THE CONCRETE BRIDGE MAGAZINE

Winter 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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C R E AT I V E C O N C R E T E C O N S T R U C T I O N 34 | ASPIRE Winter 2019 T here are numerous situations in bridge construction where it is necessar y to determine the in-place compressive strength of the concrete. These include removal of formwork, application of post-tensioning, transfer of force from pretensioned strands, termination of cold weather protection, and opening a bridge to traffic. Failure to achieve the specified compressive strength using the standard-cured cylindrical specimens is an unplanned situation that occasionally occurs. This article is part of a series of articles that pr ovide brief descriptions of methods to determine the in-place compressive strength and resources for more information. A more detailed description of these and other methods is provided in In-Place Methods to Estimate Concrete Strength (ACI 228.1R-03). 1 Field-Cured Cylinders In this method, standard concrete cylinders are left at the job site for curing in, as nearly as practicable, the same manner as the concrete in the structure. 2 In practice, this usually means placing the concrete cylinders on or near the member or under the sheet-curing material, if used. This method may be used for cast-in-place concrete as well as precast concrete. For cast-in- place concrete, it is the simplest method; however, at best, the cylinders only represent the concrete near the surface of the member. For precast heat- cured concrete, the cylinders are generally placed under an insulated cover and receive curing more similar to that of the member. Concrete Cores This method is usually used when the standard- cured cylinders fail to achieve the specified compressive strength and no more cylinders are available for later age tests. It is the most direct method to determine the compressive strength of concrete in a structure because the test is made on the actual concrete that has been subjected to the in-place curing environment. Nevertheless, the core locations must be carefully selected because all parts of the member may not have been subjected to the same curing conditions. The core locations must avoid reinforcement, other embedments, and highly stressed areas. The strength measured on the cores must also be carefully evaluated. For example, according to the American Concrete Institute's (ACI's) Building Code Requirements for Structural Concrete (ACI 318-14) and Commentary (ACI 318R-14), 3 concrete in an area represented by core tests shall be considered structurally adequate if the average strength of three cores is equal to at least 85% of the specified strength and if no single core strength is less than 75% of the specified strength. No similar provision is included in the American Association of State Highway a n d Tr a n s p o r t a t i o n O f f i c i a l s ( A A S H T O ) specifications, 4 although individual project specifications may address it. Match Curing Match curing is a system for curing standard concrete cylinders—usually 4 × 8 in.—at the same temperature as that measured in a concr ete member. 5,6 Commer cial systems include a temperature sensor in the member, a c o n t r o l l e r, s p e c i a l i n s u l a t e d c y l i n d e r molds with built-in heating systems, and a temperature sensor in the mold. The controller is used to control the heating systems so that the temperature in the cylinder matches the temperatures measured in the member. The system may also include a means to record and read the member temperatures and the cylinder temperatures. Noncommercial systems have also been used with this technique. Some precast concrete plants have used heated cabinets for this purpose rather than individual molds. T h e m e t h o d i s p a r t i c u l a r l y u s e f u l fo r d e t e r m i n i n g t h e c o n c r e t e c o m p r e s s i v e strength at early ages in the fabrication of precast concrete members or for determining the compressive strength of concrete at a critical section in a member. The location of the temperature sensor in the member is particularly important because temperatures can vary throughout a member and temperature influences the rate of strength gain. Because the match-curing system only provides heat to the cylinders and cannot cool them, the temperature of the cylinders may exceed that of the member if the temperature of the member decreases at a faster rate than that of the cylinders. If this happens, the cylinder strengths may not entirely represent the in-place concrete strength. Determining In-Place Concrete Compressive Strength by Dr. Henry G. Russell, Henry G. Russell Inc. It [the concrete core technique] is the most direct method to determine the compressive strength of concrete in a structure because the test is made on the actual concrete that has been subjected to the in-place curing environment. The [match-curing] method is particularly useful for determining the concrete compressive strength at early ages in the fabrication of precast concrete members or for determining the compressive strength of concrete at a critical section in a member. Sensors at the Black Ankle Valley Bridge were also used to monitor test cylinders for curing compliance and verification of strengths. Photo: John Gnaedinger, Con- Cure LLC.

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