Common problems encountered during concrete placement, probable causes, and recommended corrective actions are presented in Table 8.1.
Chapter 8 – Concrete Placement, Finishing, Texturing and Curing
Problem Probable Cause(s) Action
Premature stiffening of concrete with little evolution of heat.
False setting cement. Do not add water. Plasticity can be restored with additional mixing. Notify cement supplier.
Premature stiffening of con- crete with evolution of heat, lack of working time.
• Cement with too little or the wrong form of sulfates.
• Class C fly ash with high Al2O3 content.
• High placement tempera- tures.
• Lignosulfonates in water-re ducing admixture.
• Triethanolamine (TEA) in water-reducing admixture.
• Use of accelerator.
• Wrong mixture design for hot weather (high cement content, Type III cement, no SCMs).
• Dry, sorptive aggregates absorbing water from the mixture.
• Hot (fresh) cement.
• If using an accelerator, stop using it or reduce the dosage.
• Reduce the placement tem- perature of the concrete by any convenient method(s).
• In hot weather, use the hot weather mixture design.
• If using Class C fly ash, re- duce the dosage.
• Make sure the aggregates are damp at the time of batching.
• Switch to a water reducer that does not contain lignosulfonates or tri- ethanolamine (TEA). (Con- sult the admixture supplier for advice.)
Slump out of specifications or varying.
• Change in water content or aggregate grading.
• The concrete temperature is too high.
• Check aggregate moisture contents and grading.
Stockpiles should be of con- sistent grading and aggre gates need to be moist.
Make sure batch water is adjusted for aggregate moisture content.
• Check whether extra water was added at the site.
• Perform mixer uniformity test.
• Note the batch time on the concrete delivery ticket.
Haul times should not ex- ceed allowable time.
Table 8.1 Concrete Placement Troubleshooting
Table 8.1 Concrete Placement Troubleshooting (continued)
Problem Probable Cause(s) Action
Slump loss greater than 1 in.
between the plant and the paver.
• False setting tendency.
• Material incompatibility.
Check cement composition, mixing time, and admixture compatibility.
Inconsistent air content. • Variations in pozzolans (Class C fly ash can vary in carbon content, which strongly affects the required dosage of air-entraining ad- mixture).
• Change in cement source, type, or brand.
• Change in sand grading.
• Inadequate or variable mix- ing due to worn mixer blades, an overloaded mixer, or varying mixing times.
• Concrete temperature ef- fects.
• If using Class C fly ash, ex- pect variations in the re- quired dosage of the air- entraining admixture. Moni tor air contents closely and adjust admixture dosages as necessary.
• If the air contents drop be- tween the cool morning and hot afternoon, it may be due to the change in concrete temperature. In that case, increase the dosage of air- entraining admixture as the temperature rises.
• If a sudden change seems permanent, look for a change in the materials sup- plied (cement, admixtures, etc.).
• Check the sand stockpile to see whether the grading has changed.
• Examine the mixer (fins) and mixing procedures.
• Contamination of one of the ingredients with organics can also effect a sudden change in the required dosage of the air-entraining admixture. Try to isolate the source.
Excessive concrete tempera- ture.
• Ingredients may be hot at batching; check the temper- ature of aggregates, ce- ment, and SCMs.
• Long haul times.
• Hot weather.
• Follow hot weather concret- ing practice, as appropriate.
• Minimize haul times.
Chapter 8 – Concrete Placement, Finishing, Texturing and Curing
Table 8.1 Concrete Placement Troubleshooting (continued)
Problem Probable Cause(s) Action
Failure to set. • Organic contamination.
• Excessive retarder.
• Excessive water reducer.
• Retarder not dispersed.
• Cold weather.
• Check for contamination of water, aggregates, and equip- ment.
• Reduce dosage of retarder and water reducer.
• Improve mixing to disperse re- tarder.
• Follow cold weather concreting practices, if appropriate.
Sticky mixture. • Use of higher dosages (>5 per cent) of silica fume.
• Sand too fine.
• Using wood float on air-entrained concrete.
• Change sand source.
• Use magnesium or aluminum floats.
Honeycombing. • Hot weather may induce prema- ture stiffening.
• Inadequate vibration.
• Changes in aggregate grad ing will affect workability.
• Dry aggregates.
• High paver speed.
• Follow hot weather concreting practices, if appropriate.
• Check that all vibrators are working properly, and at the right frequency and amplitude.
Paver speed should not be too high.
• Check aggregate grading.
Edge slump. • Poor and/or nonuniform con- crete.
• Improper operation of paving equipment.
• Verify mixture design and batching procedures.
• Check aggregate grading and moisture.
• Check concrete placement pro- cedures.
Popouts • Unsound aggregates.
• Clay balls.
• Check aggregates for sound- ness.
• Check for inter-mixing of aggre- gate with soil.
Scaling or dusting. • Over-finishing.
• Premature finishing.
• Early freezing of concrete.
• Improve finishing technique.
• Protect concrete from freezing.
Concrete damaged by freezing needs to be removed and re- placed.
Table 8.1 Concrete Placement Troubleshooting (continued)
Problem Probable Cause(s) Action
Plastic shrinkage cracking.
• Excessive loss of moisture from fresh concrete.
• Use an accelerator to make concrete set faster.
• Protect concrete from loss of mois- ture both before and after placement:
fog spray or immediate application of evaporation retardant or curing com- pound.
• Make sure absorptive aggregates are kept moist.
• Refer to hot weather concreting prac- tices, if appropriate.
Random cracking. • Shallow sawcut/ late sawing.
• Dowel misalignment.
• Bonding with stabilized base.
• Sudden cold front.
• Excessive joint spacing.
• Saw sooner and check sawcut depth.
• Check dowel alignment.
• Take cores to check interface bond.
• Review joint spacing.
Raveling of sawcut.
• Sawing too soon. • Wait longer to saw.
• Check blade compatibility.
• Use early-entry, dry saw.
Joint spalling. • Excessive hand finishing.
• Trying to fix edge slump of low spots by hand manipulated concrete.
• Nonuniform concrete, result- ing in wavy longitudinal joint that spall when sawed.
• Collateral damage from equip ment, slipform paver tracks, screeds, etc.
• Improve construction practice.
Low strength con- crete samples.
• Errors in batching and/or mix- ing of concrete.
• Incompatibility between ce- ment and air-entraining admix- ture causing coalescence of air voids around aggregate particles.
• Improper sample preparation, curing, handling, or testing.
• Verify entire process of making, cur- ing, handling, and testing. Flexural specimens are particularly vulnerable to poor handling and testing proce- dures.
• Verify entire batching and mixing process.
• Trial batches can help eliminate the possibility of incompatibility. A quick visual examination of a cut specimen will identify any coalescence of air voids.
Chapter 8 – Concrete Placement, Finishing, Texturing and Curing
Table 8.1 Concrete Placement Troubleshooting (continued)
Problem Probable Cause(s) Action
Smoothness problems. • Nonuniform concrete.
• “Stop and go” paver operation.
• Too much or too little concrete in front of paver.
• Frequent use of construction headers.
• Use of light paver.
• Check batching procedures.
• Check aggregate grading.
• Improve construction proce- dures.
• Add extra haul trucks, if neces- sary, or slow paver.
• Improve paver operation.