27-07-2012, 11:58 AM
SEISMIC PERFORMANCE COMPARISON OF A HIGH
SEISMIC PERFORMANCE COMPARISON OF A HIGH.doc (Size: 1.61 MB / Downloads: 25)
1. INTRODUCTION
Ash is a byproduct obtained during the combustion of coal. Fly ash is generally obtained from the chimneys of coal-fired power plants. Depending on the amount of calcium, silica, iron, and alumina content of the ash there are two classes of fly ash as defined by ASTM C618, specifically Class C and Class F fly ash. Class C fly ash has high-calcium content, and its carbon content is usually less than two percent, while Class F fly ash has a low-calcium content with a carbon content usually less than five percent. Fly Ash, due to its pozzolanic properties is often used as an additive to Portland cement in concrete production. The use of fly ash in concrete increases the strength and durability of the concrete and also decreases the heat of hydration and permeability of the concrete. The use of fly ash in concrete helps to reduce environmental pollution, because for every ton of fly ash used to replace Portland cement in the manufacture of concrete, there is a reduction of carbon dioxide emissions which is, for example, equal to the amount of carbon dioxide generated from the average automobile during a two-month period [1]. Since the majority of SO2 emissions into the atmosphere are due to coal fired power plants, many coal fired power plants in the United States are now utilizing spray dry absorbers for the reduction of these SO2 gas emissions. The result is SDA which has material and behavioral properties similar to fly ash, but a different chemical makeup. In this process alkali sorbents such as lime (Cao) or calcium hydroxide (Ca(OH)2) are mixed with water to form an aqueous slurry [2]. This slurry is sprayed into the flue gas in a cloud of fine droplets. SO2 is then captured with this sorbent and is dried by the heat of the flue gases.
The dried mix of the sorbent and SO2 is collected. The ash utilized in the project described in this paper was from the Platte River Power Authority’s Rawhide Power Plant (RPP) which uses the SDA system. The ash obtained from RPP power plant has a unit mass of 2.1 g/cc, and, due to its high sulphur content its chemical properties and mineralogical properties [3] are slightly different, and, therefore, it cannot be classified as Class C ash. There have been numerous studies conducted on the use of ash in concrete. Swamy et al. [4] conducted tests on reinforced concrete fly ash concrete beams and slabs containing normal weight aggregates and light weight aggregates. The results of their tests showed that concrete with fly ash can exhibit structural performance similar to that of conventional concrete with adequate safety factors used in existing design codes at the time. The results of their study also showed that structural concrete components can be designed to incorporate fly ash at quantities as high as 30 percent cement replacement, by weight. Joshi et al. [5] studied the engineering properties of non air- entrained concrete. Laboratory tests were conducted on both fly ash concrete and ordinary Portland cement concrete specimens.
Based on properties such as compressive, flexural, indirect tensile strengths, and additional nondestructive tests, it was concluded that fly ash concrete could be used as a construction material for the core of a gravity dam and for pavement sub base. Hussain and Rasheeduzzafar [6] conducted accelerated corrosion tests on reinforced concrete specimens made of plain cement concrete and fly ash blended cement concrete. The results of the test showed superior corrosion resistance of fly ash concrete when compared to plain cement concrete. Pigeon and Malhotra [7] designed four high-volume fly ash-compacted concrete mixes by fixing the amount of fly ash to the total cementitious material content. Laboratory investigations were carried out on air entrained and non-air-entrained concrete mixes, and the results showed that frost resistance of air-entrained concrete mixes was slightly more than that of non-air-entrained concrete mixes. The results of this study recommended the use of air entrainment for roller-compacted high-volume fly ash concretes. Dinelli et al. [9] conducted experiments to find the possibility of partial or complete substitution of traditional aggregates in light weight concrete with aggregates made of fly ash.
The results of their experiments demonstrated that traditional aggregate could be substituted with aggregate made of fly ash. Fernandez-Jimenez et al. [10] studied the durability of alkali-activated fly ash (AAFA) cement under different conditions and in a number of aggressive environments such as deionized water, ASTM sea water, sodium sulphate, and acidic solutions. Studies were also made with respect to alkali-silica reaction-induced expansion. Weight loss, compressive strength, variations in volume, presence of the products of degradation, and micro structural changes were the chief parameters which were studied. The results of the study showed that AAFA cement pastes performed satisfactorily in aggressive environments, and the degradation of the materials resulting from such processes was distinctly different from that of the ordinary Portland cement paste. The AAFA mortars were found to be compliant with the 16- day expansion limit stipulated in ASTM standard C 1260-94 on potential alkali-silica reactivity. Van de Lindt et al. [11] carried out a study to investigate the possibility of increasing the thermal efficiency of a light frame residential structure through the addition of fly ash scrap tire fiber composite to traditional fiberglass insulation in light-frame wood residential construction. They found that the fly ash-scrap tire composite not only provided a sustainable supplement to traditional insulation but also helped to significantly reduce the environmental issues associated with the disposal of these materials by diverting them from a landfill. Other numerous studies have been conducted over the past decades with most of them focusing on fly ash concrete and its use as a concrete additive.
1.1 The objective of this study:-
Evaluate the seismic behavior of concrete portal frames when replacing fifty percent of their cement content with spray dryer ash (SDA) and comparing that with the seismic behavior of ordinary Portland cement concrete frames when subjected to the same ground motions. Figure 1 shows the plan view of the three storey office building that served as the example building for this study. The building was designed for seismic load conditions per ASCE 7-05 [12] and seismic detailing according to ACI 318-05 [13] as if it were situated in Los Angeles, California. A mid bay portal frame was selected as the prototype frame, and, in total, four similar 1/3 scale models of this frame were constructed for testing. Two frames were constructed with fifty percent SDA concrete and the other two frames were constructed with ordinary Portland cement concrete
2. Design and Construction
2.1. Frame Design.
The frame tested on the shake table was selected from the center bay of a three story office building having three bays in both the X and Y directions as shown in Figure 1. The office building was selected such that there were no plan irregularities or vertical irregularities. A 200mm (8 inch) thick reinforced concrete slab was assumed for the load calculations on beams. Design loads and load factors were selected as per the seismic load combinations from ASCE 7-05 [12]. The prototype frames were selected for the design such that two 1/3 scaled frames were able to be placed parallel to each other and tested on the shake table. The frames were designed as reinforced concrete special moment frames (SMF) for seismic resistance as per seismic detailing provisions of ACI 318-05 [13]. The material strengths assumed for the design were ASTM Grade 60 steel, fy = 414MPa (60 ksi), and ordinary type II Portland cement concrete having a 28-day compressive strength of 27.6MPa (4000 psi).