Bldg Materials-common Concrete Problems

COMMON CONCRETE PROBLEMS

COMMON CONCRETE PROBLEMS

Problem Cause

Prevention & CorrectionsExcessive BleedingExcess water in the mixReduce water content

Not enough fines in the mixIncrease the cement or sand

Aggregate SegregationToo high slumpAdd superplasticizer

Over vibrationReduce vibration in each location

Not enough vibrationIncrease vibrator at close intervals

Too long of drop for placementUse appropriate drop chutes

Lack of homogeneity in the mixIncrease mix time, add air entrainment

Sticky FinishToo much air entrainmentReduce air and/or fines

Surface drying too fastSpray water on subgrade and forms

Setting too fastHigh temperaturesUse cooler water in mix or cool aggregate with water

High cement contentAdd water reducing retarder

Trucks getting hot from sunSprinkle mixing drums during wait or reschedule deliveries for minimum wait time

Setting up too slowMix too leanIncrease cement content or use accelerator

Cold/wet subgradeInsulate and or protect before placement of concrete

Plastic shrinkage crackingToo rapid loss of water. High winds, heat or low humidityFog spray at curing. Do not use vapor retarder. Set up wind breaks

Low yield Wrong ratio of water, cement and aggregatesHave test run

FLAT SLABS

Shrinkage cracksWrong spacing and depth of contraction jointsCut joints the depth of slab. Space joints at 2 times the thickness

Late joint sawingPlan to saw the same day as pour, within 4 hours

No isolation joints at walls or columnsInstall isolation joints

Excessive shrinkageReduce water in mix

Hairline cracksToo high slump of concreteReduce water

Too early trowelling or too much trowellingReduce the methods of trowelling by contractor

Rapid surface dryingCure concrete immediately after finishing

BlisteringSlab closed too soon, trapping air and waterDelay floating and trowelling

CurlingUneven drying between top of slab and lower parts Use lower slump

WALLS

HoneycombNot enough vibrationReduce height of the pour. Insert vibrator more often. Add superplasticizers

Surface voidsAir or water trapped against formworksReduce air entrainment. Vibrate as close to form as possible