Mass Concreting Tplaces Mass Concrete, Specifically As It Relates Structural to Reinforced Concrete Elements in Buildings

here are many issues that arise when one Goals of Mass Concreting Tplaces mass concrete, specifically as it relates structural to reinforced concrete elements in buildings. The ACI definition of mass concrete names heat These elements, which most typically include generated within the member due to the heat reinforced mat foundations, pile caps, footings, of hydration as a core concern. It is this heat, if PRACTICES piers, and transfer elements, differ from other not controlled, that can have detrimental effects reinforced structures such as dams and retaining on the concrete element. These could include practical knowledge beyond walls due to high stresses, quantities of steel rein- cracking, delayed ettringite formation (DEF), the textbook forcement, and the use of high strength concrete. and other issues. Another core concern with mass Mass concrete element placement in buildings, concreting is the potential formation of cold joints therefore, presents unique challenges. in the element. This is a problem more common to building structures as the use of higher strength Mass Concrete: When It Applies concretes will rapidly accelerate the setting time of the concrete. The definition of mass concrete per the American ® Concrete Institute (ACI) is “any volume of con- Temperatures crete with dimensions large enough to require that measures be taken to cope with generation of heat ACI 301 provides two recommended tempera- from hydration of the cement and attendant ture limits: volume change to minimize cracking” (ACI a) The maximum concr ete temperature shall 116R and ACI 301). Although no specific not exceed 160 degrees Fahrenheit during Mass Concreting thickness is given,Copyright a member thickness of three curing, and feet is often used as the threshold dimension for b) The maximum temperature differential when mass concreting procedures are to be set between the center and surface of placement By J. Benjamin Alper, P.E., S.E., in motion. Note that there may be circumstances shall not exceed 35 degrees Fahrenheit. and Cawsie Jijina, P.E., SECB where members with a thickness less than three These limits are in place to avoid delayed ettrin- feet will call for mass concreting procedures. The gite formation (DEF) which can cause cracking final determination as to whether mass concreting and reduce concrete strength. Although these procedures should be followed is at the discretion of temperature limits may not seem significant, they the Structural Engineer of Record (SER). can often be difficult to achieve, especially the restriction on the temperature differential. Beginning concrete placement with a lower ini- J. Benjamin Alper is an Associate at ACI 207 magazinetial concrete temperature is recommended to keep Severud Associates and serves as the S TThe ACIR Committee U 207 providesC several T reports U the concrete R from Ereaching upper-temperature Quality Control Officer for Severud Associates’ inspection services. He can for guidance in the use of mass concrete. ACI limits. Typical methods to keep concrete tempera- be reached at [email protected]. 207.1, Guide to Mass Concrete, is the primary tures low include the introduction of crushed ice resource for guidance in the use of mass con- to the mix in lieu of some of the mixing water crete for structural building elements. Most and the use of precooled aggregate. For times Cawsie Jijina is a Principal at Severud of the data and concerns related to the use of when the ambient temperatures are high, it may Associates and serves as the Deputy mass concrete were formulated Technical Director for Severud from lessons learned during the Associates’ inspection services. He can United States golden era of dam be reached at [email protected]. construction. It was during the construction of these massive structures more than one hun- Review the following publications for dred years ago that issues related additional information, considerations, to the placement of mass con- and approaches for mass concreting. crete began to emerge. During ACI 207.1 – Guide to Mass Concreting dam construction, the strengths ACI 207.2 – Report on Thermal and Volume of concrete required were low Change Effect on Cracking of Mass Concrete (typically under 5,000 psi) ACI 207.4 – Cooling and Insulating Systems compared to the requirements for Mass Concrete for strength of current build- ing elements, where concrete strengths can exceed 10,000 psi. Although many of the rec- ommendations from ACI 207 may be useful for these types of structures, other recommenda- tions from ACI 207 may not be practical for building elements. Nighttime foundation mat placement with multiple points of placement.

STRUCTURE magazine12 April 2018 set prior to additional concrete being placed next to it. Maximizing the number of points of placement keeps concrete flowing as much as possible. The pre-placement meeting may reveal that, due to site restrictions, the forma- tion of a cold joint in an element is highly probable. The SER may need to consider the addition of strategically placed construc- tion joints within the structural element to minimize the risk of a cold joint forming, and provide for the reliable transfer of forces. The use of a retarding admixture or a set Completion of mat placement; insulation and tarps Thermocouple and wiring installed delaying admixture is important to prolong being placed. with mat slab. the workability period of concrete and can help control temperatures. A typical procedure is to be necessary to post-cool the concrete via Monitoring Concrete start the early concrete® trucks with the maxi- embedded cooling coils. Other than when Temperatures mum recommended quantity of admixture insulation is used, the use of curing water on and then slowly taper off the admixture during the slab is essential to prevent water loss and The monitoring of concrete temperatures concrete placement. Tapering the admixture assist in the curing operation. in mass concreting is commonly achieved during placement helps keep the concrete In a normal concrete element, a tempera- through the use of thermocouples. These sen- workable and also supports constructabil- ture gradient is created between the interior, sors are buriedCopyright within the structural element ity for the contractor. The added fluidity of where the heat of hydration is trapped, and prior to the concrete placement operation. the concrete late in the placement increases the exterior surface of the concrete that is Often, the SER generates a three dimensional the pressure head on the formwork system. transferring heat to the atmosphere. In colder grid that allows the temperature gradient to Additionally, for most elements, laborers must weather, insulation is added to the top of the be plotted throughout the cross-sectional area remain on site until finishing and curing proce- slab to maintain the thermal temperature of the element. The sensors report the tem- dures are completed. If the retarding admixture gradient and prevent the top from cooling perature within the element and enable the is used in its full dosage until the end of the more rapidly than the core. Trying to main- engineer to verify that the temperatures have placement, the staff needs to wait on site for tain this same 35-degree differential when the remained within the desired range from the many additional hours beyond the pour until interior of the concrete is “insulated” with initial concrete placement until final concrete they can walk on the concrete and begin fin- several feet of concrete above and below it, curing operations are complete. When tem- ishing/curing. Tapering the quantity of the zones that are also generating heat, is difficult peraturesmagazine approach the limits of the desired admixture allows for more efficient staffing. when the exterior of the concreteS is exposed T to Rrange, changesU can beC made toT the curingU A licensedR structuralE engineer or an ACI colder atmospheric temperatures, even with procedures, such as the addition or removal of certified technician observing the placement the use of insulating blankets. Note that the insulating layers to balance the temperatures. is recommended to help prevent the forma- 35-degree limit is a general recommendation tion of cold joints. The engineer, in concert to prevent deterioration of the concrete due to Cold Joints with the contractor, can use observations to the separation of concrete layers, but it is often make strategic real-time changes to the pour at the discretion of the Engineer of Record A cold joint forms when concrete already sequencing. The contractor can come back whether higher temperature differentials may placed within the element begins to set even with a small amount of concrete to refresh be tolerated. as subsequent layers of concrete are still being an area that is starting to set and potentially placed on top of it. The formation of a cold form a cold joint. The pour can then continue. Mix Designs joint negates the monolithic properties that Unfortunately, even with the best planning, are always desired and, unlike a planned and things can go wrong – a pump breaks, the Many recommended changes to the concrete formed construction joint, it is typically hap- concrete delivery is delayed, and so forth. It mix for mass concreting reduce or slow the heat hazard in its locations and is horizontal or is critical that someone on site, typically the of hydration to limit temperature change and on a slope. The failure plane created within inspector or engineer, record and document thereby lower the amount of crack formation. the structural element is often hard to docu- the location of any cold joints that form. It is critical to reduce the quantity of cement in ment, as it usually happens in the middle of This allows the Engineer of Record to more the mix. The replacement of cement with slag the concrete placement when operations are accurately evaluate the impact to the structure and fly ash (and now ground glass pozzolans) the most hectic. and provide the most efficient remediation, can help maintain required strengths. The use Avoiding cold joints takes prior planning. if one is required. of larger aggregates can also help; however, this During a pre-placement meeting before can often be difficult as the congestion of steel the start of mass concreting operations, it Conclusion reinforcement and the method of concrete is important to plan and discuss the order placement may restrict the aggregate sizes. of concrete placement and the number of This article describes general approaches to Admixtures used for air entrainment, water placement points. The order in which the address some of the mass concreting concerns reduction, set time, and shrinkage reduction, concrete is intended to be placed should be that are particularly relevant to building struc- strength, and durability can be adjusted to slow strategic to avoid leaving specific areas for tural systems. Several ACI publications are or reduce the heat of hydration. extended periods where concrete can start to available for more information.▪

STRUCTURE magazine13 April 2018