From the experimental results of determining the splitting tensile strength (f sp ) of 105 standard concrete cylinders of 70MPa grade, for two types of short and long fibers, the analysis gives the correlation function between the splitting compressive strength and the steel fiber content. Using the univariate linear regression method and Minitab V17 statistical analysis software to find the relationship between the splitting compressive strength ( f sp ) and the fiber content (V f ).
The regression results found coefficients A and B for two cases of steel fiber reinforcement, short fiber (L f /D f = 63.63) and long fiber (L f /D f = 80) shown in Figure 2.32 and Figure 2.33. The equations with large correlation coefficients r 2 are r 2 = 86.3% (short fiber type), r 2
f sp =5.426+2.950V f
Splitting compressive strength (f sp ,MPa)
= 86.0% (long fiber type) and these values are all greater than 80%. This shows that the regression models are suitable and statistically significant.
f sp =5.813+3.755V f
Splitting compressive strength (f sp ,MPa)
Figure 2.32. Data processing results of the sample using short fibers (l f /d f =63.63)
Figure 2.33. Data processing results of the sample using long fibers (L f /D f =80)
f '
c
f '
c
From the above experimental results, we have the coefficients A=5.426 and B=2.95 for the short fiber case (L f /D f =63.63) and the coefficients A=5.813 and B=3.755 for the long fiber case.
(L f /D f =80). If we leave it in the form A= a
and B= b (L f /D f)
then with concrete level
f' c = 70MPa, in both cases above, the values of coefficients a, b are the same (a=0.6); b=0.55). Therefore, the thesis provides a model for calculating the splitting compressive strength of high-strength concrete reinforced with steel fibers ( f sp ) as equation (2-79) for both short and long fibers.
f 0.6 f ' 0.55 f V f '
sp c d fc
f
L(2-78) |
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Pre-tax Profit of Bidv Tien Giang in the Period 2011-2015 zt2i3t4l5ee zt2a3gsnon-credit services, joint stock commercial bank zt2a3ge zc2o3n4t5e6n7ts At that time, the Branch had to set aside a provision for credit risks, which reduced the Branch's income. Chart 2.2. Pre-tax profit of BIDV Tien Giang in the period 2011-2015 Unit: Billion VND 140 120 100 80 60 40 20 0 63.3 80.34 89.29 110.08 131.99 2011 2012 2013 2014 2015 Profit before tax (Source: Report on the implementation of the annual business plan of the General Planning Department of BIDV Tien Giang [24]) However, through chart 2.2, it can be seen that BIDV Tien Giang's profit is still increasing continuously, and its operating efficiency is currently leaking. This is a contribution of non-credit services, and this service segment will be increasingly focused on growth by BIDV Tien Giang to ensure the highest profit safety because credit activities have many potential risks. At the same time, focusing on developing non-credit services is consistent with one of the contents of restructuring the financial activities of credit institutions in the project "Restructuring the system of credit institutions in the period 2011-2015" approved by the Prime Minister in Decision No. 254/QD-TTg dated March 1, 2012 [14]: "Gradually shifting the business model of commercial banks towards reducing dependence on credit activities and increasing income from non-credit services". 2.2. Current status of non-credit service development at BIDV Tien Giang. 2.2.1. BIDV Tien Giang has deployed the development of non-credit services in recent times. Along with the development of the Head Office, BIDV Tien Giang's products and services are constantly improved and deployed in a diverse manner to ensure provision for many different customer groups in the area: individual customers, corporate customers, and financial institutions. Typical services are as follows: Payment services, treasury services, guarantee services, card services, trade finance, other services: Western Union, insurance commissions, consulting services, foreign exchange derivatives trading, e-banking services,... 2.2.1.1. Payment services: In accordance with the Prime Minister's Project to promote non-cash payments in Vietnam [15], banks in Tien Giang province have continuously developed payment services to reduce customers' cash usage habits through card services and electronic banking services such as: salary payment through accounts, focusing on developing card acceptance points, developing multi-purpose cards, paying social insurance by transfer, paying bills through banks, etc. Chart 2.3. Net income from payment services in the period 2011-2015 Unit: Million VND 6000 5000 4000 3000 2000 1000 0 3922 4065 4720 5084 5324 2011 2012 2013 2014 2015 Net income from payment services (Source: Report on the implementation of the annual business plan of the General Planning Department of BIDV Tien Giang [24]) Along with the technological development of the entire system, BIDV Tien Giang has a payment system with a fairly stable transaction processing speed, bringing many conveniences to customers. The results of observing chart 2.3 show that the income from payment services that the Branch has achieved has grown over the years but the speed is not high and the products are not outstanding compared to other banks. Domestic payment products such as: Online bill payment, electricity bills, water bills, insurance premiums, cable TV bills, telecommunications fees, airline tickets, etc. bring many conveniences to customers. Regarding international payment, this is an indispensable activity for foreign economic activities, BIDV Tien Giang is providing international payment methods for small enterprises producing agriculture, aquatic food and seafood that have credit relationships with banks in industrial parks in Tien Giang province such as: money transfer, collection, L/C payment. 2.2.1.2. Treasury services: BIDV Tien Giang always focuses on ensuring treasury safety and currency security, always complies with legal regulations, and minimizes risks in operations such as: counting and collecting money from customers, receiving and delivering internal transactions, collecting from the State Bank (SBV) or other credit institutions, receiving ATM funds, bundling money, etc. BIDV Tien Giang's treasury service management department is always fully equipped with modern machinery and equipment such as: money transport vehicles, fire prevention tools, money counters, money detectors, magnifying glasses, etc. to ensure absolute safety in treasury operations, immediately identifying real and fake money and other risks that may affect people and assets of the bank and customers. In addition, implementing regulation 2480/QC dated October 28, 2008 between the State Bank of Tien Giang province and the Provincial Police on coordination in the fight against counterfeit money, in the 3-year review of implementation, BIDV Tien Giang discovered, seized and submitted to the State Bank of Tien Giang province 475 banknotes of various denominations and was commended by the Provincial Police and the State Bank of Tien Giang province [17]. Chart 2.4. Net income from treasury services in the period 2011-2015 Unit: Million VND 350 300 250 200 150 100 50 0 105 122 309 289 279 2011 2012 2013 2014 2015 Net income from treasury services (Source: Report on the implementation of the annual business plan of the General Planning Department of BIDV Tien Giang [24]) However, as shown in Figure 2.4, income from treasury operations is not high and fluctuates. Specifically, in the period 2011-2013, net income increased and increased most sharply in 2013, then in the period 2013-2015, there was a downward trend. This fluctuation is due to the fact that fees collected from treasury services are often very low and can even be waived to attract customers to use other services. 2.2.1.3. Guarantee and trade finance services: BIDV Tien Giang, thanks to the advantages of the province and the favorable location of the Branch, has continuously focused on developing income from guarantee services and trade finance. Chart 2.5. Net income from guarantee and trade finance services in the period 2011-2015 Unit: Million VND 14000 12000 10000 8000 6000 4000 2000 0 5193 5695 2742 3420 8889 3992 11604 12206 5143 5312 2011 2012 2013 2014 2015 Net income from guarantee services Net income from Trade Finance (Source: Report on the implementation of the annual business plan of the General Planning Department of BIDV Tien Giang [24]) Through chart 2.5, we can see that BIDV Tien Giang's income from guarantee services and trade finance has grown over the years. The reason is: Among BIDV Tien Giang's corporate customers, the construction industry is the industry with the highest proportion of customers after the trading industry, this is a group of customers with potential to develop guarantee services. The second group of customers is corporate customers in the fields of agricultural production, livestock and seafood processing with high import and export turnover in the area. are the target of trade finance development. In addition, BIDV Tien Giang also focuses on continuously developing these customer groups to increase revenue for many other products and services in the future. 2.2.1.4. Card and POS services: As a service that BIDV Tien Giang has recently developed strongly, it can be said that this is a very potential market and has the ability to develop even more strongly in the future. Card services with outstanding advantages such as fast payment time, wide payment range, quite safe, effective and suitable for the integration trend and the Project to promote non-cash payments in Vietnam. Cards have become a modern and popular payment tool. BIDV Tien Giang early identified that developing card services is to expand the market to people in society, create capital mobilized from card-opened accounts, contribute to diversifying banking activities, enhance the image of the bank, bring the BIDV Tien Giang brand to people as quickly and easily as possible. BIDV Tien Giang is currently providing card types such as: credit cards (BIDV MasterCard Platinum, BIDV Visa Gold Precious, BIDV Visa Manchester United, BIDV Visa Classic), international debit cards (BIDV Ready Card, BIDV Manu Debit Card), domestic debit cards (BIDV Harmony Card, BIDV eTrans Card, BIDV Moving Card, BIDV-Lingo Co-branded Card, BIDV-Co.opmart Co-branded Card). These cards can be paid via POS/EDC or on the ATM system. In addition, with debit cards, customers can not only withdraw money via ATMs but also perform utilities such as mobile top-up, online payment, money transfer,... through electronic banking services. In order to attract customers with card services, BIDV Tien Giang has continuously increased the installation of ATMs. As of December 31, 2015, BIDV Tien Giang has 23 ATMs combined with 7 ATMs in the same system of BIDV My Tho, so the number of ATMs is quite large, especially in the center of My Tho City, but is not yet fully present in the districts. Basic services on ATMs such as withdrawing money, checking balances, printing short statements,... BIDV ATMs accept cards from banks in the system. Banknetvn and Smartlink, cards branded by international card organizations Union Pay (CUP), VISA, MasterCard and cards of banks in the Asian Payment Network. From here, cardholders can make bill payments for themselves or others at ATMs, by simply entering the subscriber number or customer code, booking code that service providers notify and make bill payments. Chart 2.6. Net income from card services in the period 2011-2015 Unit: Million VND 3500 3000 2500 2000 1500 1000 500 0 687 1023 1547 2267 3104 2011 2012 2013 2014 2015 Net income from card services (Source: Report on the implementation of the annual business plan of the General Planning Department of BIDV Tien Giang [24]) Through chart 2.6, it can be seen that BIDV Tien Giang's card service income is constantly growing because the Branch focuses on developing businesses operating in industrial parks, which are the source of customers for salary payment products, ATMs, BSMS. Specifically, there are companies such as Freeview, Quang Viet, Dai Thanh, which are businesses with a large number of card openings at the Branch, contributing to the increase in card service fees [25]. Table 2.6. Number of ATMs and POS machines in 2015 of some banks in Tien Giang area. Unit: Machine STT Bank name Number of ATMs Cumulative number of ATM cards POS machine 1 BIDV Tien Giang 23 97,095 22 2 BIDV My Tho 7 21,325 0 3 Agribank Tien Giang 29 115,743 77 4 Vietinbank Tien Giang 16 100,052 54 5 Dong A Tien Giang 26 97,536 11 6 Sacombank Tien Giang 24 88,513 27 7 Vietcombank Tien Giang 15 61,607 96 8 Vietinbank - Tay Tien Giang Branch 6 46,042 38 (Source: 2015 Banking Activity Data Report of the General and Internal Control Department of the Provincial State Bank [21]) Through table 2.6, the author finds that the number of ATMs of BIDV Tien Giang is not much, ranking fourth after Agribank Tien Giang, Dong A Tien Giang, Sacombank Tien Giang. The number of POS machines of BIDV Tien Giang is very small, only higher than Dong A Tien Giang and BIDV My Tho in the initial stages of merging the BIDV system. Besides, BIDV Tien Giang has a high number of cards increasing over the years (table 2.7) but the cumulative number of cards issued up to December 31, 2015 is still relatively low compared to Agribank, Vietcombank, Dong A (table 2.6). div.maincontent .content_head3 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: bold; text-decoration: none; font-size: 14pt; } div.maincontent .p { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 14pt; margin:0pt; } div.maincontent p { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 14pt; margin:0pt; } div.maincontent .s1 { color: black; font-family:"Courier New", monospace; font-style: normal; font-weight: normal; text-decoration: none; font-size: 14pt; } div.maincontent .s2 { color: black; font-family:"Times New Roman", serif; font-style: italic; font-weight: normal; 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Sample Population, Sampling Technique and Data Processing
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Spss Data Processing Results of Survey Form for Former Students
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Statistical Data Processing Results of Class 2A and Class 2B
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Mobile Phone Usage in Hanoi Inner City Area zt2i3t4l5ee zt2a3gsconsumer,consumption,consumer behavior,marketing,mobile marketing zt2a3ge zc2o3n4t5e6n7ts - Test the relationship between demographic variables and consumer behavior for Mobile Marketing activities The analysis method used is the Chi-square test (χ2), with statistical hypotheses H0 and H1 and significance level α = 0.05. In case the P index (p-value) or Sig. index in SPSS has a value less than or equal to the significance level α, the hypothesis H0 is rejected and vice versa. With this testing procedure, the study can evaluate the difference in behavioral trends between demographic groups. CHAPTER 4 RESEARCH RESULTS During two months, 1,100 survey questionnaires were distributed to mobile phone users in the inner city of Hanoi using various methods such as direct interviews, sending via email or using questionnaires designed on the Internet. At the end of the survey, after checking and eliminating erroneous questionnaires, the study collected 858 complete questionnaires, equivalent to a rate of about 78%. In addition, the research subjects of the thesis are only people who are using mobile phones, so people who do not use mobile phones are not within the scope of the thesis, therefore, the questionnaires with the option of not using mobile phones were excluded from the scope of analysis. The number of suitable survey questionnaires included in the statistical analysis was 835. 4.1 Demographic characteristics of the sample The structure of the survey sample is divided and statistically analyzed according to criteria such as gender, age, occupation, education level and personal income. (Detailed statistical table in Appendix 6) - Gender structure: Of the 835 completed questionnaires, 49.8% of respondents were male, equivalent to 416 people, and 50.2% were female, equivalent to 419 people. The survey results of the study are completely consistent with the gender ratio in the population structure of Vietnam in general and Hanoi in particular (Male/Female: 49/51). - Age structure: 36.6% of respondents are <23 years old, equivalent to 306 people. People from 23-34 years old accounting for the highest proportion: 44.8% equivalent to 374 people, people aged 35-45 and >45 are 70 and 85 people equivalent to 8.4% and 10.2% respectively. Looking at the results of this survey, we can see that the young people - youth account for a large proportion of the total number of people participating in the survey. Meanwhile, the middle-aged people including two age groups of 35 - 45 and >45 have a low rate of participation in the survey. This is completely consistent with the reality when Mobile Marketing is identified as a Marketing service aimed at young people (people under 35 years old). - Structure by educational level: among 835 valid responses, 541 respondents had university degrees, accounting for the highest proportion of ~ 75%, 102 had secondary school degrees, ~ 13.1%, and 93 had post-graduate degrees, ~ 11.9%. - Occupational structure: office workers and civil servants are the group with the highest rate of participation with 39.4%, followed by students with 36.6%. Self-employed people account for 12%, retired housewives are 7.8% and other occupational groups account for 4.2%. The survey results show that the student group has the same rate as the group aged <23 at 36.6%. This shows the accuracy of the survey data. In addition, the survey results distributed by occupational criteria have a rate almost similar to the sample division rate in chapter 3. Therefore, it can be concluded that the survey data is suitable for use in analysis activities. - Income structure: the group with income from 3 to 5 million has the highest rate with 39% of the total number of respondents. This is consistent with the income structure of Hanoi people and corresponds to the average income of the group of civil servants and office workers. Those People with no income account for 23%, income under 3 million VND accounts for 13% and income over 5 million VND accounts for 25%. 4.2 Mobile phone usage in Hanoi inner city area According to the survey results, most respondents said they had used the phone for more than 1 year, specifically: 68.4% used mobile phones from 4 to 10 years, 23.2% used from 1 to 3 years, 7.8% used for more than 10 years. Those who used mobile phones for less than 1 year accounted for only a very small proportion of ~ 0.6%. (Table 4.1) Table 4.1: Time spent using mobile phones Frequency Ratio (%) Valid Percentage Cumulative Percentage Alid <1 year 5 .6 .6 .6 1-3 years 194 23.2 23.2 23.8 4-10 years 571 68.4 68.4 92.2 >10 years 65 7.8 7.8 100.0 Total 835 100.0 100.0 The survey indexes on the time of using mobile phones of consumers in the inner city of Hanoi are very impressive for a developing country like Vietnam and also prove that Vietnamese consumers have a lot of experience using this high-tech device. Moreover, with the majority of consumers surveyed having a relatively long time of use (4-10 years), it partly proves that mobile phones have become an important and essential item in people's daily lives. When asked about the mobile phone network they are using, 31% of respondents said they are using the network of Vietel company, 29% use the network of of Mobifone company, 27% use Vinaphone company's network and 13% use networks of other providers such as E-VN telecom, S-fone, Beeline, Vietnammobile. (Figure 4.1). Figure 4.1: Mobile phone network in use Compared with the announced market share of mobile telecommunications service providers in Vietnam (Vietel: 36%, Mobifone: 29%, Vinaphone: 28%, the remaining networks: 7%), we see that the survey results do not have many differences. However, the statistics show that there is a difference in the market share of other networks because the Hanoi market is one of the two main markets of small networks, so their market share in this area will certainly be higher than that of the whole country. According to a report by NielsenMobile (2009) [8], the number of prepaid mobile phone subscribers in Hanoi accounts for 95% of the total number of subscribers, however, the results of this survey show that the percentage of prepaid subscribers has decreased by more than 20%, only at 70.8%. On the contrary, the number of postpaid subscribers tends to increase from 5% in 2009 to 19.2%. Those who are simultaneously using both types of subscriptions account for 10%. (Table 4.2). Table 4.2: Types of mobile phone subscribers Frequency Ratio (%) Valid Percentage Cumulative Percentage Valid Prepay 591 70.8 70.8 70.8 Pay later 160 19.2 19.2 89.9 Both of the above 84 10.1 10.1 100.0 Total 835 100.0 100.0 The above figures show the change in the psychology and consumption habits of Vietnamese consumers towards mobile telecommunications services, when the use of prepaid subscriptions and junk SIMs is replaced by the use of two types of subscriptions for different purposes and needs or switching to postpaid subscriptions to enjoy better customer care services. In addition, the majority of respondents have an average spending level for mobile phone services from 100 to 300 thousand VND (406 ~ 48.6% of total respondents). The high spending level (> 500 thousand VND) is the spending level with the lowest number of people with only 8.4%, on the contrary, the low spending level (under 100 thousand VND) accounts for the second highest proportion among the groups of respondents with 25.4%. People with low spending levels mainly fall into the group of students and retirees/housewives - those who have little need to use or mainly use promotional SIM cards. (Table 4.3). Table 4.3: Spending on mobile phone charges Frequency Ratio (%) Valid Percentage Cumulative Percentage Valid <100,000 212 25.4 25.4 25.4 100-300,000 406 48.6 48.6 74.0 300,000-500,000 147 17.6 17.6 91.6 >500,000 70 8.4 8.4 100.0 Total 835 100.0 100.0 The statistics in Table 4.3 are similar to the percentages in the NielsenMobile survey results (2009) with 73% of mobile phone users having medium spending levels and only 13% having high spending levels. The survey results also showed that up to 31% ~ nearly one-third of respondents said they sent more than 10 SMS messages/day, meaning that on average they sent 1 SMS message for every working hour. Those with an average SMS message volume (from 3 to 10 messages/day) accounted for 51.1% and those with a low SMS message volume (less than 3 messages/day) accounted for 17%. (Table 4.4) Table 4.4: Number of SMS messages sent per day Frequency Ratio (%) Valid Percentage Cumulative Percentage Valid <3 news 142 17.0 17.0 17.0 3-10 news 427 51.1 51.1 68.1 >10 news 266 31.9 31.9 100.0 Total 835 100.0 100.0 Similar to sending messages, those with an average message receiving rate (from 3-10 messages/day) accounted for the highest percentage of ~ 55%, followed by those with a high number of messages (over 10 messages/day) ~ 24% and those with a low number of messages received daily (under 3 messages/day) remained at the bottom with 21%. (Table 4.5) Table 4.5: Number of SMS messages received per day Frequency Ratio (%) Valid Percentage Cumulative Percentage Valid <3 news 175 21.0 21.0 21.0 3-10 news 436 55.0 55.0 76.0 >10 news 197 24.0 24.0 100.0 Total 835 100.0 100.0 When comparing the data of the two result tables 4.4 and 4.5, we can see the reasonableness between the ratio of the number of messages sent and the number of messages received daily by the interview participants. 4.3 Current status of SMS advertising and Mobile Marketing According to the interview results, in the 3 months from the time of the survey and before, 94% of respondents, equivalent to 785 people, said they received advertising messages, while only a very small percentage of 6% (only 50 people) did not receive advertising messages (Table 4.6). Table 4.6: Percentage of people receiving advertising messages in the last 3 months Frequency Ratio (%) Valid Percentage Cumulative Percentage Valid Have 785 94.0 94.0 94.0 Are not 50 6.0 6.0 100.0 Total 835 100.0 100.0 The results of Table 4.6 show that consumers in the inner city of Hanoi are very familiar with advertising messages. This result is also the basis for assessing the knowledge, experience and understanding of the respondents in the interview. This is also one of the important factors determining the accuracy of the survey results. In addition, most respondents said they had received promotional messages, but only 24% of them had ever taken the action of registering to receive promotional messages, while 76% of the remaining respondents did not register to receive promotional messages but still received promotional messages every day. This is the first sign indicating the weaknesses and shortcomings of lax management of this activity in Vietnam. (Table 4.7) div.maincontent .s1 { color: black; font-family:"Times New Roman", serif; font-style: italic; font-weight: normal; text-decoration: none; font-size: 14pt; } div.maincontent .s2 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: bold; text-decoration: none; font-size: 14pt; } div.maincontent .s3 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: bold; text-decoration: none; font-size: 14pt; } div.maincontent .p { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 14pt; margin:0pt; } div.maincontent p { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 14pt; margin:0pt; } div.maincontent .s4 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 9.5pt; vertical-align: 6pt; } div.maincontent .s5 { color: black; 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Thus, the tensile contribution of concrete is as equation (2-79).
sp c
ff(2-79) |
Therefore, the contribution of steel fiber reinforcement to the split compressive strength of steel fiber reinforced concrete is as follows:
L f V fspf d fc
f
0.55 '(2-80) |
The direct tensile strength and split compressive strength of CST concrete proposed in ACI 544-4R88 standard [32] have the following relationship:
2 / 3product
ff(2-81) |
Therefore, formula (2-80) is multiplied by a factor of 2/3, the result of calculating the contribution of steel fiber reinforcement to the axial tensile strength of reinforced concrete is as equation (2-82)
L f V ff D fc
f
0.37 '(2-82) |
2.3.3. Proposed shear strength calculation model of CST reinforced concrete beams and problem block diagram
Model for calculating shear resistance of reinforced concrete beams CST:
From the experimental results of determining the residual tensile strength (after cracking) of CĐCCST concrete as in equation (2-82), substitute this equation into (2-68) and (2-69), the argument is
The project gives the formula to predict the shear strength of CST reinforced concrete beams as (2-83). To consider the influence of beam effects and arch effects if the ratio a/d < 2.5, the formula is multiplied by 2.5d/a as in equation (2-84). From there, the shear strength for this type of beam can be calculated as (2-85) and (2-86).
V f V L f f f , when a/d ≥ 2. 5ncf D cz szcr f
' 0.37' cotton(2-83) | |
V 2.5 d / a ( f ' 0.37 V L f f ' cot f cot ) , when a/d < 2.5; ncf D cz szcr f | (2-84) |
V ( f ' 0.37 V L f f ' cot f cot ) bd , when a/d ≥ 2. 5 ncf D cz szcr etc f | (2-85) |
V 2.5 d / a ( f ' 0.37 V L f f ' cot f cot ) bd ,when a/d < 2.5; ncf D cz szcr etc f | (2-86) |
Calculations based on the model proposed by the thesis can easily predict shear strength without the need for experimental data like some models proposed in international standards. It is completely possible to predict the crack angle and deformation in longitudinal reinforcement in reinforced concrete beams. It is possible to fully investigate the influence of factors such as concrete grade, steel fiber content, a/d ratio, etc. on shear strength through this model.
Block diagram of the problem of calculating the shear resistance of reinforced concrete beams CST
The shear strength of the above high-strength concrete beams according to the model proposed in the thesis follows equations (2-85) and ( 2-83). The block diagram is shown in Figure 2.34.
Begin
Input data: b, h, V f , f c ',f y, L f/ /D f , As, d a
Select ε x (0< ε x <0.003)
Estimated θ: 29 7000 x
Calculate: 0.37 L f V
0.4
f
D
f
f ' ;
c
f
1 1500
(1 V ) ;
f ci
x
0.31
0.18f'c 24w
d a 16
Calculate: f 1 ' cidissolve zf szcr; ' ci ci ffcolumn; f x ( cicolumn f fcolumn) / x
Calculate ε' x =f x /E x
Wrong
Ɛ' x ≈ Ɛ x
When Ɛ' x - Ɛ x <1/1000 stop looping
n
c
f D
c
z szcr
etc.
n
c
f D
c
z szcr
etc.
v
Correct
V ( f ' 0.37 V
L f
f ' cot f
column) bd ,
when a/d v <2.5;
f
V 2.5 d / a ( f ' 0.37 V
L f
f ' cot f
column) bd , when a/d ≥2.5
f
End
Figure 2.34. Block diagram for calculating shear strength for CST reinforced concrete beams
Chapter 2 Conclusion
- Selecting a semi-empirical model to predict the shear strength of CST reinforced concrete beams is very important. There are many semi-empirical models that can predict the shear strength of CST reinforced concrete beams, however, the researcher chose a simple modified Compression Field model to predict the shear strength of CST reinforced concrete beams due to the above analysis.
- In the simple modified compression field model, the tensile work of concrete is considered. For reinforced concrete beams, in addition to the tensile participation of the concrete after cracking, there is also the participation of steel fiber reinforcement.
- As we know, after the concrete cracks, the steel fiber reinforcement begins to participate in tensile strength. The contribution of the steel fiber reinforcement after the concrete cracks depends on the adhesion between the fiber and the concrete. The factors affecting the tensile strength after cracking of the reinforced concrete are fiber content, fiber shape, fiber length and concrete grade. In which, fiber content is the most important factor that greatly affects the tensile strength of the reinforced concrete. Therefore, the construction of the function of tensile strength depending on fiber content and other parameters to evaluate the contribution of steel fiber reinforcement to the tensile strength after cracking was carried out by the researcher. Because it is difficult to directly pull the reinforced concrete sample, the standard cylindrical splitting test was carried out.
- NCS used 2 types of fibers as presented above, with varying fiber content, designed the composition for a high-strength concrete mixture of 70MPa. Adjusted the composition for 7 CST BTCĐC mixes and conducted casting of 105 samples for splitting and 21 samples to test the compressive strength of each calculated mix.
- Based on the results of the split compression test with 105 samples of CST reinforced concrete pillars, using the linear regression method, a formula was developed to calculate the split compression strength of CST reinforced concrete after cracking according to the fiber content (2-82).
- Combining the model with the model for calculating the shear resistance of the selected reinforced concrete beam in section 2.2.3, the researcher has built a formula for calculating the shear resistance of the reinforced concrete beam for the two cases a/d ≥ 2.5 and a/d < 2.5 as in (2-85) and ( 2-83).
- Build a block diagram for the problem of calculating the shear strength of reinforced concrete beams as shown in Figure 2.34.
- Surveying the influence of parameters on the shear strength of CST reinforced concrete beams through the calculation model shows that the shear strength of CST reinforced concrete beams increases rapidly when the fiber content increases. Fiber length also significantly affects the shear strength of CST reinforced concrete beams. The higher the concrete grade, the greater the influence of steel fibers on the shear strength. The inclination angles of the main compressive stress (θ) in the beams calculated according to the proposed model are all less than 45 o .
Chapter 3.
EXPERIMENTAL RESEARCH ON SHEAR BEHAVIOR OF CST CONCRETE REINFORCED BEAMS
Experimental objectives
Chapter 3 studies the experimental behavior of reinforced concrete beams with design dimensions to verify the model proposed by the researcher in chapter 2. The study of the shear behavior of CST reinforced concrete beams with different CST reinforced concrete mixes is to evaluate the influence of fiber content and fiber type on shear strength. In addition, the experimental study on real beams in chapter 3 also aims to evaluate the behavior in the beam including crack angle, deformation in the compressed concrete, deformation in longitudinal reinforcement and stirrups when subjected to load until failure. The beam size is selected to match the jacking capacity and design standards. The beam must be calculated to arrange the main longitudinal reinforcement and stirrups so that the beam only experiences shear or bending failure within the shear span (a) without being damaged by bending. Casting of CST reinforced concrete beams according to the design dimensions is carried out to verify the shear strength calculation formula proposed above. The 4-point bending beam model was used to test the reference beam according to ASTM C78 [39]. The concentrated force positions were placed at a distance a≥2d from the center of the support. Where d is the distance from the center of the tensile reinforcement to the upper edge of the beam. The concrete grade used in the study was 70MPa, which is consistent with the concrete grade of the sample test. DRAMIX steel fibers with double-hook bending at different sizes were used in the study. Two types of fibers were used: Dramix 3D 80/60 BG with a strength of 1225MPa and Dramix 3D 65/35 BG with a strength of 1345MPa according to DIN 17140D9 and EN 10016-2-C9D standards with low carbon content. The experimental results include data on the shear strength of the beam, the load-deflection curve, the deformation of the concrete in the compression zone, the deformation in the longitudinal reinforcement and the deformation in the stirrups. Experimental data were compared with theory including: shear strength, failure model, and inclination angle of main compressive stress.
Experimental beam design
3.2.1. Selection of experimental reinforced concrete beam structure
Beam height: According to the provisions in clause 8.3.4.1 in TCVN11823- 2017 standard [4], which stipulates a simple design method for non-prestressed beams or beams with a height of less than 400mm, the values of θ=45 o and β=2 can be taken. Therefore, this is a special case and not a general case. In the thesis, two types of beams are selected.
with height h=450mm and h=400mm to conduct the survey with the purpose of testing beams of the same size. Beam width is chosen b=150mm, based on reference to some experimental models that the authors have researched and selected.
The distance from the concentrated force to the support is a=750mm for beam H=400mm as shown in Figure 3.2 and a=780cm for beam H=450mm. The effective height d=hd o , where do o is the distance from the center of gravity of the longitudinal reinforcement to the bottom edge of the beam cross-section.
The BTCST mixture is calculated as in Appendix 1. When casting each beam, take a sufficient amount of concrete mixture to cast a cylindrical sample to check the concrete grade of that beam.
Sample 2
The number of main reinforcements and the strength of the main reinforcements are selected so that when tested, they are only damaged by shear and not damaged by bending. The longitudinal reinforcement is selected from steel with a diameter of 22mm, with a quantity of 4 bars. The longitudinal reinforcement in the beam uses steel from Hoa Phat Company, with a steel grade of 520 MPa according to TCVN 1651-2008 or ASTM A6115/A615M-08a (USA). The steel samples were taken for testing at the Testing Center of the University of Transport. The results of the steel tensile test are shown in Figure 3.1. The average yield strength of the test samples is f y =512MPa.
Sample 1
Tensile stress, MPa
Tensile stress, MPa
Tensile stress, MPa
Tensile strain, %
Tensile strain, %
Sample 3
Tensile strain, %
Figure 3.1. Stress-strain relationship diagram during main longitudinal tensile test