constituents u mixture of aggregate and paste u paste30 to 40% u portland cement7% to 15% by Vol. u water 14% to 21% by Vol. u Aggregates 60% to 70% u coarse aggregates u Fine aggregates u Admixtures
Portland Cement u Dry powder of very fine particles u forms a paste when mixed with water u chemical reaction-Hydration u glue u paste coats all the aggregates together u hardens and forms a solid mass
Water u needed for two purposes: u chemical reaction with cement u workability u only 1/3 of the water is needed for chemical reaction u extra water remains in pores and holes u results in porosity u Good for preventing plastic shrinkage cracking and workability u Bad for permeability, strength, durability.
Aggregates u cheap fillers u hard material u provide for volume stability u reduce volume changes u provide abrasion resistance
Admixtures u chemical u set retarders u set accelerators u water reducing u air entraining u mineral u fly ash u silica fume u slags
Properties of fresh concrete u Workability u ease of placement u resistance to segregation u homogeneous mass u Consistency u ability to flow
Slump Test u Inverted cone u fill it up with three layers of equal volume u rod each layer 25 times u scrape off the surface 8” 4” 12”
Slump test results u stiff 0-2” u massive sections, little reinforcement u use vibration u medium2-5” u columns, beams, retaining walls u Fluid5-7” u heavily reinforced section, flowable concrete
Factors affecting slump u water cement ratio u w/c = weight of water / weight of cement example: weight of water mixed at the plant 292 lbs. weight of cement 685 lbs./cu. yard w/c = 292/685 = 0.43
water cement ratio if you add 10 gallons of water per cubic yard at job site, then: extra water 10 gallons/cubic yard * (3.8 liters/gallon) * (2.2 lbs./kg) *( 1kg/liter) = 83.77 lbs. total water 282 + 83.77 = 365.77 new w/c = 365.77 / 685 = 0.534 >> 0.43
Factors affecting slump- paste content u constant water cement ratio u increase paste content u increase slump u NO GOOD u constant cement content u increase water content u increase slump u NO GOOD
Factors Affecting Slump- Water Content u Add water at the constant cement content, w/c increases, slump increases. u Add water at a constant water cement ratio, have to increase cement as well, slump increases.
Admixtures u set retarding admixtures u set accelerating admixtures u water reducing admixtures u superplasticizers u air entraining admixtures
Factors affecting slump u Aggregates u grading the larger the particle size, the higher the slump for a given paste content
effect of aggregate size 1” Consider a single aggregate the size of 1”x1”x1”
Compute the surface area as you break up the particles volume = 1 cubic in surface area = 6 square inches volume = 1 cubic in surface area = 1.5*8= 12 square inches block surface area = 0.5*0.5*6=1.5 block surface area = 1*1*6= 6
Water accumulation on surface Examine the concrete surface
Interaction between bleeding and evaporation surface water Evaporation Bleed water Bleed water = evaporation
Too much evaporation leads to surface cracking no surface water Evaporation Bleed water < Evaporation drying
Side diagram of surface contraction Wants to shrink Does not want to shrink
Free Shrinkage, causes volume change, but no stresses before shrinkage After Shrinkage
Restrained Shrinkage- creates stresses, which may cause cracking
Restrained shrinkage cracking Parallel cracking perpendicular to the direction of shrinkage
Bleeding and its control u Creates problems: u poor pumpability u delays in finishing u high w/c at the top u poor bond between two layers u causes u lack of fines u too much water content u Remedies u more fines u adjust grading u entrained air u reduce water content
Causes of Plastic Shrinkage Cracking u water evaporates faster than it can reach the top surface u drying while plastic u cracking
Plastic Shrinkage Cracking- Remedies u Control the wind velocity u reduce the concrete’s temperature u use ice as mixing water u increase the humidity at the surface u fogging u cover w/polyethylene u curing compound u Fiber reinforcement
Curing u The time needed for the chemical reaction of portland cement with water. u Glue is being made. u concrete after 14 days of curing has completed only 40% of its potential. u 70 % at 28 days.
Curing tips u ample water u do not let it dry u dry concrete = dead concrete, all reactions stop u can not revitalize concrete after it dries u keep temperature at a moderate level u concrete with flyash requires longer curing
Temperature effects on curing u The higher the temperature the faster the curing u best temperature is room temperature u strongest concrete is made at temperature around 40 F.(not practical) u If concrete freezes during the first 24 hrs., it may never be able to attain its original properties.
Temperature effects on curing u real high temperatures above 120 F can cause serious damage since cement may set too fast. u accelerated curing procedures produce strong concrete, but durability might suffer. u autoclave curing.
Your consent to our cookies if you continue to use this website.