Concrete as a Building Material Building Foundations Concrete as a Building Material

Concrete As a Building Material Used for thousands of years. Romans were experts with concrete Tremendous compressive strength. Resistant to chemicals. Will not rot or be damaged by insects. Hardens even under water. Cured properly, withstands extreme heat and cold. Can be formed into almost any shape Widely available and fairly inexpensive. In residential used primarily for foundations. Also for walks, driveways, entry steps, floors, & kitchen countertops

Concrete Basics Strength & usefulness depends on quality & type of materials in mix. Concrete made by mixing cement, fine aggregate (sand), course aggregate (gravel or crushed stone), & water in proper proportions. When combined a chemical reaction (hydration) causes concrete to harden. Hydration- generates heat as the concrete cures. Aggregates are inert (inactive) ingredients. Cement & water are active ingredients. If all 4 ingredients are not present it’s not concrete. If missing course aggregate it is mortar or grout.

Portland Cement Cement in modern concrete is a manufactured substance. Various types have different strength characteristics. Consists of compounds of lime mixed with silica and alumina. Raw materials are crushed & ground to powder. Mixed in various proportions then heated to 2700ºF or more in a kiln. Heating causes small lumps called clinkers. Gypsum is added to clinker then pounded into fine powder we call cement.

5 Basic Types of Cement Type I (standard) gen. construction purposes – long setting time Type II (modified) gen. construction purposes – less heat during hydration, resists break down when exposed to sulfates Type III (high-strength) forms must be removed quickly or concrete in service quickly – gains strength faster Type IV (low heat) used on large projects (dams) – Low heat in hydration prevents cracking caused by temperature changes Type V (sulfate-resistant) will be exposed to highly alkaline conditions & sulfates

Specialty Cements Self-leveling – flows like thin syrup – poured on radiant floor heating systems Hydraulic – expands when mixed with water, hardens in minutes – plugs holes & cracks in foundations Anchor – fast setting – secures railings & hardware in holes drilled in concrete – high compressive strength Resurfacing – repairs damaged concrete – fine aggregate can be spread in thin layers

Aggregates Granular material – sand, gravel or crushed stone. Fine – washed sand or other material up to ¼” in diameter. Course – pea gravel, crushed stone or other material larger than ¼” diameter Must be clean & free of dirt, clay or vegetable matter – these reduce concrete strength

Water Must be clean & free from oil, alkali, or acid. Should be suitable for drinking with no contaminants. Sugar for example prevents concrete from hardening Ratio of water to cement VERY important. Too mush water decreases compressive & tensile strength.

Hydration Aggregates must be mixed thoroughly with cement before water is added. Hydration begins when water is added. Concrete does not dry. It Cures! Concrete must be kept moist during early stages of hydration. If it gets too dry during hydration it will weaken your final product.

Moist-Curing Improves strength of concrete. Surface kept moist for at least 7 days after placement. Delay removing formwork. Cover concrete with material that retains moisture or spray lightly with water or chemicals to slow evaporation Gains most strength in 28 days after placement. Continues to gain strength for many years after.

Admixtures Ingredient other than main 4 added to change physical or chemical characteristics. May make more workable or increase strength Air-entraining – adds tiny bubbles. Increases durability if exposed to moisture & frequent freeze/thaw cycles. Improves workability. Retarding – Slows hydration. Useful in hot weather or when difficult to finish pour before concrete sets up.

Admixtures continued Accelerating – Increases rate of gaining strength. Useful if must use quickly. Water-reducing – reduces amount of mixing water without reducing workability. Makes concrete stronger. Super-plasticizing – makes concrete flow easily or significantly increases strength.

Colorants Color sometimes added to concrete used as a finished surface. Place standard, uncolored concrete then add colored layer over it Dust powdered colorant over surface of wet concrete. As troweled flat & smooth colorant is absorbed into surface.

Check Your Knowledge If a material contains fine aggregate, cement and water, but not course aggregate, what is it called? What is a clinker? What is the purpose of using high-strength (Type III) cement? What is an admixture?

Working with Concrete Pour often used to describe process of putting wet concrete into position. Industry prefers term “placed” Concrete measured by the cubic yard. Frequently shortened to yard by builders. One cubic yard of concrete contains 27 cubic feet.

Mixing Can be mixed on site from raw materials or pre-mixed dry ingredients. Can also be ordered ready mixed. Mixing often done in wheelbarrow with a mixing hoe (hoe with holes in the blade) Do not add water all at once. Pour in 1/2 & mix evenly, pour in another ¼ & mix evenly, add remaining water gradually and mix in. Reduces chance of adding too much water.

Using Pre-Mixed Materials Comes in 60, 80, or 94 lb sacks. Mixed with water a 60-lb sack makes 1 cubic foot of concrete. Because water triggers hydration process, must be stored in dry area. Best if indoors. If not possible, cover with waterproof tarp and stack off ground & arranged tightly to limit air circulation Small amounts of moisture can cause cement to become lumpy. Lumps that can’t be broken by squeezing in hand means pre-mix is bad and should not be used.

Mixing on Site If mixed on job quantities of cement & aggregates must be figured separately for each cubic yard needed. Suggested proportions may be obtained from tables. Should wear respirator when mixing dry ingredients to prevent lung damage.

Using Ready-Mix Most concrete supplied to job sites by ready-mix plants. Concrete ingredients are poured into rotating drum & mixed as truck travels to site. Concrete slides down metal chutes and is placed in forms. Method is most economical if at least 2 cubic yards are ordered. Most charge extra for smaller amounts. Ordered by # of bags of cement per cubic yard. 5-bag mix is minimum for most work. If high-strength is needed or for steel reinforced 6-bag mix is common

Using Ready Mix continued May also order by compressive strength 2500 or 3500 psi(pounds per square inch) Where concrete will be exposed to severe weathering building codes require stronger & more durable concrete. E.g. sidewalks, exposed basement walls, porch slabs, carport slabs, & garage slabs.

Slump Test Test to measure consistency of concrete. Concrete straight from mixer is poured into small sheet-metal cone of specific dimensions. Speared with a rod to remove air pockets, then cone is removed. Measurements taken of how much unsupported mass slumps or loses it’s conical shape. The greater the slump the wetter the concrete.

Placement Should be poured continuously and kept fairly level throughout poured area. Air pockets should be removed by vibrating, tamping, or spearing with spade In cold weather the use of heated water & aggregate during mixing is good practice. In severely cold weather insulation or heat is necessary until it sets up. Usually may be poured directly from truck. In some cases (truck can’t get close enough) may be piped in with long flexible pipes from a pump truck.

Check Your Knowledge What four characteristics of concrete are controlled by the amount of water in proportion to the amount of cement? How much concrete does a 60-lb. sack of pre-mix yield when mixed with water? What is the purpose of a slump test? What precaution should be taken when placing concrete in hot weather, & why?

Reinforcing Steel When steel is embedded in concrete it is called reinforced concrete. Has excellent compression strength & good tensile strength. Footings, slabs & walls nearly always reinforced with steel. May be purchased as bars (rebar) or welded-wire fabric. Rebar has patterned surface to help concrete grip steel. Used most often in footings & walls Welded-wire fabric used mostly in slabs.

Reinforcing Steel continued Rebar comes in 20’ lengths – may be cut (hacksaw, cutting torch or rebar shear) or bent on site – diameter varies according to tensile strength needed. Welded wire fabric is an open mesh of wires running perpendicular to each other. Most common size is 6 x 6. Usually placed on wire chairs to keep centered in slab

Fiber Reinforcement Short synthetic fibers mixed with concrete to reinforce it. Help reduce shrinkage cracking and may increase resistance to impact & abrasion.

Check Your Knowledge Why is steel embedded in concrete? What is the purpose of the patterned surface on rebar? Name the two basic types of steel reinforcement. Where are they used? What is the most common size of welded-wire fabric used in residential construction?