Mechanical Properties of Silica Fume Concrete in Marine Environment

Concrete is a common material used in marine structures. Cement, as a concrete-forming substance, contributes to global CO2 emissions. Reducing cement by adding Silica Fume transforms concrete into green concrete without reducing its quality. Concrete construction in the marine area is commonly affected by abrasive factors such as water penetration, waves, and sea currents. Concrete materials have good quality, as shown by their mechanical properties. This study examined how Silica Fume affected the mechanical characteristics of concrete. The specimens were made with variations of Silica Fume 5%, 7%, and 10% by weight of Cement, then cured with seawater. Tests are carried out for mechanical properties such as compressive strength, flexural strength, permeability, and abrasion mass loss. The test results showed that the compressive and flexural strength values achieved good results, and the weight loss and abrasion coefficient values were better than ordinary concrete.


Introduction
Concrete in the marine environment often experiences damage due to harsh environmental components such as water penetration and abrasion by waves and ocean currents, concrete material with better concrete performance and mechanical properties is needed.[1].The performance of coastal structures can decline due to marine environment actions like sea waves.The cyclical force of sea waves will affect buildings along the seaside, eroding concrete structures [2].In addition to these environmental considerations, vehicle and cargo operations at the wharf also impact the harbour's construction, particularly for plates subject to vehicle wheel-induced surface erosion/abrasion.
Marine construction needs more precise, defined requirements for concrete construction, such as high quality and low permeability, or, in other words, a low void ratio.The maximum water penetration limit is 5 cm for medium aggression and 4 cm for high aggression, according to DIN 1045-2.Because the chemicals in seawater are damage to construction, these structures need waterproofing materials to prevent chloride penetration into the concrete [3].
Concrete used in marine conditions must meet better quality for compressive strength than construction on the land.According to studies on how seawater affects concrete's compressive strength, immersing concrete consistently for 28 days will lower that strength's value.The compressive strength of concrete submerged in seawater in this investigation after seven days was 200 MPa, but by the end of the test, it had decreased to 140 MPa.[4,5] Concrete technology innovation is always needed to meet the need for high-performing concrete.Without lowering economic value, the innovative concrete is supposed to be of better quality and durability in corrosive conditions.Incorporating a part or different components into the Cement makes it possible to produce high-quality concrete resistant to extreme conditions [6].Cement manufacture, a component in creating concrete, is closely related to using concrete in buildings.A large amount of CO2 emissions will be produced during the production of Portland Cement [7].Blended Cement, which substitutes Portland Cement (PC) partially or entirely, is predicted to cut PC production, significantly lowering carbon dioxide emissions and mitigating the effects of global warming.Environmental, economic, and social effects are three sustainability elements that can be resolved by green concrete.The amount of PC replacement material, production process, method, performance, and sustainability impact are the major indicators of green concrete.[8].
This study examines how Silica Fume blended cement affects concrete's mechanical characteristics in marine environments.

Materials
Silica Fume (SF) is produced when quartz and coke are reduced in an electric furnace to create silicon and ferrosilicon alloys.Most Silica Fume waste was released into the atmosphere before the mid-1970s.Despite its environmental impact, research findings show that Silica Fume can be combined with other concrete ingredients.Silica Fume can be added to concrete, enhancing its performance [9].
Adding Silica Fume into concrete can also increase the concrete mechanical properties.However, replacing cement with Silica Fume, which exceeds 10% of cement's weight, can reduce the strength of and elasticity modulus of concrete.The compressive strength value can be increased by adding 5% or more Silica Fume.[10].In this study, the amount of replacement of cement with Silica Fume in concrete was 5%, 7%, and 10%, which was determined based on the results of several other previous studies.
The cement used is type II, which is a type of cement that is resistant to moderate Sulfate influences [11].The coarse aggregate used comes from quarries Bale Endah Bandung, and fine aggregate comes from Cimalaka, Sumedang, Indonesia.

Concrete Mix Design
Concrete mix planning to determine specifications for the concrete mix using SNI 7656-2012.Based on SNI 2847 -2019 for concrete with exposure class S1 and cement type II, the minimum f'c value required is 28 MPa.Construction in a marine environment requires a concrete compressive strength value exceeding 28 MPa.In this study, the compressive strength value of the planned concrete was 30 Mpa.
The results of testing the concrete ingredients give the results of the composition of the concrete mixture as follows: Following the mixing of the concrete materials, a slump value test is performed to determine the significant viscosity level related to the workability of the concrete.The slump value obtained from the three types of mixture variations is 9 cm for the 5% Silica Fume mixture, 9.5 cm for concrete with 7% Silica Fume, and the slump value is 7.5 cm for the 10% Silica Fume variation.The slump value achieved still meets the requirements.The slump value of concrete with various Silica Fume mixtures is as follows:

curing
Specimens were treated with two treatments: immersing them in fresh water and partly in seawater.Immersion of seawater specimens aims to determine seawater's effect on concrete performance.The seawater source comes from Ranca Buaya Beach, Garut Regency, Indonesia.The test results on seawater salinity amounted to 6.83%.The immersion of the specimens was adjusted to the compressive strength and flexural strength tests, namely 28 days, 56 days, and 90 days.

Parameter
Test Method units Results

Salinity Test Results
Alpha

Compressive Strength Test
The concrete cylinder specimen has a 15 cm diameter and a 30 cm height.The compressive strength test followed the Indonesian Testing Standard.Compressive stress (f'c) is expressed as follows: Where P is load and A is cross section of specimen concrete

Flexural Strength Test
The flexural strength test is carried out using a simple test on concrete, given a load of two points above the beam.The test uses the SNI 4431-2011 and ASTM C1609 standards.The flexural test of the concrete used is the 4-point bending test.In this test, the loading of the test object beam is given at each junction point of the concrete beam span.The flexural strength test object is loaded until the concrete beam collapses or breaks.The specimen's dimensions are 15 x 15 x 60 cm.The flexural strength value of concrete is calculated according to the following formula:

Abrasion Test
The Tanifuji Concrete Abrasion Machine Manual and SNI 3419-2008, describing laboratory abrasion tests, are the primary reference documents used to conduct wear tests.The test object is a block with the following measurements: width x length x height = 4 x 6 x 15 cm.
The samples were tested in three stages: one, two, and three hours.To determine the weight loss due to abrasion testing, the weight of the test object is measured at each time step.The abrasion volume (Vn) and concrete coefficient are calculated using the following formula.
(3) W0 is the mould and specimen weight pre-testing (kN), and Wn is the mould and sample weight following testing (kN).

Coefficient of abrasion
Where K is the Abrasion coefficient (cm 3 /cm 2 ), V is the volume of the sample (cm 3 ), and A is the sample surface area (cm 2 ) Where Ka is the abrasion coefficient (cm3/cm2), and Ki is the abrasion coefficient of each specimen (6 pieces).

Compressive Strength Test Results
The compressive strength concrete samples are three for each variation and treatment type immersed in fresh or seawater.The samples were tested for 28, 56, and 90 days.SF concrete's design compressive strength (f'c) was attained after 28 days.The targeted average compressive strength value (f'cr) was achieved by all concrete with replacement variations of 5%, 7%, and 10% at 28 days.
The seawater-immersed SF concrete with the best strength value is a 10% substitution in concrete.The average compressive strength concrete value is 48.3 MPa.SF concrete immersed in freshwater with 7% SF replacement is the best SF concrete with the biggest compressive strength value at 90 days.Ordinary concrete submerged in freshwater is referred to as COC, ordinary concrete submerged in seawater as COS, Silica Fume substituted concrete soaked freshwater as CSC, and Silica Fume concrete soaked seawater as CSS.The percentage value indicates how much Silica Fume has replaced the concrete's cement weight.
Figure 4 shows that the Silica Fume substitution slightly decreased the compressive strength value compared to ordinary concrete on the 90th day.However, the compressive strength achieved exceeded the f'c and f'cr, so it still meets the quality requirements of concrete for use.The results of obtaining seawater-immersed concrete's compressive strength are marginally lower than freshwater-immersed concrete but still exceed the intended concrete's quality, and this matter shows seawater's impact on concrete.

Flexural Strength Test Results
Flexural strength testing is carried out to determine the flexibility of concrete against bending moments using a Flexural Testing Machine.Flexural test beam specimens underwent flexural strength testing after 28, 56, and 90 days in concrete.CFOC is a bending test beam soaked in fresh water, and CFOS is a bending test beam soaked in seawater.CFSC is a bending test beam soaked in fresh water, and CFSS is an immersed beam in seawater; the percentage number shows the amount of Silica Fume substitution for the weight of cement in the concrete.
The flexural strength value of concrete with Silica Fume also has a range that is not too far from the value of ordinary concrete, where the flexural strength achieved at 90 days is in the field of 5.0 MPa to 6.79 MPa.

Abrasion Test Results
The abrasion test was carried out for three hours, where the weight loss was recorded every hour of the test.The concrete abrasion test results are as follows: COAC is a specimen of abrasion immersed in freshwater, and COAS is a specimen immersed in seawater.Additionally, CSAC represents freshwater specimens and CSAS represents seawater specimens.The results of the abrasion test findings on concrete containing 5% Silica Fume showed that mass loss due to abrasion increased slightly from 107.2 gr in ordinary concrete (COA) to 120.2 gr in concrete containing 5% Silica Fume (CSAC-5).The average weight loss by abrasion in 10% Silica Fume substitute concrete was 11.97 gr.
The average weight lost from abrasion in concrete immersed in seawater decreased from 130.17 gr in ordinary concrete (SOA) to 126 gr with 5% SF substitution (CSAS-5) and 115.4 gr with 7% SF substitution (CSAS-7).With 10% SF (CSAS-10), the average mass loss value of concrete increased to 133.8 gr.
The abrasion coefficient (Ka) of Silica Fume concrete submerged in seawater with cement replacement is 5% and 7% higher than OPC submerged in seawater.This data shows that 5% and 7% Silica Fume substitution makes concrete more resistant to abrasion than ordinary concrete affected by seawater.

Relationship between compressive strength and modulus of elasticity of concrete
The modulus of elasticity of concrete is a principal factor for estimating building deformation and determining the modular ratio, n, used for the cross-sectional design of structural components subject to bending.The modulus of elasticity of concrete is directly proportional to the square root of the compressive strength of ordinary concrete strength.
Calculation of the elastic modulus of concrete uses two ways: first, using the basic equation, which is a function of the compressive strength and specific gravity of concrete [12]; the second way is to use the empirical formula in SNI 2847 -2019 and ACI 318 which is a function of the compressive strength of concrete during the compressive strength test.
Where Ec is the elastic modulus (MPa); k1 is the correction factor for coarse aggregate; k2 is the correction factor for added materials (substitutes);is the compressive strength of concrete (MPa), and  is the specific gravity of concrete (kg/m 3 ).The following table shows the gain of Ec based on the compressive strength and age of the concrete.The modulus elasticity of concrete substituted with Silica Fume for various variations at 28 days of age has an Ec value range of 24,161 MPa to 26,407 MPa.The Ec value for multiple variations of Silica Fume is 25,996 MPa to 27,2903 MPa at 90 days of concrete.These Elasticity Modulus values are below ordinary concrete because of the added material correction factor of 0.95.The Ec value from the calculation results is also below the Ec value of SNI 2847-2019.However, Noguchi provides a limit range for the confidence interval value for the elastic modulus from the research results; it can increase the calculated values by 20%.The results of the Ec value after correction and comparison with the Ec value based on SNI 2847 or ACI 318 have an intense correlation value.In concrete with the addition of Silica Fume, the correlation between the calculated Ec value and the SNI 2847 Ec has a correlation coefficient value of r = 0.99, meaning that the two values are very close, and the coefficient value achieved is intense.The correlation coefficient Ec and fc' calculation results are also intense, namely r = 0.99.Overall, the elastic modulus value of concrete with Silica Fume increases linearly with increase in compressive strength.
Another researcher also researched the concrete compressive strength with substitute materials for concrete of different qualities.The results achieved for all concrete variations were that the compressive strength at 28 days reached the maximum value at a mixture of 30%.[13] The research conducted also did not discuss the achieved concrete Ec value.The use of different types of cement is a possible factor that makes the difference in the optimum variation in this research from the results of other researchers where other researchers used OPC type I, while this study used OPC type II.

Compressive Strength and Flexural Strength Relationship
An intense correlation exists between Silica Fume substitute concrete's compressive and flexural strength.The test results both have a strong correlation level, as shown by the analysis of the flexural strength and compressive strength data variables; the coefficient of correlation for f'c and  still within the intense correlation interval (r =0.56).

Figure 8. Relationship between fc' and 
The compressive strength and flexural strength relationship in concrete is linear; an increase in compressive strength will also increase flexural strength and vice versa.Nath et al. researched compressive strength and flexural strength in concrete, where the relationship both of them is in the form of a linear function where compressive strength will increase as flexural strength increases.[14]

Compressive Strength and Abrasion Coefficient Relationship
SF concrete substitution has a coefficient of abrasion (ka) of 0.25 cm 3 /cm 2 to 0.28 cm 3 /cm 2 .For ordinary concrete with seawater immersed, ka reaches 0.29 cm 3 /cm 2 ; when SF substitution is given to the concrete, ka decreases to 0.25 cm 3 /cm 2 at a 7% SF variation.The abrasion coefficient of concrete can be lower if Silica Fume is added to concrete soaked in seawater.

Conclusion
Concrete with Silica Fume substitution can reach and meet the compressive strength (f'c) and the aveage compressive strength target (f'cr).Concrete with SF replacement has the highest compressive strength value at 5% SF addition with fresh water immersion, namely 46.7 MPa after 28 days and 49.4 MPa after 90 days.The best compressive strength findings in seawater-immersed concrete were obtained with the 5% SF replacement variation of 40.7 MPa after28 days and increased to 47.7 MPa after 90 days of concrete age.Flexural-strength value  achieved beyond the required flexural strength value.The weight loss and abrasion coefficient values are better than ordinary concrete in Silica Fume substitution concrete soaked in seawater.Overall, the mechanical properties obtained by Silica Fume substitution concrete reach the required values and have better abrasion resistance in seawater.

Acknowledgments
This research was conducted with Balai Besar Bahan dan Barang Teknik (B4T) Bandung and Balai Sabo, Yogyakarta, Indonesia.We are grateful for the cooperation and assistance of all colleagues.

Figure 9 .
Figure 9. Relationship between f'c and Ka.

Figure 9 .
Figure 9. describes that decreasing f'c values due to adding SF and immersion in freshwater has little effect on the abrasion coefficient achieved.Ka value slightly increases with adding SF variations and the decrease in concrete compressive strength.Ayoob et al. also analyzed the abrasion rate due to several factors, namely different cement types.The study used CEM I and III cement types, water/cement ratio influencing factors, Silica Fume addition, and fiber addition factors.According to their research, adding Silica Fume to concrete is favorable because it will lessen mass loss from abrasion.[15] [1] Pratiwi W D Putra F D D Triwulan Tajunnisa Y Husin N A and Wulandari K D, 2021 A review of concrete durability in marine environment IOP Conf.Ser.Mater.Sci.Eng.1175, 1 p. 012018.[2] Wahyudi S I and Adi H P, 2018 Evaluating Environment, Erosion and Sedimentation Aspects in Coastal Area to Determine Priority Handling (A Case Study in Jepara Regency, northern Central

Table 1 .
Silica Fume Variation Concrete Mix Design per m 3 .