# Why is larger concrete cover normally used in marine concrete?

Knowing the dry density of soil or pavement is important to evaluate the degree of compaction achieved during the construction process. A field density test is a common test used to determine the field density of the soil or pavement. The principle of field density test is the replacement of soil excavated materials by the sand, the sand density is known so we can calculate the volume of the sand required to fill the hole. By knowing the volume of the hole excavated in soil and the weight of soil, we can calculate the in-situ density. The in-situ density equals the weight of excavated materials divided by the volume of excavated material. Apparatus  Sand pouring cylinder equipment

Small sand pouring cylinder equipment: this suitable for fine and medium-grained soil. It consists of a cylinder having a diameter of 100 mm and a length of 380 mm with an inverted funnel at one end and a shutter to open and close for the entry of sand. Also, there will be a metal tray for the excavation of s…

Design a one-way slab for the inside of a building using the span, loads, and other data given in Figure 1. Normal-weight aggregate concrete is specified with a density of 145 pcf. (assuming cover 3/4 in) Figure 1

The minimum thickness for one-way slab simply supported=L/20 using table 1(ACI 9.5.2.1) h=10/20=0.5=6in

Table 1

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now we will calculate d=(6-3/4(cover)-1/4(estimated as half diameter of reinforcement) d=5 in
now we will calculate dead load concrete density=145pcf, Usually 5 pcf is added to account for the weight of reinforcement, so 150 pcf is used in calculating the weight of a normal-weight concrete member. dd=(6in*ft/12)*1*150/=75 lb/ft LL=200*(1)=200lb/ft Wu=1.2*75+1.6*200=410 lb/ft
maximum moment for simply supported span =(Wu*L^2)/8 =(410*10^2)/8=8/=5,125lb-ft=61,500lb-in
now we can calculate ρ
ρ=(0.85*fc’/fy)*(1-√(1-(2*Rn/0.85*fc’))

Rn=Mu/(ϕ*b*d^2) Rn=(61500)/(0.9*12*(5^2))=227.7

ρ=… Da structure can be called determinate if the equilibrium equation provides enough and sufficient condition for equilibrium. if all forces for a structure can be determined using equilibrium equations only, this structure is determinate, but if the unknown forces are more and cant be determined using equilibrium equations then this structure is indeterminate.in general we can determine if a structure is statically determinate or indeterminate by drawing free body diagram for a structure or part of it and comparing a number of unknown forces and moment component with number of available equation of equilibrium. for coplanar structures, we have  three equilibrium equations, if n is the total number of parts and r is the number of unknown forces and moment component then:

r=3*n determinate r>3*n indeterminate

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statically indeterminate structure cant be solved using equilibrium equations, so…

In a truss system, some members are not carrying any force. This called zero-force member. This member may added to increase truss stability. Identifying these members will simplify the process of analyzing truss. the determination of zero force member can be done by inspecting of truss joints, and there are two casesif we check the joint c for truss shown in figure 1, this joint has no external load, and two members are connected to this joint at a right angle. if we sum force in y-direction Σfy=0, we get FCD=0, similarly in x-direction FCB=0. For joint A, no external load applied to this joint. If we sum forces in y-direction Σfy=0, we get FAB=0. Similarly, in x-direction FAE=0. Figure 1

A quick look at this truss. We can notice joint c and joint d with no external load. Inspecting joint C will be a bit difficult because 4 members are connected to joint c. Starting with joint D. member DF is perpendicular to member DE and CD. if we sum the forces in the y-direction. As showing in fig…

Pile cap used to transfer the loads from superstructure to the piling. The pile cap is thick concrete mat rests on piles. It is part of the foundation and used to distribute the loads over the piles. Piles used when the soil bearing is not enough to carry loads of the structure.

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Pile cap construction procedure Excavation around piles: After 7 days of the casting of the pile the concrete will gain approximately, 70% of it is compressive strength. The soil around piles will be removed to the required levels and dimension that allows construction of the pile cap. A step shall be made if the excavation is more than 1.5 m to prevent soil collapse. Pile head cutting: Concrete will be removed up to cut off level. If concrete under the cut off level is unsound. It shall be removed, and the pile will be repaired. Pile must be free from cracks and unsound concrete.

Blinding concrete: after completion o…

Flaky and elongated aggregates can adversely affect concrete. Flaky and elongated aggregates are difficult to compact. Therefore using flaky and elongated aggregates will reduce concrete workability, which required more amount of water to produce a workable mix. Increasing water content will reduce concrete strength and durability. Moreover, these aggregates are not robust, which leads to lowering concrete strength. Figure 1

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The Flakiness Index of aggregates is the percentage by weight of particles whose least dimension (thickness) is less than three- fifths (0.6times) of their mean dimension. This test does not apply to sizes smaller than 6.3mm.

The Elongation index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than nine-fifths (1.8times) their mean dimension. Flakiness and elongation tests are not applicable to sizes smaller than 6.3…

Let consider a slab supported by beams and columns, to begin structural analysis for each member, we need to know the amount of load transferred to this member, the amount of load transferred to any member can be determined using tributary area(loading) concept. the geometry of a structure will determine the shape of the tributary area. figure 1

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In a one-way system, slab delivers the load to supports by one-way action. the dimension of slab shall satisfy the following equation to act as one-way slab L2/L1>2. let consider floor system in figure 3, L2/L1 for beam AB L2/L1=15/5=3, therefore slab will transfer the loads to beam in one-way action, tributary area for beam AB, E-f will be as shown in figure 3, loads of 100*2.5=250Lb/ft(linearly distributed over the whole length of the beam, for beam CD, the load will be 100*5=500Lb/ft, beam CD will share loads from both sides because it is lo…

The combination of structural elements and materials called structural system. the structural system may be composed of one or more of the four basic types of structure: Truss: truss used for structure with large span where the depth is not important. a truss consists of slender elements, arranged in a triangular shape, planner truss refers to truss where all elements lie in the same plane frequently used for bridges. space truss is composed of elements extending in three dimensions. Truss converts loads that cause truss to bend into tension and compression forces. due to this advantages truss used fewer materials compared to beam, also it is composed of slender and long elements, Truss can be used to cover spans from 9m(30ft) to 122 m(400ft).

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bridge construction:How to become a bridge engineer Figure 1

Cable and arches: cables are used to support long span where the using of truss is not feasible and could lead to a significant increase in …

strength reduction factors used to reduce the calculated strength of structure member to account uncertainties arises from the material used, inaccuracies of placing steel, inaccurate dimensions of a structure member, inaccurate design calculation and any other factor. ACI code 9.3 provide different values of ∅ for different situation. I will mention some of these values0.90 for tension-controlled beams and slabs0.75 for shear and torsion in beams0.65 or 0.75 for columns0.65 or 0.75 to 0.9 for columns supporting very small axial loads0.65 for bearing on concrete

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For compression controlled section ∅=0.65 because the section is brittle. it will fail suddenly without any major deflection or warning. in addition to lower ductility, compression controlled sections are sensitive to concrete strength variation. also, compression controlled section will supporter larger loads compared to tension c…

The cracking moment is the moment corresponding tensile bending stress at which concrete will start to crack. reinforcing steel cross-sectional area is 2% or less of concrete beam cross-section. the area of reinforcing steel is minimal compared to concrete. therefore, it will not cause a major change to concrete beam properties as long as the beam uncracked. bending stress for concrete at distance y from a neutral axis can be calculated using elastic beam theory

f=M.y/I

where  f is the bending stress at distance y M is the moment applied to the beam y is the distance from the neutral axis   I is the moment of inertia for beam

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bridge construction:How to become a bridge engineer course concrete modulus of rupture is defined as the bending tensile strength at which concrete start to cracks. ACI provides us with the equation for calculating the modulus of rupture

fr=7.5λ√fc’ (in psi) fr=0.7λ√fc’ (in Mpa) where λ is equal 1 for nor…