Print this page Why Concrete Quality Assurance is Important?
Eugene Washington, P.E.
Course Outline
1.
Introduction
2. Mix design
3. Batch Plants
4. Transit Mixers
5. Slump Consistency
6. Compression Cylinders
7. Air Content
8. Cold Weather Concerns
9. Hot Weather Concerns
10. Concrete Placing
11. Forming Concrete
12. Wet Finishing
13. Curing Concrete
14. Dry Finishing
15. Conclusion
This course includes
a True-False quiz at the end.
In order for the Inspector to be effective as a leader of quality concrete construction, that person must know the reasons for the many field tests and observations that are continually made during the concrete construction process. Using proper and consistent test procedures is vital to achieving high quality concrete structures. The inspector must also make detail orientated observations, knowing when to ask questions and when to give direction.
All too often the contractor and the inspector view each other as enemies. The inspector believes the contractor will do anything to cut costs. The contractor sees the inspector as an obstructionist only interested in getting something for nothing. Nobody wins in such situations. The owner is usually has to defend excessive claims, late completion, and the contractor loses profit and the inspector's career may be at a dead end.
On the other hand, experienced and knowledgeable inspectors are highly respected members of the construction team. They know what works and what to watch for. They often spot errors and suggest corrections before major problems develop. The best inspectors help keep the work progressing smoothly while encouraging and insuring quality and safe work practices. Disputes and change orders are minimized. Early project completion and under budget costs are often the result of the unheralded but expert inspectors.
The expert inspector knows when to give the field crews latitude and when to insist on special effort. The crews respect, understand, and appreciate an inspector's sound judgement. These crews are motivated to excel in both productivity and quality. Mutual pride in the work can result in some amazing accomplishments. The expertise demonstrated by the expert inspectors is not found in any specification or manual, it is gained by years of observation, doing and learning. The expert knows why the specifications are important and how they should be interpreted.
Sometimes an inexperienced person is given an inspection manual, virtually no training and sent to the field on their own. This action just creates all sorts of animosity and problems. It is not the green inspector's fault that the inspection manual is at odds with the project specifications and industry practice. This just results in inane pronouncements due to ignorance. This brings us to the thrust of this course: why the specifications for concrete are important and when. No specification is perfect nor can it address every conceivable concrete construction situation, so the inspector must be flexible, knowledgeable and reasonable.
The intent of the
specifications for reinforced concrete is to generate structural integrity and
aesthetic appearance. The architect and design engineer establishes the required
properties and surface treatment of the reinforced concrete. This selection
of important concrete properties generates the complex specifications that balance
the cost, blending, local aggregate properties, transportation, placing, workability,
durability and strength of the concrete. The quality assurance personnel should
know what the test results mean and why they are important.
Course Content
The quality assurance starts with the concrete mix design. The concrete strength is a function of cement content, water to cement ratio, aggregate properties and aggregate gradation. There are numerous additives available to give the concrete special properties. Water reducing admixtures are used to enhance strength. Air entraining agents are used to enhance concrete durability and resist the freeze-thaw spalling. Superplasticizers are used to achieve high slump with a low water-cement ratio and high concrete strength. Retarders and accelerators are used to control the initial set duration of the concrete mix design. There are shrink control additives that can be used. Fly ash can be used to replace a small amount of the cement and reduces the heat of hydration and lower cost.
Concrete is an inert mineral mixture of basically cement and stone aggregates. Only when water is introduced does the chemical process begin. First the water lubricates the mixture so that for a short period it can be blended, transported, placed and wet finished. This initial period is usually limited to about 90 minutes. At that time the initial set begins and continued working of the concrete will reduce the ultimate strength by breaking the weak bonds that are beginning to form. When cement and water are mixed together an exothermic chemical reaction starts immediately. The wet paste starts to harden by bonding the water to the cement. This reaction releases heat and accelerates the reaction. Under normal ambient conditions the concrete will gain about seventy percent of its design strength in the first seven days. At 28 days the concrete will usually achieve at least its design strength. Concrete will then slowly gain additional strength for many years.
The normal structural mix design is a compromise of strength and workability. Concrete consists of cement, sand, gravel, water and various additives. The ideal slump range for most normal applications is 3 to 4 inches. Normal mix designs require about 300 lbs of water per cubic yard of concrete to achieve that slump. The ideal water to cement ratio is about of 0.3 pounds of water to one pound of cement to fully hydrate the cement. That means the concrete would need 1000 lbs of cement or 10.6 sacks of cement per cubic yard. That is way too much cement. That much cement will result in shrinkage and other problems. Most normal structural concrete is designed for 3,000 to 5,000 psi 28-day compressive strength and 470 to 658 pounds (5 to 7 sacks) of cement per cubic yard. The extra 342 to 530 lbs of cement would cost an additional $14.00 to $22.00 per cubic. High strength concrete is often limited by the shear strength of the coarse aggregates, so there is no point in trying to design water cement ratios beyond the aggregate strength. 4,000-psi aggregate is common, but special effort and cost is usually required to produce high strength aggregate. Usually a 3-inch slump is needed to effectively pump and wet finish concrete. In order for the concrete to be pumped the mix must contain about 1,200 pounds of sand per cubic yard. A concrete mix that does not contain sufficient sand will be harsh and very difficult to finish because the coarse aggregates will be exposed on the surface. Without sufficient sand the vibration consolidation effort will let the fines and cement settle to fill the aggregate voids at the bottom of the pour and cause a segregation of only coarse aggregate at the top of the pour.
Another concern is the aggregate size. While large aggregate will allow a reduction of cement to obtain the same concrete strength, one-inch aggregate is about the maximum size that can be effectively pumped. Mass concrete structures, such as dams may use up to 6-inch cobbles for the coarse aggregate. The aggregate size must be at least 30% smaller than the rebar minimum clear spacing. If the aggregate can not move freely around the rebar and other embedded items, rock pockets and segregation will occur. I have had cases where the concrete was so heavily reinforced that the concrete had to be a 3/8-inch maximum size pea gravel and superplasticized just to place the concrete around the rebar.
Project quality concrete assurance usually starts at the batch plant. The plant should be certified by the by a Weights and Measures Agency. The plant should be inspected regularly. The equipment should be in good working order with a minimum of build up of grout and soil. There are basically two types of concrete batch plants: dry batch and wet batch. The dry batch plant is most commonly used for commercial concrete, where the proportioned concrete contents and then water is conveyed into a transit mixing truck. While the truck travel to the discharge point the slowly revolving barrel mixes the concrete to a uniform consistency. Usually at least 70 truck barrel revolutions is needed to properly blend the concrete mix. The starting revolution count should be noted on the delivery slip. The quality assurance team should note the count for every load delivered, especially if the transit time from the plant to placing site is less than 10 minutes. If water or additives are added at the job site an additional 70 revolutions are required to blend the concrete.
The wet mix batch concrete plant is often used on remote or specific job sites where commercial plants are not convenient or special consistency is required. The wet mix plants are of two types: continuous feed and premix plants. These plants combine the functions of both the batch plant and the transit mixing truck. The continuous feed is used for such placements as Roller Compacted Concrete (RCC) and Soil Cement Concrete (SCC). All the concrete components are continuously added at a prescribed rate into the upper end of a gently sloping drum. The drum internal paddles rotate and mix the concrete as it slowly passes to the discharge end of the drum. This continuous feed process can easily generate 250 to 500 cubic yards of concrete per hour. The main draw back is that it is impossible to maintain the close tolerances usually demanded of a commercial batch plant. I have seen specifications that want a soil cement product; high production and the associated cost savings, but demand structural concrete consistency. Some designers simply do not understand the realities of the process.
The premix plant usually batches a fixed amount of concrete, in the order of four cubic yards. This type of plant is used when very high quality and consistency is required. The advantage of the wet mix plants have is that the transportation to the pour is not limited to only a transit mixing truck. Delivery from the batch plant to placement site can be by conveyor, pump, dump truck, crane and bucket or any combination.
Occasionally yield tests should be performed immediately on the delivered concrete. These tests will confirm the concrete component proportions, gradations and aggregate quality. It is better to find out is there is a mistake before a major and a disaster is discovered a month later by compression testing. Once we suspected a plant was shorting us on volume. We built a 10 cubic yard box and ordered 10 cubic yard of concrete. Only about 9 cubic yards was discharged from the truck. We had no problem after that.
Each concrete transit mix truck should be inspected for grout build-up and fin wear. The truck barrel revolution counter and water meters should be checked periodically. The truck manufacturer will recommend a minimum number of barrel revolutions and barrel speed to properly blend the concrete components. Worn fins and/or grout build up will reduce the ability of the truck to properly blend the concrete mixture. Poorly blended concrete will show a wide variation of slump and segregation within the batch load. Obviously, this will result in loss of uniform strength and durability.
If the mix is unworkable (too low a slump or harsh) the finished product will suffer from poor finished appearance, poor consolidation and rock pockets. If the slump is too high, weak concrete, high shrinkage and segregation can occur. A consistent slump will result in a uniform consolidation and surface appearance. Some specifications require that no water be added at the job site, only at the batch plant. This requirement is actually counter productive to achieving consistent quality. The aggregates are almost always stored in the open and the moisture content will vary considerably in the pile. The outside of the pile may be very dry and the inside very wet. Each loader bucket full will get a different mix of wet and dry aggregates. This variance in aggregate moisture content can be nearly impossible to control at the batch plant to within an inch plus or minus slump. A gallon per cubic yard change in the water content will change the slump about an inch. The other reason is that there will be varying amounts of evaporation during the transit mixer travel time. By forcing the dispatcher at the batch plant to guess at the correct water weight within 7 pounds per truckload results in wide variation in slump from truck to truck, some too wet and some too dry to use. This uncontrollable slump variation only causes delays, added cost and reduced concrete quality. The best way is to have the batch plant mix the concrete slightly dry and add water at the job site. With a little experience, one can judge the slump within a half of an inch by looking into the slowly revolving transit mixer drum.
The cone slump
test is a simple job site procedure, but it can easily be botched and the results
made unrepresentative. The slump test should be made as the concrete is being
placed at the job site. The slump test is a measure of proper water content.
A high slump means that there is too much water in the mix and low strength
and excessive shrinkage may result. If the slump is too low the concrete may
be unworkable. The test must be made on a hard, level surface that is vibration
free, well away from the construction activity. The concrete must be properly
consolidated in the cone. The cone must be removed with a slow and steady motion.
I have seen 3-inch slump concrete measured as 5-inch slump due to jerky cone
removal. I once watched a slump test done on a wooden pier with the transit
mixers pounding by within five feet. The actual 4-inch slump was being read
as a 6 to 7-inch slump. The testing guy was trying to reject the concrete because
he didn't know what he was doing and refused to learn. He was removed from the
project.
The proper making and handling of concrete compression cylinders is important.
The cylinders should represent the in place concrete strength. The sample concrete
should not be taken from the first or last cubic yard of concrete that is discharged
from the transit mixer truck. That first and last concrete is naturally slightly
different from the average in the batch. The cylinder must be thoroughly consolidated
by tamping in three lifts with a rod thrust in the concrete 25 times in each
lift. The top must be screeded off and tamped. If cylinder cans are used as
a mold, the plastic cover should be immediately installed and sealed with tape.
This is to prevent drying of the concrete. Then the cylinders should be immediately
placed in a protective rack that is away from vibration, direct sunlight and
freezing temperatures.
Usually the next day after casting the test cylinders, they are taken to the test lab for curing. The job site handling and storage is critical. The cylinders should not be moved until they are transported to the lab. Cylinders that are left in the work area and subjected to impact and vibration will often show low compressive strength. The cylinders should be carefully set in a padded box when transported and driving the transport vehicle should be smooth and impact free. We had a problem with low break strengths when the driver of the transport pickup just dropped the test cylinders in the bed and let them roll around loose. Those cylinders broke at about 20% less than the cylinders that were properly transported. Once at the lab the cylinders are put in a high humidity room and kept at 70 degrees Fahrenheit until broken. Before breaking the cylinders are capped with a Sulfur cushion at each end. This is done to ensure an even distribution of the applied compression force. On one job had a problem with low breaks that was hard to explain until it was discovered that the thermostat on the Sulfur pot was miscalibrated.
When low cylinder breaks is realized an investigation should be immediately initiated. There are several possibilities. The concrete may have been misbatched. The cylinder making, handling and breaking process may be faulty. Unfortunately, this is usually not discovered until the concrete has been placed for 28 days. Even when the problem has been corrected and the concrete is found to be slightly below design strength at 28 days there are several potential solutions to salvaging the already placed substandard concrete. Only when all other alternatives are exhausted should the concrete be demolished. Demolition is very expensive and disruptive to the project. An engineered analysis may show that full concrete design strength may not be necessary at that location. A 90-day core test may show that the concrete has gained sufficient strength. It may be possible to install additional reinforcement or buttressing. Positive action should be taken to correct any faulty concrete and testing. But reasonable investigation and consideration to the circumstances of the error should be granted.
When it is important to be able to know the next day or so the concrete strength, the cylinders should be cured in the same environment as the pour they represent. The cylinder rack should be placed with the pour and under the curing blankets so the test cylinders will experience the same temperature that the pour cures under. The cylinders are then taken directly to the testing lab for immediate breakage to test if sufficient concrete strength has been achieved to strip soffit forms or start posttensioning.
This early strength test breakage of cylinders is particularly important in cycled box culvert forms. The box culverts are usually designed to have a minimum of ten feet of dirt covering them. This caused a load on the top slab of about 1,200 psf. The weight of the top slab of the box culvert is usually only nine inches or so thick, which means it weights only about 120 psf. That is only 10% of the design load. That means the concrete slabs will easily be self supporting when the concrete reaches 750-psi or 19% of a 4,000 psi, 28 day strength mix design. Unless the weather is very cool, normal concrete will cure over night to at least a 750-psi strength and allow the stripping of the box culvert forms the first thing next morning. The only concern is increased creep deflection. That concern can be eliminated by adding reshores to the soffit soon after the forms are stripped and advanced. There are several ways to reshore the top deck. One way is to use simple timber posts that are wedged or screw jacked tightly into place. The only concern is that the reshore must be designed to support at least 60% of the slab weight between the walls.
The field air test is made to ensure that the concrete mix contains the appropriate air content. Usually, an air content of 4% to 6% by volume is desired for structural concrete. The mixing process will naturally entrain about 2% air. An air-entraining agent is added to the mix to increase the air content. These agents are often temperature sensitive and the dosage needs to be adjusted for weather conditions. If the concrete contains too much air, it will lose strength. If the concrete contains too little air, it will lose durability, especially the resistance to the freeze-thaw cycle.
The air test is a simple measure of the change in volume of the concrete as it is placed under air pressure in a sealed container. If the test is not performed correctly, unreliable results will be generated. The container must filled to the brim with consolidated concrete and screeded level. Voids in the concrete or an improperly filled container will give show high false air content. The container must be kept clean and seal airtight. The gauge must be periodically calibrated and the port must be free of grout. The hand pump must stay sealed and not bleed off air pressure.
When concrete freezes before it can partially hydrate, the water expands as it forms ice crystals and will breaks up the matrix of the concrete. Such frozen concrete will never gain full strength. Concrete must be kept from freezing for at least three days. The cement hydration reaction will continue down to about 25 degrees Fahrenheit once a partial cure has taken place. The rate of hydration cure very slow when the concrete is near freezing. The concrete hardens and gains strength by the chemical reaction between the water and the cement. The reaction is exothermic; meaning the reaction generates heat energy, the heat of hydration. Even in extreme cold weather, a properly insulated large mass of concrete will generate enough heat to keep the concrete warm enough to cure at its normal rate without an external heat source. Thin walls and slabs often require heaters in below freezing weather because the heat of hydration is dissipated too quickly. Concrete should not be placed on frozen ground or previous concrete that has not had the surface heated to well above freezing. Foundations should be started below the frost line or extended into the permafrost far enough so that expansion of water freezing will not heave the foundation.
During freezing weather the test cylinders cured in the laboratory may not represent the true strength of the concrete in the structure. Thin soffit supported slabs should be monitored for low temperature and may require in situ testing for strength gain to ensure sufficient strength has been achieved prior to stripping the falsework.
Concrete in cold weather is usually warmed to at least 50 degrees Fahrenheit to allow the hydration reaction to generate heat quickly enough to initiate the strength gain of the concrete in a reasonable time. The use of retarding admixtures should be carefully monitored during cold weather. I have seen two large wall forms burst when because an overdose of retarding admixture was mistakenly introduced at the batch plant. The form failures were catastrophic and endangered the crew. Often hot water is introduced to raise the temperature of the concrete mix. The water should not be any hotter than about 150 degrees Fahrenheit. If the water is too hot, it can cause a flash set of the cement. Some times in extreme cold even the aggregates must be heated because the water content is only about 10% by weight of the aggregates. When water is 70 degrees cooler that the aggregate, the temperature of the mix is reduced only about 15 to 20 degrees. Concrete that is allowed to cure at about fifty degrees Fahrenheit for the first month will actually gain a higher strength in the end than if the concrete is cured at a high temperature.
Even the thermometers must be checked for accuracy. We recently had a case where an inspector was rejecting concrete loads because the thermometer being used was about 10 degrees inaccurate. A claim has been filed to the project owner for the lost time and costs for several thousand dollars over a device that cost less than $5.00. No matter how the claim will be settled, all the involved parties have already lost time, money, and/or credibility. Attention to such minor detail is very important.
The specifications usually state the concrete must be placed in final position in the forms within 1.5 hours after water and cement is batched. This is appropriate when the concrete temperature is high, 85 degrees Fahrenheit or so. We have placed concrete that was three hours old when the concrete temperature was between 50 and 60 degrees and no loss of strength was realized. The reason for this disparity of placement times is due to the fact that the hydration cycle is highly temperature sensitive. Steam curing of concrete at 180 degrees will cause the concrete to gain 7-day strength in about as many hours. The point being that if an unforeseen problem delays placement of the concrete for a short while, consideration should be given to the temperature of the concrete before automatically rejecting the concrete.
Hot weather concrete concerns are just as important as cold weather concrete placement. Flash sets can occur that causes cold joints and prevents the wet finish of the surface. Cold water is often introduced to cool the concrete mixture. Aggregate piles are sprinkled and even ice and liquid Nitrogen are used to cool the concrete. In very hot weather, it is best to place the concrete at night when the ambient temperature is the coolest. This also helps the finish operation by eliminating the direct exposure to the sun drying effect. In hot, dry windy conditions the surface can dry out in seconds and cause surface shrinkage cracks and dusting of the surface. The surface cement loses the needed hydration moisture and remains as fine particles. If this is caught soon enough, misting the surface with cool water can reverse the drying process and seal the cracks. If rapid drying weather conditions are present, then the concrete surface should be kept damp by using a cool water mist until the wet finishing operation is complete and the curing procedures are installed.
Massive concrete is a particular concern because the hot weather accelerates the heat of hydration reaction. The internal temperature of the concrete can be so great that the thermal expansion will cause tension cracking of the much cooler surface concrete. The thermal gradient from the surface to the center of the concrete mass becomes a serious concern when the least thickness of the concrete is five or more feet and the other dimensions are much larger. Special efforts or often required to control the thermal gradient. The cement content can be reduced by the partial substitution of flyash and type IV low heat of hydration of cement. Sometimes cooling water is circulated through conduits that are imbedded in the concrete. Cement content should be kept to minimum by adding water reducing and retarding admixtures. Superplasticizers can be used to reduce the cement content and maintain the required ultimate strength. Often water curing the mass concrete is the worst thing that can be done. The water can only cools the surface and increases the damaging thermal gradient.
In any weather conditions massive concrete structures can be damaged by the temperature differential from the surface to the interior. If the temperature difference is more than 35 F degrees in the mass concrete the surface is likely to crack in tension. Even in hot weather, insulation blankets may be required for the first week or two to keep the surface of the concrete warm enough.
Another phenomenon is false set of the concrete. This usually occurs immediately after placing and the concrete hardens suddenly. False sets are not a cement hydration, but a weak Plaster-of-Paris like chemical reaction that is not a part of the concrete strength gain. The concrete will still be workable and cold joints will not be caused. Superplasticized concrete demonstrates this characteristic by hardening almost immediately after vibration is stopped. The concrete can be walked on without leaving a footprint, but can be safely liquefied by vibration, so cold joints are not created.
Proper placing techniques are critical to achieving quality concrete. It costs no more to correctly place concrete. There is no good reason to cut corners in the placement effort. Usually the cost to repair the defects far exceeds the cost of placing the concrete properly the first time. Concrete should be freely dropped no more than six feet into the form to eliminate segregation. The concrete should be placed in about two foot lifts. The concrete should be immediately vibrated after placement. Thorough vibration will virtually eliminate rock pockets and bug holes on the formed surface of the concrete. It is difficult to over vibrate the concrete, but it is easy to get in a hurry and under vibrate the concrete. Special vibration effort should be employed near corners, embeds and blockouts to eliminate voids and rock pockets.
A good observation is to watch the contact line between the concrete and the form. When air and water stop being spit from the contact line, the concrete has been vibrated enough. It is the trapped water and air that causes the bug holes, if not vibrated out. This vibration effort takes only a few seconds. The concrete should be vibrated on about two feet on centers with the normal 2-1/2" vibrator and close to the form surface, just inside of the rebar mat. The vibrator should extend a few inches into the previous lift of concrete to ensure no joint in the concrete is created. An experienced inspector will often guide inexperienced placing crews and demonstrate proper concrete placing and vibrating techniques. The owner, contractor, crews, and inspectors will be rewarded and should take great pride in a job well done when the forms are stripped and high quality concrete is apparent.
Another vital inspection role is the pre-pour form check. It is very disheartening to prepare for a couple of weeks, make a big pour and then discover the anchor bolt groups are seriously misplaced. A double, even a triple check is well worth while. One of the first checks is the reinforcing bar yield strength, size, and arrangement. The spacing should be accurate within about a quarter of an inch. There should be the correct design cover for bond and corrosion protection. The hooks and lap lengths should be checked. The bars should be reasonable clean. Tests have been run on concrete bond to rebar. The bond between rebar and concrete is almost entirely mechanical. Minor rust, mill scale, cement splatter and even form oil has virtually no effect on the bond strength. The reinforcement needs only to be reasonably clean. Sand blasting the rebar to white metal is a waste of time and money.
The bearing surface of the pour should be clean and compacted if on soil, or roughened if on concrete for good bearing and bond. The bearing surface should be dry, if concrete or just damp if soil. Concrete adheres better to a dry concrete surface than a wet one. A just damp soil will not allow the mix water to migrate out of the concrete too quickly. The elevations of the foundations and finishing screeds should be carefully checked.
The next check is the forms. They must be suitable for the pour loads, finish, and required tolerances. The forms must be properly tied and/or braced. It may be desirable for a buried footing to pour directly against the soil to enhance lateral load resistance. If the soil will not stand vertically, long enough to make the pour, when forms are needed. These buried footing forms can be a bit sloppy as long as they do not encroach on the neat dimensions of the concrete or interfere with an external structure. On the other hand, an architectural column or wall may require exacting tolerances, special finishes, and textures.
The forms must be designed and built to resist the pouring pressures generated. A massive footing that is large and deep will often experience differential pour pressure from one side to the other. Large foundation mats range from a few hundred cubic yards to several thousand cubic yards. The proper way to pour these massive pours is to use the rolling face technique. The technique is to start at one side and bring the first lift of two feet out about ten to twenty feet. Then the second lift is brought out to within about five feet of the leading edge of the first lift. The succeeding lifts are staggered the same few feet until the pour is brought to finish grade. Then the entire pour is advanced in five to ten foot increments. The rolling face technique has several advantages. The possibility of a cold joint is eliminated for the continuous pour. Even if a major breakdown forces a halt to the pour, the cold joint preparation area is minimized. If other problems slow the pour rate, shortening the pour advancement increments will prevent the formation of cold joints. The rolling face pour places a full hydraulic concrete load on just the starting face. The means through ties are worthless because the differential pressure will cause the pressure forms to push out and pull the far side forms in. In such instances the forms should be supported and braced independent of the opposite side.
The other major advantage is that the finishers can start to wet finish and curing operations just a few minutes after the pouring starts. This makes the whole pouring and finishing effort much more efficient. It also greatly reduces the risk and cost of pour stoppages due to breakdowns and weather, such as unexpected rainstorms.
A few years ago on a power generating plant the effectiveness of the rolling face pour technique was dramatically illustrated. Another contractor and we were building nearly identical plants with about 1,000-cubic yard concrete turbine foundation mats. We exchanged invitations to view each others pour. They poured their mat a few days before we poured ours. They used through ties, which forced them to bring the pour up in six-inch layers to keep the entire concrete surface of about 7,500 square fresh to prevent cold joints from forming and protect the form from unbalanced loads. They started the pour at 6:00 AM with a crew of 15 laborers and finishers. The pour was continuous and not finished until about 2:00 AM the next morning, over twenty hours of pour and finish time.
A few days later we started our pour using the rolling face technique at 7:00 AM with a crew of eight laborers and finishers. We had the pour complete and the cure in place by 1:00 PM, only six hours after the start of the pour. They invested over 300 crew hours in their pour, while we invested only 50 pour and finish crew hours. At $50.00 straight time wages per crew-hour, we spent only $2,500 on pour and finish wages. The other contractor had their crew on double time for half the time, so their average cost was about $65.00 per crew hour. They spent about $19,500 on the pour. This is a difference of $17,000 or 680% in labor cost to pour the identical foundation.
Other significant loses were also realized. The equipment and everyone on the project lost the next day because of exhaustion. The concrete pumping cost and additional $2,500 more for their pour. While they were worn out and frustrated, we started partying at 2:00 PM and had a thoroughly proud and happy crew. The total extra cost to them may have exceeded $40,000 just for that one pour.
Their inspectors where not happy with the progress or the finished product and required extra finishing effort. Our inspectors were kept fully informed of our plans well in advance and did everything they could to ensure success and quality. They played a vital roll to help make the plan work. Their knowledge and advice were invaluable. They celebrated and partied with us, enjoying the same well-deserved pride in accomplishment as the crew did.
Recently our crews
preformed a 5,200 cubic yard pour using the best techniques and three batch
plants and placing crews. The pour was completed in eleven (11) hours and at
less than half the estimated crew cost. It proved that a well-planed and organized
operation is worth the effort.
The differential pour pressure can be realized in thick wall forms at intersections
and corner. A four-sided hollow box will have less concrete contact area on
the inside form than the out side forms, but the same pressure. If this issue
is not addressed, the exterior forms will bulge out, pulling the inside forms
with them. This can cause the pour to be rejected, demolished and then rebuilt
at great expense. Often through ties and/or external bracing is required to
counter the differential pour pressure. We have often been saved a great deal
of cost and embarrassment by an alert inspector.
Pour pressure is a function of the concrete density, rate of concrete rise in the form, and the concrete temperature. The standard ACI formulas takes all three of the factors into account. Normal concrete at a mild temperature will generate about 600 to 750 pounds per square foot pressure on forms more than five feet high, at a pour rate or rise of four feet per hour. This four-foot per hour pour rate can generate more than a thousand pounds of pressure during cold weather. The forms are designed for a specific pour pressure and the pour rate is adjusted for the concrete or ambient temperature, whichever is lowest.
Another factor that is rarely addressed is that retarding admixtures can keep the concrete in a liquid state much longer than usual. This is especially true for cold weather. This can greatly increase the pour pressure. We have had forms blow out when the combination of too fast a pour rate, retarded concrete and cool weather doubled pour pressure. It is appreciated when the inspector questions the crew and alerts them to those factors.
For the reasons of pour pressure and stiffness to maintain alignment, wall forms should be designed for at least 750 pounds of pour pressure per square foot. The standard two by four wood form with 5,000-pound snap ties spaced at two-foot centers will support about a 1,000-psf pour pressure. Columns are designed to support up to a 3,000-psf pour pressure. Most commercial steel forms are designed to handle a pour pressure of at least 1,000-psf. When a particularly flat wall is required, the form sheathing can control the pour rate. In some situations where the sunlight is nearly parallel to the wall, the bulge of the form between studs will be highlighted by a shadow pattern on a smooth wall.
The form dimensions, elevations, and survey control should be carefully checked. There are many examples of concrete structures being built in the wrong place or the wrong size. I once saw an entire major bridge abutment build ten feet too low because of a survey error. When an alert inspector spots such errors, they can be easily corrected before major cost and delays occur.
Forms are usually made of wood or steel, although fiberglass and water proofed cardboard forms are often used for small round column forms. Wood forms have the advantage of being made to any shape and are usually less expensive to fabricate than steel forms. Wood forms that are sheathed with standard BB form plywood will last for 4 to 10 reuses, depending on the care of handling, maintenance and the quality of surface required. If many reuses of wood form are planned, then special MDO or HDO plywood should be used for sheathing. Wood forms are much lighter than steel forms and require smaller cranes to move than steel forms. It is much easier to attach block outs, embedded item, textures and make form penetrations with wood forms.
Steel forms have the advantage of stiffness to maintain better alignment and often have much higher erect and strip productivity due to wider tie spacing. If a very smooth wall surface is required, steel forms may be needed. It is also easier to vibrate the air and water causing bug hole out of the steel form face. All forms should be check for a complete tie and bracing system and that form oil has been properly applied.
It is becoming
more common for concrete surfaces to be textured by a bas relief pattern, even
pictures are created. If wood patterns are used, they must have at least a 30-degree
bevel on all edges. The edges should be sanded smooth and sealed with epoxy
or waterproof enamel paint.
If those precautions are not used the pattern will be very difficult to strip
and will often spall the concrete when stripped. In fact, all forms should have
relief points to ease the stripping process and reduce the chance of spalling
the concrete and damaging the forms.
All embeds, blockouts, pipes and conduits that are cast in by the concrete should be thoroughly checked for proper location, orientation, support and accuracy before the pour is released for concreteing. All chamfer strips should be in place. On large complex, projects lift drawings should be produced. These lift drawings are made for each individual pour. They should include:
1. All pour dimensions
and survey control references.
2. Concrete mix to be used and pour volume should be shown.
3. The form design pour pressure should be listed.
4. Pour rates for a range of expected temperatures should be included.
5. Concrete placing methods should be noted, such as tailgate, crane, belt,
or pumping.
6. If access or reach is a concern a placing plan should be made, showing the
locations of the placing equipment.
7. Form areas and type of form to be used.
8. Reference to the form lift drawing.
9. Required surface finishes for wet and dry finishing.
10. Contact surface requirements such as water or sand blasting.
11. Shear keys and waterstop should be detailed.
12. Reference the rebar detail drawings for that particular lift.
13. At embedded items should be located and detailed.
14. All pipes and conduits should be detailed and located.
The prime contractor should generate the lift drawings with layout plans that locates all lift drawings and construction joint locations. These drawings should be distributed to every subcontractor and craft involved with the pour. Well before the planed start of the lift construction all comments, suggestions and conflicts should be resolved. If possible, the preliminary lift drawings should also be issued to the design engineer, architect, and inspectors for comment. Comprehensive and well-prepared lift drawings keep everybody informed and help eliminate costly mistakes of omission. Another huge advantage is the Quality assurance team can easily spot errors as they occur and the prepour inspection can be done in a minimum of time and confusion.
The time to correct conflicts and establish procedures is well before the work starts. On one project that we did, the civil, electrical and mechanical were separate prime contracts. There was very little coordination of the various contracts. The 6" electrical conduits were cast in per the electrical drawings, which did not show the equipment. After we completed our contract, it was discovered that the conduits and machinery were incompatible. In another incident, we were not shown the huge radiation-proof bank vault type doors until after the concrete work was complete. More than 1,000 cubic yards of heavily reinforced concrete had to be demolished and replaced. The project had several incidents of miscommunication that increased the project cost by more than 50%. In this case, the design team was at fault for failing to communicate with the field team and each other. The project quality assurance team could have prevented all of the costly mistakes if there had been better coordination and cooperation.
There are several types of wet finishes that can be specified. A buried footing may only need screeding. An industrial floor may require a super flat finish. Normal industry practice achieves a flatness of about ¼" in ten feet. Due to survey tolerance, screed straightness, wet finish effort, concrete shrinkage and deflection; the best that can normally be achieved by steel troweling is about 3/16" variation in ten feet. If flatter surfaces are required, special toppings or grinding are usually employed. If non-skid surfaces are needed, the surface can be broomed or textured by several methods to achieve the desired result. Usually after screeding the surface is tamped or floated to push the coarse aggregate to just below the surface. Then the troweling and texturing can be uniformly applied to the sand-cement surface. This action prevents the tearing of the surface caused by the trowel displacing the coarse aggregate at the surface.
Proper curing the concrete for the first week or two is very important. The purpose of curing is to prevent migration of water through the concrete surface. If the surface dries to quickly the cement loses the needed hydration water. This causes a weak surface subject to cracking and dusting. Introducing too much water, especially if flowing, will increase the water cement ratio of the surface cement and reduce strength and durability. There are several effective ways to properly cure concrete surfaces: water cure, spray on membrane compounds and covering the surface with sealed visqueen are some of the commonly used methods. Membrane compound cure can prevent the adhesion of paints and coatings. Water curing can actually weaken the surface by increasing the water to cement ratio. This will reduce the durability of the surface concrete. Visqueen is a very good way to cure concrete. The only drawback is that it is easily torn and must be maintained in a sealed condition to prevent rapid evaporation of the surface. If the surface dries out too quickly it will dust and micro crack due to loss of hydration water. Some of the best results I have seen is the combination of spray-on silicon based floor hardener and Visqueen. If the surface is to be painted or bonded then any membrane curing compounds should not be used unless the surface is to be heavily sand blasted.
Curing efforts in cool damp weather can be minimal. Curing concrete at temperatures about 40 F to 50 F degrees is ideal for long term strength and durability. Concrete can cure at a temperature of 25 F degrees. However, when concrete temperatures are maintained below 50 F degrees the concrete should be monitored for strength gain using field cured cylinders before structural loads are applied.
Curing concrete in freezing weather requires special attention. Concrete should not be placed on any frozen surface. The concrete must be heated or insulated so that the heat of hydration will cure the concrete enough so that freezing will not cause damage. The concrete should be cured at least 72 hours at about 70 F degrees before being allowed to be exposed to freezing temperatures. Rapidly or prematurely removing the insulation or heating can cause thermal shock. If the surface is exposed to rapid temperature decrease while the internal temperature is high the resulting surface thermal shrinkage can cause cracking. The heat or insulation should remain in place until the internal temperature of the concrete is no more than 35 F degrees above the ambient temperature.
Proper curing in hot dry windy weather is vitally important. A hot dry wind can damage the concrete surface in just seconds. Curing efforts should immediately follow behind the pour and finish work or as soon as the forms are stripped. Usually vertical wall type forms can be stripped the next day unless there is a structural reason for them to remain in place longer. Forms left in place for more than a few days will bond to the concrete and the surfaces of the concrete and forms are often damaged by the stripping effort. Often during very hot weather, above 100 F degrees), the placing is done at night to take advantage of any daily cooling.
Dry finishing of
concrete can be minimal or very extensive depending on the esthetic requirements.
In any case it is best to perform the plugging, patching and any sacking required
as soon as the forms are stripped. Well formed and placed concrete usually requires
minimal finishing effort, so it is worth a little extra effort to do quality
forming and placing of exposed concrete surfaces. Normally there will be some
color difference in the concrete surfaces. This is due to different ages and
slight proportioning variances in the concrete contents. If this is a concern,
the owner may want a special surface preparation, such as sandblasting, sack
rub finish or painting.
Course Summary
Concrete Quality assurance is a team effort requiring precise testing, detailed planning, expert observation and professional craftsmanship from mining to ribbon cutting. The quality of the concrete can be compromised at every stage of the process by poor workmanship, lapse of proper testing or failure to prepare for contingencies. When each team member expertly performs their task a pride of accomplishment is realized by all. For that reason alone it is important that everyone know why their job is important and the effects caused by applying their efforts.
Bibliography
1. Building Code
Requirements for Structural Concrete (ACI 318-95)
2. Reinforced Concrete Design, 1965 by Chu-Kia Wang and Charles G. Salmon
3. Handbook of Heavy Construction, 2nd edition, 1971 Edited by John
A. Havers and Frank W. Stubbs, Jr.
4. Design and Control of Concrete Mixtures by the Portland Cement Association,
13th Edition
5. Concrete Manual by the Bureau of Reclamation, 8th Edition - Revised
Once you finish studying the
above course content,
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DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDHonline.com or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered professional engineer. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.
