{"id":86360,"date":"2019-10-11T19:11:02","date_gmt":"2019-10-11T23:11:01","guid":{"rendered":"https:\/\/zonda-develop.go-vip.net\/concrete-construction\/article\/troubleshooting-common-slab-problems_o"},"modified":"2019-10-11T19:11:02","modified_gmt":"2019-10-11T23:11:01","slug":"troubleshooting-common-slab-problems_o","status":"publish","type":"post","link":"https:\/\/zonda-develop.go-vip.net\/concrete-construction\/how-to\/construction\/troubleshooting-common-slab-problems_o\/","title":{"rendered":"Troubleshooting Common Slab Problems"},"content":{"rendered":"

While some of the things that cause problems with concrete slabs have changed over the past 20 years, many (maybe most) have not. Looking back though the Concrete Construction <\/i>archives, we find many articles that are as relevant today as they were when written. Here, then, are three common problems with concrete slabs and solutions based on previous CC<\/i> articles. We\u2019ll cover three more of these in the next issue.<\/p>\n

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Scott Tarr \/ North S.Tarr Concrete Consulting<\/p>

This project had variable slump and setting behavior so the finisher had to squeegee to move the excess bleed water off the slab in some areas. This crew did a great job not finishing the bleed water back into the surface.<\/p>\t\t<\/figcaption>\n\t<\/figure>\n\n

Problem: Excessive Bleeding<\/b>
Bleeding is a result of the heavy components in concrete (sand and aggregate) settling and pushing the extra water of convenience to the top. Bleeding isn\u2019t always bad. It lowers the water-cement ratio and densifies the concrete. But concrete that bleeds too fast or for too long can cause a number of problems: rock jams in pump lines, sand streaks in walls, weak horizontal construction joints, and voids beneath rebars and
aggregate particles.<\/p>

Even when bleeding isn\u2019t excessive, finishing concrete flatwork at the wrong time causes different problems. Finishing before bleed water has evaporated can cause dusting, craze cracking, scaling, and low wear resistance. Working bleed water into the surface also increases permeability, allowing water, de-icing salts, and other harmful chemicals to enter the concrete more easily.<\/p>

Solution:<\/b> Excessive bleeding can be avoided. Don\u2019t add too much water to the concrete. Most of the water added to make placing easier bleeds out of the concrete. Any time saved during placement will be lost while waiting for the bleed water to evaporate. Place concrete at the lowest possible slump. If you need a higher slump to speed placement, consider using a superplasticizer. Add additional fines in the concrete, such as:
\u2022 <\/b>More finely ground cement. Concrete made with high early strength (Type III) cement bleeds less because the cement is ground finer than normal (Type I) cement.
\u2022 <\/b>More cement. At the same water content, rich mixes bleed less than lean mixes. But also consider that higher-strength slabs can curl more.
\u2022 <\/b>Fly ash or other pozzolans. However, Bruce Suprenant advises finishers to consider the effect of pozzolans on fresh concrete properties. The set time and bleeding of the fresh concrete mix can change so adjust finishing practices accordingly.
\u2022 <\/b>If concrete sands don\u2019t have much material passing the No. 50 and 100 sieves, blend in a fine blow sand at the batch plant.
\u2022 <\/b>For air-entrained concrete, use the maximum allowable amount of entrained air. Entrained air bubbles act like additional fines. Air entrainment also lowers the amount of water needed to reach a desired slump.<\/p>

To explore bleeding in more depth, check out these articles:<\/p>