Concrete Basics Presented By L&M Construction Chemicals, Inc. I. What is concrete II.What are the ingredients and how do they interrelate III. Factors controlling the ratio of ingredients IV.Concrete problems V.Rules of thumb for concrete
What is Concrete Concrete is a mixture of portland cement, fine and coarse aggregate and water. This mixture may or may not have added to it chemical and mineral admixtures, air- entraining agents, and steel or plastic fibers.
Fly Ash reacting with calcium hydroxide
Polypropylene Fiber: Coilated - Fibulated
Polypropylene Fiber: Monofilament
PORTLAND CEMENT Cement in its dry form is made up of discreet individual particles. Only approximately 65% of the cement in concrete ever hydrates. It is the water to cement ratio that determines the strength of a concrete mix and not the amount of cement in the mix alone. The only other requirement is that there is sufficient cement paste to coat all the aggregate particles. The cement particles when mixed with water do not dissolve they become dispersed.
PORTLAND CEMENT TYPES ASTM C -150 Standard Specification for Portland Cement Type I Normal Type IA Normal Air Entraining Type II Moderate Sulfate Resistance Type IIA Moderate Sulfate Resistance Air Entraining Type III High Early Strength Type IIIA High Early Strength Air Entraining Type IV Low Heat of Hydration Type V High Sulfate Resistance
HYDRATION Hydration is the chemical reaction between portland cement and water that produces crystalline structures. Two of which are Tobermortie and Ettringite. Ettringite Tobermorite
HEAT OF HYDRATION Heat of hydration is the heat generated when cement and water react. The approximate amount of heat generated during the first seven days, based on 100% for Type I, Portland Cement, is as follows: Type IImoderate 80% to 85% Type IIIhigh early strength up to 150% Type IVlow heat of hydration 40% to 60% Type Vsulfate resistant 60% to 75%
SETTING OF PORTLAND CEMENT The degree of stiffening of a mixture of cement and water after which placement should not be attempted. Initial Set: Final Set: The time after which the mixture should not be worked. If the mixture is disturbed, strength development will be adversely affected.
FALSE SETTING OF PORTLAND CEMENT The rapid development of rigidity in a freshly mixed portland cement paste without the evolution of much heat. Plasticity can be regained by further mixing without addition of water or loss of strength. False Set: A form of gypsum CaSO 4 + 2H 2 O in which three-quarters of the chemically bound water has been driven off by heating. Plaster of Paris: CaSO 4 + 1/2H 2 O
The Function of an Aggregate in Concrete 1. Contributes volume to the concrete mix 2. Contributes its own physical strength to the concrete mix 3. Affords a good bond plane for the cement
The Characteristics of a Good Aggregate 1. Must be chemically inert 2. Free of organic matter 3. Must have a good grading ( particle-size distribution) 4. Must have good particle shape 5. Must have a low volume of voids 6. Grading and particle shape must work together to reduce the effects of aggregate thixotropy 7. Particle surface texture and strength must afford a good bond plane for the cement
The proper size air voids are produced when the total volume of air in the concrete mix equals 5 to 7 % of the total volume of the concrete resulting in an air void every inches in the cement paste. Air Entrainment for Protection 0.016”
The Function of Air Entrainment in Concrete Air is placed in concrete to protect the concrete from internal expansive forces. These forces are produced when water is frozen to form ice and when alkali-silica reactivity occurs. Hardened cement paste is very rigid and will disintegrate, if internal expansive pressure becomes too great. Entrained air in concrete protects concrete by producing void areas in the cement paste in which the expanding materials can escape. This reduces the internal expansive forces thereby preventing damage to the cement paste.
AIR CONTENT: The total volume of air in a concrete mix is expressed as a percentage of the total volume of the mix. The air in a concrete mix is found in one of two forms: The natural air in the mix. It is approximately 1/2% to 3 %. This form of air does not give freeze-thaw protection. This air is chemically placed in the mix for freeze- thaw protection. This form of air may vary from 4% to 8% depending on the level of protection needed. Entrapped air: Entrained Air:
LAWS & RULES All concrete mixes behave in accordance specific laws and rules. To understand these laws and rules is to better understand the interworkings of concrete.
LAWS & RULES Abrams Law (Water Cement Ratio Law): For a given amount of cement as the water content is increased the strength of the mortar or concrete mix decreases. The optimum amount of water for strength is 1/4 lb. per lb. of cement.
LAWS & RULES For a given water cement ratio, the strength of a concrete mix remains the same regardless of the aggregate content of the mix. As long as there is sufficient cement paste to coat all the aggregate particles. Constant Strength Rule:
LAWS & RULES For a given aggregate and consistency, the quantity of mixing water per cubic yard of concrete will remain essentially constant, regardless of the cement content. Constant Water Content Rule:
LAWS & RULES The water requirements of a concrete mix, for a given consistency, will increase as the surface area of the aggregate increases per unit volume of concrete. Water Requirement Rule:
LAWS & RULES For a given concrete mix of given consistency, the water requirements of the concrete mix will decrease as the air content increases. Air Content vs Water Content Rule:
LAWS & RULES Air Content vs Strength Rule: For a given concrete mix of given consistency, as the air content increases the strength of the concrete mix will decrease. As the cement content of a concrete mix increases, the rate of strength loss will increase.
BLISTERING: Concrete Problems The irregular raising of a a thin layer of material at the surface of placed concrete or mortar during or soon after, completion of the finishing operation. This condition is caused by the premature closing of the surface and the entrapment of air or water just below the surface. Blisters
CARBONATION: Concrete Problems Carbonation of concrete is a process by which carbon dioxide from the air penetrates the concrete and reacts with hydroxides, such as calcium hydroxide, to form carbonates.
CRAZING: Network of fine Cracks. Also referred to as map cracking. Cause by early drying of the concrete surface. Concrete Problems
CRUSTING: Concrete Problems The drying out of the surface of concrete or mortar before set occurs. This condition gives a false sense of setting. The surface of the slab will not have enough moisture at the surface of the finishing operation.
CURLING: Concrete Problems Curling is the distortion of an originally essentially flat member into a curved shape. This warping is caused by differences in temperature or moisture content between the top and bottom of the slab.
PLASTIC SHRINKAGE CRACKS: Concrete Problems Plastic shrinkage cracks form prior to final set. They are the results of rapid evaporation of moisture from the surface of the concrete slab.
SCALING: Concrete Problems The deterioration of the surface of concrete. Primary causes low air content, exposer of concrete surface while wet to freezing and thawing and repeated use of deicing salts.
Rules of Thumb for Concrete
RULES OF THUMB FOR CONCRETE Water vs. Slump Change: The addition of 1 to 1 1/2 gallons of water per yard of concrete will increase the slump by one inch. Water vs. Strength: One gallon of water added to a yard of well mixed and proportioned concrete mix will decrease the compressive strength approximately 300 to 450 psi.
RULES OF THUMB FOR CONCRETE Minimum Water Cement Ratio: Minimum water cement ratio is approximately It takes approximately 1/4 pound of water to hydrate one pound of cement. Water of Convenience: Water in addition to that of the minimum water required for hydration but required for a workable consistency.
Temperature Change vs. Setting Time: An increase of 20 o F will decrease the set time by one- half. A decrease of 20 o F will double the set time. Yield vs. Air Content: A increase of one percent air will increase the yield of one yard of concrete by 0.27 cubic feet. Air Content vs. Strength: As the concrete mix increases in air content, starting at approximately 5%, each percent of air will result in a strength loss of approximately 5% at a mid-range cement factor. RULES OF THUMB FOR CONCRETE
The strength required for one freeze thaw cycle: If the concrete is properly entrained the minimum compressive strength required to resist damage from one freeze thaw cycle is 500 psi. Temperature vs. Strength Gain: As the temperature of concrete decreases, the rate of strength gain decreases. As the concrete temperature approaches 40 o F the rate of strength gain virtually stops. RULES OF THUMB FOR CONCRETE