Concrete Man made stone

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 slump cone rod concrete

Slump test Slump Ruler

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) = lbs. total water = new w/c = / 685 = >> 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.

Factors affecting slump-paste content Low paste content Harsh mix High paste content Rich mix

ball bearing effect-start starting height

ball bearing effect-end slump

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

Break it up further

Compute the surface area 0.5 in 0.25 in surface area = 0.25*0.25*6*8*8=24

Larger particles, less surface area, thicker coating, easy sliding of particles

Smaller particles, more surface area, thinner coating, interlocking of particles

Effect of aggregate size

Angularity and surface texture of aggregates angular and rough aggregate smooth aggregate river gravel

Temperature fresh concrete aggregatespaste

Bleeding

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.