Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.
Issue link: http://concrete.epubxp.com/i/791382
76 • March 2017 www.concreteproducts.com • The minor variation in water content, within 1 percent of optimum moisture content, had minimal effect on the strength of RCC in a laboratory investigation. • High fines (passing No. 200 sieve) contents, although within spec- ification limits, result in lower compressive strength compared to lower fine contents. All mixtures with various fine contents could be satisfactorily compacted. • The use of Class F fly ash at 15 percent and 25 percent by weight of cement did not adversely affect early strength development. RCC MIX DESIGNS Mixture proportions for the Virginia Stafford and Richmond roller-compacted concrete mixes (lb./yd.) PURE HYDRATED ALUMINA CAN FORESTALL ALKALI-SILICA REACTIVITY Pure hydrated alumina as a cement substitute can quell alkali-sil- ica reactivity (ASR) in cured concrete, say Tiffany Szeles, Farshad Rajabipour and Shelly Stoffels, The Pennsylvania State University, State College, Pa., and Jared Wright, Walker Restoration Consultants, Seattle, in their technical paper, Mitigation of Alkali-Silica Reaction by Al(OH) 3 [alumina]. "Recent trends and forecasts on availability of fly ash and slag for use in concrete suggest a need to seek reliable alternative options for mitigation of alkali-silica reaction (ASR)," the authors write. "One such option may be aluminum-based admixtures." Past studies have shown that supplementary cementitious materials (SCMs) that contain alumina (Al 2 O 3 ) are more effective at mitigating ASR than SCMs purely rich in silica (SiO 2 ), they say, citing their research on pure hydrated alumina, Al(OH) 3 as a cement replacement to isolate its role in ASR mitigation. Their objectives were to determine if pure hydrated alumina can successfully mitigate ASR, and investigate five hypothesized mechanisms by which it may mitigate ASR. To determine if Al(OH) 3 can mitigate ASR, length change mea- surements per ASTM C1293 were conducted on concrete prisms. Based on their tests, Szeles, Wright, Rajabipour and Stoffels con- cluded pure hydrated alumina forestalled ASR by reducing pH and alkalis in concrete pore solution, consuming and reducing dissolved Ca and portlandite, and most of all, reducing silica disso- lution and damage to aggregates at high pH. Less functional were its altering the composition of ASR gel and creating innocuous gels, and reducing water and ion transport by reducing porosity and pore size of cement paste. FLY ASH CAN MINIMIZE DEICER DAMAGE IN PCC PAVEMENTS Fly ash can reduce deicer damage to concrete pavements by sup- pressing formation of expansive calcium oxychloride, say Prannoy Suraneni, Vahid J. Azad, O. Burkan Isgor, and Jason Weiss, Oregon State University School of Civil and Construction Engineering, in their peer-reviewed paper, Use of Fly Ash to Minimize Deicing Salt Damage in Concrete Pavements. "Premature damage has been observed at the joints in numerous concrete pavements where calcium chloride and magnesium chloride deicing salts have been used," the authors say. "This damage occurs due to a reaction between the deicing salt and the calcium hydroxide in the hydrated cement paste." This reaction leads to the formation of an expansive product known as calcium oxychloride, they write. "The use of supplementary cementitious materials as a replacement for cement has been proposed to reduce the calcium hydroxide that is available in the mixture to react with the deicing salts," Suraneni, Azad, Isgor, and Weiss say. "Reducing the calcium hydroxide can reduce the amount of calcium oxychloride that forms." In this study, mixtures representative of paving-grade concrete were made with cements and fly ashes from across the country. Cal- cium hydroxide amounts were determined using thermogravimetric analysis and calcium oxychloride amounts were determined using low-temperature differential scanning calorimetry. Various replacement levels of fly ash demonstrated that the main influence on the amounts of calcium hydroxide and calcium oxy- chloride that form is the replacement level of fly ash. Their study found that 40 percent replacement of fly ash should be sufficient to limit damage due to calcium oxychloride formation in well over 90 percent of the concrete mixtures. "Fly ash replacement is also known to cause a significant reduction in chloride diffusivity, especially at later ages, this would further reduce damage by reducing chloride ingress," Suraneni, Azad, Isgor and Weiss report. They concluded that a prescriptive specification requiring 35 percent cement replacement by volume with fly ash will reduce the damage due to calcium oxychloride formation. They also proposed a performance specification to limit the calcium oxychloride formation to below 15 g/100 g paste. Examples of joint damage in concrete pavements; damage due to chloride deicers reacting with calcium hydroxide in portland ce- ment concrete to form expansive calcium oxychloride can be quelled by higher substitutions of fly ash for cement. TECHNICAL TALK BY TOM KUENNEN Alumina works: scanning electron microscope images of a highly degraded 100 percent ordinary portland cement (OPC) ASTM C1293 concrete prism at 630 days (left, corresponding to 0.2 percent lin- ear expansion), and an intact 80 percent OPC/20 percent Al(OH) 3 concrete prism at 900 days (right, corresponding to 0.04 percent expansion). MICROGRAPHS: Szeles, Wright, Pajabipour, Stoffels IMAGES: Suraneni, Azad, Isgor and Weiss