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Mike Murray Concrete Solutions Inc cold weather mark mead
- 1. Mike Murray Concrete Solutions Inc. Independent Contractor / Concrete/CSI This is outline of procedures and guidelines prepared to aid in establishing a business and construction plan for COLD weather floor construction. For Contractors, Ready mix concrete suppliers & General Contractors 1. Protection and curing 2. Finishing and formwork 3. Subgradeand reinforcementfor cold weather 4. Mix designs and admixtures 5. Troweling slabs in cold weather 6. What happens when concrete freezes 7. Suggested temperatures for placement and post placement . This outline and guide will cover procedures for ACI 306, Cold weather, but will also cover formwork screeding and protection of slabs after placement. We will provide, in this guide an emphasis on cold weather placement. For suspended and slab on ground concrete slabs. The purpose of this guide is to provide needed information and procedures, when mixes must be installed in cold weather. Many projects cannot simply wait nor afford for structure to be enclosed, or the weather to warm. We hope this information will provide needed parameters whereby concrete can be successfully placed, cured and protected in cold weather. Cold Weather Concrete Placement of Finished Concrete slabs with Troweled finishes
- 2. The following is a guide to aid contractors, ready mixsuppliers and owners, with information and guidelines for pouring, protection and curing methods for hard troweled surfaces in cold weather. Monitoring for many years, overallcosts of cold weather protection are considered not excessivewhen measured against waiting time, and the benefits of having the concrete, poured and placed into service. Concrete mixture proportions in weather conditions are influenced by four factors Strength Durability Workability Economy Durability = the ability of the concrete to resistthe environmentin which it will be placed/ this is important to the owner. Strength = the ability of the concrete to carry imposed loads/ this is important to engineer. Workability= this is the ability to be properly placed consolidated and finished/ this is important to the concrete contractor. Economy = this is total costs to place the concrete adverseweather may increase costs/this is important to owners and general contractors. Strength, durability workability and economy may many times be in conflict with each other. So the question is always contemplated should corners becut and by whom? A. Of coursethe correctanswer is always no. Even more so on suspended slab wherea failure could put lives at risk! The following guide offers proven methods and outlines what constitutes a successfulprojectin cold weather.
- 3. Cold weather concrete procedures Each year the question arises, it has warmed up outside, so I should be able to place the concrete justas I would in July, so its above 50 and my subs are waiting to go to work. ????Is A.Itremains the same each year you cant! In July, the workers and concrete most likely will not need a jacket to stay warm. The same cannot be said for workers and concretein cold weather. Concrete by justits nature, requires water in order for hydration to begin. It is very sensitiveto temperature changes and may be permanently damaged when temperatures fall below freezing and stay. When temperatures move back and forth this also presents May issues. Suspended slabs create other complications. Precautions should be taken to keep the concrete warmbelow the deck. Compare this to bridgedecks when temperature hover around 27/f ice forms on the surfaceabove. A warmjacket on a body feels much better on ones skin. The jacket keeps the main portion of the body warmwhen protecting concrete. How are edges and concrete exposed to steel protected? If your toes, feet, hands, fingers and head become cold it doesnttake long for that warmjacket to not provideas much protection. Think protection when placing concrete. How wecure and maintain temperature will be critical to the longevity and durability of the finished concrete structure. Withoutsome type of protection concrete can be damaged by frostwe can compareconcrete to a golf green and justwait for it to warmup. Or with concrete we can institute procedures, to both protect and warmthe temperature. Concrete achieves its hardened state by hydration not drying. Should temperature fall and stay below 29 f permanent damage may occur. ACI DEFINES COLD WEATHER 1. Cold weather conditions exist when the air temperature has fallen or expected to fall below 40during the protection cureperiod. The protection period is defined as the length of time the concrete is required to be protected while it is affected by cold weather.
- 4. Lower temperatures also have tremendous impact on concrete strength gains. (This is imperative for owners, contractors and sub-contractors as to when shoring and forms may be removed) If freezing occurs beforeconcrete has achieved a minimum PSI pounds per squareinch the concrete may not be suitable for its intended use. Should ice formin voids, permanent damagewill most likely occur. Hydration Understanding this process is the key if successfulconcreteplacement is be obtained. Hydration is a chemical reaction not a drying process, itrequires water to begin. The reaction is exothermic it is a process that releases energy in the concrete heat fromthe inside. What Happens When Concrete does Freeze? The pore water in the concrete will begin to freezeat around 30f. As some water begins to freeze, the ion in the unfrozen water in the concrete reducing the freeing point of the hydration water. When temperature is near the 27 f point the pore water freezes virtually stopping the hydration process and thus strength gain is terminated. Internalforces are then at work when water is allowed to freeze(frozen water expands 9 percent) this force becomes injurious to long term reliability of the concrete. Most suspended slabs do not have air entrainment so there is little if any roomfor expansion of water as with an exterior mix designed for outside work. Loss of strength should be expected. Slower development time. Longer time frame for formremoval. Concrete is more proneto damagefrom other trades. Concrete is more proneto weather damage as set time is decreased. Use of air entrained concrete is desirablein mostcold weather concrete placements. Concrete that is not air entrained will be more susceptible to damage, when construction schedules do not allow for structureto be enclosed mandatory measures mustbe put into the construction schedule then adhere to!
- 5. Should mixes be altered to allow for air in floors, finishing approaches will also be altered. Hard trowel finishing with machines, will most likely lead to delamination of the slabs surface. Itis essential that all parties involved know that changes are made for durably concerns and finishes may be differentin both texture and color shades. SIDENOTE air should not be added for slabs receiving hard troweled surfaces as delamination will likely occur. AC1 301 302 & 360 Intentions of Cold Weather Concrete Placement Avert damageto concrete at early stages by avoiding freezing. Initiate plans for safe removalof forms.
- 6. Providecuring methods that allow for normalstrength gains without using excessiveheating. Limit temperature changes of more than 20f during any 24 hour time span to prevent any thermal cracking. Deliver a protection plan with the intended preservation of the concrete structurein mind. For every 20f lessening in new concrete temperature, the setting time for finishing will double. This also has two fold consequenceas well. The time frame wheredamage may occur has also doubled as well as the time for finisher to begin their operations and protection plan. For one to fully understand strength the normal28 day window is inappropriate, if concrete has been damaged by insufficient curing and protection. Conversely mixes containing SCM (supplemental cementitious materials) Fly Ash both F and types and, Slag may greatly slow compressivestrength on test cylinders. Concrete that is unprotected from freezing until its PSI has reached a minimum of 500 will most likely suffer strength and durability issues during its life/cycle. Conversely concrete that is allowed to reach 500 PSI may gain strength slowly, but will matureto its desired strength. 3500 PSI is required for cycles of freezing and thawing when wet conditions exist. Terminating cold weather protection is based on the concretes strength is NOT a randomnumber of days. Suggested Practices and Viewpoints for Cold Weather Before pouring have a cold weather plan, that will be both used and enforced. Use of third party inspectors or laboratorys conducting strength and temperature tests should be highly considered. ASTM C 1064 temperature ASTM C 231 air testing in field ASTM C 143 slump of concrete ASTM C 31 making test cylinders ASTM C 39 strength evaluation of cured cylinders
- 7. Curing and protection after finishing when concrete strength is required as in a suspended slab, temperatures above 50f is required for predetermined time. Where concrete strength is not in question. Lower temperatures equal longer curing period. Records of concrete temperature should be kept and recorded ACI 301 keeping the concrete not air temperature is a MUST! Heated enclosures are also a mustdo servicefor suspended slab subjectin cold weather. Enclosures should be kept both weatherproof and windproof. Heaters must be properly vented to avoid surfacecarbonation. Itis not necessary to overheat under slab weatherproof structures to speed up set. This may lead to other issues such as thermal shock, cracking and carbonation. Moisture loss at temperatures below 50f should not be a concern. CAUTION Before concrete placement proceeds, heaters should be turned on and properly vented. Pouring & Placement Procedures Slump and Water to cement ratio, the slump ASTM C 143 will only determine the consistency of the concrete. The Water to cement ratio will determine the strength. GOOD 246 lb. water 歎 564 lb. of cement = 0.44 BAD 318 lb. water 歎 564 lb. of cement = 0.56 Lower slumps and W/C ratios result in reduced set time, increased strength if properly cured and less bleeding Deposit concrete into concrete this avoids segregation. Strike off or screed to predetermine grades form setting and initial screeding operations will havethe greatest effect on both floor flatness and levelness. Use hot water in winter time this improves set time and is useful as cold weather also means less hours of daylight to performplacement and finishing operations. Testing concrete before placement into forms is relatively easy task.
- 8. 1. Place the device into the concrete so sensing section is suppressed 3 inches into the concrete. 2. Lightly press the fresh concrete around the device. 3. Leave in concrete for 2 minutes. 4. Read the temperature with the device in place in the concrete. DO NOT REMOVE TO READ! Use ACI tables 306 for placement, this guide offers the needed temperature according to slab thickness and aggregate size. Also use this guide for setting time of concrete temperature guide. Setting time of concrete at various temperatures Temperature Approximate set time 70f 6 hours 60f 8 hours 50f 11 hours 40f 14 hours 30f 19 hours 20f little if any set will occur and concrete will begin to freeze
- 9. Cold weather issues in concrete construction can be traced to complications and lack of communication and failure to comprehend low air temperatures and failure to protect and keep concrete temperatures above 55f. Though suspended decks will not freeze as slab on ground subgrades will. Cold reinforcement, decking materials and imbedded items can contribute to rapid temperate changes during placement. Suspended slabs can be built with numerous decking and shoring materials. All methods should be carefully reviewed for cold weather placement. As well as any from removaland reshoring methods. Refer to ACI 347 for complete details on both shored and unshored decks. Most ready mix concrete will arriveon site with temperature in rangeof 68-72f how this temperature is handled fromdischargeis vital to success and curing of the concrete structure. Longer haul times may slightly cool the concrete as well refer to ASTMC 94 if longer distances in cold climates and water may create issues. This specification limits haul times to one and one-half hours and 300 revolutions on the drum. PREPARATIONOF SUBGRADESFOR SLABS ON GROUND Rule # 1. Is a well-known fact NEVER pour on frozen ground. Pouring in 60- 75f temperatures are idea conditions and perhaps one may forgetthe fact seasons change. Very cold weather can make concrete work much more difficult and will present numerous challenges for ready mix suppliers and contractors alike. Despite of many obstacles when contractors and suppliers work together, these issues can be overcome. Pouring on frozen ground , snow or ice Casting concrete in low temperatures requires that certain safeguards betaken. Many ready mix suppliers offer heated water and in some cases materials can be heated. Do no assumewhen bidding jobs these added items dont bring additional costs to the project.
- 10. Frozen ground will settle when it thaws. This settlement can lead to unwanted cracking. Add to fact this will also lead to crusting surfacewhich can then lead to difficult finishing. The top seems hard but the underlying concrete poured on frozen ground remains in extended plastic state. So it is never recommended that any concrete be poured upon a frozen surface. Slower curing times When temperatures are below 50f the concrete will set slow. Below 40f hydration comes to halt, strength gain is very drawn-out. Cold Weather Concrete Mix Designs Mixes should be reviewed periodically to insure mix provides the best in durability and strength requirements for its use. Never overheatthe concrete to speed up set times. Avoid thermal differences of morethan 40f. Grading of aggregates Aggregates fall into two classes Coarse3/8 inch and up, Fine 村 inch and down. A uniformed graded systems provides important statistics. Lower water demands Lower cement demands Better workability Equal proportions Uniformity = less potential for segregation Less bleeding Increased pump ability and finishing smaller spaces for chemical admixtures such as water-reducing admixtures to work Why use water-reducing admixtures 1. Decreasewater content and lower water to cement ratio 2. Increaseworkability 3. Reduce cement 4. All of the above plus 6-12 percentreduction in water
- 11. 5. In somecases high range water reducing admixtures may reduce water content by 35% Accelerators These should always be reviewed and selected in best wellbeing of the concrete. There are two types; they are classified as Chloride and non-Chloridethey offer. Speeding up of the preliminary hydration process. Give better controlover finishing time. Improveearly strength gains. Useful in cold weather. They are not an antifreeze agent! CALCIUMCHLORIDE 1. Economical 2. Can cause discoloration in concrete 3. Corrosion issues on reinforced slabs 4. Can lead to early shrinkage = cracks and settlement over reinforcement Non-chloride 1. More expensive 2. Shrinkage issues as well 3. Not as fast setting 4. May be the only option when suspended and reinforced slabs are done Cold weather mixes should follow specifications, with the attitude of no time to read them of understand them or too far behind them to follow them if you dont take time read them. THE LAWYERS WILL!!!!!!!!!!!!! The subgrades MUSTbe free of frostnot frozen or any ice or snow present, on subgradeforms and reinforcement. Keep decks and subgrades protected, place when there is no morethan 30f differencein temperatures, cover work at night to avoid frost. Retempering of mixes on site. Why is it needed? Find out and know why. Curing and protection Curing
- 12. The impending strength of the concrete will only be fully developed if fresh concrete is cured in an adequate time frame prior to being opened into service. Curing recommendations are generally 3-7 days as MINIMUMwhen the concrete temperature is 70f 3 days may be enough 50f and up 5 days for slabs on ground ONLY!! Curing with insulating concrete blankets provides a very effective method for protection R values are listed so thickness and layers can be easily calculated. Edges and corners are mostvenerable to allow cold air to enter. Properly weight edges and lapped splices (6 inch min lap). Stipulating a hand rail with sign DO NOT REMOVETARPS may aid in protection!! ACI 306R-88 Concrete placed during cold weather will develop appropriate strength and durability to fulfill intended servicerequirements. This will only occur if the concrete is batched, placed, finished, cured and protected. How the protection is prescribed is dependent on the ambient temperature. Items to evoke before placement Monitor the weather forecast Monitor the concrete temperature Monitor the subgradeof deck temperature ( within 30 of each other) Monitor rate of evaporation If possibleset pour times at warmestpartof the day Plan for rapid weather changes and prepare for it Check to be sureall materials are on hand for protection Inspectcuring blankets for tears and possibleice or snow contamination Verify no frostis presentin subgrade Diagramtravel time for ready mix delivery Rate of evaporation While this may be thought of as only Hot Weather alarm, nothing could be farther fromthe truth. Low humiditys, wind speeds and both air and concrete temperatures should be closely evaluated in cooler conditions as well. Rapid moistureloss at the concretes surfaceshould continually be worry. Protection of
- 13. paste or wear surfaceis what is viewed, walked and worked on, it is also what the owners firstsee as well. Determining rate of evaporation in concrete slabs Air temperature Concrete temperature Relative humidity Wind speed Air Temp Concrete Temp Humidity Wind Speed Evaporation rate 89/F 72 /F 36% 19 MPH 0.147/Sq-ft/hr. Charts and computer programs can now be used to calculate these four factors and determine the ROV so early precautions can be put into place. Design for suspended slabs These are covered by ACI 318, 421, and 347. Therearealso tolerance requirements set by ACI 117. Cracking is always a concern as well as deck material and shoring and shore removal. These segments of construction mustbe carefully followed and specifications and curing and temperature methods strictly monitored and tracked. Keeping and maintaining favorabletemperatures are not an option but a stringent requirement for these systems. There are not only requirements for removalof forms and shoring, but there should be required strength for loading of the suspended slab. Both field cured cylinders and maturity methods can predict concrete strength at very accurate levels. For minimum temperature requirements refer to ACI 306 table 7.3.1 for detailed information on thickness, cement content and insulation values. ACI does state that both shored and unshored decks cannot meet mostflatness requirements for slab on ground floors. References 1. ACI 301 2. ACI 302 3. ACI 304 4. ACI 306
- 14. 5. ACI 308 6. ACI 318 7. ACI 332 8. ACI 347 9. ACI 360 10.ACI 117 11.PCA Design and controlof concrete Mike Murray President Mike Murray Concrete Solutions Inc. / IndependentContractor Concrete CSI / PastPresident of DecorativeConcrete Supply 11 Cheddar Circle Bella Vista AR 72715 913 634 2267 murraydecorative@aol.com www.mikemurrayconcretesolutions.com Memberships in ACI, ICRI, Arkansas and Kansas Ready mixAssociations Concrete Promotional group of Kansas City Concrete Promotional group of Ozarks ACI Missouri, Kansas, Arkansasand Nebraska chapters ACI Fellow ACI Flatwork finisher and technician ID # 00095595 ACI Commercial finisher ID # 00095505 ACI Field grade I technician ID # 00095595 ACI Special inspector ID # 00095595 ACI Strength and lab technician ID # 00095595 ICRI moisturetesting technician Pervious concreteFinisher and technician Voting member of ACI 120 303 304
- 15. 308 310 610 / Field GradeI testing 640 / Flatwork certification Concrete Construction Liaison Committee