Our industry prides itself on quality, both in materials and workmanship. I doubt if any tradesman wakes up in the morning and says to himself, "Today I think I'll include some defects in my work." So why do we accept uncontrolled cracks in concrete slabs as unavoidable? It's a result that no one wants and no one wants to explain, especially to an angry client.
我们甚至用一个陈词滥调来为这种现象辩解:“混凝土有两种作用——它会变硬, and it cracks." But I disagree with that claim. 事实上, as a commercial construction superintendent, I have poured 15,000-square-foot slabs containing 260 cubic yards of concrete without a single crack. 我所做的和其他成千上万的商业承包商每天所做的没有什么不同.
住宅混凝土通常不需要满足与大多数商品混凝土相同的质量要求, but nevertheless, 你可以使用我将描述的技术和原则来消除板坯中不受控制的开裂. If you're pouring your own slabs, this information is essential. If you are a general contractor or builder, you can use this information to more effectively supervise and assist your subcontractor. 真正的, the concrete work may legally be within the subcontractor's scope of work, but it's your project, and there's precious little to be done if the slab comes out poorly. Better to take care of problems before they happen. 如果我必须用一个词来总结好的具体工作的秘诀，那就是“一致性”." For a successful outcome, you must exercise the same level of care at every stage of the process — design, preparing the subgrade, 放置, 完成, and curing.
如果浇筑后地基沉降，就会增加裂缝的可能性. It seems obvious, but this is the cause of far too many problems. Several things can contribute to subgrade settlement, 但两个主要的罪魁祸首是跨板区域承载力不同的土壤和路基压实不当.
Always make sure the soil is free of topsoil and organic matter. If you have any doubt about the bearing capacity of the soil you're building on, check with the local building department or other area builders. If you're still not sure, get the soil tested. 你花几百美元做土壤分析和压实测试，比起回去修复破损的地基根本不算什么.
As the subgrade elevation is being established, make sure that the soil is uniformly and properly compacted (see "Soil Compaction Basics," 3/94). 压实度将以最佳含水量(通常为95%或更高)下的百分比表示。, and it's worth verifying. If you decide not to do a compaction test, 至少运行板篡改或跳跃千斤顶，直到几乎没有留下印象与每个连续的传球. It is imperative that soil be compacted in lifts, ideally about 6 inches at a time and never more than 10 inches. If you attempt to compact greater depths, 表面看起来很好，但下面的土壤仍然没有压实，最终会沉淀下来.
Also, check for uniform bearing capacity of the subgrade. 挖掘机是否因为第一次挖得太深而把几桶沙子扔进了一个角落? Will one corner of the slab rest on rock while the remainder is over sand? 现在最好是用合适的土壤代替有问题的填充物，这样在适当的压实后，填充物就会像其余的路基一样. What constitutes a suitable soil will vary with local conditions. If you are in doubt, get a professional opinion.
Optimal moisture content happens in a narrowly defined range, so while you may need to add water to dry soils for compaction, take care not to add too much. 有时, 由于不可预测的天气条件和常规的分级和排水困难, residential sites get too wet before compaction can take place. 什么构成太湿将因土壤类型而异，但它不一定是泥泞的太湿. 如果你在土壤中看到泵动的迹象，或者在压实设备附近的土壤接近液化, you must stop and correct this condition. 在这一点上，你的选择是有限的:要么等待土壤变干，要么过度挖掘并更换它.
适当准备的路基应在板的整个区域均匀压实, leveled to within 1/2 inch.
Flat on the bottom. It's helpful to picture your slab upside down. If it's almost as smooth on the bottom as it is on the top, the battle is half won. If it looks like a crater on the moon, then your crack has already begun. Soil must be graded to the same level across the full width of the slab.
适当准备的路基应在板的整个区域均匀压实, leveled to within 1/2 inch. Your subgrade should be level within 1/2 inch (that's a 12.5% variation in your slab thickness on a 4-inch slab). Where subgrade elevation is inconsistent, then the slab will vary in thickness. This variation will cause it to cure unevenly, which will stress the concrete, increasing the likelihood of a crack.
The shape of the slab can greatly affect cracking. An inside corner, for example, is a pressure point and, as far as the concrete is concerned, a great place to crack. 试着用收缩缝(也叫控制缝)把你的板子分成对称的正方形，而不是用很多可重复进入的角来创造不规则的形状. Contraction joints can be hand-tooled right after the bleed water leaves the surface, cut with a saw right after 完成, 或在湿混凝土中使用专有的塑料或金属接头材料之一形成. No matter how you make them, all contraction joints work the same way. By creating an intentionally weakened point in the slab, 由不可避免的收缩(由于干燥或温度变化)引起的应力在可预见的情况下得到缓解, controlled manner.
收缩缝的深度应该是楼板的四分之一，或者是4英寸楼板的1英寸. 更少一些, and it may not function as designed; any more, and you will unnecessarily weaken the slab, risking vertical displacement of the concrete if the subgrade should move.
When laying out a slab, keep in mind that squares are better than rectangles. 如果你必须使用矩形，确保长边不超过短边的1.5倍. Where it isn't possible to avoid closely spaced re-entrant corners, additional steel reinforcing may help control cracks.
When laying out contraction joints on a typical 4-inch residential slab, 8- to 12-foot spacing is reasonable. Under ideal conditions, spacing up to about 15 feet between joints may be possible.
For a finished appearance at the slab edge, 在台锯上刻下纤维板，这样顶部1/2英寸可以很容易地在浇注后折断. Then finish the joint with sealant.
If the slab is contained by walls, 您必须在基础的内周长周围提供伸缩缝(也称为隔离缝)，以使板独立于墙壁移动. 同样的道理也适用于结构柱——如果它们在楼板的区域内的话, they should be surrounded with expansion joints.
Residential expansion joints are often made with 1/2-inch-thick fiberboard strips. The material can be flush with the surface of the slab if appearance is no concern. For a more finished look, 在台锯上运行纤维板的长度，以在顶部边缘(将在表面标高处)下方1/2英寸处得分，这样您可以在浇注后轻松移除顶部1/2英寸. Then use a caulk to seal the slab edge.
Remember that the slab must be isolated from the footing, too. Either sand or 15# felt between the slab and footing works well as a bond breaker.
用毛毡纸或砂子将基脚和板坯之间的粘结断开，有助于防止板坯移动时开裂.
Proper placement of reinforcement is critical. At the very least, reinforcing mesh, 管道, and cables should be suspended so that none lie on the bottom of the poured slab. Wire mesh should be in the middle of the slab if it is to do its job properly. Pipe or conduit laid directly on the ground will dramatically weaken the slab; it has the same effect as scoring a ceramic tile before snapping it. 然而，同样的管道悬挂在下面一英寸的混凝土将没有效果.
Placing rebar and 管道 so concrete can flow underneath them prevents stress risers.
Make use of steel or plastic rebar chairs to get the rebar in the right place. Do not use 砖, 因为它会过快地从周围的混凝土中吸收水分并产生应力点.
Measure the exact volume of concrete required, then add 10% as a buffer. If you underestimate and have to call for a short load, not only do you waste time, but you may get cracks. 确保混凝土供应商有所需的卡车来供应你的项目，而不必在装载之间等待一个小时. Excessive delay can result in a hot load. If the mix comes out of the chute hot, then send it back.
Although few residential contractors use them, slump tests ensure that the concrete coming out of the truck is not too wet. If better flow is needed, add a superplasticizer.
衰退. Proper water content is one of the most important elements of good quality concrete; adding water to concrete to make it flow has a measurable negative result. A slab mix should not have a slump greater than 3 to 4 inches. If smoother flow is required, add superplasticizer, which gives the effect of a 6- to 8-inch slump without weakening the mix.
Concrete is actually a delicate material until it has fully cured. How you treat it during placement is crucial to your success. To avoid segregating the 总 from the cement paste, never drop concrete more than 6 feet, use the proper concrete tools (not steel rakes), and don't drag or vibrate the concrete into place. If the soil is particularly dry or you're working in hot, windy weather, you should wet the subgrade, but not enough to create mud or leave standing water. In cold weather, do not place concrete on frozen ground.
Concrete should be bull-floated as soon as it has been screeded.
很快就会出现渗水，但要确保在完成之前就消失了.
确保在现场有足够的人力来保持浇筑混凝土的连续性. If 放置 a slab takes too long, the first concrete placed will begin to set up well before the last concrete placed. This makes it difficult to finish the slab in one continuous process.
Make sure concrete is screeded and bull-floated as it's placed, but don't allow 完成 to begin until the bleed water has disappeared. If bleed water is worked into the concrete while 完成, the surface will be weakened. For the same reason, don't allow finishers to add water in an attempt to ease 完成.
为了保证施工顺利进行，在浇筑混凝土时，用手将混凝土板的四周浮起来是个好主意.
A well-organized, well-timed, uninterrupted work flow is essential. 当混凝土被放置时，沿着墙壁和障碍物周围用手铲是一个很好的开始.
所有的放水应该在任何手工处理或动力铲开始之前离开表面.
Even with a power trowel on the job, some hand-完成 is necessary around obstructions.
Have the power trowel fueled up and ready to go before you need it. If you exercise a little patience before power trowelling is begun, 您可以避免替换较软的混凝土，并在成品板上造成潜在的低点.
If there is any chance of freezing, blankets are used to protect the subgrade before 放置 the slab.
The fresh concrete is then covered for at least three days.
Don't forget to take care during the curing stage. Initially, the temperature will rise quickly, peak, and then start to drop. This process is stressful on a new slab and can cause failures. Wet curing keeps the slab from drying out too fast during this process. Cover a fresh slab with a layer of burlap immediately after 完成, and keep it wet with a lawn sprinkler for at least three days (ideally seven days). Or, cover the slab with a layer of plastic. Wet curing will ensure an even and controlled cure. In cold weather, make sure that fresh concrete is protected from freezing.
