竹石
清·郑板桥
咬定青山不放松，立根原在破岩中；
千磨万击还坚劲，任尔东西南北风。

（图片来自网络）

标题用了郑板桥的诗句，但是改动了一个字。其原因来自最近一个新闻报道《浙江义乌百万豪宅被曝用毛竹代替钢筋 清晰可见》，具体的内容是一些居民发现阳台的护栏居然是用毛竹填充的，考虑到房价百万之巨，这毛竹实在是有点‘举重若轻’了！网上自然是一片骂声，房地产商自然又要背上‘草菅人命’之名了。竹子属于草科的一种，说是草菅人命倒也是贴切的很。

不过作为材料研究者，我倒认为这个竹片替代钢筋的混凝土材料是一个很有创意的想法。竹子作为自然界最强韧的纤维植物，自古以来就被人用来做各种器皿和容器，而且也是古代和近代百姓主要的运输工具：小小一根扁担居然可以负重近百公斤而长时间不折断！由于中国绝大领土都在热带和亚热带地区，竹子已然成为人民生活中不可或缺的家庭必备物品。我还清楚地记得小时候外婆家门后就放着几根扁担，外婆和外公也都是在竹篾社工作，编织竹席、竹篮、竹凳、鱼篓，甚至还有抓鱼用的竹鱼蒿。外公自然是没有零用钱给我们买商店里的玩具，但是每次用竹子做成的刀剑都会让我们成为岳飞一般的战将！

实际上，竹子作为现在建筑中的填充材料并非完全儿戏，而是有一定的实际意义。首先是竹子价格便宜，对于某些钢材缺乏的地方是最佳的选择。竹子生长周期短，短短几月就成为世界上最高的草。当然最关键的是竹子的拉伸强度很可观，大约有50兆帕左右，虽然不及钢铁，但是已经是自然界有机材料中最强韧的纤维梯度材料。竹子很轻，这样从轻型化来考虑具有优势。然而，竹子最致命的缺陷在于易受虫类叮咬，这对抗弯强度影响是决定性的。另外竹子容易吸水，作为复合填充材料显然也不是好消息。

美国实际上很早就对竹片填充混凝土的可行性做了研究，根据海军的城市工程实验室在1966年的研究‘BAMBOO REINFORCED CONCRETE CONSTRUCTION’，竹子作为非支撑型墙面还是存在可行性，但是需要满足一定的填充条件：

Non-bearing concrete walls should have a thickness of not less than 5 inches and not less than 1/30 the distance between the supporting or enclosing members; they should be reinforced with at least 3/4-inch-diameter culms on 6-inch centers in both vertical and horizontal directions. This reinforcement should be provided as a one-layer mat in the middle of the wall. Two bamboo culms 1/2 inch or more in diameter should be placed above and at the sides of openings, and two 3/4-inch-diameter culms 4 feet long should be placed diagonally across the corners of openings.

近年来得克萨斯州大学也开展了一系列的相关工作，其中对于竹子的本征强度以及竹节的影响做了原创性的研究。有兴趣的见论文《PERFORMANCE EVALUATION OF BAMBOO REINFORCED CONCRETE BEAMS》，作者为LEENA KHARE。从实验结果上来看，竹节似乎对于竹片的拉伸强度影响规律不太明显。微观上来说竹节中纤维的密度更高但是取向性较差，竹体经常会先承受较大的拉伸应变然后传递到竹节区域。我在2008年年初回湖南的时候曾经仔细观察了大雪后竹子的断裂，绝大部分都在竹干中间折断，说明竹节部位的抗弯强度肯定要比竹筒部位大。当然，原始竹子竹节中间是存在一层隔膜的，这和作为填充物时又有所区别。在填充了竹片以后，混凝土的抗弯强度和空白样品比一般都增强了2.5倍，在2-3%的填充量下达到最优值。一个有意思的对比结果是和填充钢筋的效果之比，在4%填充量时居然达到钢筋混凝土效果的66%，低填充量时也达到了很高的抗压载荷能力。

（摘自LEENA KHARE论文）

美国农业部在1953年也给过一些实际操作的建议：

1. Bamboo reinforcement in concrete beams does not prevent the failure of the concrete by cracking at loads materially in excess of those to be expected from an unreinforced member having the same dimensions.

2. Bamboo reinforcement in concrete beams does increase the load capacity of the member at ultimate failure considerably above that to be expected from an unreinforced member having the same dimensions.

这条说明竹片有效果

3. The load capacity of bamboo reinforced concrete beams increased with increasing percentages of the bamboo reinforcement up to an optimum value.

这条说明存在最优值

4. This optimum value occurs when the cross-sectional area of the longitudinal bamboo reinforcement was from three to four percent of the cross-sectional area of the concrete in the member.

最优截面比大约在3-4%

5. The load required to cause the failure of concrete beams reinforced with bamboo was from four to five times greater than that required for concrete members having equal dimensions and with no reinforcement.

断裂强度可以提高4，5倍

6. Concrete beams with longitudinal bamboo reinforcement may be designed to carry safely loads from two to three times greater than that expected from concrete members having equal dimensions with no reinforcement.
安全载荷可增强2到3倍
7. Concrete beams reinforced with unseasoned bamboo show slightly greater load capacities than do equal sections reinforced with seasoned untreated bamboo. This statement was valid so long as the unseasoned bamboo had not dried out and seasoned while encased in the concrete when the load was applied.

到底是经年的竹子好还是新鲜的好还存在争论，重点是在混凝土中不能因为吸水而导致竹片载荷传递。

8. When unseasoned untreated bamboo was used as the longitudinal reinforcement in concrete members, the dry bamboo swelled due to the absorption of moisture from the wet concrete, and this swelling action often caused longitudinal cracks in the concrete, thereby lowering the load capacity of the members. These swell cracks were more likely to occur in members where the percentage of bamboo reinforcement was high. This tendency was reduced by the use of high early strength concrete.

9. The load capacity of the concrete members reinforced with bamboo vary with the dimensions of the members.

10. The unit stress in the longitudinal bamboo reinforcement in concrete members decreased with increasing percentage of reinforcement.

11. The ultimate tensile strength of the bamboo in bamboo reinforced concrete members was not affected by changes in the cross-sectional area of the members so long as the ratio of breadth to depth was constant but was dependent on the amount of bamboo used for reinforcement.

12. Members having optimum percentage of bamboo reinforcement (between three and four percent) are capable of producing tensile stresses in the bamboo of from 8,000 to 10,000 pounds per square inch.

13. In designing concrete members reinforced with bamboo, a safe tensile stress for the bamboo of from 5,000 to 6,000 pounds per square inch may be used.

14. Concrete members reinforced with seasoned bamboo treated with a brush coat of asphalt emulsion developed greater load capacities than did equal sections in which the bamboo reinforcement was seasoned untreated or unseasoned bamboo.

15. When seasoned bamboo treated with a brush coat of asphalt emulsion was used as the longitudinal reinforcement in concrete members, there was some tendency for the concrete to develop swell cracks, especially when the percentage of bamboo reinforcement was high.

16. Care should be exercised when using asphalt emulsion as a waterproofing agent on seasoned bamboo as an excess of the emulsion on the outer perimeter of the culm might act as a lubricant to materially lesson the bond between the concrete and bamboo.

17. Concrete members reinforced with unseasoned sections of bamboo culms, which had been split along their horizontal axes, appeared to develop greater load capacities than did equal sections in which the reinforcement consisted of unseasoned whole culms.

18. Concrete members reinforced with seasoned sections of bamboo culms, which had been split along their horizontal axes and treated with a brush coat of asphalt emulsion, developed considerably higher load capacities that did equal sections in which the reinforcement was split sections of seasoned untreated bamboo.

19. When split sections of seasoned untreated large diameter culms were used as the reinforcement in a concrete beam, longitudinal cracks appeared in the concrete due to the swelling action of the bamboo. This cracking of the concrete was of sufficient intensity as to virtually destroy the load capacities of the members.

20. When unseasoned bamboo was used as the reinforcement in a concrete member, the bamboo seasoned and shrank over a period of time while encased in the concrete. This seasoning action of the bamboo materially lowered the effective bond between the bamboo and concrete with a lessening of the load capacities of the members.

21. Increasing the strength of the concrete increases the load capacities of concrete members reinforced with bamboo.

22. Concrete members reinforced with seasoned bamboo treated with methylolurea did not develop greater load capacities than did equal sections in which the bamboo reinforcement was seasoned culms treated with a brush coat of asphalt emulsion.

23. The load capacities for concrete members reinforced with unseasoned, seasoned or seasoned and treated bamboo culms, were increased by using split bamboo dowels as the diagonal tension reinforcement along the sections of the beams where the vertical shear was high.

24. The load capacities for concrete members reinforced with unseasoned, seasoned or seasoned and treated split sections of bamboo were increased by the use of a combination of split dowels and the bending up of the upper rows of the split bamboo from the bottom of the beam into the top and covering sections of the beams where the vertical shear is high.

25. Ultimate failure of bamboo reinforced concrete members usually was caused by
diagonal tension failures even though diagonal tension reinforcement was provided.

26. A study on the deflection data for all the beam specimens tested indicated:

(a) That the deflections of the beams when tested followed a fairly accurate straight line variation until the appearance of the first crack in the concrete.

(b) Immediately following the first crack, there was a pronounced flattening of the deflection curve (probably due to local bond slippage) followed by another period of fairly accurate straight line variation, but at a lesser slope, until ultimate failure of the member occurred. This flattening of the deflection curve was more pronounced in the members where the amount of longitudinal bamboo reinforcement was small.

(c) In all cases noted, the deflection curve had a lesser slope after the appearance of the first crack in the concrete, even though high percentages of bamboo reinforcement were used.

27 No pronounced variations were observed when the behavior of bamboo reinforced concrete members under flexure and having "tee" sections was compared with that of equal members having rectangular sections.

28 Bamboo reinforced concrete members under flexure and consisting of "tee" sections were no more effective than were equal rectangular sections, provided the breadth of the stem of the "tee" section was equal to that of the rectangular section and the effective depth of both were the same.

这些资料只是从材料学研究上有参考意义，对于中国建筑行业竹材填充混凝土是不是存在标准，是不是有具体的指导达到指定的强度要求，这些都是构成义乌案例关心的焦点。作为建筑非支撑墙到底需要多大的强度也是一个问题，这个施工队是不是充分考虑了竹子设计上的优缺点。如果假定他们都考虑到了，那么我倒希望国家都来通报赞扬一下这个承包商。

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