ACI-COMPILATION-22-1993.pdf
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1、Compilation 22 American Concrete Institute Copyright American Concrete Institute Provided by IHS under license with ACI Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 02:15:47 MSTNo reproduction or networking permitted without license from IHS -,-,- Mineral Admix
2、tures AC1 Compilation 22 Contents 3 Admixtures - Whats New on the Market, by 43 John M. Scanlon 7 Properties of Shrinkage-Compensating Silica- Fume Concrete Made with Type K Cement, by Ziad Bayasi and Rab in portland-cement concrete, the biner is a mixture of port- land cement and water.” It does no
3、t say that concrete will weigh 144 pounds per cubic foot (2306 kg/m3) or have a compressive strength of 3000 psi (21 MPa). It does not say that concrete will have an initial setting time of six hours or a final setting time of nine hours. It does not say that the concrete must freeze below 32 F (O C
4、) or, for that matter, boil at 212 F (100 C). It does not say that the concrete shall have a modulus of elasticity of 3.0 x lo” psi. The point is, the industry has tradition- ally been placing additional require- ments on hydraulic cement concrete so that the composite material will do whats require
5、d by the user and attain the particular hardened properties that are desired by the engineer or owner of the structure in question. Based on the needs of the contractor in constructing concrete structures and the designers requirements for specific hardened concrete properties, standard practices, s
6、ampling procedures, test methods, and specifications have been established to assist in ensuring that the concrete develops the desired proper- ties. These standards have responded to the needs of the concrete industry for decades, but now technology demands more. Now the users and owners are de- ma
7、nding greater capabilities from their concrete. They need concrete that can: Gain strength, without freezing, down to and below 20 F (-7 C). Remain workable for hours or days without hardening, and then set on de- mand. Remain cohesive while being dropped through water, without the use of a tremie.
8、e Attain a modulus of elasticity of 7500 ksi (51 GPa) at age 28 days. Attain a strength of 20,000 psi (138 MPa) at age 56 days. e Provide positive protection of em- bedded metais from corrosion. Chemical admixtures2 Freeze protection admixture AC1 Technical Committee Report 306R-88, “Cold Weather Co
9、ncreting,” recommends that concrete placed in sec- tions of 12 in. (300 mm) or less be placed at a temperature of at least 50 F (10 C) and be maintained at 50 F (10 C) for the duration of the protection pe- riod. Such requirements actually cause many contractors to curtail concrete construction, esp
10、ecially when ambient temperatures fall below freezing. Only dedicated contractors, knowledgeable in cold weather construction, attempt con- crete construction below 20 F (-7 C). In addition, AC1 306R recommends that a 12 in. (300 mm) section of concrete placed above ground, and containing 500 lb/yd
11、(296 kg/m3) of cement, be protected with insulation having a thermal resistance (R) rating of 6 hr.ft*.F/Btu when the ambient tempera- ture is expected to drop to 20 F (-7 C). Curtailing concrete operations during low ambient temperatures is expensive and may not be cost-effective. Freeze- protectio
12、n admixtures have been devel- oped that can be used to prevent the con- crete from freezing, down to an ambient temperature of approximately 20 F (-7 C), without insulation. In addition, these innovative freeze-protection ad- mixtures comply with all requirements of ASTM C 494, “Standard Specifica-
13、tion for Chemical Admixtures for Con- for Type C accelerating, and for Type E water-reducing and accelerating admixtures. Typical test results are pre- sented in Table 1. The data in Table 2 illustrate that this admixture does not promote corrosion of steel in concrete, even at a dosage of 120 fl od
14、cwt (78 cm3/kg) of cement. Freeze-protection admixtures have been used in construction for approxi- mately five winters with no known in- cidents of concrete freezing. Many specifiers are now requiring its use, so concrete construction will most likely continue throughout the winter months. The park
15、ing garage of the Chrysler Technical Center, Detroit, Michigan, re- quired 40,000 yd3 (30,600 m3) of con- crete to be placed all year around. A Resented at the 1992 National Concrete Engineering Conference, March 30 - April I, Chicago, Illinois. MINERAL ADMIXTURES 3 Copyright American Concrete Insti
16、tute Provided by IHS under license with ACI Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 02:15:47 MSTNo reproduction or networking permitted without license from IHS -,-,- A C 1 COMP*22 *t m 0662949 0508006 OLT m Parameter Water content (maximum % of control) T
17、ime of setting (hcmin) Initial Final Compressive strength, (minimum % of Control) 3-day 7-day 28-day -month 1 -year Flexural stmngth, (minimum % of control) 3hY 7-daY 28day Length change Table 1 - Freeze-protection admixture laboratory evaluation results (ASTM C 494 requirements). TYP E aCCelerator
18、and accelerating Xtuio - 95 90.8 TYpeC w a t e r reducing Freee-prteti at least 1 : 0 0 eariier and not more than 330 eariier at least 1:00 earlier 202 d e r 227 wlii 125 125 136 1 0 0 1 1 0 1 2 6 100 1 0 0 1 1 6 90 100 I 1 9 90 1 0 0 121 1 1 0 1 1 0 1% 1 0 0 I00 1 0 9 90 1 0 0 1 0 3 Relative durabi
19、lity factor (minimum) I 80 1% Table 2 - Time-to-corrosion test results. +Thiocyanate ion (SCN-) is 71 $96 of NaSCN I freeze-protection admixture served as an all-temperature accelerator, in addi- tion to protecting the concrete during the winter. In the summer, at ambient temperatures of 80 to 90 F
20、(27 to 32 C), 5 fl odcwt (3 cm3/kg) of the freeze-pro- tection admixture was used for the con- crete to attain high early strength. All of this concrete attained compressive strengths exceeding 5,000 psi (34.5 MPa). A 40 percent cost savings was real- ized in using a freeze-protection ad- mixture on
21、 the Corning Glass Works, State College, Pennsylvania. At a dosage of 90 fl odcwt (59 cm3/kg), the admixture was used to protect the con- crete from freezing and permit the slabs to be finished within 4 hours at ambient temperatures around 20 F (-7 C). This particular freeze-protection ad- mixture c
22、ontains sodium thiocyanate (NaSCN). Its effects on corrosion of steel in concrete when used at different dosages was evaluated by using the Southern Climate Accelerated Test Method.5 The results of this long-term test are shown in Table 2. Extended set control admixture system Conventional retarding
23、 and accelerating admixtures have been in use for many years. These admixtures are used to re- tard or accelerate the setting time of concrete by a few hours. According to ASTM C 494, the set alteration is lim- ited to a minimum of 1 hour and a max- imum of 3% hours, whether it is retar- dation or a
24、cceleration. Attempts have been made to extend the setting time of concrete beyond the typical retardation times achieved with conventional retarding admixtures.) Al- though the setting times were length- ened in these attempts, the workability of the concrete was compromised be- cause of the inabil
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