Materials - Concrete Test

Concrete sets in 3 basic stages. State the time in days, the % of strength achieved, and the significance of each stage

Concrete cures by hydration, the chemical bonding of water and cement, not by drying. Curing takes place over a long period of time. Concrete is designed for the strength it reaches over 28 days. [1] At 3 Days - 50% of the design strength. Can walk on [2] At 7 Days - 75% of the specified compressive strength. [3] At 28 Days - 100% of the design strength. Able to remove formwork

Define and Discuss Creep in Concrete

Concrete is subject to plastic and drying shrinkage and CREEP. When placed under longterm compressive stress, concrete will shorten over months or years. Generally, concrete shortens 1/16" for every 10 1/2 feet of building height.

Define and Explain: Slump Test and Cylinder Test

Slump Test: a metal cone is filled with concrete and tamped with a rod. The cone is then lifted off, and the amount that the concrete slumps is measured compared the the height of the cone. Cylinder Test: Inserting a concrete test cylinder into a structural testing machine, where it will be crushed to determine its strength.

Name three examples of Supplementary Cementicious Materials (SCMs) for concrete and their effects

Supplementary cementing materials contribute through hydraulic and pozzolanic activity. Pozzolans - react with Tricalcium Aluminate (CA3) and control the setting time and comp. strength. They are industrial by products, and their use as a partial replacement reduced the cement needed. 3 SCMs: [1] Fly Ash - a fine powder that is a waste product from coal-fired plants. It replaces 15-20% of cement in concrete. Benefits: increases strength, reduces permeability, increases Sulfate resistance. [2] Slag Cement - a by-product of steel manufacturing. It replaces 50% Benefits: improved workability, increases strength, reduces permeability [3] Silica Fume - by product of electronic semiconductor chip manufacturing Benefits: extremely high strength concrete that has low permeability.

Define and state the importance of a "well graded mix; draw a section through a sample showing fine and course aggregate"

Well Graded - an aggregate that incorporates sizes from the maximum to the minimum specified so as to fill most of the voids. [A] It should be more economical, and require less resources. [B] The aggregate is what lends the concrete mix the majority of its compressible strength.

Concrete Advantages and Disadvantages

[1] Concrete is durable material, long lasting, suitable for reuse. [2] Concrete is non-combustible, makes it safe and able to withstand high temperatures [3] Concrete has the ability to be molded and cast into almost any desired shape. [1] During construction, other materials must be used for formwork and reinforcing. [2] Concrete cracks really easily, because it has low tensile strength [3] Concrete is heavy in weight and needs a lot of steel [4] Concrete can have efflorescence.

What are the 3 benefits of "air entraining" concrete, and explain

[A] Improved workability, during placement of concrete. The bubbles work as a sort of lubricant for the aggregates and large particles. [B] With adjustments, lightweight air entrained concrete can achieve the same structural strength as normal concrete. [C] Air entrainment improves frost resistance, reduced bleeding, permeability and laitance Bleeding - movement of water to the surface of a slab during curing Laitance - the movement of fine aggregate to the surface of a slab during curing Frost Resistance - Air Entraining reduces freeze-thaw damage by giving water a place to expand in.

12. Define, contrast and explain the following

[A] Pre-tensioning Prestressing concrete is accomplished in two different ways. Pre-tensioning is used with precast members: - High strength steel strands are stretched tightly between abutments in a precasting plant, and the concrete member is cast around it. - After the concrete has cured to a minimum strength, the strands are cut off at either end of the member. [B] Post Tensioning - Unlike pretensionsing, which is done in a factory, post-tensioning is done on site. - High strength steel strands (called tendons) are covered so they don't bond with the concrete. The cables are slack before being tensioned. - Each tendon is anchored to a steel plate at each end - Once the concrete has cured, A jack tenses up the steel cable, and the concrete, and the concrete is compressed.

Name 4 Major Effects of Aggregate on Concrete

[A] Round Smooth aggregate [B] Grading a mix with different sized aggregates allows the concrete to pack together more efficiently. If all the aggregate were the same size, then gaps would be left between particles. [C] The composition, shape and size of aggregate affect the workability, durability, strength, weight and shrinkage. [D] Aggregate influences the appearance of the cast surface. [E] Lightweight aggregates - "structural lightweight concrete is made from shale (crushed and heated) and plastic. 25% less dense than normal concrete, but nearly as strong

For Concrete Slab Finishing, Define...

[A] Strike off/screed. The 1st operation in finishing the slab is to strike off or screed the concrete by drawing a stiff plank of wood or metal across the top edges to create a level surface. This is done in a sawing way. [B] Floating - immediately after striking off concrete, the slab receives floating. This step is usually done by hand. If there is going to be a finish applied to the top of the concrete slab, like tiles, it may be left unfinished [C] Trowelling - for a completely smooth, dense surface, the slab must also be trowelled. This is done immediately after the second floating either by hand or using a smooth, rectangular steel trowel. [D] Control Joints - control joins are placed in concrete slabs to control random cracking. A fresh concrete mixture is fluid but as it hardens there is reduction in volume or shrinkage. When shrinkage is restrained, tensile stress develops. Control joints are placed where there will be weaknesses.

What is the significance of the water/cement ratio? Explain

[a] Lower water-cement ratio leads to better strength and durability. Low water cement ratios make denser concrete and stronger that shrinks less during curing. [b] High water-cement ratios are often favored by concrete workers, because its easier to pour. But the concrete is weaker. [c] Concrete must be made with the right amount of water Usually, the water cement-ratios range from 0.45 to 0.60 by weight. The weight of water is between 45 to 60% the weight of the cement.

Draw / Render a cross section through a concrete beam noting the following terms and explain the purpose of each.

[picture is part of Divyas study guide] [a] Concrete - provides compressive strength [b] rebar - provides tensile strength. 0.5" in diam. technically it can be #3 to #18 (?) [c] Cover - the bottom bars are placed horizontally near the bottom of the beam, leaving concrete below and to the sides as cover. The cover protects the steel from corrosion and fire. [D] Spacing - spacing between the bars provides enough space to be filled with coarse aggregate. [E] Chamfer - helps avoid sharp concrete edges.