature reduction, the temperature gradientswere controlled with the insulation regimen following placement.

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References

American Concrete Institute (2005), ACI 207.1R-05 Guide to Mass Concrete,Michigan

American Concrete Institute (2010), ACI 301-10 Specifications for Structural Concrete,Michigan

American Society of Testing and Materials (2010), ASTM C1074 - 10a StandardPractice for Estimating Concrete Strength by the Maturity Method, Pennsylvania

American Society of Testing and Materials (2008), ASTM C618 - 08a StandardSpecification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use inConcrete, Pennsylvania

American Society of Testing and Materials, (2010), ASTM C494 / C494M - 10aStandard Specification for Chemical Admixtures for Concrete, Pennsylvania

American Society of Testing and Materials (2008), ASTM C403 / C403M - 08 StandardTest Method for Time of Setting of Concrete Mixtures by Penetration Resistance,Pennsylvania

Bamforth, P.B., Mass Concrete , Concrete Society Digest No 2

Concrete Engineering Specialists (CES) Report to Shaw Group, 2010