Materials I - Concrete

Concrete sets in three basic stages. State the time in days and % of strength achieved at the end of each stage. Explain the significance of each stage.

Concrete cures by hydration, the chemical bonding of water and cement, and not by simple drying. The curing reactions take place over a long period of time. Concrete is commonly designed on the basis of the strength that it reaches after 28 days. I. 3 days lead to 50% of its design strength - can walk on concrete. II. Minimum of 7 days of 75% of the specified compressive of flexural strength is attained. III. 28 days - 99% of its design strength, able to remove from framework

What is the significance of the water/cement ratio? Explain each point.

I. Lower water-cement ratio leads to better strength and durability. Yet it makes it more difficult to place cleanly into form work. This can be remidied by using plaztisizers. Lower water/cement ratios make concrete that is denser and stronger, it also shrinks less during curing. II. High water-cement ratios are often favored by concrete workers. It produces a fluid mixture that is easy to place, but can be lower quality in strength and surface qualities. It is important that concrete is formulated with the right quantity of water for each situation, enough to ensure workability but not not effect the properties of the cured material. For common concrete applications the water-cement ratio ranges from .45 to .60 by weight, meaning that the weight of the water in the mix does not exceed 45-60 percent of the cement,

define and explain the following terms Slump test Cylinder test

I. Slump Test measuring concrete slump. The hollow metal cone is filled with concrete and tamped with a rod according to a standard procedure. The cone is carefully lifted off allowing the wet concrete to slump under its own weight the slump is measured is the manor shows in the drawing. II. Cylinder Test. inserting a standard concrete test cylinder into a structural testing machine where it will be crushed to determine its strength.

what are three benefits of "air entraining" when applied to concrete. Explain

I. WORKABILITY air entraining improves the work ability during the placement of the concrete. The bubbles work as a lubricant for the aggregate and large particles in the concrete mix. II. DOES NOT COMPROMISE STRENGTH with the approperate adjustments to the formulation of the mix, lighweight air-entrained concrete can achieve the same structural strength as normal concrete. III. FROST RESISTANCE air entraining reduces freeze/thaw damage by providing openings for the freezing water to expand into.

Name the 4 major effects of aggregate on concrete.

I. WORKABILITY Round, smooth aggregate gives the best work ability, because the particles with sharp corners tend to interfere with each-other as the concrete flows. II. DENSITYGrading the mix with different sized aggregates allows the concrete to pack together more efficiently. If all of the aggregate were the same size then the gaps would be lift between the particles. III. WEIGHT a. structural lightweight aggragates are made from materials such as shale which is crushed heated and plastic. During the heating process water expands the soften particles making them porus. This results in a 25% less dense concrete than otherwise, nut not nearly as strong. b. Non structural lightweight concrete are made for roof toppings which results in a concrete which is 75-89% less dense than otherwise. This is made from expanded vermiculite or perlite IV. COST In order to keep the cost of the concrete down, the mix design should call for as much aggregate and as little cement as possible, while keeping in mind strength and other factors.

for concrete slab finishing define the following terms strike off floating troweling control joints

STRIKE OFF first step. Draw a stiff plank of wood or metal across the top edges of the formwork to achieve a level concrete surface. This is done end to end in a sawing motion of the plank that avoids tearing the projecting pieces of the course aggregate. as the accumulated concrete is pushed along the surface, the excess will fill out the lower points in the concrete surface. Floating second step. After the concrete is stikes off, the slab recieves the inital floating. This is performed by hand using flat surfaced tools 4-10 feel long called bull floats or darbies. They pass these tools along the surface to flatten and consolidate its surface. This step levels the surface but still leaves a rough texture on the surface, and depending on the application can be left to cure ot further smoothing can occur, Troweling third step. This step ensures a completely smooth, dense surface. This is done using a smooth rectangular trowel or a rotary power trowel. This tool is run along the surface removing discrepancies, and rough texture. Control joints. control joints are placed in concrete slabs to control random cracking. As the concrete cures, shrinkage occurs. When shrinkage is restrained tensile stresses develop within the concrete. Control joints are places within the concrete to create weakened planes where the concrete can crack in a straight line below the finished concrete surface.

Define and state the importance of a "well graded mix" + Draw a section through a sample showing fine and course aggregation

Well Graded an aggregate that incorporates sizes from the maximum to the minimum specified so as to fill most of the voids. A well-graded aggregate is economical. Concrete that is well graded is easier to place, compact and finish. Strength, the aggregate is what lends the concrete mix the majority of its compresive strength, so in a well-graded mix, the aggregate should be held in suspension, will as little cement as possible. This aggregate should be held in suspension with as little cement as possible, acting as a binder to achieve maximum structural capacity.

Advantages and Disadvantages of concrete

adv. DURABILITY: Concrete is durable and long lasting in buildings. It is not likely to corrode, decay or rot. It also has a relatively low maintenance requirements. FIRE RESISTANT: concrete is non-combustible which makes it fire safe and able to withstand high temperatures. VERSATILITY: concrete has the ability to be shaped and molded or cast into almost any desired shape. disadv. CRACKING: Concrete has a low tensile strength and must be reinforced heavily. It also expands and contracts with changes in temperature, hence expansion joint's bust be provided to avoid cracks. WEIGHT: Concrete is a heavy material and requires a large amount of steel construction to compensate for its self weight. MOISTURE: Concrete is not entirely impervious to moisture and contains soluble salts which may cause efflorescence.

Name three examples of supplementary cementitious materials (SCM's) for concrete. And state their effects on concrete

fly ash: A fine powder that is waste product from coal-fired power plants. The most common pozzolan in concrete. It is used as a replacement level of 15-20% of cement. Benifits. It increases the concrete strength and decrees permeability, reduces temperature during curing, reduces the amount of mixing water needed, and improves workability. Slag cement: A by-product of the steel industry. it is formed during the liquidification of iron in the blast furnace. It is used as a replacement up to 50% benifits: It improves workability, increses strength, reduces permibility, reduces tempature during curing and improves sulfate resistance. Silica Fume: A by-product of electronic semiconductor chip manufacturing. Benifits: high-strength concrete and low permability

define and discuss creep as it pertains to concrete

in addition to plastic and drying shrinkage, concrete is subject to long term creep. When placed under sustained compressive stress from its own weight, the weight of the other permeate building components, or the force of pre-stressing, the concrete will gradually and permanently shorten over a period of months or years. In some circumstances, this dimensional change is of sufficient magnitude that it must be accounted for in the design and detailing of the building. frame. The shrinkage of the concrete combine with other factors affecting the movement of the masonry require that horizontal movement between the cladding and the building structure. If these joints are not provided if if they are too narrow and have insufficient capacity to absorb movement, the cladding system can fail is it becomes compressive, in part, by shortening of the concrete. structure. As a rule of thumb site cast concrete building frames an be expected to shorten in height under the influence of their won weight and other dead leads at the rate of 1.16th inch for every 10 feet of building height.

define, contrast and explain the following using sketches pre-tensioning post tensioning

pre-tensioning pretensioning is used with precase concrete members. High strength steel strands are stretches tightly between abutments in a precast plant. The concrete is cast around the stretched steel. After the concrete has cured to a specified strength, the steel strands are cut from either end. This releases the external tension on the steel, allowing it to recoil slightly, which squeezes all of the concrete of the member into compression. this allows the member to take on a decided camber (lenghwise arching) which counters the loards wich the member will be subsequestly subjected to. The abutments needed to hold the tensioned strands prior to pouring the concrete are expensive to constuct unless done at a larger scale. This is why this practice mostly takes place at a precast plant. Post-tensioning Post tensioning occurs within the building sight. The steel strands are covered in a steel or plastic tube to prevent them from bonding to the concrete, and are tensioned until the concrete has cured. each strand is anchored to a steel plate embedded in one end of the beam or slab. A hydraulic jack is used to tension the strand, which is then anchored to a plate on the other side of the member before the jack is removed.